FEL2022

21 Aug 2022, 17:00 → 26 Aug 2022, 13:00 Europe/Zurich

Auditorium Generali (Trieste Convention Centre)

Luca Giannessi (Elettra), Michele Svandrlik (Elettra)

FEL2022 Trieste, Italy, August 22-26

On behalf of the International Executive Committee of the FEL Conference series, we are pleased to announce the 40th International Free Electron Laser Conference (FEL2022), to be held at the Trieste Convention Center (TCC) in Trieste, Italy, from August 22th to August 26th, 2022.

FEL 2022 will focus on recent advances in free electron laser theory and experiments, electron beam, photon beam, and undulator technologies, and applications of free electron lasers.

This edition is organised by Elettra Sincrotrone Trieste, an international, multidisciplinary research centre specialised in the generation of synchrotron and free-electron laser radiation together with their applications in material and life sciences.

The conference programme will include an optional tour of FERMI and the Elettra Storage ring.

Visit the official website at https://www.fel2022.org for further information.

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• Conference photos
• FEL2022-0-logo-header.jpg
• FEL2022-0-poster-home.jpg
• FEL2022-0-volumes-at-a-glance-cover.pdf
• FEL2022 Abstract Booklet
• Proceedings Cover
• Programme (synoptic table)

Sunday 21 August

17:00 Registration

18:00 Welcome Reception

Monday 22 August

08:45 Welcome

In memory of M. Billardon

1 In Memory of M. Billardon

Speaker: Marie-Emmanuelle Couprie (Synchrotron Soleil)

First lasing

Convener: Francesca Curbis (Lund University)

2 The Hard X-Ray Self-Seeding System at the European XFEL

This contribution describes, on behalf of the HXRSS team, design, installation, commissioning and operation of the Hard X-Ray Self-Seeding (HXRSS) system at the SASE2 FEL line of the European XFEL. We have reached up to mJ-level self-seeded pulses at 9-10 keV and the tested operational range is 6-13 keV. The setup can work in burst mode, that is following the bunch pattern of the European XFEL. The peculiarities of the European XFEL, that are high-repetition rate and long, tuneable undulators will be discussed, together with the impact of two-chicanes simultaneous seeding on the crystal heat loading. A discussion on possible future developments, including the production of self-seeded radiation at a harmonic of the fundamental, will complement the description of the current performance of the system.

Speaker: Gianluca Geloni (European XFEL GmbH)

3 First Lasing of Athos, the Soft X-Ray FEL Beamline of SwissFEL

Athos is the soft X-ray FEL beamline of SwissFEL at PSI in Switzerland. Its novel undulator layout consists of short Apple-X modules, capable of providing full polarization control, interleaved with short magnetic chicanes. This flexible configuration allows for many unique operational modes, giving control over FEL properties such as peak power, pulse duration and longitudinal coherence. This contribution presents the first lasing results of Athos, including SASE and some of the special operation modes.

Speaker: Eduard Prat (Paul Scherrer Institut)

4 Attoseconds at Harmonics at the European XFEL: First Results at SASE3

We report on the first observation of short, single-spike events generated at the SASE3 beamline of European XFEL via the “attosecond at harmonics method”. The approach was first proposed in [1]. We created bunching in the linear regime at around 0.5 keV and then, after bunching optimization by means of a magnetic chicane, we amplified the 4th harmonic bunching with a part of the undulator set to 2 keV. Due to the non-linear transformation of the bunching during the harmonic jump, radiation generated using this scheme occasionally exhibits single spike spectra (about a percent of the shots, which makes it attractive to use the method at high repetition-rate FELs). We expect those to correspond to single spikes in time-domain. We replicated the experiment numerically with the help of the GENESIS code.

Speaker: Andrei Trebushinin (European XFEL GmbH)

5 First Lasing of the COXINEL Seeded Free Electron Laser Driven by the HZDR Laser Plasma Accelerator

The COXINEL line has been designed at Synchrotron SOLEIL for electron beam manipulation in view of a seeded free electron laser using Laser plasma acceleration (LPA). After first studies on electron beam transport and undulator radiation in the spontaneous emission regime using LPA from Laboratoire d’Optique Appliquée (Ecole Polytechnique, France), the line has been moved to the HZDR, Dresden, Germany, for high quality LPA electrons driven by the DRACO laser. We report here on the demonstration of a seeded FEL at 275 nm driven by the HZDR LPA.

Speakers: Dr Marie-Emmanuelle Couprie (Synchrotron SOLEIL), Alexander Debus (Helmholtz-Zentrum Dresden-Rossendorf), Arie Irman (Helmholtz-Zentrum Dresden-Rossendorf), Charles Kitegi (Synchrotron SOLEIL), Christoph Eisenmann (Helmholtz-Zentrum Dresden-Rossendorf), Driss Oumbarek Espinos (Osaka University), Fabrice Marteau (Synchrotron SOLEIL), Julien Gautier (Laboratoire d'Optique Appliquée), Jurjen Couperus Cabadag (Helmholtz-Zentrum Dresden-Rossendorf), Maxwell LaBerge (The University of Texas at Austin), Michael Kuntzsch (Helmholtz-Zentrum Dresden-Rossendorf), Olena Kononenko (Laboratoire d'Optique Appliquée), Rene Gebhardt (Helmholtz-Zentrum Dresden-Rossendorf), Richard Pausch (Helmholtz-Zentrum Dresden-Rossendorf), Simon Grams (Helmholtz-Zentrum Dresden-Rossendorf), Uwe Helbig (Helmholtz-Zentrum Dresden-Rossendorf), Victor Malka (Weizmann Institute of Science)

6 SASE and Seeded FEL Powered by PWFA Electron Beam

We report on our first SASE and Seeded lasing achieved with electron beam accelerated with beam driven plasma wake-field acceleration technique.

Speaker: Vladimir Shpakov (Istituto Nazionale di Fisica Nucleare)

7 First Lasing of UCLA High Efficiency THz FEL

Here we report on the first lasing of the high efficiency THz FEL operating at the UCLA Pegasus laboratory. The FEL is operated in the zero-slippage regime where a circular waveguide is used to match the radiation and electron-beam velocities in a 0.96 m long tapered helical undulator, allowing resonant interaction with the ultrashort 200-pC 5.5-MeV electron beam from the RF photogun over an extended region. Electron-beam spectrum measurements, supported by energy and spectral measurement of the terahertz FEL radiation, indicate an average energy efficiency of ~10%, with some particles losing >20% of their initial kinetic energy.

Speaker: Pietro Musumeci (University of California, Los Angeles)

8 First Lasing of the Shanghai Soft X-Ray FEL User Facility

Here we report on the first lasing of the Shanghai Soft X-ray FEL user facility (SXFEL). We have achieved the saturation of SASE at 2 nm based on FEL-I and saturation of harmonic lasing self-seeding at 3 nm based on FEL-II. We have also achieved the first lasing of EEHG with a wavelength as short as 5.3 nm.

Speaker: Zihan Zhu (Shanghai Institute of Applied Physics)

9 First Lasing of the THz SASE FEL at PITZ

The Photo Injector Test Facility at DESY in Zeuthen (PITZ) develops a prototype of an accelerator-based high-power tunable THz source for pump-probe experiments at the European XFEL. The PITZ injector is also the site for the development and preparation of the high-brightness electron source for the main linac of the European XFEL and has the same pulse train structure as the X-ray photon source of the XFEL. For the proof-of-principle experiments on high-power THz generation an LCLS- I undulator (on loan from SLAC) is installed in the tunnel annex downstream of the existing accelerator. The extension of the beam line consists of a bunch compressor and a collimation system in the main PITZ tunnel, as well as a matching section, the undulator and the THz diagnostic setup in the tunnel annex. A Self-Amplified Spontaneous Emission (SASE) FEL is used to generate the THz pulses. High radiation power can be achieved by utilizing high charge (up to several nC) electron bunches from the PITZ photo injector. A beam energy of ~17 MeV is used to generate THz radiation with a centre wavelength of 100 μm. The transport of this space charge dominated electron beam and its thorough matching into the planar LCLS-I undulator with a strong vertical focusing is one of the project challenges. The installation of the first THz beamline setup was finished in summer 2022 and com-missioning with electron beam started. A specially developed procedure for a high charge beam matching into the undulator was successfully tested resulting in a first THz pulse generation. The start-up THz diagnostics is based on pyrodetectors. First measurements of the THz generation from 1,2 and 3 nC bunches have been taken, the statistics properties analysis corresponds to the expected SASE performance. The gain curve for the 3 nC case reflects the onset of saturation regime.

Speakers: Mikhail Krasilnikov (Deutsches Elektronen-Synchrotron DESY at Zeuthen), Raffael Niemczyk (Deutsches Elektronen-Synchrotron DESY at Zeuthen)

10:35 Coffee & Exhibition

FEL Prize

Convener: Vladimir Litvinenko (Stony Brook University)

10 Echo Effect in FELs Accelerators and Elsewhere

In recent years, echo-enabled harmonic generation (EEHG) demonstrated that it is capable to upshift the seed frequency in an FEL by almost two orders of magnitude. In this presentation, I will talk about the EEHG concept in FELs and show its connection to the echo effect known in other areas of physics (such as spin echo, plasma echo, echo in accelerators, etc.). The physics of the EEHG will be discussed as well is its major limitations.

Speaker: Gennady Stupakov (SLAC National Accelerator Laboratory)

11 Shaping and Control of Radiation Properties with Seeded FELs

The use of external lasers to initiate the FEL process has been introduced as a way to prepare the electron beam before entering the FEL radiator and to allow the generation of radiation pulses with increased brightness in shorter undulator. Over the recent years, new methods have been proposed and implemented to extend the capabilities of external seeding toward shorter wavelengths.
The experience gained at the FERMI seeded FEL user facility has boosted the developments of experimental techniques taking full advantage of the high degree of coherence of FEL pulses and the possibility to accurately control the radiation properties such has amplitude and phase.
Recent results together with future plans for externally seeded FEL facilities will be reported.

Speaker: Enrico Allaria (Elettra-Sincrotrone Trieste S.C.p.A.)

12 Microbunching of Relativistic Electron Beams

One of the fundamental facets of microbunching in relativistic electron beams is the potential for generation of coherent radiation at the wavelengths that characterize that periodic longitudinal modulation. This microbunching is an inherent process in the free-electron laser (FEL) mechanism for both single-pass and oscillator configurations. Besides the FEL output, diagnostics of these microbunched electron beams can be performed using coherent optical transition radiation (COTR) and imaging techniques in the former case. In these cases, the COTR from the microbunched portion of the beam in 6-D space generally dominates the images. Other mechanisms include the longitudinal-space-charge-induced microbunching in ultra-bright beams and laser-induced microbunching such as observed in laser wakefield accelerator beams. More recently, we consider the diagnostics of the TESSA** FEL concepts where a seed laser co-propagating with the electron beam through a short modulator and chicane may result in bunching fractions of >10 % leading to COTR enhancements of >22 million. Examples of these past, present, and future investigations will be discussed.

**Tapering Enhanced Super-radiant Stimulated Amplification (TESSA)

Speaker: Alex Lumpkin (Argonne National Laboratory)

13 Laser Controlled Free-Electron Lasers

Laser manipulation of electron beams is important for controlling free-electron lasers. In this talk, I will discuss how strong seeding can enable powerful, efficient FELs and high gradient acceleration when paired with strong tapering. I will then show how laser driven optical compression led to attosecond X-ray FEL pulses at LCLS and plans to use lasers to generate high repetition rate femtosecond and attosecond X-rays at LCLS-II.

Speaker: Joseph Duris (SLAC National Accelerator Laboratory)

14 Laser Assisted Beam Manipulation Techniques for Fully Coherent and Ultra-Short FEL Generation

Laser assisted beam manipulation has been extensively used in modern high gain FELs to tailor the output properties. In this talk, I will focus on several advanced techniques on high harmonic generation and ultrashort pulse generation with assistant of external UV or IR lasers. With the help of state-of-the-art laser technologies, it’s very likely these techniques will open new opportunities for coherent light sources.

Speaker: Chao Feng (Shanghai Advanced Research Institute)

12:55 Lunch

FEL Theory

Convener: Avraham Gover (University of Tel-Aviv)

15 Population Inversion X-Ray Laser Oscillator at LCLS

Cavity-based XFEL systems will potentially offer much higher spectral quality of the hard x-ray beam compared to traditional XFEL SASE and self-seeded sources. A promising cavity-based concept is the population inversion x-ray laser oscillator, dubbed XLO, where the SASE beam is used as a pump, and a transition metal serves as a gain medium. We will report on the progress in design and construction of the XLO, using LCLS as an x-ray pump, being developed by a SLAC, CFEL, University of Hamburg, University of Wisconsin, and UCLA collaboration. Initially, XLO will be demonstrated at the Coherent X-ray Imaging (CXI) LCLS end-station, as a two pass Regenerative Amplifier operating at the Copper Kalpha1 photon energy of 8048 eV. In the later phase of the project, it will utilize LCLS multi-bunch mode, with up to 8 x-ray pulses. Finally, XLO will generate fully coherent transform limited pulses with about 50 meV FWHM bandwidth. We expect the XLO will pave the way for new user experiments, e.g. in inelastic X-ray scattering, parametric down conversion, quantum science, X-ray interferometry.

Speaker: Mr Aliaksei Halavanau (SLAC National Accelerator Laboratory)

16 Attosecond Polarization Modulation of X-Ray Radiation in a Free-Electron Laser

Polarization is a fundamental property of light used in experiments to probe various properties of matter such as the chirality of molecules and crystal structures. There is increasing interest in generating bespoke radiation pulses for experiments with increasingly complex structures of polarization. At short wavelengths, free electron lasers offer an avenue to control the polarization structure at the point where the radiation is emitted through manipulation of the electron beam, removing the requirement for polarizing optics not readily available at x-ray wavelengths. This talk discusses a method for manipulating the polarization of FEL generated light based on temporal intensity modulation of radiation emitted in orthogonally polarized undulators. Simulations demonstrate the method can produce radiation that switches between orthogonal polarization states at attosecond timescales. Implementation of this ultra-fast polarization switching would provide a valuable new tool to the scientific community.

Speaker: Jenny Morgan (SLAC National Accelerator Laboratory)

17 Proposal for a Quantum Free Electron Laser Driven by Ultracold Electrons

Operation of a Quantum Free-Electron Laser (QFEL) could provide fully coherent X- and gamma-rays in a compact setup. Imperative to experimental realization is allowing for decoherence of either spontaneous emission or space-charge to take place, having opposing constraints [1]. Here, for the first time, we discuss a comprehensive QFEL model that takes into account both decoherence effects. Then, we use this model to investigate the ultracold electron source (UCES) [2] as a potential QFEL electron injector. The UCES, based on near-threshold photoionization of laser-cooled and trapped atomic gas, has the unique property of allowing highly charged electron bunches to be extracted while maintaining ultralow transverse emittance. We find that the ultracold electron bunches meet the stringent requirement for potential QFEL operation with commercially available laser systems.

Speakers: Brian Schaap (Technische Universiteit Eindhoven), Jom Luiten (Technische Universiteit Eindhoven), Mr Sander Schouwenaars (Technische Universiteit Eindhoven)

18 Quantum Diffusion Due to Coherent Radiation

Quantum diffusion is caused by the recoil effect that a particle experiences when it emits a photon [1]. Quantum diffusion due to the synchrotron radiation in high-energy electron and positron circular accelerators defines the main parameters of the beam: its energy spread and hence the bunch length, as well as the horizontal emittance. It is calculated as a single particle effect assuming incoherent radiation. This assumption is not valid in FELs where the radiation is coherent. In this work, we develop theory of the quantum diffusion in coherent radiation and show that it leads to the energy diffusion of the particles that is correlated between the different positions in the bunch.

Speaker: Gennady Stupakov (SLAC National Accelerator Laboratory)

Monday posters: Coffee & Exhibition

19 Laser-Induced Gas Breakdown at KU-FEL

Laser-induced breakdown has been observed in various combination of gases and lasers. It is known from previous studies that breakdown occurs when the number of electrons exceeds a threshold value through the following stages: generation of seed electrons, acceleration of electrons by inverse Bremsstrahlung, and avalanche multiplication of electrons by impact ionization.
We have observed laser-induced breakdown of gases at KU-FEL, the FEL oscillator at Kyoto University. In the thermionic cathode mode (2856 MHz repetition rate, 10 um), breakdown was observed in air, nitrogen, and argon, while no discharge was observed in the photocathode mode operation (29.75 MHz, 9 um). The difference in the two operation modes can be explained by the diffusion of electrons between the micropulses.
This work was supported by MEXT Q-LEAP (JPMXS0118070271), IAE ZE Research Program (ZE2022B-23) and JSPS KAKENHI (22H03881).

Speaker: Ryoichi Hajima (National Institutes for Quantum and Radiological Science and Technology)

20 A Combination of Harmonic Lasing Self-Seeded FEL with Two-Color Lasing

The free-electron laser FLASH at DESY can produce SASE-FEL pulses in the extreme ultraviolet to the soft X-ray region. The flexibility of the variable gap undulators in the FLASH2 beamline opens a wide range of scientific opportunities. Different advanced lasing schemes have been tested in the past years, like the frequency doubler scheme, two-color lasing, and harmonic lasing self-seeded FEL (HLSS). A recent user experiment required parameters not yet provided: a similar power in the fundamental and the third harmonic. To fulfill these requirements, a new way of lasing had to be developed ad hoc. A combination of HLSS and two-color lasing has been identified as the appropriated scheme to deliver a tailored two-color beam to the user experiment. In this article we describe difficulties of the setup and discuss the results achieved.

Speakers: Juliane Roensch-Schulenburg (Deutsches Elektronen-Synchrotron), Siegfried Schreiber (Deutsches Elektronen-Synchrotron), Mathias Vogt (Deutsches Elektronen-Synchrotron), Marion Kuhlmann (Deutsches Elektronen-Synchrotron), Evgeny Schneidmiller (Deutsches Elektronen-Synchrotron)

21 An Attosecond Scheme Overcoming Coherence Time Barrier in SASE FELs

In Self-Amplified Spontaneous Emission Free Electron Laser (SASE FEL) based short-pulse schemes, pulse duration is limited by FEL coherence time. For hard X-ray FELs, coherence time is in a few hundred attosecond range while for XUV and soft X-ray FELs it is in the femtosecond regime. In this paper the modification of so-called chirp-taper scheme is developed that allows to overcome the coherence time barrier. Numerical simulations for XUV and soft X-ray FEL user facility FLASH demonstrate that one can generate a few hundred attosecond long pulses in the wavelength range 2 - 10 nm with peak power reaching hundreds of megawatts.

Speaker: Evgeny Schneidmiller (Deutsches Elektronen-Synchrotron)

22 An X-Ray Regenerative Amplifier Free-Electron Laser Using Diamond Pinhole Mirrors

X-ray free-electron lasers (FELs) rely on SASE due to the lack of seed lasers and the difficulty in obtaining mirrors. Progress in diamond crystal Bragg mirrors enables the design of x-ray FEL oscillators. Regenerative amplifiers (RAFELs) are high gain/low-Q oscillators that out-couple most of the optical power. An x-ray RAFEL based on the LCLS-II at SLAC using a six-mirror resonator out-coupling 90% or more through a pinhole in the first downstream mirror is analyzed using the MINERVA and OPC to model the optical field within the undulator and the remainder of the resonator respectively.1 Results show substantial powers at the fundamental (3.05 keV) and 3rd harmonic (9.15 keV).

1. H.P. Freund, P.J.M. van der Slot, and Yu. Shvyd’ko, “An X-Ray Regenerative Amplifier Free-Electron Laser Using Diamond Pinhole Mirrors,” New J. Phys. 21, 093028 (2019).

*This research was supported under DOE Contract DE-SC0018539. Work at Argonne National Laboratory was supported by the U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357. We also thank the University of New Mexico Center for Advanced Research Computing, supported in part by the National Science Foundation, for providing high performance computing resources used for this work.

Speaker: Henry Freund (University of New Mexico)

23 Analyses Supporting the 2-Color Upgrade to the IR FEL at FHI Berlin

This paper provides a summary of the analyses that led to the definition of the 2-color upgrade of the IR FEL at FHI Berlin. We briefly cover several different aspects of the design, beginning with the beam dynamics of the second far-IR beamline, engineering considerations of that physics design, and the FEL physics that defined the short-Rayleigh range undulator as well as aspects of the undulator design itself. Additionally, we touch on the approach to 2-color commissioning with pulse picking, as well as considerations for the far-IR optical transport to users. The status of commissioning is described in a parallel paper at this Conference by W. Schöllkopf et al.

Speakers: Alan Todd (AMMTodd Consulting), William Colson (WBC Physics), David Dowell (SLAC National Accelerator Laboratory), Stephen Gottschalk (STI Magnetics LLC), John Rathke (JW Rathke Engineering Services), Thomas Schultheiss (TJS Technologies), Lloyd Young (LMY Technology), Wieland Schöllkopf (Fritz-Haber-Institut der Max-Planck-Gesellschaft), Marco De Pas (Fritz-Haber-Institut der Max-Planck-Gesellschaft), Sandy Gewinner (Fritz-Haber-Institut der Max-Planck-Gesellschaft), Heinz Junkes (Fritz-Haber-Institut der Max-Planck-Gesellschaft), Gerard Meijer (Fritz-Haber-Institut der Max-Planck-Gesellschaft), Gert von Helden (Fritz-Haber-Institut der Max-Planck-Gesellschaft)

24 Analysis of Ultra-Short Bunches in Free-Electron Lasers

Free-electron lasers (FELs) operate at wavelengths from millimeter waves through hard x-rays. At x-ray wavelengths, FELs typically rely on self-amplified spontaneous emission SASE emission which contains multiple temporal “spikes” that limit the longitudinal coherence of the optical output; hence, alternate methods that improve on the longitudinal coherence of the SASE emission are of interest. In this paper, we consider electron bunches that are shorter than the SASE spike separation.1 In such cases, the spontaneously generated radiation consists of a single optical pulse with improved longitudinal coherence than is found in typical SASE FELs. To investigate this regime, we use two FEL simulation codes. One (MINERVA) uses the slowly-varying envelope approximation (SVEA) which breaks down for extremely short pulses. The second (PUFFIN) is a particle-in-cell (PiC) simulation code that is considered to be a more complete model of the underlying physics and which is able to simulate very short pulses. We first anchor these codes by showing that there is substantial agreement between the codes in simulation of the SPARC SASE FEL experiment at ENEA Frascati. We then compare the two codes for simulations using electron bunch lengths that are shorter than the SASE spike separation. The comparisons between the two codes for short bunch simulations elucidate the limitations of the SVEA in this regime but indicate that the SVEA can treat short bunches that are comparable to the cooperation length.

1. L.T. Campbell, H.P. Freund, J.R. Henderson, B.W.J. McNeil, P. Traczykowski, and P.J.M. van der Slot, “Analysis of Ultra-Short Bunches in Free-Electron Lasers,” New. J. Phys. 22, 073031 (2020).

*The research used resources of the Argonne Leadership Computing Facility, which is a DOE Office of Science User Facility supported under contract DE-AC02-06CH11357. We also thank the University of New Mexico Center for Advanced Research Computing, supported in part by the National Science Foundation, for providing high performance computing resources used for this work .Funding is also acknowledged via the following grants: Science and Technology Facilities Council (Agreement Number 4163192 Release #3); ARCHIE-WeSt HPC, EPSRC grant EP/K000586/1; John von Neumann Institute for Computing (NIC) on JUROPA at Jülich Supercomputing Centre (JSC), project HHH20. The authors acknowledge helpful discussions with L. Giannessi

Speaker: Henry Freund (University of New Mexico)

25 Bringing Genesis to the Cloud with Sirepo

Genesis is widely used in the free electron laser community as a simulation tool for studying both simple and complex FEL systems. Until now, this has necessitated learning the command line interface, which can be challenging for new users. Sirepo Genesis provides an intuitive graphic interface for building Genesis simulations in the browser that can then be run using our cloud computing services. Our interface also provides the ability to export simulations for command line use, simulation post-processing with publication quality graphics and the power to share their results with the click of a button to anyone, anywhere, in the world. This poster describes our new GUI and highlights the notable features that have been developed.

Speaker: Christopher Hall (RadiaSoft LLC)

26 Corrugated Structure System for Fresh-Slice Applications at the European XFEL

Fresh-slice lasing using wakefield induced time-dependent orbit oscillation is capable of producing high intensity two-color XFEL pulses and high power short pulses at femtosecond level. At the European XFEL, a corrugated structure system for fresh-slice applications for both the hard x-ray beamline SASE1 and the soft x-ray SASE3 beamline is being developed and implemented. In this contribution, we present the novel design of the corrugated structure and the application to advanced lasing schemes.

Speaker: Weilun Qin (Deutsches Elektronen-Synchrotron)

27 Demonstration of Large Bandwidth Mode with a Spatially Tilted Beam at SwissFEL

X-ray absorption spectroscopy (XAS) with a SASE signal can be improved if the full XAS and reference spectrum are taken on a shot-to-shot basis, thus eliminating the impact of the intrinsic SASE fluctuations in the spectrum. This can be further improved if the FEL pulse has the frequency information encoded in its spatial position. The spatial encoding is achieved when a spatially tilted electron beam with a strong energy chirp is injected into a focusing-free undulator channel. We report on the demonstration of this mode at the hard X-ray beamline Aramis at SwissFEL. Possible applications and an outlook for further studies are discussed.

Speaker: Sven Reiche (Paul Scherrer Institut)

28 Detail Study for the Laser Activating Reflective Switch for THz Free Electron Laser

THz free electron laser at SANKEN, Osaka university generates a train of THz pulses with the interval of 27 MHz in the repetition of 5 Hz. The number of pulses in a train is about 100. Single pulse energy exceeds 200 \muJ at the carrier frequency of 4.5 THz. To extract a single pulse from the train, the reflective switch of the electron-hole plasma on the surface of Gallium Arsenide wafer driven by the Ti:sapphire laser pulse was constructed and the characteristics of the switch is studied. By evaluating also the characteristics of silicon and germanium wafers, the comparison experiments are performed. In addition, the study for carrier dynamics with the time scale of microseconds by measuring the variations of reflected and transmitted THz pulses with the interval of 27 MHz are being conducted. We report the recent results of the switching for the THz pulse and its time evolution in this conference.

Speaker: Keigo Kawase (National Institutes for Quantum and Radiological Science and Technology)

29 Evolution of Microbunching in Drift Sections

The typical layout adopted in a seeded harmonic generation free-electron laser is based on radiator undulators placed immediately after the dispersive section, where the bunching is created.
With the advent of new and more complex seeding schemes, this solution cannot always be implemented and cases, where the bunched beam needs to be propagated in free space before entering the radiator, should be investigated.
The evolution of the density modulation in a drift may also plays a role on long intra-undulator sections in short wavelength FELs.
We report here on recent studies aimed at investigating the impact of bunching evolution in a drift space on coherent harmonic emission. Experimental results collected at the FERMI free-electron laser are compared with theoretical and numerical predictions.

Speakers: Shaukat Khan (TU Dortmund University), Alexander Brynes (Elettra-Sincrotrone Trieste S.C.p.A.), Carlo Spezzani (Elettra-Sincrotrone Trieste S.C.p.A.), Enrico Allaria (Elettra-Sincrotrone Trieste S.C.p.A.), Eugenio Ferrari (Deutsches Elektronen-Synchrotron), Evgeny Schneidmiller (Deutsches Elektronen-Synchrotron), Giuseppe Penco (Elettra-Sincrotrone Trieste S.C.p.A.), Luca Giannessi (Elettra Sincrotrone Trieste and Istituto Nazionale di Fisica Nucleare), Primoz Rebernik Ribic (Elettra-Sincrotrone Trieste S.C.p.A.), Simone Spampinati (Elettra-Sincrotrone Trieste S.C.p.A.), Filippo Sottocorona (Elettra-Sincrotrone Trieste S.C.p.A. and University of Trieste), Giovanni Perosa (Elettra-Sincrotrone Trieste S.C.p.A. and University of Trieste), Mauro Trovo (Elettra-Sincrotrone Trieste S.C.p.A.), Giovanni De Ninno (Elettra-Sincrotrone Trieste S.C.p.A. and University of Nova Gorica)

30 FEL Performance of the EuPRAXIA@SPARC LAB AQUA Beamline

The AQUA beamline of the EuPRAXIA@SPARC_LAB infrastructure consists of a Free-Electron Laser facility driven by an electron beam with 1 GeV energy, produced by an X-band normal conducting LINAC followed by a plasma wakefield acceleration stage, with the goal to deliver variable polarization photons in the 3-4 nm wavelength range. Two undulator options were considered for the AQUA FEL amplifier, a 16 mm period length superconducting undulator and an APPLE-X variable polarization permanent magnet undulator with 18 mm period length. The amplifier is composed by an array of ten undulator sections 2m each. Performance associated to the electron beam parameters and to the undulator technology is investigated and discussed.

Speaker: Federico Nguyen (Ente per le Nuove Tecnologie, l'Energie e l'Ambiente)

31 First Commissioning of the Proof-of-Principle Experiment on a THz SASE FEL at the PITZ Facility

Research and development of an accelerator-based THz source prototype for pump-probe experiments at the European XFEL are ongoing at the Photo Injector Test Facility at DESY in Zeuthen (PITZ). A proof-of-principle experiment to generate THz SASE FEL radiation using an LCLS-I undulator driven by an electron bunch from the PITZ accelerator has been prepared. After four years of designs and construction, the first commissioning with an electron beam was started in July 2022. This paper presents and discusses the experience and results of the first commissioning.

Speakers: Mikhail Krasilnikov (Deutsches Elektronen-Synchrotron DESY at Zeuthen), Anja Sandmann-Lemm (Deutsches Elektronen-Synchrotron DESY at Zeuthen), Christopher Richard (Deutsches Elektronen-Synchrotron DESY at Zeuthen), Dr Mikhail Yurkov (Deutsches Elektronen-Synchrotron), Evgeny Schneidmiller (Deutsches Elektronen-Synchrotron), Bernward Krause (Deutsches Elektronen-Synchrotron), Markus Tischer (Deutsches Elektronen-Synchrotron), Pavel Vagin (Deutsches Elektronen-Synchrotron)

32 First Study of Fresh-Slice Multi-Stage Amplification at SwissFEL

We report on the first demonstration of generating high-power and short FEL pulses using the fresh-slice multi-stage amplification scheme at Athos, the soft X-ray beamline of SwissFEL. We use a transversely tilted electron beam traveling through the unique Athos layout with magnetic chicanes between every two undulator modules. Our first results show the production of FEL radiation with pulse energies of few hundreds microjoules and pulse durations at the femtosecond level. This operation mode will allow us to advance the scientific opportunities of single-particle imaging experiments.

Speaker: Guanglei Wang (Paul Scherrer Institut)

33 Flexible Operation Modes for EuXFEL

A major challenge in single-linac - multiple undulator setups like EuXFEL is the generation of individual shaped photon pulses, in particular, when working in a mode where a single pulse train, or cw stream, feeds all undulator lines. This work presents the experimental verification of a flexible delivery scheme producing photon pulses for each of the three undulator lines with their electron bunches individually shaped in charge, compression and optics on a single RF pulse burst.

Speakers: Marc Guetg (Deutsches Elektronen-Synchrotron), Bolko Beutner (Deutsches Elektronen-Synchrotron), Julien Branlard (Deutsches Elektronen-Synchrotron), Frank Brinker (Deutsches Elektronen-Synchrotron), Winfried Decking (Deutsches Elektronen-Synchrotron), Ingmar Hartl (Deutsches Elektronen-Synchrotron), Raimund Kammering (Deutsches Elektronen-Synchrotron), Dirk Lipka (Deutsches Elektronen-Synchrotron), Nils Lockmann (Deutsches Elektronen-Synchrotron), Najmeh Mirian (Deutsches Elektronen-Synchrotron), Evgeny Schneidmiller (Deutsches Elektronen-Synchrotron), Henrik Tünnermann (Deutsches Elektronen-Synchrotron), Thomas Wamsat (Deutsches Elektronen-Synchrotron), Gianluca Geloni (European XFEL GmbH), Natalia Gerasimova (European XFEL GmbH), Naresh Kujala (European XFEL GmbH), Svitozar Serkez (European XFEL GmbH)

34 Gaussian Random Field Generator SERVAL: a Novel Algorithm to Simulate Partially Coherent Undulator Radiation

We propose a computationally-efficient algorithm to calculate the field of partially coherent synchrotron radiation pulses from undulators. Wavefront propagation simulations play a pivotal role in designing beamline optics at new synchrotron radiation sources. However, they do not account for the stochastic behaviour of the initial radiation field, which is due to shot noise in the electron beam with finite transverse size and divergence. We present an algorithm that allows us to obtain and propagate radiation fields containing multiple transverse stochastic modes within undulator resonance. The proposed algorithm relies on a method for simulating Gaussian random fields. We initially generate the field as Gaussian white noise, and then we restrict its extent in the direct and in the reciprocal domains by using averaged radiation size and divergence. Strictly speaking, this procedure shapes the correct correlation function of the field only under the assumption of quasi-homogeneity. However, we show that the method can be applied with reasonable accuracy also outside of this assumption. We check consistency of the algorithm with the help of well-established approaches in simulating partially coherent undulator fields. Finally, the proposed method is well-suited for educational purposes.

Speakers: Andrei Trebushinin (European XFEL GmbH), Gianluca Geloni (European XFEL GmbH), Svitozar Serkez (European XFEL GmbH)

35 Generation of X-Ray Vortex Beams in a Free-Electron Laser Oscillator

Light with orbital angular momentum (OAM) provides new insights into a wide range of physical phenomena and has engendered advanced applications in various fields. Additionally, research interest in X-ray OAM has been rapidly increasing. Here, we report a straightforward method capable of generating intense OAM beams from an X-ray free-electron laser oscillator (XFELO). This method leverages Bragg mirrors and longitudinal-transverse mode coupling to enable mode selection in a conventional XFELO configuration, thereby natively producing fully coherent hard X-ray beams carrying OAM. Simulation results indicate that fully coherent hard X-ray OAM beams can be generated without the need for optical mode converters. This simple approach can significantly advance the creation of X-ray OAM while stimulating the development of novel experimental

Speakers: Haixiao Deng (Shanghai Advanced Research Institute Chinese Academy of Science), Jiawei Yan (European XFEL GmbH), Nanshun Huang (Shanghai Institute of Applied Physics)

36 Improvement of XFEL Brightness at PAL-XFEL

We (PAL-XFEL) have improved the PAL-XFEL performances remarkably since the user-service operation in 2017. We achieved the self-seeded XFEL with a peak brightness of 3.2 × 10^35, the highest to date, and reached a SASE FEL intensity of 3.2 mJ due to the improved beam emittance of 0.3 mm-mrad. The statistics of the SASE FEL intensities over the past three years show that the SASE FEL performance is critically dependent on the injector emittance.

Speaker: Teyoun Kang (Pohang Accelerator Laboratory)

37 Lasing Performance of the European XFEL

The European XFEL operates with 3 different SASE FELs served by variable gap undulators. In addition, the electron energy of the superconducting linear accelerator is varied between 8.5 and 16.5 GeV to cover a photon energy range from 500 eV to 30 keV. We will present SASE performance data collected over the past 5 years of operation and compare them with theoretical predictions.

