FEL2024 - 41st International Free Electron Laser Conference

18 Aug 2024, 17:00 → 23 Aug 2024, 13:20 Europe/Warsaw

POLIN

Pawel Krawczyk (National Centre for Nuclear Research)

FEL 2024 - 41^st International Free Electron Laser Conference

Odpis_Aktualny_KRS_0000171393.pdf

Sunday 18 August

18:30 Registration

19:00 Welcome Reception

Monday 19 August

Opening

08:00 Opening

1 In memoriam: Mike Poole (1945-2023)

Speaker: Richard Walker (Diamond Light Source)

2 In Memoriam: Siegfried Schreiber (1959-2024)

Speaker: Mathias Vogt (Deutsches Elektronen-Synchrotron DESY)

First Lasing, New FEL projects and Facility Reports: Session-1a

Convener: Pawel Krawczyk (National Centre for Nuclear Research)

3 Demonstration of a reliable high gain free electron laser driven by a laser plasma accelerator

Compact free electron laser (FEL) technology enabled by plasma-based accelerators is a rapidly maturing technology with several milestone demonstrations in the last several years. Still, critical work is needed to bridge the gap from proof of concept experiments to reliable operation of laser plasma accelerator (LPA) driven FELs. At the BELLA Center, we have a dedicated facility equipped with a 100 TW laser coupled to an electron beam transport section that culminates in a 4m long, strong focusing undulator. Recent efforts have enabled the production of reliable high quality e-beams which has in turn enabled exponential SASE gain of order 10^3 at 400 nm with gain lengths <20 cm.

This work was supported by the U.S. Department of Energy (DOE) Office of Science, the Office of Basic Energy Sciences, and the Office of High Energy Physics, under Contract No. DE-AC02-05CH11231, and through a CRADA with Tau Systems

Speaker: Samuel Barber (Lawrence Berkeley National Laboratory)

4 Commissioning of Echo-Enabled Harmonic Generation at the DELTA Storage Ring

Echo-enabled harmonic generation (EEHG) has been proposed as a seeding method for free-electron lasers but can also be employed to generate ultrashort radiation pulses at electron storage rings. With a twofold laser-electron interaction in two undulators (the modulators), each followed by a magnetic chicane, an electron phase space structure with a high harmonic content is produced, which gives rise to coherent emission of radiation at short wavelengths. The duration of the coherently emitted radiation in a third undulator (the radiator) is given by the laser pulse lengths. Thus,the EEHG pulses can be three orders of magnitude shorter and still more intense than conventional synchrotron radiation. At the 1.5-GeV synchrotron light source DELTA at TU Dortmund University, the worldwide first implementation of EEHG at a storage ring was achieved by reconfiguring an electromagnetic undulator. First commissioning results are presented.

Speaker: Shaukat Khan (TU Dortmund University)

5 LCLS-II first lasing

A report on the LCLS-II first lasing will be presented.

Speaker: Zhirong Huang (SLAC National Accelerator Laboratory)

6 LCLS facility report.

The LCLS facilityreport, including FEL operations, new capabilities, and future development will be presented.

Speaker: Zhirong Huang (SLAC National Accelerator Laboratory)

09:40 Discussion

7 PAL-XFEL facility report

This talk presents the recent progress and operation status of the PAL-XFEL, focusing on the accelerator and beamline performance, along with recent developments. The facility is now accommodating approximately 70 user experiments annually. Significant efforts to improve FEL (Free Electron Laser) intensity have successfully increased pulse energies to over 2 mJ in SASE mode and 1 mJ in self-seeding mode, with stable long-term operation. Pulse stability for long term operation was achieved through changing the injector laser, chiller, X-band LLRF controller, and the method for Gun phase shifter from LLRF to the laser RF phase shifter. The diverse beamline instruments were also conducted for various sample environment for XSS(X-ray Scattering and Spectroscopy), NCI(Nano Crystallography and coherent Imaging), and SSS(Soft X-ray Scattering and Spectroscopy) experiment hutches. Ongoing developments such as the fresh-slice FEL, electron beam collimation via laser heater, and the construction of a second hard X-ray line will also be shortly discussed.

Speaker: MyungHoon Cho (Pohang Accelerator Laboratory)

8 The COXINEL Demonstrator upgrade

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, where the first LPA driven seeded FEL has been demonstrated at 275 nm [1]. The seeded FEL has been extensively reproduced. Towards operation at shorter wavelength, LPA electron beam parameters and satbility is being improved [2], and a 3 m long cryogenic permanent magnet undulator is under finalization.
[1] M. Labat, J. Couperus Cabadağ, A. Ghaith, A. Irman et al, Nat. Photon. 17(2), 150-156 (2022). https://doi.org/10.1038/s41566-022-01104-w
[2] A. Irman, "Reduction of the electron-beam divergence of laser wakefield accelerators by integrated plasma lense ", this conference

Speaker: Marie-Emmanuelle Couprie (Synchrotron soleil)

10:06 Discussion

9 The European XFEL

has been in operation since 2017. It features a superconducting linear accelerator capable of achieving a maximum energy of 17.5 GeV. The accelerator can deliver up to 27,000 electron bunches per second, distributed into ten equidistant short bursts, each lasting 600 microseconds. It simultaneously serves three FEL undulators, covering photon energy ranges from 300 eV to 30 keV, with pulse energies in the millijoule range. In addition to standard SASE operation, the facility offers hard X-ray self-seeding, two-color operation, variable polarized soft X-rays, and ultra-short photon pulses down to the sub-femtosecond range.

Speaker: Winfried Decking (Deutsches Elektronen-Synchrotron DESY)

10 The UK XFEL conceptual design and options analysis project

The UK is conducting a multi-stage project to analyse the case for major investment into XFELs, through either developing its own facility or by investing at existing machines. The project’s 2020 Science Case identified a clear need for ‘next-generation’ XFEL capabilities including near-transform limited x-ray pulses across a wide range of photon energies and pulse durations; evenly spaced high-repetition rate pulses; and a high-efficiency facility with a step-change in the simultaneous operation of multiple end stations. The project is developing a conceptual design to meet these requirements, significantly aided by collaboration with international XFELs. It is also guided by an extensive ongoing user engagement programme of Townhall meetings and other activities. Both the science requirements and the emerging conceptual design are expected to be of general interest to the community

Speaker: David Dunning (Science and Technology Facilities Council)

11 Status of the Shanghai Soft X-ray Free-Electron Laser facility

The Shanghai Soft X-ray Free-Electron Laser (SXFEL) facility is the first X-ray FEL facility in China, completed in 2022 and officially opened for operation in 2023. It is based on a 1.5 GeV normal conducting high gradient C-band linac and currently contains two FEL beamlines and five experimental stations. The SXFEL can cover the whole water window range with the shortest wavelength of 2 nm. This report will give a brief introduction of the basic information of the SXFEL facility, its current operation status, the advanced FEL experiments that have been conducted and planned for the future.

Speaker: Chao Feng (Shanghai Advanced Research Institute)

10:40 Discussion

10:45 Coffee break

First Lasing, New FEL projects and Facility Reports: Session-1b

Convener: Pawel Krawczyk (National Centre for Nuclear Research)

12 FERMI 2.0: status and perspectives for the upgrade of FERMI

FERMI was built as a facility using high-gain harmonic generation to provide coherent light down to the soft X-ray region of the spectrum. After some fourteen years of operation, a number of possibilities have been exploited that go beyond the simple control of the spectral quality of the emitted pulses. In fact, the seed has offered the possibility to control pulse duration and phase, intrapulse polarization, to generate multiple pulses for the implementation of multiple colour schemes, to coherently control and to synchronise ultrashort pulses to an unprecedented level with an external laser system. FERMI has now entered an upgrade phase to FERMI 2.0. The ultimate goal of the upgrade plan is to extend the spectral range of the facility to cover the water window and beyond, and to reduce the minimum pulse duration below the characteristic lifetime of the light element core hole electrons, while preserving the uniqueness of FERMI: the possibility to control the properties of the radiation by seeding the FEL with an external laser system. An overview of the current status and future perspectives of FERMI is presented.

Speaker: Dr Luca Giannessi (Istituto Nazionale di Fisica Nucleare)

13 Status PoLFEL

At NCBJ in Otwock-Świerk a THz facility is under construction. It will deliver THz radiation in the frequency range from 0.5THz to 5THz (600μm - 60μm). Its superconducting cw-operating linac will provide electron bunches at energy up to 60 MeV and charge in the range of 20pC–250pC. In the first stage, electron bunches will be generated in a room temperature electron gun. For the second stage, a superconducting gun will be implemented, and the bunch repetition rate will be raised from 100Hz to 50kHz. The THz radiation will be generated in a permanent magnet tunable gap undulator operating in the superradiant mode. Beamline will be completed in 2025. According to the present schedule, the commissioning will take place in 2026. In the presented article, the design and technical parameters of the THz facility is discussed.

Speaker: Robert Nietubyc (National Centre for Nuclear Research)

MOAI10_slides.pdf

MOAI10_slides.pptx

14 SwissFEL: Status Report

We present the status of SwissFEL operation for the year 2023. It includes the performance and uptime statistics and the major causes of downtime.  The progress in advance machine and results form user experiments are shown.

Speaker: Sven Reiche (Paul Scherrer Institute)

11:40 Discussion

15 The FLASH facility status

The FLASH facility is driven by a superconducting linac producing 5000 electron bunches per second (1 MHz burst @ 10 Hz) at a max. electron beam energy of 1.35 GeV. The bunches can be distributed between 2 FEL beamlines and a dedicated beamline for beam driven plasma acceleration research. With more than 8000 h of beam operation, of which roughly 4500 h are dedicated to user experiments, FLASH hosts 40 experiments per year. With a strong upgrade programme called FLASH2020+, the facility is adjusting its scope to facilitate the increasing demands of next generation experiments. Already available to users is an increased photon energy in the fundamental (up to 390 eV) and at variable polarisation in 3rd harmonic via afterburner (up to 950 eV). In a currently ongoing shutdown, the transformation of FLASH1 to the first high repetition rate externally seeded FEL beamline for user experiments is in focus and will pose drastic changes to the beam properties available to users. The high complexity of parallel SASE and seeded operation, necessitating vastly different electron beam properties, has recently been successfully tackled demonstrating EEHG lasing in parallel to FLASH2 operation.

Speaker: Lucas Schaper (Deutsches Elektronen-Synchrotron DESY)

16 Status of SACLA

SACLA has continuously provided stable and high-quality XFEL light for users at three beamlines with a cumulative user operation time of more than 6,000 hours per year. Recently, the improvement of the hardware components, the installation of the various feedback controls, and the implementation of machine learning (ML) tools have enabled us to enhance the stability and to achieve a quick and systematic optimization of the XFEL beam parameters at the SASE and self-seeding operation conditions. As a scientific highlight, an extreme focus of 7 nm with an unprecedented X-ray intensity of 1e22 W/cm^2 has been achieved with a novel focusing system. The commissioning of the next-generation detector CITIUS has recently started.

Speaker: Makina Yabashi (Japan Synchrotron Radiation Research Institute)

17 Facility report on KU-FEL.

Kyoto University Free Electron Laser (KU-FEL) facility has been developed for energy-related research by the Institute of Advanced Energy, Kyoto University. There are two accelerator-driven infrared coherent light sources in the facility. One is the oscillator-type FEL whose wavelength range is 3.4 to 26 micro-m[1]. The other one is the THz-Coherent Undulator Radiation (THz-CUR) source [2] whose frequency range is 0.1 to 0.4 THz. In addition to the accelerator-driven light sources, several solid-state laser sources can be used together. The facility is open to domestic and international users. In fiscal year 2023, 18 external user groups used the facility for their research. The current status and results of recent upgrade projects for improving the performance of the light sources will be presented.
[1] H. Zen et al., Physics Procedia 84, 47-53 (2016).
[2] S. Krainara et al., Rev. Sci. Instrum. 90, 103307 (2019).

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

12:15 Discussion

18 Construction Status of the SHINE Project

SHINE is a high repetition rate X-ray FEL facility under construction in Shanghai, China. This facility is based on an 8 GeV CW superconducting linac and plans to build 3 X-ray FEL lines and 10 experimental stations in phase-I, covering the photon energy range of 0.4–25 keV. The groundbreaking of the SHINE project was made in April 2018, currently its constructions of civil engineering and utilities are close to completion, and manufactures of the components of the facility and installation of the superconducting linac system are in course. With the current construction schedule, SHINE user experiments are expected to start in 2027. The status of the project is presented.

Speaker: Zhentang Zhao (Shanghai Synchrotron Radiation Facility)

19 Shenzhen Superconducting Soft X-ray Free Electron Laser(S3FEL)

Since FLASH first lasing at 2005, Free Electron Laser (FEL) has been proved a unique and essential EUV and X-ray source for basic research. However FEL could not serve as many users as Synchrotron Radiation source limited by total user beam time due to linear accelerator (LINAC) instead of storage ring. High repetition rate electron LINAC using superconducting cavity could increase the operation efficiency and benefit more users, for example FLASH and EXFEL at DESY. With better quality of superconducting cavity X ray FEL facilities based on continuous wave operation of superconducting cavity have been launched, such as LCLS II at SLAC and SHINE at ShanghaiTech University in hard X-ray. High repetition-rate X-ray FELs, with combination of high average power, ultrashort pulses and coherence, are revolutionary observational tools that will improve our understanding in diverse fields of science and technology. S3FEL is proposed in the city of Shenzhen and approved in 2023. Its superconducting LINAC consists of 25 sets of TESLA type 1.3 GHz module and 2 sets of 3.9 GHz module to produce evenly distributed electron pulses with 2.5 GeV at the maximum repetition rates of 1MHz. At the first stage three undulator lines with different FEL schemes will be built to deliver bright soft X-ray photon pulse from 40 to 1200 eV to users, which is complementary to SHINE project.

Speaker: Weiqing Zhang (Dalian Institute of Chemical Physics)

12:40 Discussion

12:50 Lunch break

FEL theory: Session-2

Convener: Avraham Gover (Tel Aviv University)

20 Impact of space charge in externally seeded FEL

In high-gain harmonic generation (HGHG) FEL, the amplification process is triggered by an extrernal seed laser that creates periodic energy modulation in the electron phase-space, which is then converted into a density modulation in a dispersive section. This density modulation presents strong current spikes at the seed laser wavelength that leads to the harmonic bunching in the forthcoming radiator. Hower, these spikes can also induce non-negligible space charge effect to the beam that under certain conditions can have a strong impact on the evolution of the fel process in the long FEL radiator line tuned at a harmonic of the seed laser wavelength.
We present a detailed comparison of experimental data observed at FERMI FEL with numerical simulations highlighting how critical the influence of the space charge can be on the amplification process.

Speaker: Eléonore Roussel (Laboratoire de Physique des Lasers, Atomes et Molécules)

21 Three-dimensional theory of soliton-like superradiant free-electron lasers

The soliton-like superradiant regime of free-electron lasers (FEL) offers a promising path towards ultrashort pulses, beyond the natural limit dictated by the bandwidth of the high-gain FEL instability. In this work we present a three-dimensional theory of the superradiant regime. Our work takes advantage of recent developments in non-linear FEL theory to provide a fully analytical description of soliton-like superradiance. Our theory proves the existence of a diffraction-dominated steady-state regime in which the superradiant peak power grows indefinitely while leaving the pulse duration and on-axis intensity almost unchanged. These results are in excellent agreement with three-dimensional simulations and are supported by recent experimental results at the Linac Coherent Light Source. This work advances non-linear FEL theory and provides a theoretical framework for the next generation of attosecond x-ray FELs.

Speaker: River Robles (Stanford University)

22 Quantum effects in free electron - light interaction

We present a fundamental QED theory for superradiance of bunched electrons - the enhanced coherent spontaneous emission of a correlated electron beam in an FEL configuration or in a variety of other free-electron radiation mechanisms. We derive the buildup of a quantum radiation state by multiple electrons in a configuration of a narrow beam of quantum electron wavepackets passing by a dielectric high quality factor micro-resonator.
As in Dicke’s superradiance of atoms, we show that the mysterious correlation between the emissions of individual electrons originates from their entangled coupling to the same quantum radiation state. Using an iterative process for the buildup of photons in a single mode quantum radiation state by an ensemble of modulation-correlated quantum-electron wavepackets, we confirm the quadratic growth of the photons number with the number of electrons N. Furthermore, we find nonclassical features in the measurable quadrature phase-space of the radiation state and its photon statistics.

Speaker: Bin Zhang (Tel Aviv University)

23 Predicting XFEL performance using neural networks with physics constraints

Predicting X-ray Free Electron Laser (XFEL) performance using Genesis simulation code is standard approach in designing future XFELs. Running this code is time consuming that slows down exploration of the parameter space during the design stage. Thus, using surrogate models based on machine learning techniques is often employed. These models however do not know about physics behind the simulations and make predictions that violate physics constraints. This contribution reports on training neural networks constraint by physics that predict XFEL performance and could be used as surrogate models for XFEL designs.

Speaker: Petr Anisimov (Los Alamos National Laboratory)

16:20 Coffee break

FEL Prize Talks

Convener: Bruce Carlsten (Los Alamos National Laboratory)

24 Advancements in Light Source Research with the Duke FEL

Storage-ring based free-electron lasers (FELs) are a eneration of FELs that predate the current high-gain, single-pass FELs driven by advanced linear accelerators. The Duke FEL is such an oscillator FEL that has provided a rich environment for light source research. In this talk, we will recount the diverse territories explored using this FEL. The Duke FEL has allowed us to produce coherent radiation over a wide range of wavelengths, from infrared (2 microns) to vacuum ultraviolet(below 170 nm), and to establish high-gain operation (about 50% per pass) using a distributed optical klystron. With this FEL, we demonstrated the first FEL polarization control and manipulation using non-optical means. Later, we developed a new FEL configuration to achieve full control of beam polarization, producing a linearly polarized FEL beam with a rotatable direction of polarization using two crossed helical undulators of opposite helicities. We explored two-color FEL operation, achieving
simultaneous lasing in the infrared (near 720 nm) and ultraviolet (360 nm), including harmonic lasing and wavelength tuning.We also demonstrated the first structured light generation by producing orbital angular momentum beams in the coherent superposition of opposite helicities. Furthermore, we developed the Duke FEL as the photon driver for producing Compton gamma rays at the High Intensity Gamma-ray Source (HIGS), the highest flux Compton gamma-ray source in the world for nuclear research. The capabilities developed for the Duke FEL have enabled the development of new gamma-ray capabilities for a wide range of nuclear physics research, from nuclear astrophysics to few-nucleon reaction and nuclear structure research, to fundamental symmetry studies.

This work is partially supported by the US DoE grant #DE-FG02-97ER41033

Speaker: Ying Wu (Duke University)

25 A one-dimensional model of superradiant spike saturation

In this talk I will discuss some of the colleagues I have had the pleasure of working with and some of the research we have carried out in the field of Free Electron Lasers. In particular, I will focus on superradiance and give a 1-dimensional description of the highly non-linear saturation process of a single superradiant spike. The sub-wavelength pulse structures that evolve and the very high powers involved require simulations using an un-averaged FEL simulation code. The scaling of the superradiant spike saturation process is shown to be in good agreement with a simple hypothesis that saturation occurs when an initially resonant electron propagating through the pulse can lose sufficient energy within one half of an undulator period to propagate a full resonant radiation wavelength relative to the pulse. The saturation process is demonstrated using animations of the simulations.

Speaker: Brian McNeil (University of Strathclyde)

26 Some techniques for radiation generation, diagnostics and simulation I find interesting

Speaker: Svitozar Serkez (European X-Ray Free-Electron Laser)

27 Beam Shaping for Advanced Operation Modes in Free-Electron Lasers

In this prize talk, I will present recent advancements in beam shaping techniques that have enabled advanced operational modes in free-electron lasers (FELs). The discussion will cover key developments, including beam current profile shaping for LCLS and LCLS-II, and the "two-bunch" scheme for generating two-color FELs and THz-FEL pulses. Additionally, I will delve into the methods of cathode shaping for attosecond FEL generation. The talk will conclude with an exploration of the new challenges faced in superconducting-based FEL facilities.

Speaker: Zhen Zhang (SLAC National Accelerator Laboratory)

28 Novel operations of high-repetition-rate X-ray free-electron lasers.

The development of X-ray free-electron lasers (XFELs) marks a major leap forward in exploring matter at atomic and subatomic scales. Recently, high-repetition-rate XFELs based on superconducting linacs have emerged as a leading frontier in the field, with the potential to greatly expand the range of FEL applications. However, these advancements also present substantial challenges in machine design and operation. In this talk, I will present our recent explorations of externally seeded FELs, attosecond XFELs, and multi-beam-energy operations at high repetition rates.

Speaker: Jiawei Yan (European X-Ray Free-Electron Laser)

Tuesday 20 August

SASE-FEL: Session-3

Convener: Gianluca Geloni (European X-Ray Free-Electron Laser)

29 Commissioning and operation of a CW X-ray SASE FEL at SLAC

LCLS-2 is the first FEL operating in CW mode. During the first stage commissioning including injector, linac and FEL, we successfully achieved the initial goal in the summer of 2023, with operating the beam in the linac at 93kHz, and demonstrated FEL at 1kHz. Since then, operation-based commissioning for performance improvement has started. We will present the latest commissioning progress, covering topics like machine performance improvement and benchmark, rate ramping up and special short pulse mode.

Speaker: Yuantao Ding (SLAC National Accelerator Laboratory)

30 FLASH2020+: transformation of FLASH1 to a high repetition rate externally seeded FEL for users

The FLASH2020+ project is driving the current transformation of the FLASH facility. The next step is the transition of FLASH1 from a SASE to a high repetition rate externally seeded FEL beamline and will significantly expand the facilities capabilities. The generation of near transform limited pulses at superior spectral and pulse energy stability at full FLASH repetition rate of 1 MHz burst together with variable polarization will continue to broaden the facilities user community. In a 14 month lasting shutdown, that started in June 2024, the complete FLASH1 beamline will be replaced by new hardware tailored to external seeding. By upgrading the linear accelerator in a preceding shutdown the necessary foundation regarding electron beam properties has already been enabled and is available to the benefit current user experiments. In this contribution we report on the FLASH2020+ incorporated existing and upcoming alterations to the FLASH facility as well as to project progress with respect to the current as well as following near- and midterm installations.

Speaker: Lucas Schaper (Deutsches Elektronen-Synchrotron DESY)

31 First experience of using corrugated structures at high repetition-rate x-ray free-electron lasers

Wakefield structures using metallic corrugated plates have demonstrated their capabilities in high-energy electron beam manipulation and diagnostics at low repetition-rate X-ray free-electron laser (XFEL) facilities. There has been a significant effort recently to utilize such devices at a MHz repetition rate. Here we first give an overview of the theoretical modeling and experimental achievements with corrugated structures in XFELs, then discuss the challenges in high repetition-rate XFELs and present our first experience of using such devices at the European XFEL. We show the generation of femtosecond hard X-ray FEL pulses with a few spectral spikes and operation up to a few hundred pulses at MHz repetition rate.

Speaker: Weilun Qin (Deutsches Elektronen-Synchrotron DESY)

32 Terawatt-attosecond hard X-ray free-electron laser pulse generation at the European XFEL

Despite significant advancements in generating attosecond pulses in the extreme ultraviolet and soft X-ray regimes, achieving high-power attosecond pulses at Ångstrom wavelengths has remained a considerable challenge. The generation of intense attosecond hard X-ray pulses is pivotal for probing the structural and electronic dynamics of matter with unprecedented precision. Recently, we proposed and experimentally demonstrated a new method to generate terawatt-attosecond pulses at Ångstrom wavelengths using X-ray free-electron lasers (FEL). This presentation will detail our recent experiments at the European XFEL, showcasing the successful production of stable high-power single-mode hard X-ray FEL pulses.

Speaker: Jiawei Yan (European X-Ray Free-Electron Laser)

10:35 Coffee break

Seeded FEL: Session-4

Convener: Francesca Curbis (Lund University)

33 Better a chicane today than an undulator tomorrow?

Sometimes free-electron laser beamlines cannot be built under the “infinite money approximation”. This also holds true for FLASH2020+. Here, owning to resource optimization, the amount of APPLE-III type radiators to be installed in the current 2024/25 shutdown has been reduced from 11 to 6+3 re-used planar undulators from the sFLASH/Xseed experiments. While adding undulators is possible in future upgrades a key question is which strategy proves best.
In the contribution we consider a cascaded scheme that takes advantage of the lattice, by filling some of the empty spaces with chicanes instead of undulators. We show that this approach is beneficial both in terms of flexibility and performance for the Users.

