FEL2026 - 42nd International Free Electron Laser Conference

23 Aug 2026, 09:00 → 28 Aug 2026, 17:00 America/Los_Angeles

Zhirong Huang (SLAC National Accelerator Laboratory)

On behalf of the International Executive Committee of the FEL Conference series, we are pleased to announce the 42nd International Free Electron Laser Conference (FEL2026). The conference will be held at Li Ka Shing Center located on Stanford campus from August 23rd to August 28th, 2026. 

FEL2026 will focus on recent advances in free electron laser theory and experiments, electron beam, photon beam and undulator technologies, and applications of free electron lasers.

FEL2026 will be hosted by SLAC National Accelerator Laboratory and Stanford PULSE Institute. Stanford is the birthplace of the free-electron laser. SLAC houses two premier X-ray FEL facilities (LCLS and LCLS‑II) in the US. The conference program will include an optional tour of the LCLS and LCLS-II. 

Stay tuned for more conference information by following this official conference website (www.fel2026.org). Email questions should be addressed to fel2026@slac.stanford.edu. 

Sunday 23 August

15:00 Registration Starts

17:00 Welcome Reception

Monday 24 August

First Lasing and Special Topics (Invited)

1 In Memoriam: Todd Smith

Place holder for the Special talk in memoriam for Todd Smith (Stanford).

Speaker: Alan Schwettman (Stanford University)

2 XFEL science overview and outlook

Place holder for Kelly's talk.

Speaker: Kelly Gaffney (SLAC National Accelerator Laboratory)

FEL Prize Talks (Invited)

3 Which is better, small or large? -Lessons we learned at operations of SACLA-

The Compact X-ray Free-Electron Laser (SACLA) initially faced severe criticism that it would be unable to achieve effective laser amplification. However, SACLA smoothly achieved laser amplification after its construction and has become one of the world’s most widely used FEL facilities, capable of conducting independent experiments simultaneously with its three FEL beamlines. This presentation focuses on the achievements obtained by the unique systems SACLA was the first in the world to introduce. The presentation also discusses the advantages of building a compact facility, as well as the considerations of a large-scale facility.

Speaker: Hitoshi Tanaka (RIKEN SPring-8 Center)

4 SACLA XFEL Facility: Now and What's Next

Since the commissioning in 2011, SACLA has made continuous efforts to improve stability and XFEL performance with a focus on user operation. A variety of XFEL operation modes have been offered to users, such as two-color, self-seeded and attosecond XFEL. In parallel with XFEL operation, SACLA has been also playing an unique and important roll as a low-emittance injector of a storage ring, aiming at the ongoing SPring-8-II upgrade project. In this talk, the R&D strategy to date and future vision of SACLA will be outlined.

Speaker: Toru Hara (RIKEN SPring-8 Center)

5 Toward zeptosecond FELs

Recent advancements in FEL science and technologies have made it possible to generate attosecond pulses in many FEL facilities by means of various methods. Such "attosecond FELs" definitely contribute to the progress of ultrafast science, by improving the temporal resolution in time-resolved measurements; however, we should recall that human desires are limitless! The next target may probably be the realization of "zeptosecond FELs", which is in principle possible by reducing the pulse length of angstrom FELs ultimately down to a few cycles. In this talk, I will review the theoretical and experimental studies carried out to realize ultrashort and few-cycle FELs together with the prospectives toward the zeptosecond FELs.

Speaker: Takashi Tanaka (RIKEN SPring-8 Center)

6 Electron and photon beam diagnostics and wakefield studies with corrugated structures

Wakefield structures can generate strong transverse chirps (beam tilts) and thereby enable time-resolved beam diagnostics similar to transverse deflecting rf structures, but at a much reduced cost. Electron beam current and FEL power profile measurements using rectangular corrugated structures are routinely performed at SwissFEL and other facilities. Some of the data analysis methods required to perform the nonlinear mapping of streaked transverse coordinates to time coordinates are based on an analytical wakefield model. The analytical model was benchmarked against wakefield measurements performed under variations of streaking strength and beam optics, allowing for an assessment of the model applicability in the context of beam diagnostics. Another data analysis approach relies on on the current profile being known through other diagnostics devices to deduce the mapping function, which greatly reduces the complexity. This presentation gives an overview of the wakefield studies and wakefield-based beam diagnostics performed and developed at SwissFEL.

Speaker: Dr Philipp Dijkstal (Paul Scherrer Institute)

FEL Theory and Machine Learning (Invited)

7 Space Charge Studies for Enhanced SASE at the Soft X-ray Beamline Athos at SwissFEL

We present studies on enhanced SASE at the soft X-ray beamline Athos at SwissFEL. The generation of a series of current spikes with 790 nm spacing is injected into the Athos undulator beamline. Due to strong space charge forces the optimum performance occurs with a strong positive taper of the undulator field. There is a clear modal structure in the observed spectra, indicating the presence of a train of attosecond FEL pulses with no intrinsic phase-locking. The space-charge dynamics causes an elongation of the current spike and a redshift in the bunching spectra, which determines the optimum taper gradient

Speaker: Sven Reiche (Paul Scherrer Institute)

8 Homogeneity and Criticality of the time-dependent FEL action

We derive a resonant effective action for the time-dependent free-electron laser. The action treats longitudinal time dependence and finite-bunch effect within a single self-consistent framework. After canonical normalization, the action is shown to be homogeneous under Pierce parameter rescaling. Such property directly yields scaling laws for the field, without explicitly finding or solving the equations of motions.
Within the same framework, we show that the onset of weak superradiance is identified with a dynamical critical point at which the system acquires an emergent scaling symmetry.

Speaker: Dr Giovanni Perosa (European X-Ray Free-Electron Laser)

9 Neural networks and differentiable-simulation based complete 6-dimensional phase space reconstruction using standard accelerator elements

Control of 6-dimensional beam phase space is one of the most important steps for high-brightness X-ray Free Electron Lasers (XFELs). Such beam manipulation can be properly performed through accurate beam diagnostics. Recently, generative phase space reconstruction (GPSR) based on neural networks and differentiable simulations has been developed for robust phase space diagnostics, including coupled information. We incorporated GPSR method into conventional beam diagnostics techniques, such as quadrupole and RF cavity phase scans, to enable more efficient and complete 6-dimensional phase space reconstruction using standard accelerator elements. Both simulations and experimental demonstrations at the Pohang Accelerator Laboratory Free Electron Laser facility show that the reconstructed phase space accurately predicts not only the training and test datasets but also other phase spaces further downstream in the beamline that are not included in the GPSR training. These observations reveal that the reconstructed phase space closely resembles the ground-truth physical distribution and opens a path for robust beam diagnostics using elements readily available in many accelerator facilities.

Speaker: Seongyeol Kim (Pohang Accelerator Laboratory)

10 Overview and outlook of applying AI/ML to accelerator and FEL research

Author will add more later

Speaker: Auralee Edelen (SLAC National Accelerator Laboratory)

Monday Poster Session

11 A Modular Framework for Integrated Start-to-End Simulation of Seeded Free-Electron Lasers

Modern software systems increasingly require modular, reusable components that scales across diverse application domains. In accelerator physics, machines often rely on specialized simulation codes tailored to distinct segments of the accelerator, making integrated start-to-end studies complex and fragmented. Here, we introduce GEIST (Genesis–Elegant Interface Simulation Toolset), an interdisciplinary library designed for linear accelerator-based free-electron laser (FEL) simulations, with particular emphasis on seeded FEL schemes. GEIST provides a unified framework for coupling established simulation tools, enabling consistent data exchange and streamlined workflows across otherwise incompatible codes.
To support this flexibility, we present PHYSYNC, a decoupled library that has evolved from within GEIST into a standalone, general-purpose solution. PHYSYNC enables consistent transformation and mapping of physical parameters — such as momentum, energy, and spatial coordinates — across different simulation environments. Its design supports large-scale distributions with one-to-one correspondence between particles and distribution elements, making it applicable not only within GEIST but also across a broader range of simulation workflows and external libraries. Together, GEIST and PHYSYNC exemplify how domain-specific software can evolve into modular ecosystems, improving reusability while supporting sustainability-conscious studies in accelerator modeling.

Speaker: Pardis Niknejadi (Deutsches Elektronen-Synchrotron DESY)

12 Advancing Seeded-FEL Performance at FERMI: Operational Status and Future Plans

With the completion of the early phase of the FERMI 2.0 upgrade program, the FERMI facility has significantly increased its flexibility, enabling the extension of its operational capabilities. The new FEL-1 layout, supporting both HGHG and EEHG, has substantially broadened the FEL-1 tuning range. This advancement is accompanied by improved spectral brightness and enhanced stability, achieved through the reliable implementation of EEHG, which is also available for user operation.
In parallel, the upgrade of the FERMI linac with the installation of new high-gradient S-band accelerating cavities has enabled routine operation at higher electron beam energies. This has been effectively exploited to extend the FEL-2 tuning range toward higher photon energies. Furthermore, developments in the seed laser system have enabled preliminary experiments with seeding at 200 nm, further enhancing short-wavelength operation capabilities.
Thanks to these advancements, operation at the nitrogen and oxygen absorption edges has been successfully demonstrated, and measurements have confirmed the generation of higher harmonics extending beyond 1 keV.
These results demonstrate that the current machine configuration is already capable of meeting the growing demand from the scientific user community for stable, coherent radiation in this spectral range. At the same time, they provide essential input for guiding the next phases toward the full realization of the FERMI 2.0 upgrade program.

Speaker: Primoz Rebernik Ribic (Elettra-Sincrotrone Trieste S.C.p.A.)

13 AI-assisted FEL tuning for attosecond pulse generation

At LCLS, attosecond xFEL pulses (“XLEAP” mode) allow users to probe electronic process on their natural timescale.* However, XLEAP is much more sensitive than conventional SASE operation to drifts and jitter in the RF phases, laser timing, bunch charge, and beam orbit. As a result, XLEAP setup and pulse energy optimization is highly variable and difficult to replicate even minute-to-minute, leading to delays in user experiments and unpredictable time commitments for operators.

In this work, we explore archival data to identify sources of variation driving XLEAP performance. This allows us to frame XLEAP as an optimization problem in high-dimensional control space with conflicting objectives, few diagnostics, and time dependence on scales from the sampling rate to minutes and hours. We consider how machine learning techniques can be used to effectively tune the machine in this complicated landscape.

Speaker: Aaron Reed (SLAC National Accelerator Laboratory)

14 Attosecond spike formation from slice-resolved bunching dynamics in sign-reversing chirped undulator tapering

We investigate attosecond spike formation in an x-ray free-electron laser driven by sign-reversing chirped undulator tapering in a strongly non-adiabatic regime. When the total detuning changes appreciably over one gain length, the longitudinal dynamics is most transparently described through slice-resolved bunching evolution. We introduce a reduced physical picture that organizes the process into three stages: chirp-filtered onset of microbunching near selected modulation slices, asymmetric dephasing across the modulation, and delayed post-reversal emission from comparatively fresh slices. Time-dependent three-dimensional simulations with Athos-like soft-x-ray parameters support this picture and show localized soft-x-ray spike trains with FWHM durations of about 200–300 as and peak powers in the tens-of-gigawatts range, while reference configurations without chirp-based tapering remain multi-femtosecond. The same framework also clarifies an important boundary case: strongly nonlinear sawtooth modulation does not generate isolated spikes under the simulated conditions studied here.

Speaker: Longdi Zhu (École Polytechnique Fédérale de Lausanne)

15 Attosecond two-color x-ray free-electron lasers with dual chirp-taper configuration and bunching inheritance

Generating and synchronizing two-color attosecond x-ray pulses is vital for ultrafast electronic studies. We propose a novel dual chirp-taper free-electron laser scheme exploiting bunching inheritance to generate attosecond pulse pairs. An electron beam with a laser-induced sinusoidal energy chirp traverses tapered main and afterburner undulators. This produces two-color pulses separated by femtoseconds and tens of eVs. Bunching inheritance enables a remarkably short afterburner, minimizing source-point distance and easing beamline focusing. Comprehensive analysis confirms excellent radiation stability against SASE shot noise and few-cycle laser CEP jitter. This robust approach advances attosecond pump-probe techniques and coherent control of quantum systems.

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

16 Attosecond xFEL structure originating from dispersive space charge waves in a low energy injector

Attosecond FEL pulses can be generated by limiting lasing to a single high-current spike with a duration comparable to the FEL coherence length. At LCLS-II, the XLEAP program generates such a distribution by overlapping a short laser pulse onto the conventional cathode drive laser. Here, we demonstrate that this initial density perturbation drives a pair of counter-propagating space-charge waves that split and travel along the bunch within the low-energy injector. Tuning of the modulation delay and amplitude allow us to select a single spike suitable for downstream compression and attosecond SXR lasing.

Speaker: David Cesar (SLAC National Accelerator Laboratory)

17 Beam optics design for ERL-based EUV-FEL

Energy-Recovery Linac (ERL) based Free-Electron Laser (FEL) has been a promising solution for the high-power EUV light source for future semiconductor lithography. KEK has been developing the key components of the accelerator system and designing the prototype machine. Beam optics design is important for realizing the required beam performance in the FEL section, for safely transport the beam to the dump, and for recovering the energy ideally. We will present various items considered in the designing work.

Speaker: Yosuke Honda (High Energy Accelerator Research Organization)

18 Beam Shaping via Latent-Space Optimization of Longitudinal Phase Space in XFELs

Control of the longitudinal phase space (LPS) of electron beams is essential for the flexible operation of X-ray free-electron lasers, but remains challenging because the measured beam distribution is high dimensional, noisy, and sensitive to machine drift. We develop and experimentally demonstrate a latent-space approach for online LPS shaping at the Linac Coherent Light Source.

Speaker: Zihan Zhu (SLAC National Accelerator Laboratory)

19 Cathode laser temporal shaping for LCLS-HE

The photon energy range of the LCLS-HE upgrade relies heavily on the achievable longitudinal beam brightness. The final peak current and correlated energy spread are both limited by the longitudinal phase space at the exit of the injector, non-linear compression, and collective effects. The initial charge distribution off the cathode effects all of these factors. However, determining an ideal cathode laser temporal profile requires optimization of the entire accelerator configuration. In order to simplify this problem, an idealized longitudinal phase space at the undulator entrance is analytically backtracked to the injector exit. In Astra simulations, the initial charge distribution and injector settings are optimized to match the longitudinal phase space from backtracking while maintaining a small projected emittance. The remainder of the accelerator is simulated in Elegant utilizing the lattice configuration determined by backtracking. The Free Electron Laser performance is simulated in Genesis using the start-to-end beam. Comparing with the nominal Gaussian cathode laser temporal profile, we find significant improvement in both the FEL pulse energy and maximum photon energy.

Speaker: Nicholas Sudar (SLAC National Accelerator Laboratory)

20 Coherent soft X-ray radiation up to 1.6 keV by external seeding at SwissFEL

X-ray free electron lasers (XFELs) are modern photon sources which generate widely tuneable, high-brilliance, ultrafast laser pulses. External seeding schemes transfer the coherence properties of optical laser sources to the FEL pulses, while performing a photon energy upconversion. In this contribution, we show the latest external seeding results from the soft X-ray beamline at SwissFEL, Athos. With a 200 pC, 3 GeV electron beam and two ultraviolet (266 nm or 4.66 eV) seed lasers, we obtained an echo-enabled harmonic generation (EEHG) signal at photon energies between 320 and 450 eV, with pulse energies between 20 and 10 uJ. Furthermore, we employed “fresh-bunch” schemes. In one case, we further amplified the EEHG signal to reach up to 75 uJ at 320 eV. In another case, we employed additional chicanes for a second harmonic upconversion, achieving coherent emission at photon energies up to 1.6 keV. This corresponds to a total harmonic conversion factor close to 350. Our results constitute an essential step towards fully coherent and controllable FEL outputs across the entire soft X-ray range. In future work we intend to exploit longitudinally coherent XFEL pulses to facilitate various spectroscopy and pump-probe schemes, as well as to achieve coherent control of quantum states in the soft X-ray regime.

Speakers: Dr Philipp Dijkstal (Paul Scherrer Institute), Wenxiang Hu (Paul Scherrer Institute), Sven Reiche (Paul Scherrer Institute)

21 Compact Energy Recovery Linac-Based FEL for EUV Lithography

Lithography is a critical component of chip manufacturing in the semiconductor industry. The conventional laser-produced plasma (LPP) approach is energy-inefficient and poses potential environmental issues. In contrast, an energy recovery linac (ERL) based free-electron laser (FEL) offers a cleaner, more efficient alternative by recycling electron beam energy after FEL radiation. In this paper, we propose a compact ERL-based FEL that fits within a single building. We will report preliminary beam dynamics simulation results and discuss the potential challenges and future work required to develop such a facility.

Speaker: Ji Qiang (Lawrence Berkeley National Laboratory)

22 Conceptual Design Studies of a PITZ-Like THz FEL for Pump–Probe Applications at the European XFEL

The Photo Injector Test Facility at DESY in Zeuthen (PITZ) is developing a THz free electron laser (FEL) as a potential source for pump–probe experiments at the European XFEL. In a recent proof-of-principle experiment using the LCLS-I undulator and a 17 MeV/c electron beam from the PITZ injector, pulse energies exceeding 100 μJ at 3 THz were demonstrated for bunch charges up to 2.4 nC. These results confirmed the feasibility of the PITZ-based THz FEL concept; however, the achievable performance was limited by the multipurpose nature of the existing PITZ facility, which is not optimized for THz generation.
A dedicated THz FEL facility driven by a PITZ-like injector is studied by means of numerical simulations. In comparison with the existing PITZ beamline, the proposed machine is designed with beam energies of up to 45 MeV, a tunable bunch compressor, and two APPLE-II undulators covering superradiant operation in the 0.1–1 THz range and SASE operation in the 1–30 THz range. Based on GENESIS 1.3 and start-to-end simulations, pulse energies of approximately 800 μJ at 10 THz in SASE mode for 1 nC bunches and about 35 μJ at 1 THz in superradiant mode for 200 pC bunches with 1.5 ps rms length are expected. In addition, further studies on seeding techniques and facility optimization are currently being carried out.

Speaker: Mikhail Krasilnikov (Deutsches Elektronen-Synchrotron DESY)

23 Current horn suppression using laser heater shaping for FEL generation

High-brightness free-electron laser (FEL) generation critically depends on both the transverse and longitudinal properties of the electron beam, such as transverse emittance, slice energy spread. These beam parameters can be significantly degraded due to collective effects, e.g., wakefield, coherent synchrotron radiation. Such effects are pronounced due to the high current spikes, so-called current horns, formed at the head and tail of the bunch during bunch compression. To suppress the current horn formation, in general, a collimator is used in the middle of bunch compressor and removes bunch slices at both ends. However, the use of collimator is restricted in high-power machines and can introduce additional wakefield effects, complicating stable beam operation. As an alternative approach, current horn suppression using laser heater shaping has been proposed. In this presentation, we will report the experimental results of the current horn suppression via laser heater shaping and its effects for FEL generation.

Speaker: Chang-Kyu Sung (Pohang Accelerator Laboratory)

24 Demonstration of direct-amplification enabled harmonic generation in an ultraviolet free-electron laser

We report the experimental demonstration of direct-amplification enabled harmonic generation in an ultraviolet free-electron laser (FEL) driven by a low-intensity seed laser. By employing a versatile undulator configuration that enables seed amplification and harmonic generation within a unified setup, we achieved over 100-fold energy gain of the seed and observed exponential growth at the second harmonic. The results demonstrate that a sufficiently long modulator cannot only amplify a weak seed but also induce strong energy modulation of the electron beam, enabling efficient harmonic bunching. This method markedly relaxes the power requirements on external seed lasers and presents a viable route toward high-repetition-rate, fully coherent FELs.

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

25 Design and Implementation of a Tapering Enhanced Superradiant Source at THz Frequencies

Here we present an analytical model theory for time domain analysis and design of Tapering Enhanced Superradiant (TES) FEL at zero-slippage condition. The model is implemented for optimization of an ongoing experimental project of a THz radiation source based on the 6 MeV ORGAD RF-Accelerator at Ariel University.

The design of the setup is based on a twenty-period planar undulator made of five-period uniform part and a fifteen-period amplitude tapered part. The radiation is guided in a rectangular waveguide, where due to the dispersion of the waveguide mode it is possible to operate at zero-slippage condition, namely, the group velocity of the mode at the phase synchronism frequency is equal to the axial velocity of the electron bunch. Since the bunch duration is much shorter than the radiation period, its spontaneous emission is superradiant (emits in proportion to the number of electrons squared). At the end of the uniform undulator, the bunch gets trapped by its own emitted radiation, and due to the tapering in the second undulator section it extracts additional radiative energy from the beam, which continues to stay trapped at the zero-slippage condition.

Speaker: Amir Weinberg (Tel Aviv University)

26 Differentiable 1D FEL Simulation for Gradient-Based Optimization and Inference

We present a differentiable 1D free-electron laser (FEL) simulation framework based on automatic differentiation. In contrast to conventional FEL codes that provide only forward simulation, the differentiable formulation enables efficient evaluation of gradients of FEL observables with respect to beam, undulator, and model parameters. This makes it possible to apply gradient-based methods to optimization, sensitivity analysis, parameter fitting, and inverse problems within a physics-based FEL mode

Speaker: Jingyi Tang (Stanford University)

27 Evaluating ML architectures for latent beam representation at LCLS

Machine-learning surrogate models for accelerators require compact beam representations that preserve physically meaningful structure while remaining useful for predictive and tuning tasks. In this work, multi-projection beam images from simulated LCLS injector data are studied as a representation-learning problem. The goal is to evaluate how ML model architecture choices impact the degree to which beam distributions, represented via multiple 2D projections, can be compressed into a low-dimensional latent-space vector while retaining important structure, such that the projections can be reconstructed from the latent vector with acceptable accuracy. We examine how model choice affects reconstruction quality and latent-space behavior. Results show that the beam projections are highly compressible, with low reconstruction error across the models studied, while architectural differences lead to clear tradeoffs between reconstruction fidelity and latent-space regularity. Initial latent-space analysis and ongoing neighborhood exploration aim to test whether local perturbations in latent space correspond to smooth variations in decoded beam structure. This study helps identify promising representation-learning approaches for later work in accelerator model calibration, adaptation to measured data, and model-assisted beam tuning.