Speakers: Winfried Decking (Deutsches Elektronen-Synchrotron DESY at Zeuthen), Dr Matthias Scholz (Deutsches Elektronen-Synchrotron DESY at Zeuthen)

38 Low-Emittance Beam Injection from SACLA to SPring-8

The SACLA linear accelerator has been successfully used as a full-energy injector of the SPring-8 storage ring since 2020. In order to perform the beam injection in parallel with XFEL operation, three accelerators are virtually constructed in a control system. Thus the electron beam parameters, such as the beam energy, are independently tuned for the beam injection and the two XFEL beamlines. By shutting down dedicated old injector accelerators, the electricity consumption has been reduced by roughly 20-30 % and its maintenance cost is no more necessary. SACLA will also provide the electron beam for the future SPring-8-II, which requires low-emittance beams for injection due to its small beam aperture. In this presentation, we summarize the beam injection scheme developed at SACLA including observed emittance increase at a beam transport caused by quantum excitation of synchrotron radiation, a synchronization system between the two accelerators, and a purification method of the electron bunch in the storage ring.

Speaker: Toru Hara (RIKEN SPring-8 Center)

39 MINERVA Code Release Announcement

MINERVA is a 3-D, time-dependent simulation code of FEL amplifiers, low-gain/high-Q and high-gain/low-Q oscillators, optical klystrons (including high-gain harmonic generation) and SASE configurations [1-7]. Oscillator simulations are done in conjunction with OPC [8]. MINERVA uses the Message Passing interface on Linux, Macintosh and Windows systems and has been successfully benchmarked against many experiments. Particle dynamics are treated using the full Lorentz force equations to track particles through the optical and magnetostatic fields. Hence, MINERVA treats both fundamental and (linear and nonlinear) harmonic generation from first principles. The optical field is a superposition of Gaussian modes using the slowly-varying envelope approximation in which the x- and y-components of the field are integrated independently, and tracks the particles and fields as they propagate along the undulator line from the start-up through linear growth and into the nonlinear regime using either 2nd or 4th order Runge-Kutta integrators. MINERVA includes 3-D descriptions of planar, helical, and elliptical undulators (including a model of an APPLE-II undulator) with the fringing fields in the entry/exit transition regions. Magnetostatic field models for quadrupoles and dipoles are also included. As such, MINERVA implicitly simulates the evolution of the polarization of the optical field through an arbitrary sequence of undulators. MINERVA and OPC can be downloaded from
MINERVA: https://gitlab.utwente.nl/tnw/ap/lpno/public-projects/MINERVA/-/releases
OPC: https://gitlab.utwente.nl/tnw/ap/lpno/public-projects/Physics-OPC/-/releases
as well as user manuals, release notes and sample scripts showing to run MINERVA/OPC.

1. H.P. Freund, P.J.M. van der Slot, D.L.A.G. Grimminck, I.D. Setya and P. Falgari, “3-D, time-dependent simulation of FELs with planar, helical, and elliptical undulators,” New J. Phys. 19, 023020 (2017).
2. H.P. Freund and P.J.M. van der Slot, “Studies of a terawatt x-ray FEL,” New J. Phys. 20, 073017 (2018).
3. H.P. Freund, P.J.M. van der Slot, and Yu. Shvyd’ko, “An x-ray Regenerative Amplifier FEL using diamond pinhole mirrors,” New J. Phys. 21, 093028 (2019).
4. L.T. Campbell, H.P. Freund, J.R. Henderson, B.W.J. McNeil, P. Traczykowski, and P.J.M. van der Slot, “Analysis of ultra-short bunches in FELs,” New. J. Phys. 22, 073031 (2020).
5. H.P. Freund and P.J.M. van der Slot, “Variable polarization control in FELs,” J. Phys. Commun. 5, 085011 (2021).
6. P.J.M. van der Slot and H.P. Freund, “3-D, time-dependent analysis of high- and low-Q FEL oscillators,” Appl. Sci. 11, 4978 (2021).
7. H.P. Freund, D.C. Nguyen, P.A. Sprangle, and P.J.M. van der Slot, “3-D, time-dependent simulation of a Regenerative Amplifier FEL,” Phys. Rev. ST-AB 16, 010707 (2013).
8. J.G. Karssenberg, P.J.M. van der Slot, I.V. Volokhine, J.W.J. Verschuur and K.-J. Boller “Modeling paraxial wave propagation in FEL oscillators,” J. Appl. Phys. 100, 093106 (2006).

Speakers: Henry Freund (University of New Mexico), Peter van der Slot (University of Twente)

40 Modelling of Sub-Wavelength Effects in a FEL Oscillator

Previous studies of FEL oscillators typically use averaged simulation codes which cannot model sub-wavelength effects, such as Coherent Spontaneous Emission from the electron pulse. In this paper, the unaveraged FEL simulation code Puffin is used with the optics code Optical Propagation Code to model the FEL in three dimensions, enabling sub-wavelength effects to be modelled at the FEL interaction and cavity length scales. The parameters used are very similar to those of the IR-FEL of [1].
Results show that CSE does drive the FEL interaction during the start-up phase in the cavity. Further, cavity detuning effects at the sub-wavelength scale can have an effect upon the FEL output from start-up through to the steady state output.
While the effects are demonstrated here at the fundamental level, it can be expected that they may be reduced due to limitations such as electron beam and/or cavity length jitter at the wavelength scale. Such effects will need to be further investigated.

Speakers: Pornthep Pongchalee (University of Strathclyde, Cockcroft Institute and ASTeC, STFC Daresbury Laboratory), Brian McNeil (University of Strathclyde and Cockcroft Institute)

41 Nonlinear Spectroscopy at the THz-Beamline TeraFERMI

TeraFERMI is a THz beamline at the free-electron laser (FEL) FERMI. After passing the FEL's undulator, the electron bunches are refocused on a thin dielectric slab and generate coherent transition radiation (CTR), namely strong THz pulses. TeraFERMI provides single-cycle pulses with a broadband spectrum in the range from 0.1 THz to 6 THz and strong peak electric fields with up to 4 MV/cm or peak magnetic fields of up to 1 T well-suited for nonlinear experiments driving systems out of equilibrium. The low repetition rate of 50 Hz allows the systems under study to fully relax to their equilibrium state between consecutive pulses, thereby avoiding unwanted thermal effects, a feature which is particularly requested in interdisciplinary experiments in biophysics or chemical physics. A particular property of the THz radiation (CTR-sources) is the radial polarization, which allows for studies with longitudinal spectroscopy exploiting an electric field polarization parallel to the propagation direction of focused beams. Furthermore, all time-resolved experiments benefit from the excellent synchronization between THz-pulses and a local near-infrared laser with a low timing jitter of 66 fs. In this contribution we report about the latest progress at TeraFERMI including a recently implemented diagnostic station as well as experimental setups we provide at TeraFERMI.

Speaker: Johannes Schmidt (Elettra-Sincrotrone Trieste S.C.p.A.)

42 Optical-Cavity Based Seeded FEL Schemes toward Higher Repetition Rate and Shorter Wavelengths

More and more high-gain SASE FELs operate at high repetition rates, either in burst or in continuous wave mode of operation, offering an unprecedented number of electron bunches per second. External seeding techniques provide high quality FEL pulses of full coherence and shot-to-shot stability but cannot keep up with MHz repetition rates of such FELs due to their dependence on the seed laser repetition rate. One attractive solution to overcome this limitation is to employ an optical cavity to store radiation that acts as a seed for the electron bunches arriving at high repetition rates. Such a scheme not only allows seeded operation at multi-MHz repetition rates but also introduces the possibility to achieve seeded radiation at shorter wavelengths, overcoming the hurdle of insufficient power availability of seed laser systems in the vacuum ultraviolet (VUV) wavelength range. Here, we present different optical-cavity-based schemes and we give an overview of their unique capabilities together with simulation results.

Speakers: Georgia Paraskaki (Deutsches Elektronen-Synchrotron), Sven Ackermann (Deutsches Elektronen-Synchrotron), Gianluca Geloni (European XFEL GmbH), Dr Bart Faatz (Shanghai Advanced Research Institute), Bo Liu (Shanghai Advanced Research Institute), Chao Feng (Shanghai Advanced Research Institute), Hao Sun (Shanghai Advanced Research Institute)

43 Optimization of Waveguide and Wire-Grid-Polarizer for Waveguide-Based Optical Resonator of Compact THz FEL

At KAERI we are developing compact Terahertz(THz) Free Electron Laser(FEL) for a commercial application like security inspection. We are using a waveguide-based optical resonator for our FEL system. Firstly, we report on the selection of low loss and small cross-section waveguide candidate to enhance the gain of THz FEL. We performed a detailed analysis of waveguides with different shapes and size via COMSOL Multiphysics simulation. Based on the simulation result, we found that a waveguide with a special eye-shaped cross-section has a very small cross-sectional area of 4 mm$^2$ at full width at half maximum (FWHM) and a very low wave loss of less than 2.5% for 1-m propagation at an operating wavelength of 300–600 µm.
Secondly, we accomplished the optimization of the Wire-Grid-Polarizer (WGP) design parameter via COMSOL Multiphysics code, we calculated the optimized values of grating length and grating period of the WGP i.e., 20 µm and 100 µm, which have less than 10% of loss for the wavelength of 300–600 µm. The material which we used for WGP simulation is Tungsten coated with gold because it has a high extinction ratio and transmission as compared with common grating material.

Speaker: Varun Pathania (Korea Atomic Energy Research Institute)

44 Origin of Echo-Enabled Harmonic Generation

Echo-Enabled Harmonic Generation (EEHG) became a very promising and very popular technique after original publication [1]. As it commonly happens, this was the reinvention of already known technique, which was not broadly known in FEL community. EEHG vaguely reminded me of theory developed at Novosibirsk Institute of Nuclear Physics (BINP), but I was not sure that my memory is correct. Recently, BINP made their preprint available on the web and I was able to confirm that technique proposed by I.G. Idrisov and V.N. Pakin [2,3] based on the same principles as described in [1]. In this presentation I would like to briefly review this original invention and to give historic perspective to EEHG.

[1] G. Stupakov, Phys. Rev. Lett. 102, 074801 (2009)
[2] I.G. Idrisov and V.N. Pakin, “High efficiency cascade bunching using a single frequency modulation” Preprint 80-197 of Institute for Nuclear Physics, October 3, 1980, Novosibirsk, Russia (in Russian)
[3] I.G. Idrisov and V.N. Pakin, “High efficiency bunching of ultra-relativistic beams using magnetic compressors”, Preprint 80-192 of Institute for Nuclear Physics, October 3, 1980, Novosibirsk, Russia (in Russian)

Speaker: Vladimir Litvinenko (Stony Brook University)

45 Origin of the Complex Beam Profile of a Hole-Coupled Free Electron Laser Oscillator

Infrared FEL oscillators generally use hole-coupling to extract intracavity laser power. The hole-coupling inherently causes a non-Gaussian beam profile at user stations, which are more than 10 m apart from the coupling hole. It is due to the existence of the Airy pattern in the extracted laser beam. We demonstrated that the beam profile can be changed from a non-Gaussian to a nearly Gaussian distribution by removing the Airy pattern in the experiments and physical optics calculations [1]. This work was supported by MEXT Q-LEAP (JPMXS0118070271).

Speakers: Heishun Zen (Kyoto University), Hideaki Ohgaki (Kyoto University)

46 Ponderomotive Prebunching for Spontaneous Superradiant and Stimulated Thomson Scattering

Compact sources offering high-brightness radiation in the EUV to X-ray regime are highly desired. Thomson scattering, in which an electron beam colliding with a laser pulse produces radiation, is a source of X-rays of increasing prevalence in modern labs, complementing large scale facilities like synchrotrons and X-ray free electron lasers. By imposing a density modulation on the electron beam the brilliance of a Thomson source can be enhanced by orders of magnitude via superradiant emission. However, microbunching at the beam energy relevant to Thomson sources is a challenge that has yet to be met. Here, we analytically and numerically analyze electron beam modulation via the ponderomotive force from the copropagating beat wave formed by two laser pulses at different frequencies. First, we find that energy modulation favorably scales with electron beam energy, but is limited by the interaction length imposed by the finite size of the laser pulses. Next, we quantify the brightness of a Thomson source including a ponderomotive buncher that is optimized for superradiant emission. Last, we investigate under which conditions the spontaneous superradiant Thomson regime transitions into a stimulated Thomson (FEL-)regime, potentially allowing for even further increase of source brightness.

Speaker: Brian Schaap (Technische Universiteit Eindhoven)

47 Proposed FEL Schemes and their Performance for the Soft X-Ray Free Electron Laser (SXL) at the MAX IV Laboratory

The existing MAX IV 3 GeV linac could drive, with minor improvements, a soft X-ray Free Electron Laser and the aim of the SXL project has been so far to deliver a conceptual design of such a facility in the 1—5 nm wavelength range.
The project was initiated by a group of Swedish users of FEL radiation and the design work was supported by the Knut and Alice Wallenberg foundation and by several Swedish universities and organizations (Stockholm, Uppsala, KTH Royal Institute of Technology, Stockholm-Uppsala FEL center, MAX IV laboratory and Lund University).
In this paper we will focus on the baseline FEL performance based on two different accelerator operation modes (medium and short pulses) and give some hints of future developments after the first phase of the project such as 2 color/2pulses and HB-SASE.

Speaker: Francesca Curbis (Lund University)

48 Protected Mirrors Enabling Storage Ring FEL Lasing below 170 nm

In a storage ring free-electron laser (FEL) the cavity mirrors have to resist the harsh operational conditions due to high-energetic and background radiation in an ultra-high vacuum environment. For the wavelength between 120 and 190 nm only fluoride materials are suitable as coating material for high reflective mirrors. However, used in the bare form, they are not stable for extreme FEL operation conditions. Until this work, it was not possible to achieve the lasing below 176 nm with an oscillator FEL. The collaboration between DUKE University/TUNL and Laser Zentrum Hannover e.V. has recently demonstrated the storage-ring FEL lasing between 169.6 and 176.7 nm. For this work, different coating techniques such as ion beam sputtering, thermal evaporation, and atomic layer deposition were employed to produce samples of a single-layer, multilayers, and a version with a protection layer. All samples were irradiated using Duke FEL undulators and characterized with VUV spectrometry from 140 to 230 nm. We have found that the $\text{SiO}_2$-protected fluoride coatings have good thermal stability and radiation resistance. Several sets of mirrors have been coated and used to demonstrate the FEL lasing in a new VUV range with a reasonable lifetime. These mirrors have also been used to generate 120 MeV gamma rays via Compton scattering. The newly developed coating strategies are expected to enable the storage-ring FEL to operate in even shorter wavelengths.

Speaker: Leif Kochanneck (Laser Zentrum Hannover e.V.)

49 Quantum State Features of the FEL Radiation from the Occupation Number Statistics

The coherence of free-electron laser (FEL) radiation has so far been accessed mainly through first and second order correlation functions. Instead, we propose to reconstruct the energy state occupation number distribution of FEL radiation, avoiding the photo-counting drawbacks with high intensities, by means of maximum likelihood techniques based on the statistics of no-click events. The theoreticla framework, numerical results and the proposed experimental set-up for the verification of our pedictoins are illustrated. Though the ultimate goal regards the FEL radiation statistical features, the interest of the proposal also resides in its applicability to any process of harmonic generation from a coherent light pulse, ushering in the study of the preservation of quantum features in general non-linear optical processes.

Speaker: Prof. Daniele Bajoni (University of Pavia)

50 Report on the FELIX Wavelength Range Extension

The FELIX Laboratory, located at Radboud University in Nijmegen, The Netherlands, is operating a suite of FELs serving an international user community with infrared and THz radiation from 5 to 1500 micron operating three FELs in parallel and providing beam to 16 dedicated user end stations. Recently, FELIX has upgraded its most frequently used FEL-2 beamline. The 38 period, 65 mm Halbach-type SmCo undulator, originally built for the UK-FEL project in the mid 80's, which had been successfully used for the short-wavelength FEL for almost 30 years, was replaced by a new undulator. This 50 period, 40 mm NdFeB hybrid undulator was built in close collaboration with STI magnetics and the FEL group of FHI/MPG in Berlin. Together with a new resonator cavity it allows an extension of the fundamental range from 5 µm to sub-3 µm, while keeping the desirable good spectral overlap with the longer wavelength FEL-1 branch. The upgraded FEL-2 beam line produced first light at the end of April 2022 and commenced serving regular user experiments in early May. Initial results concerning gain and wavelength range which are quite satisfactory will be presented together with the first user application.

Speakers: Victor Claessen (FELIX Laboratory), Stephen Gottschalk (STI Magnetics LLC), Britta Redlich (FELIX Laboratory), Bryan Willemsen (FELIX Laboratory), Marije Barel (FELIX Laboratory), René van Buuren (FELIX Laboratory), Alexander van der Meer (FELIX Laboratory), Paul Pijpers (FELIX Laboratory), Michel Riet (FELIX Laboratory), Wouter Stumpel (FELIX Laboratory), Guus Tielemans (FELIX Laboratory), Arjan van Vliet (FELIX Laboratory), Bryan Willemsen (FELIX Laboratory)

FEL2022 MOP33 screen resolution.pdf

51 SASE Optimization Approaches at FLASH

The free-electron laser FLASH at DESY can produce SASE-FEL pulses in the extreme ultraviolet to the soft X-ray regime. A superconducting linear accelerator drives two undulator lines (FLASH1 and
FLASH2). The FLASH1 undulator beam line contains six fixed gap undulator which implies that the SASE wavelength can only be changed via the electron beam energy, while FLASH2 contains twelve variable gap undulators. Preparing different charges and compression schemes to the two parts of the bunch trains for the two undulator beamlines allows to adjust the phase space in wide range and meeting the various requirements of photon pulse trains properties. In order to improve the SASE performance reference files for standard energies and standard charges are regularly prepared. In the FLASH2 undulator beamline beam-based alignment and phase shifter scans have been applied to improve SASE operations and FEL beam quality. Improving set-up and tuning procedures allow to decrease setup times and optimize performance and stability. Procedures and optimization of FEL parameters towards a reliable SASE-FEL operation as well as the achieved results are discussed.

Speakers: Siegfried Schreiber (Deutsches Elektronen-Synchrotron), Mathias Vogt (Deutsches Elektronen-Synchrotron), Juliane Roensch-Schulenburg (Deutsches Elektronen-Synchrotron), Marie Czwalinna (Deutsches Elektronen-Synchrotron), Katja Honkavaara (Deutsches Elektronen-Synchrotron), Arvid Eislage (Deutsches Elektronen-Synchrotron), Vitali Kocharyan (Deutsches Elektronen-Synchrotron), Marion Kuhlmann (Deutsches Elektronen-Synchrotron), Rolf Treusch (Deutsches Elektronen-Synchrotron), Johann Zemella (Deutsches Elektronen-Synchrotron)

52 SASE-FEL Stochastic Spectroscopy Investigation on XUV Absorption and Emission Dynamics in Silicon

High-resolution emission/absorption spectroscopy with picosecond time resolution appears to be strategic in fundamental matter physics investigation as well as in functional materials characterization. Such a method typically requires a pulsed radiation source and high energy resolution, along with a large data statistic. In this work we demonstrate the possibility to retrieve high resolution absorption and emission spectra with picosecond time resolution, by exploiting the stochastic nature of the wide-band self-amplified FEL radiation provided by FERMI. In this work we get advantage of the two spectrometers present on the TIMEX beamline to reconstruct a 2D emission/absorption spectrum of a Si sample. To do so, we applied the singular value decomposition on the single-pulse incoming and outgoing spectra; by applying Tikhonov regularization, we were able to obtain spectra with an energy resolution of few tens of meV. In addition, we performed a time resolved characterization of the Si L23-edge and Si emission line at 99.3 eV by pumping the Si sample with visible laser below damage threshold. The result of this measurement allow us to claim for a bond softening phenomenon on the picosecond time-scale.

Speakers: Dario De Angelis (Elettra-Sincrotrone Trieste S.C.p.A.), Daniele Fausti (Università degli Studi di Trieste, Elettra-Sincrotrone Trieste), Flavio Capotondi (Elettra-Sincrotrone Trieste S.C.p.A.), Kevin Prince (Elettra-Sincrotrone Trieste S.C.p.A.), Yishay Klein (Physics Department and Institute of Nanotechnology and advanced Materials, Bar Ilan University, Ramat Gan, 52900, Israel), Riccardo Mincigrucci (Elettra-Sincrotrone Trieste S.C.p.A.), Emanuele Pedersoli (Elettra-Sincrotrone Trieste S.C.p.A.), Emiliano Principi (Elettra-Sincrotrone Trieste S.C.p.A.), Sharon Shwartz (Physics Department and Institute of Nanotechnology and advanced Materials, Bar Ilan University, Ramat Gan, 52900, Israel), Cristian Svetina (Paul Scherrer Institut), Elia Razzoli (Paul Scherrer Institut), Jacopo Stefano Pelli Cresi (Elettra-Sincrotrone Trieste S.C.p.A.), Laura Foglia (Elettra-Sincrotrone Trieste S.C.p.A.), Ivan Vartaniants (Deutsches Elektronen-Synchrotron)

53 Short FEL Pulses with Tunable Duration from Transversely Tilted Beams at SwissFEL

FEL pulses with an easily tunable duration are of great benefit to user experiments with high requirements on the temporal resolution. A transverse beam tilt is well suited to shorten the pulse duration in a controlled manner. We consider three methods of tilt generation: rf deflecting structures, lattice dispersion in combination with an energy chirp, and transverse wakefields from C-band accelerating cavities. We use monochromator scans in combination with an energy-chirped beam to diagnose the reduction in pulse duration.

Speakers: Philipp Dijkstal (Paul Scherrer Institut), Eduard Prat (Paul Scherrer Institut), Sven Reiche (Paul Scherrer Institut)

54 Signatures of Misalignment in X-Ray Cavities of Cavity-Based X-Ray Free-Electron Lasers

Cavity-based x-ray free-electron lasers (CBXFEL) will allow use of
optical cavity feedback to support generation of fully coherent x-rays
of high brilliance and stability by electrons in undulators. CBXFEL
optical cavities comprise Bragg-reflecting flat crystal mirrors, which
ensure x-rays circulation on a closed orbit, and x-ray refractive
lenses, which stabilize the orbit and refocus the x-rays back on the
electrons in the undulator. Depending on the cavity design, there are
tens of degrees of freedom of the optical elements, which can never be
perfectly aligned. Here [1], we study signatures of misalignment of the
optical components and of the undulator source with the purposes of
understanding the effects of misalignment on x-ray beam dynamics,
understanding misalignment tolerances, and developing cavity alignment
procedures. Betatron oscillations of the x-ray beam trajectory are
one of the characteristic signatures of cavity misalignment. The
oscillation period is in the general case a non-integer number of
round-trip passes of x-rays in the cavity. This period (unlike the
amplitude and offset of the oscillations) is independent of the type
of misalignment and is defined by cavity parameters. The studies are
performed on an example of a four-crystal rectangular cavity [2] using
analytical and numerical wave optics as well as ray-tracing
techniques.

References:
[1]. Peng Qi and Yuri Shvyd'ko, Phys. Rev. Accel. Beams, 25 (2022) 050701
[2]. G. Marcus, et al., CBXFEL R&D: A Joint Argonne National Laboratory and SLAC National Laboratory Collaboration, FEL2019, doi:10.18429/JACoW-FEL2019-TUD04

Speaker: Yuri Shvyd'ko (Argonne National Laboratory)

55 Simulation Studies for the ASPECT Project at European XFEL

Intense attosecond pulses generated by x-ray free-electron lasers (XFEL) are promising for attosecond science, for example, to study the quantum mechanical motion of electrons in molecules. This paper presents numerical simulations of the generation of attosecond soft and hard x-ray FEL pulses with the chirp-taper and Enhanced SASE schemes, based on the parameters of the European XFEL. To overcome the coherence time barrier, a modification of the chirp-taper scheme [1] is used in the case of soft x-rays. The results show that several hundred attosecond pulses can be obtained at photon energies of both 700 eV and 6 keV.

Speakers: Jiawei Yan (European XFEL GmbH), Gianluca Geloni (European XFEL GmbH), Christoph Lechner (European XFEL GmbH), Dr Ye Chen (Deutsches Elektronen-Synchrotron), Marc Guetg (Deutsches Elektronen-Synchrotron), Evgeny Schneidmiller (Deutsches Elektronen-Synchrotron), Svitozar Serkez (European XFEL GmbH), Dr Christoph Heyl (Deutsches Elektronen-Synchrotron)

56 Simulation Studies of Superconducting Afterburner Operation at SASE2 Beamline of European XFEL

European XFEL is a multi-beamline x-ray free-electron laser (FEL) user facility driven by a superconducting accelerator with a nominal photon energy range from 250 eV to 25 keV. An afterburner undulator based on superconducting undulator technology is currently being planned to enable extension of the photon energy range towards harder x-rays. This afterburner undulator would be installed downstream of the already operating SASE2 FEL beamline, emitting at the fundamental or at a harmonic of the upstream SASE2 undulator. In this contribution we present a first simulation study of the impact of undulator mechanical tolerances for operation of the afterburner undulator at the fundamental of SASE2.

Speaker: Christoph Lechner (European XFEL GmbH)

57 Smart*Light: a Tunable Inverse Compton Scattering (ICS) X-Ray Source for Imaging and Analysis

A tunable, tabletop, Inverse Compton Scattering (ICS) hard X-ray source is being designed and built at Eindhoven University of Technology as part of a European Interreg program between The Netherlands and Belgium. This compact X-ray source will bridge the gap between conventional lab sources and synchrotrons: The X-ray photon energy will be generated between 1 and 100 keV with a brilliance typically a few orders of magnitude above the best available lab sources. SmartLight will find applications in material science, cultural heritage and medical imaging.
In the ICS process photons from a laser pulse bounce off a relativistic electron bunch, turning them into X-ray photons through the relativistic Doppler effect. In the first phase SmartLight will use a 100 kV DC photogun as electron source and compact X-band linear accelerator technology developed by the CLIC program from CERN to accelerate the electrons further to an energy of 30 MeV. A 12 mJ, 800 nm, 100 fs laser pulse will be focused to a 5 spot and interact with the electron pulse that will be also focused to a spot of about 5 m resulting in an X-ray photon flux of 105 photons per pulse at 1 kHz rep rate, in 1% bandwidth, with an energy up to 40 keV.
The setup is currently under construction. First light is expected in 2022.

Speaker: Jom Luiten (Technische Universiteit Eindhoven)

58 Spectrometer-Based X-Ray Free-Electron Laser Pulse Duration Measurements of Chirped Beams

Accurate measurements of the x-ray pulse duration produced by x-ray free-electron lasers (XFELs) typically rely on longitudinal electron beam phase space diagnostics, e.g. in a transverse deflecting cavity or TCAV, or from measurements of spectral correlations. All of the known spectral methods share the weakness that they will underestimate the pulse length in the case that the FEL spectrum is broadened due to the electron beam having an energy chirp. We present a statistical analysis of FEL radiation in the presence of a linear electron beam energy chirp which extends previous results by including an accurate description of the FEL gain process. In doing so, we show that with measurements of the spectral intensity correlations and the average spectrum, one can reconstruct the x-ray pulse length, e-beam chirp, and spectrometer resolution. Our approach is validated by comparison with 1D FEL simulations.

Speaker: River Robles (SLAC National Accelerator Laboratory and Stanford University)

59 Status of the Free-Electron Laser User Facility FLASH

FLASH, the free-electron laser user facility at DESY, delivers XUV and soft X-ray radiation for photon experiments since 2005. It is driven by a superconducting linear accelerator, and has two undulator lines (FLASH1 and FLASH2). A third electron beam line hosts the plasma wakefield experiment FLASHForward. Presently, the FLASH facility is undergoing an extensive refurbishment and a substantial upgrade (FLASH2020+). In this paper we summarize the FLASH operation in 2019 - 2021, and report on the main upgrades realized in a long installation shutdown from November 2021 to summer 2022.

Speakers: Katja Honkavaara (Deutsches Elektronen-Synchrotron), Siegfried Schreiber (Deutsches Elektronen-Synchrotron), Mathias Vogt (Deutsches Elektronen-Synchrotron), Juliane Roensch-Schulenburg (Deutsches Elektronen-Synchrotron), Marion Kuhlmann (Deutsches Elektronen-Synchrotron), Lucas Schaper (Deutsches Elektronen-Synchrotron), Christopher Gerth (Deutsches Elektronen-Synchrotron), Rolf Treusch (Deutsches Elektronen-Synchrotron), Johann Zemella (Deutsches Elektronen-Synchrotron)

FEL2022-Poster-FLASHStatus-MOP37.pdf

60 Superradiant Amplification to Produce Attosecond Pulses in Soft X-Ray Regime via Linear Reverse Taper within Undulator Section

Laser pulses of sub-femtosecond duration can be used to track the motion of electrons in the inner shell, which is needed in a variety of advanced experiments. Although this has been accomplished in XUV and hard X-rays in a free-electron-laser facility, it remains a challenge in the soft X-ray region due to the relatively high photon energies and large slippage in the undulator. In this contribution, we present a method to achieve a pulse sequence of ∼ 120 attosecond each in average at 293.8 eV photon energy (4 nm wavelength), which covers the K-shell absorption of Carbon. The key is to create a linear undulator taper within each undulator module by rotating a transverse gradient undulator (TGU) at a small angle. The TGU technique is usually referred to minimise the energy spread effect in Laser-driven plasma accelerator, while in this paper we demonstrate that it can also be used to generate short pulses.

Speakers: Mr Longdi Zhu (Paul Scherrer Institut), Sven Reiche (Paul Scherrer Institut), Eugenio Ferrari (Deutsches Elektronen-Synchrotron)

61 Terahertz Tuning of Dirac Plasmons in Bi2Se3 Topological Insulator

Topological insulators are a class of materials which have raised a great interest over the last decade, thanks to their intriguing conduction properties. Indeed, they are insulating in the bulk and metallic at the surface. Moreover, these metallic surface states have linear Dirac dispersion [1], thanks to which topological insulators show nonlinear THz behaviour similar to the case of graphene.
Bi$_2$Se$_3$ is among the most promising topological insulators, since its band structure provides only one Dirac cone, while the bulk gap is pretty large (about 300 meV) [2].
It has been demonstrated that by terahertz-infrared spectroscopy it is possible to detect the Dirac surface state by patterning thin films of Bi$_2$Se$_3$ with ribbons of width from 2 to 20 μm. In this way, a Dirac plasmon is excited and its dispersion recovers very well the theoretical dispersion, calculated by using the parameters of the Dirac carriers [3].
In this scenario, we present here our investigation on the nonlinear regime of patterned films of Bi$_2$Se$_3$ with ribbons of width of 4 and 20 μm. By exploiting the intense THz electric field of the TeraFERMI beamline, we were able to induce a nonlinear behaviour of the Dirac plasmon. Indeed, we observed a redshift of the plasmonic peak as the incoming THz electric field increases [4].

Speaker: Johannes Schmidt (Elettra-Sincrotrone Trieste S.C.p.A.)

62 Two Color Upgrade of the IR FEL at FHI Berlin

Since coming on-line in November 2013, the Fritz-Haber-Institut (FHI) der Max-Planck-Gesellschaft (MPG) Free-Electron Laser (FEL) has provided intense, tunable infrared radiation to FHI user groups. It has enabled experiments in diverse fields ranging from bio-molecular spectroscopy to studies of clusters and nanoparticles, nonlinear solid-state spectroscopy, and surface science, resulting in 85 peer-reviewed publications so far. A significant upgrade of the FHI FEL is now nearing completion. A second short Rayleigh range undulator FEL beamline has been added that will permit lasing from < 5 microns to > 160 microns in the far IR. Additionally, a 500 MHz kicker cavity has been installed. It will permit simultaneous two-color operation of the FEL from both FEL beamlines over an optical range of 5 to 50 microns by deflecting alternate 1 GHz pulses into each of the two undulators. We will describe the upgraded FHI FEL physics and engineering design and present the current status of commissioning.

Speakers: Wieland Schöllkopf (Fritz-Haber-Institut der Max-Planck-Gesellschaft), Marco De Pas (Fritz-Haber-Institut der Max-Planck-Gesellschaft), Heinz Junkes (Fritz-Haber-Institut der Max-Planck-Gesellschaft), Gert von Helden (Fritz-Haber-Institut der Max-Planck-Gesellschaft), William Colson (Naval Postgraduate School), David Dowell (SLAC National Accelerator Laboratory), Stephen Gottschalk (STI Magnetics LLC), John Rathke (Advanced Energy Systems), Tom Schultheiss (TJS Technologies), Alan Todd (AMMTodd Consulting), Lloyd Young (LMY Technology), Sandy Gewinner (Fritz-Haber-Institut der Max-Planck-Gesellschaft), Gerard Meijer (Fritz-Haber-Institut der Max-Planck-Gesellschaft)

63 Two-Color FEL by Laser Emittance Spoiler

A novel and noninvasive method for two-color x-ray emission is demonstrated at SwissFEL. In the setup, a laser emittance spoiler pulse is overlapped with the primary photocathode laser to locally spoil the beam emittance and the FEL emission. This results, together with a chirped electron pulse, in X-ray emission at two colors. High-energy, high stability, independent control of the duration and of the intensity of the two colors is demonstrated. The laser emittance spoiler enables shot-to-shot selection between one and two-color FEL emission and further, it is compatible with high repetition-rate FELs, as it does not contribute to beam losses.

Speakers: Carlo Vicario (Paul Scherrer Institut), Christopher Arrell (Paul Scherrer Institut), Simona Bettoni (Paul Scherrer Institut), Andreas Dax (Paul Scherrer Institut), Philipp Dijkstal (Paul Scherrer Institut), Martin Huppert (Paul Scherrer Institut), Eduard Prat (Paul Scherrer Institut), Sven Reiche (Paul Scherrer Institut), Alexandre Trisorio (Paul Scherrer Institut), Alberto Lutman (SLAC National Accelerator Laboratory)

64 Unaveraged Simulation of Superradiance in FEL Oscillators

Generation of few-cycle FEL pulses with a high extraction efficiency was achieved at JAERI-FEL [1] and KU-FEL [2]. The observed lasing can be understood as superradiance, radiation from bunched electrons in the slippage region. In the superradiance FEL oscillators, the high-extraction efficiency is accompanied by significant energy variation of the electrons during the undulator. Therefore, numerical studies of such FELs should be conducted by unaveraged simulation codes, in which macro-particles are not bound to bunch slices. In this paper, superradiant FEL pulse evolution in the FEL oscillators is studied by using one-dimensional [3] and three-dimensional [4] simulation codes.
This work was supported by MEXT Q-LEAP (JPMXS0118070271) and JSPS KAKENHI (22H03881).

Speaker: Ryoichi Hajima (National Institutes for Quantum and Radiological Science and Technology)

65 Variable Polarization States in Free-Electron Lasers

Free-electron lasers (FELs) produce different optical polarizations including linear, elliptic and circular polarizations corresponding to the polarizations of the undulators used. X-ray FELs depend upon long undulator lines consisting of a sequence of short undulators. Linearly polarized undulators are most commonly used; hence the optical output is linearly polarized. Elliptic or circular polarizations are possible by varying the undulator orientation. Alternately, APPLE-II or Delta undulator designs produce undulating magnetic fields with arbitrary polarizations. We present a three-dimensional, time-dependent formulation that self-consistently includes two optical orientations and, therefore, treats any given sequence or combination of undulator including undulator imperfections and degradation.1 There are two principal characteristics of the formulation that underpin this capability. First, particles are tracked using the full Newton Lorentz force equations with analytic models of the undulators fields. This permits an accurate model of the interaction of the electrons with a large variety of undulator fields and orientations. Second, the electrons can couple simultaneously to two independent electromagnetic polarizations and, therefore, the optical polarization evolves self-consistently along the undulator line. We present the numerical model and give some examples using prevailing undulator configurations.

1. H.P. Freund and P.J.M. van der Slot, “Variable Polarization Control in Free-Electron Lasers,” J. Phys. Commun. 5, 085011 (2021).
   *This research used resources provided by the University of New Mexico Center for Advanced Research Computing, supported in part by the National Science Foundation.