Speaker: Eugenio Ferrari (Deutsches Elektronen-Synchrotron DESY)

34 Conversion to EEHG of the FEL-1 line at FERMI: commissioning results and first experience with user’s operations

The conversion of the FERMI FEL-1 line to Echo Enabled Harmonic Generation (EEHG) has been successfully completed, and the new scheme is currently undergoing commissioning and testing for user operation. This achievement marks a significant milestone in the facility's upgrade process, aimed at extending the covered spectral range to include the entire water window and beyond. Thanks to this upgrade, the FEL-1 line can now deliver fully coherent ultra-short pulses with gigawatt power levels down to a wavelength of 10 nm. This enhancement is accompanied by improvements in spectral quality and stability, characteristics unique to the EEHG process.
The updated FERMI FEL-1 is the first user facility operating in the spectral range of 20-10 nm utilizing the EEHG scheme. It will also serve as an ideal test bench for conducting new machine studies in anticipation of future developments. In this report, we present the results obtained during the commissioning phase and the initial user experiments.

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

35 Progress with seeding ATHOS, the soft X-ray FEL at SwissFEL

The soft X-ray FEL beamline ATHOS at SwissFEL has been upgraded for Echo-enabled Harmonic Generation (EEHG) to provide coherent signals up to 1 keV photon energies. This presentation reports on the current demonstration of the EEHG principle, its challenges to implement it at a beam energy of 3 GeV and the outlook on studies to further improve the electron beam quality, namely reducing the intrinsic energy spread.

Speaker: Sven Reiche (Paul Scherrer Institute)

36 High-repetition-rate seeded free-electron laser enhanced by self-modulation

High-repetition-rate and fully coherent soft X-ray free-electron lasers (FELs) are crucial for time-resolved spectroscopic experiments. However, it is difficult to operate seeded FELs at a high repetition rate due to the limitations of present state-of-the-art laser systems. We have proposed and carried out a series of experimental studies on self-modulation based seeded FELs at the Shanghai Soft X-ray Free Electron Laser (SXFEL) facility to significantly relax the requirements for seed lasers. Our findings demonstrate that this new method not only decreases seed laser requirements but could also largely enhance the harmonic up-conversion efficiency, paving the way for the realization of high-repetition-rate, fully coherent soft X-ray FELs. In this presentation, we will summarize our progress* in self-modulation based seeded FELs, including harmonic self-modulation and ultrashort ultrahigh harmonic pulse generation.

Speaker: Hanxiang Yang (Shanghai Institute of Applied Physics)

12:50 Lunch break

37 RadiaSoft: Computation, Controls & Machine Learning for Free Electron Lasers

RadiaSoft applies machine learning techniques to design particle accelerators, radiation sources, and real-time control systems. Recent projects and new ideas are discussed, with an emphasis on free electron lasers and emerging applications, such as ptychography.

FEL oscilators & IR-FEL: Session-5

Convener: Dr Aliaksei Halavanau (SLAC National Accelerator Laboratory)

38 The cavity based FEL project at the European XFEL

We present the first commissioning results of the XFEL laser oscillator (XFELO) demonstrator project, a joint European XFEL and DESY effort. XFELOs promise unprecedented coherence, stability and Brilliance in the hard X-ray regime. Their successful realization would mean a leap forward for the
field of FELs, opening new experimental opportunities [1] and facilitating the notoriously demanding experiments at FEL facilities due to the increase in reproducibility. The demonstrator setup constructed at the European XFEL facility aims to realize the first ever true multi-bunch XFELO by achieving seeding with a bandwidth of 0.02eV at a fixed X-ray photon energy of 7 keV. Our demonstrator project shall showcase the capability to produce XFEL photon pulses with mJ-level pulse energies and high pulse-to-pulse stability. The scope of the project with its inherent challenges as well as its current status shall be presented in this contribution.

Speaker: Patrick Rauer (Deutsches Elektronen-Synchrotron DESY)

39 Progress towards construction of cavity-based XFEL at SLAC

Cavity-based free-electron lasers (CBXFELs) offer the potential to significantly enhance the stability and coherence of FELs. The CBXFEL project is a collaborative effort between SLAC, Argonne, and RIKEN, focused on constructing a 65-meter rectangular X-ray cavity at the LCLS. The primary goal is to demonstrate low-loss cavity ring-down and, ultimately, achieve two-pass FEL gain. This talk presents recent simulation and measurement results that support these objectives, including efforts related to FEL gain and the delivery of two electron bunches separated by 624 RF buckets. Additionally, we report on progress in testing and installing X-ray optics and diagnostics components, the Bragg mirror nano-positioning and diagnostics stages, and the X-ray return line.

Speaker: Dr Aliaksei Halavanau (SLAC National Accelerator Laboratory)

40 Active Q-switched X-Ray Regenerative Amplifier Free-Electron Laser

Despite tremendous progress in x-ray free-electron laser science over the last decade, future applications still demand fully coherent, stable X rays that have not been demonstrated in existing X-ray FEL facilities. Cavity-based x-ray free electron lasers (CBXFELs) such as the x-ray regenerative amplifier FEL (XRAFEL) and the XFEL oscillator (XFELO) are poised to revolutionize the landscape. In this presentation, I'll introduce some innovative schemes for CBXFELs. First I will talk about the active Q-switched XRAFEL. By using simple electron-beam phase space manipulation, we show this scheme is flexible in controlling the x-ray cavity quality factor Q and hence the output radiation. This section encompasses numerical simulations and planned experiments. In the second part, I will discuss a novel concept staging XRAFEL and XFELO to generate hard X-ray pulses with very narrow bandwidth (~meV) while maintaining high pulse energy (~mJ).

Speaker: Jingyi Tang (Stanford University)

41 First operation of a two-color mode in a dual-oscillator infrared free-electron laser

Since 2013 the infrared FEL at the Fritz Haber Institute (FHI FEL) has been providing intense, pulsed mid-infrared (MIR) radiation continuously tunable from <3 μm to >50 μm for in-house users. In 2023 an additional short-Rayleigh-range far-infrared (FIR) FEL was commissioned lasing from <5 μm to >170 μm. In addition, a 500 MHz kicker cavity has been installed downstream of the electron accelerator. It deflects the electron bunches alternatingly left and right by an angle of ±2° thereby splitting the high-repetition-rate (1 GHz) electron bunch train into two bunch trains of half the repetition rate each; one is steered to the MIR FEL and the other one to the FIR FEL. The wavelengths in both FEL’s can be tuned independently over wide ranges of up to a factor of four by undulator gap variation. In addition, 2-color operation is also available at reduced repetition rates (e.g. 55.5 MHz of both MIR and FIR pulses). Furthermore, two additional small dipole magnets upfront and behind the kicker cavity permit conventional single-color operation of either the MIR or the FIR FEL when the 500 MHz kicker field is off. Regular user operation in 2-color mode is scheduled to start in 2024.

Speaker: Wieland Schöllkopf (Fritz Haber Institute of the Max Planck Society)

Poster session: Coffee and poster session

42 THz Superradiant Waveguide FEL, Design Considerations

Theoretical analysis and design considerations of a THz superradiant FEL are presented. The superradiant emission scales like the number of electrons N squared, enhancing spontaneous emission by a factor N. The longitudinal dimension (duration) of the beam diminishes the excited mode energy at high frequencies through the bunching coefficient.
We demonstrate the design considerations considering the parameters of the ORGAD compact (6 MeV) hybrid-accelerator of Ariel and Tel Aviv universities. The accelerator length in this setup is only 60 cm long and the undulator is 80 cm long. For this configuration it is possible to attain appreciable levels of radiation energy in the low frequency branch of the dispersion relation (below 1 THz) and in the high frequency branch (3.2-3.4 THz).
The analytical calculations of radiation excitation compare well with numerical computations using UCLA GPT-FEL including space-charge effects. The space charge effect can be mitigated by using a ribbon beam configuration. When the space-charge effect is negligible, the bunching parameter and the high frequency THz emission limits are still limited by the intrinsic energy spread of the gun.

Speaker: Amir Weinberg (Tel Aviv University)

43 Noise-based modelling of partially coherent radiation with an arbitrary Wigner function

We present an algorithm for modelling single instances of partially coherent radiation using their Wigner distribution. This algorithm is based on an approach for simulating Gaussian random fields, where randomly generated noise is constrained by the effective radiation size in the conjugate Fourier domains. This approach, utilizing Fourier transform methods, significantly enhances computational efficiency compared to Monte Carlo methods. The final result is a single stochastic instance of the field. With the proposed algorithm, one can simulate a variety of sources including but not limited to synchrotron and linear-regime free-electron laser radiation, as well as thermal light sources. The resulting field can be propagated through optical elements. We confirmed analytically that the results of our algorithm follow the correct ensemble-averaged intensities and correlation functions. In addition, we present simulations of use cases such as a double slit experiment, as well as free-electron laser radiation in a linear regime. Finally, the algorithms provide illustrative examples ideal for teaching the concept of coherence.

Speaker: Andrei Trebushinin (European X-Ray Free-Electron Laser)

44 Impact of local bunching factor on the performance of THz FEL

Experimental studies on THz FELs at the Photo Injector Test facility at DESY in Zeuthen (PITZ) have revealed that the THz radiation pulse energy is significantly higher than predictions by Genesis1.3 simulation using the standard shot noice model. To understand the underlying mechanism, the bunching factors of individual slices with a length of the resonant wavelength (local bunching factors), have been calculated considering the actual current profile within each slice. It is found that the current profile contributes to the bunching factors several orders of magnitude higher than that from the shot noise of a uniform current profile within each slice. This effect is pronounced near the half-maximum of the beam current profile, where the current gradient is particularly strong, while the current value remains significant. Meanwhile, the bunching phase along the slice also becomes stable other than randomly distributed. The current profiles of electron beams from start-to-end simulations have been used to introduce a local bunching factor for each slice in the Genesis1.3 simulations, and the results will be presented in this paper.

Speaker: Mikhail Krasilnikov (Deutsches Elektronen-Synchrotron DESY)

45 High-resolution synchrotron radiation topography characterization of diamond monochromator for hard X-ray free electron laser

In this paper, the results of high-resolution X-ray topography characterization of monocrystal diamond plate with (100) crystal surface orientation used as high-quality monochromator of high-heat-load free electron lasers are reported. The monocrystal diamond plate was grown using high-pressure high-temperature method and fabricated by laser-cutting. The intrinsic crystal quality of the diamond surface was studied using sequential X-ray diffraction method of synchrotron radiation in weak-dispersive and non-dispersive configuration and the rocking-curve topography of the lattice was obtained. The variations of the rocking-curve width and peak position measured with 7.4 μm spatial resolution and ~10-7 energy resolution over a 0.5 mm×0.5 mm selected region was found to be less than 0.5 μrad, which was suitable for applications in wavefront-preserving high-heat-load crystal optics.

Speaker: Yixuan Liu (Shanghai Institute of Applied Physics)

46 Transition Layer of the Relativisyic Strophotron

The Klein – Gordon equation of motion in the transition layer of a free-electron laser of the relativistic strophotron type is studied. The condition of suddenly switching-on interaction is found: a smallness of the transition layer at the entrance compared to the distance traveled by the electron in longitudinal direction in one period of transverse oscillations in strophotron. The results correspond to the results of classical description. It is shown that in an electric strophotron, under the assumption of a suddenly switching-on interaction, the transverse energy of an electron at the moment undergoes a jump. However, the total energy is conserved, since at the same moment the longitudinal energy of the electron has the same jump with opposite sign.

Speaker: Koryun Oganesyan (A. Alikhanyan National Laboratory)

47 Installation of pulsed quadrupole magnets for independent envelope tuning of multiple beamlines

The transverse beam envelope matching inside undulators is important for XFELs. SACLA operates two XFEL beamlines, and the beam energy and longitudinal compression are adjusted bunch-by-bunch in accordance with the XFEL parameters of each beamline. Since transverse focusing strengths depend on the beam energy and RF phases, the transverse envelope varies from bunch to bunch, and mismatch occurs to the undulator beamlines. Up to now, the beam envelopes had been adjusted and re-matched after a beam switchyard, but it takes time to properly adjust the beam optics of all beamlines. In order to facilitate this procedure, 15 pulsed quadrupole magnets have been installed in the downstream section of the linear accelerator. With these magnets, the electron bunch energy can be freely changed between 5 GeV and 8 GeV maintaining the identical transverse envelopes for all beamlines. Another 6 pulsed quadrupole magnets are also introduced for the bunches injected to SPring-8 and fine optics tuning. In this presentation, the strategy of introducing pulsed quadrupole magnets and the results of beam tuning will be presented.

Speaker: Toru Hara (RIKEN SPring-8 Center)

48 Analysis of temporal and spatial coherence of the radiation at FLASH FEL beamlines

We perform analysis of a typical practical case of FLASH operation with the number of radiation modes in the range pulse M ~ 4 ... 8. Standard tuning procedure (for maximum pulse energy at full undulator length) results in visible energy chirp along the electron bunch such that spectrum bandwidth is increased to about 1% fwhm, visibly wider than natural FEL bandwidth. These features were taken in the current consideration. Simulation results with numerical simulation code FAST are illustrated for the radiation wavelength 18 nm, 13.5 nm and 8 nm. As it was predicted earlier, maximum value for the degree of transverse coherence at FLASH1 is in the range of 0.8, and it is a bit better for the case of FLASH2. Actual value is dependent on the stage of amplification process, and can be visibly lower in the deep post saturation regime. Several experiments on measurements of the degree of transverse coherence have been performed at FLASH1 and FLASH2 so far, and we believe that the results presented here will be useful for analysis of experimental data. From our side we compare simulation results with coherence characteristics measured with statistical methods*, and find good agreement.

Speaker: Dr Mikhail Yurkov (Deutsches Elektronen-Synchrotron DESY)

49 Lasing of the elliptically polarized undulators in SXFEL

Shanghai soft X-ray free electron laser facility(SXFEL) is the first coherent X-ray light source in China with a wavelength covering the water window. With the help of a pair of elliptically polarized undulators(EPU) in the seeding line, the output laser can be switched between linear polarization and circular polarization, meeting the needs of different users. After tuning the planar undulators upstream and adjusting the gap and phase shift of the EPUs, FEL pulses at the wavelength of 6.75nm with both polarization states can be observed after the EPUs. With a proper reverse taper on these planar undulators, the intensity of the circularly polarized radiation is found much higher than linearly polarized radiation. Vertically polarized radiation can also be observed in a similar way. The pulse energy and the polarization purity are left to be measured.

Speaker: Zhangfeng Gao (Shanghai Synchrotron Radiation Facility)

50 First spectral measurements of single-pass high-gain THz FEL at PITZ

The Photo Injector Test facility at DESY in Zeuthen (PITZ) develops a prototype tunable high-power THz source for pump-probe experiments at the European XFEL. A single-pass high gain THz free-electron laser (FEL) was realized at PITZ. A THz beam line based on a strongly focusing planar LCLS-I undulator is used for proof-of-principle experiments. The first lasing at the center frequency of ~3 THz demonstrated high gain with radiation pulse energies exceeding 100 microjoules. Electron beams with bunch charge of 2-3 nC and mean beam momentum of ~17 MeV/c were used to generate high-energy THz pulses. Recently, a narrow-band spectrum has been measured using a Fourier Transform Infra-red (FTIR) spectrometer based on a reflective lamellar grating. The experimental results, including the gain curves as well as spectral properties along them, are presented.

Speaker: Mikhail Krasilnikov (Deutsches Elektronen-Synchrotron DESY)

51 Status of FAST-XPD: Photon data base for the European XFEL

XPD project - photon data base for the European XFEL is a joint effort of DESY and European XFEL*. The project has been launched in 2009, and is in use by users since 2012. The goal of the XPD project is to provide users with full temporal, spatial, and spectral patterns of the radiation fields for different regimes of the European XFEL operation. Tracing of these fields with advanced user simulation tools from the source (undulator) via photon beam line (mirrors, gratings, etc) with subsequent simulation of user experiment (interaction with a sample, simulation of physical processes, simulation of detection process of related debris like photons, electrons, and ions) is of key importance for planning future user experiments. Full 3D maps of the radiation fields are stored in the mass storage service dCache (disc cache system) at DESY. A web application (https://in.xfel.eu/fastxpd) allows to pick up a selected photon pulse data in the HDF5 format for any given XFEL operation mode (electron energy, charge/photon pulse duration, active undulator range etc) suitable for statistical analysis, propagating through the optical system, interaction with the sample, etc.

Speaker: Dr Mikhail Yurkov (Deutsches Elektronen-Synchrotron DESY)

52 3D theory of micro-bunching instabilities driven by space-charge

Microbunching - or short-wavelength – instabilities are well-known for drastic reduction of the beam quality, its filamentation and strong amplification of the noise in a beam. Space charge and coherent synchrotron radiation (CSR) are the leading causes for such instability. I will present rigorous 3D theory of such instabilities driven by the space-charge forces and define the condition when our theory is applicable for an arbitrary accelerator system with 3D coupling. Finally, I will derive a linear integral equation describing such instability and identify conditions it can be reduced to an ordinary second order differential equation. I will illustrate this theory with a few examples of computer simulations. This work is extension of the theory developed in [1].

Speaker: Vladimir Litvinenko (Stony Brook University)

53 A Development of Nanosecond Pulse Power Modules

A nanosecond multi-bunch mode in FEL extends the laser capabilities. There are several critical components to be added to a baseline of the multi bunch FEL (for example, LCLS/LCLS-II). One of the components is the system that properly control the individual bunch orbit in the linac. The individual bunch orbit control based on the RF amplitude and phase modulation is limited by the bandwidth. Powerful and fast solid-state switches driving a transmission line kicker are needed to breakthrough these limitations. They must be stable working in the pulse mode and capable of generating power from MW to GW and rise/fall time from 10x psec to 100 nsec. The pulse dynamic processes in ferromagnetic and semiconducting materials can support a 1 MW/ns switching speed stably, reliable, and efficiently. We will present developments and our results of such modules.

Speaker: Anatoly Krasnykh (SLAC National Accelerator Laboratory)

54 A HIGH GAIN HARMONIC GENERATION FREE ELECTRON LASER DRIVEN BY A COMPACT LASER PLASMA BEAMLINE

Laser-wake field accelerators (LWFAs) are potential candidates to produce intense relativistic electron beams to drive compact free electron lasers (FELs) in VUV and X-ray regions. The High-Field Physics and Ultrafast Technology Laboratory at National Central University (NCU) is actively developing a compact LWFA-based high gain harmonic generation (HGHG) FEL aim at coherent extreme ultraviolet (EUV) radiation. However, the high divergence and excessive energy spread of the LWFA electron beam increase the difficulties in both beam transportation and radiation power gain. Here we present a start-to-end simulation to study the feasibility of a compact beamline based on the experimental data of the NCU LWFA group with electron energy of 250MeV. Numerical results indicate that a 4th harmonic radiation gain at 66.5nm wavelength can be obtained with excessive energy spread. Giving a strongly monochromatic beam with a few percent bandwidth or smaller within a shorter saturation length compare to the SASE results.

Speaker: Shan-You Teng (National Central Univeristy)

55 A high-energy XFEL scheme based on the CEPC linear accelerator

The Circular Electron Positron Collider (CEPC) is a large-scale scientific project consisting of three accelerators: a 30 GeV Linac, a full-energy booster, and a collider operating at four different energy modes. The CEPC Linac is a normal-conducting linear accelerator with an energy of 30 GeV and repetition frequency of 100 Hz. To fully utilize the potential of the Linac, during the injection interval, it can be function as a high-energy XFEL that produces photon energies greater than 50 keV. To achieve this, a new injector system based on a C-band RF electron gun is added to the linear accelerator, along with two bunch compression sections and beamlines. This paper proposed the high-energy XFEL scheme and detailed design.

Speaker: Dr Cai Meng (Chinese Academy of Sciences)

56 A Method for Passive Streaker LPS Reconstruction

Understanding the electron beam distribution in the longitudinal phase space (LPS) is crucial for Free Electron Laser (FEL) facilities. Conventionally, LPS diagnostics utilize radio frequency (RF) deflecting structures to streak the electron beam transversely, mapping the longitudinal bunch distribution onto a transverse plane for observation. However, RF structures are complex and costly, especially for high-energy machines like the European XFEL. Wakefield structures have emerged as a promising alternative, offering simplicity in construction and minimal maintenance costs. However, they suffer from nonlinear streaking, requiring image reconstruction for LPS distribution. Several iterative algorithms have been developed for LPS reconstruction using passive wakefield streakers in recent years. This paper proposes a simple, computationally efficient method tailored for cases with known beam current profiles.

Speaker: Sergey Tomin (Deutsches Elektronen-Synchrotron DESY)

57 A Pulsed-Optical Timing Distribution System Featuring Reconfigurable Synchronization of 40 Remote Locations

State-of-the-art XFELs generate bright, ultrashort X-ray pulses enabling cutting-edge sub-femtosecond resolution time-resolved pump-probe experiments. Key requirement is an utmost precise timing distribution and synchronization between pulsed lasers and microwave sources distributed across a km-scale facility. Fiber-based pulsed optical timing distribution systems (TDS) are an ideal solution delivering sub-fs timing precision over km-range. A pulsed-optical TDS was deployed at the SLAC National Accelerator Laboratory as part of the overall timing and synchronization system of the Linac Coherent Light Source II** and was recently upgraded to a high-link-capacity large-scale turnkey system. The TDS has 7 stabilized timing links, each with a two-stage bidirectional EDFA and an out-of-loop delay line providing a high precision timing signal to 40 remote locations with adjustable time delay. An 8x40 fiber optic MEMS switch is deployed to efficiently re-route the timing signal to any remote location without the need for physical intervention by an operator. We present the technical design and out-of-loop timing characterization of the TDS achieving an RMS timing jitter of about 1 fs.

Speaker: Mr Max Rückmann (Cycle (Germany))

58 A RESONATOR FOR A HIGH-GAIN X-RAY FEL OSCILLATOR

Current x-ray free-electron lasers (XFELs) rely on single-pass amplification starting from noise or a seed and require long undulator lines to reach saturation. An oscillator configuration allows for multi-pass amplification, using shorter undulator lines, and the filtering properties of both the resonator and the gain will improve the transverse and longitudinal coherence. Here we consider a four-mirror resonator comprising of four identical diamond Bragg reflectors, reflecting on 45 degrees, and two compound reflecting lenses to create a stable resonator for an XFEL. As an example, we use the parameters of LCLS II to analysis the resonator using OPC and OPC-MINERVA for initial study of the XFEL oscillator performance.

Speaker: Dr Peter van der Slot

59 A review of space charge models for high brightness electron injectors

Achieving lower beam emittance through effective space charge compensation is critical for enhancing beam brightness and luminosity in accelerator physics. Traditional solenoid-based methods are limited to linear compensation and often introduce chromatic aberrations. Our study aims to develop novel techniques that compensate for both linear and non-linear space charge forces. As a first step, we reviewed and scrutinised existing space charge models. We employ Particle-In-Cell (PIC) simulations, Fast Fourier Transform (FFT) methods, and the OPAL framework to model electric fields and implement compensation schemes. Preliminary results demonstrate that our models align well with overall trends when compared to OPAL, an open-source tracking code. This research holds promise for applications in particle colliders, X-ray free electron lasers (FELs), and medical treatments, potentially improving beam quality in these advanced technologies.

Speaker: Oliver Betteridge (Cockcroft Institute)

60 A Systematic Approach to Obtain Reliable Wake Functions from CST-calculated Wake Potentials for Multi-particle Simulations

Bunch compressors are frequently employed to boost beam brightness in accelerator facilities such as e+e-
linear colliders and free electron lasers (FELs). However, the energy chirp or correlated energy spread intro-
duced into the beam by the chirper linac remained after bunch compression is undesirable in some applica-
tions. Wakefield structures known as dechirpers are designed to manipulate electron distribution
in longitudinal phase space to remove the associated energy spread of the compressed bunches. In this re-
port, we present our studies by using planar dielectric-lined waveguide (DLW) as a compact and effective
dechirper for removing of drive beam energy chirp in NSRRC EUV FEL facility and the development of a sys-
tematic approach to obtain an appropriate wake function for simulation of multi-particle dynamics from the
wake potentials of a general structure calculated by commercially available codes-CST. The wake function of
dechirper extracted by proposed process was consistent with analytic wake function. This wake function of
dechirper combined with ELEGANT or ImpactT shown the 45keV/m of energy spread will be removed after
the operation of round DLW dechirper.

Speaker: Dr Wei-Yuan Chiang (National Synchrotron Radiation Research Center)

61 A Workflow for End-to-end Modeling of FELs

SLAC and RadiaSoft are partnering to provide integration support for two parallel workflows that support end-to-end modeling and machine learning integration for accelerators. LUME, Light Source Unified Modeling Environment, has been developed by SLAC to facilitate end-to-end modeling for machine tuning and optimization. This workflow includes the integration of machine learning surrogate models. In parallel, RadiaSoft is developing a workflow, Omega, for chaining simulations developed using different Sirepo applications. This tool allows users to import simulations built in Sirepo and connect them into an end-to-end simulation. Our collaboration is focused on integrating these two workflows in order to provide the community with a unified toolbox for online modeling of light sources. Our poster will provide an overview of the software tools and showcase their use for modeling FELs.