Speaker: Kiley Gruenberg (SLAC National Accelerator Laboratory)

28 Experimental Study of Third-Harmonic Laser Heating in the LCLS-I Injector

Laser heaters are commonly used in free-electron laser (FEL) injectors to suppress microbunching instabilities by increasing uncorrelated slice energy spread. We investigate using a third-harmonic laser–undulator interaction in the LCLS-I injector for this purpose. Measurements of the central slice energy spread as a function of laser power are performed for both fundamental and third-harmonic interactions, enabling a direct comparison of heating efficiency and scaling behavior. While the fundamental interaction exhibits the expected dependence on laser power, the third-harmonic configuration shows distinct deviations from expected scaling, suggesting increased higher-order and three-dimensional effects. The experimental results are compared with GENESIS simulations of realistic beam distributions, showing qualitative agreement and highlighting the limitations of simplified models for higher harmonic couplings. These results demonstrate the feasibility of third-harmonic laser heating in an operational injector, potentially allowing for heater functionality at lower than design beam energies or constrained machine configurations.

Speaker: Ahmed Osman (SLAC National Accelerator Laboratory)

29 Generalization of the Ming Xie shot noise formula to treat different pulse profiles

The Ming Xie* formula for shot noise has been widely used to estimate the performance of SASE free-electron lasers and has been compared with various simulation codes with great success. However, the formula is directly applicable only to a single slice of the electron beam and it would be useful to generalize the formula to treat arbitrary temporal bunch shapes. Ming’s formula applies predicts the noise power Pnoise scales as Ib to the 2/3power. If we assume that the current is time-dependent with some temporal shape, then we can integrate over time in the bunch to get the energy emitted by the entire bunch. This generalization has been applied to two SASE (LCLS and SPARC) FELs and compared with experimental measurements and simulations with the MINERVA FEL code with good agreement.

Speaker: Dr Henry Freund (University of Maryland, College Park)

30 Generating Coherent Undulator Radiation from CEP-stable Pre-bunched Pulses

Coherent undulator radiation from pre-bunched electron beams can generate carrier-envelope phase (CEP) stable radiation pulses. Leveraging the development of multi-bunch concepts and pulse-by-pulse bunch shaping, we propose using CEP-stable coherent undulator radiation to seed the FEL instability and generate fully coherent phase-stable radiation pulses via double reflection. In this poster, we present theoretical calculations of this concept using the reciprocity theorem, as well as 3D numerical simulations using PUFFIN. We find two different regimes with different amplitude scaling behaviors determined by the angular distribution of the radiation relative to the central cone.

Speaker: Arnav Swaroop (SLAC National Accelerator Laboratory, University of California, Berkeley)

31 Generation of fresh-slice self-seeded hard X-ray FELs at PAL-XFEL

We present the experimental results on the generation of fresh-slice self-seeded hard X-ray free-electron laser (FEL) pulses at PAL-XFEL. The self-seeding scheme significantly enhances the spectral brightness of FELs and has therefore been extensively verified and widely applied in user science over the years, whereas the fresh-slice technique enables higher pulse power by reducing the pulse duration while maintaining high pulse intensity. By mitigating the trade-off between the seed FEL intensity and the slice energy spread of the electron beam, the fresh-slice method can further improve the efficiency of the self-seeding scheme. Consequently, the combined scheme enables the generation of high-power and high-brightness FELs, potentially enabling diffraction-before-destruction in experiments and investigation of ultrafast dynamics. Recently, the fresh-slice scheme exploiting a corrugated structure was implemented at PAL-XFEL and combined with the self-seeding scheme for the generation of hard X-ray FEL pulses. In this presentation, the experimental results of the fresh-slice self-seeded HXFEL generation will be reported.

Speaker: Chang-Kyu Sung (Pohang Accelerator Laboratory)

32 Generation of Sawtooth Correlations for Brighter Compact Light Sources

We present a method that uses transverse wigglers to impart arbitrary correlations in transverse phase space. This approach becomes particularly powerful when combined with emittance exchange, which converts the transverse phase space into the longitudinal phase space. In contrast to conventional single-sinusoidal correlations, a sawtooth correlation can maximize the fraction of particles contributing to the peak current of a bunch train. Simulations indicate that 90% or more of the particles can be compressed into short, periodic density spikes. We are currently preparing an experimental demonstration of this method. In this work, we introduce the concept and present simulation results supporting its feasibility. We also discuss a key experimental challenge: the feedback control of transverse wigglers during operation.

Speaker: Alex DeSimone (Northern Illinois University)

33 Generation of tunable, phase-locked hard X-ray pulse pairs

First tunable, phase-locked, ultrafast hard X-ray (PHLUX) pulse pairs were generated at PAL-XFEL. A fresh-bunch self-seeded FEL, driven by an electron beam that was shaped with a slotted foil and a corrugated wakefield structure, generates coherent radiation that is intensity-modulated on the femtosecond time scale. We measure a stable frequency comb generated by a phase-locked pulse pair (to within a shot-to-shot phase jitter corresponding to 0.1 attoseconds) at a photon energy of 8.3 keV, with a pulse energy of several tens of microjoules, a freely tunable relative phase, a pulse delay tunability between 5 and 17 fs, and an individual pulse duration at the few-femtosecond level. Such pulse pairs are suitable for a wide range of applications, including coherent spectroscopy and hard X-ray quantum optics experiments. More generally, these results represent an important step towards a hard X-ray arbitrary waveform generator.

Speakers: MyungHoon Cho (Pohang Accelerator Laboratory), Chi Hyun Shim (Pohang Accelerator Laboratory), Dr Philipp Dijkstal (Paul Scherrer Institute), Wenxiang Hu (Paul Scherrer Institute)

34 Generation of very soft x-ray at the LCLS

In recent years there is a growing interest in FEL-based generation of very soft x-rays, due to increased scientific and industrial applications. In this proceeding, we investigate the feasibility of very soft x-ray generation at the Linac Coherent Light Source (LCLS). We focus on the mechanisms of FEL tuning, diagnostics, and practical aspects of user delivery of such light. We demonstrate our recent experimental results and discuss photon energy calibration. Finally, we provide an outlook for future experiments at the LCLS of this kind.

Speaker: Aliaksei Halavanau (SLAC National Accelerator Laboratory)

35 GINGER-3D -- Code Summary and Some Particular Results

The venerable GINGER code (first virtual light in 1985) has been extensively updated and extended with new features including an ADI-based Cartesian radiation field solver, a more robust, transversely-resolved, longitudinal space charge solver, and a new Python-based, GUI-oriented post-processor (XG3DpyPP). Serious efforts have been directed toward optimizing run times including support for parallel execution through MPI and minimizing output storage requirements, e.g., substituting multi-axis projections for full x-y field dumps. The code handles harmonics and simultaneous multiple polarizations including OAM emission. Simple run time comparisons in x-y-z-t mode indicate factors of 2X or greater speedup relative to Genesis-4 and slowdowns of ~5X relative to the original axisymmetric field solver. XG3DpyPP runs both interactively as a Tkinter-based GUI (alternatively in terminal mode via a Python interpreter) and non-interactively via command line script control. We present some sample results concerning (1) the dependence of spontaneous emission upon transverse grid resolution and (2) longitudinal space charge effects on both strongly-bunched HGHG e-beams and the recent LBNL laser-plasma wakefield accelerator SASE FEL. In the first case, longitudinal phase evolution within the chicane before an HGHG modulator is likely of comparable* importance to that occuring after the chicane and should not be neglected.

Speaker: Dr William Fawley (Lawrence Berkeley National Laboratory)

36 Hard X-ray self-seeding design for SwissFEL

One goal of the recently proposed SwissFEL facility upgrade
(SwissFELplus) is to enable the generation of longitudinally coherent
FEL pulses across a large photon energy range at the the hard X-ray
beamline Aramis. We consider to implement the Forward Bragg Diffraction self-seeding approach, where a diamond crystal in transmission geometry filters the SASE signal generated in the first undulator section, and where the second undulator section further amplifies the coherent seed. We performed calculations and ideal beam simulations to determine which crystal cuts and Bragg reflections provide the best longitudinal coherence for all photon energies between 3 and 14 keV. According to simulations, a high degree of longitudinal coherence is possible for photon energies above 6 keV, whereas the coherence for lower photon energies suffers from an increased FBD bandwidth. Furthermore, we discuss the implementation of advanced modes, among them the fresh bunch self-seeding to control the self-seeded FEL pulse duration and a scheme to achieve phase-locked ultra-short FEL pulse sequences.

Speaker: Dr Philipp Dijkstal (Paul Scherrer Institute)

37 Harmonic modulation for ultra-wide tuning range terahertz free-electron laser via frequency-beating laser modulation

Broadband tunability is a key pursuit for terahertz (THz) free-electron lasers, yet it is often constrained by the narrow energy acceptance of laser modulators in prebunched schemes. Here, we propose a harmonic modulation scheme to decouple this constraint. Taking the Electron Source Test Facility at Dalian as a specific case study, our simulations demonstrate that by matching the laser field to higher-order harmonic resonances of the undulator, we can obtain efficient energy modulation across a significantly wider electron energy range (40--130 MeV), surpassing the limit of 71--130 MeV imposed by the fundamental resonance. This technique facilitates continuous frequency tuning from 0.5 to 30 THz using a single modulator-radiator chain, establishing a versatile method for ultra-broadband THz generation.

Speaker: Lingjun Tu (Institute of Advanced Light Source Facilities Shenzhen)

38 High-Repetition-Rate Tunable UV Laser System for External Seeding of FLASH

We present recent progress on the SLASH laser system, a high-repetition-rate tunable femtosecond UV laser developed for external seeding of the FLASH free-electron laser (FEL). The system features a dual-branch architecture: a third-harmonic generation arm (343 nm) and a tunable OPCPA-based arm (297–317 nm), enabling flexible pulse control for advanced seeding schemes like echo-enabled harmonic generation (EEHG). Two about 30 and 40 meter long UHV beam transport lines deliver synchronized UV pulses to the accelerator, with 600 µm waist radius in the two modulator structures, where overlap with the electron beam was established. These developments mark a key milestone toward externally seeded operation at FLASH.

Speaker: Ingmar Hartl (Deutsches Elektronen-Synchrotron DESY)

39 Laser-Induced Structured Electron Beam

Microbunching enables laser-like radiation generation from charged particle beam,
and is one of the main driving forces advancing accelerator light sources in the past
decades. The most prominent example is free-electron laser. A natural approach of microbunching
creation is to imprint a coherent phase space correlation on the electron
beam using a conventional quantum laser, whose information can then be preserved for
an infinitely long time in a symplectic dynamical system. This is the idea behind various
laser-based harmonic generation schemes developed in particle accelerators. In this
paper, we study laser-induced microbunching in a systematic way, including both energy
modulation and angular modulation-based schemes. Beam evolution in both phase
space and spectral space is presented to appreciate the physical essence of these schemes
in multiple respects.

Speaker: Xiujie Deng (Tsinghua University)

40 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 elliptically polarized undulators(EPU), 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 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)

41 Live Longitudinal Modeling for LCLS and LCLS-II

Digital twins and online physics models are becoming increasingly important for the operation of modern X-ray free-electron lasers, where high-brightness beam delivery depends on precise control of the longitudinal phase space. At LCLS and LCLS-II, small RF phase drifts in the early linac can produce large changes in bunch compression, causing discrepancies between offline model predictions and the live machine state. We report the development of a live longitudinal modeling workflow based on the fast beam longitudinal tracking code BELT, integrated with LCLS and LCLS-II process variables through a LUME-based interface. The model reads live machine settings, performs rapid longitudinal tracking through the accelerators, and enable physics-informed feedback procedure to iteratively update the model until the live simulation is consistent with the measured machine state. This approach turns BELT from an offline design and optimization tool into an online, measurement-constrained digital twin for longitudinal beam dynamics. The workflow provides a practical path toward real-time longitudinal phase-space prediction, model-based tuning, and automated feedback for high-repetition-rate FEL operation.

Speaker: Jingyi Tang (Stanford University)

42 Machine Learning Optimization of E-Beam Transport in the ORGAD Accelerator

The 6MeV ORGAD Accelerator at Ariel University is driving a THz Superradiant FEL. The radiation is emitted superradiantly (in proportion to the number of electrons squared) in a rectangular waveguide-undulator section. The condition for the spontaneous emission to emit superradiantly is that the duration of the electron bunch σ_t is much shorter than the optical period (2π/ω) of the radiation as the beam propagates along the undulator. This requires proper choice of the acceleration phase and energy chirp in the hybrid RF gun. In addition, the parameters of the coils and quads along the transport linemust be adjusted to keep the beam cross-section dimensions small enough to enter the waveguide. Both goals are susceptible to space-charge limitations which tend to expand the beam in both transverse and longitudinal dimensions. This requires optimization of many transport parameters that are mutually dependent. In this paper we suggest an optimization method, using machine learning Bayesian optimization of the entire beamline with the goal of attaining minimal bunch duration in the undulator under limitation of good transport along the beamline. Full 3D general particle tracer (GPT) simulations, including space-charge effect, were applied using realistic field-maps which were measured in the lab. Thirty simulation iterations sufficed to arrive to optimal beam transport design.

Speaker: Amir Weinberg (Tel Aviv University)

43 Machine-learning-based four-dimensional phase space reconstruction for future S3FEL electron beam diagnosis

Four-dimensional (4D) transverse phase space (TPS) reconstruction of electron beam is an important issue in accelerator beam diagnosis, since the 4D-TPS is vital for beam manipulation and cannot be directly determined from limited two-dimensional screen measurements. A machine-learning-based differentiable reconstruction framework is developed to infer the hidden phase-space distribution, and it has been verified with ELEGNAT simulation code. The reconstructed 4D-TPS can be used to resolve the 4D emittance and analyze the x-y transverse coupling. What’s more, it can support creating the surrogate model for practical accelerator beam tracking. This framework is being examined on a superconducting continuous-wave Electron Source Test Facility. In the future, it will also be applied to the under-construction Shenzhen Superconducting Soft-X-ray Free Electron Laser (S3FEL) facility for beam diagnosis and manipulation.

Speakers: Cheng-xin Wu (Institute of Advanced Light Source Facilities Shenzhen), Zhenbiao Sun (Institute of Advanced Light Source Facilities Shenzhen)

44 Mitigating the Sensitivity to Seed-Laser Dispersion in Seeded Free-Electron Lasers via Direct-Amplification Enabled Harmonic Generation

High-gain harmonic generation (HGHG) is known to be critically sensitive to residual group-delay dispersion (GDD) and third-order dispersion (TOD) in the seed laser, which distort the temporal profile and degrade the harmonic up-conversion spectrum. The feasibility of the direct-amplification enabled harmonic generation (DEHG) scheme has been experimentally demonstrated [Phys. Rev. Lett., 2025]. In the long modulator of DEHG, the seed is first directly amplified to a long, relatively flat radiation pulse. Owing to the slippage between electrons and radiation, each longitudinal electron slice samples the laser field across many optical cycles, thereby averaging out the fine temporal distortions induced by GDD and TOD. Consequently, the energy modulation is governed by the seed spectral envelope rather than the instantaneous intensity, making the subsequent harmonic bunching highly insensitive to dispersion. In addition, the prolonged interaction length intrinsically homogenizes the transverse modulation distribution across the electron beam, further stabilizing the output. These effects enable DEHG to preserve nearly transform-limited harmonic spectra without the need for precise dispersion compensation, significantly relaxing seed laser requirements and positioning DEHG as a robust, practical route for high-repetition-rate seeded FEL facilities.

Speaker: Lanpeng Ni (Shanghai Institute of Applied Physics)

45 Neural Network Quantum Molecular Dynamics for Simulating Excited-State Water Chemistry

X-ray free-electron lasers enable femtosecond-resolution probing of photoinduced chemical dynamics. Interpreting these experiments requires atomistic simulations of excited-state systems on matching timescales - a task where ab initio molecular dynamics remains prohibitively expensive. We present a Neural Network Quantum Molecular Dynamics (NNQMD) framework that bridges this gap by training E(3)-equivariant graph neural networks on DFT calculations of hole-doped water, mimicking the ionization conditions created by intense FEL pulses at liquid interfaces. Three architectures - NequIP, MACE, and Allegro - are benchmarked on a 324-atom water system across four charge states. Molecular dynamics simulations driven by the learned potentials remain stable even under extreme 16e excitation, capturing bond-breaking and dissociation events. The resulting pair distribution function differences (ΔPDF) reproduce signatures of bond softening and solvent shell expansion consistent with FEL pump-probe measurements. This framework provides a scalable ML-driven simulation tool for predicting and interpreting ultrafast structural dynamics observed at XFEL facilities, and establishes a foundation for reinforcement-learning-guided steering of catalytic pathways in FEL experiments.

Speaker: Samuel Sahel-Schackis (SLAC National Accelerator Laboratory, Stanford University)

46 Numerical Analysis of FEL Pulse-Energy RMS Jitter in FBD-Based HXRSS

Forward Bragg diffraction (FBD)-based hard X-ray self-seeding (HXRSS) is a promising method for producing narrow-bandwidth and stable hard X-ray FEL pulses. However, the pulse-energy stability of self-seeded FELs can be influenced by several temporal effects, including the FBD filter response, the electron bunch duration, the seed-pulse overlap, and the self-seeding chicane delay. In this work, we numerically investigate the origin of FEL pulse-energy rms jitter in FBD-based HXRSS under baseline conditions without undulator tapering and without bunch energy chirp. By excluding these additional optimization and compensation effects, the study focuses on the intrinsic temporal factors that determine the FEL energy fluctuation.

Speaker: Kookjin Moon (Pohang Accelerator Laboratory)

47 Numerical Studies of Trapped-Ion Stability for Various Bunch-Gap Patterns and Beam Loading in ERL-FEL

Ions produced by collisions between high-energy electron beams and residual gas molecules can be trapped along the beam trajectory by the strong electric potential well of the electron beam, leading to detrimental effects on beam dynamics. Ion trapping is therefore a relevant issue for high-duty continuous-wave (CW) machines, such as energy recovery linacs intended for use as high-average-current FEL drivers.
To mitigate trapped ions, we investigate the introduction of bunch gaps between bunch trains as an alternative to clearing electrodes, motivated by concerns over wake fields excited by electrode structures. The use of bunch gaps is well established in storage rings, where periodic focusing from successive bunches is interrupted, rendering ion trapping unstable. In linac-based FELs, however, the introduction of bunch gaps gives rise to variations in beam loading in accelerating cavities, which is likely to affect beam and RF stability.
In this study, assuming an average beam current of 10 mA and a bunch repetition frequency of 162.5 MHz, the stability of trapped ions is numerically evaluated for various bunch-gap patterns using a linear optics model. In addition, beam-loading variations in accelerating cavities are analytically estimated under conditions where ion trapping is expected to be suppressed. The feasibility of introducing bunch gaps for ion clearing in the designed ERL-FEL machine at KEK is discussed, considering the associated beam-loading effects.

Speaker: Naoto Yamamoto (High Energy Accelerator Research Organization)

48 Online X-ray Characterization Toward Closed-Loop FEL Control at LCLS-II

High-repetition-rate x-ray free-electron lasers operating at MHz rates generate data volumes that preclude offline analysis as a sole experimental strategy, demanding streaming diagnostics capable of real-time decision support. We present a hybrid neural architecture for online characterization of attosecond x-ray pulses from angular-streaking diagnostics at LCLS-II, integrating convolutional with bidirectional recurrent neural networks to perform single-shot denoising, SASE sub-spike classification, and sub-spike separation extraction at demonstrated throughputs exceeding 10 kHz. Ongoing developments are pushing throughput significantly higher while expanding model capabilities toward direct inferencing of pulse temporal structure, sub-spike timing and spectral phase, enabling online x-ray pulse reconstruction. Looking ahead, we discuss how such diagnostic pipelines, coupled with programmable upstream controllers such as photoinjector laser shaping,** establish the sensing layer of a closed-loop adaptive control architecture for next-generation autonomous FEL operation.

Speaker: Jack Hirschman (Stanford University)

49 Physics Design of S3FEL Superconducting Linear Accelerator

Shenzhen Superconducting Soft X-Ray Free Electron Laser (S3FEL), a superconducting accelerator facility aimed at providing soft X-ray FEL pulses with a repetition rate of 1 MHz, is currently under construction. These FEL pulses originate from the electron beam with an energy of 2.5 GeV generated in a continuous-wave superconducting linear accelerator. This paper comprehensively describes the physics design of the linac, detailing the physical layout, the longitudinal working point, the transverse lattice, and finally the corresponding particle tracking simulations. Based on these studies, a detailed summary of the beam performance is presented.

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

50 Radiation friction in a relativistic electron beam passing through an undulator.

The radiation friction force acting on a relativistic electron in incoherent fields of spontaneous undulator radiation of other beam electrons moving in a spatially periodic magnetic field of the undulator is considered. An expression for the radiation friction force, both for the initial stage of the pre-Brownian motion of electrons and for the kinetic stage is obtained. Considering a helical undulator in the approximation of a small value of the undulator parameter, the dependences of the radiation friction force on both the value of the longitudinal momentum and the distance traveled by the electrons in the undulator were found. The dependence of the friction force on the value of the initial thermal energy spread is analyzed. It is shown that the radiation friction force is proportional to the density of the electron beam. Radiative friction and diffusion may lead to a change in the initial distribution of electron momenta, and consequently affect on the formation of stimulated emission.

Speaker: Vladimir Ognivenko (National Science Centre Kharkiv University)

51 Returning European XFEL to user operation after the 2025 long shutdown

Following the long installation and maintenance period in the second half of 2025, the European XFEL accelerator complex was successfully recommissioned and returned to user operation. During the shutdown, numerous modifications and upgrades were implemented across the accelerator and associated infrastructure, including improvements to the cryogenic system, the injector and the undulator beamlines, to name just a few.
The restart phase required coordinated recommissioning and optimization efforts to restore stable operation of the new electron source, the superconducting linac and the FEL beamlines.
Despite the large number of hardware and software changes introduced during the shutdown, the facility successfully recovered its previous performance. In several operational aspects, FEL performance already exceeded the levels achieved before the shutdown.
This contribution summarizes the major accelerator modifications performed during shutdown, the recommissioning strategy, the challenges encountered during startup, and the operational performance achieved during the first months after restart.

Speaker: Matthias Scholz (Deutsches Elektronen-Synchrotron DESY)

52 Simulation model calibration and optimization of the beamline based on generative phase space reconstruction methodology

Precise beam manipulation and preservation of beam quality along the beamline are essential to produce high-brightness X-ray Free Electron Lasers (XFELs). These tasks can be effectively carried out through accurate beam diagnostics. In addition, a calibrated simulation model enables investigation of the optimal beamline conditions against collective effects such as coherent synchrotron radiation and longitudinal space charge, which cause nonlinear distortions in the beam phase space. At the Pohang Accelerator Laboratory (PAL) XFEL facility, complete six-dimensional phase space reconstruction using neural networks and differentiable simulations has been successfully demonstrated along the first bunch compressor section. In this paper, we present simulation and experimental demonstrations of simulation model calibration using the reconstructed phase space information as input beam parameters. We show how to perform model calibration with multiple wire-scan measurements obtained under various beamline conditions, together with several optimization methods such as evolutionary strategies and Bayesian optimization. In addition, we further discuss model uncertainty and the sensitivity of model elements. Furthermore, we present the optimal beamline settings that preserve beam quality along the undulator section. It is expected that this model calibration can be incorporated into the digital twin frameworks in the future for online, virtual diagnostics and robust optimizations.