Speakers: Henry Freund (University of New Mexico), Dr Peter van der Slot (University of Twente)

Tutorial 1: How to expand your research network and write a successful project proposal

Convener: Cecilia Blasetti (Elettra-Sincrotrone Trieste S.C.p.A.)

66 LEAPS Overview and Opportunities

Networking, access and project opportunities for FEL and Laser research in Europe

Speaker: Britta Redlich (FELIX Laboratory)

67 Laserlab-Europe Access and Networking

Networking, access and project opportunities for FEL and Laser research in Europe

Speaker: Sylvie Jacquemot (Ecole Polytechnique)

18:00 Questions

68 Calls Overview and Best Practices Examples

Networking, access and project opportunities for FEL and Laser research in Europe.

Speaker: Cecilia Blasetti (Elettra-Sincrotrone Trieste S.C.p.A.)

18:40 Any other questions

Tuesday 23 August

SASE FELs

Convener: Evgeny Schneidmiller (Deutsches Elektronen-Synchrotron)

69 Cascaded Amplification of Attosecond X-Ray Pulses: Towards TW-Scale Ultrafast X-Ray Free-Electron Lasers

The natural time scale of valence electronic motion in molecular systems is on the order of hundreds of attoseconds. Consequently, the time-resolved study of electronic dynamics requires a source of sub-femtosecond pulses. Pulses in the soft x-ray domain can access core-level electrons, enabling the study of site-specific electron dynamics through attosecond pump/probe experiments. As time-resolved pump/probe experiments are nonlinear processes, these experiments require high brightness attosecond x-ray pulses. The X-ray Laser-Enhanced Attosecond Pulses (XLEAP) collaboration is an ongoing project for the development of attosecond x-ray modes at the Linac Coherent Light Source (LCLS). Here we report development of a high power attosecond mode via cascaded amplification of the x-ray pulse. We experimentally demonstrate generation of sub-femtosecond duration soft x-ray free electron laser pulses with hundreds of microjoules of energy. In conjunction with the upcoming high repetition rate at LCLS-II, these tunable, high intensity attosecond capabilities enable new nonlinear spectroscopic techniques and advanced imaging methods.

This work was supported by US Department of Energy Contracts No. DE-AC02-76SF00 and the Basic Energy Sciences Accelerator and Detector Research Program.

Speakers: Paris Franz (Stanford University), Zhaoheng Guo (Stanford University), River Robles (Stanford University), Dorian Bohler (SLAC National Accelerator Laboratory), David Cesar (SLAC National Accelerator Laboratory), Xinxin Cheng (SLAC National Accelerator Laboratory), Taran Driver (SLAC National Accelerator Laboratory), Joseph Duris (SLAC National Accelerator Laboratory), Andrei Kamalov (SLAC National Accelerator Laboratory), Siqi Li (SLAC National Accelerator Laboratory), Razib Obaid (SLAC National Accelerator Laboratory), Nick Sudar (SLAC National Accelerator Laboratory), Anna Wang (SLAC National Accelerator Laboratory), Zhen Zhang (SLAC National Accelerator Laboratory), James Cryan (SLAC National Accelerator Laboratory)

70 Short Pulses and 2-Color Capabilities at the SASE3 FEL Line of the European XFEL

European XFEL offers a unique combination of high electron
energy and soft X-ray SASE3 undulator resonant to low photon energies with
high K parameter. The long undulator allows us to employ split-undulator
scheme to deliver pump-probe radiation to users. Pulse energies depend on
photon energy and range from 100uJ at 2200eV to 900uJ at 600eV per pulse
respectively. We plan to install optical delay line in the chicane to
reliably scan through zero delay. Strongly compressed 16.5GeV electron
beam combined with maximized orbit dispersion allowed us to generate 1keV-
and 1mJ-order radiation with predominantly less than 2 spectral spikes.
This operation mode was also employed to generate 100uJ-order pump-probe
pulses.

Speaker: Svitozar Serkez (European XFEL GmbH)

71 Demonstration of Enhanced FEL Performance with Optical Klystron and Helical Undulators

This contribution presents the experimental demonstration of improved performance of an X-ray free-electron-laser (FEL) using the optical klystron mechanism and helical undulator configuration in comparison to a standard planar undulator without optical klystron. The demonstration has been carried out at Athos, the soft X-ray beamline of SwissFEL. Athos has variable-polarization undulators and small magnetic chicanes placed between every two undulator modules to fully exploit the optical klystron. It is shown that, for wavelengths between 1 and 3 nm, the required length to achieve FEL saturation is reduced by about a factor of two when using both the optical klystron and helical undulators, with each effect accounting for about half of the improvements. Moreover, it is shown that a helical undulator configuration provides a 20% or higher saturation power than planar undulators. This work represents an important step towards more compact and high-power FELs, rendering this key technology more efficient, affordable, and accessible to the scientific community.

Speakers: Christoph Kittel (Paul Scherrer Institut), Marco Calvi (Paul Scherrer Institut), Eduard Prat (Paul Scherrer Institut), Dr Guanglei Wang (Paul Scherrer Institut), Dr Nicholas Sammut (University of Malta)

72 Two-Colored FEL Generation Using Phase Shifters at Undulator

Phase shifters at undulator line are usually used for optimizing FEL intensity by setting 'in-phase' by matching the FEL pulse and electrons phases. π-offset so called ‘out-phase’ may suppress FEL intensity at the resonant frequency, therefore the 'out-phase' condition is an unwanted state. However, this 'out-phase' setting can arise side band spectrums. This phenomena can be explained by the theory of the spontaneous radiation or low-gain FEL, and it expects that these side band spectrums have two main spectrums with the spectrum difference determined by the number of undulator period. This poster shows amplification of the two-colored spectrum seeded by the spontaneous spectrum feature. Results of two colored FEL is studied by simulations and experiments are performed at PAL-XFEL showing it’s intensity grows exponentially along the number of undulator segments and reaches the saturation resulting in hundreds μJ energy.

Speaker: Myung Hoon Cho (Pohang Accelerator Laboratory)

10:35 Coffee & Exhibition

Seeded FELs

Convener: Ryoichi Hajima (National Institutes for Quantum Science and Technology)

73 Coherent and Ultrashort Soft X-Ray Pulses from Echo-Enabled Harmonic Cascade FEL

Shanghai Soft X-ray FEL facility (SXFEL) is the first X-ray FEL facility in China. Various external seeding techniques have been adopted for improving the performance of SXFEL. Here we report on the first demonstration of echo-enabled harmonic cascade (EEHC) for generating coherent and ultrashort soft X-ray pulses. Benefiting from the superiority of low sensitivity to the electron beam imperfections and flexible output pulse control of EEHC, nearly transform-limited soft X-ray pulses with tunable pulse durations have been successfully generated. These experiments exceed the current limitations of external seeding techniques and may open up new opportunities for extending external seeded FEL to shorter wavelength range.

Speaker: Chao Feng (Shanghai Advanced Research Institute)

74 Enhanced Self-Seeding with Ultrashort Electron Beams

We describe a new method to produce intensity stable, highly coherent, narrow-band x-ray pulses in self-seeded free electron (FEL) lasers. The approach uses an ultrashort electron beam to generate a single spike FEL pulse with a wide coherent bandwidth. The self-seeding monochromator then notches out a narrow spectral region of this pulse to be amplified by a long portion of electron beam to full saturation. In contrast to typical self-seeding where monochromatization of noisy self-amplified spontaneous emission pulses leads to either large intensity fluctuations or multiple frequencies, we show that this method produces a stable, coherent FEL output pulse with statistical properties similar to a fully coherent optical laser. With self-consistent, start-to-end simulations we show that laser heater shaping and cathode shaping techniques both can produce the electron beam current profile needed for the enhanced self-seeding scheme.

Speakers: Zhen Zhang (SLAC National Accelerator Laboratory), Erik Hemsing (SLAC National Accelerator Laboratory), Mr Aliaksei Halavanau (SLAC National Accelerator Laboratory)

75 Comparison of Transverse Coherence Properties in Seeded and Unseeded FEL

The transverse coherence of the source is an important property for FEL experiments. Theory and simulations indicated different features for seeded and unseeded FELs but so far no direct comparison has been pursued experimentally on the same facility.
At FERMI one has the unique possibility to test both configurations (SASE and seeding) within the same operating conditions.
In this contribution we present the experimental results of the characterization of transverse coherence with special attention to the evolution of such property.

Speakers: Mihai Pop (Lund University), Nicola Mahne

76 First Observation of Laser-Beam Interaction in a Dipole Magnet

Recently, a self-modulation scheme was proposed and experimentally demonstrated for enhancing energy modulation in seeded FELs [1], thereby significantly reducing the requirement of an external laser system. Driven by this scheme, an electron beam with a laser-induced energy modulation as small as 1.8 times the slice energy spread is used for lasing at the 7th harmonic of a 266-nm seed laser in a single-stage high-gain harmonic generation (HGHG) setup and the 30th harmonic of the seed laser in a two-stage HGHG setup. Moreover, using this scheme, we report the first observation of the laser-beam interaction in a pure dipole magnet [2] in which the electron beam energy modulation with a 40-keV amplitude and a 266-nm period is measured. We demonstrate that such an energy modulation can be used to launch a seeded FEL, that is, lasing at the sixth harmonic of the seed laser in a high-gain harmonic generation scheme. The results reveal the most basic process of the FEL lasing and open up a new direction for the study and exploitation of laser-beam interactions.

Speaker: Jiawei Yan (European XFEL GmbH)

12:50 Lunch

FEL Oscillators and IRFELs

Convener: Ying Wu (Duke University)

77 Observation of Burnham-Chiao Ringing with Pi-Phase Jumps in a High-Efficiency Superradiance FEL Oscillator

At the mid-infrared free electron laser oscillator in Kyoto University (KU-FEL), high extraction efficiency (9.4%) operation has been achieved [1] by introducing the dynamic cavity desynchronization technique [2] and photocathode operation of a thermionic RF gun [1]. Because of the interaction between the electron beam and FEL electromagnetic field, a maximum electron energy decrease of 16% was observed. The measured energy decrease was consistent with the measured FEL spectrum. The FEL pulse structure under the high extraction efficiency operation was obtained by a phase retrieval based on the result of fringe resolved autocorrelation measurement [3]. As the result, it was confirmed that the FEL pulse has several sub-spikes after the main spike having a 4.2-cycle pulse length at the wavelength of 11 $\mu$m. Moreover, the neighboring spikes has 180-degree different optical phases, i.e. $\pi$-phase jumps. The appearance of the sub-spikes and the $\pi$-phase jumps are the specific feature of the Burnham-Chiao ringing (or Superradiance ringing) [4], which has been predicted by numerical simulations [5] but not yet fully characterized in experiments. The ringing and the $\pi$-phase jumps are clear evidence of the periodic acceleration and deceleration of the microbunched electrons. In this talk, we present an overview of few-cycle FEL lasing with the high extraction efficiency, >9%, and the details of FEL pulse measurements to reveal the Burnham-Chiao ringing. This work was supported by MEXT Q-LEAP (JPMXS0118070271).

Speakers: Heishun Zen (Kyoto University), Hideaki Ohgaki (Kyoto University), Ryoichi Hajima (National Institutes for Quantum and Radiological Science and Technology)

78 FEL Lasing Below 170 nm Using an Oscillator

While the linac based single-pass FEL has been successfully operated
in the EUV and x-ray regions for about two decades, the oscillator
FEL has been limited to operating in the longer wavelength region
due to the limitation of high-reflectivity, thermally stable, and
radiation-resistant short-wavelength mirrors. With Duke storage ring FEL,
we have recently extended the shortest lasing wavelength
of the oscillator FEL to 168.6 nm. We have demonstrated lasing wavelength
tuning from 168.6 to 179.7 nm with excellent beam stability.
This progress has been made possible by developing a new FEL configuration
with substantially reduced undulator harmonic radiation on the FEL mirror,
a thermally stable FEL optical cavity, and a new type of high-reflectivity
fluoride-based multilayer coating with a protective capping layer.
Employing this VUV FEL in Compton scattering, we have also produced the
first 120 MeV gamma rays at the High Intensity Gamma-ray Source (HIGS).

Ref: Y.K. Wu et al., J. Appl. Phys. 130, 183101 (2021); doi: 10.1063/5.0064942
This work is partially supported by DOE Grant No. DE-FG02-97ER41033.

Speaker: Prof. Ying Wu (Duke University)

79 FEL Resonance of Circular Waveguide Modes

The THz gap is a region of the electromagnetic spectrum where high average and peak power radiation sources are scarce while scientific and industrial applications grow in demand. Free-electron laser coupling in a magnetic undulator can provide radiation generation in this frequency range, but slippage effects require the use of relatively long and low current electron bunches in the THz FEL, limiting the amplification gain and output peak power. We show how a circular waveguide in a meter-long strongly tapered helical undulator can be used to match the radiation and e-beam velocities, extracting energy from an ultrashort 200 pC 5.5 MeV electron beam along the entire undulator. E-beam spectrum measurements, supported by energy and spectral measurements of the THz FEL radiation, indicate an average energy efficiency of 10% with some particles losing >20% of their initial kinetic energy.

Speakers: Andrew Fisher (University of California, Los Angeles), Pietro Musumeci (University of California, Los Angeles)

TUCO3_slides.pptx

80 Synchronized Terahertz Radiation and Soft X-rays Produced in a FEL Oscillator

We present a scheme to generate synchronized THz and Soft X-ray radiation pulses by using a Free-Electron Laser Oscillator driven by a high repetition rate energy recovery linac. The backward THz radiation in the oscillator cavity produces naturally synchronized Soft/Hard X rays via Thomson back-scattering by interacting with a successive electron bunch. The performances of this dual source are illustrated by means of dedicated simulations assessing the capability of the scheme for typical wavelengths of interest, namely up to 50 µm for the short-THz radiation and close to the water window at 3 nm for the X-rays.

Speakers: Marcel Ruijter (Istituto Nazionale di Fisica Nucleare), Vittoria Petrillo (Università Statale degli Studi di Milano and Istituto Nazionale di Fisica Nucleare), Alberto Bacci (Istituto Nazionale di Fisica Nucleare), Illya Drebot (Istituto Nazionale di Fisica Nucleare), Marcello Rossetti Conti (Istituto Nazionale di Fisica Nucleare), Andrea Rossi (Istituto Nazionale di Fisica Nucleare), Luca Serafini (Istituto Nazionale di Fisica Nucleare), Sanae Samsam (Istituto Nazionale di Fisica Nucleare and University La Sapienza of Rome), Dr Michele Opromolla (Università Statale degli Studi di Milano and Istituto Nazionale di Fisica Nucleare)

One-to-one meetings with experts in project building 1

One-to-one meetings with experts in the fields, who will give you operative instructions on how to prepare successful project proposals to obtain access to European laser and FEL laboratories and/or to get grants for researchers from all over the world.

81 Euraxess – an European Network to Support Mobility and Career Development for Researchers

Get information on how to apply for research jobs in Europe.

Speaker: Anna Comini (Consorzio per l'AREA di ricerca scientifica e tecnologica di Trieste)

82 Horizon Europe Programmes and Grants for All Stages of Research Careers

Get introduction and assistance on funding possibilities for researchers from all over the world

Speaker: Mrs Marina Kozlik Mercatelli (Consorzio per l'AREA di ricerca scientifica e tecnologica di Trieste)

83 Laserlab-Europe

Become aware of transnational access support tools, joint research and training opportunities offered by the Laserlab-Europe Consortium

Speaker: Sylvie Jacquemot (Ecole Polytechnique)

84 Wayforlight and Beamtime Proposals

Discover the wayforlight.eu portal to find out the most suitable instruments for your research and get useful tips to draft a successful beamtime proposal

Speaker: Cecilia Blasetti (Elettra-Sincrotrone Trieste S.C.p.A.)

Tuesday posters: Coffee & Exhibition

85 Experimental Demonstration of Temporally Shaped Picosecond Optical Pulses for Driving Electron Photoinjectors

Next-generation electron photoinjector accelerators, such as the LCLS-II photoinjector, have increasingly tight requirements on the excitation lasers, often calling for tens of picosecond, temporally flat-top, ultraviolet (UV) pulse trains to be delivered at up to 1 MHz. We present an experimental demonstration of temporal pulse shaping for the LCLS-II photoinjector laser resulting in temporally flat-top pulses with 24 ps durations. Our technique is a non-colinear sum frequency generation scheme wherein two identical infrared optical pulses are imparted with equal and opposite amounts of spectral dispersion. The mixing of these dispersed pulses within a thick nonlinear crystal generates a second harmonic optical pulse that is spectrally narrowband with a designed temporal profile*. In experiment we achieve upwards of 40% conversion efficiency with this process allowing this to be used for high average and peak power applications. These narrowband pulses can then be directly upconverted to the UV towards use in driving free electron laser photocathodes. Additionally, we present a theoretical framework for adapting this method to shape optical pulses driving other photoinjector based applications.

86 Real-Time Programmable Shaping for Electron and X-Ray Sources

The next generation of augmented brightness XFELs, such as LCLS-II, promises to address current challenges associated with systems with low X-ray cross-sections. Typical photoinjector lasers produce coherent ultraviolet (UV) pulses via nonlinear conversion of an infrared (IR) laser. Fast and active beam manipulation is required to capitalize on this new generation of XFELs, and controlling the phase space of the electron beam is achieved by shaping the UV source. However current techniques for such shaping in the UV rely on stacking pulses in time, which leads to unavoidable intensity modulations and hence space-charge driven microbunching instabilities [1]. Traditional methods for upconversion do not preserve phase shape and thus require more complicated means of arriving at the desired pulse shapes after nonlinear upconversion [2]. Upconversion through four-waving mixing (FWM) allows direct phase transfer, convenient wavelength tunability by easily changeable phase matching parameters, and also has the added advantage of greater average power handling than traditional χ(2) nonlinear processes [3, 4,]. Therefore, we examine a possible solution for e-beam shaping using a machine learning (ML) implementation of real-time photoinjector laser manipulation which shapes the IR laser source and then uses FWM for the nonlinear upconversion and shaping simultaneously. Our presentation will focus on the software model of the photoinjector laser, the associated ML models, and the optical setup. We anticipate this approach to not only enable active experimental control of X-ray pulse characteristics but could also increase the operational capacity of future e-beam sources, accelerator facilities, and XFELs.

References:
[1] S. Bettoni, et al. “Impact of laser stacking and photocathode materials on microbunching instability in photoinjectors”, Phys. Rev. Accel. Beams 23, 024401 (2020)
[2] Lemons, Randy, et al. “Dispersion-controlled Temporal Shaping of Picosecond Pulses via Non-colinear Sum Frequency Generation.” Phys. Rev. Accel. Beams 25, 013401 (2022)
[3] P. Zuo, T. Fuji, and T. Suzuki, "Spectral phase transfer to ultrashort UV pulses through four-wave mixing," Opt. Express 18, 16183-16192 (2010)
[4] John E. Beetar, M. Nrisimhamurty, Tran-Chau Truong, Yangyang Liu, and Michael Chini, "Thermal effects in molecular gas-filled hollow-core fibers," Opt. Lett. 46, 2437-2440 (2021)

Speaker: Mr Jack Hirschman (Stanford University)

87 Simulations of Ultrahigh Brightness Beams from a Plasma Photocathode Injector

Plasma photocathode injectors may enable electron beams with normalised emittance at the nm-rad level from a Plasma Wakefield Acceleration (PWFA) stage [1]. These electron beams typically have kA-level peak currents leading to ultrahigh 5D brightness beams with the potential to drive advanced light sources [1]. The feasibility of the plasma photocathode was demonstrated at FACET-I at SLAC [2]. Further experimental campaigns are gradually aiming toward ultrahigh 5D and 6D brightness beams at FACET-II [3]. However, a series of milestones must be reached before these beams can be utilised for XFELs. For example, electron beams accelerated in plasma-based accelerators inherently have a significant energy chirp due to the GV/m accelerating gradients involved. Since energy chirp and energy spread can be detrimental to the high-gain FEL interaction, advanced approaches have been developed for energy spread minimisation of the initially ultrahigh 5D brightness beams towards ultrahigh 6D brightness [4]. Here we show within the framework of the PWFA-FEL project that it may also be possible to produce ultrahigh 5D brightness beams with reduced energy spread using beam-loading. We present results aiming at a trade-off between reduced energy spread, increased peak current, and increased emittance and their application to a soft XFEL in the water window.

Speaker: Lily Berman (University of Strathclyde)

88 A Pulse Shaper for Direct Generation of 515 nm 3D Ellipsoidal Pulses at PITZ

In this paper, a cathode laser pulse shaper at 515 nm is presented that will be used for emittance optimizations. In case alkali antimonide photocathodes are used, the shaped green pulses can be applied directly for photoemission while Cs₂Te photocathodes requires second harmonic generation to provide UV laser pulses. Recent tests of CsK₂Sb photocathodes in the high gradient RF gun at PITZ are first steps for the future usage of green laser pulses, which would simplify the requirements for the photocathode laser system, especially for CW operation cases envisioned in future. As long the alkali antimonide photocathodes are not in regular use yet, the laser pulses need to be converted into the UV. The green pulse shaper still simplifies the laser system since two conversion stages from IR to green to UV were needed in the past, which dilutes the quality of the shaped laser pulses. In this paper, a pulse shaper for direct generation of 515nm 3D ellipsoidal pulses is presented that is expected to further improve the beam emittance generated by ellipsoidal laser shaping.

Speaker: Christian Koschitzki (Deutsches Elektronen-Synchrotron DESY at Zeuthen)

89 A Novel Method for Generating High-Repetition-Rate and Fully Coherent EUV Free-Electron Laser

High-brightness extreme ultraviolet (EUV) light source is strongly required for high-resolution photoelectron spectroscopy, imaging experiments, and EUV lithography. In this work, the self-modulation technique is introduced into seeded FELs, such as high-gain harmonic generation (HGHG), to significantly reduce the requirement of the seed laser power by enhancing coherent energy modulation. Numerical simulations demonstrated that the modified HGHG configuration with the self-modulation technique could generate high-repetition-rate, fully coherent, stable, and kilowatt-scale EUV pulses at a more compact linac-based light source.

90 ACE Injector for Burst Mode Operation in a ICS Source

At Eindhoven university an inverse Compton scattering (ICS) source is being built. The ICS source consists of a 100kV photo gun electron injector, X-band accelerator, and interaction laser. One of the first upgrades for this ICS source is operating in a so-called burst mode. In burst mode, the electron injector is replaced by the advanced continuous electron (ACE) injector and a Fabry-Perot cavity is added to the laser. Both systems work in a 100 nanosecond long burst. Significantly increasing the current x-ray yield and the brilliance of the ICS source.
The ACE injector works by generating a continuous beam with a high current and low emittance through thermionic emission. The continuous electron beam is then chopped into a pulsed beam by a combination of a dual-mode elliptical RF cavity and a knife-edge. The dual-mode cavity uses both the fundamental mode (1.5 GHz) and its second harmonic (3.0 GHz) to increase the duty cycle of the chopping process to approximately 30% with a minimal loss of beam quality. Finally, a second dual-mode elliptical RF cavity compresses the pulse length of the bunches, preparing the beam for injection into an X-band linear accelerator.

Speaker: Rick van den Berg (Technische Universiteit Eindhoven)

91 An XFELO Demonstrator Setup at the European XFEL

An X-ray free-electron laser oscillator (XFELO) is a next generation X-ray source promising radiation with full three-dimensional coherence, nearly constant pulse to pulse stability and more than an order of magnitude higher spectral flux compared to SASE FELs. In this contribution, the concept of an R&D project for installation of an XFELO demonstrator experiment at the European XFEL facility is conceptually presented. It is composed of an X-ray cavity design in backscattering geometry of 133 m round trip length with four undulator sections of 20 m total length producing the FEL radiation. It uses cryocooled diamond crystals and employs the concept of retroreflection to reduce the sensitivity to vibrations. Start to end simulations were carried out which account for realistic electron bunch distributions, inter RF-pulse bunch fluctuations, various possible errors of the X-ray optics as well as the impact of heat load on the diamond crystals. The estimated performance and stability derived from these simulations shall be reported and foreseen issues shall be discussed.

Speakers: Patrick Rauer (Deutsches Elektronen-Synchrotron), Winfried Decking (Deutsches Elektronen-Synchrotron), Daniele La Civita (European XFEL GmbH), Andreas Koch (European XFEL GmbH), Harald Sinn (European XFEL GmbH), Immo Bahns (European XFEL GmbH), Jan Grünert (European XFEL GmbH), Liubov Samoylova (European XFEL GmbH), Massimiliano Di Felice (European XFEL GmbH), Maurizio Vannoni (European XFEL GmbH), Jörg Rossbach (University of Hamburg)

92 Analysis of Spectral Contents in Hard X-Ray Self-Seeded Free-Electron Laser Operation at the European XFEL

Recently, Hard x-ray self-seeding (HXRSS) operations at the European X-ray free-electron laser (EuXFEL) opened a pathway towards the application of pulses with high spectral density (in terms of ph/eV per pulse) in the fields of applied physics, chemistry and biology, where the coherent radiation spectrum is essential. The spectrum of hard x-ray self seeding pulses is generally accompanied by a pedestal around the central seeded photon energy. The pedestal contains two separate components: normal self-amplified spontaneous (SASE) and sideband emissions that can be ascribed to long-wavelength modulations of the electron beam. The pedestal limits the spectral purity and can impact some user applications. In this report, we analyze the purity of HXRSS pulses in the presence of microbunching instability. We look at the spectral contents after and before saturation, and display the contribution of the pedestal in the HXRSS spectrum.

Speakers: Najmeh Mirian (Deutsches Elektronen-Synchrotron), Tianyun Long (Deutsches Elektronen-Synchrotron)

93 ARIA, a VUV Beamline for EuPRAXIA@SPARC_LAB

EuPRAXIA@SPARC_LAB is a new Free Electron Laser (FEL) facility that is currently
under construction at the Laboratori Nazionali di Frascati of the INFN. The electron
beam driving the FEL will be delivered by an X-band normal conducting LINAC followed
by a plasma wakefield acceleration stage. It will be characterized by a small
footprint and include two different plasma-driven photon beamlines. In addition to
the soft-X-ray beamline, named AQUA and delivering ultra-bright photon pulses for
experiments in the water window to the user community, a second beamline, named
ARIA, has been recently proposed and included in the project. ARIA is a seeded FEL
line in the High Gain Harmonic Generation configuration and generates coherent and
tunable photon pulses in the range between 50 and 180 nm. Here we present the
potentiality of the FEL radiation source in this low energy range, by illustrating
both the layout of the FEL generation scheme and simulations of its performances.

Speakers: Dr Michele Opromolla (University of Milan and INFN-Milan), Vittoria Petrillo (Universita' degli Studi di Milano), Federico Nguyen (Ente per le Nuove Tecnologie, l'Energie e l'Ambiente), Andrea Selce (Ente per le Nuove Tecnologie, l'Energie e l'Ambiente), Francesco Stellato (Sezione di Roma Tor Vergata), Marcello Coreno (Elettra-Sincrotrone Trieste S.C.p.A. and CNR-ISM), Massimo Ferrario (Istituto Nazionale di Fisica Nucleare), Andrea Ghigo (Istituto Nazionale di Fisica Nucleare), Dr Luca Giannessi (Elettra Sincrotrone Trieste and Istituto Nazionale di Fisica Nucleare), Augusto Marcelli (Istituto Nazionale di Fisica Nucleare), Fabio Villa (Istituto Nazionale di Fisica Nucleare), Alberto Petralia (Ente per le Nuove Tecnologie, l'Energie e l'Ambiente), Dr Zeinab Ebrahimpour (Istituto Nazionale di Fisica Nucleare)

94 Brilliant X-Ray Free Electron Laser Driven by Resonant Multi-Pulse Ionization Injection Accelerator

Laser Wakefield Accelerators are now sufficiently mature to provide GeV scale/high-brightness electron beams capable of driving Free Electron Laser (FEL) sources. Here, we show start-to-end simulations carried out in the framework of the EuPRAXIA project of a Free Electron Laser driven by an LWFA accelerator in the Resonant Multi-Pulse Ionisation Injection (ReMPI) framework. Simulations with this model using a 1 PW Ti:Sa laser system and a 20 cm long capillary, show the injection and acceleration of an electron beam up to 4.5 GeV, with a slice energy spread and a normalized emittance below $4\times 10^{-4}$ and 80 $nm \times rad$, respectively. The transport of the beams from the capillary exit to the undulator is provided by a matched beam focusing with a marginal beam-quality degradation. Finally,
3D simulations of the FEL radiation generated inside an undulator show that $\approx 10^{10}$ photons with central wavelength of $0.15\, nm$ and peak power of $\simeq 0.3\, GW$ can be produced for each bunch. Our start-to-end simulations indicate that a single-stage ReMPI accelerator can drive a high-brightness electron beam having quality large enough to be efficiently transported to a FEL undulator, thus generating X-ray photons of brilliance exceeding $10^{25} ph/s/mm^2/0.1\%bw$

Speakers: Dr Paolo Tomassini (CNR-INO and ELI-NP), Dr Luca Giannessi (INFN-LNF, Elettra Sincrotrone Trieste), Anna Giribono (Istituto Nazionale di Fisica Nucleare), Federico Nguyen (Ente per le Nuove Tecnologie, l'Energie e l'Ambiente), Dr Leonida A. Gizzi (CNR-INO and INFN-PI)

95 Calculation of the CSR Effect on EEHG Performance

Externally seeded FELs can produce fully coherent short-wavelength pulses with the advantage of higher shot-to-shot stability and spectral intensity than SASE radiation. For the FLASH2020+ project, the Echo-Enabled Harmonic Generation (EEHG) seeding technique achieves seeded FEL radiation in the XUV and soft X-ray range down to wavelengths of 4 nm. The implementation of the EEHG requires precise phase space manipulations in the seeding section of the beamline, which would make the performance of the EEHG sensitive to the collective effects, such as Coherent Synchrotron Radiation (CSR) in some working range. Therefore, it is essential to consider the CSR in EEHG simulations and to understand its impact on the electron beam properties. In this work, we compare different methods for calculating CSR and investigate the mechanism of its effect on the EEHG performance.

Speaker: Dmitrii Samoilenko (University of Hamburg)

96 Characterization of Coherent Seeded FEL Pulses in the Presence of Incoherent Electron Beam Energy Modulations

Over the last few years tremendous progress has been gained in the theoretical understanding and experimental demonstration of seeded FELs . The ultimate spectral limit of seeded FEL, however, remains unclear, because of the broadening and distortions induced in the output spectrum by residual broadband energy modulations in the electron beam.
In this talk, we present the mathematical descriptions of the impact of broadband energy modulations on the EEHG, HGHG and self seeding bunching spectrums produced by the microbunching instability through both the accelerator and the FEL line. We will show the agreement of our models with the systematic experimental characterization seeded FEL spectrums in FERMI and Eu-XFEL. Using experimental data of EEHG FEL performance in FERMI in the photon energy range 130–210 eV, we demonstrate that amplification of electron beam energy distortions primarily in the EEHG dispersive sections explains an observed reduction of the FEL spectral brightness proportional to the EEHG harmonic number. Local maxima of the FEL spectral brightness and of the spectral stability are found for a suitable balance of the dispersive sections’ strength and the first seed laser pulse energy[1].
[1] Physical Review Accelerators and Beams 24, 8, 2021

Speaker: Najmeh Mirian (Deutsches Elektronen-Synchrotron)

97 Chirped Pulse Amplification in a Seeded FEL: Towards the Generation of High-Power Few-Femtosecond Pulses Below 10 nm

In optical conventional lasers, chirped pulse amplification (CPA) has become an extremely powerful technique for the generation of ultrashort pulses in the infrared and visible spectral ranges. In this contribution we report the successful implementation of CPA in a seeded XUV FEL. A second experiment, using a two-stage harmonic generation scheme (FERMI FEL-2) has the objective to generate coherent and phase-tailored few-femtosecond FEL pulses, with gigawatt peak power in the sub-10 nm spectral range. This second experiment is still in progress. We will discuss the main scientific and technical bottlenecks and the implications.

Speakers: David Garzella (Elettra-Sincrotrone Trieste S.C.p.A.), Giovanni De Ninno (Elettra-Sincrotrone Trieste S.C.p.A.)

98 Comparison of the Spectro-Temporal Properties of Echo-Enabled and High-Gain Harmonic Generation Free-Electron Laser Pulses at the 15th Harmonic

The external seeding scheme Echo-Enabled Harmonic Generation (EEHG) utilizes two modulators and two chicanes to manipulate the longitudinal phase space of an electron beam to achieve bunching at higher harmonics of the seed laser wavelength. Different combinations of energy modulation and longitudinal dispersion can result in the same amount of bunching at a certain harmonic. This study investigates the impact of the choice of the energy modulation amplitudes on the bunching properties and the spectro-temporal characteristics of the free-electron laser (FEL) radiation. Finally, a comparison between EEHG and the single modulator-chicane seeding scheme High-Gain Harmonic Generation (HGHG) at the 15th harmonic of the seed laser wavelength is presented. The corresponding numerical modelling and simulations are performed within the parameter range of the future upgrade of the FEL user facility FLASH at DESY.

Speaker: Fabian Pannek (University of Hamburg)

99 Conditioning and High Power Test Results of First 3.0 m High Gradient Structure for FERMI Linac Energy Upgrade

ERMI is the seeded Free Electron Laser (FEL) user facility at Elettra laboratory in Trieste, operating in the VUV to soft X-rays spectral range. In order to extend the FEL spectral range to shorter wavelengths, an upgrade plan for increasing the Linac energy from 1.5 GeV to 2.0 GeV is actually going on. After successful testing of the short prototype of new high gradient S-band accelerating structure up to an accelerating gradient of 40 MV/m, a full length 3.0 m HG structure has been built in collaboration with Paul Scherrer Institute (PSI). In the first step, two such new structures would be installed in place of S0a and one deflector at K15 increasing the beam energy to 1.7 GeV. In the next phase 14 new HG structures would replace the present Backward Travelling Wave sections reaching to the final goal of 2.0 GeV. Currently first 3.0 m HG structure is under conditioning and high power testing at Cavity Test Facility of Elettra. In this paper we report the low power measurement results as well as conditioning results of 3.0 m HG structure.

Speaker: Nuaman Shafqat (Elettra-Sincrotrone Trieste S.C.p.A.)

100 Control of the Longitudinal Phase and Benchmarking to HBSASE

Improvement of the longitudinal coherence in the proposed Soft Xray FEL, the SXL, for the MAX IV Laboratory is an important design aspect to enhance the user case. One of the main considered methods is HBSASE. However the final compression in the MAX IV acceleratos is done at full energy, and thus leaving an energy chirp in the electron pulse. This chirp in longitudinal phase space has to be removed for an efficient implementation of HBSASE. In this paper we show in simulations how the phase space is improved by first overcompressing the pulse, and then correct it by a two-plate wake field de-chirper. The resulting pulse is then shown to have qualities such that, by HBSASE, a significant narrowing of the FEL bandwidth is achieved at 1 nm.

Speakers: Francesca Curbis (Lund University), Mihai Pop (MAX IV Laboratory), Sverker Werin (MAX IV Laboratory), Saeid Pirani (Lund University)

101 Design Considerations of High Repetition Rate VUV FEL

A new concept of a high repetition rate VUV FEL is discussed. The FEL is envisioned to operate in the wavelength range from 50 to 250 nm with pulse energies of about 30 µJ throughout the wavelength range, and a pulse length of a few 100 fs. The SRF LINAC technology developed and used at the Helmholtz-Zentrum Dresden-Rossendorf for the Radiation Source ELBE is planned to be used for the driver-accelerator. This allows operating an electron beam with an average current of 1 mA on the order of magnitude, pulse repetition rate of up to 10 MHz, and the bunch charge of 100 pC, as used for the FEL design. We consider using the HGHG to allow the generation of fully coherent pulses. The high repetition rate electron beam makes it possible to construct an FEL oscillator that would be used as the high repetition rate seed of the HGHG amplifier. In the proposed scheme, the SRF LINAC provides beams for the seeding oscillator and the HGHG amplifier simultaneously. The described FEL would create new experimental regimes, not available at any other photon source. These could result in transformative changes in physical chemistry studies in the gas phase and at the interfaces, e.g., heterogeneous catalysis.