Speaker: Christopher Hall (RadiaSoft (United States))

62 Active Control of the Energy Chirp of a Relativistic Electron Beam

Many applications in modern accelerators require short electron bunches with high peak current. To achieve this high current, a large energy chirp must be imposed on the bunch so that the electrons are compressed when they pass through a chicane. In existing linear accelerators, this energy chirp is imposed by accelerating the beam off-crest from the peak fields of the RF cavities, thereby reducing the average accelerating gradient. It is a cost-inefficient solution that results in an increase in the facility footprint and reduced beam quality A promising solution, known as the Transverse Deflecting Cavity Based Chirper (TCBC) [1], presents an alternative method for actively imposing a substantial energy chirp onto an electron beam in an accelerator, without the need for off-crest acceleration. The TCBC consists of 3 transverse deflecting cavities, which together impose an energy chirp while canceling out the transverse deflection. An experiment is being developed to validate this concept at the Argonne Wakefield Accelerator (AWA) facility. Here, we explain the concept, show preliminary simulations, and report on progress related to the implementation of the experiment at AWA.

Speaker: Nikolai Yampolsky (Los Alamos National Laboratory)

63 Adaptive beam current feedback system at the Shanghai Soft X-ray FEL facility

Despite the good short-term performance of electron beam control, the long-term stability and control accuracy of the particle beams still limit the performance and application range of the FEL due to the thousands of coupling parameters as well as the uncertainty and time-varying nature of the electron distribution generated by the photocathode. Many challenges are still faced when confronted with the precise control of the particle beams in the long term. This paper is to present our research efforts on the adaptive beam feedback system and to show how the system performs in the SXFEL facility.

Speaker: Hang Luo (Shanghai Advanced Research Institute)

64 Advanced Hard X-ray Self-seeding operation schemes at the European XFEL

As a scheme to increase longitudinal coherence in SASE based FELs, hard X-ray self-seeding (HXRSS) has demonstrated its capability of delivering above 1mJ/eV peak pulse intensity in the hard X-ray range. To further increase its operational range and tunability, several advanced HXRSS operation schemes are under investigation at the European XFEL. The studies on 2nd harmonic generation, two color HXRSS and phase locked HXRSS are presented here.

Speaker: Shan Liu (Deutsches Elektronen-Synchrotron DESY)

65 Analysis of a decade of pioneering user operation at the FERMI Free-Electron Laser facility

This study presents a comprehensive analysis of user operations at the FERMI free-electron laser (FEL) facility spanning a period of 10 years. Located in Trieste, Italy, FERMI is the first seeded FEL light source dedicated to users, delivering since 2011 high quality extreme ultraviolet and soft X-ray beams for a wide range of scientific experiments based on diffraction, scattering and spectroscopic techniques. The highly coherent, ultra-short, ultra-bright, almost Fourier-transform limited FEL light pulses can be synchronized to an external laser with unprecedented precision and controlled in phase, coherence, wavelength(s), duration, line-width and polarization, all with remarkable reproducibility and stability. The analysis of the FEL parameters made available to the FERMI user community over the years demonstrates the extensive exploitation of its unique potential and the evolution of the FEL operating modes, highlighting FERMI's pioneering role in cutting-edge scientific research, as well as its commitment to continuous innovation and international collaboration.

Speaker: Laura Badano (Elettra-Sincrotrone Trieste S.C.p.A.)

66 Analytical study of wakefields of short beams with large transverse size in planar corrugated structures

Planar corrugated wakefield structures are widely used in X-ray FEL facilities for passive dechirping and transverse deflecting. The wakefields of these structures have previously been analytically studied, assuming the electron beam is short and has a small transverse beam size. However, the transverse beam size can be notably large in some practical cases, for example, when a slotted foil is inserted upstream. We present analytical formulas, based on the existing wakefield theory, that are valid also for large transverse charge distributions. We find good agreement with ECHO2D simulations.

Speaker: Wenxiang Hu (Paul Scherrer Institute)

67 Automatic electron optics compensation for FODO lattices affectcted by undulator focusing

Within the FLASH 2020+ upgrade project of the FLASH facility in Hamburg, the original FLASH beamline (FLASH1) will be completely remodelled and equipped with variable gap APPLE-III undulators, capable of controlling the polarization state of the FEL radiation.In the course of enhancing the operability of FLASH, the influence of the variable undulator focusing in both planes will be compensated by an automatic correction algorithm.The aim is to maintain a stable optics while minimizing optical changes.
In this article, we discuss the basic stability concepts of a ``FUDU'' (focussing quadrupole, undulator, defocussing quadrupole, undulator) lattice, and compare several analytical approximations and numerical approaches.

Speaker: Mathias Vogt (Deutsches Elektronen-Synchrotron DESY)

68 Bayesian optimization for generating attosecond X-ray FEL pulses carrying orbital angular momentum

Attosecond X-ray pulses carrying orbital angular momentum (OAM) are as a powerful tool for investigating various ultrafast phenomena, offering unique insights into the dynamics of matter at the atomic and molecular level. The self-seeded FEL with OAM (SSOAM) method providing a new way to produce attosecond X-ray vortices pulses with high intensity. In this study, we present our recent progress on optimizing the generation of high-power attosecond X-ray vortices using multi-objective Bayesian optimization.

Speaker: Chenzhi Xu (Shanghai Institute of Applied Physics)

69 Beam Based Undulator Trajectory Alignment at European XFEL

Precise alignment of the undulator trajectory onto a straight line is a per-requisite for high fidelity SASE operation, which in turn also enables non-standard operation modes like self-seeding or two-color operation. At European XFEL electron and photon beam based method are combined with a step-by-step performance based optimization to ensure that the trajectory is on a straight line over most of the length of the about 250 m long undulator system.

Speaker: Matthias Scholz (Deutsches Elektronen-Synchrotron DESY)

70 Beam dynamics study on low-emittance electron beams generated via a combined transverse and temporal photocathode laser shaping approach at the European XFEL

Low-emittance electron beams play an essential role in improving the machine performance and extending the capabilities of existing free-electron lasers. The generation of such high-quality electron bunches relies on an overall optimization of the photoinjector, and most importantly, on the desired properties of the cathode drive laser. Appropriate shaping of the drive laser, namely, controlling the laser intensity distribution both temporally and spatially, has emerged as a promising technique to reduce the emittance of electron beams. A strategic combination of two-dimensional (2D) transverse and one-dimensional (1D) temporal shaping of the drive laser pulse may efficiently bring down the overall bunch emittance very close to its fundamental limit. In this work, we present beam dynamics simulations conducted for the European XFEL photoinjector so as to optimize the transverse projected and sliced emittance of the bunch under various transverse and longitudinal laser pulse configurations. The obtained results confirm the impact of lengthening the cathode drive laser pulse on both the projected and sliced emittance of the optimized bunches, and demonstrate the advantage of a longitudinal flat-top shaping over a conventional Gaussian pulse. It is also shown that a combined 2D spatial and 1D temporal shaping technique can minimize the central slice emittance of the bunch to the level of the cathode thermal emittance.

Speaker: Meng Cai (Shanghai Institute of Applied Physics)

71 Beam energy measurement at the European XFEL with high-performance trajectory fitting

Knowledge of the beam energy is important for practically all particle accelerators. This contribution presents the method used at the European XFEL since more than 8 years. It relies on high-performance trajectory fitting of beam position monitor data against an online model of the accelerator lattice. Unlike most other methods, it can be applied to virtually any section of the lattice with sufficient dispersion without the need for manual adaptation. Our implementation is fast enough to measure the energy of many thousands of bunches per second in realtime on conventional x86 server hardware shared with other processes. A short outline of the technique is presented and illustrated with data from the accelerator.

Speaker: Lars Froehlich (Deutsches Elektronen-Synchrotron DESY)

72 Bunch length control with a beam energy control unit

The bunch-to-bunch energy control of the electron beam is crucial in the continuous wave X-ray free-electron lasers facility(XFEL). Recently, a delay system based on double bend achromat (DBA) was proposed for the Shanghai High-Repetition-Rate XFEL and Extreme Light Facility(SHINE) to achieve this goal. On this basis, we further optimize this structure to realize the bunch length control while maintaining the electron beam qualities. In this paper, we will discuss the related lattice design and simulations.

Speaker: Liuyang Wu (Shanghai Advanced Research Institute)

73 Bunch length measurement based on Cherenkov radiation spectrum at PolFEL and Solaris

The PolFEL THz free electron laser project comprises 80 MeV cw-linac furnished with warm S-band electron gun and 2 Rossendorf-like cryomodules. Besides bringing the beam to undulator, inverse Compton scattering interaction point, and finally to the dump, the beam diagnostics system is dedicated to metallic superconducting photocathodes development, in particular gun performance characterization. Bunch length will be measured in the injector section, behind the bunch compressor, and in each linac branch, behind the Wakefield linearizer at the undulator entrance. The bunch length is evaluated from sub-THz coherent Cherenkov radiation spectral distribution. Radiation emitted from a punched radiator will be analyzed with Martin–Puplett interferometer and measured with a broadband detector, both located on the breadboard at linac. A prototype will be preliminary measured with laboratory sub-THz source at IOE-MUT and subsequently at the Solaris linac with 0.5 GeV electrons.

Speaker: Dmytro But (National Centre for Nuclear Research)

74 Chromatic index to find a working point for a 4th generation synchrotron light source

The design and tuning of a storage ring for a fourth-generation synchrotron light source is very demanding. Recently, some research groups have considered techniques based on quasi-invariants of motion to address this task. This contribution presents tools, based on a quasi-invariant of motion method, for the description and optimisation of the quality of electron dynamics in a storage ring. An overview of this quasi-invariant formalism in the context of electron dynamics in storage rings for synchrotron light sources is presented. Quasi-invariant surface techniques to study and optimise the quality of the dynamics of a particular model are shown in detail. The relevance of the chromatic index for the study and tuning of the cell to determine the working point of a machine is highlighted. These techniques are implemented to optimise the horizontal electron dynamics generated by a ring model based on a 7BA cell, with 20 cells, 81 pm rad emittance and approximately 490 m circumference, and the results are presented.

Speaker: Dr Edgar Sanchez (Universidad Nacional Autónoma de México)

75 Code for modelling synchrotron radiation in a waveguide

We present a study that characterizes synchrotron radiation in the presence of a waveguide (the vacuum pipe). Building on previous work*, we have developed a code for calculating synchrotron radiation (SR) that accounts for boundary conditions. Our approach employs the Green's function technique to solve field equations under a paraxial approximation. We cross-checked the outputs of our code with the Synchrotron Radiation Workshop (SRW) in the free-space case and identified the sources of the observed differences. As a result, we offer a Python library for calculating synchrotron radiation with any given Green's function under specific boundary conditions. This tool will be used for field characterization and optimization of SR sources, especially in the THz and far-infrared ranges where the influence of vacuum components on radiation becomes significant.

Speaker: Andrei Trebushinin (European X-Ray Free-Electron Laser)

76 Commissioning of Echo-Enabled Harmonic Generation at the DELTA Storage Ring

Echo-enabled harmonic generation (EEHG) has been proposed as a seeding method for free-electron lasers but can also be employed to generate ultrashort radiation pulses at electron storage rings. With a twofold laser-electron interaction in two undulators (the modulators), each followed by a magnetic chicane, an electron phase space structure with a high harmonic content is produced, which gives rise to coherent emission of radiation at short wavelengths. The duration of the coherently emitted radiation in a third undulator (the radiator) is given by the laser pulse lengths. Thus,the EEHG pulses can be three orders of magnitude shorter and still more intense than conventional synchrotron radiation. At the 1.5-GeV synchrotron light source DELTA at TU
Dortmund University, the worldwide first implementation
of EEHG at a storage ring was achieved by reconfiguring
an electromagnetic undulator. First commissioning results are presented.

Speaker: Shaukat Khan (TU Dortmund University)

77 Commissioning results of X-band linear compression system at Dalian Coherent Light Source

Dalian Coherent Light Source (DCLS), a vacuum ultraviolet free-electron laser (FEL) facility, has been delivering high-brightness FEL pulses to the scientific community for over seven years. To further enhance the electron bunch quality and consequently improve FEL lasing performance, an X-band system for linear bunch compression has been recently installed and commissioned at DCLS. This system primarily consists of an 11.424 GHz radiofrequency (RF) microwave power supply and a 0.6-meter-long normal conducting RF structure. This paper provides an overview of the commissioning status of this system, offering a detailed comparative analysis of the experimental results concerning electron beam and FEL lasing performance.

Speaker: Xinmeng Li (Dalian Institute of Chemical Physics)

78 Compact prebunched waveguide THz-FEL

Free-electron-lasers fill a critical gap in the space of THz-sources as they can reach high average and peak powers with spectral tunability. Using a waveguide in a THz FEL significantly increases the coupling between the relativistic electrons and electromagnetic field enabling large amounts of radiation to be generated in a single passage of electrons through the undulator. In addition to transversely confining the radiation, the dispersive properties of the waveguide critically affect the velocity and slippage of the radiation pulse. Here we report on an experiment carried out at the UCLA Pegasus laboratory where the spectral properties of a compact waveguide THz FEL are characterized using electro-optic sampling based reconstruction of the time-domain waveform. The data shows simultaneous lasing at two different frequencies and the high frequency component in the pulse can be enhanced by injecting in the undulator an electron beam prebunched on the scale of the resonant radiation wavelength.

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

79 Comparing 266 nm and 200 nm seeding at the FERMI double-cascade HGHG FEL

FEL Seeding in High Gain Harmonic Generation scheme has been used for more than a decade with excellent results. The shortest wavelength delivered for user operation is so far around 4 nm based on double-cascade fresh bunch seeding at the FERMI FEL2. The implemented seed wavelength range in this case is 240-267 nm, which provides a good compromise between not too high harmonic order to reach 4 nm, and good performance of the nonlinear processes to generate the seed pulse. Using shorter seed wavelength would be advantageous to reach the shortest FEL wavelengths, there are however challenges in terms of available seed pulse energy and quality. We present a setup based on the developed at FERMI scheme for fourth-harmonic generation which allows to provide seed pulses switchable between 266 and 200 nm with comparable parameters.The planned measurement campaign is expected to provide qualitative comparison between the performance of seeding at the two wavelengths in generating FEL pulses in the 4 and sub-4 nm region. In parallel with the energy upgrade of FERMI to above 1.5 GeV, 200 nm seeding may become a route to reach and offer to users seeded FEL light at the Nitrogen K-edge.

Speaker: Miltcho Danailov (Elettra-Sincrotrone Trieste S.C.p.A.)

80 Compression of attosecond x-rays with space-charge induced chirp

Single-spike SASE radiation exhibits partial temporal coherence, creating an opportunity to compress pulses below the slippage length limit. In this case, the FEL pulse length will be limited by the coherent bandwidth. We consider how arbitrary control of the electron beam chirp can be used to first create a large coherent bandwidth through the chirp-taper dynamics; and secondly to compress that pulse in an afterburner by creating a mismatch between the chirp of the bunching and of the radiation.

Speakers: Agostino Marinelli (SLAC National Accelerator Laboratory), David Cesar (SLAC National Accelerator Laboratory)

81 Consideration of HB-SASE and HP-SASE at the SXFEL

X-ray free-electron laser is developing towards the sources with ultra-high intensity and ultra-short duration. The SXFEL facility is operated to generate the soft x-ray FEL with sub-GW-level peak power and nearly 100 fs pulse duration. It also has taken into consideration to shorten the pulse duration and increase the peak power. In this contribution, we analyzed the schemes possibility of high brightness SASE and high power SASE, from which high power SASE based on fresh-slice technique utilizing a dechirper is more feasible at the SXFEL. The schematic design and FEL simulation is presented, which demonstrates that an FEL pulse with peak power of 10 GW-level and pulse duration of 10s fs-level is possible to be generated.

Speakers: Tao Liu (Shanghai Advanced Research Institute), Zhangfeng Gao (Shanghai Synchrotron Radiation Facility)

82 Creating microbunched electrons for a coherent soft x-ray ICS source

Imposing a density modulation, or microbunching, on an electron beam may improve the brightness of an inverse Compton scattering (ICS) source by orders of magnitude via superradiant emission. We analytically and numerically analyze a new method, termed ponderomotive bunching, to create microbunching on a relativistic electron beam. Here, an electron beam interacts with a copropagating beat wave formed by two laser pulses. Via the ponderomotive force, the beat wave imposes an energy modulation onto the electron beam, which is transformed into a density modulation.

We show that pondermotive bunching can create microbunching for relativistic electron beams. Additionally, effects of the electron beam emittance, energy spread, and laser pulse shape, on the quality of the microbunching is studied. Using these analytical results, verified by particle tracking simulations, we propose a coherent ICS source based on ponderomotive bunching.

Speaker: Coen Sweers (Eindhoven University of Technology)

83 Dark current study of a normal conducting VHF band RF gun

The room temperature VHF RF gun which can provide high brightness beams with MHz repetition rate is one of the candidates for the electron source of a high-repetition-rate FELs.The VHF RF gun operates at continuous wave (CW) mode with high cathode gradient and high accelerating voltage. The dark current is critical to the performance of the downstream SRF cavities, undulators and beam line electronic equipments. In this paper, we will introduce the efforts to reduce dark current, including the dimension optimizations of plug vicinity and cavity inner surface treatment techniques. Besides, we adopted stainless steel instead of oxygen free copper as the material of cathode plug hole trying to decrease the field emission strength. The trajectories of the field emissions from high electric field regions were tracked. The dark current energy depositions on anode and cathode nose were calculated. The dark current transmission ratio from the gun to the downstream faraday cup was simulated. We also analyzed the plug insertion depth effect on the dark current transmission. Plugs with different insertion depth were fabricated. The dark current measurements will be reported in this paper.

Speaker: Mr xing heng wang (Shanghai Institute of Applied Physics)

84 Design considerations for the free electron laser oscillators of Anhui University

An infrared free electron laser user device will be constructed at the Anhui University in Hefei. The facility integrate free electron laser, strong magnetic field, and low-temperature environment aiming at materials science and other fields research. It consists of two FEL oscillators driven by one normal-conducting S-band linac and five experimental stations. The two oscillators generate the mid-infrared and far-infrared lasers covering the spectral range of 3-40 μm and 30-200 μm, respectively. In this paper the main considerations on optimizing design, especially reducing diffraction loss and mitigating power gap issues, are briefly presented, and basic design parameters of the FEL oscillators are given.

Speaker: Dr Qika Jia (University of Science and Technology of China)

85 Design of a four-mirror bow-tie cavity based free electron laser oscillator for eliminating spectral gaps

FELiChEM is an infrared free electron laser (FEL) user facility located in China, covering a wavelength range of 2-200 μm. Many spectral gaps were measured which the user experiments did not expect, especially in the far-infrared wavelength region from 50-200 μm. In this paper, we propose to apply a bow-tie cavity resonator instead of the conventional resonators to eliminate all the spectral gaps. Numerical simulation results demonstrate that this innovative resonator configuration can effectively eliminate the spectral gaps and significantly enhance the performance of long-wavelength lasers.

Speaker: Yuanfang Xu (University of Science and Technology of China)

86 Design of a tunable radio frequency elliptically polarized undulator for SXFEL user facility

Circularly polarized free electron laser (FEL) pulses are strongly demanded to satisfy plenty of experiments, but static magnetic elliptically polarized undulators(EPU) are too limited to achieve shorter undulator periods. In this paper, the potential of polarization control using the radio frequency (RF) undulator afterburner is discussed for Shanghai soft X-ray FEL user facility(SXFEL-UF). The parameters of the afterburner in the 2nm undulator line are obtained by theoretical predictions and numerical simulations. Then the general description of the proposal, and the optimization of the RF undulator cavity are presented. The coupler design is still in progress, leaving more work to do for a proper rotating electromagnetic field.

Speaker: Zhangfeng Gao (Shanghai Synchrotron Radiation Facility)

87 Design of the post-linac beam collimation at the switchyard section of SHINE

The operation of X-ray free-electron laser facilities necessitates the use of electron beams characterized by both high repetition rates and high energy, thereby elevating radiation safety concerns attributable to potential beam loss. Beam collimation is employed to protect the undulator and other elements by removing the beam halo in operation, as well as absorbing the off-axis beam during machine failure. A beam collimation system in the switchyard is designed to protect the undulator and beam pipe in the SHINE. A tracking simulation with a large initial distribution provides a result of the collimation efficiency. Detailed simulation studies evaluating the available collimation design limits for the acceptance of the undulator are described.

Speaker: Bingyang Yan (Shanghai Institute of Applied Physics)

88 Development of array-type secondary electron emission monitor toward single-shot determination of extraction efficiency of oscillator FELs

The extraction efficiency is one of the key parameters of an FEL oscillator. In the conventional way of extraction efficiency measurement in Kyoto University FEL (KU-FEL), temporal evolutions of the electron beam energy distribution in a macro-pulse with and without FEL lasing were measured by a Faraday cup placed after an energy analyzer[1]. Then the extraction efficiency is evaluated from the difference between the instantaneous average energy with and without the FEL lasing. Due to the scanning nature, the conventional way needs a long measurement time. To enable single-shot determination of the extraction efficiency, we developed a monitor that enables us to measure the temporal evolution of the electron beam energy in a macro-pulse by using an array-type secondary electron emission monitor[2]. The monitor consists of 24 ribbon-shaped electrodes and 2 shielding electrodes are placed after the energy analyzer magnet. The beam energy evolutions in a macro-pulse with and without FEL lasing were measured in a single-shot with <100-ns temporal resolution. This monitor will enable single-shot determination of the extraction efficiency of FEL oscillators.

Speaker: Zhuang Bi (Kyoto University)

89 Development status of the fiber length stabilizers for the laser arrival-time measurement

The Shanghai soft X-ray Free-Electron Laser facility (SXFEL) is a fourth-generation linac-based light source, capable of producing X-ray pulses with duration of tens of femtosecond. The photocathode laser and the seed laser for external seeding FEL therefore have tight requirements for relative arrival time to the machine and electron bunch. To reach required energy and wavelength to drive photocathode, as well as for external seeding FEL, further optical amplification and frequency conversion is needed. the femtosecond-stable pulsed optical reference, which are delivered via fiber length stabilizers. In this paper, we present the development status of the fiber length stabilizers for the laser-arrival time measurement.

Speaker: Jinguo Wang (Shanghai Advanced Research Institute)

90 Developments for the High-Duty-Cycle Upgrade of the European XFEL

DESY is developing different scenarios for a high-duty-cycle upgrade of the superconducting linac of the European XFEL. These scenarios include the current 10-Hz “burst mode” at 16.5 GeV (~1% duty cycle) up to full CW operation at ~7 GeV (100% duty cycle), with an intermediate 10% duty cycle option which could achieve ~15 GeV. The linac upgrade itself requires an additional sixteen CW-ready cryomodules, a complete overall of the RF power source as well as a new cryoplant. Timescale for the upgrade is ~2030.

Speaker: Winfried Decking (Deutsches Elektronen-Synchrotron DESY)

91 DFCSR: A Fast Calculation of 2D/3D Coherent Synchrotron Radiation in Relativistic Beams

Coherent Synchrotron Radiation (CSR) is regarded as one of the most important reasons that limit beam brightness in modern accelerators. Current numerical packages containing CSR wake fields generally use 1D models, which can become invalid in extreme compression regimes. On the other hand, the existing 2D or 3D codes are often slow. Here we report DFCSR, a novel particle tracking codes which can simulate 2D/3D CSR and space charge wakes in relativistic electron beams 2 or 3 orders of magnitude faster than conventional models like CSRtrack. We showed benchmark simulations and compared the results with existing models. The tracking code is written in Python and C programming languages with human-friendly input styles and is open-sourced on GitHub. It can serve as a powerful simulation tool for the design of next-generation accelerators.

Speaker: Jingyi Tang (Stanford University)

92 Dispersion Elimination of Electron Beam from a Microtron Accelerator for Terahertz Free Electron Laser

The Korea Atomic Energy Research Institute has developed a 5-MeV compact microtron accelerator to construct a high-power terahertz (THz) free electron laser (FEL) for field use. Good quality electron beam is essential for generating strong THz laser beam. Optimizing the electron beam was a priority for ensuring the performance and stability of the final THz FEL beam. The compact microtron accelerator has components that are difficult to adjust. The controllable factors are the RF power, microtron magnetic field strength, and the relative position between microtron RF cavity and beam extractor. We found that part of electron beam hit the exit gate of the extractor. To solve this problem, an adjustable guide line for the RF assembly including a magnetron, waveguide components and cavity was installed to stably relocate the cavity for optimizing the beam extraction to an FEL beamline. The beam exiting the microtron inevitably has large dispersion due to 180-degree rotation. Using ASTRA code, we determined the strengths of the quadrupole magnets in the beamline to minimize the dispersion after bending the beam 90 degrees. After bending, electron beam enters a strong electromagnet undulator and waveguide-mode resonator. After accurately characterizing the beam and undulator specifications, we plan to inject the electron beam into the undulator to attempt THz laser oscillation.