Speaker: Seongyeol Kim (Pohang Accelerator Laboratory)

53 Status of the high-power, tunable THz FEL source at PITZ

Development of a high-power, tunable THz source for pump–probe experiments at the European XFEL is currently underway at the Photo Injector Test Facility at DESY in Zeuthen (PITZ). With the present PITZ setup, FEL radiation in the 1–5 THz range can be generated using an LCLS-I undulator. Proof-of-principle experiment has been successfully demonstrated with high-charge electron beams of several nC at ~17 MeV/c, producing a narrow-band ~3 THz radiation*. A key focus of the ongoing program is the controlled transport and matching of several-nC, low-energy electron beams into the undulator, where space-charge effects and beam quality preservation are particularly challenging. Advanced photocathode laser pulse shaping is employed to optimize the longitudinal and transverse beam distributions, enabling improved matching and enhanced FEL performance. To demonstrate tunability, PITZ has also generated FEL radiation at 5 THz. Ongoing experiments aim to maximize the pulse energy at this frequency. A dedicated diagnostic setup has been implemented for comprehensive characterization of the THz radiation.

Speaker: Namra Aftab (Deutsches Elektronen-Synchrotron DESY)

54 Study of Instantaneous Surface Current Distribution on an Echelette Type Grating for Coherent Smith-Purcell Radiation

The Smith-Purcell effect is a well-known radiation emitted when an electron beam propagates the vicinity of a diffraction grating surface. Apparently, the surface current model* successfully interprets the process of Smith-Purcell radiation (SPR) generated through an Eshellet-type grating with a small blazing angle. According to the surface current model, the SPR intensity rapidly increases as the observing angle approaches the traveling direction of electrons. In fact, since the longitudinal formfactor of the surface current (charge) distribution is considered to be identical to that of the electron bunch, coherent SPR has been investigated as a candidate for a non-destructive tool of diagnosing bunch length.
We have investigated the distribution of surface current on the metal grating induced by the single electron employing a classical electromagnetism approach for electric field propagation. As a result, we found that the longitudinal distribution of surface current is not a delta-function and has a long tail, which mean the surface current formfactor is not 1 and much suppressed in entire frequency range. Considering the finite longitudinal distribution of electrons in a short bunch, a higher frequency component of coherent SPR is much suppressed. We will report the detailed calculation of the surface current with simple math. Effects of finite beam size are also discussed.

Speaker: Hiroyuki Hama (Tohoku University)

55 TDC-based longitudinal manipulator

Transverse deflecting cavities (TDCs) have always been interesting beamline elements that couple a particle’s longitudinal position and transverse coordinates. While they have been widely adopted for longitudinal diagnostics and other specialized purposes, such as emittance exchange, several longitudinal manipulation methods using TDCs have been introduced during the last six years. These include longitudinal linear and nonlinear chirp control and profile shaping. While each method was proposed independently, our recent work showed that integrating all three functions is feasible, which allows operation of the linac on-crest, removal of harmonic linearizers, and suppression of CSR effects by profile shaping. We are investigating the implementation of this integrated scheme in a more realistic beamline and present the status of ongoing simulation studies.

Speaker: Alex DeSimone (Northern Illinois University)

56 The UK vision for next‑generation XFELs

UK scientists are amongst the leading users of XFELs, which enable revolutionary atomic-level imaging and ultrafast time resolution far beyond other methods. Looking to the future, the UK community have developed a science and technology case and a conceptual design for a next-generation XFEL. Such a facility will be able to leverage maturing technologies to deliver vast increases in research productivity, through enhanced data rates, throughput, and X-ray quality. The design couples extremely high repetition rate operation (100 kHz – 1 MHz) of a range of accelerator, laser and end station technologies with the emergence of AI, and significantly increases multiplexing capabilities. Such a facility would be capable of driving deep and wide-ranging impact across all areas of science and technology and major sectors of the economy. The project has analysed options to realise these next-generation capabilities through either a new facility or co-developing upgrades at existing international projects. This contribution provides an overview of the design.

Speaker: David Dunning (Science and Technology Facilities Council)

57 Fast Electron Beam Steering for MHz X-ray Beam Sharing

The European XFEL comprises three SASE beamlines that branch to eight endstation instruments. Each beamline currently operates a single instrument exclusively, fundamentally limiting the total beamtime available for experiments. In this talk, we propose an electron beam steering approach that can enable simultaneous operation of multiple scientific endstations on a single undulator beamline. By avoiding the thermal and mechanical limitations of X-ray optical multiplexing, our method allows flexible beam distribution between instruments at up to MHz repetition rates.

We describe the implementation of fast beam steering at the European XFEL, characterise the joint electron–photon beam response to fast steering, and specify key optical constraints. Finally, we show how our method can be coupled with advanced experimental schemes to unlock new scientific opportunities and evaluate the potential impact of our approach on the design of existing and emerging XFELs beamlines.

Speaker: Trey Guest (European X-Ray Free-Electron Laser)

Tuesday 25 August

SASE FEL (Invited)

58 Reverse undulator tapering for polarization control and short pulse generation

Reverse undulator taper is a unique method allowing to strongly suppress radiation intensity at the exit of SASE undulator while maintaining microbunching at a significant level. Recently, it was used to generate high-purity circularly polarized radiation from dedicated afterburners (Apple-X at the European XFEL and Apple-III at FLASH, the latter operates at the 3rd harmonic of SASE undulator). Chirp-taper compensation is another method where a reverse taper is typically used to generate attosecond pulses in X-ray FELs. Recent results from the European XFEL on generation of extremely short (~ 100 as) and intense (~ 10 TW) pulses in soft X-ray regime are presented. Finally, so-called excessive reverse taper (beyond the compensation condition) helps to generate short pulses well below cooperation length limit. The validity of the concept is illustrated by the experimental results from FLASH.

Speaker: Evgeny Schneidmiller (Deutsches Elektronen-Synchrotron DESY)

59 Sub-Femtosecond X-ray Free-Electron Laser Driven by Additional Bunch Compression via Reverse-Energy Chirp at SACLA

We demonstrate sub-femtosecond hard X-ray free-electron laser (XFEL) pulse generation at SACLA, achieved through a robust and flexible scheme for additional electron bunch compression. The method leverages a reverse-energy chirp, driven by wakefields within the accelerating structures and longitudinal space charge effects. By employing a tunable R56 optics in a two Double Bend Achromat (DBA) dogleg, we further compress electron bunches just before the undulators. A key advantage is the ability to adjust the longitudinal pulse duration while keeping the downstream beam orbit and transverse envelope stationary, ensuring that lasing conditions remain optimized throughout the tuning process. This allows for a rapid and reproducible transition between standard SASE and sub-femtosecond operation modes without the need for extensive beam tuning. Experimental characterization using a single-shot spectrometer and ASE-based diagnostics confirms that this additional compression successfully produces pulse durations below 1 fs (FWHM) with pulse energies up to 60 uJ. This scheme provides a practical and scalable approach for attosecond-regime XFEL science, applicable to various facilities.

Speaker: Eito Iwai (Japan Synchrotron Radiation Research Institute, RIKEN SPring-8 Center)

60 Coherent Twin-Pulse Generation at XFELs: Dual- Injector-Laser Production of Electron Bunch Pairs for SASE, HXRSS and PWFA

The recently commissioned dual NEPAL photocathode laser systems at EuXFEL and FLASH enable the generation of twin electron bunches with unprecedented flexibility. Each laser pulse can be independently tuned in pulse energy, temporal and longitudinal profile, and relative timing, allowing for electron bunch production in the same or any two 0.77 ns spaced RF buckets. This capability unlocks novel operation modes at both facilities, such as: Twin SASE or self-seeded hard X-ray pulses without split-and delay unit and driver-witness bunch production for laser-plasma wakefield acceleration. At EuXFEL, twin X-ray pulse operation is currently being considered for first users in October this year.

Speaker: Ingmar Hartl (Deutsches Elektronen-Synchrotron DESY)

SASE FEL (Contributed)

61 Enhanced generation of single-spike hard x-ray free-electron laser pulses with lower charge and shorter electron beams in the injector

Standard x-ray free-electron lasers (FELs) generate pulses with total durations of tens of femtoseconds and with time and spectral profiles consisting of multiple randomly distributed spikes. Strongly compressing an electron beam is a typical approach to produce shorter and coherent FEL pulses. We have advanced this method by starting at the injector of the FEL facility with electron beams with lower charges and shorter durations than in standard configurations. This leads to shorter electron beams with reduced energy spread after full compression and, consequently, to shorter and higher-quality FEL pulses. By operating with electron beams at the injector with charges of a few pC and rms durations of 360 fs, we show the generation of hard x-ray FEL radiation with practically all pulses having a single spike and a duration estimated from spectral measurements of about 300 as (full-width-at-half-maximum values). The demonstration was conducted at SwissFEL, the FEL at the Paul Scherrer Institute in Switzerland. Our work represents a simple way to enhance the production of single-spike events in x-ray FEL facilities, paving the way to achieve fully coherent hard x-ray FEL pulses with unprecedented durations.

Speakers: Alexander Malyzhenkov (European Organization for Nuclear Research), Eduard Prat (Paul Scherrer Institute), Gian Luca Orlandi (Paul Scherrer Institute), Sven Reiche (Paul Scherrer Institute), Tobias Weilbach (Paul Scherrer Institute)

62 Operational experience and challenges of a CW X-ray FEL at SLAC

Driven by a continuous-wave superconducting linear accelerator, a high-repetition-rate x-ray FEL has been build at SLAC. After commissioning in 2022 and 2023, early user operation was started in 2024. By the end of 2025, the machine was operated with up to 93,000 Hz beam rate, supporting 3 soft x-ray user stations. We will report the beam and FEL performance, operation observations and challenges in this paper.

Speaker: Christopher Zimmer (SLAC National Accelerator Laboratory)

Seeded FEL (Invited)

63 Few-femtosecond extreme-ultraviolet pulses and pulse pairs with tunable topological properties

We will present the first results of an experiment performed at the FERMI FEL to generate few-femtosecond extreme-ultraviolet (XUV) pulses using a recently developed slippage-compensation scheme in externally seeded free-electron lasers. Building on this approach, we will also show the results of a theoretical study aimed at extending the scheme to generate multiple few-femtosecond XUV pulses, in particular pulse pairs, with tunable topological properties and adjustable temporal delay. Simulations show that the topological charge l of each pulse can be independently set to l=−1,0,1, while the delay can be tuned from a few to several tens of femtoseconds. Compared with superradiance, the proposed scheme offers greater flexibility, as it does not require driving the FEL into saturation and suppresses SASE emission from the unseeded portion of the electron beam, which can be substantial at longer wavelengths. Combined with pulse-to-pulse polarization switching, the proposed approach could enable XUV studies of circular and helical dichroism in chiral systems and magnetic materials, as well as high-resolution imaging, in the few-femtosecond regime.

Speaker: Primoz Rebernik Ribic (Elettra-Sincrotrone Trieste S.C.p.A.)

64 First Lasing and Stable Operation of a Direct-Amplification Enabled Harmonic Generation Free-Electron Laser

First lasing and stable operation of a direct-amplification enabled harmonic generation free-electron laser were achieved, with the harmonic number up to 12th from a single-stage modulation and 30th from two-stage modulations employing the beam-echo effect. This work demonstrates for the very first time that high harmonic FEL generation can be facilitate using only one single seed laser with significantly reduced power. Using only one single seed laser makes the operation both simplified and more stable. The substantially reduced seed laser power means high-repetition-rate external seeded XFEL may be feasible now. Moreover, the most promising prospect is that, this scheme relights the path towards the high repetition-rate short-wavelength external seeded XFELs using directly the short wavelength high-order harmonic generation pulses from noble gases.

Speaker: Zheng Qi (Shanghai Advanced Research Institute)

Seeded FEL (Contributed)

65 Coherent Undulator Radiation Seeding of High-Gain Harmonic Generation

We propose and simulate a scheme for generating coherent short-wavelength radiation exploiting the intrinsic coherence of emission from a compressed, high-current electron beam. When the bunch length is shorter than the resonant wavelength, the undulator radiation is temporally coherent. This radiation is extracted and optically refocused onto a second, longer electron beam, acting as a coherent seed which drives a large energy modulation. A dispersive section converts this modulation into microbunching, enabling harmonic upconversion to shorter wavelengths via high-gain harmonic generation.

Speaker: Jenny Morgan (SLAC National Accelerator Laboratory)

66 HXRSS at the European XFEL: Progress from SASE2 to SASE1

The first high-repetition-rate hard X-ray self-seeding system was installed on the SASE2 beamline of the European XFEL in 2019. It has since been used to deliver self-seeded radiation pulses in burst mode over the photon-energy range from 6 to 18 keV, reaching a maximum spectral density of approximately 2 mJ/eV at 7.5 keV. User operation started in 2021, and an increasing fraction of user time has since been allocated to HXRSS mode, reaching about 50% in 2025. Building on this success, a self-seeding system has recently been installed on the SASE1 beamline. During the initial commissioning, self-seeded radiation at 9.3 keV was demonstrated within only a few hours, highlighting the maturity and transferability of the HXRSS concept. In this talk, we will discuss the current operational capabilities, remaining issues and limitations, lessons learned from SASE2 operation, and possible next steps for further development.

Speaker: Matthias Scholz (Deutsches Elektronen-Synchrotron DESY)

67 Recent development of Hard X-Ray Self-Seeding design and experiment tests for LCLS-II-HE high repetition rate operation

To overcome the thermal load challenges at the high-rep rate HXR self-seeding FELs, we summarize the recent design and experimental test progress. The LCLS-II-HE is designed to have cryo-cooled HXRSS system. To validate the design and prototype, experiment tests have been carried out on the diamond and the related mounting and clamping schemes. Wavefront sensors and single-shot spectrometers are used to quantitatively characterize these thermal-acoustic effects on crystals and HXRSS FEL. We will report recent design updates, experimental testing, hardware-readiness progress, and planned future validation for the LCLS-II-HE hard X-ray self-seeding system and related X-ray optics.

Speaker: Ye Hong (SLAC National Accelerator Laboratory)

FEL Oscillators and Infrared FEL (Invited)

68 Lasing of a Cavity Based X-ray Source

Classical optical lasers provide intense, coherent light in the visible and infrared regions, but extending this concept to X-rays has been particularly challenging due to a lack of suitable gain media and mirrors. Current hard X-ray free-electron laser (XFEL) facilities overcome this difficulty using a relativistic high-peak-current electron bunch as gain media for a self-amplified spontaneous emission in a single pass. This implies the use of long lines of undulators, delivering a very high brightness beam but with a noisy, multi-spiked temporal and spectral profile. Cavity-based XFELs (CBXFELs) were thought to close this gap by recirculating spectrally filtered X-ray pulses in a cavity, similarly to what is done in optical lasers. The first-ever lasing of one of these configurations, the XFELO, was realized at European XFEL in May 2025. The diamond-based Bragg cavity was tuned at 6.952 keV in a 132.8 meter round-trip and a ring-up across successive bunches was observed, producing spectrally pure, microjoule-level pulses.

In this talk, we show more details about the setup used for such a demonstrative result, with insights into the commissioning and the challenges that were overcome during the alignment and the tuning of the cavity until lasing was achieved. An insight into the next steps will also be illustrated.

Speaker: Maurizio Vannoni (European X-Ray Free-Electron Laser)

69 Continuous THz Band Coverage through Precise Electron Beam Tailoring in Free-electron Lasers

High-power, continuously tunable narrowband terahertz (THz) sources are essential for advancing nonlinear optics, THz-driven material dynamics, and ultrafast spectroscopy. Conventional techniques typically impose a trade-off between pulse energy and frequency tunability. Here, we demonstrate a novel free-electron laser approach that overcomes these limitations by pre-modulating a relativistic electron beam with a frequency-beating laser pulse and leveraging bunch compression along with collective effects to enhance microbunching. Experimental results demonstrate that this technique generates narrowband THz emission with continuous frequency tunability from 7.8 to 30.8 THz, achieving pulse energies up to 385 μJ while maintaining spectral bandwidths between 7.7% and 14.7%. Moreover, the method exhibits exceptional robustness and scalability, highlighting its unique ability to bridge the long-standing THz gap and offering a promising solution for diverse cutting-edge scientific applications.

Speaker: Kaiqing Zhang (Shanghai Synchrotron Radiation Facility)

FEL Oscillators and Infrared FEL (Contributed)

70 Cavity-based XFEL at LCLS

The cavity-based XFEL (CBXFEL) is a novel X-ray source based on an X-ray Free-Electron Laser (XFEL) encased in an optical cavity. This concept, when built, will offer substantial improvements in X-ray brightness and coherence compared to existing high-gain SASE sources. A joint project is underway at SLAC National Accelerator Laboratory, in collaboration with Argonne National Laboratory and the RIKEN SPring-8 Center, to demonstrate CBXFEL technology with the first seven hard X-ray undulators of the LCLS. We report on recent progress in CBXFEL construction efforts. We describe the initial optical alignment of the cavity Bragg mirrors, the performance of the photon diagnostics, and future plans for CBXFEL lasing. We especially focus on the challenges of CBXFEL operation and our solutions. Finally, we demonstrate that our measurements are corroborated by theoretical and numerical calculations, further proving the cavity-based concept.

Speakers: Aliaksei Halavanau (SLAC National Accelerator Laboratory), Zhirong Huang (SLAC National Accelerator Laboratory)

71 High Harmonic Generation Driven by Mid-Infrared Free-Electron Lasers at 2 µm and 5 µm

High harmonic generation (HHG) has emerged as a powerful method for producing coherent femtosecond-to-attosecond light in the VUV, EUV, and X-ray regions. Since the harmonic cutoff energy and the upconversion phase matching limits increase with the driving laser wavelength, mid-infrared (MIR) drivers are uniquely advantageous for extending the emission toward soft X-ray wavelengths, and potentially even into the hard X-ray regime. In this study, we exploit MIR-FEL oscillators to investigate harmonic generation in gaseous media. We report on experiments using LEBRA-FEL at Nihon University and KU-FEL at Kyoto University, operating at wavelengths of 2 µm and 5 µm, respectively. By focusing 80-120 fs micro-pulses onto argon and oxygen targets, we observe, for the first time, harmonics up to the 7th order using advanced 20-120 µJ-class MIR-FELs and high gas pressures for phase matching. The experimental results reveal distinct spectral differences between the harmonics produced in gases and the intrinsic harmonics of the FEL. Furthermore, the measured macro-pulse temporal profiles of all harmonics exhibit significant deviations from the fundamental FEL pulse, providing clear evidence of the nonlinear dynamics inherent in the upconversion process. These results represent an essential milestone toward the realization of phase-matched HHG in the VUV, EUV, and X-ray spectral regions using MIR-FEL drivers.

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

Tuesday Poster Session

72 A Digital Twin Framework for Particle Accelerators

We present a software framework for digital shadows (one‑way data flow) and digital twins (two‑way closed‑loop synchronisation) for particle accelerators, linking physical machines with virtual counterparts to enable predictive modelling, automated control and improved facility design. The architecture comprises a coherent chain of tools that maintain a unified description of the accelerator. At its core, the LAURA ontology defines the accelerator lattice, while the SIMBA simulation framework utilises LAURA-defined models to perform start‑to‑end particle tracking across multiple codes. These elements integrate with pyCATAP for control‑system access and SARABI, which produces soft EPICS IOCs from the LAURA lattice, ensuring consistency across virtual and physical systems. This architecture is under active development for the CLARA accelerator, the ISIS injector, and other accelerator facilities: we present digital shadow results and the initial steps to digital twinning. The approach establishes a foundation for a community‑driven, interoperable digital twin ecosystem for particle accelerators.

Speaker: David Dunning (Science and Technology Facilities Council)

73 Accelerator Modeling Ecosystem for LCLS

The accelerator modeling ecosystem at SLAC automates much of the work involved in generating the information that feeds survey & alignment teams, engineering groups, and the controls system. Starting from the same lattice files and modeling codes used to develop Twiss optics and calculate beam properties, quantities including beam stay clears and element locations are produced. These quantities are then processed into various coordinate systems and uploaded into a central database. SLAC is in the later stages of a project to modernize this ecosystem, replacing legacy mad8-based utilities with Bmad-based utilities, and also consolidating around python-based post-processing. Here we provide an overview of this ecosystem, and steps taken to ensure that updates to the ecosystem be transparent to operations.

Speaker: Michael Ehrlichman (SLAC National Accelerator Laboratory)

74 Active Q-Switched XRAFEL with Start-to-End LCLS-II-HE Beams

Cavity-based X-ray free-electron lasers (CBXFELs) are promising sources of fully coherent, stable, high-brightness hard X-rays, but practical implementation at LCLS-II-HE must account for beam-power and beam-dump limitations. In previous work, we proposed an active Q-switched X-ray regenerative amplifier FEL, where electron-beam phase-space manipulation controls the effective cavity quality factor. Here we ran start-to-end simluations using start-to-end LCLS-II-HE project parameters. Three-dimensional FEL simulations show that active Q-switching enables CBXFEL operation with a reduced electron-beam repetition rate, while allowing radiation to recirculate in a low-loss cavity between amplification shots. This lowers beam-power requirements while preserving efficient intracavity buildup and controlled outcoupling. The results support active Q-switching as a practical path toward coherent, narrow-band, high-average-brightness hard X-ray generation at future high-repetition-rate FEL facilities.

Speaker: Jingyi Tang (Stanford University)

75 An x-ray regenerative amplifier free-electron laser with a step-tapered undulator*

Regenerative amplifier x-ray free-electron lasers constitute alternative configurations for fourth generation light sources which are typically operated in self-amplified spontaneous emission (SASE) mode. However, SASE exhibits relatively large fluctuations in the power and spectral properties from shot to shot, whereas XRAFELs provide a much more stable source of hard x-ray photons. Here, we study the performance of an x-ray cavity under consideration at the Stanford Linear Accelerator Facility using both uniform and step-tapered undulators with either transmissive or hole out-coupling.

Speaker: Dr Henry Freund (University of Maryland, College Park)

76 Automation strategy and status at European XFEL

Modern FEL operation increasingly relies on advanced software tools, automation, and data-driven optimization methods. At the European XFEL, a strategy is being developed to improve operational efficiency, overall FEL performance, and to reduce the dependence on experts during non-standard operation modes.

Recent developments include automated scanning and tuning procedures, the use of large language models for improved documentation and access to various knowledge bases, as well as operator support via chat-based interfaces, and agentic AI systems for performance optimization.

In addition, efforts are ongoing to combine individual optimization tools and software services in order to automate larger and more complex operational workflows.