Speaker: Pavel Evtushenko (Helmholtz-Zentrum Dresden-Rossendorf)

102 Development and Test Results of Multi-Alkali Antimonide Photocathodes in the High Gradient RF Gun at PITZ

Multi-alkali antimonide photocathodes can have high quantum efficiency similar as UV sensitive (Cs2Te) photocathodes, but with the advantages of photoemission sensitivity in the visible region of the light spectrum and a significant reduction in the mean transverse energy of photoelectrons. A batch of three KCs2Sb photocathodes was grown on molybdenum substrates via a sequential deposition method in a new preparation system at INFN LASA. Afterwards, the cathodes were successfully tested in the high gradient RF gun at PITZ. This contribution summarizes the experimental results obtained in both the preparation chamber and the RF gun. Based on those findings, we are now optimizing the recipe of KCs2Sb and NaKSb(Cs) photocathodes for lower field emission and longer lifetime, and the measurements for the latest photocathodes with the improved recipe are also presented.

Speaker: Sandeep Mohanty (Deutsches Elektronen-Synchrotron DESY at Zeuthen)

103 EEHG Seeding Scheme at SwissFEL ATHOS FEL

In order to improve the brightness and coherence of the soft x-ray FEL line of SwissFEL (Athos), components for an Echo Enabled Harmonic Generation (EEHG) scheme are currently in preparation. The first components have been installed to allow first ESASE operation test in Spring 2022. This first stage consists in a 10 mJ class seed laser, a U200 modulator with individual control of each half period and a four electromagnets dipole chicane (R56 < 800 um). The large magnetic chicane and the second modulator are still in preparation for an installation by end 2022. This paper will give a technical description of the different systems as well as preliminary results of the commissioning with beam.

Speakers: Romain Ganter (Paul Scherrer Institut), Alexandre Trisorio (Paul Scherrer Institut), Carlo Vicario (Paul Scherrer Institut), Eduard Prat (Paul Scherrer Institut), Kittel Christoph (Paul Scherrer Institut), Gabriel Aeppli (Paul Scherrer Institut), Christopher Arrell (Paul Scherrer Institut), Hans-Heinrich Braun (Paul Scherrer Institut), Marco Calvi (Paul Scherrer Institut), Adrian Cavalieri (Paul Scherrer Institut), Andreas Dax (Paul Scherrer Institut), Philipp Dijkstal (Paul Scherrer Institut), Eugenio Ferrari (Paul Scherrer Institut), Nicole Hiller (Paul Scherrer Institut), Martin Huppert (Paul Scherrer Institut), Pavle Juranic (Paul Scherrer Institut), Xiaoyang Liang (Paul Scherrer Institute), Stefan Neppl (Paul Scherrer Institut), Eduard Prat (Paul Scherrer Institut), Sven Reiche (Paul Scherrer Institut), Didier Voulot (Paul Scherrer Institut)

104 Energy-Chirp-Based Outcoupling Scheme for X-Ray Regenerative Amplifier FEL

Cavity-based X-ray Free Electron Lasers (FELs) such as the X-ray regenerative amplifier FEL (XRAFEL) [1] and the X-ray FEL oscillator [2] have drawn great interest as a means of producing high-brightness, fully coherent and stable hard x-ray pulses for high-repetition rate FELs [3]. However, high efficiency outcoupling of the stored cavity x-ray radiation remains challenging. Here we present a novel XRAFEL design to achieve efficient cavity outcoupling or Q-switching by introducing energy chirp in the electron beam while leaving the high-quality X-ray optics intact. During the FEL interaction, electron beam with an linear energy chirp can be slightly compressed or decompressed by the undulator, which leads to a gradual shift of radiation frequency outside the bandwidth of the Bragg crystal for efficient outcoupling. Our simulation results show that substantial power can be outcoupled from the X-ray cavity driven by chirped electron beams at 100 kHz repetition rate. We also discuss parameter tradeoff in such an XRAFEL scheme and a practical way to achieve the desired fast chirp control by a small, normal-conducting RF station in the LCLS-II [4].

[1] Z. Huang and R. D. Ruth. PRL96, 144801 (2006).
[2] K.-J. Kim, Y. Shvyd'ko, S. Reiche, PRL100 244802 (2008).
[3] G. Marcus, et al., PRL125, 254801 (2020).
[4] M. Nasr, et al., in proceedings of IPAC'16 (Busan, Korea,2016).

Speaker: Zhirong Huang (SLAC National Accelerator Laboratory)

105 Facility Concept Outlines for a UK XFEL

In early 2019, the UK initiated a project to develop the science case for a UK XFEL, featuring a diverse team of UK scientists and international advisors. Accelerator scientists were engaged to highlight potential future accelerator developments and to develop concept outlines for a facility design meeting the requirements for world-leading capabilities. The UK XFEL Science Case, featuring the concept outlines, was published in late 2020. Subsequent exercises further demonstrated the support of the UK community and the project is anticipated to enter a more detailed design phase. The concept outlines are reviewed and potential next steps are outlined.

Speakers: David Dunning (Science and Technology Facilities Council), James Clarke (Science and Technology Facilities Council), Louise Cowie (Science and Technology Facilities Council), Neil Thompson (Science and Technology Facilities Council)

106 FAST-GREENS: a High Efficiency Free Electron Laser Driven by Superconducting RF Accelerator

In this paper we'll describe the status of the FAST-GREENS experimental program where a 4 m-long strongly tapered helical undulator with a seeded prebuncher is used in the high gain TESSA regime to convert a significant fraction (up to 10 %) of energy from the 240 MeV electron beam from the FAST linac to coherent 515 nm radiation. We'll also discuss the longer term plans for the setup where by embedding the undulator in an optical cavity matched with the high repetition rate from the superconducting accelerator (3,9 MHz), a very high average power laser source can be obtained. Eventually, the laser pulses can be redirected onto the relativistic electrons to generate by inverse compton scattering a very high flux of circularly polarized gamma rays for polarized positron production.

Speaker: Pietro Musumeci (University of California, Los Angeles)

107 FERMI FEL-1 Upgrade to EEHG

In order to meet the user request of extending the FERMI FEL spectral range over the whole water window, we are developing an upgrade strategy that is based on the implementation of the Echo Enabled Harmonic Generation (EEHG) scheme.
The FERMI upgrade strategy is structured as follow: during a first phase, the single cascade FEL-1 branch will be adapted to operate either in EEHG or in HGHG. This upgrade can be achieved with relatively low cost and impact on FERMI operations and will improve the spectral range, spectral quality and scheme flexibility of FEL-1. Furthermore, it will provide a versatile test bench opening the possibility to explore in details the EEHG scheme potentialities and address many of the possible issues related to the second and more critical phase of the upgrade project: the upgrade of FEL-2. These two phases will proceed in parallel to the LINAC upgrade to increase the nominal energy. Solutions aiming at a peak beam energy of 1.8 and 2.0 GeV are under study.
In this contribution we will focus on the upgrade of the FEL-1 branch that has already started and is foreseen to provide light to users with the new configuration by spring 2023.

Speakers: Carlo Spezzani (Elettra-Sincrotrone Trieste S.C.p.A.), Enrico Allaria (Elettra-Sincrotrone Trieste S.C.p.A.), Laura Badano (Elettra-Sincrotrone Trieste S.C.p.A.), Filippo Bencivenga (Elettra-Sincrotrone Trieste S.C.p.A.), Carlo Callegari (Elettra-Sincrotrone Trieste S.C.p.A.), Flavio Capotondi (Elettra-Sincrotrone Trieste S.C.p.A.), Davide Castronovo (Elettra-Sincrotrone Trieste S.C.p.A.), Paolo Cinquegrana (Elettra-Sincrotrone Trieste S.C.p.A.), Marcello Coreno (Elettra-Sincrotrone Trieste S.C.p.A.), Miltcho Danailov (Elettra-Sincrotrone Trieste S.C.p.A.), Giovanni De Ninno (Elettra-Sincrotrone Trieste S.C.p.A.), Paolo Delgiusto (Elettra-Sincrotrone Trieste S.C.p.A.), Alexander Demidovich (Elettra-Sincrotrone Trieste S.C.p.A.), Simone Di Mitri (Elettra-Sincrotrone Trieste S.C.p.A.), William Fawley (SLAC National Accelerator Laboratory), Mario Ferianis (Elettra-Sincrotrone Trieste S.C.p.A.), Giulio Gaio (Elettra-Sincrotrone Trieste S.C.p.A.), Gabor Kurdi (Elettra-Sincrotrone Trieste S.C.p.A.), Marco Lonza (Elettra-Sincrotrone Trieste S.C.p.A.), Marco Malvestuto (Elettra-Sincrotrone Trieste S.C.p.A.), Michele Manfredda (Elettra-Sincrotrone Trieste S.C.p.A.), Claudio Masciovecchio (Elettra-Sincrotrone Trieste S.C.p.A.), Ivaylo Nikolov (Elettra-Sincrotrone Trieste S.C.p.A.), Giuseppe Penco (Elettra-Sincrotrone Trieste S.C.p.A.), Gregory Penn (Lawrence Berkeley National Laboratory), Giovanni Perosa (University of Trieste, Elettra Sincrotrone Trieste), Kevin Prince (Elettra-Sincrotrone Trieste S.C.p.A.), Primoz Rebernik Ribic (Elettra-Sincrotrone Trieste S.C.p.A.), Claudio Scafuri (Elettra-Sincrotrone Trieste S.C.p.A.), Nuaman Shafqat (Elettra-Sincrotrone Trieste S.C.p.A.), Paolo Sigalotti (Elettra-Sincrotrone Trieste S.C.p.A.), Alberto Simoncig (Elettra-Sincrotrone Trieste S.C.p.A.), Filippo Sottocorona (University of Trieste, Elettra Sincrotrone Trieste), Luca Sturari (Elettra-Sincrotrone Trieste S.C.p.A.), Takashi Tanaka (RIKEN SPring-8 Center), Mauro Trovo (Elettra-Sincrotrone Trieste S.C.p.A.), Marco Veronese (Elettra-Sincrotrone Trieste S.C.p.A.), Roberto Visintini (Elettra-Sincrotrone Trieste S.C.p.A.), Marco Zangrando (Elettra-Sincrotrone Trieste S.C.p.A.), Federico Gelmetti (Elettra-Sincrotrone Trieste S.C.p.A.), Bruno Diviacco (Elettra-Sincrotrone Trieste S.C.p.A.), Simone Spampinati (Elettra-Sincrotrone Trieste S.C.p.A.)

108 First Demonstration of Parallel Operation of a Seeded FEL and a SASE FEL

The FLASH facility houses a superconducting linac powering two FEL beamlines with MHz repetition rate in 10 Hz bursts. Within the FLASH2020+ project, which is taking care of facility development, one major aspect is the transformation of one of the two FEL beam lines to deliver externally seeded fully coherent FEL pulses to photon user experiments. At the same time the second beam line will use the SASE principle to provide photon pulses of different properties to users.
Since the electron beam phase space conducive for SASE or seeded operation is drastically different, here a proof-of-principle experiment using the existing experimental seeding hardware has been performed demonstrating the possibility of simultaneous operation.
In this contribution we will describe the setup of the experiment and accelerator, and discuss the chances and limitations of the experimental seeding hardware. Finally, we will discuss the results and their implications also for the FLASH2020+ project.

Speakers: Sheida Mahmoodi (University of Hamburg), Mehdi Mohammad Kazemi (Deutsches Elektronen-Synchrotron), Samuel Hartwell (Deutsches Elektronen-Synchrotron), Lucas Schaper (Deutsches Elektronen-Synchrotron), Mathias Vogt (Deutsches Elektronen-Synchrotron), Andreas Thiel (University of Hamburg), Siegfried Schreiber (Deutsches Elektronen-Synchrotron), Dmitrii Samoilenko (University of Hamburg), Philipp Amstutz (Deutsches Elektronen-Synchrotron), Eugenio Ferrari (Deutsches Elektronen-Synchrotron), Wolfgang Hillert (University of Hamburg), Fabian Pannek (University of Hamburg), Georgia Paraskaki (Deutsches Elektronen-Synchrotron), Juliane Roensch-Schulenburg (Deutsches Elektronen-Synchrotron), Enrico Allaria (Elettra-Sincrotrone Trieste S.C.p.A.), Florian Christie (Deutsches Elektronen-Synchrotron), Sven Ackermann (Deutsches Elektronen-Synchrotron), Pardis Niknejadi (Deutsches Elektronen-Synchrotron)

109 FLASH2020+ Project Progress: Current installations and future plans

The FLASH2020+ project has started to transform the FLASH facility to broaden the facility profile and meet demands of future user experiments.
In a nine-month lasting shutdown until August 2022 the linear accelerator of the FLASH facility has, among others, been upgraded with a laser heater, new bunch compressors and new modules. The latter results in an energy upgrade to 1.35 GeV allowing to reach sub 4 nm wavelength. In the following 14-month lasting shutdown starting mid 2024 the FLASH1 FEL beamline will be completely rebuild. The design is based on external seeding at MHz repetition rate in burst mode allowing for coherent tuneable FEL radiation in wavelength and polarization by installation new APPLE-III undulators. Post compression of the beam downstream of the radiators will allow for high quality THz generation and together with the new experimental endstations and pump probe lasers provide a unique portfolio for next generation user experiments.

Speakers: Lucas Schaper (Deutsches Elektronen-Synchrotron), Philipp Amstutz (Deutsches Elektronen-Synchrotron), Nicoleta Baboi (Deutsches Elektronen-Synchrotron), Karolin Baev (Deutsches Elektronen-Synchrotron), Martin Beye (Deutsches Elektronen-Synchrotron), Christopher Gerth (Deutsches Elektronen-Synchrotron), Ingmar Hartl (Deutsches Elektronen-Synchrotron), Katja Honkavaara (Deutsches Elektronen-Synchrotron), Jochen Mueller-Dieckmann (Deutsches Elektronen-Synchrotron), Rui Pan (Deutsches Elektronen-Synchrotron), Elke Ploenjes-Palm (Deutsches Elektronen-Synchrotron), Olaf Rasmussen (Deutsches Elektronen-Synchrotron), Juliane Roensch-Schulenburg (Deutsches Elektronen-Synchrotron), Evgeny Schneidmiller (Deutsches Elektronen-Synchrotron), Siegfried Schreiber (Deutsches Elektronen-Synchrotron), Kai Tiedtke (Deutsches Elektronen-Synchrotron), Markus Tischer (Deutsches Elektronen-Synchrotron), Sven Toleikis (Deutsches Elektronen-Synchrotron), Rolf Treusch (Deutsches Elektronen-Synchrotron), Mathias Vogt (Deutsches Elektronen-Synchrotron), Lutz Winkelmann (Deutsches Elektronen-Synchrotron), Mikhail Yurkov (Deutsches Elektronen-Synchrotron), Johann Zemella (Deutsches Elektronen-Synchrotron)

110 Free Electron Laser Seeded by Betatron Radiation

The possibility of using a plasma accelerated electron beam to generate Free Electron Laser (FEL) radiation has recently been proven. In the plasma acceleration process an intense broadband spectrum radiation in the X ray region, the betatron radiation, is produced by the electron beam passing through the ionized gas.
In this paper it is proposed to use this radiation, suitably monochromatised, as a seed to stimulate the emission in the Free Electron Laser on the fundamental frequency and on the higher harmonics. This scheme could be adopted from all FEL injected by plasma accelerated electron beams via particle or laser wakefield acceleration.

Speaker: Andrea Ghigo (Istituto Nazionale di Fisica Nucleare)

111 Frequency Mixing Experiments at the European XFEL

Frequency mixing was studied experimentally at SASE3, the soft X-ray undulator of the European XFEL. Two frequencies were generated in the first part of the undulator in alternating K configuration. The mixing process occurred in the second part with detuned undulator segments used to generate R56. Finally, the difference frequency was radiated and amplified in a third part of the SASE3 undulator. Experiments were performed at several electron energies (11.5 GeV, 14 GeV, and 16.5 GeV) with frequency mixing generation at photon energies between 500 eV and 1.1 keV. Pulse energies were on the mJ level, depending on the length of the radiator part. A practical application of frequency mixing at European XFEL is a possible extension of the operating range of the SASE3 undulator towards lower photon energies, by using a relatively short afterburner with longer period.

Speaker: Dirk Noelle (Deutsches Elektronen-Synchrotron)

112 Frequency Pulling in a Superradiant FEL Amplifier

Free-electron lasers producing ultrashort pulses with high peak power are a resource to extend ultrafast non-linear spectroscopic techniques into the extreme-ultraviolet–X-ray regime. A super radiant cascade was proposed as a method to shorten the pulse duration in seeded FEL. Pulses shorter than the typical duration supported by the FEL gain bandwidth of the FEL amplifier in the linear regime were measured at FERMI. In these conditions we also observed a strong frequency pulling phenomenon that that will be discussed in this contribution.

Speaker: Najmeh Mirian (Deutsches Elektronen-Synchrotron)

113 Future Upgrade Strategy of the FERMI Seeded FEL Facility

ABSTRACT: FERMI is implementing a development plan to keep the facility in a world-leading position on the base of the requests coming from the user community and the advises from the Scientific Advisory Council and the Machine Advisory Committee. The ultimate goal of this plan consists in doubling the maximum photon energy available and in reducing the pulse duration below the characteristic lifetime of the atomic core levels in the source spectral range. An upgrade of FERMI aimed at reaching the oxygen K-edge requires a profound modification of the FEL configurations and of the main components of the machine, including the linac and the undulator lines. One of the most promising approaches for this upgrade is to implement the echo-enabled harmonic generation (EEHG) scheme, relying on two external lasers to precisely control the spectrotemporal properties of the FEL pulse. The conversion to EEHG of the first stage of the double-stage harmonic cascade presently in use on FEL-2, would allow to reach harmonics as high as 120, enabling to generate coherent pulses down to 2 nm. The main aspects of the upgrade strategy will be discussed in this contribution.

Speakers: Enrico Allaria (Elettra-Sincrotrone Trieste S.C.p.A.), Laura Badano (Elettra-Sincrotrone Trieste S.C.p.A.), Filippo Bencivenga (Elettra-Sincrotrone Trieste S.C.p.A.), Carlo Callegari (Elettra-Sincrotrone Trieste S.C.p.A.), Flavio Capotondi (Elettra-Sincrotrone Trieste S.C.p.A.), Davide Castronovo (Elettra-Sincrotrone Trieste S.C.p.A.), Paolo Cinquegrana (Elettra-Sincrotrone Trieste S.C.p.A.), Marcello Coreno (Elettra-Sincrotrone Trieste S.C.p.A.), Miltcho Danailov (Elettra-Sincrotrone Trieste S.C.p.A.), Alexander Demidovich (Elettra-Sincrotrone Trieste S.C.p.A.), Giovanni De Ninno (Elettra-Sincrotrone Trieste S.C.p.A.), Paolo Delgiusto (Elettra-Sincrotrone Trieste S.C.p.A.), Simone Di Mitri (Elettra-Sincrotrone Trieste S.C.p.A.), Bruno Diviacco (Elettra-Sincrotrone Trieste S.C.p.A.), William Fawley (Elettra-Sincrotrone Trieste S.C.p.A.), Mario Ferianis (Elettra-Sincrotrone Trieste S.C.p.A.), Giulio Gaio (Elettra-Sincrotrone Trieste S.C.p.A.), Federico Gelmetti (Elettra-Sincrotrone Trieste S.C.p.A.), Dr Luca Giannessi (Elettra Sincrotrone Trieste and Istituto Nazionale di Fisica Nucleare), Gabor Kurdi (Elettra-Sincrotrone Trieste S.C.p.A.), Marco Lonza (Elettra-Sincrotrone Trieste S.C.p.A.), Marco Malvestuto (Elettra-Sincrotrone Trieste S.C.p.A.), Michele Manfredda (Elettra-Sincrotrone Trieste S.C.p.A.), Claudio Masciovecchio (Elettra-Sincrotrone Trieste S.C.p.A.), Ivaylo Nikolov (Elettra-Sincrotrone Trieste S.C.p.A.), Giuseppe Penco (Elettra-Sincrotrone Trieste S.C.p.A.), Kevin Prince (Elettra-Sincrotrone Trieste S.C.p.A.), Emiliano Principi (Elettra-Sincrotrone Trieste S.C.p.A.), Primoz Rebernik Ribic (Elettra-Sincrotrone Trieste S.C.p.A.), Claudio Scafuri (Elettra-Sincrotrone Trieste S.C.p.A.), Nuaman Shafqat (Elettra-Sincrotrone Trieste S.C.p.A.), Paolo Sigalotti (Elettra-Sincrotrone Trieste S.C.p.A.), Alberto Simoncig (Elettra-Sincrotrone Trieste S.C.p.A.), Simone Spampinati (Elettra-Sincrotrone Trieste S.C.p.A.), Carlo Spezzani (Elettra-Sincrotrone Trieste S.C.p.A.), Luca Sturari (Elettra-Sincrotrone Trieste S.C.p.A.), Mauro Trovo (Elettra-Sincrotrone Trieste S.C.p.A.), Marco Veronese (Elettra-Sincrotrone Trieste S.C.p.A.), Roberto Visintini (Elettra-Sincrotrone Trieste S.C.p.A.), Marco Zangrando (Elettra-Sincrotrone Trieste S.C.p.A.), Takashi Tanaka (RIKEN SPring-8 Center), Gregory Penn (Lawrence Berkeley National Laboratory), Giovanni Perosa (University of Trieste, Elettra Sincrotrone Trieste), Filippo Sottocorona (University of Trieste, Elettra Sincrotrone Trieste)

114 Generation of a Sub-Picosecond Sheet Electron Beam Using a 100 fs Laser

The biggest benefit of DC photoelectron-gun driven by the sub-picosecond laser is that such type of guns can be operated with the current density much higher than the Child’s low limitation. We demonstrated 0.3 nC bunch generation by irradiating a 100 fs Ti:sapphire laser focused to 0.1 square-cm area onto a tangsten photocathode installed in a diode type 40 kV DC gun. The drawback is the strong Coulomb repulsive force by which electrons may suffer the emittance degradation in the vicinity of the cathode. To reduce the repulsive force at the cathode surface, we are trying to generate a “sheet-like” photoelectron bunch. In our experiments, electron bunches are generated by irradiating the laser pulse shaped in an ellipse onto the photocathode. The ellipticity is set in the range of 0.03-0.05 while the most of sheet-beam experiments were conducted with the ellipticity about 0.1. The smaller the ellipcity, the longer the circumference; this may reduce the radial electric field on the electron bunch side-wall. Moreover the electron bunch shape is rather a “line” than a “sheet” due to the short duration of the drive laser pulse. We conducted a preliminaly experiment and observed that the elliptical photo-electron bunch had much larger divergence angle in the minor axis direction. In the presentation, experimental results, the numerical simulation on the particle motion and the design of the sheet-photo-electron DC gun will be discussed.

Speakers: Prof. Makoto Asakawa (Kansai University), Prof. Soichiro Yamaguchi (Kansai University), Prof. Makoto Nakajima (Osaka University), Mr Yuga Karaki (Kansai University), Mr Hiroki Matsubara (Kansai University), Mr Yuki Miyajima (Kansai University), Mr Ryosuke Michishita (Kansai University), Mr Tomohiro Shirai (Kansai University), Mr Ryosuke Horii (Kansai University), Ms Shizuki Yoshimatsu (Kansai University)

115 Generation of High-Power Free-Electron Laser Pulses with Orbital Angular Momentum

The generation of x-ray pulses carrying orbital angular momentum from an x-ray free-electron laser (FEL) has attracted considerable attention due to the ability to directly change atomic states and develop new material characterization techniques. In this contribution, we report a new method for generating intense x-ray vortices. The method is based on the widely used self-amplified spontaneous emission scheme and does not require additional helical undulators or external laser systems. It can therefore in principle be employed by all existing XFEL facilities with limited hardware additions.

Speakers: Jiawei Yan (European XFEL GmbH), Gianluca Geloni (European XFEL GmbH)

116 High Brightness Self-Seeded X-Ray FEL and Its Applications at PAL-XFEL

Nearly fully coherent hard X-ray self-seeded (HXRSS) free-electron laser (FEL) pulses with an unprecedented peak-brightness and a narrow spectrum using the forward Bragg-diffraction (FBD) monochromator has been provided. We have achieved outstanding performance of HXRSS FEL over photon energy range covering from 3.5 keV to 14.6 keV at PAL-XFEL. Furthermore, an averaged energy of seed FEL of ~1mJ is obtained in the range from 5 keV to 10 keV. With these pulses single-shot coherent imaging (SSI) experiment and serial femtosecond crystallography (SFX) were performed. We developed x-ray energy scanning program with the help of double crystal monochromator (DCM), which results in improved spectral impurity and fully calibrated energy scale. With this energy scanning program, we have conducted test experiments such as resonant inelastic X-ray scattering (RIXS) and X-ray emission spectroscopy (XES), femtosecond time resolved X-ray absorption near edge structure (TR-XANES). In this presentation, we present recent experimental results by using the hard X-ray self-seeded FEL with energy scanning at PAL-XFEL.

Speaker: MyungHoon Cho (Pohang Accelerator Laboratory)

117 High Harmonic Lasing Using Attosecond Electron Pulse Combs in Photon-Induced Near-Field Electron Microscopy

Attosecond laser pulses in the extreme ultraviolet/soft X-ray (XUV/SXR) spectral regions are presently available for attosecond pump-probe spectroscopy and extreme ultraviolet lithography for chip manufacturing, ultrafast atomic-scale microscopy, and nonlinear X-ray optics. There are two main approaches to produce attosecond light pulses: high-harmonic generation (HHG) in gas-phase or solid-state matter based on the three-step model, and X-ray free-electron lasers (XFELs) based on self-amplified spontaneous emission (SASE) and laser seeding processes of relativistic free electrons traveling through an undulator. Here, we propose a novel route of producing attosecond laser pulses, based on the generation of attosecond electron pulse trains in photon-induced near-field electron microscopy (PINEM), combined with the SASE principle for light amplification. Our scheme relies on high-density nanotip arrays emitting dense electron bunches that are subsequently modulated with a PINEM-type interaction, enabling high-gain for amplification of XUV/SXR high harmonic radiation. Our PINEM-HHG mechanism using attosecond electron pulses can serve as promising ultra-bright extreme ultraviolet/soft X-ray attosecond laser sources.

Speakers: Prof. Ido Kaminer (Israel Institute of Technology), Prof. Michael Krueger (Israel Institute of Technology), Dr Yiming Pan (Israel Institute of Technology)

FEL2022-poster-HHG-PINEM.pdf

118 High Repetition Rate Seeded Free-Electron Laser with a Harmonic Optical Klystron in High-Gain Harmonic Generation

External seeding techniques like high-gain harmonic generation (HGHG) and echo-enabled harmonic generation (EEHG) have been proven to be able to generate fully coherent radiation in the EUV and X-ray range. However, towards seeding at a high repetition rate, the repetition rate of current laser systems with sufficient power for seeding is limited to the kilohertz range. One attractive solution to this limitation is to reduce the required seed laser power. In this contribution, we will present a harmonic optical klystron scheme with high gain harmonic generation. With the harmonic optical klystron scheme as the seeding technique, the required seed laser power is decreased, and higher harmonics than in a standard single-stage HGHG can be achieved.

Speakers: Hao Sun (Shanghai Institute of Applied Physics), Georgia Paraskaki (Deutsches Elektronen-Synchrotron), Bart Faatz (Shanghai Advanced Research Institute), Chao Feng (Shanghai Advanced Research Institute), Bo Liu (Shanghai Advanced Research Institute)

119 High Repetition Rate, Low Noise and Wavelength Stable OPCPA Laser System with Highly Efficient Broadly Tunable UV Conversion for FEL Seeding

Within the FLASH2020+ project the FLASH VUV/XUV FEL facility at DESY (Hamburg, Germany) is currently undergoing a major upgrade to become the first high repetition rate, fully coherent FEL light source worldwide [1]. To reach this goal, one of the two in parallel operated FEL branches will be seeded at a fixed wavelength at 343 nm in a first step (SEED 1) and tunable between 297 nm to 317 nm in a second step (SEED 2) following the two-color Echo-Enhanced Harmonic Generation (EEHG) scheme [2]. The seed laser system is designed to deliver UV pulse energies > 50 µJ and > 100 µJ for SEED 1 and SEED 2, respectively, and with 6000 pulses in one second (1 MHz pulse trains in 600 µs - 10 Hz bursts). In combination with the EEHG seeding principle, this will allow for the generation of high harmonics corresponding to XUV FEL pulses with photon energies of more than 300 eV (down to 4 nm in wavelength). In order to exploit the full capabilities of the narrow-band fully coherent FEL pulses for 24/7 scientific user experiments, the seed laser needs to provide broadly tunable, high power UV laser pulses with pulse durations of 50 fs, excellent beam quality and exceptional high short and long-term stability in respect to the seeding wavelength (< 2e-4), pulse – pulse energy (< 2%) and pointing jitter (< 20 µrad). Altogether, the requirements on the laser system are beyond state-of-the-art.
We will present the concept as well as the first experimental results of our novel high-power seed laser system based on a 5 kW Inno-Slab CPA pump laser system, optical parametric chirped pulse amplification (OPCPA) and a highly efficient UV conversion scheme. An extensive numerical study based on a 3+1 dimensional start-to-end simulation code (chi3D) allows for a precise predictions of system performance in terms of output power, tunability, beam quality and stability in respect to the measured input parameters and respective statistical and systematic fluctuations. The theoretical results are confirmed by first experimental studies being in excellent agreement in terms of UV conversion efficiency, beam quality and the predicated improvement of the pulse-to-pulse stability compared to the OPCPA stability. The insides of this study had major impact on the conceptual design of the laser system, especially the dispersion concept and the best implementation of user controls, such as power attenuation and fast wavelength control, etc.
[1] M. Beye, ed., “FLASH2020+: Making FLASH brighter, faster and more flexible : Conceptual Design Report.” Deutsches Elektronen-Synchrotron, DESY, Hamburg, 2020. DOI: 10.3204/PUBDB-2020-00465
[2] L. Schaper, ed.al, “Flexible and Coherent Soft X-ray Pulses at High Repetition Rate: Current Research and Perspectivesal” Appl. Sci. 2021, 11, 9729. https://doi.org/10.3390/app11209729

Speakers: Tino Lang (Deutsches Elektronen-Synchrotron DESY at Zeuthen), Samuel Hartwell (Deutsches Elektronen-Synchrotron DESY at Zeuthen), Jiaan Zheng (Deutsches Elektronen-Synchrotron DESY at Zeuthen), Ingmar Hartl (Deutsches Elektronen-Synchrotron DESY at Zeuthen), Lucas Schaper (Deutsches Elektronen-Synchrotron DESY at Zeuthen), Mehdi Mohammad Kazemi (Deutsches Elektronen-Synchrotron DESY at Zeuthen), Mr Nhat-Phi Hoang (Deutsches Elektronen-Synchrotron DESY at Zeuthen), Eugenio Ferrari (Deutsches Elektronen-Synchrotron DESY at Zeuthen), Enrico Allaria (Elettra-Sincrotrone Trieste S.C.p.A.)

120 Hybrid LWFA-PWFA Staging for Beam Quality Booster

Beam-driven plasma wakefield accelerators (PWFAs) offer a unique regime for the generation and acceleration of high-quality electron beams to multi-GeV energies. Here we present an innovative hybrid staging approach, deploying electron beams generated in a laser-driven wakefield accelerator (LWFA) as drivers for a PWFA, integrated in a particularly compact setup. This scenario exploits the capability of LWFAs to deliver shortest, high peak-current electron bunches [1] with the prospects for high-quality witness beam generation in PWFAs [2]. The feasibility of the concept is presented through exemplary particle-in-cell simulations, before describing experimental results from extensive campaigns performed at high-power laser facilities; ATLAS (LMU, Munich), SALLE-JAUNE (LOA, Paris) and DRACO (HZDR, Dresden). Using few-cycle optical probing we captured clear images of beam-driven plasma waves in a dedicated plasma stage, allowing us to identify a non-linear plasma-wave excitation regime. Trailing the plasma waves, the impact of ion motion to the transverse modulation of the plasma density was observed over many picoseconds [3]. Furthermore, we demonstrate for the first time the acceleration of distinct witness beams in such LWFA-driven PWFA (LPWFA) setup [4,5], showcasing an accelerating gradient on the order of 100 GV/m. These milestones pave the way towards compact sources of energetic ultra-high brightness electron beams as well as a miniature model for large scale PWFA facilities.

Speaker: Arie Irman (Helmholtz-Zentrum Dresden-Rossendorf)

121 Impact of Electron Beam Energy Chirp on Optical-Klystron-Based High Gain Harmonic Generation

External seeding schemes allow the generation of stable and fully coherent free electron laser (FEL) radiation but can be limited in repetition rates in orders of tens of Hz. This limitation is mainly posed by limited average power of the seed lasers that are required to provide hundreds of MW peak power to modulate the electron bunches. An optical-klystron-based high gain harmonic generation (HGHG) scheme, which can be implemented in several existing and upcoming seeded FEL beamlines with minimal to no additional installations, overcomes this limitation by greatly reducing the required seed laser power. In this work, we carefully study the scheme with detailed simulations that include imperfections of electron beam properties such as a quadratic electron beam energy chirp that characterizes existing FEL facilities. We discuss the optimization steps that in these conditions ensure successful operation, opening the path towards exciting science at FELs with fully coherent and high repetition rate FEL radiation.

Speakers: Georgia Paraskaki (Deutsches Elektronen-Synchrotron), Lucas Schaper (Deutsches Elektronen-Synchrotron), Evgeny Schneidmiller (Deutsches Elektronen-Synchrotron), Enrico Allaria (Elettra-Sincrotrone Trieste S.C.p.A.)

122 Improving the Realistic Modeling of the EEHG Seed Section in Start to End Simulations

A tunable and multicolor light source with near Fourier-limited pulses, controlled delay, and fully coherent beam with precisely adjustable phase profiles enables state-of-the-art measurements and studies of femtosecond dynamic processes with high elemental sensitivity and contrast. The start-to-end simulations efforts aim to take advantage of the available global pool of software and past and present extensive efforts to provide realistic simulations, particularly for cases where precise and fine manipulation of the beam phase space is concerned. Since, for such cases, tracking of beams with billions of particles through magnetic structures and handover between multiple codes are required, extensive realistic studies for such cases are limited. Here we will describe a workflow that reduces the needed computational resources and share studies of the EEHG seed section for the FLASH2020+ [1] project.