Speaker: Myung Jin Bae (Korea Atomic Energy Research Institute)

93 EEHG and HGHG FEL performance in the extreme ultraviolet spectral range

Fermi is a seeded FEL and has been operating in High Gain Harmonic Generation mode for more than a decade. Recently, the FERMI FEL-1 line, which normally delivers photons from 100 nm to 20 nm, has been upgraded to operate in Echo Enabled Harmonic Generation mode, extending the spectral range down to 10 nm and even beyond.
A measurement campaign was carried out to characterise the FEL performance in EEHG in terms of pulse energy and spectral quality. This provided a unique opportunity to compare experimentally the FEL performance in EEHG and HGHG under very similar beam and diagnostic conditions.

Speaker: Giuseppe Penco (Elettra-Sincrotrone Trieste S.C.p.A.)

94 Electron beam instrumentation for EuPRAXIA

Plasma-driven FELs require electron beams of high quality to facilitate effective FEL amplification, mirroring the stringent beam property requirements of FELs driven by radio frequency structures. Unique to plasma accelerators, however, are specific challenges such as dealing with the small beam size in the plasma, and managing a significant energy chirp within the beam. The study offers a comparative analysis of existing techniques and introduces novel instrumentation solutions that optimize beam quality and stability, ensuring a stable and reliable operation of EuPRAXIA.

Speaker: Rasmus Ischebeck (Paul Scherrer Institute)

95 Electron Beam Instrumentation for SwissFEL: Advancements and Future Directions

SwissFEL has been in user operation since 2019. Central to its ongoing success and enhancement is the continuous development of the properties of the electron beam, which requires ever more accurate instrumentation. This poster provides a comprehensive overview of the latest advancements in electron beam instrumentation at SwissFEL. which are pivotal for improving the facility's operational reliability, enhancing the resolution of the instruments, and for enabling new lasing modes.
These enhancements not only contribute to the robustness and efficiency of SwissFEL but also pave the way for new avenues of scientific inquiry and experimentation. The poster will further discuss the implications of these improvements for future research and the potential they hold for the evolution of free electron laser facilities worldwide.

Speaker: Rasmus Ischebeck (Paul Scherrer Institute)

96 Electron bunch shaping by laser heater for attosecond XFEL

The pulse duration of the X-ray free-electron laser (XFEL) relies on the pulse duration of the electron bunch. The energy distribution of the electron bunch can be manipulated by using the laser heater in the purpose of generating attosecond pulse duration electron bunch current profile. Therefore, the resultant electron bunch current profile after the bunch compressor chicanes is programmable by the laser parameters. We performed the electron bunch shaping experiment by using the laser heater at PAL-XFEL. The specific FEL lasing condition using the laser heater shaped electron bunch is investigated using ELEGANT and GENESIS simulations.

Speaker: Kookjin Moon (Pohang Accelerator Laboratory)

97 Electron Bunch Spacing with Extreme Laser Heating for FEL improvement at PAL-XFEL*

All XFEL machines utilize multiple magnetic Bunch Compressors (BC) for e-bunch compression. Since the non-linear energy chirp among bunch slices, current peaks occur in the head and tail of bunches, which generate collective effects spoiling the core slices. To remove current peaks, collimators at BCs are used to collimate head and tail slices in FEL machines operating at an approximate hundred Hz. However, the electron collimation is impractical to implement in MHz machines due to heat and radiation issues. Therefore, we study to suppress the current peaks in the head and tail with extreme laser heating. The energy spread of bunch slices is significantly increased by the high-power IR laser in a Laser Heater (LH). The collective effects from head and tail slices are almost disappeared by diluting these slices with intense dual laser pulses in the LH. In this paper, we present the bunch spacing with extreme laser heating in simulations and experiments. Also, we present the FEL improvement achieved through this bunch spacing.

Speaker: Haeryong Yang (Pohang Accelerator Laboratory)

98 Emittance Optimization Studies at PITZ with Laser Pulse Shaping

The PITZ Facility at DESY Zeuthen focuses on the development & optimization of high-brightness e-beam sources for FELs like the Eu-XFEL. The main scientific program is generation of intense e-beams characterized by small transverse emittance from the photo injector, a critical requirement for modern FELs. At PITZ, a thorough study of factors influencing emittance growth is carried out. Emittance growth due to space charge can be well controlled by precise laser pulse shaping. Astra simulations are utilized for the optimization studies to minimize emittance for various laser temporal & transverse profiles. A benchmarking analysis of Gaussian and flattop laser profiles is presented. The lasing process in XFEL occurs predominantly through highly charged slices with low emittance, assumed to originate from the longitudinal core of the electron bunch in the photo injector. The transverse emittance of these segments is of major importance. Therefore, the optimization of not only the projected emittance but also the slice & core are carried out through Astra. The results of these comprehensive optimization studies will be discussed in terms of further improving the performance of XFEL.

Speaker: Sumaira Zeeshan (Deutsches Elektronen-Synchrotron DESY)

99 ERFORMANCE ESTIMATES FOR A COMPACT THZ FEL

Free-electron lasers in the terahertz region that produce high peak and average power require small diameter, low-emittance, high voltage electron beams. This paper presents a 1.5-2 MV, 100-200 A thermionic cathode e-beam source for compact megawatt range peak power, multi-kilowatt average power, high repetition rate THz FELs. The beam generation system includes a high-quality Pierce gun followed by a four-stage acceleration section followed by a magnetic beam compression lens. A specific design for a 1.0 THz FEL is presented. The injector possesses various power supply switching advantages and the paper also includes remarks on very high-voltage multi-stage depressed collectors for overall efficiency enhancement. Performance estimates are included throughout the paper.

Speaker: Henry Freund (University of New Mexico)

100 Experiment Timing at High Repetition Rate Free Electron Lasers

Ever since the first pump-probe experiment at LCLS in 2009, it has been clear that determining X-ray/optical relative arrival times is critical to performing precision measurements. In the decade of ensuing developments at LCLS, we have pioneered many of the XFEL timing measurement schemes used across the world. Currently, to measure timing at the soft x-ray end stations, an incident optical laser is split into two time delayed pulses, crossed at an angle with respect to the x-rays at a target, and the arrival time of the x-rays is geometrically encoded on the reflected light. The pulses are then recombined and the spatial position of the onset of phase and amplitude changes provides the arrival time. The x-ray targets are designed utilizing thin film interference effects to produce a large x-ray induced reflectivity change. We have used these methods at LCLS for x-ray pulse energies as low as few a microjoules and the results will be presented. Further, we will show some future arrival time measurement schemes that will overcome the drift between the optical pump and arrival time measurements, which is quickly becoming one of the main limitations in experiment time resolution.

Speaker: James Glownia (SLAC National Accelerator Laboratory)

101 Experimental application of the multi-profiles method for the measurement of projected emittance at SXFEL

High-brightness electron bunches with small transverse emittance are required to drive X-ray free-electron lasers (FELs). For the measurement of the transverse emittance, the quadrupole scan and multi-profiles method are the two most common methods. The quadrupole scan method is more flexible in freely choosing the data points during the scan, while the multi-screen method allows on-line emittance measurements. The latter is especially the case for high repetition-rate FELs, such as the SHINE, which offer the possibility of on-line diagnostics. The quad scan method that has been previously used by SXFEL to measure projected emittance is a more homogeneous and non-comparative way. Recently, algorithms and procedures for measuring projected emittance by the multi-profiles method have been developed and implemented, which can give the SXFEL more options for measuring projected emittance and improve the accuracy of measurements. In this paper, we present the latest results of the multi-profiles method of projected emittance measurement on the SXFEL, and compare it with quad scan method, further discussing the implementation of on-line diagnostics at the SHINE.

Speaker: Jun-Jie Guo (Shanghai Zhangjiang Laboratory)

102 Experimental design challenges for single-shot electro-optical measurements of electron bunch shapes at high repetition rates

DESY and PhLAM are developing electro-optical measurements systems aiming at single-shot operation and high (MHz+) acquisition rates to characterize electron bunch shapes of short wavelength FELs as well as the temporal properties of pulsed THz sources. We review here the design strategies developed at DESY, and that allow the latest high-resolution measurements strategy based on diversity techniques DEOS to be implemented. The basic principle relies on the well-known spectral decoding technique, where the electric field of interest modulates a chirped laser pulse, whose optical spectrum is eventually recorded. However the actual performance of such electro-optic monitors crucially relies on key points of the design, in particular the management of optical signals with small polarization ellipticity, temporal interleaving, and the performance of fast (MHz+) readout cameras. This paper will outline design rules, challenges and solutions, that enable the implementation of DEOS in an effective manner at FLASH and the European XFEL as well as at other THz sources.

Speaker: Bernd Steffen (Deutsches Elektronen-Synchrotron DESY)

103 Exploring non-linear longitudinal phase space correlations: emittance-preserved bunch compression scheme for the UK XFEL low energy beamline

We investigated the versatility of manipulating the nonlinear longitudinal phase space for an electron injector including a VHF gun, bunch compressor cavities and a booster section. The research focuses on minimising the bunch length through RF compression while preserving the emittance produced after the gun. We will present the best results obtained from the OPAL simulations. The findings of this study will provide input to the conceptual design and option analysis for the UK XFEL.

Speaker: Oznur Apsimon (Cockcroft Institute)

104 FAST Injector Modeling for the TESSA experiment

The FAST facility at Fermilab provides particle accelerator R&D with > 150 MeV electrons from an RF linac out of a PITZ style photoinjector. The TESSA tapered undulator FEL experiment [1] will make use of the FAST accelerator and requires careful control of beam properties with small emittances. We present modeling studies of the photoinjector electron beam dynamics using the Opal code, comparing to recent multi-slit diagnostic emittance measurements. We include computed initial electron beam distributions as well as modeling of the emittance diagnostic. Misalignments of the solenoid and accelerating cavity are taken into account using data from a recent survey. Phase and solenoid current are scanned in simulation to compare with the measurements. Improved modeling for better agreement between measured and computed emittance is expected to help ensure success of the TESSA experiment.

[1] Musumeci, P et. al. (2022). FAST-GREENS: A High Efficiency Free Electron Laser Driven by Superconducting RF Accelerator. JACoW, IPAC2022.

Speaker: David Bruhwiler (RadiaSoft (United States))

105 Fast modeling of regenerative amplifier free-electron lasers

The regenerative amplifier FEL (RAFEL) promises to greatly increase the brightness and stability of single pass x-ray FELs. One of the critical challenges of the x-ray RAFEL is maintaining electron-optical overlap over the relatively large (hundreds of meters) footprint of the system. Numerical modeling of x-ray RAFELs with angular and positional errors is critical for designing stable cavities, as well as to predict signatures of specific misalignment effects. Full-scale simulations of x-ray FELs are incredibly time consuming, making large-scale parameter searches intractable on reasonable timescales. We present a semi-analytical model that allows to investigate realistic scenarios - x-ray cavity without gain (“cold cavity” or x-ray FEL oscillator) and x-ray RAFEL in the presence of angular/positional errors and electron trajectory oscillation. We especially focus on fast modeling of the FEL process and x-ray optics, while capturing effects pertaining to actual experimental setups at the Linac Coherent Light Source (LCLS) at SLAC. Such a method can be used to explore RAFEL at other wavelengths by suitable replacement of the optics modeling.

Speaker: River Robles (Stanford University)

106 FERMI 2.0: status and perspectives for the upgrade of FERMI P

FERMI has started the upgrade phase to FERMI 2.0. The ultimate goal of the upgrade plan is to extend the spectral range of the facility to cover the water window and beyond, and to reduce the minimum pulse duration below the characteristic lifetime of the light element core hole electrons. One of the main requirements of this upgrade is to preserve the uniqueness of FERMI: the possibility to control the properties of the radiation by seeding the FEL with an external laser system.
Much of the upgrade will affect the linac for an increased energy and improved beam quality, and the FEL-2 line. FEL-2 currently operates as a two-stage HGHG FEL. In the future FEL-2 will accommodate a first stage that can be configured as an EEHG FEL and a HGHG second stage with a shorter-period final amplifier. In a first phase of the upgrade, FERMI FEL-1 has been converted to an EEHG configuration and the linac energy was increased to 1.5 GeV at 50 Hz repetition rate (above 1.65 at 10 Hz). The experience with EEHG on the FEL-1 line will be an important step towards the final realisation of the FERMI FEL as a reliable source of highly coherent radiation at ~2 nm and below.

Speaker: Dr Luca Giannessi (Istituto Nazionale di Fisica Nucleare)

107 Fiber Bragg-Grating strain measurements on the APPLE – X undulators for SABINA

SABINA (Source of Advanced Beam Imaging for Novel Applications) is a new IR/THz FEL source under construction at the INFN Laboratori Nazionali di Frascati (LNF). The FEL is a single-pass amplifier designed to operate in the 10-100 THz spectral range using the SPARC_LAB infrastructure. The amplifier, consisting of three Apple-X undulator modules built by KYMA S.p.A., will deliver pulse energies of tens of microJoule with variable polarisation. The undulators were delivered to the LNF in 2023. In collaboration with ENEA, the mechanical structure was investigated for stability and deformations by strain measurements based on optical methods. The measurements have shown a clear deformation of the structure in the presence of varying magnetic forces. However, our conclusion is that the magnitude of this deformation is well within the tolerances required for the undulator functionality.

Speaker: Ilaria Balossino (Istituto Nazionale di Fisica Nucleare)

108 First EEHG lasing at FLASH and future prospective

External seeding via Echo-Enabled Harmonic Generation (EEHG) has been established for the first time at FLASH. This major achievement is a critical milestone towards FLASH2020+, the upgrade project for the FLASH facility, which will enable external seeding at MHz repetition rates at a user facility. Compared to standard SASE operation, the spectral quality and the longitudinal coherence are drastically improved and will ultimately allow for the next generation of user experiments. In this contribution, we will report on the latest highlights of the seeding efforts and present the foreseen performance of the new FLASH1 after the upgrade.

Speaker: Eugenio Ferrari (Deutsches Elektronen-Synchrotron DESY)

109 First Light at the Israeli THz Superradiant Free Electron Laser

We present first light emission from the Israeli THz superradiant FEL based on the ORGAD accelerator. The source was designed to operate beyond 1 THz. However, we measured superradiant emission near 0.6 THz with radiative energy output of 1.5 uJ.
This FEL operates in the FEL center of Ariel and Tel-Aviv universities. Designed and constructed in collaboration with UCLA, it is a hybrid RF gun with accelerating energies of 3.5 to 8.5 MeV. It is 64 cm in length. With charge of up to 330 pC. The electron bunch is 150 fsec. The planar undulator is 80 cm long. At radiation frequency of 0.6 THz, the super-radiant emission condition was satisfied (bunch duration less than quarter period of the radiation). When this condition is met, all electrons in the bunch emit in phase with each other. Hence the total radiation energy is proportional to the square of the number of electrons, not to the number of electrons as in conventional spontaneous emission. The bunch duration is limited by space-charge beam effect in the undulator, limiting attainment of superradiace at higher frequencies. This will be improved by ongoing work, adjusting the beam transport parameters and undulator injection.

Speaker: Ariel Nause (Ariel University)

110 First trial of scraper out-coupling in Kyoto University Free Electron Laser

Kyoto University Free Electron Laser (KU-FEL) achieved a record high extraction efficiency of 9.4% [1]. For further increase of the extraction efficiency, reduction of the optical cavity loss or increase of the FEL gain is required [2]. One possible way to reduce the optical cavity loss while keeping the reflection bandwidth of the cavity mirror is changing the out-coupling way from current hole-coupling to scraper out-coupling with an insertable mirror which has been implemented in JAERI-FEL [3]. By changing the hole-coupling mirror having a 1-mm diameter hole in its center to a mirror without the hole, the total optical cavity loss can be largely reduced. Then out-coupling of the intracavity power to the outside cavity is performed by an insertable scraper mirror. For this purpose, an insertable scraper mirror has been introduced in KU-FEL. By changing the out-coupling way, the optical cavity loss of the KU-FEL was reduced from 3.9 to 2.2% at the wavelength of 11 micro-m. In a test experiment, the optical cavity loss could be controlled from 2.2 to 19% by changing the insertion amount of the scraper mirror.

Speaker: Heishun Zen (Kyoto University)

111 FOMO: A Tool For Start-to-End Simulations of the FERMI Free-Electron Laser

Simulations of particle accelerators are fundamentally important both to help in understanding measurements, and to plan future experiments. Start-to-end simulations, which begin with the initial generation of a particle distribution in a gun, up to the final beam energy, often require switching between codes to account for the differences between models. Tools which are able to switch between codes seamlessly along the accelerator lattice can therefore be very useful. This contribution presents a simulation tool that can simulate the FERMI free-electron laser injector, linac, and FEL lines with the complexity of creating individual lattice and input files abstracted away. All simulation files and pertinent information are saved to a shared database, in order to help users keep track of simulations that have been run. This tool is therefore able to streamline and simplify the start-to-end simulation process, to improve the ways in which data are stored, and to provide direct comparisons between simulations and measurements.

Speaker: Alexander Brynes (Elettra-Sincrotrone Trieste S.C.p.A.)

112 Fresh-sliced FEL generation at PAL-XFEL

A flat dechirper is used for fresh-slice generation of hard X-ray FEL pulses at PAL-XFEL. The electron beam is transversely deflected with a 1m long corrugated plate positioned just before the undulator beamline. The beamline is divided into two parts that contain respectively 8 and 13 undulator modules, and between which a self-seeding chicane introduces a variable electron beam time delay. In this presentation we show simulations and experimental results. In a recent experimental test we successfully operated the fresh-slice FEL with and without self-seeding. To simulate the electron beam deflection, we modified the well-known wakefield formulas from (K. Bane et al., PRAB 19 084401 (2016)) to achieve higher accuracy for long bunches, which matches well with numerical calculations. Our ongoing efforts are focused on obtaining a fresh-slice FEL intensity in comparison to the standard SASE configuration.

Speaker: MyungHoon Cho (Pohang Accelerator Laboratory)

113 Gamma Spectrometer Measurements at the European XFEL Undulator System

This work presents measurements of the GR05+ Gamma Spectrometers placed in two different undulator cells of the SASE1 Undulator System at the European X-Ray Free Electron Laser (XFEL). Because of their small size, it is possible to register energy spectra near the beam pipe and undulator permanent magnets. Additionally, the characterization of this CZT solid-state detector is included, as well as the dependence of its response on the different detector orientations relative to the radiation source. It shows that for the lower energy range, the spectrometer's signal can decrease even more than 50% compared to the reference conditions. Simultaneous measurements in two undulator cells (upstream and downstream) show the difference between the radiation field in different parts of the undulator system. Simulations confirm that the radiation field in the upstream part of the SASE1 Undulator System consists mostly of bremsstrahlung radiation.

Speaker: Olga Falowska-Pietrzak (Stockholm University)

114 Generating the strong field terahertz radiation at shanghai soft X-ray free electron laser

Coherent, wide-tunable frequency and high intensi-ty terahertz (THz) source is under preparation at the Shanghai Soft X-ray free-electron laser facility (SXFEL). The electron bunches modulated by frequen-cy beating light can generate coherent, wide-tunable, high intensity THz radiation from 0.1 to 30 THz through wigglers. The electromagnetic wiggler with peak magnetic field up to 1.75 T is adopted and the parameters of the wiggler are optimized to ensure the generation of strong field THz radiation. Due to the limitation of layout space of the SXFEL, the length of wiggler is limited within 5 meters. By properly in-creasing the charge of the electron beam, the THz pulse energy can be kept at mJ level under the proposed different parameters of the wiggler. In this article, we will present the possible layout of THz source on the SXFEL and the S2E simulation of THz radiation of mJ magnitude within the 5-meter wiggler.

Speaker: Kaiqing Zhang (Shanghai Synchrotron Radiation Facility)

115 Generation of an attosecond FEL-quality beams in plasma wakefield accelerators

Plasma wakefield accelerators (PWFA) have showcased remarkable acceleration gradients, reaching tens of GeV per meter. In this method, the accelerating structure is a highly nonlinear charge-density wave in a plasma, which is excited by an ultrarelativistic electron beam. Advancements in generating FEL-quality, attosecond electron beams represent the forefront of this field. In this work, we introduce a novel approach to inject a high-quality electron beam using beam-induced ionization injection with a driver-injector beam configuration. We will explain the physical underpinnings of this design using analytical plasma wakefield theory and present supporting Particle-In-Cell simulation results that show the potential for creating beam with ~500 attosecond duration, hundreds of nanometer emittance, and less than 1% energy spread. We will present the prospects of realizing this beam experimentally at FACET II facility at SLAC. Finally, parameters of attosecond X-ray FEL driven by this beam and simulated by Genesis will be presented.

Speaker: Jiayang Yan (Stony Brook University)

116 Generation of high-power attosecond X-ray FEL pulses carrying orbital angular momentum

X-ray beams, carrying orbital angular momentum (OAM), are emerging as a powerful tool to probe matter. Recently, the self-seeded FEL with OAM (SSOAM) method has been proposed to generate high-power X-ray OAM pulses, which places the traditional optical elements in the linear regime of the FEL amplification process before saturation to reduce the thermal load of the optical element. In this work, we propose to utilize the SSOAM scheme to produce attosecond X-ray vortices with high intensity. Numerical simulations demonstrate the X-ray OAM pulses with peak powers of more than one hundred gigawatts and a pulse duration of the order of hundred attoseconds can be achieved using the proposed method.

Speaker: Chenzhi Xu (Shanghai Institute of Applied Physics)

117 Generation of Short Wavelength at Variable Polarization in FLASH2

FLASH is an XUV and soft X-ray free-electron laser user facility at DESY. In the FLASH2 main undulator beamline radiation down to 3.2 nm wavelength in the fundamental has been demonstrated. In order to efficiently generate soft X-ray radiation at the third harmonic of the FLASH2 main undulator wavelength with tunable polarization, an additional APPLE-III type undulator has been installed downstream of the FLASH2 SASE undulator section. This so-called afterburner allows to reach wavelength down to 1.33 nm with selectable polarization. The afterburner undulator also serves as a full-scale prototype for the FLASH1 seeding radiators to be installed in 2025. This paper describes the successful commissioning with beam, the demonstration of the amplification of the third harmonic and the generation of different polarizations and operation of the afterburner undulator.

Speakers: Juliane Roensch-Schulenburg (Deutsches Elektronen-Synchrotron DESY), Mathias Vogt (Deutsches Elektronen-Synchrotron DESY)

118 Generation of ultrashort THz radiation pulses using chirped micro-bunching and tapered undulators

Ultrashort, intense terahertz radiation sources hold tremendous potential for various industrial and scientific applications. However, the bandwidth of FEL based pulses is limited due to slippage effects in the undulator. In this paper we study a solution to generate very short and intense THz pulses using chirped microbunching combined with tapered undulators in order to effectively compress the radiation pulse length. Tunable chirped microbunching can be generated in a compact setup using a properly shaped mask at a high dispersion plane in the electron beam transport prior injection into a tapered undulator. With the help of numerical simulation, we study the radiation pulse length and peak power scaling of this scheme showcasing its ability to produce few-cycle pulses with high peak power at 10THz.

Speaker: Yining Yang (Tsinghua University)

119 Harmonic generation in a terawatt X-ray free-electron laser

Terawatt XFELs represent the frontier in further development of x-ray sources and require high current densities with strong transverse focusing. In this paper, we investigate the implications/potentialities of TW XFELs on the generation of harmonics at still shorter wavelengths and higher photon energies. The simulations indicate that significant power levels are possible at high harmonics and that these XFELs can be an important coherent source of hard x-rays through the gamma ray spectrum. For this purpose, we use the MINERVA code which self-consistently includes harmonic generation. Both helical and planar undulators are discussed for fundamental at 1.5 Å and we study the associated harmonic generation. Tapered undulators are needed to reach TW fundamental powers, but the taper does not enhance the harmonics. Nevertheless, the harmonics reach substantial powers. Simulations indicate that, for the parameters under consideration, peak powers of the order of 180 MW are possible at the 5th harmonic with a photon energy of about 41 keV and still higher harmonics may be generated at substantial powers. Such high harmonic powers are certain to enable a host of enhanced applications.

Speaker: Henry Freund (University of New Mexico)

120 Harmonic generation with homeopathic seeding at FERMI

The use of an external laser to initialize the FEL process at the harmonics of the laser has been demonstrated to be very efficient for improving the FEL performance. Thanks to the external seeding FEL can reach saturation in a shorted undulator compared to SASE FELs, but also gain in terms of FEL spectral brightness and stability.
An efficient harmonic conversion in the FEL process requires powerful laser pulses that are not compatible with new generation of linear accelerators and FELs aiming at MHz repetition rate. The limit can be overcome with the use of a self-amplification of the laser pulse before the FEL harmonic generation. This scheme, also known as OK-HGHG, has been previously studied and the principle experimentally demonstrated. In this work, conducted at the FERMI FEL-1, we have performed a detailed characterization of the FEL performance in the OK-HGHG configuration. Our studies clearly show the benefits allowing to reduce the seed power by 3 orders of magnitude but also highlights some possible issues related to the microbunching amplification. Results are reported together with a discussion on implication on future seeded FELs.