This paper summarizes the overall concept, the current implementation status, and first experiences with AI- and machine-learning-related tools in daily accelerator operation.

Speaker: Matthias Scholz (Deutsches Elektronen-Synchrotron DESY)

77 Autonomous operation of the DIAG0 diagnostic line for 6D phase-space monitoring at LCLS-II

Characterizing the full 6-dimensional phase-space distribution of beams from the LCLS-II photoinjector is essential for understanding and optimizing downstream accelerator performance. Long-term monitoring of this distribution is equally important for detecting drifts in machine state and implementing timely corrective actions. Continuous phase space characterization during routine operation demands reliable tomographic diagnostic measurements and fast, efficient reconstruction methods. In this work, we demonstrate the first fully autonomous 6-dimensional beam-tomography system deployed on the DIAG0 parasitic beamline at LCLS-II. Using machine-learning-based control algorithms, the system autonomously configures DIAG0 and executes tomographic manipulations within operational constraints, adaptively re-optimizing beamline parameters and scan ranges in response to changes in the incoming beam. Tomographic measurements are streamed to the S3DF computing cluster where generative analysis methods reconstruct the phase-space distribution. We demonstrate that this framework produces detailed 6-dimensional beam reconstructions at a cadence of one reconstruction every 5 to 10 minutes, enabling real-time, multi-hour monitoring of injector beam evolution with unprecedented fidelity. These results represent a significant step toward fully autonomous operation of accelerator beamlines with real-time beam diagnostics for current and next-generation accelerator facilities.

Speaker: Dylan Kennedy (SLAC National Accelerator Laboratory)

78 Characterization of Energy and Energy Spread of Electron Beams from the High-Brightness Photoinjector at NSRRC

The energy and energy spread of electron beams from the NSRRC photoinjector were studied for the generation of ultrashort bunches using velocity bunching for a superradiant THz source. A dipole-magnet-based spectrometer was employed to measure the beam energy and relative energy spread from the energy-dispersed beam profile observed on a downstream diagnostic screen. To achieve high energy resolution, a dedicated diagnostic beamline consisting of two quadrupole magnets and one dipole magnet was designed and optimized. Beam dynamics in this section were investigated using the IMPACT simulation code to optimize optics settings and evaluate their impact on the measured energy spread. This report presents experimental results on the characterization of the NSRRC photoinjector energy spread, providing a basis for further injector optimization.

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

79 Comment on ‘‘Laser mode complexity analysis in infrared waveguide free-electron lasers”

In the article of Rui Prazeres [Phys. Rev.Accel. Beams 19, 060703 (2016)], hereafter the article and the authors, one of the conclusions drawn by the author of the article is that there is a limit to the length of the waveguide, exceeds which a phenomenon of “mode disorder” appears in the cavity. Our analysis indicates that the phenomenon should be caused by the instability of the optical cavity, and by simultaneously increasing the radius of curvature of the cavity mirrors to ensure that the stability condition is still met, the limitation of the waveguide length could be eliminated.

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

80 Development of a hybrid prism spectrometer with linear dispersion for coherent-radiation-based longitudinal diagnostics of ultrashort electron bunches

Diagnosing the longitudinal profile of ultrashort electron bunches, now reaching femtosecond and attosecond durations in modern Free Electron Lasers and Laser Wakefield Accelerators remains a major challenge, as conventional tools, such as Transverse Deflecting Cavities, lack the required temporal resolution. Coherent radiation spectroscopy provides an attractive alternative. Radiation, generated by such short bunches is coherent from the optical to the far‑infrared wavelength regions, allowing the use of readily available detection tools. We present a hybrid spectrometer that combines optical to far‑infrared detection in a single platform. The system integrates a commercial spectrometer, covering 0.4 - 1 um region with a custom far‑infrared spectrometer based on a dispersive prism and a pyroelectric detector array covering 1 - 20/40 um region, depending on the prism material. A modular cage‑based optomechanical design ensures stability and straightforward alignment. Although the prism can produce a wide spectrum in a "single order" contrary to the grating based spectrometers, it suffers from non-linear dispersion. To overcome this we develop a compound‑prism configuration using far‑infrared‑transparent materials to achieve nearly linear dispersion across the target band. We report the instrument design, calibration, dispersion‑linearization analysis and initial coherent transition radiation spectra, along with planned upgrades and future applications.

Speaker: Konstantin Kruchinin (SLAC National Accelerator Laboratory)

81 Development of a Sheet-Beam Electron Source for Terahertz Smith–Purcell Radiation

We report on the development of a sheet-beam electron source for terahertz Smith–Purcell radiation experiments. A photoemission-based electron gun driven by a femtosecond Ti:sapphire laser produces an elliptical beam of approximately 0.1 mm × 8 mm at 5 keV.
To control beam divergence along the short axis, a focusing electrode was introduced. Beam profiles were measured using a microchannel plate with a phosphor screen and a wire grid monitor.
The results show that increasing the focusing voltage improves beam confinement, consistent with simulations. The wire grid measurements agree with MCP observations.
These results demonstrate the feasibility of generating a controlled sheet beam suitable for Smith–Purcell radiation experiments. Further studies are underway to explore alternative photoemission approaches, including edge-enhanced photoemission geometries and magnetic focusing schemes.

Speaker: Makoto Asakawa (Kansai University)

82 Dispersion tuning in linear accelerators using microbunching damping

Microbunching damping in relativistic electron beams is strongly influenced by inverse dispersive transport terms that couple transverse phase-space coordinates to the longitudinal displacement. In this work, we investigate the use of microbunching damping as a diagnostic and optimization tool for lattice tuning in linear accelerators. In particular, residual dispersion is minimized by maximizing the observed microbunching amplification.

Two representative cases are studied: tuning of a dogleg quadrupole strength and transverse sextupole positioning in a W-chicane. For both systems, experimental measurements are compared with numerical particle tracking simulations and analytical predictions.

The results demonstrate that microbunching-based dispersion characterization can serve as a faster alternative to conventional dispersion measurements, enabling more efficient lattice optimization.

Speaker: Sergei Kladov (University of Chicago)

83 Efficiency enhancement in regenerative amplifier free-electron lasers using a tapered undulator*

Recent progress in short wavelength FELs from the EUV through x-rays has opened new avenues for industrial and research applications. Most such FELs rely on SASE in which the optical pulse grows from noise in a single pass through the undulator. However, this results in significant shot-to-shot fluctuations in the power and spectrum. Oscillators are under consideration to stabilize the noise associated with SASE. In particular, a high-gain/low-Q oscillator, i.e., a regenerative amplifier free-electron laser (RAFEL), is one possible concept. In this paper, we present the first analysis of efficiency enhancement in a RAFEL with a long, tapered undulator line and demonstrate substantial enhancements in the brightness of the optical output. Here, we consider a high average power EUV RAFEL at 13.5 nm and show that the performance exceeds that of single-pass, tapered self-amplified spontaneous emission. This points the way to high power tapered undulator x-ray RAFELs.

Speaker: Dr Henry Freund (University of Maryland, College Park)

84 Electron Beam Instrumentation for Advanced FEL Schemes in SwissFEL

In addition to the standard FEL operation of SwissFEL, we are developing several advanced FEL modes, where we tailor the phase space of the electron beam before sending it into the undulator. This includes ultra-short pulse and large-bandwidth modes, echo-enabled harmonic generation, and a scheme to generate trains of attosecond FEL pulses.
These modes require beams with a high 6d phase space density, or a reduced slice energy spread. We will present the continued development of electron beam instrumentation to support the base operation, and to enable the generation of advanced FEL schemes.

Speaker: Rasmus Ischebeck (Paul Scherrer Institute)

85 Enabling Arbitrary Color Attosecond X-ray Pump-Probe Experiments with the LCLS Soft X-ray Delay Line

The Linac Coherent Light Source (LCLS) enables pioneering studies of ultrafast phenomena by producing X-ray pulses with attosecond durations and continuously tunable wavelengths. However, X-ray pump-probe experiments are limited by achievable time delays. While existing magnetic chicanes offer wide delay ranges, they introduce a minimum slippage of 300 as for harmonic schemes, which grows to 5 fs for arbitrary-color experiments. This prevents continuous scanning through zero delay (t=0) and obscures the initial, often crucial, moments of interaction.
We present the Soft X-ray (SXR) Delay Line, an instrument designed to overcome these limitations. The device is an optical chicane using a 3-bounce mirror geometry to provide a 14 fs tunable delay for 250-1250 eV photons. Fine delay tuning is achieved via a piezo-actuator, offering a hardware resolution of tens of attoseconds, while a motorized pitch correction system ensures sub-µrad pointing stability.
Acting as a high-resolution microstepper to the magnetic chicane, the SXR Delay Line enables continuous delay scanning through t=0. This capability overcomes the 5 fs slippage barrier for arbitrary-color schemes and, by allowing use of the full undulator, permits higher probe pulse intensities. The system opens new capabilities for investigating fundamental charge migration and electron correlation dynamics with unprecedented temporal fidelity.

Speaker: Sean OTool (Stanford University)

86 Extreme high-order harmonics from an FEL-oscillator

In an FEL oscillator, it is not uncommon to see spectral content at the 3rd and even 5th harmonic of the fundamental lasing wavelength. This arises from on-axis emission from the undulator at the fundamental and odd harmonics. Under proper conditions, FEL oscillations also at the 2nd harmonic are possible, but even harmonics are strongly suppressed due to off-axis emission from the undulator. In the results here from the FELBE IR FEL, we demonstrate spectral emission from an IR FEL oscillator at both odd and even harmonics and remarkably, up to extremely high orders (> 50th order).

We have found that the harmonics, while falling rapidly in power at low orders, exhibit a plateau-like behavior at higher orders. The power and range of the harmonics increases with increasing K-parameter. While lasing in the mid-IR, the harmonics reach into the near-IR and visible. This allows imaging of the harmonics with a standard CMOS array, where we see a transverse mode that is zero on center and appears more like a Laguerre-Gaussian mode. Measurements of the high harmonics will be described along with an outline of potential applications for this surprising spectral content.

Speaker: J. Michael Klopf (Helmholtz-Zentrum Dresden-Rossendorf)

87 Fault-tolerance analysis for an injector of continuous-wave free-electron lasers

High-repetition-rate free-electron laser (FEL) facilities such as LCLS-II rely on continuous-wave (cw) injectors to deliver high-quality electron beams to multiple undulator lines simultaneously. Ensuring injector reliability under nonideal conditions is critical for sustained user operations. In this work, we investigate the fault tolerance of a representative cw photoinjector by analyzing its performance under various degraded scenarios, including reduced gun energy, radio-frequency (rf) cavity failures, solenoid field constraints, and early-stage energy gain requirements. Using multiobjective optimization with a genetic algorithm framework (Xopt), we assess achievable beam quality—focusing on transverse emittance and bunch length—across a range of fault conditions. Our results reveal that while certain failures (e.g., buncher off) significantly impact emittance, coordinated adjustments to laser parameters, rf phases, and cavity gradients can still partially recover performance. The injector demonstrates resilience to a variety of perturbations, confirming its robustness for high-demand cw FEL operations.

Speaker: Zhen Zhang (SLAC National Accelerator Laboratory)

88 Feasibility Modeling of Medical LINAC Driven Free Electron Lasers

This work investigates the feasibility of using commercial medical
linear accelerators as electron sources for compact free electron laser
systems in the mid infrared regime. Representative beam parameters were
defined using product specifications and engineering estimates, then
compared with the infrared free electron laser at the Fritz Haber
Institute. Resonance conditions and Pierce parameter estimates were used
to evaluate expected gain limitations, followed by computational
simulations using GENESIS. The simulations show that resonance and
limited amplification at 16 𝜇m can be achieved. However, medical LINAC
beam quality, especially energy spread, limits efficient single pass
operation. These results suggest that state of the art FEL performance
is difficult without beam conditioning, but application specific mid
infrared sources can be feasible through improved configurations such as
optical seeding or oscillator operation.

Speaker: Mauro Siebeneichler Leffever (University of Nevada, Las Vegas, Varian Medical Systems (United States))

89 Generation of frequency-tunable superradiant THz free electron laser at NSRRC

NSRRC has been dedicated to providing the superradiant THz free electron laser (FEL) for scientific research users in recent years. Linac-based coherent superradiant THz FELs produced by the NSRRC high brightness photoinjector, which has been installed in the Accelerator Test Area (ATA), is served for users. Users in different research fields have different requirements for THz FEL specifications, such as THz energy, THz frequency and so on. In general, changing the undulator gap is the common method to tune the frequency. However, in the photoinjector system, frequency tunability of superradiant THz FELs can also be achieved by changing the electron energy through varying rf power injected into the linac and correspondingly generating ultrashort electron beams with different bunch length by velocity bunching technique. In this report, the demonstration of the frequency-tunable superradiant THz FEL covering the range from 0.4 to 2 THz is presented.

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

90 Gun5: A New Generation of Normal-Conducting RF Photogun for Long-Pulse Operation at the European XFEL

A new generation of normal-conducting 1½-cell L-band RF photoguns (Gun5) has been developed at DESY for long-pulse operation at the European XFEL. Compared to the previous generation (Gun4), Gun5 supports RF pulses up to 1 ms at 10 Hz while maintaining an accelerating gradient of 60 MV/m at the cathode (~70 kW average RF power in the gun cavity). Key design improvements include an elliptical cavity geometry with increased shunt impedance, enhanced distributed water cooling for mitigation of pulsed RF heating, and two RF pickups in the full cell enabling direct field control for improved amplitude and phase stability. RF asymmetries are reduced by a dual-sided power coupler feeding a coaxial antenna. Three Gun5 cavities have been commissioned at PITZ, with one currently in operation at the European XFEL. Results on conditioning, dark current, and beam emittance at 250 pC, together with operational experience, are presented.

Speaker: Mikhail Krasilnikov (Deutsches Elektronen-Synchrotron DESY)

91 Impact of fiducial magnets near the PITZ L-band electron source

The Photo-Injector Test facility at DESY in Zeuthen (PITZ) is designed to characterize and optimize L-band photo-electron sources. As such, it has many diagnostics and tools available to measure the beam quality close to the gun. To ensure all components near the gun are properly aligned, four pairs of fiducial magnet nests were installed around the gun; approximately 50 cm from the beam axis. It was initially assumed that this distance would be sufficient to keep the magnetic field effects of the beam negligible. However, recent measurements demonstrate that these magnets significantly influence the electron beam dynamics. Comparing measurements with the fiducial magnets in place and removed shows the magnets significantly impact the beam trajectory, the laser’s beam-based alignment on the photo-cathode, and increase the transverse emittance by approximately 30%.

Speaker: Frank Stephan (Deutsches Elektronen-Synchrotron DESY)

92 Improvement of KU-FEL performance by a new 1.6-cell photocathode RF gun with a semiconductor cathode

A new 1.6-cell photocathode RF gun has been installed at the Kyoto University Free-Electron Laser (KU-FEL) facility to increase its peak power for high-harmonic generation (HHG) in gas. During the 2022 Phase I commissioning, which utilized a copper photocathode, the electron bunch charge was limited to 60 pC due to low quantum efficiency (QE). To address this, a semiconductor thin film (CsTe with a CsBr protective layer) was subsequently deposited onto the copper cathode plug to increase the QE. This upgrade increased the electron bunch charge to 550 pC and achieved an FEL micro-pulse energy of 120 micro-J at a wavelength of 5 micro-m at the KU-FEL user station #1. The achieved micro-pulse energy was 2.4 times as high as that obtained with 200-pC electron bunches from the old 4.5-cell RF gun. By focusing the FEL pulse, we could observe 3rd harmonic generation in oxygen more clearly than previous. This work was supported by MEXT Q-LEAP (JPMXS0118070271).

Speaker: Heishun Zen (Kyoto University)

93 Laser-modulation-driven x-ray pulse shaping in regenerative amplifier free-electron lasers

We present a robust method for generating custom-shaped, coherent hard X-ray pulses in regenerative amplifier free-electron lasers (RAFELs) using laser-induced energy modulation of the electron beam. A temporally shaped optical modulation imprints an optical-wavelength energy pattern on the beam, which interacts with the recirculated seed in the X-ray cavity to generate spectral sidebands. These sidebands lie outside the Bragg reflection bandwidth and are extracted as outcoupled radiation, while the seed is retained for continued amplification, enabling a novel mechanism for both output coupling and pulse shaping. This approach allows deterministic control over the spectral and temporal properties of the emitted X-rays, including multicolor generation, twin-pulse structures, pulse duration tuning, and complex temporal waveforms. Importantly, it does not require modification of the RAFEL cavity or optics and is fully compatible with high-repetition-rate operation, offering a practical pathway to deliver coherent, shaped and on-demand X-ray pulses for advanced time-resolved and nonlinear X-ray science.

Speaker: Zhen Zhang (SLAC National Accelerator Laboratory)

94 Nanosecond Radio-Frequency Pulse Driven Photogun for Very Hard X-ray FELs

One pathway to producing high brightness electron beams for future light sources is to use a radio-frequency (rf) driven high field photogun to rapidly accelerate photoemitted electrons to the relativistic regime and preserve the brightness. However, the highest attainable field is limited by rf breakdowns of materials used in a photogun. Shortening rf pulse duration feeding into a photogun provides a viable pathway to achieve high field and prevent rf breakdowns. Here we propose and investigate Compressed Ultrashort Pulse Injector Demonstrator (CUPID), a nanosecond rf pulses driven photogun powered by a klystron and rf pulse compression system capable of achieving 300 MW at 20 ns duration, to produce bright electron beams with high electric field. We introduce the design of the CUPID photogun and designed performance to achieve a very high cathode field, followed by beam dynamics studies of a photoinjector formed by CUPID photogun and other beamline components. We show a proof-of-concept start-to-end simulation of the CUPID photoinjector paired with the existing Linac Coherent Light Source (LCLS) copper accelerator free-electron laser (FEL) to demonstrate achievable mJ-scale pulse energy for very hard x-ray photons at 40 keV or higher. Finally we summarize the results from current high power tests of prototype cavities and outline the future directions this project can take to validate our approach to upgrading the capabilities of LCLS.

Speaker: Ankur Dhar (SLAC National Accelerator Laboratory)

95 Observation of strong multi-pulse effects on spectral Broadening of long-wavelength infrared FEL pulses in a KRS-5 plate

The post-compression of femto-second pulsed lasers utilizing spectral broadening in solid and gas is now commonly used for obtaining few-cycle pulses. In the long-wavelength infrared (LWIR) region, solid targets such as ZnSe, and KRS-5 has been utilized*. I performed spectral broadening experiment with a 3-mm-thick KRS-5 plate at the FEL wavelength of 11 micro-m to see what would happen when the spectral broadening technique utilized for FEL pulses whose micro-pulse repetition rate was 29.75 MHz. I could observe near infrared components (1.5-2.6 micro-m), which induced by the spectral broadening in the KRS-5 plate, using a InGaAs detector. When the KRS-5 plate was placed before the focusing point of the focus point of the LWIR-FEL, strong multi-pulse effects were observed due to nonlinear absorption in the KRS-5 plate. The strong multi-pulse effects and nonlinear absorption can be understood by the accumulated carrier induced by multi-photon excitation of previous micro-pulses. This work was supported by MEXT Q-LEAP (JPMXS0118070271).

Speaker: Heishun Zen (Kyoto University)

96 Optimization of SRF CW Photoinjector Brightness for Optimal Performance of the XFEL

Beam dynamics studies have been performed to optimize the brightness of a superconducting radio-frequency (SRF) photoinjector for the CW option of the European XFEL. A superconducting L-band 1.6-cell cavity is considered as a high-brightness electron source capable of CW operation and meeting stringent beam quality requirements. Due to the limited achievable accelerating gradient in the SRF gun and the fixed layout with the gun separated from the solenoid, photocathode laser pulse shaping is used to mitigate space-charge-driven emittance growth. A 4D integral brightness metric, combining slice emittance and current profile, is used as the optimization objective. Multi-objective optimization using ASTRA is performed to minimize transverse emittance and maximize brightness for various temporal and transverse laser distributions. Optimized working points are tracked through start-to-end simulations up to the undulator entrance. A local gain-length model identifies relevant lasing electron bunch slices after three-stage compression, which are correlated with the 4D brightness of the longitudinal core at the injector exit, providing a figure of merit. Different laser pulse profiles are compared in terms of achievable performance.

Speaker: Mikhail Krasilnikov (Deutsches Elektronen-Synchrotron DESY)

97 Performance of the superconducting linac RF for LCLS

The Linac Coherent Light Source superconducting linac (LCLS-SC) at SLAC National Accelerator Laboratory, built during the LCLS-II project, has been in operation since 2023, with user experimental delivery commencing in 2024. In 2026, the LCLS-II-HE project is adding 23 additional 1.3 GHz cryomodules, bringing the LCLS-SC maximum electron beam energy from 4 GeV to 8 GeV. This poster will provide an overview of operational experiences, challenges, and performance analysis of the SRF cavities and associated systems during the first few years of LCLS-SC operation and their impact on beam dynamics and stability.

Speaker: Sebastian Aderhold (SLAC National Accelerator Laboratory)

98 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. The other one is the THz-Coherent Undulator Radiation (THz-CUR) source 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 2026, 21 external user groups will use 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)

99 Progress towards control of the carrier envelope phase in an FEL-oscillator

FELs have a long history of delivering intense tunable narrowband THz and IR radiation for a vast array of applications. Nearly all THz/IR FELs are oscillators that utilize an optical resonator to achieve the extremely high spectral brightness of transform-limited optical pulses. Due to the dynamics of the FEL process, FEL-oscillators must operate with an optical resonator “detuned” from perfect synchronization with the electron beam. This results in slippage of the carrier envelope phase (CEP) from pulse-to-pulse.

Recently, we have gained more understanding of the CEP slippage and in collaboration with the PhLAM Team (Uni de Lille) and partners at DESY, we are testing methods to measure the shot-to-shot CEP of the FELBE FELs. These methods have demonstrated the power of these techniques of metrology, and have also revealed that control and stabilization of the FEL CEP should be possible. This opens the door for the exploration of coherently driven phenomena requiring a CEP-stable driving field from the THz/IR FEL. These techniques also have vast potential for shot-to-shot electron beam diagnostics from other types of sources (e.g. CTR, CSR, THz generation from waveguides).

Speaker: J. Michael Klopf (Helmholtz-Zentrum Dresden-Rossendorf)

100 Python wire scanner analsysis validation using historical LCLS data

As the SLAC high-level applications environment begins migrating towards Python-based frameworks, rigorous validation against legacy tools is critical for operational continuity. We present a comprehensive benchmarking study comparing a new Python wire scanner analysis framework (slac-tools) against legacy MATLAB software at LCLS using 379 historical datasets from routine operations since 2022.