Speakers: Pardis Niknejadi (Deutsches Elektronen-Synchrotron), Sven Ackermann (Deutsches Elektronen-Synchrotron), Philipp Amstutz (Deutsches Elektronen-Synchrotron), Martin Dohlus (Deutsches Elektronen-Synchrotron), Eugenio Ferrari (Deutsches Elektronen-Synchrotron), Tino Lang (Deutsches Elektronen-Synchrotron), Georgia Paraskaki (Deutsches Elektronen-Synchrotron), Lucas Schaper (Deutsches Elektronen-Synchrotron), Evgeny Schneidmiller (Deutsches Elektronen-Synchrotron), Siegfried Schreiber (Deutsches Elektronen-Synchrotron), Mathias Vogt (Deutsches Elektronen-Synchrotron), Mikhail Yurkov (Deutsches Elektronen-Synchrotron), Wolfgang Hillert (University of Hamburg), Fabian Pannek (University of Hamburg), Dmitrii Samoilenko (University of Hamburg), Sven Reiche (Paul Scherrer Institut), Francesca Curbis (Lund University), Mihai Pop (Lund University)

123 LAPLACIAN: A Step Forward for Compact LPA Based Electron Accelerators

The LAPLACIAN (Laser Acceleration Platform as a Coordinated Innovation Anchor) experimental facility inside the MIRAI project framework is the Japanese answer to the global effort for the development of compact accelerators based on laser plasma acceleration (LPA) for its application to free electron laser (FEL). Situated in the SPRING-8 site, LAPLACIAN aims for the generation of X-ray FEL with relativistic electrons (GeV) from an LPA source in a beamline of under 10 m. Even after the recent demonstration of by the SIOM group, achieving proper electron beam parameters in a consistent manner and a reliable coupling with the undulator is non-trivial and still under research. In LAPLACIAN, multiple gas targets and LPA schemes are being studied, including a planned multiple plasma stages setup for GeV electron energies combined with magneto-optics for coupling. In this talk, an overview of the current facility status and some future plans will be given. In addition, we will report in some of the already achieved results and the new planned beamline.

Speaker: Driss Oumbarek Espinos (Osaka University)

124 Laser-Based Seeding of SwissFEL Athos

In the scope of the HERO ERC project, we are implementing a laser-based seeding scheme at the SwissFEL soft X-ray Athos beamline to generate fully coherent X-ray FEL pulses. With this perspective, we designed and built a new laser facility. It consists of a terawatt-class, femtosecond laser system based on Titanium Sapphire technology with wavelength tuning capability, an optical transfer line as well as a launching optical setup and diagnostics to spatially and temporally overlap the laser and the electron bunch inside the modulator, where the seeding process occurs. We present an overview of the facility with details of the laser performance as well as first commissioning results with the electron beam.

Speakers: Dr Alexandre Trisorio (Paul Scherrer Institut), Dr Sven Reiche (Paul Scherrer Institut), Dr Chris Arrell (Paul Scherrer Institut), Adrian Cavalieri (Paul Scherrer Institut), Dr Andreas Dax (Paul Scherrer Institut), Dr Chris Michael Deutschendorf (Paul Scherrer Institut), Dr Philipp Dijkstal (Paul Scherrer Institut), Mr Edwin Divall (Paul Scherrer Institut), Dr Romain Ganter (Paul Scherrer Institut), Rasmus Ischebeck (Paul Scherrer Institut), Dr Nicole Hiller (Paul Scherrer Institut), Dr Martin Huppert (Paul Scherrer Institut), Dr Pavle Juranic (Paul Scherrer Institut), Dr Stephan Neppl (Paul Scherrer Institut), Dr Eduard Prat (Paul Scherrer Institut), Cezary Sydlo (Paul Scherrer Institut), Dr Carlo Vicario (Paul Scherrer Institut), Dr Didier Voulot (Paul Scherrer Institut), Gabriel Aeppli (Paul Scherrer Institut), Dr Eugenio Ferrari (Deutsches Elektronen-Synchrotron)

125 LCLS-II MHz X-Ray Temporal Shaping

Shaping techniques traditionally used to produce few femtosecond and even sub femtosecond soft X-ray FEL pulses at LCLS do not scale well to high repetition rates. Here we present the progress of the LCLS-II X-ray temporal shaping project which uses infrared and ultraviolet picosecond lasers to shape the electron beam of the LCLS-II superconducting linac. Quickly switching these shaping lasers on and off will enable multiplexing different beams to different beamlines.

Speaker: Joseph Duris (SLAC National Accelerator Laboratory)

126 Measurements of Slice Energy Spread at Low-energy Photoinjectors

The slice energy spread of the electron beam is one of the key parameters for high performance of linac-driven free electron lasers (FELs). The simulated uncorrelated energy spread in modern XFEL photoinjectors with beam energies of many tens of MeV is on the order of a few keV or even less. Thus, accurate measurement of the slice energy spread is not trivial. Two recent studies on high energy (>100 MeV) photoinjectors at SwissFEL and European XFEL have reported much higher slice energy spread than expected at their XFEL working points (200 – 250 pC). A new method for measuring slice energy spread at a lower beam energy (∼20 MeV) is proposed and demonstrated at the Photo Injector Test facility at DESY Zeuthen (PITZ). The contribution will summarize previous results obtained on high energy injectors and then review the details of the technique used at PITZ as well as the experimental results for 250 pC, which are considerably lower than the results measured at high energy injectors.

Speaker: Mikhail Krasilnikov (Deutsches Elektronen-Synchrotron DESY at Zeuthen)

127 Non-linear Harmonics of a Seeded FEL at the Water Window and Beyond

The advent of free electron lasers (FELs) in the soft and hard X-ray spectral region has opened the possibility to probe electronic, magnetic and structural dynamics, in both diluted and condensed matter samples, with femtosecond time resolution. In particular, FELs strongly enhanced the capabilities of several analytical techniques, which took advantage of the high degree of transverse coherence provided. FELs based on the harmonic up-conversion of an external seed laser are characterised also by a high degree of longitudinal coherence, since electrons inherit the coherence properties of the seed. At the present state of the art, the shortest wavelength delivered to user experiments by an externally seeded FEL light source is about 4 nm. We show here that pulses with a high longitudinal degree of coherence (first and second order) covering the water window and with photon energy extending up to 790 eV can be generated by exploiting the so-called nonlinear harmonic regime, which allows generation of radiation at harmonics of the resonant FEL wavelength.
Moreover, we report the results of two proof-of-principle experiments: one measuring the oxygen K-edge absorption in water ($\sim$ 530 eV), the other analysing the spin dynamics of Fe and Co through magnetic small angle x-ray scattering at their L-edges (707 eV and 780 eV)

Speakers: Giuseppe Penco (Elettra-Sincrotrone Trieste S.C.p.A.), Alberto Simoncig (Elettra-Sincrotrone Trieste S.C.p.A.), Alessandro Gessini (Elettra-Sincrotrone Trieste S.C.p.A.), Alexander Brynes (Elettra-Sincrotrone Trieste S.C.p.A.), Enrico Allaria (Elettra-Sincrotrone Trieste S.C.p.A.), Alexander Demidovich (Elettra-Sincrotrone Trieste S.C.p.A.), Antonio Caretta (Elettra-Sincrotrone Trieste S.C.p.A.), Boris Vodungbo (Sorbonne Université, CNRS, Laboratoire de Chimie Physique – Matière et Rayonnement, LCPMR, Paris 75005, France), Carlo Callegari (Elettra-Sincrotrone Trieste S.C.p.A.), Carlo Spezzani (Elettra-Sincrotrone Trieste S.C.p.A.), Claudio Masciovecchio (Elettra-Sincrotrone Trieste S.C.p.A.), Dario De Angelis (Elettra-Sincrotrone Trieste S.C.p.A.), Eléonore Roussel (Laboratoire de Physique des Lasers, Atomes et Molécules), Emanuele Pedersoli (Elettra-Sincrotrone Trieste S.C.p.A.), Emiliano Principi (Elettra-Sincrotrone Trieste S.C.p.A.), Emmanuelle Jal (Sorbonne Université, CNRS, Laboratoire de Chimie Physique – Matière et Rayonnement, LCPMR, Paris 75005, France), Filippo Bencivenga (Elettra-Sincrotrone Trieste S.C.p.A.), Filippo Sottocorona (University of Trieste, Elettra Sincrotrone Trieste), Flavio Capotondi (Elettra-Sincrotrone Trieste S.C.p.A.), Gabor Kurdi (Elettra-Sincrotrone Trieste S.C.p.A.), Giovanni De Ninno (Elettra-Sincrotrone Trieste S.C.p.A.), Giovanni Perosa (University of Trieste, Elettra Sincrotrone Trieste), Giulio Gaio (Elettra-Sincrotrone Trieste S.C.p.A.), Ivaylo Nikolov (Elettra-Sincrotrone Trieste S.C.p.A.), Jan Luning (Sorbonne Université, CNRS, Laboratoire de Chimie Physique – Matière et Rayonnement, LCPMR, Paris 75005, France), Laura Badano (Elettra-Sincrotrone Trieste S.C.p.A.), Laura Foglia (Elettra-Sincrotrone Trieste S.C.p.A.), Luca Giannessi (Elettra-Sincrotrone Trieste S.C.p.A.), Marcel Hennes (Sorbonne Université, CNRS, Laboratoire de Chimie Physique – Matière et Rayonnement, LCPMR, Paris 75005, France), Marco Malvestuto (Elettra-Sincrotrone Trieste S.C.p.A.), Marco Zangrando (Elettra-Sincrotrone Trieste S.C.p.A.), Mauro Trovo (Elettra-Sincrotrone Trieste S.C.p.A.), Michele Manfredda (Elettra-Sincrotrone Trieste S.C.p.A.), Miltcho Danailov (Elettra-Sincrotrone Trieste S.C.p.A.), Paolo Cinquegrana (Elettra-Sincrotrone Trieste S.C.p.A.), Pelli Cresi Stefano (Elettra-Sincrotrone Trieste S.C.p.A.), Primoz Rebernik Ribic (Elettra-Sincrotrone Trieste S.C.p.A.), Riccardo Mincigrucci (Elettra-Sincrotrone Trieste S.C.p.A.), Simone Di Mitri (Elettra-Sincrotrone Trieste S.C.p.A.), Simone Spampinati (Elettra-Sincrotrone Trieste S.C.p.A.), Valentin Chardonnet (Sorbonne Université, CNRS, Laboratoire de Chimie Physique – Matière et Rayonnement, LCPMR, Paris 75005, France), Benedikt Roesner (Paul Scherrer Institut), Christian David (Paul Scherrer Institut), Panagiota Bougiatioti (Paul Scherrer Institut), Maurizio Sacchi (Synchrotron Soleil), Simone Dal Zilio (CNR-IOM, Istituto Officina dei Materiali)

128 Optimization of the FAST LINAC for a GREENS FEL Experiment

The FAST-GREENS FEL experiment is aimed at demonstrating extraction efficiencies of greater than 10%. This is accomplished with a high-power seed laser and an aggressively tapered undulator to compensate for the energy loss in the electron beam. A proof of concept experiment will be conducted at the Fermilab Accelerator Science and Technology Facility (FAST) using an undulator specifically built for this purpose. To support this experiment, the LINAC requires a unique setup that optimizes the longitudinal current distribution while preserving emittance in the presence of CSR and space-charge effects. This paper summarizes the beam dynamics optimization performed in support of TESSA and provides the nominal working point for the FEL experiment.

Speaker: Christopher Hall (RadiaSoft LLC)

129 Phase-Locked Hard X-Ray Self-Seeding FEL Study for the European XFEL

Phase-locked pulses are important for coherent control experiments. Here we present theoretical analyses and start-to-end simulation results for the generation of phase-locked pulses using the Hard X-ray Self-Seeding (HXRSS) system at the European XFEL. As proposed in Ref. [1], the method is based on a combination of self-seeding and fresh-slice lasing techniques. However, at variance with Ref. [1], here we exploit different transverse centroid offsets along the electron beam. In this way we may first utilize part of the electron beam to produce SASE radiation, to be filtered as seed and then generate HXRSS pulses from other parts of the beam applying appropriate transverse kicks. The final result consists in coherent radiation pulses with fixed phase difference and tunable time delay within the bunch length. This scheme should be useful for applications such as coherent x-ray pump-probe experiments.

Speakers: Tianyun Long (State Key Laboratory of Nuclear Physics and Technology), Ye Chen (Deutsches Elektronen-Synchrotron), Winfried Decking (Deutsches Elektronen-Synchrotron), Shan Liu (Deutsches Elektronen-Synchrotron), Najmeh Mirian (Deutsches Elektronen-Synchrotron), Weilun Qin (Deutsches Elektronen-Synchrotron), Jiawei Yan (European XFEL GmbH), Senlin Huang (State Key Laboratory of Nuclear Physics and Technology), Kexin Liu (State Key Laboratory of Nuclear Physics and Technology), Gianluca Geloni (European XFEL GmbH)

130 Photocathodes for the Electron Sources at FLASH and European XFEL

The photoinjectors of FLASH at DESY (Hamburg, Germany) and the European XFEL are operated by laser driven RF-guns. In both facilities cesium telluride photocathodes are successfully used since several years. We present recent data on the lifetime, quantum efficiency (QE), and dark current of the photocathodes currently in operation. In addition we present recent design changes in the photocathode transfer systems in order to further improve the cathode handling.

Speakers: Dr David Juarez-Lopez (Deutsches Elektronen-Synchrotron), Dr Siegfried Schreiber (Deutsches Elektronen-Synchrotron), Dr Laura Monaco (Istituto Nazionale di Fisica Nucleare), Dr Sven Lederer (Deutsches Elektronen-Synchrotron), Dr Daniele Sertore (Istituto Nazionale di Fisica Nucleare), Dr Frank Brinker (Deutsches Elektronen-Synchrotron)

131 Ponderomotive Scattering of Sub-Picosecond Ultracold Electron Bunches

We are developing an ultrafast and Ultracold Electron Source (UCES), based on near-threshold, two-step, femtosecond photoionization of laser-cooled rubidium gas in a grating Magneto Optical Trap (MOT). This source delivers stable ultrafast electron bunches with a unique combination of high bunch charge and low transverse emittance ~1.9 nm·rad, demonstrating the cold electron temperature ~25 K.

Recent development focused on long term stabilizing the electron beam. By pulsing the high voltage accelerator potential, the effects of surface charge buildup in the accelerator structure are mitigated and secondary electron emission as a result of ion impacts on the cathode is prevented.

This made a high resolution ponderomotive scattering measurement possible, in which a 1.1 mJ, 25 fs, 800 nm laser pulse is focused onto the electron bunch to a waist of 5.9 μm in vacuum. The ponderomotive force scatters the electrons which can be detected in the transverse profile. In this way the electron bunch length inside the self-compression point of the UCES has been measured to be 735±7 fs. Some wavelength dependent temporal structure originating from the ionization process could be observed.

Speaker: Tim de Raadt (Technische Universiteit Eindhoven)

132 Preparatory Experimental Investigations in View of EEHG at the DELTA Storage Ring

At DELTA, a 1.5-GeV electron storage ring operated by the TU Dortmund University, the seeding scheme CHG (coherent harmonic generation), the counterpart to HGHG (high-gain harmonic generation) without FEL gain, is used to provide ultrashort pulses in the femtosecond regime at harmonics of the seedlaser wavelength. To provide higher harmonics and thus shorter wavelengths, it is planned to upgrade the short-pulse facility to the EEHG (echo-enabled harmonic generation) scheme, which has yet not been implement at any storage ring. To install the needed three undulators and two chicanes, about a quarter of the storage ring needs to be modified. The paper presents the layout of the envisaged EEHG facility and the demo project SPEED (Short-Pulse Emission via Echo at DELTA) where all components are realized in a single undulator.

Speaker: Arjun Radha Krishnan (TU Dortmund University)

133 Quantum to Classical Transition of Free Electron Interaction with Light

We present a theory for free electrons interaction with radiation in both classical and quantum regimes and delineate their transition, based on a model of quantum electron wavepacket (QEW). The theory has general validity for a wide range of free electron interaction and radiation sources, including Free Electron Lasers, Cerenkov radiation, and transition radiation. We exemplify our analysis with the schemes of Smith-Purcell radiation and dielectric laser acceleration (DLA). These interactions, which were studied in terms of point particle physics, have a quantum nature in a phenomenon known as “photon-induced near-field electron microscopy” (PINEM).
Our QEW model identifies three universal distinct interaction regimes: (i) near-point-particle acceleration/deceleration DLA regime, (ii) PINEM regime of multiphoton induced electron energy sidebands, and (iii) anomalous PINEM regime (APINEM) of a newly reported periodic spectral bunching. See the three regimes in Fig.2.
The formulation displays the transition of the FEL stimulated gain expression from the quantum to classical limit. Elsewhere we provided extension of the semiclassical model to quantum electrodynamics to include spontaneous emission and spontaneous superradiance by modulated QEW similar to the classical prebunched-beam superradiant FEL in the classical point-particle picture。

Speaker: Bin Zhang (University of Tel Aviv)

Bin-QEW light interaction-last.pdf

134 Radio-Frequency-Detuning Based Modeling and Simulation of Electron Bunch Train Quality

A numerical study is carried out on the quality of the electron bunch train produced from a photoinjector based on a frequency-detuning dependent gun coupler kick. The impact of the kick on the emittance of the bunch train is modelled via three-dimensional electromagnetic field maps calculated at detuned frequencies of the gun cavity within
long radio-frequency pulses. Beam dynamics simulations are performed in the so-called frequency-detuning regime. Preliminary results are presented and discussed.

Speaker: Dr Ye Chen (Deutsches Elektronen-Synchrotron)

135 Recent Developments of the Laser Oscillator Synchronization for the FERMI Seed Laser

The local laser synchronization is known to be of high importance for Free Electron facilities, affecting both machine performance and pump-probe FEL-external laser experiments. So, there has been a continuous effort to improve the timing jitter of all machine lasers. One of the main contributions to the overall timing jitter comes from the locking of the local laser oscillators to the reference signal of the facility. Here we describe the latest developments and progress in this direction related to the FERMI seed laser system. The first investigated aspects includes the characterization and optimization of the locking performance of the commercial Ti:Sapphire oscillators Vitara T and HP (Coherent). We present data on the performance of three different oscillators of this type, as well as on the effect of adding an additional cavity length control actuator. The second presented aspect is related to the plan to extend the optical synchronization layout: for some planned seed laser operation modes two Ti:Sapphire oscillator need to be synchronized simultaneously. For this purpose, studies of optimum schemes for locking the two oscillators are in progress, first results are presented.

Speaker: Paolo Cinquegrana (Elettra-Sincrotrone Trieste S.C.p.A.)

136 RF Performance of a Next-Generation L-Band RF Gun at PITZ

A new generation of high-gradient normal conducting 1.3 GHz RF gun with 1% duty factor was developed to provide a high-quality electron source for superconducting linac driven free-electron lasers like FLASH and European XFEL. Compared to the Gun4 series, Gun5 aims for a ~50% longer RF pulse length (RF pulse duration of up to 1 ms at 10 Hz repetition rate) combined with high gradients (up to ~60 MV/m at the cathode). In addition to the improved cell geometry and cooling concept, the new cavity is equipped with an RF probe to measure and control the amplitude and phase of the RF field inside the gun. The first characterization of Gun5.1 included measurements of RF amplitude and phase stability (pulse-to-pulse and along 1 ms RF pulse). The dark current was measured at various peak power levels. The results of this characterization will be reported.

Speaker: Mikhail Krasilnikov (Deutsches Elektronen-Synchrotron DESY at Zeuthen)

137 Sensitivity of Echo-Enabled Harmonic Generation to Seed Power Variations

The external seeding technique Echo-Enabled Harmonic Generation (EEHG) consists of two undulators which are used to imprint energy modulations to an electron bunch via interaction with a seed laser. Each of these so-called modulators is followed by a chicane introducing longitudinal dispersion. Proper adjustment of the amplitudes of the energy modulations and dispersive strengths allows to achieve bunching at high harmonics of the seed laser wavelength. In the near future, this seeding scheme will be utilized in one of the beamlines of the free-electron laser (FEL) user facility FLASH at DESY to provide stable seeded radiation down to the soft X-ray regime at high repetition rate. Dedicated numerical simulations are carried out within the foreseen parameter space to investigate how variations of the energy modulations due to power fluctuations of the two seed lasers affect the bunching properties and the stability of the generated FEL radiation.

Speaker: Fabian Pannek (University of Hamburg)

138 Simulations of Seeding Options for THz FEL at PITZ

A THz FEL is in preparation at PITZ as a proof-of-principle experiment for a high power and high repetition rate THz source and as an option for THz-driven experiments at the European XFEL. Some of these experiments require excellent coherence and CEP stable THz pulses. In SASE regime the coherent properties of the FEL radiation are limited. A seeding scheme can be used instead of SASE to improve the coherent properties and shot-to-shot stability. Several options for seeding are considered in simulation for the THz FEL at PITZ: external laser pulse, pre-bunched electron beam, energy modulated electron beam and additional short spike. The results of the simulations for each method of seeding are evaluated and compared. The improvements over SASE in energy, spectral and temporal stability of the THz pulse are presented.

Speakers: Georgi Georgiev (Deutsches Elektronen-Synchrotron DESY at Zeuthen), Prach Boonpornprasert (Deutsches Elektronen-Synchrotron DESY at Zeuthen), Mikhail Krasilnikov (Deutsches Elektronen-Synchrotron DESY at Zeuthen), Xiangkun Li (Deutsches Elektronen-Synchrotron DESY at Zeuthen), Wolfgang Hillert (University of Hamburg)

139 Spectral Control of THz Super-Radiant Spontaneous Undulator Radiation Driven by Ultrashort Electron Beam with Energy Spread

S.Y. Teng(1,2), S.H. Chen(1), W.Y. Chiang(2), M.C. Chou(2), H.P. Hsueh(2), W.K. Lau(2), A.P. Lee(2), P.T. Lin(3)
1 Department of Physics, NCU, Taoyuan, Taiwan
2 NSRRC, Hsinchu, Taiwan
3 Department of Engineering and System Science, NTHU, Hsinchu, Taiwan.

Intense coherent THz radiation has been generated from an 18-period, hybrid-type U100 planar undulator as it is driven by short relativistic electron pulses produced from the NSRRC photoinjector. However, it is observed that the number of output optical pulse cycles is much less than the number of undulator periods and therefore the radiation spectral bandwidth has been broadened. It is found that the dispersion of undulator with excessive energy spread is responsible for this undesired broadening of THz radiation spectrum. In this study, instead of using rectilinear rf bunch compression (i.e. velocity bunching) in photoinjector linac, we investigate the feasibility of using nonlinear magnetic bunch compression for spectral bandwidth control of coherent THz undulator radiation.

Speakers: Shan You Teng (Department of Physics, National Central University), Shih-Hung Chen (Department of Physics, National Central University), Dr An-ping Lee (National Synchrotron Radiation Research Center), Dr Ming-Chang Chou (National Synchrotron Radiation Research Center), Dr Hsin-Pai Hsueh (National Synchrotron Radiation Research Center), Dr Wei-Yuan Chiang (National Synchrotron Radiation Research Center), Wai-Keung Lau (National Synchrotron Radiation Research Center), Mr Pao Ting Lin (National Tsing Hua University)

140 Spectro-Temporal Properties of Coherently Emitted Ultrashort Radiation Pulses at DELTA

At the 1.5 GeV synchrotron light source DELTA operated by the TU Dortmund University, the short-pulse facility employs the seeding scheme coherent harmonic generation (CHG) to produce ultrashort pulses in the vacuum ultraviolet and terahertz regime. This is achieved via a laser-induced electron energy modulation and a subsequent microbunching in a dispersive section. The spectro-temporal properties of the CHG pulses as well as the coherently emitted terahertz radiation are influenced by the seed laser parameters and can be manipulated by varying the laser pulse shape and the strength of the dispersive section. CHG spectra for different parameter sets were recorded and compared with the results of numerical simulations to reconstruct the spectra. A convolutional neural network was employed to extract the spectral phase information of the seed laser from the recorded spectra. In addition, the shaping of the coherently emitted THz pulses by controlling the seed pulse spectral phase using a spatial light modulator was also demonstrated.

Speakers: Arjun Radha Krishnan (TU Dortmund University), Benedikt Büsing (TU Dortmund University), Shaukat Khan (TU Dortmund University), Carsten Mai (TU Dortmund University), Arne Held (TU Dortmund University), Zohair Usfoor (TU Dortmund University), Vivek Vijayan (TU Dortmund University)

141 Stable Multi-Day Performance of a Laser Wakefield Accelerator for FEL Applications

We report on the operation of the DRACO Laser Driven electron source for stable multi-day operation for FEL applications. The nC-class accelerator delivers charge densities around 10 pC/MeV , <1 mrad rms divergence at energies up to 0.5 GeV and peak currents of over 10 kA [1].
Precise characterisation is paramount for controlled operation, including: spectrally resolved charge diagnostic, coherent optical transition radiation (TR) to resolve microbunch beam structures [2] and TR-based multioctave high-dynamic range spectrometry for sub-fs resolved characterisation of the 10 fs rms electron bunches [3]. Achieved stability allows for systematic exploration of demanding applications, resulting in the recent demonstration of the first LWFA based Beam-driven Plasma Wakefield Accelerator [4].
Fulfilling the high demands required for FEL operation, the COXINEL manipulation line developed at Synchotron SOLEIL has recently been installed at our facility. Based on successful beam transport of over 13000 shots within 9 experimental days during commissioning, we were able to demonstrate the very first operation of a seeded FEL driven by a laser plasma accelerator [5].

Speakers: Jurjen Couperus Cabadag (Helmholtz-Zentrum Dresden-Rossendorf), Stefan Bock (Helmholtz-Zentrum Dresden-Rossendorf), Yen-Yu Chang (Helmholtz-Zentrum Dresden-Rossendorf), Alexander Debus (Helmholtz-Zentrum Dresden-Rossendorf), Rene Gebhardt (Helmholtz-Zentrum Dresden-Rossendorf), Amin Ghaith (Helmholtz-Zentrum Dresden-Rossendorf), Uwe Helbig (Helmholtz-Zentrum Dresden-Rossendorf), Arie Irman (Helmholtz-Zentrum Dresden-Rossendorf), Alexander Koehler (Helmholtz-Zentrum Dresden-Rossendorf), Maxwell LaBerge (Helmholtz-Zentrum Dresden-Rossendorf), Richard Pausch (Helmholtz-Zentrum Dresden-Rossendorf), Thomas Püschel (Helmholtz-Zentrum Dresden-Rossendorf), Ulrich Schramm (Helmholtz-Zentrum Dresden-Rossendorf), Mr Susanne Schöbel (Helmholtz-Zentrum Dresden-Rossendorf), Dr Klaus Steiniger (Helmholtz-Zentrum Dresden-Rossendorf), Patrick Ufer (Helmholtz-Zentrum Dresden-Rossendorf), Omid Zarini (Helmholtz-Zentrum Dresden-Rossendorf), Eléonore Roussel (Laboratoire de Physique des Lasers, Atomes et Molécules), Marie-Emmanuelle Couprie (Synchrotron Soleil), Marie Labat (Synchrotron Soleil), Michael Downer (The University of Texas at Austin)

142 Status of the Seeding Upgrade for FLASH2020+ Project

In the framework of the FLASH2020+ project, the FLASH1 beamline will be upgraded to deliver seeded FEL pulses for users. This upgrade will be achieved by combining high gain harmonic generation and echo-enabled harmonic generation with a wide-range wavelength-tunable seed laser, to efficiently cover the 60-4 nm wavelength range. The undulator chain will also be refurbished entirely using new radiators based on the APPLE-III design, allowing for polarization control of the generated light beams. With the superconducting linac of FLASH delivering electron beams at MHz repetition rate in burst mode, laser systems are being developed to seed at full repetition rates. In the contribution, we will report about the progress of the project.

Speakers: Andreas Thiel (University of Hamburg), Dmitrii Samoilenko (University of Hamburg), Enrico Allaria (Elettra-Sincrotrone Trieste S.C.p.A.), Eugenio Ferrari (Deutsches Elektronen-Synchrotron), Fabian Pannek (University of Hamburg), Georgia Paraskaki (Deutsches Elektronen-Synchrotron), Ingmar Hartl (Deutsches Elektronen-Synchrotron), Jiaan Zheng (Deutsches Elektronen-Synchrotron), Johann Zemella (Deutsches Elektronen-Synchrotron), Dr Lucas Schaper (Deutsches Elektronen-Synchrotron), Margarit Asatrian (University of Hamburg), Dr Markus Tischer (Deutsches Elektronen-Synchrotron), Martin Beye (Deutsches Elektronen-Synchrotron), Dr Mehdi Mohammad Kazemi (Deutsches Elektronen-Synchrotron), Dr Pardis Niknejadi (Deutsches Elektronen-Synchrotron), Pavel Vagin (Deutsches Elektronen-Synchrotron), Samuel Hartwell (Deutsches Elektronen-Synchrotron), Dr Sheida Mahmoodi (Deutsches Elektronen-Synchrotron), Siegfried Schreiber (Deutsches Elektronen-Synchrotron), Dr Sven Ackermann (Deutsches Elektronen-Synchrotron), Tino Lang (Deutsches Elektronen-Synchrotron), Wolfgang Hillert (University of Hamburg)

143 Studies of Wavelength Control at FERMI

FEL basic theory indicates that the output wavelength of a seeded FEL operated in the HGHG configuration is determined by the wavelength of the seed laser and light is emitted when undulators are tuned to one of the harmonics of the seed laser. In a more realistic case, when taking into account the electron beam imperfections and the finite bandwidths of the seed and of the amplification process, the output wavelength is influenced by these factors and there is a small variation from this rule.
In this work, we consider the effects of the dispersive section, the curvature of the electron beam longitudinal phase-space and the frequency pulling as major contributors. We show how these quantities influence the effective final FEL wavelength. Furthermore, we show how one can reconstruct the electron beam longitudinal phase-space from the analysis of the FEL wavelength sensitivity to the seed laser delay with respect to the beam arrival time.

Speakers: Filippo Sottocorona (Università degli Studi di Trieste), Giovanni Perosa (Università degli Studi di Trieste)

144 The ASPECT Project

Attosecond pulse production is an important development focus for most major FEL facilities. Chirp/taper and eSASE schemes, both of which will shorten the pulses well below the femto-second level for both hard and soft x-rays, are proposed for implementation at EuXFEL. As a high repetition rate super conducting linac that feeds three 200m long undulator lines for parallel operation, EuXFEL presents distinct challenges but also unique opportunities for the proposed schemes.

Speakers: Marc Guetg (Deutsches Elektronen-Synchrotron), Gianluca Geloni (European XFEL GmbH), Dr Ye Chen (Deutsches Elektronen-Synchrotron), Evgeny Schneidmiller (Deutsches Elektronen-Synchrotron), Christoph Lechner (European XFEL GmbH), Jiawei Yan (European XFEL GmbH), Svitozar Serkez (European XFEL GmbH), Christoph Heyl (Deutsches Elektronen-Synchrotron)

145 The New FLASH1 Undulator Beamline for the FLASH2020+ Project

The 2nd stage of the FLASH2020+ project at DESY will be an upgrade of the FLASH1 beamline to enable HGHG and EEHG seeding with two modulator-chicane stages, and a radiator section with 11 Apple-III undulators to enable FEL radiation with controllable polarization. A key feature of FLASH, namely the capability of providing several thousand FEL pulses in the extreme UV and soft X-ray must not be compromised.
Downstream of the radiator the beamline houses longitudinal diagnostics, a double bend (quasi-) achromat to separate the electrons from the photons and divert the electron beamline from the photon diagnostics, a post-compressor, a THz-Undulator (requires an electron beam that is compressed more strongly than for seeding), and finally the dumpline, capable of safely aborting up to 100 kW electron beam power.
This article describes the conceptional and some technical details of the beamline with emphasis on the upstream part (modulators and radiator) designed for seeding.

Speakers: Johann Zemella (Deutsches Elektronen-Synchrotron), Mathias Vogt (Deutsches Elektronen-Synchrotron)

146 Towards a Seeded High Repetition Rate FEL: Concept of Seed Laser Beam Transport and Incoupling

FLASH2020+ is an upgrade project for the FLASH facility at Hamburg. A main goal of the project is to generate fully coherent soft X-ray FEL radiation at a high repetition rate (MHz). The project will utilize two external laser seeding principles in order to produced Seeded FEL with tunable wavelength from 4-60 nm. In order to achieve this goal, both HGHG (High Gain Harmonic Generation) and EEHG (Echo-Enhanced Harmonic Generation) methods provide FEL emission at harmonics of a seed laser. For HGHG, a tunable UV laser system (297-317 nm) and for EEHG a combination of the tunable UV laser and fixed wavelength (343 nm) laser system would be used to cover the whole range of wavelengths between 4-60 nm.
In this contribution, we will describe the requirements of the seed laser to initiate the seeding process and will explain the concept of seed laser beam transport and incoupling into the modulators for FEL radiation production.
The first seed laser (Seed1) with fixed wavelength is transported about 28 meters from laser lab to the incoupling chicane. The second seed laser (Seed2) with a tunable UV wavelength is transported about 35 meters.
Our concept uses a full relay imaging system and in vacuum components for the laser transport in addition to high repetition rate diagnostics to deliver, monitor and control the beam and pulse parameters at the interaction with electron beam. We investigate the technical and engineering limitations for the design and address those challenges to provide the demanding seed laser parameters for generating high repetition rate seeded FEL.

Speaker: Mehdi M. Kazemi (Deutsches Elektronen-Synchrotron)

147 Transversally Separated Crossed Polarized FEL Subpulses

The extension of four-wave mixing (FWM) technique to the extreme ultraviolet and soft X-ray ranges allows to monitor the dynamics of coherent excitations of matter, when realized with the exquisite coherent property of bright FEL pulses. We show for the first time a scheme to provide transversally separated pulses with parallel or crossed linear polarizations, realized at FERMI FEL. This configuration paves the way to explore additional features of pump&probe and FWM techniques, and, in particular, the possibility to excite a transient polarization grating on the sample. For this reason, such a technique is important the detection of circular dichroism and chiral properties of matter and the characterization of spin waves and magnons. By tailoring the electrons trajectory along the undulator line, we demonstrate the possibility of deliver balanced and stable couple of pulses with an horizontal separation of the order of millimeters at the experimental station.

Speaker: Giovanni Perosa (Università degli Studi di Trieste and Elettra-Sincrotrone Trieste S.C.p.A.)

148 Universal Tool for THz Radiation Analysis

A unique platform for a Tera Hertz Transmission Line design for a superradiant FEL is present. The smart line is controlled by Artificial Intelligence (AI) intended for a wide tunable broad-spectrum THz radiation propagation. The main goal is to transfer radiation in the most efficient way. A 3D analysis and diagnostic of radiation space-frequency tool was developed. The AI changes the functions of the mirrors in such a way that all the reflected rays will reach the target. The rays represent the electromagnetic field similar to a light field. The representation of the field in terms of rays was carried out using the Wigner Distribution Function. It allows describing the dynamics of field evolution in future propagation. This in turn helps with the initial design of the transmission line and facilitates the use of a Ray Tracing method for future processing. Thus, working in the linear and non-linear regimes. The Ray Tracing method and code is greatly enhanced using parallel processing with graphics cards.

Speaker: Michael Gerasimov (Ariel University)

Wednesday 24 August

Electron sources

Convener: Fernando Sannibale (Lawrence Berkeley National Laboratory)

149 Review of Recent Photocathode Advancements

Photocathodes are routinely used as a source of electrons in high brightness beam photoinjectors. The properties of the photocathode have a significant influence on the parameters of the electron beams and on the operation of the machines. The choice of photocathode materials is an important step in reaching the challenging requirements of modern accelerators. Recent advancements towards more performing photocathodes are here presented and discussed.

Speaker: Laura Monaco (Istituto Nazionale di Fisica Nucleare)

150 First Commissioning of LCLS-II Injector

The 1-MeV LCLS-II electron source including CW RF gun and buncher was successfully commissioned 2018-2020. Since then, a few upgrades has been implemented. Full scale of LCLS-II injector including the upgraded 1-MeV electron source, one standard 100-MeV Cryomodule, essential diagnostics for beam performance characterizations is being commissioned since late March 2022. This report will present technical progresses and challenges of the injector commissioning and measurements of e-beam performance as well as dark current.