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

121 Harnessing Artificial Intelligence for Single-Shot Measurement of Free Electron Laser Pulse Power

In the realm of Free Electron Laser (FEL) research, the accurate characterization of radiation pulse profiles is crucial for optimizing beam quality and experimental outcomes. Our presentation introduces a pioneering approach utilizing machine learning techniques for virtual diagnostic profiling of FEL radiation pulses. Our innovative Artificial Intelligence (AI) diagnostic tool leverages longitudinal phase space data from the X-band transverse deflecting cavity collected by a DAQ system at FLASH facility in DESY to reveal the shot-to-shot temporal profile of FEL pulses in real-time. Unlike conventional methods, our AI-driven approach overcomes the limitation of single-shot measurements, offering a non-invasive and efficient method for characterizing radiation pulses.
By harnessing cutting-edge machine learning models, our tool enables accurate single-shot measurements of FEL pulse power, facilitating groundbreaking research in ultrafast science. This presentation will delve into the theoretical foundations, methodology, and validation results of our virtual diagnostic tool, showcasing its potential to revolutionize FEL research and unlock new frontiers in science and technology.

Speakers: Najmeh Mirian (Deutsches Elektronen-Synchrotron DESY), Till Korten (HZDR)

122 High-harmonic generation from solid and gas targets with infrared FEL pulses

Research into high-harmonic generation (HHG) from solid-state and gas targets holds significant importance in advancing high-field photon science, particularly in the realm of attosecond VUV and X-ray pulse generation. Thanks to the recent advancements in few-cycle pulse generation in superradiant FEL oscillators [], HHG photon sources driven by infrared FEL pulses are becoming feasible. Marking a seminal milestone in FEL-HHG, we report experimental results of harmonic generation from solid and gas targets conducted at two FEL facilities: KU-FEL at Kyoto University and LEBRA-FEL at Nihon University. In these experiments, FEL pulses of 2-8 um wavelength were focused on solid-state and gas targets, and the harmonics generated were measured with a photomultiplier tube and a semiconductor detector. So far, up to 13th-order harmonics from a ZnSe plate and 7th-order harmonics from oxygen and argon gases were observed. In this talk, we will describe the details of the experimental results and future prospects for attosecond X-ray generation through FEL-HHG [*]. This work was supported by MEXT Q-LEAP (JPMXS0118070271) and JSPS KAKENHI (22H03881).

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

123 High-power THz free eelectron laser via tapering-enhanced super-radiance

FELs have made tremendous progress in the last fifty years, but the energy conversion efficiency is still in the order of 1%. The ORGAD is a hybrid S-band (2856 MHz) RF photo-cathode accelerator located at the Schlesinger Compact Accelerator Center at Ariel University, a collaborative project with Tel Aviv University. A longitudinally bunched electron beam is then injected into an undulator generating a coherent spontaneous emission, referred to as a super-radiant emission, whose energy yield is proportional to $N^2$. Here we present the design and simulations of a THz source based on the novel radiative interaction–Tapered Enhanced Super-radiance(TES), using a combined waveguided undulator structure in the THz regime (~0.5 THz). A coherent emission generated in the uniform section is used as a seed source, which interacts with the electron beam bunch in a tapered (amplitude) undulator, in the zero-slippage, realizing a significantly more powerful and efficient emission. This method avoids a complicated seed and electron pulse synchronism system. Here, this scheme is simulated starting from the Cathode according to the theoretical model, demonstrating an increased efficiency of x3.

Speaker: Leon Feigin (Ariel University)

124 High-power, Narrow-bandwidth Cavity-Based Free Electron Lasers

Cavity-based x-ray free electron lasers (CBXFELs) such as the x-ray regenerative amplifier FEL (XRAFEL) and the XFEL oscillator (XFELO) have been proposed to produce stable, fully-coherent, hard x-ray unreachable with the current XFELs. XRAFEL approach employs high-current, short-bunch electron beams to support a high-gain FEL process, resulting in substantial pulse energy. On the other hand, XFELO utilizes a low-gain FEL process with long bunch length, low current electron beams and achieves extremely narrow pulse bandwidths. Here we introduce the novel concept of staging, which combines the strength of XRAFEL and XFELO to generate hard X-ray pulses with very narrow bandwidth while maintaining high pulse energy. We first kick-start the cavity with high-current electron beams to obtain high intra-cavity power. After that, we gradually decrease the beam current and increase the bunch length until it matches the recirculated pulse length. Leveraging post-saturation tapering, the FEL power can be continuously boosted. A case study based on LCLS-II HE and CBXFEL project parameters demonstrates generating 20keV hard x-ray pulses with mJ energy and sub-10 meV bandwidths.

Speakers: Jingyi Tang (Stanford University), Madison Singleton (SLAC National Accelerator Laboratory)

125 Influence of ionizing beam irradiation on the critical parameters of superconducting elements

Superconducting materials are more and more widely used in FEL-s accelerators, which proper work strongly depends on their properties, very sensitive to the irradiation effects. The ionizing irradiation occuring in FEL-s influences the subtle structure of superconductors, especially of layered form, damaging it, which effect will have negative influence on superconducting parameters. From the other side created then nano-defects stabilize the magnetic vortices array, improving the critical current. To analysis of these opposite effects and searching for optimum of irradiation concentration is devoted presentation, in which is investigated influence of the ionizing irradiation on critical current, pinning forces and others relevant parameters of superconductors, for various strength of capturing vortex interaction. It will be discussed too influence on the critical temperature of superconductors FEL-s infrared irradiation, leading to new phonons generation. These investigations have therefore great scientific meaning connected to critical current problems in superconductots especially now at large progress of HTc superconductivity. and are closely related to the FEL-s proper work.

Speaker: Jacek Sosnowski (National Centre for Nuclear Research)

126 Initial Commission Result of LCLS II Precision Timing System

In 2023, LINAC Coherent Light Source II achieved first light, spanning over 5 km. The goal of achieving 10fs relative jitter between the experiment laser and the x-ray led to the development of new systems. We will present the timing system design, architecture, and key commission results.
The challenge of reference distribution in the hostile environment of LCLS II was addressed by using a multi-drop coaxial cable for the superconducting LINAC and stabilized radio frequency over fiber systems for the experimental hall. An in-house laser locker locks the experiment laser to the reference signal, and an S-band beam phase cavity determines the electron beam's correlation with the reference.
Initial commission results show a laser-to-x-ray jitter of around 60 femtoseconds, aiming for significant improvement with optical detection. The precision timing system is vital for LCLS II's experiments, and additional diagnostic instruments are proposed to enhance synchronization performance.

Speaker: Chengcheng Xu (SLAC National Accelerator Laboratory)

127 Iris waveguide for low-losses THz radiation propagation over long distances

We investigate an iris waveguide designed for installation in the long halls of free-electron laser facilities for THz radiation propagation for pump-probe experiments. We approach this problem theoretically and validate our results using numerical wavefront propagation techniques. An iris line, optimised for 3 THz, can propagate radiation over 370 m with a transmission coefficient (T) of 0.69. Furthermore, a line optimized for chosen energy transmits radiation even more effectively at higher frequencies, e.g., at 10 THz, T = 0.94, and it is practically lossless at 25 THz. It is important to note that radiation should be matched before entering the iris line to ensure minimal losses. The least loss is found in the fundamental mode of the iris line. We conclude our study by proposing preliminary design parameters for the THz propagation line at European XFEL.

Speaker: Andrei Trebushinin (European X-Ray Free-Electron Laser)

128 Longitudinal phase space diagnostics with a non-movable corrugated passive wakefield streaker

Time-resolved diagnostics at Free-Electron Laser (FEL) facilities, in particular electron beam longitudinal phase space (LPS) and FEL power profile measurements, provide information highly valuable for users, machine development studies, and beam setup. We investigate the slice energy resolution of passive streaker setups, in particular the effect of an energy chirp on the measured slice energy spread. Downstream of the hard X-ray SASE2 beamline at the European XFEL, these measurements are enabled by a single-plate non-movable passive wakefield streaker, essentially a rectangular corrugated plate placed inside a vacuum chamber. We show measurements with a time resolution down to a few femtoseconds, and an energy resolution down to a few MeVs.

Speaker: Dr Philipp Dijkstal (Paul Scherrer Institute)

129 Machine Learning based optimization of beam emittance in Free-Electron Lasers

The European XFEL at DESY is a world-leading research infrastructure in Hamburg (Germany),
enabling scientists to observe and investigate microstructural processes and phenomena with spa-
tial and temporal resolutions on the atomic and femtosecond scale. To improve the performance
of the accelerator and also to ensure its competitiveness with other facilities, it is essential
to optimize the European XFEL for operation in continuous-wave (CW) mode in near future.
Despite its advantages, an operation in CW mode requires a reduction of the beam energy and
is associated with an increase in the geometric beam emittance though.
To still ensure the delivery of high quality beams, we develop and
apply machine learning methods to the modeling the photoinjectors and pulse shaping within a novel approach,
resulting in the improvement of crucial beam properties such as the beam emittance.
Based on both simulated and experimental data, we will implement a Digital Twin of the EU XFEL,
enabling us to optimize the generated emittance in an online control while also considering nonlinear
space-charge effects near the photocathode while outperfoming classing optimization methods.

Speaker: Alexander Klemps (Hamburg University of Technology)

130 Machine Learning Surrogates for 2D CSR Simulations

Coherent synchrotron radiation (CSR) has been an important consideration for design of bunch compressors and other beam manipulating sections in FEL linacs. There has been increasing interest and need to move from the most common 1D CSR models to consider 2D and even 3D CSR modeling. However, these simulations are very slow and computationally intensive. Machine learning models are capable of learning to represent complicated, nonlinear systems; however, ML models often struggle to generalize well outside of their training data. In this work we explore the use of ML models to serve as quickly evaluating surrogates for expensive 2D CSR simulations. The issue of model generalization is addressed through the use of regularization terms in training and the addition of 1D CSR calculations into the ML model. Benchmarking of the ML model is shown for several bunch compression chicanes.

Speaker: Christopher Hall (RadiaSoft (United States))

131 Magnetic characterizations of the APPLE – X undulators for SABINA

SABINA (Source of Advanced Beam Imaging for Novel Applications) is a new IR/THz FEL source under construction at the INFN Laboratori Nazionali di Frascati (LNF). The FEL will be driven by the SPARC_LAB linac, a normal conducting linear accelerator delivering high-brightness beams generated in a photo catode gun. The beam peak current is increased by compression in velocity bunching regime, in order to drive the SABINA single-pass amplifier, designed to operate in the 10-100 THz spectral range. The amplifier, consisting of three Apple-X undulator modules, will deliver pulse energies of tens of microjoule with variable polarisation. The undulators received at LNF in 2023, were designed and realized by KYMA. In this contribution we present a summary of the magnetic characterization of the three devices. The measured parameters are in good agreement with the design values.

Speaker: Ilaria Balossino (Istituto Nazionale di Fisica Nucleare)

132 Measurement of first and second order longitudinal dispersion in the MAX IV bunch compressor

The MAX IV linac is used to provide sub-100 fs pulses for a Short Pulse Facility (SPF). In order to form this beam, two bunch compressors at 245 MeV and 3.0 GeV are used. The properties of these compressors have been studied and simulated in the past, but lacking longitudinal diagnostics in their proximity, measurements of their effect on the beam have been limited. We seek to ascertain the $R_{56}$ and $T_{566}$ of the transfer through the compressor, which we measure as the resulting variations in the time-of-flight as dependent on changes to the incoming energy. In this study, the raw waveforms of two BPMs situated before and after the first bunch compressor have been used to extract the time-of-flight through the compressor. Investigating the dependence of time-of-flight on the changes in energy provides the sought $R_{56}$ and $T_{566}$. The results of these new measurements have been compared with the design values and elegant simulations for the bunch compressor.

Speaker: Johan Lundquist (MAX IV Laboratory)

133 Measurement of Thermal Emittance from a Cs2Te Photocathode Based on a VHF Electron Gun

RF electron guns can produce higher quality electron beams due to their high accelerating gradients, and semiconductor photocathodes have higher quantum efficiencies. However, the thermionic emission of the material can affect the beam emittance. Therefore, it is necessary to develop cathodes with lower thermionic emission, and to measure the thermionic emission, especially under high power conditions, in order to iteratively optimize the cathode fabrication process. This paper mainly focuses on the study of thermionic emission measurement based on a VHF electron gun.

Speaker: Zipeng Liu (Shanghai Synchrotron Radiation Facility)

134 Measurements and simulations of the longitudinal wakefields at free electron laser FLASH II

Recent results of short-range longitudinal wake calculations in the undulator lines at the European XFEL predicted strong wakefield effect. Design of the vacuum system of FLASH 2 undulator is pretty much similar to that of the European XFEL. Bunch charge and the peak current are also in the same range as well. Thus, we expect comparable effects of the wakefields for both machines. Relevant experimental studies have been performed recently at FLASH 2. We traced energy of the lasing fraction of the electron bunch along the undulator by means of tracing central frequency of SASE FEL radiation. We measured radiation spectra of SASE FEL from 5 undulator modules with closed gaps at different positions along the undulator: 1-5, 2-6, …, 8-12 keeping remaining 7 modules with open gaps. We detected clear red shift of the radiation spectra along the undulator which corresponds to the energy loss of the lasing fraction of the electron bunch of 5.4 MeV at the whole undulator length (or, 450 keV per one undulator section). Here we present and discuss both, experimental results from SASE FEL and simulation results of wakefields.

Speaker: Dr Mikhail Yurkov (Deutsches Elektronen-Synchrotron DESY)

135 Measurements of particle trajectory caustics in bunch compressors.

Caustic theory has been applied to electron trajectories in particle accelerators and has been used to describe current spike formation in bunch compressors. Current spike development is sensitive to small changes in the initial longitudinal phase space distribution and longitudinal dispersion (R56, T566 and U5666) of bunch compressors. The caustic theory can be used to describe current horn suppression in chicane compression schemes, or for increasing peak current in arc-like compression schemes. In this paper we show experimentally, how current spikes could develop in the MAX IV bunch compressors from small changes in pre-BC1 accelerating phase and BC1 sextupole strength

Speaker: Adam Dixon (University of Liverpool)

136 Measurements of slice parameters for EEHG optimization

An accurate knowledge of the slice electron beam parameters such as energy spread and seed laser induced energy modulation is very critical for an optimal setting of the EEHG parameters and FEL optimization. In this work we report about a method that, based on a fitting of experimental parameters scans in coherent harmonic generation with numerical simulations or models, can provide accurate information of the electron beam properties. We present recent results obtained at FERMI FEL-1.

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

137 Method for measuring the transverse electron beam size with intensity interferometry at free-electron lasers

We demonstrate the feasibility of measuring the spatial intensity correlation function of synchrotron radiation at free-electron laser facilities. For this technique, it is sufficient to use a synchrotron radiation imager paired with an undulator commissioning crystal monochromator at Bragg's angle. By measuring the transverse intensity correlation, we retrieved the electron beam size at the undulator cell where this radiation was emitted. We tested this technique at the hard X-ray SASE1 and SASE2 beamlines of the European XFEL.

Speaker: Andrei Trebushinin (European X-Ray Free-Electron Laser)

138 Microbunching instability analysis of the three-stage magnetic compressor scheme at SHINE

In order to improve the stability of the accelerator system and to have the ability to further compress the electron beam, the space for installing a third magnetic compressor was reserved at the beginning of the physical design of SHINE. However, the three-stage compression of the microbunching instability is prone to cause the three-stage cascade amplification, and therefore it is necessary to analyse it in order to reduce the hazards to the quality of the beam. In this study, a theoretical analysis of the system MBI evolution of the SHINE three-stage magnetic compression scheme is firstly carried out by solving the three-stage iterative solution of the Volterra integral equation. Based on this theoretical model, the dependence terms with high correlation with the system MBI gain are identified, and after parameter scanning, the working point of lower MBI gain is determined. Compared with the two-stage compression of the conventional scheme, it was shown that the MBI gain caused by the LSC effect did not grow further, and the simulation results of the 3D numerical software proved the accuracy of the theoretical derivation.

Speaker: Wencai Cheng (Shanghai Institute of Applied Physics)

139 Microbunching study of the beam switchyard section of SHINE

The microbunching instability (MBI) has become a critical issue in advanced X-ray free-electron lasers, posing a considerable challenge to achieving the desired beam quality and coherence. In the multi-bend beamlines of the beam switchyard system, the MBI effect can have a large gain because of the coherent synchrotron radiation (CSR). We have performed a comprehensive simulation with different initial beam parameters at the entrance of the beam switchyard. Different beam transport designs are used to evaluates the impact of MBI gain by isochronicity and to explore methods to suppress the MBI effect. This study provides practical guidance for the design of beam switchyard optics and the limitation of upstream beam quality.

Speaker: Bingyang Yan (Shanghai Institute of Applied Physics)

140 Modular Cosmic Ray Detector (MCORD) and its potential use in various physics experiments, astrophysics and geophysics.

As part of the collaboration building a set of detectors for the new collider, our group was tasked with designing and building a large-scale cosmic ray detector, which was to complement the capabilities of the MPD (Dubna) detector set. The detector was planned as a trigger for cosmic ray particles and to be used to calibrate and test other systems. Additional functions were to be the detection of pairs of high-energy muons originating from some particle decay processes generated during collisions and continuous observation of the cosmic muon stream in order to detect multi muons events. From the very beginning, the detector was designed as a scalable and universal device for many applications. The following work will present the basic features and parameters of the Modular COsmic Ray Detector (MCORD) and examples of its possible use in high energy physics, astrophysics and geology. Thanks to its universal nature, MCORD can be used as a fast trigger, neutron veto detector, muon detector and as a tool in muon tomography.

Speaker: Marcin Bielewicz (National Centre for Nuclear Research)

141 Monte Carlo studies of BeamLoss detectors for PolFEL

The main purpose of Beam Loss Monitor (BLM) system is the early detection of incorrect beam propagation inside the beamline and, therefore, the protection of machine, its vacuum and electronic components. The beam parameters within the PolFEL facility induces the need of such a system. Each of the proposed beam loss detector, which are distributes along the PolFEL device, is built of miniature photomultiplier tube integrated with HV supplier and voltage divider, coupled with small plastic scintillator. The paper describes numerical investigation of Beam Loss Monitor position and efficiency along the designed PolFEL linear accelerator and undulator sections. In order to determine response of BLM detector for various positions within the facility, we have used the Geant4 Monte Carlo framework. Definition of the geometry used during calculations was based on the CAD design of PolFEL and detectors, and utilize gdml support built into the Geant4. During the analysis, the energy deposition within the detector, as well as optical photon production, was taken into account. Additionally, the preliminary numerical studies of the cherenkov fiber optic BLM is also showed.

Speaker: Roch Kwiatkowski (National Centre for Nuclear Research)

142 Nanometer-class longitudinally bunched beams in the AWA emittance exchange beamline

The emittance exchange (EEX) beamline provides a unique capability in transferring transverse beam density modulation into longitudinal bunching. This process can be advantageous for rapidly starting the FEL process reducing the effective length of the undulators. We investigate the feasibility of creating nanometer-scale longitudinal density modulation at the Argonne Wakefield Accelerator. We particularly focus on the case of 800 nm bunching, and subsequent radiation generation.

Speaker: Dr Aliaksei Halavanau (SLAC National Accelerator Laboratory)

143 Narrow-band SASE FEL for EUV Lithography

We present a high-slippage concept of SASE FEL that can produce narrow-band spectra at 13.5 nm for Extreme Ultraviolet Lithography (EUVL). The narrow-band SASE FEL differs from the seeded FEL which involves spectrally filtering the SASE output to produce a monochromatic seed and amplifying the coherent seed with additional undulators. Seeded FELs have been experimentally demonstrated to produce narrow-line spectra [see *]. In this paper we study a new SASE method that relies on strong slippage in the tapered undulators to produce a few longitudinal modes, thereby narrowing the output spectra without using a monochromatic seed. We present numerical simulation results that show the high-slippage SASE FEL generates a few longitudinal modes and few spectral modes. The few-mode SASE produces a relative spectral bandwidth of 0.3% which is significantly narrower than the reflectivity curve of the molybdenum-silicon EUV mirrors that are used to reflect the EUV radiation in EUVL.

Speaker: Dinh Nguyen (xLight Incorporated)

144 Nonlinear compression of long-wavelength-infrared free electron laser pulses in thick Germanium plate

For enabling attosecond X-ray pulse generation via high harmonic generation in rare gases driven by free electron laser (FEL)[1], nonlinear compression of long-wavelength-infrared (LWIR) pulses from an oscillator-type FEL in a thick Germanium plate has been demonstrated[2]. LWIR-FEL pulses with the peak wavelength in 8.5 micro-m and the pulse duration of 146 fs in full width at half maximum (FWHM) generated from KU-FEL (Kyoto University FEL) were compressed down to 106 fs by inserting an antireflection-coated Ge plate with the thickness of 30 mm. At the same time, the spectral width of LWIR-FEL pulses was broadened in the Ge plate. The simultaneous occurrence of the spectral broadening and the pulse compression is called nonlinear compression. The achieved pulse duration (106 fs) was 86% of the expected pulse duration of the FEL pulse (123 fs) after the Ge plate with the assumption that no spectral broadening occurred in the Ge plate. By the nonlinear compression of the LWIR-FEL pulses, the peak power was increased to 1.4 times of the incident LWIR-FEL pulses. This work was supported by MEXT Q-LEAP (JPMXS0118070271).

Speaker: Heishun Zen (Kyoto University)

145 Observation of coherent edge radiation for electron-bunch length monitoring during free-electron laser oscillations at KU-FEL

It has been reported that cavity-type free-electron laser (FEL) oscillations cause shape distortion in an electron bunch depending on a detuning length of an optical cavity [1]. We have studied the FEL-induced bunch distortion by observing coherent edge radiation (CER) generated at an FEL facility of Kyoto University, which has the highest extraction efficiency of an FEL oscillation [2]. We have already found that an electron bunch interacting with the cavity-type FEL elongated when the detuning length of an optical cavity was positive [3]. To extract higher-power CER from the optical cavity without interfering with FEL oscillations, we updated a system that separates the CER beam from the FEL beam in the optical cavity. Using the new system, it is expected to measure the temporal evolution of the electron bunch shape in the FEL macropulse. In the presentation, we will report on the CER separation system and the latest results of the CER measurements.

Speaker: Hideaki Ohgaki (Kyoto University)

146 Observation of high order sum frequency generations observed in polycrystalline ZnS

Finding a temporally overlapping condition is necessary for pump-probe experiments. This can be achieved by obtaining sum frequency generation (SFG) signal. Our attempt proceeded using a mid-infrared free electron laser (MIR-FEL) of 12.5 micro-m, ps-Nd:YVO4 laser of 1064 nm from Kyoto University free electron laser (KU-FEL) facility, and a polycrystalline ZnS plate at room temperature. An ideal overlapping condition of the two lasers was confirmed when the highest intense SFG signal was observed at 980 nm. Along with that, additional peaks have been observed simultaneously at 909, 846, 510, and 490 nm. Each of these appears to correspond with higher order SFGs of (2×MIR-FEL+1×ps-laser), (3×MIR-FEL+1×ps-laser), (1×MIR-FEL+2×ps-laser) and (2×MIR-FEL+2×ps-laser) respectively. It is also confirmed that these observed peaks are not associated with the vibrational Raman scattering process since there are no phonon absorption energies of ZnS [1] within our observation range.

Speaker: Hideaki Ohgaki (Kyoto University)

147 Operational experience with attosecond soft x-rays at LCLS

Attosecond physics is an emerging as one of the fastest-growing, highest-impact, areas of science at FELs. In this proceeding, we compare several methods for generating single-spike attosecond soft x-ray pulses at LCLS. Over the last several years we have measured more than 30 independent attosecond SXR configurations, across which we can explore a variety of techniques, including: cathode-modulation, laser-heater pulse shaping, and wiggler-induced changes to the beamline impedance. In each configuration, we optimize the undulator taper to selectively lase on the strongly chirped portion of a narrow-current spike in the middle of the electron beam. From the taper optimization we can then infer properties of the attosecond radiation which we compare across multiple techniques.

Speaker: David Cesar (SLAC National Accelerator Laboratory)

148 Optimization and characterization of externally seeded FLASH1

Externally seeded Free Electron Lasers (FELs), such as the upgraded FLASH1 at DESY, utilize external lasers to initiate bunching, contrasting with the self-amplification of spontaneous emission (SASE). This upgraded facility will offer simultaneous seeded and SASE pulses. Introducing a chirped electron bunch and high repetition rate seed lasers with unique spectral features will significantly influence the electron bunching and the spectral properties of the output. This work discusses our computational strategies to align model predictions closely with experimental outcomes, focusing on the dynamics of chirped bunch interactions and the integration of seed laser characteristics to optimize the performance and predictability of FEL outputs.