MATLAB .mat files were converted to HDF5 format, preserving wire encoder positions, PMT traces, and original beam size measurements. The Python pipeline—including automated profile windowing, Gaussian fitting, and beam size extraction—was applied identically to converted datasets, treating historical data as live acquisitions.

Comparison of 1062 measurements showed 62% yielded physically valid results after two-stage filtering: excluding non-physical disagreements (≥100% difference) and statistical outliers (1.5×IQR). Valid measurements demonstrated excellent agreement with median difference of 0.31% and mean of 1.38%—well below operational uncertainty. Beam sizes ranged from 20-1000 microns across multiple sectors and modes.

The Python framework is in the process of deployment for live acquisition with comparable performance. This work establishes a systematic methodology for validating modern analysis tools using real operational data and demonstrates HDF5 as a durable format for diagnostic archiving.

Speaker: Tyler Kabana (SLAC National Accelerator Laboratory)

101 STATUS OF INFRARED FEL PROJECT AT ANHUI UNIVERSITY

The infrared free electron laser project FEL-HMF of Anhui university was launched in May 2022. The facility integrate free electron laser, strong magnetic field, and low-temperature environment for material science research and other research. The project is scheduled to commission and lasing this year. The progress and the current status (results of the commissioning) of the project are reported here.

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

102 Studies on optical cavity misalignment effects in the IR-FEL at RRCAT, India

The Infrared Free Electron Laser (IR-FEL) at the Raja Ramanna Centre for Advanced Technology (RRCAT) is the first operational FEL based user facility in India, delivering radiation in the wavelength range of 12.5-50 µm. At a repetition rate of 2 Hz, a CW average power >20 mW, micro-pulse peak power ~ 10 MW, micro-pulse energy ~100 µJ and peak spectral brightness up to ~10¹⁸ photons/s/mm²/mrad²/0.1%BW has been achieved at 21.8 µm wavelength^*,^**. Lasing characterization revealed lower round-trip cavity loss, a wider detuning curve, and higher sensitivity to downstream offset than that predicted by the simulations. To understand this, detailed studies on cavity alignment sensitivity and detuning behavior were conducted through experiments and FEL simulations using GENESIS-OPC^***. Simulations predict that a static upstream mirror tilt causes a reduced alignment tolerance and low cavity loss^****. Start-to-end simulations with a 6-D electron beam distribution predict that the longitudinal beam profile significantly affects the detuning behaviour. These studies provide a sensitive diagnostic tool for optical cavity alignment, thereby helping in the performance optimization of the IR-FEL.

Speaker: Sona Chandran P (Raja Ramanna Centre for Advanced Technology)

103 Temperature controlled, space charge injected Potassium Tantalate Niobate (KTN) deflector

Potassium tantalate niobate (KTN) is a paraelectric perovskite crystal with a strong quadratic electro-optic effect near its Curie temperature, making it a promising material for high-speed optical beam deflection*. Electrode design strongly influences the electric-field uniformity, drive voltage, fringe fields, and dielectric-breakdown risk in KTN deflectors. Based on electrostatic simulations using COMSOL Multiphysics and CST Studio Suite for a KTN crystal of dimensions 5 mm × 5 mm × 0.5 mm, parallel-plate electrodes provide the most uniform electric field, whereas alternative geometries generate strong fringing fields and substantial nonuniformity. These results provided practical guidance for building a prototype of a KTN-based electro-optic deflector using silver electrodes. We present preliminary results characterizing the performance of the prototype KTN deflector.

Speaker: Timothy Suzuki (Michigan State University)

104 Temporal photocathode laser pulse shape control for EuXFEL, FLASH and PITZ

The recently installed NEPAL photocathode lasers at FLASH, PITZ and the European XFEL, deliver exceptional stability, reliability, and flexibility for XFEL operation. Their programmable spectral filter enables precise control of intensity and phase of the near-infrared seed-laser pulses which are then nonlinearly amplified and wavelength converted to UV. We developed an adaptive pulse-shaping method to use the filter for UV output temporal profile control (target: flat-top, triangular, inverted parabola or Gaussian). We utilize a differentiable physics model of the laser system which includes all nonlinearities. We iterate between refining the physics model parameters to match real-system measurements (model fitting) and refining the spectral filter parameters for temporal profile optimization (control optimization). We typically reach convergence in 50 iteration pairs.
We applied this tool to match the temporal laser pulse profiles of the two NEPAL systems at the European XFEL, leading to identical SASE X-ray output without any accelerator retuning. A recent study at the photoinjector test stand PITZ showed significant emittance dependence on laser pulse duration and shape with the potential to enhance EuXFEL performance in the near future.

Speaker: Ingmar Hartl (Deutsches Elektronen-Synchrotron DESY)

105 The Optical Synchronization System at SHINE

The Shanghai High Repetition Rate XFEL and Extreme Light Facility (SHINE) requires a facility-wide synchronization system with femtosecond-level stability to support high-brightness FEL operation and ultrafast pump–probe experiments. This paper presents the design of the optical synchronization system developed for SHINE. A mode-locked optical master oscillator is employed as the central timing reference, while ultra-low-jitter timing signals are distributed through actively stabilized optical fiber links over kilometer-scale distances.
Optical cross-correlators and balanced optical-microwave phase detectors are used for high-precision phase measurement and compensation to suppress timing drift caused by environmental perturbations. The synchronization network integrates accelerator RF systems, photoinjector lasers, seed lasers, experimental lasers, and timing diagnostics into a unified timing infrastructure. A hierarchical synchronization strategy is adopted to achieve sub-10-fs timing stability between critical subsystems.
Key technologies including stabilized optical transmission, optical-to-microwave synchronization, timing receivers, and environmental stabilization are introduced. The proposed system is designed to provide the timing precision, long-term stability, and scalability required under SHINE operating conditions and future facility upgrades.

Speaker: Jinguo Wang (Shanghai Advanced Research Institute)

106 THz radiation driven by SRF accelerator at Peking University: plan and progress

The construction of a platform for terahertz (THz) radiation research at Peking University has been completed recently. The platform consists of a DC-SRF-II photocathode gun, an superconducting radio-frequency (SRF) linac equipped with two 9-cell TESLA-type cavities, a magnetic chicane for electron bunch compression, and a 40-period undulator with taper control. By integrating with an additional THz parametric source synchronized to the photocathode drive laser, a high repetition rate THz FEL amplifier is going to be demonstrated. Furthermore, benefiting from a powerful photocathode drive laser, superradiant radiation can be studied by flexibly manipulating the temporal profiles of the electron bunches. In this work, we present our research plan and latest progress.

Speaker: Senlin Huang (Peking University)

107 Time-resolved cross-correlation measurements of a two-color dual oscillator infrared free-electron laser

The infrared free-electron laser (IR-FEL) at the Fritz Haber Institute recently commissioned a two-color upgrade, enabling simultaneous lasing with two IR FEL oscillators with independently tunable wavelength. This is realized by employing a high-frequency kicker cavity to feeding alternating electron bunches from within the 1GHz bunch train into the two separate IR FEL oscillators. Within this scheme, a high intrinsic temporal synchronization between both FEL outputs is expected*.

In our contribution, we present two-color time-resolved optical cross correlation results proving this synchronization. We also establish that the pulse duration of both FEL increases with increasing desynchronism between the oscillating optical field and the electron bunches, in accordance with earlier studies**. By leveraging the independently tunable pulse durations in each FEL, we further explore the achievable time-resolution that can be expected when the two-color FEL is employed for future IR-IR pump-probe experiments.

Speaker: AKASH CHANDRA BEHERA (Fritz Haber Institute of the Max Planck Society)

108 Title: Nonlinear Photoemission for Bright Beams in X-band Photoinjectors

Recent years have seen a growing interest in developing x-ray sources that are cheaper and more compact than conventional XFELs and synchrotrons. The Compact X-ray Light Source (CXLS) at Arizona State University achieves this using Inverse Compton scattering to produce a high-brightness, short pulse duration x-ray beam. This source operates in blowout mode, meaning a high-charge, short-bunch length electron beam self compresses into a uniform ellipsoid in phase space after emission from the cathode. Seeding this electron beam via single photon photoemission with a UV laser presents many technical challenges due to a lack of optical materials that can withstand the high intensity UV light for a long period of time. For this reason, we implemented a photoinjector scheme that uses multiphoton emission with a 515 nm femtosecond laser. Using this design, we have been able to achieve a bunch charge of 200 pC, a charge density of 700 pC/mm^2, and have measured a transverse emittance of 0.95 mm mrad and a bunch length of 750 fs downstream. Ongoing work is aimed at integrating a spatial light modulator to further reduce the electron beam emittance by adaptively shaping the laser profile.

Speaker: Kevin Eckrosh (Arizona State University, Biodesign Institute)

109 Toward Adaptive Dynamic Desynchronization in FEL Oscillators

Cavity desynchronization is important in FEL oscillators, especially in the IR and THz regimes where optical slippage is significant and laser lethargy can reduce gain under perfect synchronism. Because fast mechanical cavity tuning is difficult, several facilities have implemented dynamic desynchronization by sweeping the electron bunch repetition rate through upstream RF phase control, typically using linear phase ramps across a macropulse. Here, the University of Hawaiʻi FEL oscillator is used as an example system, and full 3D time-dependent simulations are employed to study the effect of dynamic desynchronization. Rather than assuming a linear ramp, we treat desynchronization as a pass-dependent control variable, recognizing that FEL gain and extraction efficiency evolve during oscillator buildup. This study enables exploration of timing profiles better matched to the changing FEL dynamics within the radiation power growth within a macro pulse. The results provide a basis for future adaptive and AI-assisted optimization of dynamic desynchronization in FEL oscillators.

Speaker: Levi Fisher (University of Hawaiʻi at Mānoa)

110 Towards Fully Non-Destructive Sub-Micrometer Resolution Transverse Diagnostics in FELs Using a Nano-Fabricated Wire Scanner

At the Paul Scherrer Institute (PSI), nanotechnology has been successfully employed to develop minimally invasive Wire Scanners with sub-micrometer spatial resolution for monitoring the transverse beam size at SwissFEL during lasing operation. The devices consist of a monolithic low-stress Si₃N₄ structure with a C-shaped silicon fork, integrating two perpendicular wires for full transverse beam profile reconstruction. The wires are as small as 2 µm in width and 1 µm in thickness; their length complies with the SwissFEL beam clearance.

This contribution presents dedicated beam tests performed under standard operating conditions at SwissFEL. For the first time, the results demonstrate a reliable WS operation fully compatible with machine protection constraints, with a negligible impact on lasing delivery.

A systematic comparison in terms of beam invasiveness is carried out against conventional wires of different material (tungsten, CNT) and diameter. The results show a substantial reduction in beam perturbation when using Si₃N₄ wires, confirming their suitability for FELs and marking a significant step toward truly non-destructive transverse beam diagnostics.

Speaker: Dr Francesca Addesa (Paul Scherrer Institute)

111 Transverse, Longitudinal and Intensity Jitter of the LCLS-II Beam powered by Superconducting RF

The energy of the beam is 5 to 10 times more stable than the beam powered by the LCLS Copper Linac due to the continuous wave (CW) operation of the superconducting (SC) Linac. Transversely, we have identified Injector Laser movements on the cathode of RF Gun (low frequency to a few Hz). Some 60 Hz lines from spreader kicker were seen and corrected by timing adjustments of the kicker pulse. Intensity wise we have seen a low frequency component which might be related to the transverse Laser motion, but also a higher frequency line of "1690 Hz" which seemed to be aliased down from a much higher frequency. Data are shown and discussed with mitigation results and plans.

Speaker: Franz-Josef Decker (SLAC National Accelerator Laboratory)

112 Tunable Compact Non-Coplanar X-ray Cavity for Cavity Based X-ray Free-Electron Lasers

Cavity-based X-ray free-electron lasers (CBXFELs) require tunable X-ray crystal cavities to broaden their practical utility. However, the previously proposed four-crystal bow-tie design has a lateral footprint comparable to the undulator length, limiting its deployment in accelerator tunnels. Here, we introduce a compact, broadly tunable X-ray cavity comprising six Bragg-reflecting crystals arranged in a non-coplanar three-dimensional geometry analogous to a pair of optical retroreflectors. This architecture reduces the lateral dimension to approximately 0.5 m while enabling enhanced photon-energy tunability across a wide hard-X-ray range. Although the non-coplanar geometry introduces polarization coupling absent in coplanar configurations, our theoretical analysis shows that polarization mixing vanishes at the center of the Bragg reflection and remains below 5% over a substantial portion of the reflection range, with the exact value depending on the Bragg angle. These results establish non-coplanar multi-crystal cavities as a viable and practical route toward compact, tunable X-ray resonators for CBXFEL implementation.

Representative optical designs will be presented, with details on tuning range, bandwidth, polarization mixing, and other design considerations.

Speaker: Yuri Shvyd'ko (Argonne National Laboratory)

113 Ultra-bright Cavity-based Free Electron Lasers

Cavity-based X-ray free-electron lasers (CBXFELs), including the X-ray regenerative amplifier FEL (XRAFEL) and the XFEL oscillator (XFELO), have been proposed as sources of highly coherent and stable hard X-rays. XRAFELs can generate high-peak-power X-ray pulses with bandwidths limited by Bragg-crystal filtering, whereas XFELOs provide much narrower bandwidths but at substantially lower peak power. In this work, we introduce an ultra-bright CBXFEL scheme that uses a staged configuration to simultaneously enhance peak power and reduce bandwidth. Start-to-end simulations show that this approach can be applied to high-repetition-rate FEL facilities to generate ultra-bright hard X-ray pulses, enabling a further two to three orders of magnitude improvement in spectral flux compared with existing CBXFEL designs.

Speaker: Madison Singleton (SLAC National Accelerator Laboratory)

114 A Hard X-ray Storage Ring Free Electron Laser Oscillator at the APS-U

We demonstrate that a hard X-ray storage ring free-electron laser oscillator (SRFELO) with steady-state output is feasible using nominal APS-U parameters. Numerical simulations using a planar undulator yield a 5.2 keV configuration that exceeds 6% gain, even within a strict 5-meter insertion device limit. We present a comparative study between results using an Elegant and Genesis-based python wrapper code and a 1D FEL code made internally. The code provides a self-consistent model by coupling 3 ODEs for multi-slice FEL growth with a Fokker-Planck solver to evolve electron bunch moments between passes. We observe a saturation regime where FEL-induced heating balances synchrotron radiation damping, reaching an equilibrium consistent with the Renieri limit. These results indicate that the APS-U can support a hard X-ray SRFELO, marking a promising avenue toward fifth-generation light sources.

Speaker: Emmanuel Aneke (Northwestern University)

115 Self-referenced attosecond streaking with electron-beam-driven fields in FELs

We propose a novel photoelectron streaking technique for free-electron laser facilities, enabling simultaneous, in situ characterization of attosecond soft X-ray pulses and the relativistic electron beam that generates them*.

Our device combines solid-state valence photoemission driven by FEL soft-X-ray pulses with co-propagating, THz-like transient electric field of the electron beam acting as the intrinsic streaking field. Simulations show that the resulting photoelectron trace imprints the temporal structure of both the X-ray pulse and the unipolar beam field into the photoelectron momentum distribution, enabling sub-femtosecond reconstruction. Our design is compatible with FEL beamlines and enables single-shot, non-invasive diagnostics with sensitivity to sub-GV/m electron-beam fields. By using the electron beam itself as the streaking field, the approach removes external synchronization constraints and is intrinsically tolerant to timing jitter. In contrast to electro-optic sampling and laser-driven streaking, this approach provides direct, self-referenced access to beam–photon dynamics without external timing references.

Beyond its intrinsic temporal resolution, it opens a route to quantitative measurements of coupled electron–photon dynamics at relativistic sources, with implications for beam optimization, attosecond pulse engineering**, and the direct observation of correlated ultrafast phenomena in matter, like Auger decay and resonant emission delays in solids.

Speaker: Federico Vismarra (SLAC National Accelerator Laboratory)

116 When High QE Isn’t Enough: Stabilizing LCLS-II Photocathodes

Significant progress has been made in the performance of the LCLS-II photoinjector and its photocathode subsystem. Cs₂Te photocathodes are now routinely produced with uniform, high initial quantum efficiency (QE) and Coulomb-level lifetimes. However, unexpected photocathode dynamics have emerged as an operational challenge. Over longer timescales, initially uniform QE develops spatial non-uniformities, including a persistent central damage feature. On shorter timescales, QE shows strong sensitivity to changes in laser illumination, with rapid variability that complicates machine tuning and contributes to fluctuations in FEL output. These observations point to a strong coupling between the photoemitting surface, laser illumination conditions, underlying emission physics, and downstream FEL performance. To address these issues, current efforts are prioritizing QE stability and spatial uniformity over peak QE. This includes fabricating and characterizing photocathodes under a range of growth conditions and substrate choices. Recent work has focused on high-repetition-rate laser damage studies, along with expanded post-mortem SEM/EDS analysis to better understand the underlying damage mechanisms. The overall goal is to improve photocathode robustness and enable stable, long-term FEL operation at high repetition rates.

Speaker: Theodore Vecchione (SLAC National Accelerator Laboratory)

Wednesday 26 August

Undulators and Photon Beamline Instrumentation (Invited)

117 Hard x-ray attosecond metrology via split-delay autocorrelation

I will present the design and commissioning results of a broadband hard x-ray delay line for generating attosecond pulse pairs. The system utilizes an unconventional delay adjustment mechanism, in which the relative path length between the two beams is controlled through the rotation of an x-ray mirror pair. The exit beam pointing stability is guaranteed by the mirror-pair self-error-compensating geometry, thereby enabling stable fast continuous delay scan with sub-20 attosecond precision.

As a first application, the delay line is operated as an autocorrelator. By using the instantaneous nonlinearity of non-sequential two-photon absorption, we directly characterize the pulse duration of the hard x-ray attosecond pulses at the Linac Coherent Light Source to be on average 170 attoseconds, with a dependence on the bandwidth of the pulse. This delay line will further enable attosecond x-ray pump x-ray probe capability and establish a robust platform for hard x-ray nonlinear spectroscopy and diffraction, thereby opening new opportunities to investigate valence electron dynamics at sub-femtosecond timescales.

Speaker: Yanwen Sun (SLAC National Accelerator Laboratory)

118 X-ray diagnostics for a Cavity-based XFEL

European XFEL is commonly providing user operation with self-amplified spontaneous emission (SASE) and hard x-ray self-seeding (HXRSS), ref [1]. A facility-internal project had been started to demonstrate operation of a cavity-based XFEL (CB-XFEL), ref [2], which promises to improve spectral purity and brightness significantly. The first demonstration of a CBXFEL was achieved in May 2025, ref [3]. The X-ray photon diagnostics (XPD) group designed and deployed dedicated diagnostics inside the cavity, operated these as well as diagnostics outside the cavity and set up new online data processing pipelines. The results and performance of these hardware and software elements and their contribution to the overall project success are explained.

[1] Gianluca A. Geloni, "High repetition rate hard x-ray self-seeding at the European XFEL, " Proc. SPIE PC13536, X-Ray Free-Electron Lasers:
Advances in Source Development and Instrumentation VII, PC1353604 (6 June 2025)
[2] Patrick Rauer et.al., "Cavity based x-ray free electron laser demonstrator at the European X-ray Free Electron Laser facility", Phys. Rev. Accel. Beams 26, 020701 (2023).
[3] Rauer, P., Bahns, I., Friedrich, B. et al. Lasing of a cavity-based X-ray source. Nature (2026). https://doi.org/10.1038/s41586-025-10025-x

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

119 CASPER: a Compact Arbitrary Superconducting Polarisation Emitting Radiator

To provide fully controllable elliptical polarisation to all experimental stations of the SwissFEL facility, spanning soft to hard x-rays, the PSI ID group is developing a novel undulator concept based on HTS REBCO tapes. The proposed design enables the superposition of right-handed and left-handed helical fields with comparable strength, allowing the generation of horizontal and vertical linear polarisation with similar field amplitudes and continuous tuning across all elliptical states. Owing to the compactness of the concept, polarisation rotation could also be achieved through a physical rotation of the coil assembly or its cryostat.
This presentation will introduce the new winding scheme and the underlying REBCO tape technology, and will summarise the expected magnetic field performance for parameter sets relevant to future SwissFEL upgrades. Particular emphasis will be placed on the remaining challenges and the substantial R&D effort required to establish this approach as a robust undulator technology, including issues related to persistent currents, quench protection, and automated winding processes.

Speaker: Marco Calvi (Paul Scherrer Institute)

Undulators and Photon Beamline Instrumentation (Contributed)

120 Angular streaking at high repetition rate

Probing the ultrafast motion of electrons that drive chemical reactions and determine material properties requires state-of-the-art light sources with attosecond pulse duration. Soft X-ray pulses can probe electronic densities with atomic-site specificity. The advent of X-ray free-electron lasers (XFELs) has increased the per-pulse flux of attosecond sources by orders of magnitude, enabling powerful techniques like nonlinear multidimensional X-ray spectroscopy; however, the high statistical demands of these measurements translate into extremely long acquisition times at such low-repetition-rate facilities. Here, we demonstrate the generation and temporal characterization of isolated attosecond X-ray pulses at 33.2 kHz repetition rate from a continuous-wave (CW) superconducting accelerator. This advancement in repetition rate allowed for a two-orders-of-magnitude reduction in data acquisition time shown in a representative attosecond pump-probe experiment, paving the way for mapping complex, high-dimensional ultrafast dynamics.complex, high-dimensional ultrafast dynamics.

Speaker: Veronica Guo (Stanford University)

121 Automatic alignment of the HIgh REsolution hard X-ray single shot (HIREX) spectrometer to use as a feedback system for the European XFEL beamlines

The European X-ray Free Electron Laser facility in Germany delivers pulses with femtosecond pulse lengths at repetition rates up to 4.5 MHz. The X-ray photon diagnostics group provides reliable measurement devices and feedback systems for monitoring and controlling the properties of the FEL radiation which is created by the Self Amplified Spontaneous Emission (SASE) process. The stochastic nature of this process gives rise to fluctuations in the pulse energy and spectrum from shot-to-shot. To cover these variations, the HIgh REsolution hard X-ray single shot (HIREX) spectrometer has been installed in the SASE1 and SASE2 undulator beamlines to provide feedback for machine optimization. The spectrometer is an online device, based on a bent diamond crystal as a dispersive element and an MHz-acquisition rate strip detector. The goal is to automate the repetitive setup and alignment tasks without expert involvement. The time saved could then be recuperated by users to collect more, high quality, scientific data. Here, automation involves aligning the crystals and detector in the FEL beam, while minimizing human intervention by continuously monitoring the spectrometer and making or suggesting the necessary adjustments during operation. In this contribution we will discuss the newly developed automatic alignment of the spectrometer.