Speaker: Feng Zhou (SLAC National Accelerator Laboratory)

151 Continuous-Wave Operation of a Low-Emittance DC-SRF Photocathode Gun

DC-SRF gun, a DC and superconducting rf (SRF) combined photocathode electron source, has been developed at Peking University for nearly 20 years. Recently, a low-emittance version of DC-SRF gun, DC-SRF-II, was brought into stable CW operation with a DC voltage of 100 kV and an SRF cavity gradient of 13 MV/m, under which condition the dark current was measured to be lower than 0.001 nA. Normalized RMS emittances (with 95% particles) of about 0.5 mm-mrad, 0.85 mm-mrad, and 1.25 mm-mrad have been achieved at the bunch charge of 20 pC, 100 pC, and 260 pC, respectively, and with an electron energy gain of 2 MeV. In this work we will present the detailed results of our latest experiments.

Speaker: Dr Weilun Qin (Deutsches Elektronen Synchrotron (DESY) and Center for Free Electron Science (CFEL))

152 Chirped Pulse Laser Shaping for High Brightness Photoinjectors

Photoemission laser shaping is essential for both beam brightness and advanced accelerator concepts, therefore is an important R&D at the Photo Injector Test Facility at DESY in Zeuthen (PITZ). The laser pulse shaper presented here is based on spectral amplitude modulation of chirped laser pulses. In this approach one can do temporal-spatial coupled laser shaping, i.e. 3D shaping. The laser shaping is done at 1030 nm with spatial light modulators, and then converted to 257.5 nm through harmonic generation for photoemission. Experimental results of laser pulse shaping and shape preservation through harmonic generation are presented for different cases: spatial shaping, temporal shaping and full 3D shaping. Electron beam testing results will also be presented.

Speaker: Dr Christian Koschitzki (Deutsches Elektronen-Synchrotron DESY at Zeuthen)

10:35 Coffee & Exhibition

Electron beam dynamics

Convener: Sara Thorin (MAX IV Laboratory)

153 Comparison of Eulerian, Lagrangian and Semi-Lagrangian Simulations of Phase-Space Density Evolution

Good understanding of the underlying beam dynamics is mandatory for the successful design and operation of Free-Electron Lasers. In particular, it is important that all physically relevant collective effects are adequately represented in simulation codes so that their influence on the phase-space evolution of the bunch can be calculated with sufficient accuracy at all relevant length scales. Besides coherent collective effects such as space charge or coherent radiative interaction also incoherent effects such as intra-beam scattering are suspected to have a significant impact on the efficacy of sophisticated lasing techniques.
Most of the well-known and widely-used beam dynamics codes employ the Lagrangian approach, in which the particle bunch is represented by discrete points in phase-space and track the solutions of their equations of motion. In contrast to that, in the Eulerian and semi-Lagrangian approach, the bunch is described by a numerical representation of its phase-space density function.
This contribution discusses the working principles of the three classes of simulation methods Lagrangian, Eulerian, and semi-Lagrangian and highlights their respective advantages and short-comings, when applied to the simulation of collective beam dynamics in FELs.

Speaker: Philipp Amstutz (Deutsches Elektronen-Synchrotron)

154 First Evidence of Intrabeam Scattering in an Electron Linac and Impact on Short Wavelength FELs

To date, the main obstacle to the extension of free electron lasers’ longitudinal coherence to the water window and beyond is the detrimental effect of spurious harmonic content in the longitudinal profile of electron bunches, namely the microbunching instability.
Intra-beam scattering is another (less known) collective effect that consists of multiple (small-angle) soft Coulomb scattering of electrons inside a bunch. The inclusion of this electronic scattering in MBI model has proved to be an essential step to more faithful predictions of the beam energy spread and characterization of spurious content.
Analytical expressions for intra-beam scattering in single pass linacs and multi-bend transfer lines are presented and included in two different semi-analytical description of microbunching. The overall modeling turns out to be a fast comprehensive tool for the optimization of linac-driven free-electron lasers.

Speaker: Giovanni Perosa (Università degli Studi di Trieste)

155 Energy Spread Blow-Up by Intra-Beam Scattering and Micro-Bunching at the SwissFEL Injector

High-resolution measurements of the uncorrelated energy spread at SwissFEL indicate energy spread levels much larger than predicted by state-of-the-art particle tracking. This contribution presents measurements of the energy spread at the SwissFEL injector as a function of the electron bunch charge, the optics and the longitudinal dispersion of the lattice. The results indicate that both intra-beam scattering and micro-bunching, not covered in the conventional modeling of injectors, cause a blow-up of the energy spread. The work underlines the importance of considering the energy spread in the optimization and design of high-brightness electron beam sources and the need to develop new models to adequately understand and simulate the observed physics effects.

Speakers: Eduard Prat (Paul Scherrer Institut), Paolo Craievich (Paul Scherrer Institut), Simone Di Mitri (Elettra-Sincrotrone Trieste S.C.p.A.), Thomas Lucas (Paul Scherrer Institut), Alexander Malyzhenkov (Paul Scherrer Institut), Giovanni Perosa (University of Trieste, Elettra Sincrotrone Trieste), Sven Reiche (Paul Scherrer Institut), Eugenio Ferrari (Deutsches Elektronen-Synchrotron), Philipp Dijkstal (Paul Scherrer Institut)

156 Characterization of the European XFEL Pulses in the Presence of Microbunching Instability

One of the serious issues for short electron bunches in electron beam accelerators is the microbunching instability driven by longitudinal wake fields along the accelerator. Over the last decades a tremendous effort has been made in the theoretical understanding and experimental study of the microbunching instability impact on free electron laser performances. At the European XFEL, the compression of the electron beam to high peak current is achieved through three bunch compressors. Normally, the compression factor is more than 100, resulting in around 25 fs RMS bunch length. This high compression factor transfers the microbunching wavelengths to the visible or (very) near infrared radiation wavelength. In this presentation, we discuss our recent MBI study and measurements in European XFEL. By using the matrix model for collective space charge and coherent synchrotron radiation phenomena in electron beam longitudinal phase space and considering existing theories of intera beam scattering, in single pass linacs and multi-bend transfer lines [1], we theoretically characterize the MBI after each bunch compressor at European XFEL machine. We verify our theoretical prediction with longitudinal phase space and FEL radiation measurement.

[1] G. Pasao, S. Di Mitri Scientific Reports 11:7895 (2021)

Speaker: Najmeh Mirian (Deutsches Elektronen-Synchrotron)

12:50 Lunch

Novel acceleration and FEL concepts

Convener: Amin Ghaith (Helmholtz-Zentrum Dresden-Rossendorf)

157 Free-Electron Lasing Based on a Laser Wakefield Accelerator

Laser wakefield accelerators can sustain accelerating gradients more than three orders of magnitude higher than those of radio-frequency accelerators, and are regarded as an attractive option for driving compact X-ray free-electron lasers. However, the realization of such devices remains a challenge owing to the relatively poor quality of electron beams that are based on a laser wakefield accelerator. After ten years of efforts, we present an experimental demonstration of undulator radiation amplification in the exponential-gain regime by using electron beams based on a laser wakefield accelerator. The amplified undulator radiation, which is typically centred at 27 nanometres and has a maximum photon number of around 1010 per shot, yields a maximum radiation energy of about 150 nanojoules. The results constitute a proof-of-principle demonstration of free-electron lasing using a laser wakefield accelerator, and pave the way towards the development of compact X-ray free-electron lasers based on this technology with broad applications. In future, a laboratory-scale, ultra-brilliant FEL (around 10 m in size), with the advantages of low cost (~US$5 million), high temporal resolution (femtosecond-level), high resolution (nanometre-level), and ultra-high precision timing control (less than 1 fs), could gain popularity.

Speaker: Prof. Wentao Wang (Shanghai Institute of Optics and Fine Mechanics)

158 First SASE and Seeded FEL Lasing Based on a Beam Driven Wakefield Accelerator

The breakthrough provided by plasma-based accelerators enabled unprecedented accelerating fields by boosting electron beams to GeV energies within few cm.

This enables the realization of table-top accelerators able to drive a Free-Electron Laser (FEL), a formidable tool to investigate matter at sub-atomic level by generating X-UV coherent light pulses with fs and sub-fs durations.

So far, short wavelength FELs had to rely on the use of conventional large-size radio-frequency (RF) accelerators due to the limited accelerating fields provided by such a technology.

Here we report the experimental evidence of a FEL driven by a compact (3 cm) plasma accelerator. The accelerated beams are characterized in the six-dimensional phase-space and have a quality, comparable with state-of-the-art accelerators. This allowed the observation of amplified SASE radiation in the infrared range with typical pulse energy exponential growth, reaching tens of nJ over six consecutive undulators.

On the basis of these first amplification results starting from spontaneous emission (SASE), we upgraded the setup by seeding the amplifier with an external laser. Compared to SASE, the seeded FEL pulses are characterized by a higher pulse energy, two orders of magnitude larger (up to about 1 uJ) and an enhanced reproducibility (up to about 90%) resulting in a higher shot-to-shot stability.

Speaker: Mario Galletti (Istituto Nazionale di Fisica Nucleare)

159 First Laser Plasma Accelerator Based Seeded FEL

We report the first lasing of a seeded FEL fully driven by a laser plasma accelerator. The experiment was performed at HZDR (Germany), coupling the high quality electron beams of the HZDR laser plasma accelerator with the versatile COXINEL beam manipulation line. Using an external seed at 270 nm, the FEL signal was observed at 275 nm. We explain how this slight red-shift confirms previous predictions [1], show the precise control over the FEL wavelength and give evidence of the longitudinal coherence of the emitted pulses. All experimental results are strongly supported by analytic modeling and Genesis numerical simulations. Our results substantiate the continuous progress of LPA technology to enable FEL operation and finally bring temporal coherence to those compact promising sources.
[1] M Labat et al 2020 New J. Phys. 22 013051.

Speakers: Fabrice Marteau (Synchrotron SOLEIL), Jean-Pierre Duval (Synchrotron SOLEIL)

160 Bridging the Gap of Storage Ring Light Sources and Linac-Driven Free-Electron Lasers

High-gain free-electron lasers (FELs) are driven by short, high-charge density electron beams as only produced at dedicated single pass or recirculating linear accelerators. We describe new conceptual, technical, and modeling solutions to produce subpicosecond, up to ∼100 μJ-energy extreme ultra-violet and soft x-ray FEL pulses at high and tunable repetition rates, from a diffraction-limited storage ring light source. In contrast to previously proposed schemes, we show that lasing can be simultaneous to the standard multibunch radiation emission from short insertion devices, and that it can be obtained with limited impact on the storage ring infrastructure. By virtue of the high-average power but moderate pulse energy, the storage ring-driven high-gain FEL would open the door to unprecedented accuracy in timeresolved spectroscopic analysis of matter in the linear response regime, in addition to inelastic scattering experiments.

Speaker: Simone Di Mitri (Elettra-Sincrotrone Trieste S.C.p.A.)

One-to-one meetings with experts in project building 2

One-to-one meetings with experts in the fields, who will give you operative instructions on how to prepare successful project proposals to obtain access to European laser and FEL laboratories and/or to get grants for researchers from all over the world.

161 Wayforlight and Beamtime Proposals

Discover the wayforlight.eu portal to find out the most suitable instruments for your research and get useful tips to draft a successful beamtime proposal

Speaker: Cecilia Blasetti (Elettra-Sincrotrone Trieste S.C.p.A.)

162 Laserlab-Europe

Become aware of transnational access support tools, joint research and training opportunities offered by the Laserlab-Europe Consortium

Speaker: Sylvie Jacquemot (Ecole Polytechnique)

163 Horizon Europe Programmes and Grants for All Stages of Research Careers

Get introduction and assistance on funding possibilities for researchers from all over the world

Speaker: Mrs Marina Kozlik Mercatelli (Consorzio per l'AREA di ricerca scientifica e tecnologica di Trieste)

164 Euraxess – an European Network to Support Mobility and Career Development for Researchers

Get information on how to apply for research jobs in Europe.

Speaker: Anna Comini (Consorzio per l'AREA di ricerca scientifica e tecnologica di Trieste)

Wednesday posters: Coffee & Exhibition

165 AI Methods for an Improved Evaluation of FEL Diagnosic Data

Free electron lasers (FEL) serve a broad user community in many scientific fields ranging from
atomic and molecular physics to plasma and solid state physics as well as chemistry and biology.
Many experiments could benefit from a non-destructive online photon diagnostic of the provided
x-ray pulses. Especially, for free-electron lasers that are operated in the self-amplified sponta-
neous emission (SASE) regime, where the pulse characteristics fluctuate from pulse to pulse [1],
reliable online information on the intensity, spectral distribution, and temporal structure of
each individual pulse can be crucial. A fast feedback can significantly improve an on-the-fly
evaluation of user experiments. In addition, subsequent sorting of measurement data by, for ex-
ample, intensity or wavelength can reveal signatures of physical processes that would otherwise
be hidden in the fluctuation. Finally, real-time information about the pulse can give a direct
feedback for FEL beam tuning.

Neural networks became popular as a powerful analysis tool in all categories of science [2]. This
is due to their ability to recognize complex relationships in large datasets. There are various
architectures of neural networks, each with its own focus on specific tasks. What they all have in
common is that they need to be trained during a training process in order to recognize patterns
and correlations. A special case of training is performed in unsupervised learning, where the
network does not need any expert knowledge about the data. This can be done for example with
autoencoder networks [3]. These networks consist of an encoder and a decoder. The encoder
learns during the training phase to compress data to lower dimensionality, the so-called latent
space, the decoder to reconstruct the input from this compressed representation. This means
that, given the decoder, the latent space contains all information needed to reconstruct an in-
put sample. A special form of autoencoder networks are β Variational Autoencoder (β-VAE)
networks [4], that allow to balance between the goal of a perfect reconstruction of the data and
a perfect disentanglement of the latent space vector components. These networks are found to
be able to find the key principles in an unlabeled data set, even if these principles were not
known before.

We demonstrate the usage of β-VAEs to characterize SASE X-ray pulses of the free electron
laser FLASH in Hamburg. We combine data from different diagnostic devices. We evaluate
measured data from the online photoionization spectrometer OPIS [5], that uses 4 electron time
of flight spectrometers to monitor each individual FEL pulse. In addition, we include data from
an X-band transverse deflecting mode cavity diagnostic system (XTCAV). The latter is simi-
lar to the XTCAV at the Linac Coherent Light Source [6]. This device measures the position
and kinetic energy of the electrons after they have passed the undulator and is therefore able
to monitor the differences in the temporal structure of the electron bunches due to the lasing
process. We demonstrate that a β-VAE can detect key principles in the XTCAV and the OPIS
data, like pulse duration and central wavelength and compare them to other diagnostic devices
such as data from a gas monitor device (GMD) [7] and THz field-driven streaking [8]. Without a-priori knowledge the network is able to find directly human-interpretable representa-
tions of single-shot FEL spectra, remove noise as well as reveal data artefacts and hence allows
for an improved in-depth analysis of photon diagnostics data.

Speaker: Gesa Goetzke (Deutsches Elektronen-Synchrotron)

166 Development of a Table-Top THz Free-Electron Laser with a Microtron Accelerator and a Hybrid Electro-Magnetic Undulator

We were able to realize a compact microtron accelerator with 5 MeV electron beam acceleration energy and a hybrid electro-magnetic undulator that can vary the magnetic field of 1.07 T at 0.74 T. The electron beam is accelerated by an RF electric field of a 1-cell acceleration cavity and recirculated by a uniform magnetic field of the microtron main chamber. Through the re circulation process, the electron beam energy 5 MeV, energy spread 0.5%, pulse width 5 μs, and electron beam acceleration current are 48 mA. Hybrid electro-magnetic undulator set Iron buses in hybrid planar undulator structures and generate electro-magnet effects.The undulator has an iron pole in the form of a comb and can easily install one turn of electromagnet coil using a copper tube. This undulator can adjust the applied current to adjust the magnetic field strength to 0.74–1.07T, when the standard deviation of the maximum magnetic field strength distribution is very accurate with less than 0.5%. The trajectory of the electron beam of the undulator inside has the stable less than the 3 mm.

Speaker: Sangyoon Bae (Korea Atomic Energy Research Institute)

167 Infrared Spectrometer for Microbunching Characterization

The presence of microbunching in the FERMI FEL electron beam is a known nuisance that can impact the performance of the FEL. Microbunching is generated by complex dynamics along the machine where there are several amplification sources. The use of a laser heater, at the cost of an increased energy spread, can improve the quality of the electron beam mitigating the modulation due to microbunching. A new diagnostic has been developed to provide more detailed insight into the microbunching properties. This diagnostic is essentially an Infrared Spectrometer that analyzes the spectrum of the Coherent Transition Radiation emitted when the electron beam passes through a metallic screen. The Coherent Transition Radiation replicates in its spectral content the microbunching properties which, in the case of FERMI, is peaked in a range from 1 to 10 𝜇m. A simple design based on a CaF2 prism has been preferred over the use of diffraction gratings to avoid the superposition of higher diffraction orders. A spherical mirror is employed to improve the signal-to-noise ratio and the resolution of the spectrometer. PbSe and Pyroelectric detectors are used to cover a wavelength range from 0.25 to 10 𝜇m. In this contribution, the design of the instrument together with some preliminary measurements is presented.

Speakers: Marco Veronese (Elettra-Sincrotrone Trieste S.C.p.A.), Enrico Allaria (Elettra-Sincrotrone Trieste S.C.p.A.), Alexander Brynes (Elettra-Sincrotrone Trieste S.C.p.A.), Giovanni De Ninno (Elettra-Sincrotrone Trieste S.C.p.A.), Simone Di Mitri (Elettra-Sincrotrone Trieste S.C.p.A.), Mario Ferianis (Elettra-Sincrotrone Trieste S.C.p.A.), Luca Giannessi (Elettra Sincrotrone Trieste and Istituto Nazionale di Fisica Nucleare), Giuseppe Penco (Elettra-Sincrotrone Trieste S.C.p.A.), Giovanni Perosa (University of Trieste, Elettra Sincrotrone Trieste), Eléonore Roussel (Laboratoire de Physique des Lasers, Atomes et Molécules), Carlo Spezzani (Elettra-Sincrotrone Trieste S.C.p.A.), Simone Spampinati (Elettra-Sincrotrone Trieste S.C.p.A.)

168 Upgrade of the 2 Tesla Electro-Magnet and Power Supply of the DEIMOS Beamline at Synchrotron SOLEIL

DEIMOS (Dichroism Experimental Installation for MagnetOptical Spectroscopies) is the beamline built at French Synchrotron SOLEIL facility intended for soft X-rays magnetic and natural dichroism spectroscopies. It has been designed to enable most challenging measurements in terms of X-rays sample sensitivity and signal detection level. The energies accessible on DEIMOS beamline rank from 350 eV up to 2500 eV, with all polarizations (circular left and right, linear), covering the absorption edges of the elements most relevant to the magnetic nanostructure scientific community, i.e. the first (3d) and second (4d) rows transition metals L-edges, the rare earth elements M-edges and nitrogen, oxygen and sulfur K-edges. While its main end station, a 7 Tesla cryomagnet, allows for measurements down to sub-Kelvin temperature up to room temperature, its second end station, a 2 Tesla electromagnet, is currently under renovation thanks to the partnership between Italian power supply constructor OCEM Power Electronics and French electromagnet manufacturer SEF Technologies.
The coil is made of hollow copper to allow direct cooling in order to achieve the required 2 Tesla field strength. The magnetic model of this coil has been studied and validated before manufacturing. The new power supply will have a four quadrant fast switching topology, with a high stability output. To increase the reliability, the architecture is a proven modular technology coming from previous realizations running in other facilities. Once renovated, the so-called MK2T end station will allows for fast switching (1 Hz) between +/-2 Tesla. It will be aimed to host most peculiar inserts such as variable temperature liquid cell, high temperature (1000 K) and multiferroic inserts. Commissioning is expected as early as autumn 2022 and the facility could be available to users through standard review of the SOLEIL program committee, at the upcoming call for proposal.

Speaker: Miguel Pretelli (OCEM Energy Technology)

169 Achievements and Challenges for Sub-10 fs Long-Term Arrival Time Stability at Large-Scale SASE FEL Facilities

A high temporal stability of produced photon pulses is a key parameter for some classes of experiments, e.g., those using a pump-probe scheme. A longitudinal intra bunch-train feedback system, that reduces the intra bunch-train and the train-to-train arrival time fluctuations down to the sub-10 fs level was implemented at the European X-ray free electron laser (EuXFEL). The low arrival time jitter of the electron beam is preserved in the generated photon pulses. However, over long measurement periods, additional environmental factors acting on different time scales have to be considered. These factors include the temperature, relative humidity and in case of the European XFEL ground motions due to ocean activities. Mitigation of the residual timing drifts between pump laser and FEL pulses requires additional measures to disentangle the overlaid effects. The latest results and future challenges for the long-term arrival time stabilization will be presented.

Speaker: Björn Lautenschlager (Deutsches Elektronen-Synchrotron)

170 Adhesive Technologies at Manufacture THz Mirrors

This project describes different techniques to manufacture THz mirrors with arbitrary surfaces. The research is part of the development of THz transmission line for the compact FEL-THz accelerator.
As an initial phase flat mirrors were 3D printed with FFF (Fused Filament Fabrication) and SLA (Stereolithography Apparatus). The impact of material, layer height and layer direction to mirror’s surface quality was exanimated. In addition, various metal coating was tested, for example vacuum evaporation and metal foil.
The 3D printed flat mirror’s reflection was measured in TDS (Time Domain Spectroscopy) at 1–5 THz and compared with aluminum metal plate and glass silver coated mirror.
The results approve sufficient surface and coating quality.
Further research is manufacture off-axis parabolic mirrors, validate with a beam profiling and manufacture arbitrary surface mirrors optimized to the current accelerator by machine learning.

Speaker: Mr Johnathan Ciplis (Ariel University)

FEL2022_246_Ciplis.pdf

171 Application of Machine Learning in Longitudinal Phase Space Prediction at the European XFEL

For a free-electron laser facility, the longitudinal phase space of the beam is essential to the FEL lasing performance. However, the commonly-used diagnostics device such as the transverse deflecting cavity provides a destructive way to measure the beam longitudinal properties, which is not available during beam delivery. Thus, the convolutional neural network is introduced to construct a virtual diagnostic tool that facilitates bunch-to-bunch nondestructive measurement of the longitudinal phase space distribution.

Speaker: Zihan Zhu (Shanghai Institute of Applied Physics)

172 Beam Based Alignment of a Seeded FEL

Optimal FEL gain in a seeded FEL requires the careful alignment of different components. As for SASE FELs, the gain is optimized when the electron bunch travels in a straight line along the axis of each undulator in the radiator section. We have recently developed an alignment strategy for the optimization of the FERMI FELs which combines the beam-based alignment of the magnetic elements (undulators and quadrupoles) with the collinear alignment of spontaneous emission from each undulator. The method is divided into 3 steps. In the first step, we measure the undulator spontaneous emission with a spectrometer to fine-tune each undulator gap and set the best electron beam trajectory for collinear emission of each module. In the second step, the alignment of the undulator axis on the electron trajectory previously defined is achieved by looking at the undulator focusing effect. Finally, the seed laser is superposed on the electrons and aligned to maximize the bunching along the defined direction. This procedure can lead to an improvement in the control over the electron beam trajectory and results in a more efficient FEL process characterized by more stable and larger energy per pulse and a cleaner optical mode. A description of the method with the obtained results are reported in this work

Speakers: Enrico Allaria (Elettra-Sincrotrone Trieste S.C.p.A.), Alexander Brynes (Elettra-Sincrotrone Trieste S.C.p.A.), Bruno Diviacco (Elettra-Sincrotrone Trieste S.C.p.A.), Dr Luca Giannessi (Elettra Sincrotrone Trieste and Istituto Nazionale di Fisica Nucleare), Carlo Spezzani (Elettra-Sincrotrone Trieste S.C.p.A.)

173 Beam-Splitting Normalization Schemes for Femtosecond X-Ray Absorption Spectroscopy Using Stochastic Free-Electron Laser Pulses

X-ray absorption spectroscopy (XAS) enables the study of the electronic and geometric structural properties of matter. Such investigations can now be realized with femtosecond temporal resolution owing to the availability of X-ray free-electron lasers (XFELs) [1]. However, most XFELs currently utilize self-amplified spontaneous emission (SASE), which causes strong shot-to-shot fluctuations of their intensity and spectral distribution. Consequently, SASE fluctuations represent a challenge for the precise normalization of the measured absorption signal to the incident photon flux.
Here, we have developed normalization schemes utilizing diffractive optics that overcome the SASE fluctuations. The diffractive optics are used to split the incoming XFEL SASE beam into two or three identical copies (±1 and 0th orders). By placing the (solid or liquid jet) sample in one of the diffracted beams, the absorption and reference signals are recorded simultaneously, thus enabling efficient data normalization on a shot-to-shot basis.
In this contribution, we will present diffractive optics for two normalization schemes at SASE XFELs. First, a three beam geometry based on beam-splitting silicon off-axis zone plate [2] for soft XAS implemented at the Spectroscopy and Coherent Scattering beamline at the European XFEL to study L2,3 edges of transition metals will be presented. Secondly, a two-beam configuration for hard X-ray transient absorption spectroscopy of liquid jets (K-edge, an aqueous solution of [Fe(C2O4)3]3-) will be reported. Here, a beam-splitting transmission diamond grating for focused hard X-rays in combination with bent silicon <220> crystal was experimentally tested at the ALVRA station at the SwissFEL (ΔE/E ≈ 3% bandwidth). The results demonstrate high-quality K-edge transient XAS of [Fe(C2O4)3]3- solution without the need to scan the monochromator [3]. These normalization schemes pave the way for ultrafast L- and K-edge XAS measurements of transition metals at XFELs.

Speaker: Talgat Mamyrbayev (Paul Scherrer Institut)

174 Characterisation of a Diamond Channel Cut Monochromator Designed for High Repetition Rate Operation at the EuXFEL

The European X-ray Free-Electron Laser (EuXFEL) is a unique FEL facility that provides X-ray pulses of high spectral brilliance and high photon flux at MHz repetition rate. However, the high peak power, produced in trains of up 2700 femtosecond pulses at a rate of 10 Hz, induces a periodic temperature increase of the hard X-ray monochromators, thereby reducing their transmitted intensity. To address this limitation, a diamond channel cut monochromator (DCCM) was proposed as an alternative to the currently used silicon monochromators. The heat load effect of typical EuXFEL pulses at 300 K and 100 K was simulated by finite element analysis (FEA) and indicates that the significant reduction of the transmitted intensity occurs after a higher number of pulses when compared to silicon. The DCCM first prototype was manufactured from an HPHT IIa type diamond single block and characterised by rocking curve imaging (RCI). The RCI results demonstrated the high crystalline quality of the DCCM with rocking curve widths of the same order as the width predicted by the dynamical theory and a uniformly reflected intensity over the surface. The performance as a monochromator was demonstrated by measuring the double bounce reflection. The resulting images after two successive reflections showed a diffracted beam of the same size and parallel to the incident beam and confirmed its applicability.

Speakers: Kelin Tasca (European XFEL GmbH), Liubov Samoylova (European XFEL GmbH), Raymond Barrett (European Synchrotron Radiation Facility), Anders Madsen (European XFEL GmbH), Ilia Petrov (European XFEL GmbH), Dr Angel Rodriguez-Fernandez (European XFEL GmbH), Dr Roman Shayduk (European XFEL GmbH), Thu Nhi Tran Thi (European Synchrotron Radiation Facility), Alexey Zozulya (European XFEL GmbH), Maurizio Vannoni (European XFEL GmbH)

175 Conceptual Design of the THz Undulator for the PolFEL Project

PolFEL will be the first free-electron laser facility in Poland. It will be driven with RF continuous-wave superconducting linac including an SRF injector furnished with a lead film superconducting photocathode. PolFEL will provide a wide wavelength range of electromagnetic radiation from 0.6 mm down to 60 nm. The linac will be split into three branches. Two of them will feed undulators chains dedicated for VUV, and IR radiation emission, respectively, and a single THz undulator will be settled in the third branch. The design of the THz undulator has been recently accomplished. It consists of a 1560 mm long permanent magnet’s structure ordered as a Halbach array of 8 periods. Large blocks dimensions, gap flux zeroing at full opening and 0.5 THz – 5 THz wavelengths range imposed on the undulator significantly influenced the final shape of the device, including blocks holders, girders and frame robustness unto magnetic forces, and hindered manufacturing and assembling processes. The following publication presents the challenges and solutions that were accompanying the conceptual phase.

Speaker: Jaroslaw Wiechecki (National Synchrotron Radiation Centre Solaris)

176 Considerations on Wakefield Effects in a VUV FELO Driven by a Superconducting TESLA-Type Linac

The electron-beam properties needed for successful implementation of a free-electron-laser oscillator (FELO) on a superconducting TESLA-type linac at the Fermilab Accelerator Science and Technology (FAST) facility include the intrinsic normalized emittance and the submacropulse centroid stability. We have demonstrated that short-range wakefields (SRWs) and long-range wakefields including higher-order modes (HOMs) are generated for off-axis beams in the two, 9-cell capture cavities and eight, 9-cell cavities of a cryomodule in the FAST linac. The resulting degradation of the emittance and centroid stability would impact the FELO performance. At 300 MeV and with the 4.5-m long, 5-cm period undulator, the saturation of a vacuum ultraviolet (VUV) FELO operating at 120 nm has previously been simulated with GINGER and MEDUSA-OPC using the non-degraded beam parameters. The measured electron-beam dynamics due to the SRWs (submicropulse, 100-micron head-tail kicks) and HOMs (submacropulse centroid slew of up to 100s of microns) will be presented. These are mitigated by steering on axis as guided by the minimization of the HOM signals. Simulations using MINERVA:OPC of the effects of submacropulse centroid slew on FELO performance will also be reported.

Speakers: Dr Alex Lumpkin (Fermi National Accelerator Laboratory), Henry Freund (University of New Mexico), Peter van der Slot (University of Twente)

177 Controlling Beam Trajectory and Beam Transport in a Tapered Helical Undulator

A helical undulator provides a stronger FEL coupling than common planar geometries as the beam’s transverse velocity never vanishes. However, a significant challenge lies in tuning and measuring the fields with limited access to the beam axis along the undulator. Confirming the good field region off axis is difficult without space for 3D hall probe scans, and is important for low energy beams used to create THz radiation. We present our tuning procedures developed for the meter-long THESEUS undulators, consisting of two orthogonal permanent magnet Halbach arrays shifted by a quarter period relative to one another. The hall probe and pulsed wire measurements are guided by the general field expansion of helical undulators to correctly tune fields on and near the axis.

Speakers: Andrew Fisher (Particle Beam Physics Lab (PBPL)), Jason Jin (Particle Beam Physics Lab (PBPL)), Dr Pietro Musumeci (Particle Beam Physics Lab (PBPL))

178 Correlation of Orbit Disturbance in the Photoinjector with SASE Performance at the European XFEL

We present experimental observation for the impact of an introduced orbit disturbance in the photoinjector section on the SASE performance at the European XFEL. An orbit bump is first created and then closed by the orbit feedback downstream, that is, the orbit leaving the injector section stays the same while presumably only causing a disturbance to the bunch. With the same orbit launched into the undulators, first measurement data have shown a correlation between the magnitude of the introduced orbit disturbance in the injector and the SASE intensity in the undulators. Similar behaviors are observed as well for bunch train operation. The results will be shown and the discussions are given.

Speaker: Dr Ye Chen (Deutsches Elektronen-Synchrotron)

179 Demonstration of Hard X-ray Multiplexing using Microbunch Rotation through an Achromatic Bend

Electrons in a X-ray free electron laser (XFEL) develop periodic density fluctuations, known as microbunches, which enable the exponential gain of X-ray power in an XFEL. When an electron beam microbunched at a hard X-ray wavelength is kicked, microbunches are often washed out due to the dispersion and R56 of the bend. An achromatic (dispersion-free) bend with small R56, however, can preserve microbunches, which rotate to follow the new trajectory of the electron bunch. Rotated microbunches can subsequently be lased in a repointed undulator to produce a new beam of off-axis X-rays. In this work, we demonstrate hard X-ray multiplexing in the Linac Coherent Light Source (LCLS) Hard X-ray Undulator Line (HXU) using microbunch rotation through a 10 microrad first-order-achromatic bend created by transversely offsetting quadrupole magnets in the FODO lattice. Quadrupole offsets are determined analytically from beam-matrix theory. We also discuss the application of microbunch rotation to out-coupling a cavity-based XFEL (CBXFEL) [1].

Speakers: Rachel Margraf (SLAC National Accelerator Laboratory), James MacArthur (SLAC National Accelerator Laboratory), Gabriel Marcus (SLAC National Accelerator Laboratory), Heinz-Dieter Nuhn (SLAC National Accelerator Laboratory), Alberto Lutman (SLAC National Accelerator Laboratory), Dr Aliaksei Halavanau (SLAC National Accelerator Laboratory), Zhirong Huang (SLAC National Accelerator Laboratory)

180 Design Considerations for a New Extraction Arc at the European X-Ray Free Electron Laser

It has been proposed that a new arc, called T20, should be installed for a third fan of undulators at the EuXFEL in the next decade. Due to geometric constraints this arc will need to be at a much larger angle than for the existing arc (T1). It is expected therefore that coherent synchrotron radiation effects in T20 on the bunch emittances will be considerable. To preserve the x-ray beam qualities in any downstream undulators, this effect will need to be understood and ideally mitigated. In this paper the status of the T20 beamline design is discussed, the expected downstream beam properties are shown and possible strategies for improving the beam quality are outlined.

Speaker: Stuart Walker (Deutsches Elektronen-Synchrotron)

181 Design Considerations for the Extraction Line of the Proposed Third Beamline Porthos at SwissFEL

It is planned to extend SwissFEL by a third beamline, named Porthos, operating in the hard X-ray regime. Three bunches will be accelerated within one RF pulse and distributed into the different beamlines with resonant kickers operating at the bunch spacing of a few tens of nanoseconds.
While the full extent of Porthos will not be realized before the end of this decade the extraction line from the main linac will also serve the P^3 experiment for the demonstration of a possible positron source for the FCC-ee project at CERN. We present the design of the switchyard, which will serve both purposes with only minimal changes.

Speakers: Sven Reiche (Paul Scherrer Institut), Paolo Craievich (Paul Scherrer Institut), Thomas Schietinger (Paul Scherrer Institut), Mattia Schaer (Paul Scherrer Institut)

182 Design of a New Beamline for the ORGAD Hybrid RF-Gun at Ariel University

The ORGAD Hybrid RF-gun which was commissioned in Ariel University is based on a smaller-scale prototype built at UCLA's Particle Beam Physics Laboratory (PBPL) as an on-going collaboration between the Universities. The main beamline of the hybrid S-band (2856 [MHz]) photo injector is currently driving a 150[kW], short pulse THz-FEL. In order to use the RF gun for other applications, a new and independent beam line is required. The secondary beamline is only feasible with the design of a dispersive beam-line dogleg section. High quality of the secondary beam is crucial for the designated applications such as Ultra-fast Electron Diffraction (UED). We present full 3D GPT (General Particle Tracer) simulations of this secondary beamline in which we manipulate the beam, compress the beam and maintain beam emittance and pulse duration. An optimization procedure of the design was performed to reconstruct the electron beam quality parameters after passing through the dispersive dogleg section. The optimization procedure is based on transfer matrices and simulations using realistic field-maps and fringe fields of the quadrupoles which were designed in-house, and their 3D field maps were exported using CST (Computer Simulation Technology. We present the optimization results with the improved beam quality.

Speaker: Amir Weinberg (Ariel University)

183 Design of the Innovative Apple-X AX-55 for SABINA Project, INFN Laboratori Nazionali di Frascati

Kyma S.p.A. was awarded the design and production of the APPLE-X undulator for SABINA project at INFN - Laboratori Nazionali di Frascati. SABINA (Source of Advanced Beam Imaging for Novel Applications) is a project aimed at the enhancement of the SPARC_LAB research facility. The two user lines that are going to be implemented are; a power laser target area and a THz radiation line.