Speaker: Eugenio Ferrari (Deutsches Elektronen-Synchrotron DESY)

149 Optimization of seeding from dielectric-lined waveguide for PITZ-like THz FEL

Experimental investigations of terahertz (THz) free electron lasers (FELs) at the Photo Injector Test Facility at DESY in Zeuthen (PITZ) have demonstrated the feasibility and substantial potential of high-power tunable THz sources driven by a PITZ-like accelerator. Preliminary seeding experiments indicate improvements in the lasing process, such as earlier saturation and reduced fluctuations in the pulse energy of THz radiation. The concept of a THz FEL based on a PITZ-like accelerator has been proposed as a tunable high-power THz source for pump-probe experiments at the European XFEL, which includes the possibility of seeding with a dielectric-lined waveguide (DLW). This paper presents start-to-end simulations of the DLW seeding option, demonstrating that high bunching factors can be achieved at both the fundamental frequency and higher harmonics, with a discussion of the THz FEL lasing process.

Speaker: Xiao-Yang Zhang (Tsinghua University)

150 Performance of the high-brightness SRF CW photo-electron gun

Next generation X-ray FELs (XFELs) are rapidly moving towards CW operation, which brings the new requirements on high quality of the CW electron sources. such as a superconducting RF (SRF) electron gun. While operation in SRF environment can be quite challenging, the quality of the generated beams is rather rewarding [1-4].
We report on our unique experience with the record performing SRF electron gun equipped with warm CsK2Sb photocathode. The gun is generating high charge electron bunches (up to 20 nC/bunch) and extremely low transverse emittances suitabel for X-ray FELs. We present study of the transverse beam emittance from the gun, including our experimental results and numerical simulations. We also presnt initial results of operating the gun with GaAs photocathode. In addition, we disucss accelerating scheme based on our photoinjector, which can satisfy the high requirements for the new generation FELs.
We report on our unique experiences with the only operational low emittance photo-electron SRF CW gun: we report the challenges, problems but also our breakthroughs in this challenging technology

Speaker: Vladimir Litvinenko (Stony Brook University)

151 Physical design of a VHF band normal conducting RF gun

The APEX gun experiments at LBNL have demonstrat-ed the average beam current up to 0.3 mA. The VHF band normal conducting RF gun indicates the potential to provide both high brightness and high average current beams. R&D activities for the mA-scale average beam current by using the VHF gun started in our laboratory. In this paper, the RF design, multipacting simulations, vacuum calculations, multiphysics analysis investigating the RF, thermal and mechanical properties of the VHF gun are presented.

Speaker: Guan Shu (Institute of High Energy Physics)

152 Ponderomotive laser lens for beam phase space control

Slice and normalize emittancies are the critical parameters that dictate the efficiency of SASE FEL mechanism and advanced FEL lasing schemes. The control of the emittance has been notoriously difficult and requires substantial efforts to transport a high-brightness electron beam from the cathode to the undulator. In this communication, we introduce the concept of a ponderomotive laser lens that imprints a time-dependent linear and/or nonlinear correlation into the transverse phase space of the electron beam. The laser lens is based on the ponderomotive force created by the gradient of a laser field. The slice emittance can be reduced by correcting undesired nonlinear correlations in the transverse phase space. Advanced linear emittance compensation can also be implemented using a dedicated temporal profile of the ponderomotive laser lens. The SACLA injector is used as a theoretical study case to demonstrate capabilities of the proposed ponderomotive laser lens.

Speaker: Johan Ribbing (Uppsala University)

153 Preliminary design of the beam dump line at the S3FEL

Shenzhen Superconducting Soft X-Ray Free-electron Laser (S3FEL) is a newly proposed high-repetition-rate X-ray FEL facility in China, which will be located at Guangming Science City in Shenzhen. The superconducting RF structure will provide electron beams with a few hundred kW of beam power at beam energies up to 2.5 GeV and repetition rates up to 1 MHz. The design of the dump line from the undulator exits to the beam dumps is essential not only for transporting the electrons to the beam dump but also for diagnosing the beam longitudinal phase space. The safe operation of the beam dumps should also be considered. This paper describes the current design of the dump line at the S3FEL.

Speaker: Yifan Liang (Institute of Advanced Light Source Facilities Shenzhen)

154 Preparing for XLO: optical cavity design

X-ray laser oscillator, dubbed XLO, is an ongoing project at SLAC National Accelerator Laboratory. XLO employs atomic gain medium pumped by an XFEL, to generate hard x-ray coherent, transform limited light. XLO has unique properties of very high gain per pass (similarly to the case of an undulator-/cavity- based high gain XFEL, while maintaining narrow bandwidth of XFELO. Current effort is focused on demonstrating XLO operating at 8048 eV (Copper $K_{\alpha1}$ line), using 9 keV pulse from the LCLS.

Speaker: Dr Aliaksei Halavanau (SLAC National Accelerator Laboratory)

155 Present status of Kyoto University Free Electron Laser facility

Kyoto University Free Electron Laser (KU-FEL) facility has been developed for energy-related research by the Institute of Advanced Energy, Kyoto University. There are two accelerator-driven infrared coherent light sources in the facility. One is the oscillator-type FEL whose wavelength range is 3.4 to 26 micro-m[1]. The other one is the THz-Coherent Undulator Radiation (THz-CUR) source whose frequency range is 0.1 to 0.4 THz[2]. In addition to the accelerator-driven light sources, several solid-state laser sources can be used together. The facility is open to domestic and international users. In fiscal year 2023, 18 external user groups used the facility for their research. The current status and results of recent upgrade projects for improving the performance of the light sources will be presented.

Speaker: Heishun Zen (Kyoto University)

156 Progress by the Cavity-Based FEL Project

Cavity-based free-electron lasers (CBXFELs) have the potential to dramatically improve the stability and coherence of FELs. The CBXFEL project is a collaboration between SLAC, Argonne, and RIKEN to build a 65 m rectangular X-ray cavity at the LCLS, with the goal of demonstrating low-loss cavity ring-down and, ultimately, two-pass FEL gain. This paper describes recent simulation and measurement results supporting our efforts, including those related to FEL gain and delivering two transversely aligned electron bunches separated by 624 RF buckets. Finally, we report our progress in testing and installing Xray optics and X-ray diagnostics components, the Bragg mirror nano-positioning and diagnostics stages, and the X-ray return line.

Speaker: Yuri Shvyd'ko (Argonne National Laboratory)

157 Proof-of-concept of an ultrafast source in the Soft X-ray range

At MAX IV laboratory the linac is used to inject into the two rings but also to drive the FemtoMAX beamline, which is a source of (incoherent) short pulses in the Hard X-ray range.
In this contribution we present the design and simulations for a new mode of operation of the linac that would allow to generate coherent ultra-short FEL pulses in the soft X-ray domain. Preliminary simulations indicate that the power will reach GW level with a number of photons per pulse comparable to other FELs.
The scheme will re-utilize the same infrastructure and the existing undulators. The main change will be lowering the electron energy, but still using the same bunch compressors to reach femtoseconds pulse lengths. We will take advantage of the recently installed transverse deflecting structure to characterize the longitudinal profile of the electron beam and tailor it to the new operation mode.
Further studies will include a new electron optics for the lower electron energy and the design of the detection system to demonstrate the gain.
This additional operation mode will offer an alternative to the conventional FemtoMAX source and thus expand the potential of the Short Pulse Facility.

Speaker: Francesca Curbis (Lund University)

158 Proposal for a New Approach to Undulator Tuning

In the PolFEL team, we are working on developing a new, more precise method of undulator tuning. Our method is based on an accurate model of the undulator's magnetic field distribution, described using approximation-free analytical expressions derived directly from the Biot-Savart equation. Having an accurate description of the magnetic field allows tuning to be carried out in two phases: (1) minimizing the vertical drift of the electron beam by optimizing the order of magnets, (2) vertical positioning of the magnets to ensure the uniformity of the magnetic field better than 0.1%, which is crucial for the efficient emission of the laser beam.

Speaker: Andrzej Horodeński (National Centre for Nuclear Research)

159 Proposal for an extreme ultraviolet free-electron laser with low-energy electron beam

There is an increasing demand for coherent extreme ultraviolet (EUV) radiation in imaging and spectroscopy, particularly as the next generation of EUV lithography requires high-power EUV light sources. Experimental results demonstrate that harmonic lasing in a free-electron laser (FEL) is an effective method for extending the wavelength range and producing narrow bandwidth FEL. In this contribution, we propose operating the radiation segment in conventional echo-enabled harmonic generation at high harmonic mode instead of the fundamental radiation. This approach combines phase shifting and undulator tapering to generate high-power EUV radiation. The results indicate that fully coherent EUV radiation with an average power of up to 1 kW can be generated using a 280-MeV electron beam with an average current of 10 mA.

Speaker: Hanxiang Yang (Shanghai Institute of Applied Physics)

160 R&D progress of cavity based EUV/X-ray free electron lasers at Shanghai

The development of short-wavelength, fully coherent Free Electron Lasers (FELs) represents a pivotal direction for advancing X-ray source technology. Promising solutions under consideration include cavity-based X-ray Free Electron Lasers (CBXFELs), such as the X-ray Regenerative Amplifier FEL (XRAFEL) and the X-ray FEL Oscillator. We are pursuing the development of advanced FEL schemes and the experimental validation of optical cavity technologies at SARI. This report outlines our progress in the development of CBXFEL for the hard X-ray and Extreme Ultraviolet (EUV) regions. We are currently testing the operation of a rectangular cavity with Bragg-reflecting crystals based on the SSRF. Ringdown of a small 1-meter silicon crystal cavity has been experimented, and preparations for a diamond crystal-based cavity are underway. Concurrently, we have developed a suite of EUV reflective optics utilizing multilayer technologies, aiming to provide high average power with RAFEL mode at a wavelength of 13.5 nm in the future. Additionally, we are advancing the manipulation of CBXFEL performance, including the generation of controlled polarization and Orbital Angular Momentum (OAM).

Speaker: Nanshun Huang (Shanghai Zhangjiang Laboratory)

161 Radiation dose mitigation for APPLE-X electronics at the European XFEL.

An afterburner consisting of four APPLE-X undulator modules has recently been re-installed downstream of the planar undulator in the soft x-ray beamline (SASE3) at the European XFEL. The modules were removed shortly after initial installation because of damage to electrical components caused by the spontaneous radiation produced in the planar undulator. After the damage became apparent, a study was carried out to investigate different schemes for protection of the electronic components from the planar undulator radiation. Based on the results of this study, extra synchrotron radiation absorbers were added at selected locations in SASE3, and the APPLE-X modules were re-installed. Subsequent measurements showed a significant reduction in the levels of radiation impacting the sensitive electronic components. This paper outlines the model used for the radiation simulations, presents the results of the validation of the simulations against measurements, and compares some of the different methods that were considered for protection of the sensitive electronics. Recent radiation measurements will also be presented showing the effectiveness of the additional absorbers that were installed.

Speaker: Andrew Potter (University of Liverpool)

162 Realizing Steady-State Microbunching with Optical Stochastic Crystallization

Optical Stochastic Cooling (OSC) is a state-of-the-art beam cooling technology first demonstrated in 2021 at the IOTA storage ring at Fermilab's FAST facility. A second phase of the research program is planned to run in early 2025 and will incorporate an optical amplifier to enable significantly increased cooling rates and greater operational flexibility.

In addition to beam cooling, an OSC system can be configured to enable advanced control over the phase space of the beam. An example operational mode could enable crystallization, where the particles in a bunch are locked into a self-reinforcing, regular microstructure at the OSC fundamental wavelength; we refer to this as Optical Stochastic Crystallization (OSX). OSX represents a new path toward Steady-State Microbunching (SSMB), which may enable light sources combining the high brightness of a free-electron laser with the high repetition rate of a storage ring.  Such a source has applications from the terahertz to the extreme ultraviolet (EUV).

This contribution will discuss the status of the OSC experimental program and its potential to achieve the first demonstration of SSMB during the upcoming experimental run.

Speaker: Michael Wallbank (Fermi National Accelerator Laboratory)

163 Resolving Longitudinal Information of Electron Beam and FEL pulses Using Transverse Deflecting Cavity at Dalian Coherent Light Source

Dalian Coherent Light Source (DCLS) is a free-electron laser (FEL) user facility in the vacuum ultraviolet wavelength region, operating in high-gain harmonic generation (HGHG) mode. Accurate diagnosis of the longitudinal phase space of electron bunch plays a critical role in beam tuning and FEL optimizing. Additionally, advanced user experiments have demonstrated a demand for determining the FEL pulse duration. To address this, an S-band radiofrequency transverse deflecting structure (TDS) has been installed and commissioned at the DCLS undulator exit. This paper provides a comprehensive overview of the TDS system, detailing its physical layout, diagnostic methods, and experimental results, allowing for the characterization of both the longitudinal phase space of electron bunch and the duration distribution of FEL pulse.

Speaker: Jitao Sun (Dalian Institute of Chemical Physics)

164 Second generation of multi-alkali antimonide photocathodes for high-gradient RF guns

Due to their excellent photoemissive properties, especially low thermal emittance and high sensitivity in the green wavelength, multi-alkali antimonide photocathode, particularly potassium-cesium-antimonide, have emerged as prominent photoemissive materials for the electron sources of high-repetition-rate FEL applications. To explore their feasibility of operating in a high-gradient RF gun, DESY collaborated with INFN LASA to develop multi-alkali photocathode materials. Three KCsSb photocathodes and one NaKSb(Cs) cathode were grown on molybdenum substrates using the sequential deposition method in the new preparation system at INFN LASA. Subsequently, these cathodes were transferred successfully to PITZ for testing in the high-gradient RF gun. This contribution summarizes the growth procedures and experimental results obtained from these second-generation multi-alkali antimonide cathodes.

Speaker: Sandeep Mohanty (Deutsches Elektronen-Synchrotron DESY)

165 Self-seeded hard X-ray free-electron lasers with the narrowest bandwidth at PAL-XFEL

To produce nearly fully coherent hard X-ray self-seeded (HXRSS) free-electron laser (FEL) pulses, the forward Bragg-diffraction (FBD) monochromator is utilized at PAL-XFEL. These HXRSS FEL pulses exhibited exceptional peak-brightness and a narrow spectrum, demonstrating outstanding performance across a wide photon energy range from 3.5keV to 14.6keV. To further decrease the spectral bandwidth, a thinner diamond can be utilized. Using a diamond with a thickness of 30 μm, we experimentally obtained the bandwidth of ~160 meV (FWHM) at 11.21 keV for the self-seeded scheme. The spectrum was measured using a von Hamos spectrometer in RXES (resonant x-ray emission spectroscopy). In this talk, we will present recent experimental findings on the characteristics of the hard X-ray self-seeded FEL with different Bragg condition and thinner diamond crystal.

Speaker: Inhyuk Nam (Pohang Accelerator Laboratory)

166 Self-seeding scheme for testing CBXFEL configurations

The next generation of X-ray free-electron lasers (XFELs) research is focused on the investigation of self-seeding techniques, and, more recently, X-ray cavity designs. Here, we explore the capabilities of the X-ray self-seeding (HXRSS) setup at LCLS. The self-seeding mechanism is initiated by generating SASE in the first undulator section, followed by monochromatization and seeding in the subsequent undulator sections. The electron bunch is delayed with respect to the produced seed field to temporally overlap with the monochromator wake. We then utilize the HXRSS setup to mimic the performance of recently proposed cavity-based XFEL (CBXFEL) experiment currently under construction at SLAC. When set up with 7 undulators for SASE generation, and 7 undulators for seeding, HXRSS setup approximately reproduces the conditions of the CBXFEL experiment. We investigate the self-seeding effects on the resulting photon flux, radiation bandwidth, and spectral pedestal. Our findings help optimize CBXFEL photon diagnostics and set up the lower bound on the resulting CBXFEL pulse quality.

Speaker: Madison Singleton (SLAC National Accelerator Laboratory)

167 Shot-to-shot Control of Bunch Length for High-repetition-rate XFELs

It is a critical challenge to meet the requirements for XFEL operation modes and photon properties across different undulator lines simultaneously. We propose a solution to this challenge by introducing a dipole-kicker combination in the bunch compressors, allowing for the variation of electron bunch length in continuous-wave XFEL facilities driven by a superconducting linac. Through start-to-end simulations using parameters from the Shanghai high-repetition-rate XFEL and extreme light facility, we demonstrate the feasibility of this technique.

Speaker: Bart Faatz (Shanghai Advanced Research Institute)

168 Signal Filtering Optimization for Laser Arrival Time Measurement Based on Electro-Optic Modulator

The LAM system in XFELs is crucial for measuring and compensating for time jitter in laser transmission. The EOM encodes phase information onto the reference pulse amplitude, but amplitude noise can affect measurement accuracy. In this study, we analyzed the spectral characteristics of the optical pulse signal output from the EOM using FFT, identifying primary noise sources. We then compared pulse amplitude stability under different cutoff frequencies to find an optimal filtering scheme. Experimental results showed that at a 3000 MHz cutoff frequency, the pulse amplitude maintained good stability (CV = 0.007) without significant signal distortion. This finding suggests an effective approach to suppress amplitude noise in LAM systems.

Speaker: Bowei Wu (Shanghai Advanced Research Institute)

169 Simulation study of nanostructured copper photocathodes

We present an electromagnetic characterization and optimization study of nanostructured photocathodes for electron gun applications. The study concentrates on photocathodes operated at visible to infrared wavelengths, for which an accurate simulation model is constructed. We apply a customized dispersion model for the cathode material, which can describe the measured permittivity data over a broad frequency range. Various types of nanopatterns are explored in order to understand how different geometrical parameters affect cathode reflectance. The results reveal an optimized model of nanostructured photocathodes, demonstrating improved absorptance at the target laser wavelength. Additionally, the impact of geometrical imperfections on the reflectance spectra is examined.

Speaker: Ms Margarita Bulgacheva (Technical University of Darmstadt)

170 Simulations Study of Reverse-taper Enhanced Harmonic Lasing at SXFEL

Generating so-called ‘laser-like’ X-ray in the full X-ray band has been a long-term challenge for seeded free electron lasers (FELs), which is generally limited by the up-frequency conversion efficiency and the materials of optical monochromators. Here, we demonstrate a reverse tapered undulator-enhanced harmonic lasing mechanism to possibly extend the photon coverage of the seeded FELs for fully coherent X-ray pulses in the full X-ray band. The mechanism adopts a reverse tapered undulator with a relatively longer magnetic period as the first stage to produce low-intensity X-ray pulses at the fundamental wavelength while imprinting the microbunching into the phase space of the electron beam. Besides, the technique has a relatively narrower bandwidth, which can possibly increase the repetition of the self-seeding mode.

Speaker: Yaozong Xiao (Shanghai Institute of Applied Physics)

TUP014-FRA_paper.pdf

171 Smart*Light: a high-brilliance ICS based x-ray source

High-brilliance X-ray beams provided by large-scale synchrotron facilities allow detailed visualization of the structure of materials and tissues, making them indispensable for the development of new high-tech materials in the broadest sense of the word. Unfortunately, however, because of the size and costs of these big facilities they are scarce and not very accessible, with long waiting times for precious beamtime. Smart*Light is a lab scale x-ray source based on inverse Compton scattering and aims to bridge the gap between conventional lab x-ray sources and synchrotrons.

The Smart*Light beamline consist of a 100 kV DC photo-electron gun that creates 10 pC electron bunches which are accelerated up to 30 MeV using a high gradient X-band linear accelerator. The electrons interact with a 12 mJ, 800nm laser pulse to generate x-rays up to 20 keV. Alternatively the second harmonic of the laser pulse can be utilized to extend the x-ray energy up to 40 keV.

An overview of the design and results of the commissioning will be given.

Speaker: Ids van Elk (Eindhoven University of Technology)

172 Spatio-temporal overlap procedures for seeding at FLASH

Externally seeded FEL schemes with harmonic up-conversion, e.g. Echo-Enabled Harmonic Generation (EEHG), deliver pulses with improved spectro-temporal properties and shot-to-shot stability at wavelengths down to the XUV and X-ray range. Such schemes rely on the manipulation of the longitudinal phase space of the electron beam at the wavelength of the seed laser, for which it is crucial that both beams overlap longitudinally and transversely inside a short undulator (modulator).
In preparation for the high repetition rate seeded FEL user operation planned for the FLASH2020+ upgrade of FLASH (DESY, Hamburg), EEHG is explored in the Xseed project. In this contribution, the recent optimization of the procedures for setting up and transversely maintaining the overlap is presented.

Speaker: Andreas Thiel (Universität Hamburg)

173 Speckle suppression by splitting and delaying short FEL pulses

In the application of FELs for the EUV lithography, an important question is how the coherence of the FEL light affects the imaging of the mask. As is well known, coherent light produces speckles which, in lithographic process, can distort the mask image. An effective way to suppress the speckes was pointed out in Ref. [1]. It is based on splitting the FEL beam into multiple beamlets in a way that the light from different beamlets does not overlap in time due to the short duration of the FEL pulses. In this work, using a simple model, we calculate the speckle suppression as a function of the beamlets overlay, and mathematically confirm the predictions of Ref. [1].

Speaker: Gennady Stupakov (xLight Incorporated)

174 Spectral response and quantum efficiency of rejuvenated Cesium Telluride photocathodes for high average current photoinjectors

Cesium telluride (Cs-Te) photocathodes are the current workhorse in high repetition rate free-electron lasers (FELs) around the world. Unfortunately, they are chemically highly reactive, which limits their operational lifetime and require frequent interventions for Cs-Te film replacement or rejuvenation. The precise control of the Cs-Te deposition stoichiometry ratio during the photocathode fabrication process is essential to optimise the resulting quantum efficiency (QE) and emittance of electron sources. For example, an excess of Cs or the formation of another CsxTe phase can lead to a sub-optimal electron bunch energy spread or a decrease in overall quantum yield. In this study, we analyse the photoemissive characteristics of Cs-Te photocathodes rejuvenated by co-depositing a Cs-Te layer over a degraded one. The QE of the photocathodes was measured using a 10 Hz 5 ns pulsed OPO in the wavelength range from 240 to 360 nm. This experiment allows to estimate the photoemission threshold and corresponding energy distribution of emitted electrons, and to compare with the expected spectral response using Spicer's three-step model.

Speaker: Palma Katona (European Organization for Nuclear Research)

175 Spectrotemporal shaping of attosecond x-ray pulses with a fresh-slice free-electron laser

We propose a scheme for coherently shaping attosecond x-ray pulses at free-electron lasers. We show that by seeding an FEL with a short coherent seed that overfills the amplification bandwidth, one can shape the wigner function of the pulse by controlling the undulator taper profile. The examples of controllable coherent pulse pairs and trains, as well as isolated spectrotemporally shaped pulses with very broad coherent bandwidths are examined in detail. Existing attosecond XFELs can achieve these experimental conditions in a two-stage cascade, in which the coherent seed is generated by a short current spike in an electron bunch and shaped in an unspoiled region within the same bunch. We experimentally demonstrate the production of pulse pairs using this method at the Linac Coherent Light Source.

Speaker: River Robles (Stanford University)

176 Spreader design options for a high-throughput multi-user XFEL

One option being considered to meet the needs of the XFEL user community within the UK is a new national facility capable of producing photon beams with properties identified in the UK XFEL Science Case. The case calls for a high-efficiency facility with a step-change increase in the simultaneous operation of multiple end stations, with high-repetition rate near-transform limited x-ray pulses provided across a wide range of photon energies and pulse durations. Another key capability would be to enable experiments combining hard and soft x-ray pulses, potentially from different beamlines. The beam spreader in this case would need to direct individual electron bunches, separated by a few ns, to adjacent beamlines, while maintaining the beam quality and stability needed to achieve high-power x-ray output. In this paper, we discuss possible spreader configurations and consider the technology requirements and options for critical components. As an example, we present an outline spreader optics design together with initial results from tracking studies aiming to characterise the potential performance.

Speaker: Andrew Potter (University of Liverpool)

177 Steady State and Power Buildup in a Tapered XUV FEL Oscillator

We present a simulation study of a tapered FEL oscillator, which aims at delivering high average power in the extreme ultraviolet wavelength region. The setup relies on the combination of three critical elements: a pre-buncher, a strongly tapered undulator and an optical cavity enclosing them. Such a configuration allows for efficient power extraction
from a high-brightness electron beam.
We concentrate on tuning the steady state of the oscillator, including the optimisation of the taper profile based on an analytical estimate, as well as a multi-pass optimisation of the cavity design. We briefly address the buildup from shot-noise power level.

Speaker: Margarit Asatrian (Universität Hamburg)

178 Study on Cs-Te Photocathode with Protective Film for Performance Improvement of KU-FEL

A new 1.6-cell photocathode RF gun has been installed to improve the performance of Kyoto University free electron laser (KU-FEL). A test operation using a copper cathode has been completed, and the electron bunch charge of 60 pC with 120 bunches in a macro-pulse was obtained. However, due to the copper cathode's low quantum efficiency, the electron beam performance could not reach the target of 1 nC and 200 bunches. From the performance of the photocathode drive laser, the required quantum efficiency is about 0.1%. Therefore, a Cs-Te with CsBr protective film* is a good candidate for our project since it can have enough high quantum efficiency with a reasonable lifetime. A Cs-Te and CsBr protective film deposition system has been developed and test measurement has been performed. In the presentation, we will report the characteristics of the deposited Cs-Te photocathode with CsBr protective film.