Speaker: Naresh Kujala (European X-Ray Free-Electron Laser)

Science Applications (Invited)

122 Damage-free diffraction and spectroscopy with attosecond hard X-ray pulses

We report on the realization of damage-free diffraction and spectroscopy using attosecond hard X-ray pulses from LCLS, European XFEL, and SACLA. By compressing the X-ray pulse duration to the attosecond regime, X-ray–induced electronic and structural damage is effectively suppressed during the interaction, enabling the probing of pristine atomic structures and electronic states in matter. We present several examples of damage-free measurements using attosecond pulses, along with temporal diagnostics of the pulses.

Speaker: Ichiro Inoue (The University of Tokyo)

123 Coherent nonlinear X-ray four-photon interaction with core-shell electrons

Coherent nonlinear light–matter interaction with X-rays gives access to a regime in ultrafast spectroscopy in which atomic resolution meets femtosecond and attosecond timescales. Particularly X-ray four-wave mixing, involving several resonant transitions in a single coherent nonlinear process, has the potential to provide information on the electronic states coupling, coherent electron motion,correlation and dynamics, with state and site selectivity.
We demonstrate coherent four-photon interactions with core-shell electrons using broadband SASE X-ray pulses from a free-electron laser. A square spatial mask splits the incoming beam into three cut out beams arranged in a ‘folded BOX’ geometry, enabling background free signal detection.
The all-X-ray four-wave mixing signals, measured in gaseous neon, arise from doubly resonant nonlinear processes involving Raman transitions. The 2D spectral maps (photon-in/photon-out) represent a step towards multidimensional correlation spectroscopy at the atomic scale.

Speaker: Gregor Knopp (Paul Scherrer Institute)

124 Nanoscale Polarization Transient Gratings

Progress in nanoscience increasingly depends on our ability to control light at spatial and temporal scales matching those characteristic of nanostructures. In particular, controlling the polarization of light is essential for investigating materials whose properties depend not only on light intensity, but also on the specific orientation of the electric field. Such polarization-sensitive effects are central to the behavior of magnetic and chiral materials, which are of great interest across condensed matter physics, chemistry, and materials science.
However, unlike in the visible and hard-X ray wavelenghts, no diffractive optics are available to tune light polarization in the extreme ultraviolet (EUV). To overcome this limitation, we have combined the unique TIMER instrument at the FERMI free electron laser with a tailored configuration of the FEL, to create to create periodic, polarization modulations at the nanoscale and demonstrate its capabilities by comparing the dynamics induced by this polarization transient grating with those driven by a conventional intensity grating on a thin ferrimagnetic alloy. While the intensity grating signal is dominated by the thermoelastic response, the polarization grating excitation minimizes it, uncovering helicity-dependent responses previously undetected.

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

125 Progress in soft X-ray Science with High Repetition Rate FELs

Author will add more on the abstract

Speaker: Taran Driver (SLAC National Accelerator Laboratory)

Electron Beam Dynamics (Invited)

126 Review of coherent synchrotron radiation-immune magnetic compressors

Magnetic bunch compressor is an indispensable component in free-electron laser facilities. Recently, breakthrough progress has been made in the physical modeling, understanding, and analytical analysis of the bunch compressor, enabling effective bunch compression while avoiding emittance growth induced by coherent synchrotron radiation effects. In this reivew, we will report latest progress in modeling, analysis of the CSR effects in bunch compressors and design studies on coherent synchrotron radiation-immune magnetic compressors.

Speaker: Yi Jiao (Chinese Academy of Sciences)

127 Ultra-short FEL Pulses by Laser-driven Local Bunch Compression

To generate ultra-short photon pulses in a free-electron laser (FEL) accurate control of the longitudinal phase space density of the electron bunch that drives the FEL process is required. A novel laser-based technique to manipulate the longitudinal phase space density (T. Tanaka, 2019) has been explored at FLASH. An ultra-short current spike is created by compressing part of an electron bunch via a laser-induced energy modulation with a linearly changing envelope. Suitable modulation is created by shaped laser pulses in a laser heater before the electrons reach their final energy. As the FEL process is limited to the ultra-short current spike, a strong reduction of the photon pulse duration is achieved. Creation of the current spike and the generation of ultra-short photon pulses is observed in measurements of the longitudinal phase space density downstream of the radiator undulators. Estimates of the FEL pulse durations are determined from measurements of the photon spectra obtained at FLASH. Further, simulations show that laser-driven local compression is a viable method to enable attosecond pulses at the SASE1 beamline of the European XFEL.

Speaker: Philipp Amstutz (Deutsches Elektronen-Synchrotron DESY)

Electron Beam Dynamics (Contributed)

128 Microbunching instability studies and lasing at FERMI with one-and two-stage compression scheme

We report on recent experimental sessions at FERMI FEL, characterizing the electron beam longitudinal phase space and the HGHG FEL, in single and double compression scheme. The impact of microbunching instability on the beam dynamics and lasing is recorded, and succesfully compared with predictions from a MBI instability integral model incorporating intrabeam scattering. With the addition of some optics tuning in the FERMI spreader line, such theoretical and experimental advancements have contributed to the emission of fully coherent HGHG harmonic pulses at around 1 nm.

Speaker: Giovanni Campri (Elettra-Sincrotrone Trieste S.C.p.A.)

129 Beam Halo Formation via Longitudinal-Transverse Coupling in Continuous-Wave Photoinjectors

Beam halo formation poses a critical challenge for high-repetition-rate continuous-wave (CW) free-electron lasers (FELs), directly affecting beam quality and machine protection, as observed during the LCLS-II commissioning. We identify and experimentally validate a previously unrecognized three-step mechanism for halo generation in the photoinjector, arising from coupled longitudinal-transverse dynamics in the low-energy beam. Theoretical analysis reveals that (i) the RF buncher induces an energy-radius correlation, (ii) velocity bunching transforms this correlation into hollowed density structures in the bunch head and tail, and (iii) differential overfocusing of these hollowed regions by downstream focusing forms the observed halo. This mechanism is confirmed by particle-in-cell simulations and direct experimental measurements, including controlled formation of a core-ring profile via solenoid tuning. The results establish the physical origin of the halo and demonstrate a mitigation via buncher compression tuning that reduces halo and downstream loss, supporting sustained high-repetition-rate FEL operation.

Speaker: Zhen Zhang (SLAC National Accelerator Laboratory)

Wednesday Poster Session

130 100-nm focusing of 100 eV XFEL pulses at SACLA BL1 using high-NA mirror optics

Recent advances in X-ray free-electron lasers (XFEL) have opened new opportunities for X-ray nonlinear optics and transient dynamics in condensed matter*. Tight focusing of XFEL pulses is essential for generating the high intensities required for nonlinear X-ray processes and for achieving high-spatial-resolution imaging of ultrafast dynamics. However, 100-nm focusing of soft X-rays remains challenging because it requires high-numerical-aperture (high-NA) optics with extremely high fabrication accuracy.

Here, we demonstrate 100-nm focusing using a two-dimensional (2D) Wolter type-III mirror optics composed of an ellipsoidal mirror and a hyperboloidal mirror. The 2D Wolter type-III geometry enables high-NA focusing while maintaining the required fabrication accuracy. We performed a focusing experiment at SACLA BL1 using a prototype mirror pair and achieved a focal spot size of 100 nm (vertical) × 300 nm (horizontal).

Speaker: Yu NAKATA (The University of Tokyo, RIKEN SPring-8 Center)

131 2. CXLS Early Science and the Path Toward a Compact Coherent X-ray FEL

The Compact X-ray Light Source (CXLS) at Arizona State University is a compact inverse-Compton-scattering (ICS) facility designed to provide femtosecond hard X-rays in a university laboratory environment. CXLS has entered commissioning and early-science operation, demonstrating stable operation of a 9.3 GHz X-band accelerator, synchronized picosecond laser systems, and femtosecond hard X-ray generation at 1 kHz repetition rate. Initial operation has produced hard X-rays near 9.25 keV with sub-ps duration, micron-scale source size, and timing jitter below 100 fs. Early experiments have commissioned the multimodal X-ray endstation for time-resolved diffraction and spectroscopy, including the first protein crystallography results obtained at CXLS.

This talk will summarize commissioning performance of the accelerator, RF, laser, and X-ray systems, including recent results from structural biology and quantum materials experiments. We will also discuss construction of the Compact X-ray Free-Electron Laser (CXFEL) which is underway. CXFEL extends the compact ICS architecture toward fully coherent soft X-ray pulses with femtosecond-to-attosecond duration. We’ll touch on the advanced beam manipulation methods, including nanopatterned electron beams and emittance-exchange concepts, for achieving coherent X-ray generation as well as future applications in structural biology, ultrafast chemistry, quantum materials, and multipulse/multicolor schemes.

Speaker: Nicholas Matlis (Arizona State University)

132 A Compact 13.5-nm LPA-Driven FEL Based on a 6-m Integrated-Focusing FNAPU

A 13.5-nm SASE FEL based on a force-neutral, phase-adjustable undulator (FNAPU) with integrated FODO focusing has been simulated. For an undulator period of 28 mm and K = 2.32, simulations assuming a 1-GeV electron beam with 0.5-mm·mrad normalized emittance and 3-kA peak current show that the integrated focusing maintains an rms beam size of approximately 20 μm, enabling saturation in about 6 m with a peak power of ~15 GW. This short saturation length removes the need for undulator segmentation, while the integrated focusing eliminates the need for external quadrupole magnets, enabling operation with a single undulator in a highly compact FEL configuration. The concept is well suited to laser-plasma-accelerator drivers and offers a promising route toward compact FEL facilities with potential applications in semiconductor lithography.

Speaker: Maofei Qian (Argonne National Laboratory)

133 A Digital Twin Framework for Optimization of Compact X-ray Light Source

X-ray free-electron lasers (XFELs) provide exceptional brightness and ultrafast capabilities but face high demand for beamtime. Compact X-ray light sources (CXLS) aim to reduce facility size and cost, enabling broader access to a subset of XFEL experiments and complementing large-scale facilities.

We present the development of a digital twin for key CXLS components, designed for system optimization and autonomous operation. The framework integrates physics-based models of inverse Compton scattering X-ray generation, beam transport, multilayer mirror optics, and photon-counting detectors into a unified simulation environment. Monte Carlo methods are used to model X-ray generation and propagation through the beamline, enabling rapid exploration of machine parameters and their impact on photon output.

The digital twin serves as a high-fidelity virtual environment for testing optimization algorithms, feedback schemes, and autonomous beamline strategies before deployment on hardware. This approach supports virtual commissioning, sensitivity analysis, and predictive tuning while reducing operational risk and commissioning time. The framework provides a foundation for optimization and automation in compact X-ray sources.

Speaker: Adil Ansari (Arizona State University)

134 Analysis of two color FEL operation at BL3 at SACLA

We present results of modeling of the FEL radiation used in pump probe experiment at BL3 at SACLA free electron laser. 160 pC, 8 GeV electron bunch produces two photon pulses with different photon energies (color1: 8 keV and color 2: 9 keV) when consecutively passing two undulator sections. First color is filtered by Si111 monochromator to the bandwidth of 0.02% (pulse energy is 0.1 uJ). Second color is used as is with fwhm spectrum width of 0.3% (pulse energy is 70 uJ). Width of the radiation spectrum is mainly defined by the space charge induced energy chirp along the electron bunch. Our analysis shows that parameters of SASE FEL in the experiment correspond to a large value of diffraction parameter, B ~ 400 which prevent formation of the transverse coherence for both colors. An important feature of the radiation is significant slice spatial distortions of the fields and poor pointing stability for the color 1 arising from fundamental reasons - start up of the amplification from the shot noise and radiation mode degeneration arising from large values of diffraction parameter. We perform simulations of the radiation pulses with three dimensional, time dependent FEL simulation code FAST. Then simulated data are stored in the XPD-FAST, data base of photon properties hosted by the European XFEL and DESY, and finally used for modeling of experiment. The FEL simulation results will be used for a detailed analysis of X-ray crystal diffraction data from the two-color experiment.

Speaker: Liubov Samoylova (European X-Ray Free-Electron Laser)

135 Attosecond metrology of a seeded free electron laser

We use attosecond interferometry to diagnose the temporal profile of attosecond pulse trains generated with a seeded free electron laser. A harmonic train is generated using the FERMI free electron laser with a seeding field of 266 nm. This harmonic pulse train is overlapped with a second 266 nm pulse at the Low Density Matter beamline. Two color photoemission in helium was analyzed using a novel correlation based approach to reveal harmonic phase differences in the APT. Our analysis method exploits the high kinetic energy resolution of the magnetic bottle electron spectrometer to reveal the femtosecond-scale harmonic phase variation of the attosecond pulse train. This technique has also been used to probe autoionizing resonances in the photoionization of neon.

Speaker: John Walters (Stanford University)

136 AURA - The Most Compact Broadband FTIR Spectrometer (3–600 µm) for IR-FEL and Laser Diagnostics

Spectral diagnostics of infrared and THz free electron lasers typically rely on either laser-based techniques or large benchtop FTIR systems, making alignment and beamline integration time-consuming. We present AURA, the most compact broadband FTIR spectrometer developed by WAIV FTIR GmbH, covering a continuous spectral range from 3 to 600 µm (100 THz to 0.5 THz). The instrument fits in a 130 × 150 × 80 mm housing and reaches a resolution down to 1 cm⁻¹, operating with both pulsed and CW sources.
AURA uses fully reflective optics together with broadband detection, therefore no beamsplitter exchange is required during measurement and beamsplitter absorption features contribute to the measured spectrum. Data acquisition and analysis are handled through a browser-based interface that requires no dedicated installation.
The poster will present the instrument concept, measurement results from various FELs and laser laboratories, and use cases for spectral diagnostics at IR-FEL beamlines. WAIV FTIR GmbH is a spin-off of the THz beamline at FLASH, DESY (Hamburg).

Speaker: Dr Ekaterina Jung (WAIV FTIR GmbH)

137 Biological Effects of Mid-Infrared Free-Electron Lasers: Electroretinograms from Compound Eyes of Two Arthropod Species

Free-electron lasers (FELs) are valuable light sources for examining the biological effects of mid-infrared radiation on living organisms. We investigated the compound eyes of two arthropod species, crayfish (Procambarus clarkii, Crustacea) and crickets (Gryllus bimaculatus, Insecta), using LEBRA-FEL (3–6 μm) and KU-FEL (3.4–26 μm) as radiation sources. In both cases, the mid-infrared FELs generated electroretinograms suggesting a novel mid-infrared reaction in the animals’ visual organ function and indicating the universality of arthropod compound eye properties with respect to mid-infrared radiation. A remaining task for future investigation is how and when these two species use the detected mid-infrared reaction in their behavior.

Speaker: Fumio Shishikura (Nihon University)

138 Cascaded Electron Accelerator with Terahertz Pulses

Efficient acceleration has been demonstrated using dielectric waveguides driven by multi-cycle THz pulses. Further improvements were achieved by employing sequential THz sources and waveguide structures, enabling cascaded acceleration . The cascaded THz-driven dielectric electron accelerator proposed here provides enhanced acceleration efficiency while enabling electron bunch compression. A multi-cycle THz pulse is coupled into a dielectric-lined waveguide, where the dielectric thickness is chosen to ensure phase-velocity matching for efficient acceleration. After the electrons exit the first acceleration stage, they are guided along a curved trajectory into a second waveguide using a magnetic chicane, while the THz pulse propagates in a separate waveguide. By synchronizing the electron path with the THz pulse propagation, further post-acceleration is achieved. According to our calculations, using 0.165 THz pulses with a peak electric field of 500 kV/cm (in the waveguide), a 300 keV electron bunch with an initial duration of 85 fs (@FWHM) is accelerated such that a substantial fraction of the bunch reaches energies close to 750 keV at the end of the second acceleration stage, while the bunch duration compresses to 75 fs** .

Speaker: Zoltán Tibai (University of Pecs)

139 Characteristics of SSMB Radiation

In this paper, we will report the property of SSMB radiaiton, like the power, bandwidth, and spectral brightness.

Speaker: Xiujie Deng (Tsinghua University)

140 Commissioning of X-ray optics and diagnostics for CBXFEL at LCLS

We present results from the commissioning of the X-ray optics motion and photon diagnostics systems for the cavity-based X-ray free-electron laser (CBXFEL) project, a partnership among Argonne National Laboratory (ANL), SLAC National Accelerator Laboratory, and RIKEN SPring-8. The project aims to demonstrate CBXFEL technology using X-rays produced in the first seven hard X-ray undulators of the Linac Coherent Light Source (LCLS). The CBXFEL X-ray optics positioning [1] and diagnostic [2] components have now been installed in the LCLS undulator tunnel and are currently undergoing commissioning. This work focuses on evaluating the performance of the X-ray nanopositioning system and the multiple diagnostic elements distributed throughout the X-ray cavity. Highlighted results include characterization of beam intensity and position diagnostics, multi-axis motion assessment of the X-ray optics for stable operation while maintaining the crystals’ diffraction condition, and joint operation of these subsystems to enable continuous photon transport over multiple round trips within the X-ray cavity.

[1] Deming Shu, et al. "Design of the nanopositioning and mechanical system for cavity-based X-ray free electron laser". J. Phys.: Conf. Ser. 2022 2380}012029. DOI 10.1088/1742-6596/2380/1/012029
[2] Peifan Liu, et al. "X-ray diagnostics for the cavity-based x-ray free-electron laser project" Phys. Rev. Accel. Beams, 27 (2024) 110701
DOI: 10.1103/PhysRevAccelBeams.27.110701

Speaker: Mario Balcazar (SLAC National Accelerator Laboratory)

141 Compact High-Repetition-Rate EUV to Soft X-Ray Free Electron Laser

High-brightness X-ray Free Electron Lasers (FELs) are transformative tools for scientific discovery. However, most existing FELs are kilometer-scale facilities with high construction costs and low repetition rates, limiting their accessibility and throughput. This paper proposes a compact, high-repetition-rate (MHz) EUV to 1 nm soft X-ray FEL with a footprint of less than 100 meters. The facility leverages a multi-turn recirculating linear accelerator that integrates superconducting technology with recent advances in diffraction-limited storage rings.
We present the conceptual design and analyze the impact of incoherent and coherent synchrotron radiation, demonstrating that these effects do not limit achieving high-quality electron beams. Such a compact facility would substantially reduce construction and operational costs while expanding access to these research tools. Furthermore, the design provides a potential upgrade path to hard X-ray radiation by incorporating high-accelerating gradient structures to further accelerate a portion of the MHz electron beam.

Speaker: Ji Qiang (Lawrence Berkeley National Laboratory)

142 Concept for a future light source toward coherent agile X-ray pulse generation

Increasing the number of X-ray free electron laser (XFEL) facilities and capabilities across the United States is critical to keep pace with the growing demand from the user community. We are exploring a novel concept based on two-beam acceleration technology, with a focus on delivering coherent X-ray pulses with agile and customizable properties. The Coherent Agile Pulse at Argonne X-ray Source (CAPAX) platform leverages ANL's expertise in high-gradent acceleration, bright electron source, and undulator design to enable rapid reconfiguration of key pulse parameters. This paper discusses the overall architecture of the proposed concept, its key technological building blocks, and its expected photon beam performance, with the goal of establishing CAPAX as a new paradigm for flexible, user-driven XFEL science in the US.

Speaker: Philippe Piot (Argonne National Laboratory)

143 Core-Hole Attosecond Electronic Coherence in Molecular Systems

Soft-X-ray attosecond spectroscopy at FELs provides direct access to ultrafast electronic dynamics in matter with simultaneous sub-femtosecond resolution and atomic-site specificity*. By exploiting core-level transitions, these approaches offer a localized and phase-sensitive probe of electronic coherence, extending attosecond science beyond valence dynamics into the regime of strongly correlated, site-selective excitation.

The high-repetition-rate operation of LCLS-II delivers the photon flux and peak intensity required to both initiate and resolve coherent motion of deeply bound electrons. Crucially, FELs uniquely enable attosecond pump–attosecond probe schemes at core-level energies, allowing the preparation and interrogation of non-stationary electronic states at the ultimate time resolution of molecular processes**.

Our two recent experimental user campaigns explored the creation, evolution, and decay of core-hole coherence in molecular systems, ranging from small molecules to transition-metal complexes. These attosecond pump–probe measurements reveal how quantum coherence emerges from core-excitation and how this coherence is subsequently reshaped by electron–nuclear coupling on few-femtosecond timescales.

Together, these results establish soft-X-ray attosecond spectroscopy at FELs as a uniquely powerful approach to initiate, control, and resolve electron dynamics in complex matter, defining a new experimental regime for ultrafast molecular science.

Speaker: Federico Vismarra (SLAC National Accelerator Laboratory)

144 Critical slippage effects in laser-plasma-accelerator-driven free-electron lasers*

The application to FELs has become a research interest after improvements in laser-plasma-accelerator (LPA) beam quality and reliability, [1,2]. The goal is for more compact light sources due to the high gradients in the LPA. However, the FEL output from ultra-short LPA electron bunches (at the 20-fs level) is dominated by the rapid slippage of the beam behind the optical pulse in two LPA-FEL experiments at 27 nm [1] and 420 nm [2]. When the ratio of the electron bunch length to slippage is close to one, exponential gain is limited even in an rf-linac-driven FEL at 530 nm [3]. This empirical limit is supported by simulations using the MINERVA code at 420-nm. In ref. 2, the optical pulse slips ahead of the electrons over the first 1-m of the undulator. However, the electron bunch has been modulated and after ballistic bunching further radiation is found. Hence, this acts like an optical klystron where the first meter of the undulator acts as the modulator and the remainder of the undulator is the radiator. The code’s unique feature being able to apply slippage 30 times per undulator period (instead of once as in some codes) is required to resolve the rapid slippage [2].

Speaker: Dr Henry Freund (University of Maryland, College Park)

145 Development of a Magnetic Measurement Test Stand for LCLS-II-HE Undulator Pre-Tuning

A magnetic measurement test stand has been developed to reduce the workload and time required for the final tuning of the LCLS-II-HE soft X-ray undulators. The test stand includes a touch probe for mechanical measurements, a Hall probe for local field measurements, and a stretched wire system for field integral measurements. The test stand enables early magnetic alignment and magnetic field corrections, including trajectories, phase error, and field integrals. This paper presents the system, its capabilities, and preliminary results demonstrating its use for pre-tuning undulators.