Here we present the magnetic design and a novel mechanical implementation of this APPLE-X undulator for the THz/MIR radiation line. Undulator is made from three 1.35 m long sections. Each section consist of an APPLE-X magnetic array with 55 mm undulator period, a minimum gap of 10 mm and a mechanical frame. The undulator design is both compact and lightweight. This is achieved by novel mechanical design and implementation of the multiple dynamic corrections through the motion control system.

WEP47_Poster.pdf

184 Design of the Superconducting undulator for EuPRAXIA@SPARC_LAB

EuPRAXIA@SPARC_LAB is a new Free Electron Laser (FEL) facility that is currently under construction at the Laboratori Nazionali di Frascati of the INFN. Fermilab is contributing to the project with the design, manufacturing and qualification of a prototype conduction cooled superconducting undulator (SCU) that, if successful, could be integrated in the final machine.
The design of the SCU capitalizes on the extensive experience present at Fermilab on cryomodules. Specifically, the system is based on the warm strongback concept developed for the PIP-II project which enables a modular design with multiple undulator coils integrated in a single vacuum vessel. This publication focuses on the overall design concept of the magnet system, its modularity, cost reduction potential and industrialization strategy.

Speaker: Federico Nguyen (ENEA-Frascati)

185 Determination of a High-Power Short THz Single Pulse Detector for FEL

Terahertz (THz) radiation may pass through dielectric materials, and this ability can be used for a variety of applications. Terahertz (THz) radiation is located between infrared and microwave radiations in the electromagnetic spectrum. FEL produces brief, high-power THz single pulses, and we provide a diagnostic approach for them. The electro optic efficacy is used as a detection method. For the THz pulse, a GaAs crystal is used as a detector. The THz pulse causes the electro-optical crystal to shift polarization, implying that the electro-optic sampling device detects 30psec pulses (or depending on the pulse length of the accelerator). The optical pulse from the electo-optic sampling is coupled to fiber, allowing the optical pulse to be stretched to the order of nanoseconds.

Speaker: Adnan Haj Yahya (Ariel University Center of Samaria)

186 Development of a Photoelectron Spectrometer for Hard X-Ray Photon Diagnostics at the European XFEL

Development and characterization of an angle-resolved photo-electron spectrometer, based on the electron Time-of-Flight concept, designed for hard X-ray photon diagnostics at the European free-electron laser is described. The objective with the instrument is to provide beamline users and operators with pulse resolved, non-invasive spectral distribution diagnostics, which in the hard X-ray regime is a challenge due to the poor cross-section and often very high kinetic energy of photo electrons for the available target gases. In this contribution we describe development of the device, including electron trajectory simulations, and first tests with hard X-rays at the PETRA III synchrotron where we have characterized the performance and optimized the voltage settings for resolution and electron detection efficiency. We demonstrate a resolving power of better than 5 eV up to at least 20 keV photon energy.

Speakers: Florian Dietrich (European XFEL GmbH), Jan Grünert (European XFEL GmbH), Jia Liu (European XFEL GmbH), Joakim Laksman (European XFEL GmbH), Marc Planas (European XFEL GmbH), Naresh Kujala (European XFEL GmbH), Randeer Pratap Gautam (European XFEL GmbH), Dr Sonia Francoual (European XFEL GmbH), Theophilos Maltezopoulos (European XFEL GmbH), Wolfgang Freund (European XFEL GmbH)

187 Development of Diamond-Based Pass-Through Diagnostics for Next-Generation XFELs

FELs deliver rapid pulses on the femtosecond scale, and high peak intensities that fluctuate strongly on a pulse-to-pulse basis. The fast drift velocity and high radiation tolerance properties of chemical vapor deposition (CVD) diamonds make these crystals a good candidate material for developing a high frame rate pass-through diagnostic for the next generation of XFELs. We report on two diamond based diagnostic systems being developed by a collaboration of a UC campuses and National Laboratories supported by the University of California and the SLAC National Laboratory.

For the first of these diagnostic systems, we have developed a new approach to the readout of diamond diagnostic sensors designed to facilitate operation as a passthrough detection system for high frame-rate XFEL diagnostics. Making use of the X-ray Pump Probe (XPP) beam at the Linac Coherent Light Source (LCLS), the performance of this new diamond sensor system has been characterized and compared to that of a commercially available system. Limits in the magnitude and speed of signal charge collection are explored as a function of the generated electron-hole plasma density and compared to results from a TCAD simulation.

A leading proposal for improving the efficiency of producing longitudinally coherent FEL pulses is the cavity-based X-ray free electron laser (CBFEL). In this configuration, the FEL pulses are recirculated within an X-ray cavity in such a way that the fresh electron bunches interact with the FEL pulses stored in the cavity over multiple passes. This creates a need for diagnostics that can measure the intensity and centroid of the X-ray beam on every pass around the recirculatory path. For the second of these diagnostic systems, we have created a four-channel, position-sensitive pass-through diagnostic system that can measure the intensity and centroid of the circulating beam with a repetition rate in excess of 20 MHz. The diagnostic makes use of a planar diamond sensor thinned to 43 µm to allow for minimal absorption and wave-front distortion of the circulating beam. We present results on the response and position sensitivity of the diagnostic, again measured using the LCLS XPP beam.

Speakers: Mr Rene Padilla (Santa Cruz Institute for Particle Physics), Ms Isleydys Silva Torrecilla (SLAC National Accelerator Laboratory), Dr Diling Zhu (SLAC National Accelerator Laboratory), Eric Gonzalez (Santa Cruz Institute for Particle Physics), Serguei Kachiguine (Santa Cruz Institute for Particle Physics), Forest Martinez-Mckinney (Santa Cruz Institute for Particle Physics), Simone Mazza (Santa Cruz Institute for Particle Physics), Mohammad Nizam (Santa Cruz Institute for Particle Physics), Nora Norvell (University of California, Santa Cruz), Emma Potter (Santa Cruz Institute for Particle Physics), Eric Ryan (Santa Cruz Institute for Particle Physics), Bruce Schumm (Santa Cruz Institute for Particle Physics), Michal Tarka (Santa Cruz Institute for Particle Physics), Max Wilder (Santa Cruz Institute for Particle Physics), Bryce Jacobson (SLAC National Accelerator Laboratory), James MacArthur (SLAC National Accelerator Laboratory), Dr Jen Bohon (Los Alamos National Laboratory), Dongsung Kim (Los Alamos National Laboratory), Dr John Smedley (Los Alamos National Laboratory), Carl Grace (Lawrence Berkeley National Laboratory), Tarun Prakash (Lawrence Berkeley National Laboratory), Charles Harris (Sandia National Laboratories), David Stuart (University of California at Santa Barbara), Eric Prebys (University of California at Davis)

188 Development of the RF Systems for the PolFEL Accelerator

PolFEL stands for Polish Free Electron Laser, the first FEL research infrastructure in Poland. This facility is under development, and it will operate in three wavelength ranges: IR, THz and VUV, using different types of undulators. Machine will be driven by 200 MeV linear superconducting accelerator, which will operate in both, pulsed wave (PW) and continuous wave (CW) modes. This contribution will describe the concept, current status and the first results of the RF systems development.

Speaker: Jaroslaw Szewinski (National Centre for Nuclear Research)

189 Experimental Slice Emittance Reduction at PITZ Using Laser Pulse Shaping

At the Photo Injector Test facility at DESY in Zeuthen (PITZ) photo electron guns for the use at the X-ray free-electron laser (FEL) facilities FLASH and European XFEL are conditioned. An electron beam with high current and low transverse emittance is required for high performance in an X-ray FEL.
As the lasing process occurs on the part of the electron bunch with the highest charge density the emittance of this part is of interest. A scheme to measure the slice emittance which uses a transversely deflecting structure and a single-slit scan has been developed at PITZ. This allows the beam characterisation at low beam energies and high charge densities.
The contribution shows that using laser pulses with temporal flattop shape (and temporal Gaussian shape) or temporal and transverse flattop shape lead to a reduced center slice emittance compared to an electron beam emitted using a laser pulse with temporal Gaussian and transverse flattop shape.

Speaker: Zakaria Aboulbanine (Deutsches Elektronen-Synchrotron DESY at Zeuthen)

190 Fabrication of X-Ray Gratings by Grey-Tone Electron-Beam Lithography and Thermal Oxidation of Silicon

Diffraction gratings are an essential instrument used at free-electron laser facilities in soft and tender x-ray ranges. Their application ranges from monochromators and analyzers to self-seeding and pulse compression. These gratings are typically around 50-200 mm, up to 500mm in length with pitches from a few micrometers down to a few 100 nm, made on flat or curved substrates. Blazed gratings exhibiting higher efficiency are made by the ruling technique, however, the production of high-quality blazed gratings has become a significant bottleneck due to challenges in their fabrication and few suppliers.
In this presentation, we report on a novel method for production of next-generation X-ray diffraction gratings based on gray-tone electron-beam lithography (EBL) and thermal oxidation of silicon. We can take advantage of the greatly enhanced flexibility regarding the grating design, allowing for enhanced optical performance as well as novel optical functionalities.
This new technology will enable researchers all around the world to exploit fully the unique opportunities provided by the dramatically enhanced brilliance and coherence of a new generation of light sources.

Speaker: Nazanin Samadi (Paul Scherrer Institut)

191 Feasibility of Single-Shot Microbunching Diagnostics for a Pre-bunched Beam for TESSA at 515 nm

Co-propagating a relativistic electron beam and a high-power laser pulse through a short undulator (modulator) provides an energy modulation which can be converted to a periodic longitudinal density modulation (or microbunching) via the R56 term of a chicane. Such pre-bunching of a beam at the resonant wavelength and the harmonics of a subsequent free-electron laser (FEL) amplifier seeds the process and results in improved gain in a TESSA** experiment. We describe potential characterizations of the resulting microbunched electron beams after the modulator using coherent optical transition radiation (COTR) imaging techniques for transverse size (50 micron), divergence (sub-mrad), trajectory angle (0.1 mrad), coherence factor, spectrum (few nm), and pulse length (ps). The transverse spatial alignment is provided with near-field imaging and the angular alignment is done with far-field imaging and two-foil COTR interferometry (COTRI). Analytical model results for a 515-nm wavelength COTRI case with a 10% microbunching fraction will be presented. COTR gains of 22 million were calculated for an initial charge of 1000 pC which enables splitting the optical signal for single-shot measurements of all the cited parameters.

*Tapering Enhanced Super-radiant Stimulated Amplification (TESSA)
Work supported by U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357.

Speakers: Alex Lumpkin (Argonne National Laboratory), Donald Rule (Naval Surface Warfare Center), Pietro Musumeci (University of California, Los Angeles)

192 Intrabeam Scattering Effects in the Electron Injector of the European XFEL

Intrabeam scattering (IBS) causes growth of the uncorrelated energy spread of electron bunches due to multiple small-angle Coulomb collisions over long propagation distances. As such, this effect may be a limiting factor for the beam current and therefore for the achievable photon energy in the SASE process. In addition, IBS influences the noise spectrum of the bunch, thus, interfering with microbunching instability (MBI) effects. An accurate estimation of IBS is, therefore, necessary for the proper application of so-called laser heaters for MBI suppression.
Recent experimental evidence at the FERMI linac, SwissFEL and European XFEL suggests that IBS effects in FELs are important. A large uncorrelated energy spread of the electron beam was observed that could otherwise not be reproduced in numerical simulations. So far, however, this energy spread growth could not be clearly attributed to IBS alone. This is due primarily to the nonlinear, space-charge dominated beam dynamics in the injector for which theoretical IBS models are not applicable. For this reason, we introduce a simulation approach for the full space-charge dynamics in the injector of the European XFEL including IBS effects. The approach is based on a Monte-Carlo technique for modeling Coulomb collisions within an electron bunch of arbitrary distribution. The results for the slice energy spread along the beam line are presented for various operation conditions. This allows to identify exactly the amount by which IBS contributes to the overall uncorrelated energy spread growth in the injector.

Speaker: Erion Gjonaj (Technische Universität Darmstadt)

193 Investigation of High Absorbed Doses in the Intersections of the European XFEL Undulator Systems

The European X-Ray Free Electron Laser (XFEL) operates three Undulator Systems to generate high-brilliance and high repetition X-ray pulses. Each System consists of multiple 5-m long undulator segments separated by 1.1-m long intersections. Such intersections contain vacuum systems, diagnostic and correction equipment for the electron's trajectory, and phase shifters (PS) [1] to match the phase of the electron beam and SASE photons. An array of Radfets monitors the absorbed doses at the entrance of each undulator segment since the start of operation in 2017 [2] but no dosimetry is available in the intersections. Recently, some SASE3 phase shifters stopped working and their motors had to be exchanged. This may have been caused by radiation damage. In this work we used Gafchromic films to measure radiation doses and its spatial profile in the intersections and PS vicinity. The measurements showed that significantly higher radiation doses are absorbed in the intersections as compared to the entrance of the next downstream undulator segment and significant radiation is also found near mechanical motors and electronic circuitry. We performed Monte Carlo simulations using the Geant4 code to investigate the composition of the radiation field in the intersections of the Undulator Systems and correlate it with the Gafchromic measurements and possible radiation damage to PS encoders and motors.

Speakers: Olga Falowska-Pietrzak (Stockholm University), Anders Hedqvist (Stockholm University), Fredrik Hellberg (Stockholm University), Guillermo Lopez Basurco (Universidad Autonoma de Madrid), Frederik Wolff-Fabris (European XFEL GmbH)

194 Investigation of the Beam Losses and Radiation Loads for the Implementation of a Slotted Foil at the European XFEL

Ultra-short X-ray pulses in an XFEL can be generated by means of a slotted foil inserted into a bunch compressor. There is an ongoing study into whether such a technique could be used at the European XFEL. One important factor that must be considered is whether the additional beam losses and radiation load caused by the foil is acceptable with a high repetition rate of up to 4.5MHz at the European XFEL. As there is currently no foil implemented in the European XFEL, experimental investigations were carried out by inserting a screen in the bunch compressor at the location where a foil would be inserted. Simulations have been performed using BDSIM to study losses caused by the insertion of the foil. Neutron radiation measurements and beam loss monitor readings were taken and compared with the simulations to provide validation and calibration of the simulations for the case of a slotted foil.

Speaker: Andrew Potter (The University of Liverpool)

195 Laser Plasma Accelerator Based Seeded FEL Commissioning on COXINEL at HZDR

The tremendous developments on Laser Plasma Accelerators (LPAs) have significantly improved the electron beam properties and stability making it possible to drive a Free Electron Laser (FEL). We report on the electron beam transport and manipulation using the COXINEL beamline implemented at HZDR that has recently led to the first measurements of an LPA-based seeded FEL in the UV region. Our experiment, cross-checked with ELEGANT simulations, shows that the beamline enables the handling of the large divergence via high gradient quadrupoles, reducing the slice energy spread with the help of a chicane, controlling the position and dispersion in both transverse planes using beam pointing alignment compensation and implementing the super matching optics. We also show that the beamline properly allows for the spectral tuning and spatial overlapping between the electron beam and the seed, using electron and photon beam diagnostics, and thus making LPA based FEL amplification within reach.

Speakers: Dr Amin Ghaith (Helmholtz-Zentrum Dresden-Rossendorf), Jurjen Couperus Cabadag (Helmholtz-Zentrum Dresden-Rossendorf), Prof. Ulrich Shramm (Helmholtz-Zentrum Dresden-Rossendorf), Arie Irman (Helmholtz-Zentrum Dresden-Rossendorf), Marie Labat (Synchrotron Soleil), Alexandre Loulergue (Synchrotron Soleil), Marie-Emmanuelle Couprie (Synchrotron Soleil), Eléonore Roussel (Laboratoire de Physique des Lasers, Atomes et Molécules)

196 Machine Learning Developments for CLARA

CLARA is an electron beam test facility being developed in phases at STFC Daresbury Laboratory. The first phase, with up to 35 MeV electron beam energy, has been operated since 2018 for a wide range of accelerator applications. The second phase, presently being installed, will expand the range of applications by taking the beam to 250 MeV energy and via a dog-leg to an experimental station that will feature a new high-power laser. A third phase utilising the 250 MeV beam in the straight-ahead line is also envisaged. Machine learning will play an important role in the future development of the facility, with aims to rapidly deliver bespoke beam properties, to detect and diagnose anomalies, and to provide virtual diagnostics. This paper summarises machine learning developments to date, in the areas of RF breakdown detection, photo-injector laser pulse shaping, and longitudinal phase space shaping. Studies to date have largely been offline or using simulated data but steps towards deployment are also reported.

Speakers: David Dunning (Science and Technology Facilities Council), Amelia Pollard (Science and Technology Facilities Council), Minerva Maheshwari (Science and Technology Facilities Council), Anthony Gilfellon (Science and Technology Facilities Council), William Okell (Science and Technology Facilities Council), Edward Snedden (Science and Technology Facilities Council)

197 Magnetic Field Investigation in a Compact Superconducting Undulator with HTS Tape

The superconducting undulator (SCU) based on the second-generation high-temperature superconducting (HTS) tapes is a promising application for building tabletop free-electron lasers (FELs). The short period < 10 mm undulators with a narrow magnetic gap < 4 mm are especially relevant. The advantage of the HTS tape is that it shows both high critical current density and high critical magnetic field. Each tape has 50 µm thickness and 12 mm width and is further scribed by a laser to achieve a meander structure, hence, providing the desired magnetic field pattern.

Thus, a new approach to a superconducting undulator has been presented in the past and is further developed at KIT: each coil is wound with a single 15 m structured HTS tape. As a result, 30 layers of scribed sections lay above each other, and therefore, provide the required magnetic field. The results of the magnetic field measurements together with the results of the numerical investigation will be presented and discussed.

Speakers: Daria Astapovych (Karlsruhe Institute of Technology), Nicole Glamann (Karlsruhe Institute of Technology), Andreas Grau (Karlsruhe Institute of Technology), Bennet Krasch (Karlsruhe Institute of Technology), David Saez de Jauregui (Karlsruhe Institute of Technology)

198 Measurement of Orbit Coupling by the Apple-X Undulator Modules in the Soft X-ray Beamline Athos at SwissFEL

Orbit response measurements in the soft X-ray beamline of Athos have shown coupling of the beam transport between the transverse planes, which is influenced by the on-axis field strength of the Apple-X undulator modules. A model reproduces this observation if a coupling term is included in the transport matrix of the undulator module. The presentation shows the estimate of the coupling strength as a function of beam energy, undulator field strength and orbit excitation.

Speakers: Sven Reiche (Paul Scherrer Institut), Marco Calvi (Paul Scherrer Institut), Romain Ganter (Paul Scherrer Institut), Eugenio Ferrari (Deutsches Elektronen-Synchrotron)

199 Microwave Cavity Resonance Spectroscopy of Ultracold Plasmas

Ultracold plasmas (UCPs) form a new exotic category of plasmas that can be produced by photo-ionizing laser-cooled atoms in a magneto-optical trap (MOT) near-threshold. With densities up to $10^{18}$ m$^{-3}$, temperatures as low as $\sim$100 $\mu$K for the ions, and $\sim$1 K for the electrons, they are the ideal model plasmas to study fundamental processes in plasma physics, such as (the competition between) three-body recombination, disorder-induced heating, and collisional and collisionless microwave heating.

To study these plasmas, conventional diagnostics such as Langmuir probes are not suitable, and tools from the field of atomic and particle physics are employed instead: charged particle diagnostics for electrons and ions, and laser-induced fluorescence and absorption imaging for ions. However, these diagnostics are limited by the charged particle’s time-of-flight to the detector or require optical transitions available to lasers and cameras, such as present in alkaline earth metals, to work.

At TU/e, we recently developed a novel diagnostic that combines some of the advantages provided by the previous diagnostics. The diagnostic is based on a 5 GHz resonant microwave cavity and uses the shift in the resonance frequency of the cavity, induced by the UCP, to determine the electron dynamics of the plasma. This diagnostic allows us to study the dynamics simultaneously non-destructively, very fast (ns temporal resolution), with high sensitivity, and is a potentially interesting device for other types of plasmas as well, such as plasmas induced by extreme ultraviolet irradiation.

Speaker: Dr Mark Van Ninhuijs (Technische Universiteit Eindhoven)

200 Millimeter-Wave Undulators for Compact X-Ray Free-Electron Lasers

Electromagnetic wave undulators have the advantage of a shorter period compared with the permanent magnet undulators when operating at high frequency, therefore producing FEL radiation at the same wavelength with less electron energy. This paper investigates the properties of a Ka-band microwave undulator, and the factors that affect the choice of the high-power drive sources, through the design and beam dynamic study of a 36GHz cavity-type microwave undulator proposed for the CompactLight X-ray FEL. The future research is to prototype a millimeter-wave undulator operating at ~100GHz, which will have an undulator period of about 1/10 of the state-of-the-art permanent magnet undulators. The millimeter-wave undulator will allow the generation of soft X-ray radiation at much lower beam energy, such as hundreds of MeV, enabling a reduction in the cost of a compact XFEL facility.

Speakers: Dr Liang Zhang (University of Strathclyde and Cockcroft Institute), Jack Easton (Department of Physics, SUPA, University of Strathclyde), Dr Craig Donaldson (University of Strathclyde and Cockcroft Institute), Colin Whyte (University of Strathclyde), Adrian Cross (University of Strathclyde and Cockcroft Institute), Jim Clarke (Science and Technology Facilities Council and Cockcroft Institute)

201 Optimization and Fine Tuning of Machine Parameters with Model-Less Algorithm

Despite the use in machine physics of high-performance software for calculating and predicting machine parameters, when these are applied to the real world, optimal operating point search is often necessary to obtain the desired performance.
Furthermore, small configuration changes required by FEL Users during running experiments ,lead to search new good working points in a short time.
Use of tools based on model-less algorithms such as Nelder-Mead and 1D or 2D scans allow the automatic and online search for the best fine setup of the parameters in short times.
The development of MIMOFB (Multi Input Multi Output Feedback) software used as optimizer with model-less algorithms has provided a versatile tool that can be applied in many situations.
The ability to concatenate optimizations with pre-programmed batch executions allows to develop complex optimization strategies and iterate them by refining algorithm’s parameters.
In FERMI MIMOs optimizers are currently used with good results for fine tuning the electron beam magnetic optic and trajectory by acting on quadrupoles and correctors magnets current for FEL signal optimization and terahertz parasitic signal maximization to TeraFERMI line.

Speakers: Mr Francesco Tripaldi (Elettra-Sincrotrone Trieste S.C.p.A.), Giulio Gaio (Elettra-Sincrotrone Trieste S.C.p.A.), Fabio Galassi (Elettra-Sincrotrone Trieste S.C.p.A.)

202 Optimization in the Structure of Klystron Drive Signal to Extend RF Pulse Flattop Length at the European XFEL

Currently 26 RF stations are in operation at the European X-ray Free Electron Laser (XFEL) and all RF stations can deliver sufficient power to reach maximum gradients in the accelerating modules, limited only by cavity and coupler properties. It was demonstrated that by activating a dynamic frequency shift (DFS) of the RF drive signal, the requested klystron power can be reduced by up to 20%, keeping the gradient levels unchanged. Currently the high voltage (HV) pulse has a length of 1.7ms and the RF pulse a length of 1.42ms, out of which only 0.6ms can be used for beam acceleration. Currently, the RF pulse starts when the level of klystron HV reaches 99% of the nominal voltage. If one allows the RF pulse to start at the 80% level of the nominal voltage, then the RF pulse length can be increased. In this article we will present a proposal for increasing the XFEL RF pulse flattop length using phase and amplitude compensation during the rise and fall of the HV, as well as applying DFS when filling the cavities of the accelerator. The first demonstration of the proposed procedure with the 10MW multi-beam klystrons (MBK) at the klystron test stand and at the XFEL RF station A10.L3 will be presented as well. The described procedure can be used both to increase the duration of the RF flat top as well as to shorten the duration of the HV, which could lead to energy savings.

Speakers: Dr Vladimir Vogel (Deutsches Elektronen-Synchrotron DESY at Zeuthen), Valeri Ayvazyan (Deutsches Elektronen-Synchrotron), Julien Branlard (Deutsches Elektronen-Synchrotron), Lukasz Butkowski (Deutsches Elektronen-Synchrotron), Stefan Choroba (Deutsches Elektronen-Synchrotron), Jens Hartung (Deutsches Elektronen-Synchrotron), Mr Nicholas Walker (Deutsches Elektronen-Synchrotron), Steffen Wiesenberg (Deutsches Elektronen-Synchrotron), Michael Bousonville (Deutsches Elektronen-Synchrotron DESY at Zeuthen), Andrey Cherepenko (Deutsches Elektronen-Synchrotron DESY at Zeuthen), Sebastian Goeller (Deutsches Elektronen-Synchrotron DESY at Zeuthen)

203 Orbit Jitter Analysis at SwissFEL

With the beam synchronous readout of the beam position measurement at the hard X-ray FEL beamline Aramis at SwissFEL we analyze the intrinsic orbit jitter, using a classification algorithm and Principal Component Analysis (PCA). The method sorts the jitter in a set of eigenvectors and -values. With the magnitude of the eigenvalues the impact of the different jitter sources can be estimated. From the purely stochastic results we derive also a physical interpretation by matching the linear transport functions to the eigenvectors, reconstructing the orbit jitter in terms of the center of mass jitter of the electron bunch in the transverse positions, momenta, and the mean energy. Any deviation from the theoretical prediction indicates possible wrong set values of the transport magnets or errors in the BPM calibration (sign flip or faulty amplitude calibration). We present the results and give an outlook on extending the analysis to additional channels such as charge, compression and arrival time monitors as well as the FEL output signal.

Speaker: Sven Reiche (Paul Scherrer Institut)

204 Protection of the European XFEL Undulators from the Additional Beam Losses Caused by the Insertion of a Slotted Foil

The undulators in the European XFEL are made of permanent magnets that need to be protected from beam losses that could cause demagnetisation. Under current operating conditions, beam losses in the undulators are prevented by a collimation section downstream of the main Linac and upstream of the switchyard. In the future, a slotted foil may be installed in the European XFEL to reduce the X-ray pulse length; however, the insertion of the foil will spoil the emittance of most of the bunch which increases the probability of particles scraping the collimator and continuing to be transported to the undulator section. In this paper, we report a study to assess the level of the beam losses in the undulators in the European XFEL that would be caused by a slotted foil, and to determine the optimal apertures to use in the collimators to minimise the losses. We also assess whether shielding or an additional collimator in front of the undulator could be added to the beamline to prevent the losses.

Speaker: Andrew Potter (The University of Liverpool)

205 RF Commissioning and First Beam Operation of the PolariX Transverse Deflecting Structures in the FLASH2 Beamline

In January 2021 two X-band (12 GHz) PolariX Transverse Deflecting Structures with variable streak polarization were installed into the FLASH2 beamline at FLASH. Since none of the RF components for the FLASH2-PolariX RF-distribution system nor the two PolariX structures could be pre-conditioned, RF-conditioning was and is quite tedious. Nevertheless, after 6 weeks of conditioning, we have already been able to streak the electron beam enough to start commissioning of the PolariX controls and the software.
After 4 months of conditioning in parallel to FLASH2 user operation, we achieved a stable 5.5 MW flat top of 400 ns operation. Next step will be to include RF pulse compression to achieve the design power of 22 MW. Since then operational experience with the PolariX system has continuously evolved and it has quickly become a valuable if not indispensable tool for tuning FLASH2. Even with the reduced power, a measurement resolution of 12 fs could be reached.
In this article we describe key aspects of the conditioning and commissioning process as well as the first experiences and first results of beam measurements both for SASE tuning and for dedicated micro-bunching studies.

Speakers: Mathias Vogt (Deutsches Elektronen-Synchrotron), Juliane Roensch-Schulenburg (Deutsches Elektronen-Synchrotron), Siegfried Schreiber (Deutsches Elektronen-Synchrotron), Florian Christie (Deutsches Elektronen-Synchrotron)

206 RF Conditioning and First Experiences with the PolariX TDS at PSI

In 2017, a collaboration between DESY, PSI and CERN was established with the aim of developing and building seven advanced X-Band Transverse Deflection Structure (TDS) with the new feature of variable polarization of the deflecting force. Seven deflectors were produced by PSI of which five were installed in three experiments at DESY, while the remaining two were installed in the ATHOS soft X-ray beamline in SwissFEL, with the goal to provide sub-fs resolution for soft X-ray pulse profiles. Early this year the X-band power source of the TDS for SwissFEL was completed and system commissioning started. This contribution summarizes the first deflection experiments performed.

Speakers: Paolo Craievich (Paul Scherrer Institut), Juergen Alex (Paul Scherrer Institut), Hans-Heinrich Braun (Paul Scherrer Institut), Romain Ganter (Paul Scherrer Institut), Zheqiao Geng (Paul Scherrer Institut), Roger Kalt (Paul Scherrer Institut), Thomas Klebb (Paul Scherrer Institut), Thomas Lucas (Paul Scherrer Institut), Fabio Marcellini (Paul Scherrer Institut), Ralf Menzel (Paul Scherrer Institut), Marco Pedrozzi (Paul Scherrer Institut), Eduard Prat (Paul Scherrer Institut), Sven Reiche (Paul Scherrer Institut), Wolfgang Tron (Paul Scherrer Institut), Riccardo Zennaro (Paul Scherrer Institut), Michael D'Amico (Paul Scherrer Institut), Reto Fortunati (Paul Scherrer Institut), Carl Beard (Paul Scherrer Institut)

207 Short Period Apple-X Undulator Modeling for the AQUA Line of the Future EuPRAXIA@SPARC_LAB Facility

We present the study for a short period Apple-X variable polarizing undulator, with small gap of operation and high magnetic field, which will be the base module for the AQUA line of the EuPRAXIA@SPARC_LAB FEL facility, of next realization at INFN Laboratory of Frascati. The undulator allows to achieve radiation between 3 and 5 nm, the so called water-window, with a 1 GeV electron beam energy, lower than other FELs operating in the world, so giving the possibility to have a Soft X-ray source with a full polarization control in a more cost effective way and with less required space than the state of the art devices. An overview of the magnetic design is given with the main parameters and performances in terms of the field properties, tuning capabilities and the effects on the electron beam motion.

Speakers: Alberto Petralia (ENEA Fusion and Technology for Nuclear Safety and Security Department), Mariano Carpanese (ENEA Fusion and Technology for Nuclear Safety and Security Department), Mario Del Franco (INFN Laboratori Nazionali di Frascati), Andrea Doria (ENEA Fusion and Technology for Nuclear Safety and Security Department), Federico Nguyen (ENEA Fusion and Technology for Nuclear Safety and Security Department), Luca Giannessi (Elettra Sincrotrone Trieste and Istituto Nazionale di Fisica Nucleare), Andrea Selce (ENEA Fusion and Technology for Nuclear Safety and Security Department)

208 Simulation Study of a Dielectric Beam Energy Dechirper for the Proposed NSRRC EUV FEL Facility

In this report, we present a simulation study of dielectric beam energy dechirper designed for the proposed NSRRC EUV FEL facility. As revealed from ELEGANT simulation of the high brightness driver linac system, a residual energy chirp of about 42 keV/m is left after bunch compression. It can be corrected by a capacitive dechirper structure when the bunch is slightly over-compressed. We successfully used a 1-m long corrugated pipe to remove the residual energy chirp in such simulation. However, in order to save space and for a simplified mechanical design, we consider also the usage of two orthogonally oriented planar dielectric-lined waveguide (DLW) structures to remove residual energy chirp after bunch compression. Wake fields due to this DLW dechirper has been calculated by CST code and the deduced wake function will be used in particle tracking using ELEGANT.

Speakers: Mr Chih-Kai Liu (Department of Physics, National Central University), Prof. Shih-Hung Chen (Department of Physics, National Central University), Dr Wai-Keung Lau (National Synchrotron Radiation Research Center), Dr Wei-Yuan Chiang (National Synchrotron Radiation Research Center), Mr Shan You Teng (Department of Physics, National Central University)

209 Single-Shot Temporal Characterization of XUV FEL Pulses

The free-electron laser in Hamburg (FLASH) operates in the extreme ultraviolet (XUV) and soft X-ray region, providing photon pulses of few femtosecond (fs) duration and unprecedented intensity [1]. FLASH operates in the self-amplified spontaneous emission (SASE) regime, meaning that every pulse has a unique combination of energy, spectrum, arrival time and pulse duration. Therefore, it is critical to be able to determine these parameters for each individual pulse. The THz field-driven streaking technique has the potential to deliver single-shot pulse duration information, as well as the XUV arrival time, basically wavelength-independent and over a large dynamic range (in pulse duration and FEL energy) [2, 3].

We present the results of several campaigns measuring the single-shot pulse duration over a wide range from 10 fs to 350 fs (FWHM) [3]. Here we focus on the particular difficulties in the different pulse duration regimes. Furthermore, correlations between the pulse duration and other radiation parameters as pulse energy and spectrum are compared on a single-shot and average level as well as being compared to simulations [4]. The variable gap undulators at FLASH2 also allow to study the evolution of the XUV pulse duration for the fundamental as well as for the 3rd harmonic radiation pulse as function of contributing undulators. The best agreement between measurement and simulation was found when modeling the SASE process using an energy chirped electron pulse.
Finally, a comparison of the pulse duration determined by THz streaking with an alternative pulse duration diagnostic, a transverse deflecting structure (TDS) measuring the modulation of the electron bunch (analog to [5]) is shown and the advantages, as well as limitations of both techniques, are discussed.

[1] W. Ackermann et al., Nat. Photonics 1, 336 (2007)
[2] Grguras et al., Nature Photonics 6, 852-857 (2012)
[3] R. Ivanov et al., J. Phys. B 53, 184004 (2020)
[4] I. Bermudez et al., Opt. Express 29, 10491 (2021)
[5] C. Behrens et al., Nat. Commun. 5, 3762 (2014)

Speaker: Stefan Düsterer (Deutsches Elektronen-Synchrotron)

210 Study of an ERL-Based X-Ray FEL

We propose to develop an energy-recovery-linac (ERL)-based X-ray free-electron laser (FEL). Taking advantage of the demonstrated high-efficiency energy recovery of the beam power in the ERL, the proposed concept offers the following benefits: i) recirculating the electron beam through high-gradient SRF cavities shortens the linac, ii) energy recovery in the SRF linac saves the klystron power and reduces the beam dump power, iii) the high average beam power produces a high average photon brightness. In addition, such a concept has the capability of optimized high-brightness CW X-ray FEL performance at different energies with simultaneous multipole sources. In this paper, we will present the preliminary results on the study of optics design and beam dynamics.

Speakers: Fanglei Lin (Oak Ridge National Laboratory), Jiquan Guo (Thomas Jefferson National Accelerator Facility), Vasiliy Morozov (Oak Ridge National Laboratory), Yuhong Zhang (Thomas Jefferson National Accelerator Facility)

211 The Evolution of KAOS, a Multipurpose Active Optics System for EUV/Soft X-Rays

KAOS is the flagship optics of FERMI, the first - and presently only - fully seeded Free Electron Laser facility in the world. The name stands for Kirkpatrick-Baez Active Optical System, and it has been entirely developed in-house. After progressive revisions and upgrades, it presently empowers three out of six beamlines at FERMI, and it also serves two beamlines at FLASH, Hamburg (DiProI, LDM, MagneDyn; FL23 and FL24). Although KAOS grounds on the well-established concept of Kirkpatrick-Baez mirrors, the challenges it addressed and the needs it was built for, ultimately produced a unique system with unique features: a versatile curvature control, a broad spectral range (100 nm<λ<1 nm), and a large demagnification power (>80×). These features made KAOS an essential and mandatory tool to access the new class of scientific investigations addressed by FERMI, becoming a standard in time resolved spectroscopies, holography, and diffraction. In addition, it also enabled non-custom pump-probe spectroscopy correlation, and made the first realization of transient-grating in the XUV possible.The simple and clean mechanical design combined with the assiduous attention to online wavefront diagnostics did the rest in determining the success of KAOS over time. This contribution aims at telling how KAOS was born and grew up, showing how wavefront sensing made it work at the best, and how it will face the future challenges.