Speaker: Koki Umemoto (Kyoto University)

179 Sub-femtosecond hard X-ray FEL pulses from photocathode temporal shaping at SwissFEL

At SwissFEL we employ photocathode temporal shaping to produce sub-femtosecond X-ray FEL pulses. Two optical laser pulses arrive on the photocathode with a time delay of few picoseconds, resulting in a notch at the center of the generated electron current profile. Longitudinal space charge and coherent synchrotron radiation effects that occur during beam transport and bunch compression then transform this notch into a large current spike. With this scheme (previously demonstrated at LCLS), we achieved peak currents tunable up to 12 kA after the second bunch compressor. The bunches then emitted FEL pulses, some of which contained only one spectrum spike. The photon energy was 6 keV at the hard X-ray beamline Aramis, or 535 eV at the soft X-ray beamline Athos. To our knowledge, this is the first demonstration of the method in the hard X-ray regime. Ongoing machine development is aimed at compatibility with SwissFEL's double bunch operation mode.

Speaker: Dr Philipp Dijkstal (Paul Scherrer Institute)

180 Superconducting Nb/Pb electron photocathode - towards improved adhesion of lead layer to niobium substrate

The preparation of a superconducting photocathode for a sc electron gun, composed of a photoemission lead layer deposited on niobium substrate must fulfil strict requirements not only in terms of photoemission efficiency, operation in ultra high vacuum and high-power electromagnetic field. Of fundamental importance is the practical requirement for strong adhesion of the Pb layer. It yields from a procedure of cleaning the e-gun in a stream of ultrapure water under 100 bar. Achieving good adhesion in this case is a difficult condition to meet because Nb and Pb neither form solid solutions nor even present good mutual miscibility or wettability. A new approach is presented to improve Nb-Pb adhesion by pre-implanting lead into niobium before final deposition of the photoemissive Pb layer. The performed studies indicate the effectiveness of this method in the case of Pb layers deposited by magnetron sputtering and its failure for Pb applied by using cathodic arcs. These results are discussed in the light of nanoindentation studies and universal models describing the state of intrinsic stress in the transition zone between a substrate and deposited layer.

Speaker: Jerzy Lorkiewicz (National Centre for Nuclear Research)

181 Surface plasmon enhanced photocathode R&D at DESY for CW photoinjector for future CW and high-duty-cycle European XFEL∗

The future upgrades of the European XFEL (EuXFEL) foresees continuous wave (CW) and High-Duty-Cycle (HDC) operation requiring a CW electron photoinjector. Motivated by this, a 1.6-cell superconducting radio frequency (SRF) electron gun cavity is under development at DESY. Recently, the DESY CW SRF gun cavities out of niobium with a copper cathode screwed directly to the cavity’s back wall demonstrated a peak electric on an axis field of up to 55 MV/m. The design of the DESY gun cavity requires air-stable photocathodes, limiting the choice of photocathode materials to metals. Presently, copper is the baseline photocathode material. However, photoinjector operation at high repetition rates using a copper photocathode is challenging due to laser power limitations in the UV range. Therefore, we are exploring methods to enhance the quantum efficiency (QE) of metallic photocathodes to allow higher repetition rates. In this work, we report our current progress on numerical and experimental efforts towards metal cathodes with enhanced photoemissive properties.

Speaker: Chirag Banjare (Deutsches Elektronen-Synchrotron DESY)

182 Tapering enhanced high efficiency THz waveguide oscillator

The waveguide free electron laser has demonstrated the ability to efficiently convert relativistic electron beam energy into THz radiation in a single passage through a tapered helical undulator. An oscillator configuration can further boost energy extraction efficiency surpassing single-pass state-of-the-art. Embedding the undulator in an oscillator cavity is particularly useful in combination with high repetition rate electron sources, even if at reduced peak brightness, since recirculating a fraction of the radiation as an intense seed can compensate for lower single-pass gain. In this paper, we investigate the efficiency scaling of a tapering-enhanced waveguide oscillator, showcasing its capability for frequency-tuning operation and high-efficiency generation for different wavelengths. Using a thermionic-driven beamline equipped with compression elements, our simulation results indicate a 35% efficiency at 200~GHz and a 6.67% efficiency at 1.5THz, with out-coupling of a few hundred microjoules and tens of megawatts.

Speaker: Yining Yang (Tsinghua University)

183 Temporal and spatial coherence properties of the odd harmonics of the radiation from X-ray free electron laser with planar undulator

Radiation from SASE FEL with planar undulator contains visible contribution of the odd harmonics. Comprehensive studies of the nonlinear harmonic generation mechanism have been performed in * in the framework of the one-dimensional model. General features of harmonic radiation have been determined. It was found that coherence time at saturation falls inversely proportional to harmonic number, and relative spectrum bandwidth remains constant with harmonic number. In this paper we extend studies of higher harmonics taking into account diffraction effects. We consider parameter range when intensity of higher harmonics is mainly defined by nonlinear harmonics generation mechanism. Temporal and space correlation functions, coherence time and degree of transverse coherence are calculated using results of numerical simulations with the code FAST. Simulation of the FEL process has been performed using actual number of electrons in the beam. Application of similarity techniques allowed us to derive universal dependencies for the main characteristics of the SASE FEL covering all practical range of optimized X-ray FELs**. Present studies cover results for the 1st, 3rd, and 5th harmonic.

Speaker: Dr Mikhail Yurkov (Deutsches Elektronen-Synchrotron DESY)

184 Testing and characterization of Solid-State Amplifiers for PolFEL Accelerator

PolFEL stands for Polish Free Electron Laser, the first FEL research infrastructure in Poland. It will be based on a superconducting linear accelerator using Tesla-type resonant cavities with fundamental RF frequency of 1.3 GHz. Each superconducting cavity will operate in closed loop driven by individual Solid-State Amplifier (SSA) in single cavity regulation mode. The amplifiers have been specially designed for PolFEL by Kubara Lamina S.A. company. They are designed for providing 7kW peak power in pulsed regime and 5kW of continuous wave power at 1.3 GHz. This contribution presents the test stand for the SSA, the results of long-term stability tests and characterization of the power amplifier static and dynamic behaviour.

Speaker: Tomasz Kowalski (National Centre for Nuclear Research)

185 The effect of wake field and longitudinal space charge field on the SASE performance at S3FEL

Longitudinal wake fields are generated when relativistic electron beams pass through the narrow undulator vacuum chambers. The wake field, and the relatively weaker longitudinal space charge field, may cause considerable energy variations along the beam and influence the FEL radiation process. In this work, the wake field due to finite conductivity of the vacuum chamber material and the inner surface roughness are considered. Simulations are performed using beam and undulator parameters from Shenzhen Superconducting Soft X-Ray Free-electron Laser (S3FEL). Both effects are included in the simulations and their influences on the SASE-FEL performance are investigated.

Speaker: Huaiqian Yi (Institute of Advanced Light Source Facilities Shenzhen)

186 The FLASH User Facility Between Two Upgrade Shutdowns

FLASH, the XUV and soft X-ray free-electron laser user facility at DESY, is in the transitional period between two substantial upgrade shutdowns within the FLASH2020+ upgrade project.FLASH consists of a common injector and linac and drives 3 different beamlines of which the two FEL beamlines FLASH1/2 can be operated simultaneously at 10 Hz with subtrains of typically 1 to 500 bunches within the the 600 us RF flat tops made possible by the high duty cycle of FLASH's superconducting RF.The first (2021/22) shutdown was aimed at upgrading injector and linac and equipping the beamline FLASH2 with an APPLE-III type after burner undulator, to enhance the third harmonic output and to enable controllable polarization.The next (2024/25) shutdown will focus on the complete exchange of the FLASH1 beamline to allow for externally seeded operation in the range from 60 nm down to 4 nm at 1 MHz pulse repetition rate.

We report on the operation between the two shutdowns which was, to a large extend, dedicated to FEL operation for users and on the commissioning of the new features implemented in the last shutdown with emphasis on new features for FEL operation.

Speaker: Mathias Vogt (Deutsches Elektronen-Synchrotron DESY)

187 The hard X-ray self-seeding system at the European XFEL: current status and applications

The SASE2 undulator of the European XFEL is equipped with a fully operational Hard X-Ray Self-Seeding (HXRSS) system. HXRSS pulses are delivered on a regular basis to users [1] and together with the high rep-rate capabilities of European XFEL linac allowed already for innovative applications [2]. Recently we have delivered HXRSS with pulse energy up to 1.4 mJ at 9 keV and investigated different methods for bandwidth and background controls. In this work, presented on behalf of the HXRSS team at the European XFEL and DESY, we will review its status, current capabilities and outlook.
[1] see https://www.nature.com/articles/s41566-023-01305-x
[2] see https://www.nature.com/articles/s41586-023-06491-w

Speaker: Gianluca Geloni (European X-Ray Free-Electron Laser)

188 The preliminary study of pulse energy optimization with reinforcement learning

Traditionally, the optimization of Free-Electron Laser (FEL) facilities has been performed manually by the FEL operators. This approach proves to be time-consuming due to the multitude number of parameters that require adjustments. The results of this manual optimization are highly contingent upon the operator’s experience. To address these challenges, the implementation of machine learning algorithms offers a rapid and adaptable alternative for achieving global optimization of FEL performance within limited timeframes. In this paper, a surrogate model has been constructed using neural networks to expedite simulations. Additionally, the simulation results of FEL pulse energy optimization utilizing reinforcement learning are presented.

Speaker: Li Zeng (Institute of Advanced Light Source Facilities Shenzhen)

189 The simulation of beam-based alignment at S3FEL

The proposed Shenzhen Superconducting Soft X-Ray Free-electron Laser (S$^3$FEL) is a high-repetition-rate FEL facility which is currently commencing civil construction. The overlap between the electron beam and the radiation field is one of the critical determinants during the FEL amplification process, exerting a significant influence on the quality of FEL performance. The stringent requirement for straightness in both electron beam and radiation pulse can be achieved through the utilization of beam-based alignment (BBA) techniques. To optimize FEL performance, a combined approach incorporating electron beam-based alignment (e-BBA) and photon beam-based alignment (p-BBA) will be implemented at S$^3$FEL. This paper presents theoretical analyses and simulation results regarding both e-BBA and p-BBA techniques at S3FEL.

Speaker: Li Zeng (Institute of Advanced Light Source Facilities Shenzhen)

190 The UK XFEL conceptual design and options analysis project

The UK is conducting a multi-stage project to analyse the case for major investment into XFELs, through either developing its own facility or by investing at existing machines. The project’s 2020 Science Case identified a clear need for ‘next-generation’ XFEL capabilities including near-transform limited x-ray pulses across a wide range of photon energies and pulse durations; evenly spaced high-repetition rate pulses; and a high-efficiency facility with a step-change in the simultaneous operation of multiple end stations. The project is developing a conceptual design to meet these requirements, significantly aided by collaboration with international XFELs. It is also guided by an extensive ongoing user engagement programme of Townhall meetings and other activities. Both the science requirements and the emerging conceptual design are expected to be of general interest to the community.

Speaker: David Dunning (Science and Technology Facilities Council)

191 The waveguide effect of FEL oscillator at different horizontal size

In the long wavelength range of FEL oscillator, especially in FIR & THz, the vacuum chamber is designed as waveguide to suppress the diffraction effect during propagation. Meanwhile it will also bring some other effect, such as frequency shift, spectral gap, truncation loss and so on. The waveguide effect is studied mostly based on simple parallel plate situation by setting the horizontal size of rectangular waveguide as infinity. While in our simulation it reveals that this horizontal size can also affect the output power, the spectral gap is shifted by the horizontal size slightly. This is more clearly in FIR & THz wavelength range. Besides, the simulation results of FEL oscillator in different horizontal size and mirror curve is shown. The results shows that the waveguide size should be in match with cavity mirror curve to get the best FEL oscillator performance.

Speaker: Yuanfang Xu (University of Science and Technology of China)

192 The XUV High-Gain FEL Oscillator Project at FLASH

Externally seeded FELs deliver fully coherent radiation down to the soft X-Rays with high shot-to-shot stability. They are, however, limited in repetition rate by the available seed laser systems and highest achievable harmonic.
Cavity-based FELs, on the other hand, have the potential to generate fully coherent FEL radiation at the maximum repetition rate supported by superconducting accelerators.
The XRAY Project was aimed at setting up such a cavity-based high-gain FEL oscillator at 13.5 nm wavelength and 3 MHz repetition rate. In this contribution, we report on the experimental campaign carried out at the seeding infrastructure at FLASH.

Speaker: Margarit Asatrian (Universität Hamburg)

193 Time-resolved measurements of short-range transverse wakefields in flat corrugated structures

Flat corrugated structures designed to generate strong wakefields are used for various purposes in X-ray FEL facilities, in particular for longitudinal and transverse electron beam shaping and for diagnostics. In this contribution we present time-resolved measurements of transverse (dipole and quadrupole) wakefield effects produced in flat corrugated structures by electron bunches with typical parameters in X-ray FELs (few tens of um long and multi-GeV energies). Measurements were done as a function of the distance between the beam and the corrugated plate, beam optics and bunch length. The measurements generally agree with analytical predictions from a wakefield model [K. Bane et al., Phys. Rev. Accel. Beams 19, 084401 (2016)].

Speaker: Dr Philipp Dijkstal (Paul Scherrer Institute)

194 Tolerance studies of the APPLE-X Undulators for the EuPRAXIA@SPARC_LAB AQUA Beamline

The AQUA beamline of the EuPRAXIA@SPARC_LAB free-electron laser facility is a SASE FEL designed to operate in the water window, in the 3-4 nm spectral range. The beam driving this FEL is accelerated up to 1-1.2 GeV FEL by an X-band normal conducting linear accelerator followed by a plasma wakefield acceleration stage.
The main radiator consists of an array of ten APPLE-X permanent magnet undulator modules, each 2 m long and with a period length of 18 mm.
An analysis of resistive wall wakefields and tolerances to magnetic field errors and misalignments is discussed, and their impact on the FEL performance is evaluated.

Speaker: Dr Luca Giannessi (Istituto Nazionale di Fisica Nucleare)

195 Tunable UV Laser for External Seeding of the High Repetition Rate Soft X-ray Free Electron Laser FLASH

The X-ray free-electron laser (FEL) FLASH in Hamburg I currently installing installing a new FLASH1 beamline incorporating external seeding using High Gain Harmonic Generation (HGHG) and Echo Enabled Harmonic Generation (EEHG). Our novel UV seed laser system matches FLASH burst operation pattern with 1 MHz pulse trains and 600 µs duration at 10 Hz (6000 pulses/sec). It features two beams: Seed 1 (343 nm, 500 fs, 50 µJ) and Seed 2 (297 nm - 317 nm tunable, 50 fs, 16 µJ), pushing the limits of average in-burst power and pulse energy in UV. The laser system is designed to meet the stability requirements for 24/7 availability at a operating user facility.
Here, we will present our seed laser design, which employs a green-pumped femtosecond Optical Parametric Chirped Pulse Amplification (OPCPA) and an highly efficient cascaded sum frequency conversion (ccSFG) along with the installation status. Additionally, we will discuss our 40-meter vacuum beam transport system and our strategies to handle beam pointing, high peak powers, specialized coatings, variable B-integral adjustments, timing stabilization and pulse compression.

Speaker: Dr Tino Lang (Deutsches Elektronen-Synchrotron DESY)

196 Two-color soft X-ray free-electron laser schemes at the S3FEL

The Shenzhen Superconducting Soft X-Ray Free-electron Laser (S3FEL) is a newly proposed high repetition-rate X-ray FEL facility. In this contribution, we have presented various two-color FEL schemes based on this facility, including undulator splitting, fresh slice, and afterburner configurations. Based on these schemes, there is a promising prospect to generate high-brightness two-color soft X-rays on a timescale ranging from attoseconds to femtoseconds at the S3FEL, thereby providing a novel tool for experimental users in soft X-ray pump-probe studies.

Speaker: Dr Hao Sun (Institute of Advanced Light Source Facilities Shenzhen)

197 Ultrashort large-bandwidth X-ray free-electron laser generation with a dielectric-lined waveguide

In this letter, we proposed a new scheme to generate femtosecond-level high peak current electron beam, compressed from a flat-top beam, through energy modulation based on dielectric-loaded circular waveguide is numerically implemented.. With the advantage of passiveness and dephasingless self-modulation at terahertz frequency, stable electron beam and free electron laser radiation can be generated with this regime can provide more stable bunch for following x-ray free electron lasing using current enhanced SASE scheme. About 9.5 kA peak current can be achieved through 100 mm long dielectric-lined waveguide. Three dimensional start-to-end simulations have been carried out to demonstrate the Consequently, x-ray free electron laser (XFEL) performance for those high peak current bunch is simulated, verifying its ability to generate 2 fs radiation pulses with mean peak power up to 12 GW if undulators are tapered appropriately to compensate energy modulation caused by longitudinal space charge effect and undulator wakefield.

Speaker: Zhen Wang (Shanghai Advanced Research Institute)

198 Updates to Xopt for Online Accelerator Optimization and Control

The recent development of advanced black box optimization algorithms has promised order of magnitude improvements in optimization speed when solving accelerator physics problems. These algorithms have been implemented in the python package Xopt, which has been used to solve online and offline accelerator optimization problems at a wide number of facilities, including at SLAC, Argonne, BNL, DESY, ESRF, and others. In this work, we describe updates to the Xopt framework that expand its capabilities and improves optimization performance in solving online optimization problems. We also discuss how Xopt has been incorporated into the Badger graphical user interface that allows easy access to these advanced control algorithms in the accelerator control room.

Speaker: Ryan Roussel (SLAC National Accelerator Laboratory)

199 Upgrade of the High Brightness Photo-injector System at NSRRC

NSRRC high brightness photo-injector has been built to generate intense ultrashort electron bunches for novel accelerator-based light source development. The injector is equipped with a laser-driven photocathode rf gun and a 5.2-m long S-band traveling-wave linac for beam acceleration. The electron bunch is compressed near the linac rf zero crossing phase by the so-called velocity bunching technique. Based on autocorrelation technique the bunch length is measured with the THz coherent transition radiation (CTR) generated by passing the ultrashort electron beam through a metallic foil. Before 2019 the electron bunch length is measured to be 490 fs due to insufficient linac field. Recently the original Thales TH2100A klystron was replaced by the Canon klystron to provide higher rf power in the linac. Currently, the electron bunch length is further compressed to be 240 fs when the linac field gradient increased from 9.8 to 13.8 MV/m. The detailed upgrade process and relative light source development are presented in this paper.

Speaker: Ming-Chang Chou (National Synchrotron Radiation Research Center)

200 Utilizing a pair of orthogonally oriented corrugated structures for variable polarization self-locked streaking of electrons in time.

Corrugated structures have recently been utilized for the time-resolved diagnostics of electron bunches in the several GeV energy range and free-electron-laser (FEL) pulses across several FEL facilities: SwissFEL at PSI and European XFEL at DESY. This approach is simple and cost-effective, based on the self-streaking of electrons with a transverse wakefield enhanced in such structures.

In this work, we introduce the simplified design of a corrugated streaker developed for electron bunches in the several hundred MeV range and the wide range of beam parameters of the CERN Linear Electron Accelerator for Research (CLEAR). We emphasize the potential benefits of using a pair of orthogonally oriented streakers. Firstly, variable polarization streaking can be achieved in such a configuration. Additionally, the undesired quadrupole wakefield of streaking in the vertical (or horizontal) direction with one structure can be compensated by the second streaker. This allows for a significant improvement in the resolution of the method and paves the way for cost-effective and robust temporal diagnostics for future compact FEL facilities.

Speaker: Alexander Malyzhenkov (European Organization for Nuclear Research)

201 X-ray beam diagnostics at the European XFEL

After six years of user operation, the x-ray photon diagnostics system [1] at the European XFEL has matured and contributes daily to reliable beam delivery to users with exquisite tailored x-ray pulses at MHz repetition rates in the range from below 500 eV to above 24 keV at pulse energies up to 4 mJ in the hard x-ray and > 10 mJ in the soft x-ray range, with pulse durations from ~100 fs down to below 1 fs. This contribution shortly reviews established standard FEL beam diagnostics such as gas-based pulse energy measurements [2] at MHz-rates and scintillator-based imagers in 24/7 operation, mentioning also incurred damages [3] to these, and then highlights our powerful new beam-diagnostic capabilities beyond baseline: regular operation of online hard x-ray crystal-based spectrometry [4] for self-seeded lasing [5], two-color lasing and ultra-short pulse creation, gas-based online spectrometry for both soft and hard x-rays [6], online polarization monitoring for the soft x-ray undulator line [7], diamond-based MHz-rate beam position and pulse energy measurements [8], and temporal [9] as well as hard x-ray wavefront characterization.

Speaker: Jan Grünert (European X-Ray Free-Electron Laser)

202 High-power attosecond hard X-ray generation via enhanced SASE driven by cathode laser shaping

In molecular systems, quantum-level electron motions occur on the sub-femtosecond timescale. In order to probe such dynamics, high energy attosecond pulses are required to provide the necessary spatial and temporal resolutions. At LCLS, the X-ray Laser-Enhanced Attosecond Pulses (XLEAP) collaboration develops operation modes for attosecond pulses in the soft and hard X-ray regimes for these studies. Here, we experimentally demonstrate the generation of high-power attosecond hard X-ray pulses via enhanced SASE driven by cathode laser shaping. Under this operation mode, we report pulses with tens of microjoules of energy and single-spike spectra with an average bandwidth of roughly 20 eV and an estimated pulse duration on the order of 100 as. Individual shots exhibit a bandwidth of up to 50 eV, paving the way to sub-100 as pulses with X-ray FELs.

Speaker: Veronica Guo (Stanford University)

18:00 IEC Meeting

20:00 Dinner IEC

Wednesday 21 August

Electron sources: Session-6

Convener: Laura Badano (Elettra-Sincrotrone Trieste S.C.p.A.)

203 Commissioning of the SHINE Electron Source

Shanghai high repetition rate XFEL and Extreme light facility (SHINE) is designed to be driven by an 8 GeV continuous wave (CW) superconducting RF (SRF) linac with MHz repetition rate high brightness electron beam. SHINE started electron source installation in 2023, and the first 100 pC photoelectron beam successfully reached the ~1 MeV beam dump at 1 MHz rate in December of 2023. After that, the 100 MeV injector resumed installation in January of 2024. In this report, both the electron source commissioning results and the 100 MeV injector installation progress will be presented.

Speaker: Houjun Qian (Shanghai Zhangjiang Laboratory)

204 Overview of the Center for Bright Beams photocathode work

Low Mean Transverse Energy (MTE) photocathodes play a crucial role in enhancing the performance and capabilities of Free Electron Lasers (FELs) and other advanced electron accelerator applications, enabling groundbreaking scientific research across diverse fields. Due to the requirements for the high charge density, disordered nature of photocathode materials, surface roughness and work function variations, the MTE of electrons emitted from conventional photocathodes is limited to several 100s of meV, which is nearly an order of magnitude larger than the theoretically predicted MTE limit. Moreover, most electron sources used for high-brightness applications degrade under high applied electric fields or large incident laser fluences. Therefore, robust photocathodes capable of emitting bright electron beams with significantly reduced MTE can enable higher-quality FEL output with better beam characteristics. In this talk, we will review different techniques for advanced photocathode modeling, growth, fabrication, and characterization which are used at the Center for Bright Beams (CBB, https://cbb.cornell.edu) towards brighter electron beam production.

Speaker: Oksana Chubenko (Northern Illinois University)

205 Breaking Convention: Novel Normal-Conducting Electron Sources for Higher 6D Brightness

As part of an international collaboration between PSI and INFN, two novel C-band RF photoguns have been designed and realised in the context of the IFAST programme. These RF photoguns aim to push the brightness of the next generation of FELs through an increased cathode gradient achieved through reducing the filling time of the device. In this work, we demonstrate how these newly developed photoguns will impact upon the achievable beam brightness. This begins with the presentation of the first high power testing of these new photoguns in dedicated tests stand at the Paul Scherrer Institut. The beam brightness will be modelled using beam dynamics simulations including the effects of intrabeam scattering, which is now known to greatly impact upon the sliced energy spread. Finally, as we continue to look forward and further push the machine brightness, we discuss how we can continue to push the peak cathode gradient, and consequently beam brightness, of ‘conventional’ normal-conducting electron sources using unconventional high bandwidth RF designs.

Speaker: Thomas Lucas (Paul Scherrer Institute)

206 Novel Photocathode Lasers for the Hard- and Soft- X-ray Free Electron Lasers EuXFEL and FLASH

We commissioned a new type of photocathode laser systems (NExt generation PhotocAthode Laser NEPAL) at the superconducting XFELs FLASH and EuXFEL.
The NEPAL lasers provide temporal and spatial shaped picosecond deep UV pulses enabling very low emittance electron beams, which are needed for highest X-ray photon energies. At EuXFEL we have demonstrated an excellent transverse projected emittance of 0.375 mm mrad for the routine working point of 250 pC. The new laser systems offer advanced controls, including individual control over the generated electron bunch charge and temporal and spatial pulse shaping for emittance optimization. At EuXFEL we used the temporal shaping capability to match the electron beam properties to the existing laser, allowing fast switching in case of failures. Furthermore, we are exploring advanced shaping techniques that utilize the phase-shaping capabilities of our pulse-shaper for injector emittance optimization in an ongoing R&D project. Simulations show that the advanced beam shaping capabilities of the NEPAL lasers will both allow to push the X-ray photon-energy beyond 25 keV and to enhance special operation modes, such as attosecond X-ray generation.