Speaker: Johann Eduardo Baader (SLAC National Accelerator Laboratory)

146 Experimental Design and First Results from the Plasma-Driven Attosecond X-Ray Experiment at FACET-II

The ongoing Plasma-driven Attosecond X-ray source experiment (PAX) at FACET-II aims to produce coherent soft X-ray pulses of attosecond duration using a plasma wakefield accelerator [1]. These X-ray pulses can be used to study chemical processes where attosecond-scale electron motion is important. For this first stage of the experiment[KS1.1], PAX plans to demonstrate that <100 nm bunch length electron beams can be generated using the 10 GeV, 20 um duration beam accelerated in the FACET-II linac and using the plasma cell to give it a percent-per-micron chirp. The strongly chirped beam can then be compressed in a weak chicane to sub-100nm length, producing coherent synchrotron radiation [KS2.1]in the final chicane magnet at wavelengths as low as 10s of nm. In this contribution we describe the results of recent experiments aiming to produce these ultra-compressed bunches and generate this single-cycle radiation, as well as the completion of the experimental setup of the PAX.
[1] C. Emma, X.Xu et al APL Photonics 6, 076107 (2021)

Speaker: Rafi Hessami (SLAC National Accelerator Laboratory)

147 Experimental generation of attosecond soft and hard X-ray pulses with photocathode shaping at SwissFEL

We demonstrate the generation of both soft and hard X-ray attosecond pulses at SwissFEL, based on the temporal shaping of the photocathode laser pulses. Through flexible bunch compression schemes, the electron beam current profile evolves from a double-Gaussian shape with 10 ps duration into a femtosecond-scale isolated high-current spike in the core of a tens-of-femtosecond-long electron bunch. This high-current spike enables the generation of isolated soft and hard X-ray pulses characterized by a single spectral spike with multi-eV bandwidth, consistent with attosecond pulse durations. High-resolution measurements of the electron beam longitudinal phase space, together with start-to-end simulations, provide further support for the attosecond-scale duration of the emitted X-ray pulses. In the soft X-ray regime, the polarization of the attosecond pulses can be continuously tuned from linear to circular using the APPLE-X undulator. We further demonstrate rapid switching of the soft X-ray pulse duration from sub-femtosecond to tens of femtoseconds by exploiting the time-dependent transverse phase space of the shaped electron beam.

Speaker: Zhaoheng Guo (École Polytechnique Fédérale de Lausanne, Paul Scherrer Institute)

148 FACET-II: recent experimental results and plans for R&D relevant to FEL applications

FACET-II is a National User Facility at SLAC focusing on research in the areas of advanced acceleration and coherent radiation techniques with high-energy electron beams [1]. This contribution summarizes recent experimental results relevant for FEL and high brightness beam applications including the generation of extremely high current beams through laser-electron phase space manipulation [2], and the demonstration of a plasma injector brightness transformer [3]. We also discuss present and future R&D plans for the facility including attosecond X-ray generation with plasma-chirped electron bunches [4] and the possibility of utilizing the FACET-II linac as a driver for a very high energy XFEL (up to ~50 keV photon energy) with a high energy (25 - 30 GeV) electron beam.
[1] Yakimenko et al., PRAB 22, 101301 (2019)
[2] C. Emma, N. Majernik, K. Swanson et al., PRL 134, 085001 – (2025)
[3] C. Zhang, D. Storey et al., Nature Communications volume 16, Article number: 10719 (2025)
[4] C. Emma et al., APL Photonics 6, 076107 (2021)

Speaker: Claudio Emma (SLAC National Accelerator Laboratory)

149 Force-Neutral Adjustable Phase Undulators

Conventional variable gap undulators rely on mechanically complex, force intensive support systems that limit tuning speed, alignment stability, and operational reliability—challenges that are increasingly critical for next generation free electron lasers (FELs). In collaboration with Argonne National Laboratory (ANL), RadiaBeam Technologies is advancing ANL’s Force Neutral Adjustable Phase Undulator (FNAPU) concept toward FEL deployment through the development of electromagnetic optimization tools and scalable manufacturing and quality assurance workflows, including the design and fabrication of a standalone 1 meter FNAPU prototype targeted for completion by 2027.

FNAPUs incorporate a secondary array of commercially available permanent magnets that counteract magnetic forces within the primary undulator structure, creating an inherently force neutral system. This architecture enables a compact mechanical design with simplified assembly, reduced structural loads, and improved magnetic alignment precision. These features support faster tuning, enhanced stability, and improved operational safety for FEL facilities. The modular FNAPU platform also enables advanced configurations of interest to the FEL community, including crossed undulator geometries for polarization control and multi undulator matrices for flexible X ray energy coverage, positioning FNAPU technology as a promising candidate for future XFEL and synchrotron radiation sources.

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

150 Harmonic lasing self-seeded FEL in a RAFEL cavity with multilayer mirrors

Harmonic lasing is an advanced FEL concept to produce short wavelengths with a low-energy electron beam by developing lasing at an odd harmonic of the FEL fundamental wavelength. Harmonic lasing self-seeded (HLSS) FEL uses a two-stage high-gain FEL with the first stage being a harmonic lasing FEL to produce the coherent seed for the second stage which serves as a high-gain FEL amplifier. HLSS has been theoretically studied for X-ray wavelengths and experimentally demonstrated between 4.5 and 15 nm*. Due to the SASE nature of the first stage, the harmonic seed pulses from the first stage do not have good temporal overlap with the core of the electron bunch in the second stage, thus reducing the output pulse energy and efficiency. We study an advanced version of HLSS by using a regenerative amplifier FEL (RAFEL) cavity consisting of multilayer mirrors to recirculate the harmonic wavelength in the first stage. We show in simulations that using the regenerative amplifier cavity with multilayer mirrors stretches and delays the harmonic seed pulses in time and narrows the radiation spectral bandwidth. The RAFEL harmonic seeding significantly increases the second stage output power and longitudinal coherence.

Speaker: Paris Franz (xLight)

151 Imaging the valence electron density in crystalline materials using X-ray Optical Mixing

The microscopic structure and dynamics of valence electrons determine many of a material's properties, including its optical ones. Conventional x-ray scattering can resolve the angstrom-scale electronic structure, but it is difficult to separate the valence electrons from the more localized core electrons. Here we report nonlinear x-ray scattering measurements from optically-driven valence electrons using the LCLS hard x-ray free-electron laser. We measure the phase-matched x-ray-optical sum frequency generation between two optical photons and a single x-ray photon in bulk centrosymmetric silicon.* We show that a single higher-order wave-mixing measurement gives critical information on the optically inaccessible nonlinear driven valence electron density. In particular, we measure the reduced site symmetry of the bonding electrons including their local inversion-symmetry breaking. The results have implications for understanding optically driven materials in both the strong field regime and with exotic properties.

Speaker: Chance Ornelas-Skarin (Stanford University)

152 Measurements of Vertical Polarization in the Second Harmonic of Planar Undulator Radiation

Polarization control of high-intensity coherent X-ray light is essential for many experiments, including magnetic spectroscopy and studies of chiral molecular structure. However, most free-electron laser facilities rely on planar undulators for light generation, which are generally unable to provide polarization tunability. Counterintuitively, analytical solutions of Maxwell's equations predict a vertical polarization component in the second harmonic of planar undulator radiation, a feature that offers a potential route to polarization control without the need for specialized undulator designs. Understanding the polarization structure of higher harmonics is thus crucial for extending the capabilities of these facilities. Using an angle-resolving electron time-of-flight (ARTOF) detector, we measure the out-of-plane polarization of first- and second-harmonic soft X-ray pulses at the Linac Coherent Light Source (LCLS). We present results of varying polarization structures between the fundamental and second harmonic and demonstrate that ARTOF-based measurements provide a practical method for characterizing FEL polarization. These results suggest that polarization tunability may be more broadly accessible at FEL facilities than previously recognized.

Speaker: Csaba Nemeth (SLAC National Accelerator Laboratory)

153 Modal and Vlasov Descriptions of EEHG FEL

A theoretical framework for understanding and controlling mode selection in echo-enabled harmonic generation (EEHG) free-electron lasers (FELs) at high harmonic numbers is presented. In EEHG, adjacent harmonics naturally exhibit comparable microbunching amplitudes, creating a unique opportunity to work with different spectral components. To capture this process, we benchmark orthogonal transverse-mode expansions against a Maxwell–Vlasov treatment of FEL gain. This framework enables a quantitative comparison between modal descriptions and first-principles kinetic theory, with emphasis on the role of transverse structure, detuning, and polarization in mode competition. This work serves as a blueprint for future computational methods to optimize selective amplification of neighboring harmonics. The final analysis aims to reveal the extent of transverse restructuring driven by instabilities that can break degeneracies in mode competition, reduce the demand for start-to-end simulations, and inform the feasibility of the machine setup for future experimental studies.

Speaker: Pardis Niknejadi (Deutsches Elektronen-Synchrotron DESY)

154 Modeling BELLA center FEL results

Several demonstration experiments of laser plasma accelerator (LPA) driven free electron lasers (FEL) have emerged in the past 5 years [1,2] and are paving the path towards compact light sources. The electron beams generated in plasma accelerators differ significantly from those produced in RF-based photoinjectors and linear accelerators, which drive all existing XFELs. LPAs exhibit larger energy spreads, stronger shot-to-shot fluctuations, and correlated phase space structure. Here we discuss recent efforts to model the FEL process recently demonstrated by the BELLA Center [2] using two freely available codes, Genesis 1.3 [3] and MINERVA [4]. These codes are both based on the slowly-varying envelope approximation but employ different numerical approaches to modeling the FEL interaction, with differing assumptions regarding beam dynamics and radiation field evolution. By comparing their predictions for LPA-driven beams, we aim to assess their applicability and identify key considerations for accurately modeling FEL performance in this emerging regime.

[1] W. Wang et al., Nature 595, 516 (2021).
[2] S.K. Barber et al., Phys. Rev. Lett. 135, 055001 (2025).
[3] S. Reiche, Nucl. Instrum. Methods Phys. Res. A429, 243 (1999).
[4] H.P. Freund et al., New J. Phys. 19, 023020 (2017).

Speaker: Samuel Barber (Lawrence Berkeley National Laboratory)

155 Nanoscale Ultrafast Lattice Modulation with Hard X-ray Free Electron Laser

Ultrafast optical laser-based techniques have enabled the probing of atomistic processes at their intrinsic temporal scales with femto- and attosecond resolution.
However, the long wavelengths of optical lasers have prevented their interrogation and manipulation with nanoscale spatial specificity.
Advances in hard X-ray free-electron lasers have enabled progress in developing X-ray transient grating spectroscopy — a technique that aims to coherently control elementary excitations with nanoscale X-ray standing waves.
Thus far, the realization of this technique at the nanoscale has been a challenge.
Here we demonstrate X-ray transient grating spectroscopy with spatial periods of the order of 10 nm via sub-femtosecond synchronization of two hard X-ray pump pulses at a precisely controlled crossing angle.
This creates a thermal grating and preferentially excites coherent longitudinal acoustic phonon modes with the transient grating wavevector.
Probing with a third, variably delayed X-ray pulse with the same photon energy, time-and-wavevector resolved measurements of the induced scattering intensity modulation provide evidence of ballistic thermal transport at nanometer scales.
These results highlight the potential of X-ray transient grating as a powerful platform for studying nanoscale transport in condensed matter and the coherent control of nanoscale dynamics.

Speaker: Haoyuan Li (Linac Coherent Light Source)

156 Numerical and experimental benchmarking of microbunching instability with intrabeam scattering at linac-FELs

The systematic comparison of two semi-analytical models of microbunching instability affecting electron beams in single pass or recirculating linear accelerators is reported. The comparison is comprehensive of numerous features of the instability, and enriched by three different expressions to calculate intrabeam scattering. The inclusion of this effect allows the recovery of agreement with published experimental observations, to date either in disagreement with theory, or in partial agreement by virtue of blind fitting of parameters. The work is supported by recent experimental sessions at FERMI FEL, on control of microbuching instability in single vs. double compression.

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

157 Opportunities offered by the variable polarization afterburner at the European XFEL

The variable polarization afterburner at the European XFEL was successfully commissioned in early 2024. It consists of four APPLE-X undulators installed behind the SASE3 planar undulator system, generating the free-electron laser pulses for the soft X-ray beam lines. The afterburner has now become an integral part of the European XFEL and is used for various experiments. These include experiments for users as well as experiments to improve lasing quality and explore new ideas for operating the facility. An overview of these experiments is presented.

Speaker: Suren Karabekyan (European X-Ray Free-Electron Laser)

158 Pair beams unlock beyond‑terawatt attosecond free‑electron laser pulses

Free-electron lasers (FELs) deliver the brightest coherent X-ray pulses for atomic-resolution ultrafast science, yet scaling to extreme peak powers and attosecond durations faces a fundamental barrier: in the ultrahigh-current regime, longitudinal space-charge fields impose slice-dependent energy detuning that suppresses gain and confines efficient lasing to a small bunch fraction. We demonstrate that a quasi-neutral electron-positron pair beam eliminates this self-field entirely, enabling full-bunch coherent amplification without external compensation. Three-dimensional particle-in-cell simulations of a single-pass untapered undulator confirm the mechanism in two regimes: in the soft X-ray range, the pair beam reaches ~1.85 TW at ~345 as with enhanced odd harmonics and improved spatial coherence, while the electron-only beam fails to saturate; in a high-harmonic pair-cascade configuration, ~10 TW isolated spikes of ~3.5 as emerge with coherent amplification to ~177 keV. These results establish a new operating regime for ultrahigh-power attosecond sources and a direct route to coherent gamma-ray emission beyond 100 keV, presently inaccessible to magnetic-undulator FELs.

Speaker: Cagri Erciyes (Max Planck Institute for Nuclear Physics)

159 Plasma-based compression of electron beams

Electron bunches with attosecond-scale bunch durations and high peak currents would enable new opportunities for next-generation light sources. However, to generate such short bunches requires novel compression techniques. Here we consider plasma-based compression, a method which exploits the inherently large accelerating fields in a plasma wakefield accelerator, to add an energy-time correlation to an electron bunch which exceeds conventional approaches by several orders of magnitude. A downstream dispersive element converts this chirp into temporal compression. Particle-in-cell and particle tracking simulations demonstrate that this technique can enable bunch durations approaching 10 nm and peak currents nearing 1 MA. Practical implementation, however, introduces additional constraints that will impact the compressor performance. Using beam parameters from recent experiments at FACET-II at SLAC National Accelerator Laboratory, we model plasma-based compression of 10 GeV electron bunches in a beam-driven plasma wakefield accelerator. We present simulation results showing the achievable compression under realistic conditions and identify the technical challenges that must be addressed for successful compression.

Speaker: Kelly Swanson (SLAC National Accelerator Laboratory)

160 Probing ultrafast heating and ionization dynamics in solid density plasmas with time-resolved resonant X-ray absorption and emission

Heating and ionization are among the most fundamental processes in relativistic laser--solid interactions; however, their spatiotemporal evolution remains challenging to capture experimentally. Here we present detailed diagnosis of high-intensity laser interactions with wire targets, leveraging the extreme spectral brightness of an X-ray free-electron laser in sub-picosecond time-resolved resonant X-ray emission spectroscopy and absorption imaging. Experimental results are compared with comprehensive simulations using atomic collisional--radiative models, particle-in-cell, and magnetohydrodynamics codes to elucidate the underlying physics. These multi-scale simulations reveal extreme sensitivity of basic plasma parameters with widely used models, such as temperature and ionization depth, which are able to be constrained by incorporating a detailed accounting of laser spatial profiles, pre-plasma conditions, and collisional processes. These results provide new insights into heating and ionization dynamics in the high-energy-density regime relevant to inertial fusion energy research, both as an experimental platform for accessing theoretically challenging conditions and as a benchmark for improving models of high-power laser-plasma interactions.

[1] Lingen Huang, et.al., Nature Communications, 17,3219 (2026)
[2] Lingen Huang, et.al., Matter and Radiation at Extremes, 11, 017201 (2026)

Speaker: Lingen Huang (Helmholtz-Zentrum Dresden-Rossendorf)

161 Programmable Phase Fronts for Tunable Electron Beam Interactions Using Silicon Photonics

We present a Dielectric Laser Accelerator concept for achieving tunable interactions with electron beams using silicon photonic integrated circuits to program laser phase fronts. Our photonic circuit is an 8x8 binary tree mesh of Mach-Zehnder Interferometers (MZIs), enabling routing, splitting, and recombination of light via adjustable thermal phase shifters. We couple 2.2 um, 330 fs FWHM laser pulses into eight single-mode waveguides (each with estimated peak fields of 350 MV/m) via a large, high-efficiency (~73%) Three-Wave Interaction Grating (TWIG) coupler. With our circuit architecture, we transform the input beam, with an arbitrary phase front, by aligning the phase in each waveguide channel using TWIG taps, to create a desired wave front incident on an integrated DLA. When illuminated with laser pulses, the DLA generates strong evanescent fields phase-matched to ~40 keV electrons traveling ~100 nm above the silicon surface, allowing gradients up to 75 MeV/m and energy gains on the order of 600 eV across a 128-period structure (0.81 um periodicity). DLAs have been demonstrated as a tool for acceleration, deflection, bunching, and focusing of electron beams, and engineering the incident phase front allows for switching between different interaction modes—unlocking new functionality for controlling electron beam dynamics at optical wavelength scales and kHz frequencies.

Speaker: Melanie Murillo (Stanford University)

162 Programmable Photoinjector Laser Shaping for Structured X-ray Pulse Generation at LCLS-II

Next-generation x-ray free-electron lasers operating at MHz repetition rates require fast, flexible source-level actuators for adaptive and autonomous operation.* The photoinjector drive laser, mapping directly onto the electron beam's initial longitudinal phase space, represents a powerful and largely unexplored upstream handle. Here we present the first demonstration of programmable ultraviolet pulse shaping at an operating FEL photoinjector beyond the temporal flattop, implemented at LCLS-II using dispersion-controlled nonlinear synthesis** combined with spatial-light-modulator spectral shaping. Multi-peaked UV modulations are tracked through acceleration, compression, and undulator transport, with laser-imprinted current structure clearly resolved in the compressed beam. Variance-based x-ray temporal reconstruction from transverse deflecting cavity measurements reveals structured emission profiles broadly consistent with the programmed waveform***. These results establish programmable photoinjector shaping as a viable upstream actuator, with implications for on-demand pulse structuring, multiplexed operation, and adaptive control at high-repetition-rate light sources.

Speaker: Jack Hirschman (Stanford University, SLAC National Accelerator Laboratory)

163 Recalculation of brilliance of x-ray sources via Lambert function for SASE FELs

An update version of the brilliance plot in units of 0.01% relative bandwidth is proposed, including most recent planned and running 4th generation multi-bend storage ring light sources. It is then shown that, in order to match experimental and simulated data, the calculation has to take into consideration a non-uniform electron current profile in self-amplified spontaneous emission, and the scaling of the light pulse duration with the harmonic jump in seeded architectures. The calculation of brilliance for free-electron lasers is conducted through a newfound expression for the power saturation length, which highlights its dependence on the number of cooperating electrons via the so-called Lambert function, providing an exact value in place of well-known but semi-quantitative estimations reported in the literature.

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

164 Self-seeded x-ray mixer for attosecond pulse metrology

Attosecond pump-probe experiments are a long-sought tool to observe electronic processes on their natural timescales; however, slippage during FEL amplification complicates the ability to set short delays. At LCLS, we have recently demonstrated pump-probe measurements with sub-fs delays by using harmonic pulse pairs (w/2w). A soft x-ray delay line will enable general two-color operation, but challenges remain to quickly identify time-zero and estimate pulse lengths.

Here, we present an attosecond soft x-ray cross-correlator. We consider a self-seeding type geometry in which lasing from two colors (w1, w2) is synchronized using an x-ray delay line such that the microbunching develops heterodyne components (w1 + w2). Measuring the beat frequency provides a nearly background-free measure of the cross-correlation signal. Because the mixing is done via bunching, the cross-correlation signal is not a simple measure of the x-ray overlap, but depends also on the dynamic structure of microbunching.

Speaker: Federico Vismarra (SLAC National Accelerator Laboratory)

165 Single-Cycle Waveform Control of Undulator Radiation Using Plasma-Accelerated Beams

We present simulations demonstrating single-cycle waveform control of undulator radiation using an ultrashort electron beam produced by a plasma wakefield accelerator. By operating in a regime where the resonant radiation wavelength exceeds the effective bunch length of the compressed current spike, the emitted electric field closely follows the structure of the undulator magnetic field. This enables direct shaping of the radiation on a sub-cycle level through undulator design. These simulations demonstrate chirped pulse generation and cycle-to-cycle polarization flipping in compact undulator configurations. To accurately model the realistic magnetic fields of these short undulators, the unaveraged FEL code Puffin was extended to accept externally defined 3D magnetic field maps, here generated using the RADIA magnetostatics package.

Speaker: Jenny Morgan (SLAC National Accelerator Laboratory)

166 Single-Shot Echo-Type IXS for Self-Seeded and Cavity-Based XFELs

Momentum-resolved inelastic x-ray scattering (IXS) probes the dynamical structure factor S(Q,ω) in solids, liquids, biological systems, and quantum materials, providing access to phonons, magnons, and charge, orbital, and valence excitations on meV energy scales and nm$^{−1}$ momentum scales. Although IXS is a well-established technique at storage-ring sources, its broader impact is limited by low spectral flux.

Self-seeded and cavity-based x-ray free-electron lasers (XFELs) can deliver spectral flux at least four orders of magnitude higher than storage rings [1]. Echo-type IXS uses broadband dispersive optics with refocusing (“echo”) imaging to decouple energy resolution from instrument bandwidth, enabling scanless measurements of S(Q,ω) over a broad energy-loss range. This approach supports single-shot IXS measurements with meV or sub-meV resolution using broadband incident x-rays [2].

Coupling echo-type IXS with seeded XFELs could increase signal strength by up to 10$^6$ relative to current storage-ring IXS spectrometers, opening new opportunities for high-resolution spectroscopy. We present optical designs for echo-type IXS instruments with a 20 meV single-shot spectral window at 9.83 keV, tailored to CBXFELs, including the system under development at LCLS through a joint ANL-SLAC project [3] and the CBXFEL recently demonstrated at the European XFEL [4].

Speaker: Yuri Shvyd'ko (Argonne National Laboratory)

167 Slippage and Energy Chirp Effects in the Laser-Plasma-Accelerator-Driven FEL at the LBNL BELLA Center

Free-electron lasers (FELs) driven by electron beams generated by laser-plasma-accelerators (LPAs) offer an attractive avenue towards compact, high brightness and short wavelength coherent light sources. The BELLA Center LPA FEL facility at the Lawrence Berkeley National Laboratory delivers few fs-duration, 40-80 pC electron bunches of 100 MeV energy, 4% FWHM energy spread, at a 1Hz repetition rate to the 4-meter long VISA undulator, producing 420-nm radiation via self-amplified spontaneous emission (SASE). Although the 6D electron beam brightness is sufficient for amplification, the percent-level energy spread, and the nonlinear longitudinal phase space structure, degrades FEL lasing and is a key challenge for LPA-driven FELs. A magnetic chicane is used to lengthen the bunch, reduce the slice energy spread and generate an energy chirp. We investigate the impact of slippage and the effect of large energy chirps in the BELLA FEL amplification process using both numerical simulations and theoretical models, and compare their predictions to experimental measurements.