Speaker: Michele Manfredda (Elettra-Sincrotrone Trieste S.C.p.A.)

212 The SASE3 Soft X-Ray Beamline at European XFEL: Monochromatic Operation

The SASE3 soft X-ray beamline at the European XFEL is equipped with the grating monochromator allowing to reduce SASE FEL bandwidth and to improve longitudinal coherence at the experiments in the photon energy range 250 eV - 3000 eV. The design of the monochromator is challenged by a demand to control both photon energy resolution and temporal resolution; the aim to transport close to transform-limited pulses poses very high demands on the optics quality, in particular on the grating. Presently, the monochromator operates with two gratings: the low-resolution grating is optimized for time-resolved experiments and allows for moderate resolving power of about 2000 - 5000 along with pulse stretching of few to few tens of femtoseconds RMS, and the high-resolution grating reaches resolving power of 10000 at a cost of larger pulse stretching. The examples of time-resolved experiments and experiments performed in high photon energy resolution mode are presented. In addition, being operational in spectrometer mode, the monochromator is regularly used for the spectral characterization of the FEL beam including photon pulse length retrieval.

Speaker: Natalia Gerasimova (European XFEL GmbH)

213 Upgrade to the Transverse Optics Matching Strategy for the FERMI FEL

Good control over the transverse distribution of an electron bunch is crucial for optimising the beam transport through a linear accelerator, and for improving the energy transfer of electrons to photons within the undulators of a free-electron laser (FEL). In order to achieve this, it is necessary to match, as closely as possible, the Twiss parameters of the electron bunch to the design values. This is done, in the case of the FERMI FEL, by finding the optimal quadrupole strengths in various matching sections using a particle tracking code. This contribution reports an upgrade to the matching tools in use in the FERMI control room: the functionalities of two existing programs have been merged into a single tool; and some new options are available in order to provide more flexibility when performing transverse optics matching.

Speakers: Alexander Brynes (Elettra-Sincrotrone Trieste S.C.p.A.), Mauro Trovo (Elettra-Sincrotrone Trieste S.C.p.A.), Simone Di Mitri (Elettra-Sincrotrone Trieste S.C.p.A.), Giuseppe Penco (Elettra-Sincrotrone Trieste S.C.p.A.), Enrico Allaria (Elettra-Sincrotrone Trieste S.C.p.A.), Simone Spampinati (Elettra-Sincrotrone Trieste S.C.p.A.), Giovanni Perosa (Elettra-Sincrotrone Trieste S.C.p.A.), Dr Luca Giannessi (Elettra-Sincrotrone Trieste S.C.p.A.)

WEP04_poster.pdf

WEP04_poster.pptx

214 Virtual Diagnostic for Longitudinal Phase Space Imaging for the MAX IV SXL Project

Accurate and high resolution detection of the Longitudinal Phase Space (LPS) of the electron beam is a great advantage for operating and setting up a FEL. In the case of the soft X-ray FEL being proposed at the MAX IV synchrotron facility in Lund, this information is mainly supplied by a Transverse Deflecting Cavity (TDC) which is currently being installed and scheduled for commissioning in the autumn. Performing the LPS measurement with the future TDC is limited in two regards: it is destructive and may be low in resolution as compared to the maximum compression possible in the MAX IV linac. In this project we propose using machine learning tools to implement a virtual diagnostic to retrieve the LPS information non-destructively using fast, non-invasive measurements and critical set-points in the linac as inputs for a neural network. In this paper we summarize the current progress of this project which has thus far focused on simulation studies of the TDC and the training of a virtual diagnostic using the TDC's simulated output.

Speakers: Johan Lundquist (Lund University), Sverker Werin (Lund University), Francesca Curbis (Lund University)

215 Wakefield Calculations of the Undulator Section in FEL-I at the SHINE

In free electron lasers (FEL) the accumulative effects of wakefields always lead to critical impacts on the electron bunch, resulting in an energy spread and deviation of transverse position. Thus the lasing performance will be decreased. The Shanghai high-repetition-rate XFEL and extreme light facility (SHINE) is under construction and the wakefields estimations are required. The SHINE contains three different undulator lines (FEL-Ⅰ- Ⅲ) designed for different functions. The wakefields of FEL-Ⅰ undulator section has been studied in our work before. However the wakefields of inner segments between undulators are calculated simply. In this paper, we calculate the wakefields of inner segments considering more exquisite structures in FEL-Ⅰ. We consider gradual changed connections between beam pipes of different diameters and dechirpers. We compared wakefields of different schemes of inner segments. Based on the results, we give some suggestions for the designation of the inner segments in FEL-Ⅰ.

Speakers: He Liu (Shanghai Institute of Applied Physics), Jiawei Yan (European XFEL GmbH), Haixiao Deng (Shanghai Advanced Research Institute Chinese Academy of Science), Bo Liu (Shanghai Advanced Research Institute Chinese Academy of Science)

Tutorial 2: Meeting the editor

What is the best place where you can publish your FEL-related research? Should your target be the FEL community, or do your results deserve the attention of a broader audience? How to write a perfect paper, taking advantage of the interaction with editors and referees?
Add your own questions about publication procedures and policies to the previous list and join the tutorial that will take place on Thursday 22/8 at 5:30 pm, at FEL 2022. Two special guests will be there to answer: Serena Dalena, associate editor of Physical Review letters, and Oliver Graydon, Chief Editor of Nature Photonics.

Convener: Giovanni De Ninno (Elettra-Sincrotrone Trieste S.C.p.A.)

216 Physical Review Letters

What is the best place where you can publish your FEL-related research? Should your target be the FEL community, or do your results deserve the attention of a broader audience? How to write a perfect paper, taking advantage of the interaction with editors and referees?

Speaker: Serena Dalena (American Physical Society)

217 Nature Photonics

What is the best place where you can publish your FEL-related research? Should your target be the FEL community, or do your results deserve the attention of a broader audience? How to write a perfect paper, taking advantage of the interaction with editors and referees?

Speaker: Oliver Graydon (Springer Nature)

Thursday 25 August

Electron diagnostics, timing, synchronization & controls

Convener: Marie Labat (Synchrotron Soleil)

218 Machine Learning-Based Virtual Diagnostic

Existing beam diagnostics are invasive, and oftentimes cannot operate at the required resolution. In this work we present a Machine learning-based Virtual Diagnostic (VD) tool to accurately predict the Longitudinal phase space (LPS) for every shot using spectral information collected non-destructively from the radiation of a relativistic electron beam. VD is a computational tool based on deep learning that can be used to predict a diagnostic output. VDs are especially useful in systems where measuring the output is invasive, limited, costly or runs the risk of altering the output. We show a few applications (experimental or simulated data) for high repetition-rate machine (LCLS-II) or a high-current, ultra-short bunch facility (FACET-II). Then, given a prediction, we relay how reliable that prediction is, i.e., quantify the uncertainty of the prediction. Finally, we show how VD can be used for machine optimization as aberration corrector tuning with ML-based emittance measurements.

Speaker: Adi Hanuka (Eikon Therapeutics)

219 Coherent 3D Microstructure of Laser-Wakefield-Accelerated Electron Bunches

Recent breakthroughs in laser wakefield accelerator (LWFA) technology have allowed them to drive free-electron lasers [1]. This is a significant accomplishment as LWFA electron beams are not as well controlled as beams from conventional accelerators. However, longitudinal structure in LWFA beams could be harnessed to accelerate the self-amplified spontaneous emission (SASE) process. Pre-bunched beams have been shown to achieve gain with shorter saturation length than conventional beams [2]. Because of the nature of the LWFA process, electron beams from LWFAs emerge from the plasma with preformed microstructures. The parameters of the accelerator dictate the shape, size and coherence of these features. Coherent optical transition radiation (COTR) can diagnose features in microbunched portions of the electron beam. We present experimental results across three different LWFA regimes demonstrating extreme visible microbunching (up to 10%), as well as sub mm-mrad emittance substructures in LWFA electron beams. In each regime we examined the near field COTR at eight different wavelengths from a foil directly after the end of the accelerator. Depending on the LWFA operating regime, we observe different levels of bunch substructure. How this structure evolves across optical wavelengths is also LWFA-regime dependent. The COTR point spread function model enables the annular shapes observed in the near field to be remapped as the actual 2D beam distributions [3]. We have also used COTR interferometry to measure sub mm-mrad divergence of the microbunched portion of the beam. In addition, we employed a multi-octave spectrometer to measure the spatially averaged TR spectrum from IR to near-UV wavelengths to characterize longitudinal beam shape. Wavelength-dependent variations in the size and radial distribution of the TR images can be correlated with features in the reconstructed longitudinal profile. Combining the longitudinal information acquired by the multi-octave spectrometer with multi-wavelength images of the foil, we observe features in the 3D beam that are unresolvable using other techniques. Moreover, with the aid of physically reasonable assumptions about the bunch profile, reconstructions of the 3D electron bunch distribution will be presented.

Speakers: Alex Lumpkin (Helmholtz-Zentrum Dresden-Rossendorf), Alexander Koehler (Helmholtz-Zentrum Dresden-Rossendorf), Rafal Zgadzaj (The University of Texas at Austin), Michael Downer (The University of Texas at Austin), Brant Benjamin Bowers (The University of Texas at Austin), Andrea Hannasch (The University of Texas at Austin), Maxwell LaBerge (The University of Texas at Austin)

220 Self-Synchronized and Cost-Effective Time-Resolved Measurements at X-Ray Free-Electron Lasers with Femtosecond Resolution

Temporal diagnostics of FEL pulses are generally of great benefit to FEL facilites, in particular to provide information to users and for the setup of special modes such as fresh-slice schemes. In this contribution we present FEL power profile measurements with femtosecond resolution at SwissFEL. The FEL temporal profiles are obtained from the longitudinal phase-space of the electrons after the undulator section. We use the transverse wakefields of a corrugated structure to horizontally streak the electron beam, and vertical dispersion to access the energy information. The advantages of this approach, in comparison to the standard streaking using transverse deflecting rf structures, are cost-effectiveness and stability against arrival time jitter.

Speakers: Philipp Dijkstal (Paul Scherrer Institut), Alexander Malyzhenkov (Los Alamos National Laboratory), Eduard Prat (Paul Scherrer Institut)

221 Ultimate Pulse-to-Pulse Stability in Non-Linear Bunch Compressors

Recent advances in bunch compression and FEL schemes have enabled ultrashort sub-fs electron and X-ray pulses. The timing jitter is, at best, one order of magnitude larger that the pulse duration. This can be handled by high precision pump-probe delay measurements and data sorting. However, only a small fraction of the pulses will be in the relevant time window.

The acceleration and compression in non-linear achromat bunch compressors enables cancellation of the energy and timing jitter caused by modulator high voltage (HV) ripple.
The cancellation works at a specific off-crest acceleration phase, the so-called magic angle.
We present experimental data showing the current performance at the MAX IV linac, and the benefit of operating at the magic angle.

Another major contribution to energy and arrival time jitter is lasers, both for the electron guns and the experiment, and how they are synchronized to the reference RF field. The RF distribution can either be optical or electrical.
By extracting the reference RF directly from the gun laser, we have eliminated the relative jitter between the gun laser pulses and the reference field. We show data of the improved performance in our optical master oscillator scheme.
A full synchronization system that includes the experimental lasers is under development.
Our current plan is to base the synchronization system on a continuous wave reference laser to take advantage of the high frequency of optical waves, instead of relying on the envelope of pulsed lasers.

Combining acceleration at or around the magic angle with the high-precision synchronization system we aim at a timing jitter on the order of 1 fs at the end of the linac.

Speakers: Erik Mansten (MAX IV Laboratory), Sara Thorin (MAX IV Laboratory), Mr Robin Svärd (MAX IV Laboratory), Prof. Mikael Eriksson (MAX IV Laboratory), Neven Blaskovic Kraljevic (MAX IV Laboratory), Dr Filip Lindau (MAX IV Laboratory), Dr Domenico Alj (MAX IV Laboratory), Dr David Kroon (MAX IV Laboratory), Oliver Grimm (MAX IV Laboratory), Dr Lennart Isaksson (MAX IV Laboratory), Dr Pedro Fernandes Tavares (MAX IV Laboratory)

10:35 Coffee & Exhibition

Photon beamline instrumentation & undulators

Convener: Jan Grünert (European XFEL GmbH)

222 Development of APPLE-III Undulators for FLASH

The implementation of a helical afterburner undulator at DESY's VUV-FEL source is part of the current FLASH2020+ upgrade program. The device shall be installed downstream of the present FLASH2 SASE undulators and will provide radiation with variable polarization from 1.33 nm to 1.77 nm (890-700eV) and thus also cover the L-edges of the 3d transition metals Fe, Co, and Ni. Despite a moderate energy upgrade of the machine to 1.35 GeV, the required wavelengths and tunability range can only be reached by a high magnetic performance of the undulator.
We report on design and development of an APPLE-III undulator with 17.5 mm period length operating at a minimum magnetic gap of 8 mm which will make use of a magnetic force compensation scheme. A short prototype has been built to verify and iterate both the mechanical and magnetic concept. Details on keeper design, prototype results and the tuning concept will also be discussed. The full length device is presently under construction and shall also verify this concept for the future seeding undulators at FLASH1.

Speakers: Markus Tischer (Deutsches Elektronen-Synchrotron), Pavel Vagin (Deutsches Elektronen-Synchrotron), Kathrin Götze (Deutsches Elektronen-Synchrotron), Patrick N'Gotta (Deutsches Elektronen-Synchrotron), Andreas Schöps (Deutsches Elektronen-Synchrotron)

AppleIII-Development_Tischer_FEL2022-THBI1.pdf

223 XFEL Sub-10 nm Focusing Mirror System at SACLA for Achieving 10^22 w/cm2 Intensity

The XFELs with an anomalously high peak brilliance are opening the way to a number of novel X-ray photon research paths. At SPring-8 Angstrom Compact Free-Electron Laser (SACLA) [1], the XFEL pulses with high stability and short pulse duration (6-7 fs) have been regularly provided thanks to the unique electron gun, accelerator, and undulator systems [2]. By focusing these XFELs to 1um-100nm, the peak intensity has been dramatically increased and new phenomena in hard X-ray nonlinear optics have been explored, such as observation of saturable absorption [3], two-photon absorption [4], and the atomic inner-shell laser emission [5]. To further promote the study in the ultra-intense X-ray laser field, we have developed a focusing system that achieves sub-10nm spot size and 10²² W/cm² intensity.
For the sub-10 nm focusing optics, an advanced Kirkpatrick-Baez (AKB) mirror system based on Wolter-type III geometry [6] has been adopted. The AKB consisting of one-dimensional Wolter mirrors can satisfy Abbe’s sine condition, which leads to a reduced coma aberration and a high tolerance to the incident angle error. We have designed and developed the AKB mirror system for SACLA BL3-EH4c at a photon energy of 9.1 keV. One of the remarkable challenges for the development was the fabrication of the mirrors with 1-nm accuracy. We applied an X-ray wavefront correction scheme [7] for the precise fabrication, and achieved wavefront accuracy of λ/15 rms which satisfies Maréchal’s criterion. Ptychographic probe measurements revealed the focusing spot size of 6.6 nm (horizontal) × 7.1 nm (vertical), indicating eventually attained focused intensity of 1.21 × 10²² W/cm².
References:
[1] T. Ishikawa et al., Nat. Photon. 6 (2012).
[2] For example, I. Inoue et al., Phys. Rev. Lett. 127 (2021). & T. Osaka et al., Phys. Rev. Research 4 (2022).
[3] H. Yoneda et al., Nat. Commun. 5 (2014).
[4] K. Tamasaku et al. Nat. Photon. 8 (2014). & K. Tamasaku et al., Phys. Rev. Lett 121 (2018).
[5] H. Yoneda et al., Nature 524 (2015).
[6] J. Yamada et al., Opt. Express 3 (2019).
[7] S. Matsuyama et al., Sci. Rep. 8 (2018).

Speakers: Jumpei Yamada (Osaka University, RIKEN SPring-8 Center), Kazuto Yamauchi (Osaka University), Makina Yabashi (RIKEN SPring-8 Center)

224 Ringdown Demonstration of a Low-Loss 14 m Hard X-Ray Cavity

Cavity-Based X-ray Free-Electron Lasers (CBXFELs) employ an X-ray cavity formed by crystal mirrors such that X-ray pulses receive periodic FEL-amplification and Bragg-monochromatization. CBXFELs enable improved longitudinal coherence and spectral brightness over single-pass self-amplification of spontaneous radiation (SASE) FELs [1,2] for high-repetition rate FELs. Construction and alignment of a stable low-loss cavity of Bragg-reflecting mirrors has been considered a daunting challenge and has not seen previous experimental implementation of large X-ray cavities in the hard X-ray regime. In this work, we demonstrate stable operation of a low loss 14-m-roundtrip rectangular cavity of four Bragg-reflecting diamond (400) mirrors. 9.831 keV X-rays from the Linac Coherent Light Source (LCLS) were in-coupled into the cavity via a thin diamond transmission grating. X-ray ring-down was characterized using fast photodiodes and a nanosecond-gated camera. Intra-cavity focusing was introduced to further stabilize the cavity, enabling observation of X-ray storage at >50 round trips. This experiment demonstrates feasibility of a stable low-loss hard X-ray cavity that will support future CBXFEL tests and operation [3].

Speakers: Rachel Margraf (Stanford University), Gabriel Marcus (SLAC National Accelerator Laboratory), River Robles (SLAC National Accelerator Laboratory), James MacArthur (SLAC National Accelerator Laboratory), Aliaksei Halavanau (SLAC National Accelerator Laboratory), Dr Sun Yanwen (SLAC National Accelerator Laboratory), Jacek Krzywinski (SLAC National Accelerator Laboratory), Zhirong Huang (SLAC National Accelerator Laboratory), Kenan Li (SLAC National Accelerator Laboratory), Anne Sakdinawat (SLAC National Accelerator Laboratory), Kenji Tamasaku (RIKEN SPring-8 Center), Taito Osaka (RIKEN SPring-8 Center), Diling Zhu (SLAC National Accelerator Laboratory), Takahiro Sato (SLAC National Accelerator Laboratory)

225 AC/DC: The FERMI FEL Split and Delay Optical Device for Ultrafast X-Rays Science

Free-electron lasers (FELs) are currently the most advanced class of light sources, by virtue of their unique capability to lase high-brightness and ultrashort pulses characterized by wavelengths spanning the Extreme-Ultraviolet (EUV), the Soft (SXR) and Hard (HXR) X-Ray spectral domains, alongside with temporal duration lying in the femtosecond (fs) timescale [1]. Specifically, the advent of FELs light sources has recently allowed to perform, in a time-resolved fashion approach, both established spectroscopies, daily employed at synchrotron light sources, and novel non-linear optical methods, mostly combining FELs and laser pulses. Nonetheless, the next step to push the ultrafast X-Ray science standards is widely recognized to be linked to go beyond the current time-resolved schemes, so performing experiments engaging exclusively EUV, SXR and HXR pulses. Indeed, exciting (and probing) matter at its (or nearby) electronic resonance is largely speculated to be one of the key for discriminating and revealing the microscopic mechanisms hiding behind some of the most exotic phases of physical, chemical, and biological systems. Such a challenge calls the design of optical devices capable to both split and delay (in time) FELs pulses, without impacting on their coherence properties, and fully user-friendly in terms of preserving the perfect overlap of the resulting focal spots, even in the few microns spatial domain, a well-known trademark for focusing EUV, SXR and HXR pulses at FELs light sources [2].
At the seeded FERMI FEL (Trieste, Italy) this goal is committed by the novel optical device known as AC/DC, which stands for the Auto Correlator/Delay Creator. AC/DC is purposely designed to double the incoming FEL photon beam into two exact pulse replicas, splitting it by inserting a grazing incidence flat mirror, and further delaying in time, in a controlled way, one of the two pulses, with an intrinsic temporal resolution of approximately 360 attoseconds. A detailed description of AC/DC is highlighted here. Specifically, strong emphasis is dedicated to the opto-mechanical design and the laser-based feedback system, purposely designed and implemented to compensate in real-time any potential drift and pointing mismatch affecting the FEL optical trajectory, ascribable to both mechanical imperfections and residual paraxial errors appearing during a temporal delay scan [3].

[1] Bostedt C., Boutet S., Fritz D.M., Huang Z., Lee H.J., Lemke H.T., Robert A., Schlotter W.F., Turner J.J., Williams G.J., Linac Coherent Light Source: The first five years. Rev. Mod. Phys.88, 015007 (2016)

[2] Manfredda M., Fava C., Gobessi R., Mahne N., Raimondi L., Simoncig A., Zangrando M., The evolution of KAOS, a multipurpose active optics system for EUV/Soft X-rays, Synchrotron Radiation News, 0, 0, (2022) DOI: 10.1080/08940886.2022.2066432
.
[3] Simoncig A., Manfredda M., Gaio G., Mahne N., Raimondi M., Fava C., Gerusina S., Gobessi R., Abrami A., Capotondi F., De Angelis D., Menk R., Pancaldi M., Pedersoli E., Zangrando M., AC/DC: The FERMI FEL Split and Delay Optical Device for Ultrafast X-rays Science, Photonics, 9(5), 314, 2022

Speaker: Alberto Simoncig (Elettra-Sincrotrone Trieste S.C.p.A.)

12:50 Lunch

14:10 Visit to Elettra Sincrotrone Trieste

19:00 Social Dinner and FEL Prize ceremony

Friday 26 August

User experiments

Convener: Kiyoshi Ueda (Tohoku University)

226 Probing Transient Structures of Nanoparticles by Single-Particle X-Ray Diffraction

We report on our recent experimental results of single-shot and single-particle X-ray diffraction of nanoparticles at SACLA. The single-shot diffraction data provided insights into the crystallization kinetics of Xe nanoparticles, where the nanoparticles initially crystallize in the metastable stacking-disordered phase and then transform into the stable fcc phase. In addition, we investigated the ultrafast structural dynamics of nanoparticles triggered by the irradiation of an intense laser pulse.

Speakers: Akinobu Niozu (Hiroshima University), Kiyonobu Nagaya (Kyoto University), Hironobu Fukuzawa (Tohoku University), Koji Motomura (Tohoku University), Kiyoshi Ueda (Tohoku University), Edwin Kukk (University of Turku), Claudio Cirelli (Paul Scherrer Institute), Carlo Callegari (Elettra-Sincrotrone Trieste S.C.p.A.), Michele Di Fraia (Elettra-Sincrotrone Trieste S.C.p.A.), Giorgio Rossi (Università degli Studi di Milano), Makina Yabashi (RIKEN SPring-8 Center)

227 Novel Lattice Instability in Ultrafast Photoexcited SnSe

There has been growing interest in using ultrafast light pulses to drive materials into nonequilibrium states with novel properties. Using time-resolved X ray scattering, I demonstrated that SnSe, one of the IV-VI resonantly bonded compounds, hosts a novel photo-induced lattice instability associated with an orthorhombic distortion of the rock-salt structure with space group Immm [1]. The new lattice instability is accompanied by a drastic softening of the lowest-frequency Ag phonon (TO phonon), which has previously been identified as the soft mode in the thermally driven phase transition to a Cmcm structure. Therefore, we provide a counterexample of the conventional wisdom that laser pump pulse serves as a heat dump. Density functional theory calculations reveal that the photoinduced Immm lattice instability arises from electron excitation from the Se 4p- and Sn 5s-derived bands (the lone pair orbitals) deep below the Fermi level that cannot be excited thermally.
Furthermore, I show results of non-zone-center measurements of time-resolved X-ray scattering, from which I extracted interatomic force constants in the photoexcited states. and identify a certain bond that is largely overlapped with the lone pair orbital as responsible for the observed photoinduced lattice instability. The conclusion is in contrary to the consensus that in thermal equilibrium, the resonant bonding network of chalcogen p orbitals is the main origin for lattice instability and a soft TO mode. And indeed, the photoexcited phonon modes have a significantly longer lifetime, which means less anharmonicity of the lattice, than those in thermal equilibrium, consistent with the observation that interatomic interaction driving the photoinduced lattice instability is different from the thermal.
The results have implications for optical control of the functional properties of monochalcogenides and other resonantly bonded materials. More generally, the results emphasize the importance ultrafast structural probes that reveal distinct atomic-scale dynamics too subtle for conventional spectroscopies.
This work was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences through the Division of Materials Sciences and Engineering under Contract No. DE-AC02- 76SF00515.
[1] Y. Huang et al., Phys. Rev. X, 12, 011029 (2021)

Speakers: Yijing Huang (SLAC National Accelerator Laboratory), Prof. Samuel Teitelbaum (SLAC National Accelerator Laboratory), Dr Gilberto de la Pena (SLAC National Accelerator Laboratory), Prof. David Reis (SLAC National Accelerator Laboratory), Mariano Trigo (SLAC National Accelerator Laboratory), Dr Shan Yang (Duke University), Dr Jennifer Niedziela (Duke University), Prof. Dipanshu Bansal (Duke University), Prof. Olivier Delaire (Duke University), Dr Takahiro Sato (SLAC National Accelerator Laboratory), Matthieu Chollet (SLAC National Accelerator Laboratory), Diling Zhu (SLAC National Accelerator Laboratory)

228 Ultrafast Dynamics in (TaSe4)2I Triggered by Optical and X-Ray Excitation

Dimensionality plays a key role for the emergence of ordered phases such as charge-density-waves (CDW), which can couple to, and modulate, the topological properties of matter.
In this work, we study the out-of-equilibrium dynamics of the paradigmatic quasi-one-dimensional material (TaSe$_4$)$_2$I, that exhibits a transition into an incommensurate CDW phase when cooled just below room temperature, namely at T$_{\rm{CDW}} $= 263\,K.
We make use of both optical laser and free-electron laser (FEL) based time-resolved spectroscopies in order to study the effect of a selective excitation on the normal-state and on the CDW phases, by probing the near-infrared/visible optical properties both along and perpendicularly to the direction of the CDW, where the system is metallic and insulating, respectively.
Excitation of the core-levels by ultrashort X-ray FEL pulses at 47 eV and 119 eV induces reflectivity transients resembling those recorded when only exciting the valence band of the compound - by near-infrared pulses at 1.55 eV - in the case of the insulating sub-system. Conversely, the metallic sub-system displays a relaxation dynamics which depends on the energy of photo-excitation.
Moreover, excitation of the CDW amplitude mode is recorded only for excitation at low-photon-energy. This fact suggests that the coupling of light to ordered states of matter can predominantly be achieved when directly injecting delocalized carriers in the valence band, rather than localized excitations in the core levels.
On a complementary side, table-top experiments allow us to prove the quasi-unidirectional nature of the CDW phase in (TaSe$_4$)$_2$I, whose fingerprints are detected along its $c$-axis only.
Our results provide new insights on the symmetry of the ordered phase of (TaSe$_4$)$_2$I perturbed by a selective excitation, and suggest a novel approach based on complementary table-top and FEL spectroscopies for the study of complex materials.

Speakers: Dr Wibke Bronsch (Elettra - Sincrotrone Trieste), Mr Manuel Tuniz (Università degli Studi di Trieste), Mr Giuseppe Crupi (Università degli Studi di Trieste), Mr Michela De Col (Università degli Studi di Trieste), Mr Denny Puntel (Università degli Studi di Trieste), Mr Davide Soranzio (Università degli Studi di Trieste), Mr Alessandro Giammarino (Università degli Studi di Trieste), Mr Michele Perlangeli (Università degli Studi di Trieste), Dr Helmuth Berger (Ecole Polytechnique Fédérale de Lausanne), Dr Dario De Angelis (Elettra - Sincrotrone Trieste), Mr Danny Fainozzi (Elettra - Sincrotrone Trieste), Mr Ettore Paltanin (Università degli Studi di Trieste and Elettra Sincrotrone Trieste), Dr Jacopo Stefano Pelli Cresi (Elettra - Sincrotrone Trieste), Mr Gabor Kurdi (Elettra - Sincrotrone Trieste), Dr Laura Foglia (Elettra - Sincrotrone Trieste), Dr Riccardo Mincigrucci (Elettra - Sincrotrone Trieste), Prof. Fulvio Parmigiani (Università degli Studi di Trieste), Dr Filippo Bencivenga (Elettra - Sincrotrone Trieste), Federico Cilento (Elettra - Sincrotrone Trieste)

229 FLASH2020+ Pump-Probe Laser Upgrade: Concept and Current Status

Time-resolved experiments are increasingly relevant in modern FEL user facilities. With the FLASH2020+ upgrade project, the pump-probe capabilities of the FLASH will be extended. Besides offering fixed wavelengths (1030 nm fundamental and its harmonics), tunable wavelengths are under development: sub-150 fs long tunable mid-infrared (2-5 microns) pulses for the solid-state community and sub-40 fs long tunable UV-VIS (200-500 nm) pulses for the general chemistry, atomic molecular and optical physics (AMO) communities. The optical pulses will be fully synchronized with the FEL pulses and are generated with up to a 1 MHz repetition rate in bursts of 0.6 ms in length at 10 Hz. Since we are limited by our pump-lasers available fixed average power, we can also reduce the repetition rate to 200kHz or less for delivering higher energy pulses for experiments using small density targets (such as gas phases or clusters).
Here, we present our pump-probe laser concept: from the laser front end to the beam delivery to experimental end-stations and instruments. We would be happy to receive any feedback from the users on their needs so we can adjust our concept as needed.

Speaker: Dr Skirmantas Alisauskas (Deutsches Elektronen-Synchrotron DESY)

10:35 Coffee & Exhibition

End-to-end experiments (machine driven)

Convener: Flavio Capotondi (Elettra-Sincrotrone Trieste S.C.p.A.)

230 Experiments with Phase-Controlled Multi-Pulses from FERMI

The FERMI Free Electron Laser in Trieste (Italy) has been designed and built as a seeded source, for precise control of the properties of its light pulses. Its excellent longitudinal coherence is inherited from the seed laser, and is its uppermost distinctive feature. In the realm of atomic, molecular and optical science, the use of longitudinal coherence of laboratory lasers as a time reference for precise measurements, as a control parameter for the synthesis of arbitrary waveforms, and for steering the outcome of a photophysical process, has a long history of achievements. One wishes to extend the same concepts to shorter wavelengths, because the latter provide higher spatial and temporal resolution, as well as chemical selectivity. In this talk I will present the challenges faced and the solutions found towards this goal, as well as the applications demonstrated so far, such as: coherent control of photoionization, measurement of photoemission delays, sensitive detection of weak processes, or the generation of periodic waveforms.

The results originate from the joint effort of many international laboratories and of a large number of researchers, whose work is gratefully acknowledged.

Speaker: Carlo Callegari (Elettra-Sincrotrone Trieste S.C.p.A.)

FEL22_v2.pptx

231 Observation of Coherent Electronic Motion with X-Ray Free-Electron Lasers

Electron motion is a key ingredient of every chemical processes. The natural timescale for such electronic dynamics in small molecular systems is typically in the range of tens to hundreds of attoseconds. Here I will present recent experimental results using attosecond x-ray free electron laser pulses and pulse pairs to probe ultrafast electronic motion. X-ray free-electron lasers offer continuous wavelength tunability across the soft x-ray region allowing for atomic-site specific probes of the electron density in molecular systems.

I will present our first results showing isolated attosecond soft X-ray pulses from the FEL, with peak power approaching the terawatt scale. Such high power pulses open the door for nonlinear spectroscopies such as pump/probe spectroscopy, and X-ray wave mixing. We have demonstrated the preparation of a coherent electronic wavepacket by driving stimulated X-ray Raman scattering. Combing attosecond X-ray pulses with an external laser field we are able to time-resolve the photoemission dynamics of core-level electrons in molecules, observing the coherent evolution of a wavepacket of core-excited states. I will also show the first results from a x-ray pump/x-ray probe measurement of ionization induced charge motion.

Use of the Linac Coherent Light Source (LCLS), SLAC National Accelerator Laboratory, is supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences (BES) under Contract No. DE-AC02-76SF00515. This work was supported by the U.S. DOE, Office of Science, Office of BES, Chemical Sciences, Geosciences, and Biosciences Division (CSGB).

Speakers: James Cryan (SLAC National Accelerator Laboratory), Agostino Marinelli (SLAC National Accelerator Laboratory)

232 The Role of Light Possessing Orbital Angular Momentum in Ptychographic Imaging Experiments

The use of light beams possessing orbital angular momentum (OAM) is rapidly becoming a way for probing condensed-matter systems, even in the XUV range [1]. The wavefronts of such beams are characterized by an azimuthal angular dependence of the electric field phase, associated with an OAM topological charge $\ell \neq 0$. For imaging purposes, it has been shown that OAM beams can overcome the Rayleigh criterion limit, so enhancing the theoretical resolution with respect to gaussian illumination ($\ell = 0$) [2]. We tested this feature at the DiProI beamline of the FERMI FEL by performing ptychographic experiments with a standard sample [3,4]. As predicted, the ptychographic reconstructions with OAM beams showed a higher image resolution, and the retrieved illumination functions proved to be very sensitive to optical aberrations. This study will potentially provide the basis for new characterization and diagnostic tools, since the extra degree of freedom $\ell$ can be exploited for tuning the light-matter interaction even during pump-probe experiments.

Speakers: Matteo Pancaldi (Elettra-Sincrotrone Trieste S.C.p.A.), Arun Ravindran (University of Nova Gorica), Benedikt Roesner (Paul Scherrer Institut), Charles S. Bevis (University of Pavia), Dario De Angelis (Elettra-Sincrotrone Trieste S.C.p.A.), Emanuele Pedersoli (Elettra-Sincrotrone Trieste S.C.p.A.), Flavio Capotondi (Elettra-Sincrotrone Trieste S.C.p.A.), Francesco Guzzi (Elettra-Sincrotrone Trieste S.C.p.A.), Georgios Kourousias (Elettra-Sincrotrone Trieste S.C.p.A.), Giovanni De Ninno (Elettra-Sincrotrone Trieste S.C.p.A.), Giulia F. Mancini (University of Pavia), Iuliia Bykova (Paul Scherrer Institut), Maurizio Sacchi (Synchrotron SOLEIL), Mauro Fanciulli (Lab. de Physique des Materiaux et Surfaces, CY Cergy Paris Université), Michele Manfredda (Elettra-Sincrotrone Trieste S.C.p.A.), Paolo Vavassori (CIC nanoGUNE BRTA), Stefano Bonetti (Stockholm University and Ca' Foscari University of Venice), Thierry Ruchon (Université Paris-Saclay, CEA, CNRS, LIDYL)

233 A Perfect X-Ray Beam Splitter and its Applications to Time-Domain Interferometry and Quantum Optics Exploiting FELs

Brilliant, ultrashort, and coherent X-ray FEL pulses allow investigations of dynamics at the inherent time and length scale of atoms. However, the user community still lacks access to phase-locked X-ray pulses, desirable for time domain correlation spectroscopies and coherent quantum control. Based on selective electron-bunch degradation in the accelerator, combined with two-stage, self-seeded photon emission, we propose an FEL mode generating subfemtosecond, phase-locked X-ray pulse pairs with up to 100 fs delay. Splitting the electron bunch in the accelerator, instead of photon pulses in the beamline, avoids relative phase jitter. This enables time-domain interferometry, such as the X-ray analog of the ubiquitous Fourier transform infrared spectrometer, and, more generally, all of nonlinear and quantum optics requiring coherent copies of beams.

Speaker: Sven Reiche (Paul Scherrer Institut)

12:50 Closeout