Speaker: Ingmar Hartl (Deutsches Elektronen-Synchrotron DESY)

10:35 Coffee break

Electron beam dynamics: Session-7

Convener: Yuantao Ding (SLAC National Accelerator Laboratory)

207 Chicane or arc compressors for FEL? - Experience with the MAX IV arc compressors and beyond

The MAX IV bunch compressors consist of two achromatic arcs each where linearisation of longitudinal phase space is done with a sextupole in the centre of each achromat instead of a harmonic cavity. Although the initial design choice was a cost-saving solution, simulations and 10 years of experience demonstrate that arc compressors not only perform as well as a chicane compression system but can actually do better for some beam properties, such as arrival time jitter, ultra-short bunch length and high peak current. We compare variants of both schemes, and discuss the pros and cons. We show in simulation the superior performance of arc compression schemes, especially in the regime of strong compression where CSR-driven emittance degradation and microbunching is dominant.

With the newly installed and commissioned transverse deflecting cavity system, the MAX IV compression and linearisation scheme has been studied and characterised and the results show a very stable compression scheme for the current Short Pulse Facility as well as promising ultra-short bunches for proposed FELs.

Speaker: Sara Thorin (MAX IV Laboratory)

WEBI01_slides.pdf

208 Microbunching Instability Mitigation Strategies and Diagnostic Methods

One of the key barriers to the generation of free-electron laser (FEL) pulses that are fully longitudinally coherent is the microbunching instability. Numerous methods for suppressing this instability have been studied, and recent investigations at the FERMI FEL have demonstrated that the transverse beam optics can play an important role in achieving this mitigation. Experimental and theoretical studies show this effect, and a new method for measuring the microbunching in the electron beam is presented. Future investigations are also planned which can take advantage of microbunching suppression based on beam optics.

Speaker: Alexander Brynes (Elettra-Sincrotrone Trieste S.C.p.A.)

209 Recent Progress in Steady-State Micro-Bunching Light Source Development

Steady-State Micro-Bunching (SSMB) was proposed by Ratner and Chao in 2010 to generate high-power short-wavelength coherent radiation in an electron storage ring. To advance the SSMB development, an SSMB taskforce was established in Tsinghua University since 2017, in collaboration with several institutes around the world. In this talk, the recent progress achieved by the SSMB collaboration will be reviewed. More specifically, we will report the results of the SSMB proof-of-principle experiment conducted at the Metrology Light Source and present the conceptual design of an SSMB-EUV light source which can deliver 1 kW EUV radiation. The development progress of some key technologies for SSMB will also be briefly introduced, such as the laser enhancement cavity, the electron beam injection system and the accelerating unit of an SSMB storage ring.

Speaker: Zhilong Pan (Tsinghua University)

210 First measurements of quantum diffusion in an undulator

Plasma and beam physics are usually considered as classical physics disciplines with quantum effects featuring only rarely. In particular, free electron lasers (FELs) even in the Angstrom regime (developed recently and being upgraded towards even shorter wavelengths) are well described by classical mechanics and electrodynamics. There is, however, a quantum effect that can influence the operation of these devices and limit the shortest achievable wavelength, namely energy diffusion in an electron beam due to quantum fluctuations in undulator radiation. Although this effect has been calculated theoretically, it has never been measured. In this work we present measurements of quantum diffusion effect at the European X-Ray Free-Electron Laser. The method uses a recently installed wakefield structure, which enables measurements of the longitudinal phase space after the hard X-ray undulator. The effect of quantum diffusion in the undulator is measured for the first time, and the results are in good agreement with theoretical predictions.

Speaker: Sergey Tomin (Deutsches Elektronen-Synchrotron DESY)

12:50 Lunch break

Novel acceleration and FEL concepts: Session-8

Convener: Hideaki Ohgaki (Kyoto University)

211 Plasma accelerating modules developments for the EuPRAXIA FEL user facility

The EuPRAXIA project offers the opportunity to produce free-electron laser radiations from accelerated electron beams using plasma technology in compact structures. Furthermore, the FELs radiations, in turn, represent a formidable tool to investigate matter at the sub-atomic level by generating coherent light pulses with sub-ångström wavelengths and sub-femtosecond durations. Achieving such a result is necessarily linked to the ability to design and construct dedicated devices suitable both for formation and confinement of plasmas in thin structures, which are mounted inside the particle accelerator. In recent years, these motivations have driven an intense technological development of plasma accelerating modules in order to achieve high gradients while retaining the high quality of accelerated beams, based on what is required to produce FEL radiations. Therefore, as will be discussed, discharge capillaries, gas cell or gas jets have been designed for acceleration and focusing experiments to control plasma properties, as stability, density etc., in order to optimize the interaction with electron beams. Prospects for high repetition rate module development will be also discussed.

Speaker: Angelo Biagioni (Istituto Nazionale di Fisica Nucleare)

212 Reduction of the electron beam divergence of laser wakefield-accelerators by integrated plasma lenses

We report on electron-beam collimation using a passive plasma lens, integrated directly into a laser wakefield-accelerator stage operating in the high-charge regime. The lens is created by the reshaping of the gas-density profile of a supersonic jet at the beam’s exit side. It reduces the beam’s divergence by a factor of 2 to below 1 mrad (rms), while preserving the total charge of 170 pC and maintaining the energy spread. Ultrafast probing of plasma dynamics and Particle-in-Cell (PIC) simulations reveal that the effect is induced by the focusing field of the generated beam-driven wakefield as the remnant laser intensity drops significantly. The resulting spectral-charge density, defined here as the charge per energy bandwidth and emission angle, of up to 7pC/MeV mrad played a key role in the recent experimental demonstration of free-electron lasing*. The simple and robust gas-shaping technique presented holds the potential to generate specific density profiles, which are essential for the application of adiabatic focusing or staging of accelerators.

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

213 Energy and brightness-boosted electron beams from plasma-based accelerators

Recent advancements in plasma-based accelerators have ignited "the race for wakefield-driven FELs" [1]. However, FELs at soft and hard X-ray wavelengths require electron beams with dramatically improved 6D brightness. The author presents comprehensive strategies towards generating ultra-high 6D brightness electron beams in beam-driven plasma wakefield acceleration (PWFA). These ultra-high 6D brightness electron beams may enable novel photon and fundamental science modalities [2]. One direct ramification of this new class of electron beams is a blueprint for an ultra-compact attosecond-Angstrom class FEL [3]. Further, the author will explain how these high-brightness PWFA stages can enhance the capabilities and modalities of existing and future linac-based FELs.

[1] Graydon, O. Nat. Photon. 16, 750–751 (2022).
[2] Habib, AF et al. Annalen der Physik 535.10,p. 2200655 (2023)
[3] Habib, AF et al. Nat. Commun. 14, 1054 (2023)

Speaker: Dr A. Fahim Habib (University of Strathclyde)

214 Experimental efforts towards non-collinear superradiant Compton scattering

Inverse Compton scattering by relativistic electrons off intense laser pulses provides an attractive option for compact radiation sources in the (soft) X-ray spectral range. Tunability of the radiation wavelength is greatly increased by varying the crossing angle in the interaction. On the other hand, the radiated power from these sources is typically rather low, limiting the range of applications. By imposing a spatial modulation on the electron beam the yield can be enhanced by many orders of magnitude via superradiant emission even when considering limiting contributions by spot sizes, energy spread and beam emittance. However, attaining the required density modulation at the relevant electron beam energy is still a major challenge. We will discuss our experimental efforts on two alternative methods of achieving the required density modulation for superradiant Compton scattering at the UCLA Pegasus laboratory: ponderomotive bunching by two lasers at different frequencies and attosecond velocity bunching using an s-band buncher linac in conjunction with a x-band linearizer. Our plans to observe the coherent enhancement of Compton scattering in the near future are considered.

Speaker: Brian Schaap (Eindhoven University of Technology)

16:20 Coffee break

Industrial applications of FELs: Panel discussion

Convener: Sandra Biedron (University of New Mexico)

Thursday 22 August

Electron diagnostics, timing, synchronization & controls: Session-10

Convener: Rasmus Ischebeck (Paul Scherrer Institute)

215 Measurement with sub-femtosecond resolution of electron and photon beams

Time-resolved diagnostics with sub-femtosecond resolution are of crucial importance for ultra-fast x-ray FEL applications. Radio-frequency (RF) transverse deflector structures (TDS) are typically employed to characterize the temporal properties of the electron beams driving FELs. If located after the undulator section, the TDS can measure the FEL power profile as well. In this contribution, we present measurements with sub-femtosecond resolution from C-band and X-band RF TDS at SwissFEL. Furthermore, we give an overview of other time diagnostics under development at SwissFEL, such as photon streaking and FEL phase diagnostics.

Speaker: Dr Philipp Dijkstal (Paul Scherrer Institute)

216 Efficient 6-dimensional phase space measurements using generative machine learning and applications to autonomous beam monitoring at LCLS-II

Enhancing the performance and capabilities of free electron lasers, such as LCLS-II, hinges on our ability to precisely control and measure the 6-dimensional phase space distribution of the beam. However, conventional diagnostic techniques necessitate a substantial number of measurements and computational resources to characterize a single beam distribution, using many hours of valuable beam time. In this work, we present a novel approach to analyzing experimental measurements using generative machine learning models of 6-dimensional beam distributions and differentiable beam dynamics simulations to substantially reduce the number of measurements needed to reconstruct detailed phase space distributions. We demonstrate in simulation and experiment that using our analysis technique can reconstruct detailed 6-dimensional phase space distributions using as few as 20 beam measurements with no prior training or data collection. We also discuss plans for combining this work with advanced accelerator control algorithms and parasitic beam measurements to autonomously monitor the 6-dimensional phase space distribution of the beam at LCLS-II during accelerator operations.

Speaker: Ryan Roussel (SLAC National Accelerator Laboratory)

217 ML-driven Automated Tuning of XFEL for Various Experiments and User-specific Requirements at SACLA

To satisfy recent complex requirements from XFEL applications, an automated tuning tool has been developed and regularly used for daily XFEL operations at SACLA. In a ring-based synchrotron radiation facility, an electron beam with identical characteristics is provided to many user beamlines. In contrast, in a linear accelerator-based XFEL facility with multiple beamlines, the electron beams should be properly tailored by varying energies and longitudinal distributions to meet simultaneously different experimental requirements of each beamline. In addition, since the requirements change every few days alongside the users, efficient and prompt accelerator tuning is crucially important. To address this challenge, we have developed an automated tuning system based on Bayesian optimization, and it can optimize performance indicators, which encompasses not only a central wavelength and a pulse energy, but also a spectral shape and a transverse laser profile, and so largely facilitates the delivery of XFELs that mostly meet the stringent requirements set by experimental users. This presentation will report our recent efforts and results in daily XFEL tuning at SACLA utilizing the tool.

Speaker: Eito Iwai (RIKEN SPring-8 Center)

218 Low-jitter conversion from optical references to electrical radio frequency signals

Ultra-fast science at free electron laser (FEL) facilities is pushing accelerator and FEL technology towards shorter laser pump and FEL probe pulses to resolve fast dynamics.

Ideally, the short pulses should be backed by a synchronization system that provide a pump-probe jitter that is similar to the pulse duration. This is challenging, and the achieved timing jitter is typically one order of magnitude larger than the pulse duration.

Recent developments at MAX IV are focused on the use of a low-noise optical main oscillator (OMO) as the common reference for the accelerator. The OMO optical signal is converted to electrical RF with a photo detector. The conversion does not add jitter by amplitude-to-phase coupling that can be present in photo-detector conversion. We have also enhanced the available electrical RF power from the detector by repetition rate multiplication, which shifts power in the spectral plane to the frequency of the RF system.
The combination of an OMO and direct conversion gives on the order of 1 fs relative jitter between the reference laser and the generated RF.

Speaker: Erik Mansten (MAX IV Laboratory)

219 Single-shot electro-optic measurement of bunch shapes using diversity schemes at European XFEL and FLASH

Laser-based electro-optic detection is a well-known tool for characterizing electron bunch shapes - and electric fields in general – in single-shot. However, reaching sub-ps resolution together with long acquisition length still remains a challenging problem. This motivated a search for novel conceptual approaches to the problem, based on the so-called diversity schemes* (i.e., using multiple simultaneous measurements for retrieving an electron bunch shape). In this spirit, we reconsider here the traditional technique, consisting of imprinting the electric field of interest onto an chirped optical probe, and of recording optical spectra of the modulated pulses. A key point is a totally novel approach for inverting the problem, i.e., for retrieving the bunch (or THz pulse) shape from the measurements. We will also present the first numerical and experimental tests of this single-shot measurement method. The immediate applications are bunch shape monitors at FLASH and European XFEL, however such methods have further potential in applications in characterizing THz CTR or FELs sources.

Speaker: Quentin Demazeux (Laboratoire de Physique des Lasers, Atomes et Molécules)

10:35 Coffee break

Photon beamline instrumentation & undulators: Session-11

Convener: Jan Grünert (European X-Ray Free-Electron Laser)

220 Superconducting Undulator developments at the European XFEL

Superconducting undulators (SCUs) have been and are successfully delivering x-rays in storage rings. Within its facility development strategy, European XFEL plans to implement SCUs in the upcoming years. This contribution describes different activities ongoing and planned supporting this upgrade.

Speaker: Sara Casalbuoni (European X-Ray Free-Electron Laser)

221 Diamond sensors for fast pulse-resolved hard X-ray FEL beam position and intensity monitoring

There are commercially available diamond sensors for synchrotron beams, but they are incompatible for application at XFELs. EuXFEL had launched dedicated R&D to create sensors that survive intense MHz-rate beams and deliver fast pulse-resolved position and intensity data. This presentation reports on new sensors and results of their testing with hard x-ray FEL beam, where we introduced fast e-beam orbit kicks. Future applications like integration to feedback loops with the electron machine could bring beam stability to a new level.

Speaker: Wolfgang Freund (European X-Ray Free-Electron Laser)

222 Force-Neutral Adjustable Phase Undulators

Variable gap undulators require large and complex motion systems to operate, making their tunability to generate specific radiation wavelengths cumbersome, limited, and slow. RadiaBeam Technologies is engaging in a project to advance undulator manufacturing by utilizing force-neutral adjustable phase undulator (FNAPU) technology developed by Argonne National Laboratory (ANL). This innovative approach allows high precision undulators to be more compact, cost-effective to fabricate and assemble, and safe and user friendly in alignment, manipulation and operation.
The innovation of FNAPU technology is based on the inclusion of a secondary array of permanent magnets, arranged to compensate the internal forces brought on by the main undulator array. The flexibility and compact design of FNAPUs allows for exotic applications (X-undulators) and multiple FNAPUs can be packed together to form an undulator matrix, covering extensive X-ray energy ranges and a broad range of applications, relevant to the needs of XFEL and SR communities, and beyond.

Speaker: Nathan Burger (RadiaBeam Technologies (United States))

223 Challenges for the LCLS-II HE Instrument Suites

The recent LCLS-II upgrade allows operation with bunch rates up to 1MHz for photon energies ranging from 200eV up to about 5keV. The LCLS-II-HE design will extend the high rate capability to 12keV and beyond from an 8GeV superconducting RF linac. The high energy upgrade imposes technical challenges on the X-Ray photon transport line devices to effectively deliver high average power beams to the X-Ray instruments.
This talk provides an overview of the LCLS-II-HE project and summarizes the design challenges on the photon beam transport line diagnostics, optics, and beam containment components. There are thermal and stability challenges faced by the optics deflecting the coherent FEL X-Ray pulses with minimal wave front distortion. The X-Ray pulses are manipulated by attenuators, apertures, focusing optics and monochromators, each presenting its own set of challenges.
Additionally, there are programmatic and management challenges. The project’s scope deliverables need to align with the facilities vision. Misalignment constrains experimental capability and incur expenses. We’ll touch on some areas that required realignment with the long-term vision of the LCLS HXR instrument suite.

Speaker: Eliazar Ortiz (SLAC National Accelerator Laboratory)

12:50 Lunch break

14:30 Transfer and visit to National Centre for Nuclear Research (NCBJ) or sightseeing in Warsaw or visiting the POLIN Museum

19:30 Social Dinner and FEL Prize ceremony

Friday 23 August

Advanced FEL modes and science applications: Session-12

Convener: Agostino Marinelli (SLAC National Accelerator Laboratory)

224 Fully structured light with seeded free-electron lasers

Light with precisely tailored structure in all degrees of freedom is called fully structured light. We present an experimental demonstration of coherent FSL EUV light with the `star’ type polarization topology at the FERMI free electron laser (FEL) in Trieste, Italy. Control of the polarization is obtained through the overlap of radiation emitted in orthogonally polarized helical undulators with different transverse phase distributions. The spatial polarization structure was mapped by imaging the light downstream of a polarizer which showed different distinct polarization states distributed across the spatial positions of the radiation. These states showed near complete coverage of the Poincare sphere, in good agreement with predictions.

Speaker: Jenny Morgan (SLAC National Accelerator Laboratory)

225 X-ray FEL lays the groundwork for Scandium-45 nuclear clock

Precise timekeeping is indispensable in everyday life, science, and technology. It relies on reference oscillators with stable frequencies. Atomic clocks -- the most precise time-measurement devices at present -- use spectrally very narrow resonant transitions between electronic states in atoms as their reference oscillators. With the advent of hard x-ray FELs, the use of extremely narrow resonant transitions in atomic nuclei as reference oscillators for ultra-high-precision clocks is now within reach. Nuclear oscillators are naturally more stable and more resilient to external perturbations than their atomic counterparts. Resonant excitation of a ultra-narrow transition in Scandium-45 nuclear isomer with hard x-rays became recently possible [1] due to the high spectral photon flux delivered by the European XFEL in self-seeded high-repetition-rate mode. In this talk, the results of this experiment will be presented along with discussion of further developments of hard X-ray FELs required for ultra-high precision nuclear clocks.

[1] Shvyd'ko, Yu. et al. Resonant x-ray excitation of the nuclear clock isomer $^{45}$Sc. Nature 622 (2023) 471.

Speaker: Yuri Shvyd'ko (Argonne National Laboratory)

FRAI02_slides.pdf

226 Experimental demonstration of attosecond pump–probe spectroscopy with an X-ray free-electron laser

Pump–probe experiments with subfemtosecond resolution are the key to understanding electronic dynamics in quantum systems. Here we demonstrate the generation and control of subfemtosecond pulse pairs from a two-colour X-ray free-electron laser. By measuring the delay between the two pulses with an angular streaking diagnostic, we characterize the group velocity of the X-ray free-electron laser and show control of the pulse delay down to 270 as. We confirm the application of this technique to a pump–probe measurement in core-ionized para-aminophenol. These results reveal the ability to perform pump–probe experiments with subfemtosecond resolution and atomic site specificity.

Speaker: Zhaoheng Guo (Paul Scherrer Institute)

227 Demonstration of tunable, phase-locked X-ray FEL pulses

The availability of coherent copies of free-electron laser (FEL) X-ray pulses with tunable delay will facilitate a realm of techniques, such as the X-ray analogue of Fourier transform infrared (FTIR) spectroscopy, and accelerate the development X-ray quantum optics. Here we report steps toward phase-locked, tunable X-ray FEL pulses by combining the self-seeding mechanism, the slotted foil technique and transverse beam shaping, following our proposal in PNAS 11, e2117906119 (2022). Experiments have been conducted at the PAL-XFEL facility in Pohang, South Korea, and reveal coherent interference of few-femtosecond hard X-ray pulses and achieved a tunable time delay between them of 7 to 12 fs. Our future efforts will include improving the performance and tunability of the scheme, and applying it to time-domain hard X-ray interferometry experiments.

Speaker: Wenxiang Hu (Paul Scherrer Institute)

10:35 Coffee break

Attosecond science - Nobel Prise session: Session-13

Convener: James Cryan (SLAC National Accelerator Laboratory)

228 Attosecond x-ray free-electron lasers

Free-electron lasers are the brightest sources of attosecond x-ray pulses, improving the brightness by more than six orders of magnitude with respect to table-top high-harmonic sources. This huge increase in brightness has opened new avenues for attosecond science, leading to the demonstration of non-linear x-ray spectroscopy and x-ray pump/probe experiments with attosecond resolution. Furthermore, the ability to reach the hard x-ray spectral region paves the way for direct imaging of electronic processes with x-ray scattering. In my talk I will review the development of attosecond x-ray free-electron lasers: from the pioneering theoretical work on SASE, ESASE and chirp-tapered FELs, to the recent experiments the Linac Coherent Light Source and other XFELs worldwide. I will also give my personal view on the future of the fields and discuss ongoing developments in the context of plasma-based high-brightness electron sources.

Speaker: Agostino Marinelli (SLAC National Accelerator Laboratory)

229 Attosecond Science at FELs

The development of trains and isolated attosecond pulses in the extreme ultraviolet (XUV) and soft X-ray range at free-electron lasers (FELs) has opened up new avenues for attosecond science [1,2]. These pulses possess notable characteristics such as energy tunability, high peak intensities and, in the case of seeded FELs, precise phase and amplitude control.
In this presentation, I will show how attosecond metrology approaches initially developed for high-harmonic generation (HHG)-based attosecond sources have been successfully applied for the temporal characterisation of pulse trains as well as isolated attosecond pulses at FELs. Similarly, attosecond spectropy techniques used to elucidate the occurrence of attosecond time delays in photoionisation using HHG sources are currently employed for the investigation of similar effects using FELs in the soft X-ray spectral region. Furthermore, I will demonstrate how FELs can deliver attosecond radiation with unique properties (amplitude and phase control) that outperform the current capability offered by HHG-based sources [3].

Speaker: Prof. Giuseppe Sansone (University of Freiburg)

230 A novel single-shot characterization method for attosecond FEL pulses using self-referenced spectral interferometry

The complete spectrotemporal characterization of attosecond X-ray free-electron lasers (FELs) is of great importance for the ultrafast scientific experiments. Currently, the lack of high-precision characterization methods has become a key bottleneck that limits the application of attosecond X-ray FELs to some extent. To address this issue, we proposed a novel method, demonstrated by a proof-of-principle experiment, for single-shot characterization of ultrashort FELs based on self-referenced spectral interferometry. A pair of replica pulses with suitable spectral shear can be generated by using the frequency-pulling effect, and then, the complete spectrotemporal information of attosecond FELs can be extracted from the spectral shearing interferogram of these two frequency-sheared pulses. Recently, we are planning to conduct ultrafast experiments at SXFEL in order to further validate this characterization method in the X-ray range. This would provide an excellent diagnostic approach for the optimization and fine-tuning of ultrafast FELs and future attosecond scientific experiments based on X-ray FELs.

Speakers: Prof. Chao Feng (Shanghai Advanced Research Institute), Yaozong Xiao (Shanghai Optoelectronics Science and Technology Innovation Center)

231 Applications of Attosecond Soft-X-ray pulses to Photoemission Chronoscopy and Transient Absorption

Attosecond soft-X-ray pulses can nowadays be produced either through high-harmonic generation (HHG) or free electron lasers (FELs). Whereas HHG sources achieve the shortest durations (43 as [1]), FELs achieve the highest peak intensities [2]. I will discuss recent experiments that exploit the complementarity of these attosecond sources. Combining attosecond soft-X-ray pulses from LCLS with circularly polarized infrared pulses, we have measured attosecond photoionization delays of N1s photoemission of a series of aromatic azabenzene molecules (pyridine, pyrazine, s-triazine) [3]. We have observed a systematic increase of the photoionization delays with increasing number of electronegative nitrogen atoms and with increasing symmetry of the molecular scaffold. Taking advantage of the excellent timing stability of HHG-based attosecond pulses, we have observed the decoherence and revival of charge migration in neutral silane molecules and the transfer of electronic coherence through conical intersections [4]. Exploiting the broad bandwidth of HHG-based sources, we have observed a charge-directed proton-transfer reaction in ionized urea solutions [5]. These experiments highlight the complementarity of HHG- and FEL-based sources and suggest promising perspectives for attosecond science.

References:
[1] T. Gaumnitz et al., Opt. Exp. 2017, https://doi.org/10.1364/OE.25.027506
[2] J. Duris et al., Nat. Photon. 2020, https://doi.org/10.1038/s41566-019-0549-5
[3] J.-B. Ji, Z. Guo et al., arxiv https://doi.org/10.48550/arXiv.2402.17685
[4] D. Matselyukh et al., Nat. Phys. 2022, https://doi.org/10.1038/s41567-022-01690-0
[5] Z. Yin et al., Nature 2023, https://doi.org/10.1038/s41586-023-06182-6

Speaker: Hans Jakob Woerner (ETH Zurich)

12:50 Close up