Speaker: Tehya Andersen (Lawrence Berkeley National Laboratory, University of California, Berkeley)

168 Spatially Resolved Spectral Diagnostics for Temporally Separated OAM Pulse Pairs with Different Topological Charges

There has been significant interest in generating orbital angular momentum (OAM) pulse pairs at free-electron lasers, either through direct OAM seeding schemes or via harmonic emission in helical undulators. In such schemes the pulses are temporally separated; while coherent superpositions of OAM modes can be identified from their transverse interference patterns, this approach fails when the pulses do not overlap in time. Here we present a spatially resolved spectral method that addresses this need, demonstrating that topology-dependent signatures are recovered by measuring the spectrum across selected regions of the transverse beam profile. We show that appropriate spatial filtering schemes can be identified for pulse pairs of different topological charge combinations.

Speaker: Jenny Morgan (SLAC National Accelerator Laboratory)

169 Status of development of an x-ray laser oscillator based on population inversion pumped by the LCLS-II-HE free electron laser

We report on the progress of the design and development of an X-ray Laser Oscillator (XLO) based on population inversion, to be driven by the LCLS-II-HE free-electron laser currently under construction at SLAC National Accelerator Laboratory. The XLO concept exploits the brightness, high repetition rate (up to 1 MHz), and tunability of LCLS-II-HE photon beam to generate population inversion in a transition metal gain medium via inner-shell photoionization, enabling stimulated emission at X-ray wavelengths. The resulting device, an FEL pumped X-ray laser, will generate fully coherent, transform limited photon pulses, opening new experimental areas of research in quantum optics, interferometry and more. We discuss the choice of gain medium, the design of the X-ray resonator cavity employing high-reflectivity Bragg crystals and intracavity strong focusing Kirkpatrick-Baez mirrors system, the expected gain and saturation dynamics, and the optimization of the interaction geometry for efficient energy transfer from the LCLS-II-HE pump pulses to the XLO gain medium. Numerical simulations of the population inversion dynamics and cavity round-trip gain are presented, together with an assessment of the required crystal reflectivity and mechanical stability tolerances. Potential scientific applications in ultrafast spectroscopy, quantum optics at X-ray wavelengths, and nuclear resonance scattering are outlined. The path toward a proof-of-concept experiment at LCLS-II-HE is described.

Speakers: Aliaksei Halavanau (SLAC National Accelerator Laboratory), Dr Claudio Pellegrini (SLAC National Accelerator Laboratory)

170 Stochastic Cooling Enhanced Steady-State Microbunching

In this paper, we propose to combine two promising research topics in accelerator physics, i.e., optical stochastic cooling (OSC) and steady-state microbunching (SSMB). Our study shows that such an OSC-SSMB storage ring with a circumference of 50 m and beam energy of several hundred MeVs using present technology can deliver kilowatt radiation at 100 nm wavelength. A more ambitious application of OSC in an SSMB ring can push the radiation wavelength to an even shorter wavelength, such as EUV and soft X-ray. Such a powerful compact light source could benefit fundamental science research and industry applications.

Speaker: Xiujie Deng (Tsinghua University)

171 Structured X-rays from structured light via ICS

We investigate generation of structured X-rays at ~9 keV energies via inverse Compton scattering (ICS) of 1030 nm structured light from ~20 MeV electrons. Structured light at X-ray photon energies enables entirely new ways of probing and controlling matter at the atomic and nanoscale, including accessing chiral, magnetic, and orbital-selective interactions that are invisible to conventional X-ray beams. However, generating such beams remains challenging: XFEL-based approaches require complex seeding or electron beam phase-space manipulation, while post-generation structuring with X-ray optics is limited by efficiency and damage thresholds.
ICS offers an alternative route in which the driving laser acts as an optical undulator, allowing the well-developed toolbox of optical beam shaping to be leveraged. This provides a high degree of flexibility, including the potential for rapid switching between spatial and polarization structures on a shot-to-shot basis. A key open question, however, is the extent to which transverse phase structure, such as orbital angular momentum (OAM), is preserved in the typically incoherent ICS process.
We present initial experimental results and calculations of OAM X-ray generation using a classical electrodynamics description of ICS, and identify observable signatures that can be used to diagnose the presence of OAM in the emitted radiation. These results aim to guide future experimental efforts toward structured X-ray generation in compact sources.

Speaker: Nicholas Matlis (Arizona State University)

172 The development of time-resolved nonlinear XUV/soft X-ray spectroscopy in SACLA BL1

Nonlinear spectroscopy in the XUV and soft-X-ray regions offers a unique route to combine the symmetry sensitivity of optical nonlinear processes with the element and orbital selectivity of core-level resonances. We present an experimental platform for time-resolved XUV-to-soft-X-ray nonlinear spectroscopy at FEL sources, using second-harmonic generation as a representative nonlinear probe. The platform integrates tight beam focusing, optical-pump/XUV-probe synchronization, polarization-resolved detection, and shot-to-shot normalization for weak nonlinear signals. We discuss key experimental requirements, including photon density, focus stability, pulse-to-pulse fluctuations, detector sensitivity, sample damage, and discrimination of sample-generated second-harmonic photons from source harmonic background. Our current XUV-region implementation provides a practical basis for extending the methodology toward soft-X-ray FEL operation and applications to ultrafast interfacial, semiconductor, ferroelectric, and magnetic dynamics.

Speaker: XINGYU SU (The University of Tokyo)

173 Ultrafast Resonant Single-Particle Imaging with Attosecond XFEL Pulses

Ultrafast imaging enables the investigation of nanoscale dynamics in single particles using individual X-ray pulses. Accurate interpretation of such images requires understanding the strong interaction between the X-ray pulse and the sample, particularly when the electronic structure effects is of interest.

We investigate ultrafast X-ray--induced dynamics in sodium iodide (NaI) nanoparticles using resonant single-particle imaging with ultrashort X-ray free-electron laser pulses at the SwissFEL Maloja endstation. Photon-energy scans across the I 3d absorption edge were recorded to probe the pulse-length-dependent scattering response. Single-particle diffraction patterns were reconstructed by phase retrieval to extract scattering intensity and particle size.

The measured scattering signals exhibit pronounced resonant features, with attosecond pulses providing a response closest to the ground-state electronic structure. For few-femtosecond pulses, the diffraction response is significantly modified, indicating the increasing influence of transient electronic excitations. Electronic structure simulations are employed to model X-ray--atom interactions and transient population dynamics, yielding time-dependent scattering cross sections. This combined approach provides insight into transient electronic configurations and defines the conditions under which ground-state resonant imaging can be achieved with attosecond XFEL pulses.

Speaker: Hankai Zhang (École Polytechnique Fédérale de Lausanne, Paul Scherrer Institute)

174 Undulator Tapering for Phase Synchronization Recovery in Energy-Compressed LWFA-Driven FELs

Laser wakefield accelerators (LWFAs) have emerged as promising compact drivers for free-electron lasers (FELs), owing to their ability to produce femtosecond electron beams with high peak current over centimeter-scale acceleration distances. However, their relatively large energy spread remains a key challenge for achieving high-gain FEL operation. While magnetic compression can effectively reduce the slice energy spread to a level suitable for FEL amplification, it simultaneously introduces a significant longitudinal energy chirp. This chirp leads to a continuous detuning of the FEL resonance along the planar undulator, resulting in phase slippage between the electron beam and the radiation field, reduced bunching efficiency, and degraded output power and spectral quality.
We investigate the use of a longitudinally tapered undulator to compensate for the chirp-induced resonance mismatch in a self-amplified spontaneous emission (SASE) FEL driven by an energy-compressed LWFA beam. Through three-dimensional, unaveraged simulations, we demonstrate that an optimized taper profile can effectively restore electron–radiation phase synchronization, leading to substantial improvements in both saturation power and spectral characteristics compared to the untapered case. Our results indicate that undulator tapering provides a viable and effective approach to mitigating chirp-induced performance degradation, thereby enhancing the feasibility of compact plasma-based FEL systems.

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

Thursday 27 August

Electron Sources (Invited)

175 LCLS-II injector operational performance and challenges

LCLS-II has transitioned to users operation since 2023 and recently ramped the beam rate up to 93 kHz. This paper will present LCLS-II injector operational progresses and challenges.

Speaker: Feng Zhou (SLAC National Accelerator Laboratory)

176 Sub-GV/m gradient in Short-Pulse X-Band Photogun: From First Demonstration to Recent Results and Future Developments

Normal-conducting accelerating structures capable of supporting GV/m-scale electric fields offer a compelling pathway toward compact accelerator systems. Achieving such fields in photocathode RF guns is equally essential for the production of high-brightness electron bunches. Our group has recently demonstrated ~0.4 GV/m peak electric fields on the photocathode surface of an X-band (11.7 GHz) photoemission gun (Xgun) powered by ultra-short (~9 ns) RF pulses. In this work, we present the latest experimental progress, including beam characterization through slice-emittance measurements and longitudinal phase-space diagnostics. We also discuss a novel hybrid accelerating-structure design being developed for the Xgun beamline upgrade, together with ongoing and future directions toward the high-energy beam generation at the level of 100 MeV.

Speaker: Gongxiaohui Chen (Argonne National Laboratory)

Electron Sources (Contributed)

177 An alternative method to generate CW electron beams for Hard X-ray FEL

The generation of high-quality continuous-wave (CW) electron beams with parameters suitable for hard X-ray free-electron lasers (FELs) has so far not been demonstrated experimentally. Traditionally, photoinjectors operating at very high accelerating gradients are considered necessary to meet the stringent beam-quality requirements of hard X-ray FELs. While high-gradient guns have been successfully operated in pulsed mode, extending this approach to CW operation is technically challenging, and can lead to excessive field emission (dark current) and reduced photocathode lifetime.

In this talk, I will present our results demonstrating that an alternative approach using a superconducting radiofrequency (SRF) gun with a modest accelerating gradient of approximately 15 MV/m, combined with a ballistic compression beamline, can produce CW electron beams with the required parameters. I will present a detailed description of the accelerator system and report measurements of projected (0.2 mm-mrad @ 100 pC) and slice transverse emittances (0.15 mm-mrad @ 100 pC), as well as the bunch’s longitudinal phase space. The experimental results will also be compared with start-to-end simulations.

Speaker: Nikhil Bachhawat (Stony Brook University)

178 Upstream Photocathode Laser Tailoring for Optimized Longitudinal Phase Space in High-Brightness FELs

Generating high-brightness electron beams with optimized longitudinal phase space is essential for driving next-generation free-electron lasers (FELs). However, accelerator-induced distortions often degrade beam quality and lasing performance. Here, we present a novel approach to counteract these effects by manipulating the photoemission laser’s temporal distribution. By introducing a controlled longitudinal ramp to a flattop profile via spatiotemporal shaping of the UV photocathode laser, we achieved deterministic phase space control at the LCLS-II superconducting injector. We demonstrate that this tailored optical asymmetry acts as a pre-compensation mechanism for downstream resistive wall wakefields at ~80 pC. This effectively linearizes the phase space and suppresses nonlinear energy chirp, which is the critical requirement for optimal FEL performance. These findings establish upstream spatiotemporal laser shaping as a powerful tool for directly optimizing FEL beam brightness at the source.
Zhou et al., PRAB 24, 073401 (2021)
**Zhang et al., arXiv:2601.03580 (2026)
Lemons et al., Ultrafast Sci. 5, 0112 (2025)

Speaker: HAO ZHANG (University of California, Los Angeles)

Electron Diagnostics, Timing, Synchronization, and Controls (Invited)

179 Transverse Electron Beam Size Diagnostics Based on Synchrotron Radiation

We report the observation of transverse intensity fluctuations (spikes) in synchrotron radiation from a single undulator cell at the European XFEL after monochromatization [1]. Autocorrelation analysis of the recorded events confirms that these fluctuations originate from the partial transverse coherence of the radiation. By calculating the second-order autocorrelation function, we determined the averaged transverse slice size of the electron beam along the SASE1 undulator [2]. The measurements were performed with 5.49 keV x-ray photons using a commissioning silicon monochromator with (111) Bragg reflection and an x-ray imager. This technique enables undulator cell-by-cell diagnostics of the transverse electron beam size at free-electron laser facilities using existing hardware. We demonstrate the capability to differentiate between the averaged transverse slice size and the projected beam size. This diagnostics along SASE beamlines is particularly relevant for advanced XFEL operation schemes, including attosecond pulse generation with chirp-dispersion methods, self-seeding, and two-color lasing.

[1] DOI: https://doi.org/10.1103/z89g-f7j6

[2] DOI: https://doi.org/10.1103/31gl-qyk7

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

180 Absolute characterization of sub-femtosecond electron bunches in SwissFEL using a Bunch-Compressor-Monitor

Monitoring the electron pulse duration is crucial for setting up and stabilizing the lasing in a linac-driven Free-Electron-Laser (FEL). Bunch-Compressor-Monitors (BCMs) allow for a non-invasive - albeit uncalibrated – monitoring of the electron pulse duration at the exit of a magnetic chicane by detecting and fully integrating the radiation energy emitted by the electron beam around the spectral threshold of the temporal coherence. We will present results of the experimental characterization of low-charge and sub-femtosecond electron bunches generated in SwissFEL during a SASE-lasing with a single-spike mode prevalence in the frequency domain (*). The electron pulse duration measurements were performed by applying a formal processing method (**) of the signal readouts of a BCM which is equipped with two independent and simultaneously illuminated detectors covering two partially overlapping wavelength bands. In a CW superconducting linac-driven FEL, where the non-invasiveness is a mandatory feature for the beam diagnostics, the implementation of the “BCM two-detector method” discloses the path to the use of a BCM as an absolute monitor of the electron pulse duration.

Speaker: Gian Luca Orlandi (Paul Scherrer Institute)

Electron Diagnostics, Timing, Synchronization, and Controls (Contributed)

181 Evaluation of the synchronized operation between the infrared free-electron laser and fiber-laser by measuring the balanced cross-correlation via sum-frequency generation

To demonstrate the stabilization of the carrier-to-envelope phase (CEP) of the free-electron laser oscillator via injection seeding, we are developing a fiber-laser-based, passively CEP-stabilized difference-frequency-generation mid-infrared source. The external injection seeding into the FEL requires a synchronized laser pulse with the electron bunch driving the FEL. Thus, we introduce a phase-locking system with the external radio-frequency signal into the fiber-laser system. To evaluate the performance of the synchronization, we performed a sum-frequency generation experiment using a fiber laser and an FEL at the infrared free-electron laser at the Institute of Advanced Energy, Kyoto University. The experimental technique used here is based on that previously demonstrated at the Fritz-Haber-Institut (R. Kiessling et al., Phys. Rev. Acc. and Beams 21, 080702 (2018)). In this report, we present our experimental results and discuss them. This research is carried out under the MEXT Q-LEAP program (contract number　JPMXS0118070271).

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

182 Design of a Compact Photoconductive Antenna for Direct Femtosecond-Micron Measurement of 3D Space-Charge Fields

Accurate characterization of 3D space-charge fields in ultrashort electron beams is crucial for next-generation accelerators. The complex, transient 3D space-charge and CSR fields that ultimately shape and limit high-brightness beams have yet to be directly measured, leaving critical dynamics unobserved. A compact diagnostic for the direct measurement of the field itself thus remains a key challenge.
We present a compact Photoconductive Antenna (PCA) diagnostic for direct field measurement at the LCLS. The PCA operates by using a soft X-ray (SXR) pulse, collimated by a laser-drilled aperture, to generate localized free charge carriers in a diamond sensor. The transverse space-charge field of a passing electron bunch then drives these carriers, inducing a measurable current in a nearby antenna via the Ramo-Shockley theorem.
A full 3D spatiotemporal field map is reconstructed by mechanically scanning the device for transverse (x-y) resolution and varying the X-ray arrival time using an SXR delay line for femtosecond-scale longitudinal (z) resolution. This presentation will cover the PCA's operational principle and design for LCLS. By providing a direct, femtosecond-micron measurement of the beam's vector field, rather than its scalar charge density, the PCA offers a unique capability for optimizing beam performance in advanced accelerators.

Speaker: Sean OTool (Stanford University)

Friday 28 August

Novel Accelerator and FEL Concepts (Invited)

183 Greater than 1000-fold Gain in a Free-Electron Laser Driven by a Laser-Plasma Accelerator with High Reliability

Compact free-electron lasers (FELs) based on plasma-based accelerators have been envisioned for many years. While recent milestone experiments have demonstrated their feasibility, further progress is required to establish laser-plasma-accelerator-driven FELs as reliable light sources. Demonstrating both full FEL saturation and operational reliability comparable to conventional FEL facilities is essential. Here, we report progress on both fronts. FEL gain exceeding 1000 at a wavelength of 420 nm in the self-amplified spontaneous emission regime has been measured. Lower and upper bounds for the characteristic exponential gain length were determined to be 16.7–22.5 cm. In addition, the reliability of the laser-plasma-accelerator-driven FEL, defined here as the percentage of shots exhibiting FEL gain, reached an unprecedented level exceeding 90% over the course of an hour while operating at a 1 Hz repetition rate.

Speaker: Samuel Barber (Lawrence Berkeley National Laboratory)

184 Numerical evaluation of FEL instability in the Plasma Discharge Undulator

The plasma discharge undulator (PDU) has recently been proposed as a compact, tunable, fully plasma-based undulation device. In the PDU, a high-current discharge within a capillary generates an azimuthal magnetic field providing strong linear focusing, while a controlled periodic modulation of the discharge axis acts as a geometric driving term, producing well-defined undulator oscillations at a wavelength distinct from the natural betatron motion. Proper beam injection conditions suppress collective betatron oscillations, significantly reducing the intrinsic undulator strength spread typical of conventional plasma undulators, while matched beam transport is ensured by the strong active plasma lens focusing. While the PDU is still in the experimental development phase, the feasibility of free-electron laser operation has been assessed through one-dimensional analytical estimates, including scaling relations for gain length, Pierce parameter, and the onset of longitudinal microbunching. These results are here supported by self-consistent numerical simulations of the beam–radiation interaction within the PDU, which show the emergence of FEL instability and the development of seeded longitudinal microbunching analogous to that observed in conventional magnetic undulators, establishing the PDU as a promising pathway toward miniaturized, tunable, plasma-based light sources.

Speaker: Andrea Frazzitta (Paul Scherrer Institute)

185 Development and Optimization of LWFA Electron Beams for XUV FEL Generation

Free-electron lasers (FELs) provide ultrashort, coherent, and wavelength-tunable radiation for a wide range of scientific applications. Here, we present the demonstration of a laser-wakefield-acceleration (LWFA)-driven FEL operating at a central wavelength of 40 nm. By using a compact 0.8 J laser system and carefully optimizing the plasma density, electron injection, laser wavefront, and gas-jet stability, we generated high-quality monoenergetic electron beams with excellent shot-to-shot reproducibility. Start-to-end simulations, including the acceleration, beam transport, and FEL interaction processes, show good agreement with the experimental results. The observed FEL radiation exhibits an exponential gain of approximately 20, marking an important step toward compact, tunable XUV and x-ray FEL sources.

Speaker: YANJUN GU (The University of Osaka)

186 Saturated Soft X-ray FEL based on a hollow plasma channel

With the development of plasma-based accelerators as a novel acceleration technology, beam phase space manipulation techniques based on plasma wakefields have also gradually begun to be applied, demonstrating their unique advantages in flexibility and device miniaturization compared to traditional beam manipulation techniques. Here, we demonstrate the saturation of a soft X-ray FEL driven by an electron beam that has undergone manipulations within a hollow plasma channel. We show that the plasma wakefield effectively modulates the beam energy while preserving the slice emittance and stability required for high-gain amplification. The manipulated beam drove robust XFEL at a central wavelength of 9.8 nm. The system generated stable, saturated pulses achieving saturation with a maximum single-pulse energy of 91 μJ and exhibited significant spectral tunability. Furthermore, by steering the electron beam within the plasma channel, we observed evidence of ultrafast pulse generation with durations estimated ~5 fs. This work provides a potential route for driving ultrafast XFELs.

Speaker: Fei Li (Institute of High Energy Physics)

Advanced FEL Modes (Invited)

187 Demonstration of Mode-Locked Frequency Comb for an X-Ray Free-Electron Laser

X-ray free-electron lasers (FELs) are powerful photon sources offering a wide wavelength range, subfemtosecond pulse duration, and high brightness. Most X-ray FELs are based on self-amplified spontaneous emission (SASE). SASE-FEL radiation has excellent transverse but only limited longitudinal coherence, with power and spectral profiles consisting of multiple randomly distributed spikes.
In this contribution, we present the first experimental demonstration of mode-locked SASE, which generates periodic trains of phase-locked sub-femtosecond pulses, thus providing an X-ray analog of the optical frequency comb. Our approach combines the mode-coupled SASE scheme, where magnetic chicanes between the undulator modules of the FEL increase the coherence of the output radiation, and an external optical laser that restricts the FEL amplification to periodic and short regions of the electron bunch. The work relies on evidence in the frequency and time domains for photons and electrons, respectively, and will benefit investigations of ultrafast dynamics as well as coherent spectroscopy and enable new types of experiments requiring phase-correlated X-ray pulses.

Speaker: Wenxiang Hu (Paul Scherrer Institute, ETH Zurich)

188 Recent progress in attosecond X-ray free-electron lasers: direct observation for hard X-rays, high repetition rates, and spectrotemporal shaping

Attosecond science with XFELs is a rapidly growing field. Nearly half of soft X-ray LCLS user experiments now use attosecond pulses. In this talk, I will present recent advances in the generation and characterization of attosecond pulses at the LCLS and LCLS-II, focusing on three main topics:
1. First direct observation of attosecond pulses in the hard X-ray domain.
2. Attosecond lasing in a high repetition rate, CW superconducting accelerator.
3. Spectrotemporal shaping techniques for attosecond XFEL pulses.
On the first point, I will show results from two different experiments demonstrating 9 keV pulses with sub-400-as duration, with direct measurements enabled by recent advances in broadband hard X-ray optics. I will then show the first attosecond soft X-ray lasing results from the LCLS-II at 16 kHz with pulse duration measurements using angular streaking in a circular time of flight spectrometer array. Extending on that, I’ll present two different techniques for shaping the spectrotemporal structure of attosecond pulses, highlighting experimental evidence from the LCLS-II of coherent attosecond pulse pairs and trains with tunable color separations, time delays, and relative phases. Altogether, these results open the doors to (1) attosecond scattering experiments and electronic-damage-free measurements, (2) the full realization of attosecond pump attosecond probe spectroscopy, and (3) attosecond coherent control in the X-ray domain.

Speaker: River Robles (Stanford University)

Advanced FEL Modes (Contributed)

189 Generation and Diagnostics of OAM Beams at the Shanghai Soft X-ray Free-Electron Laser Facility

A comprehensive framework for the generation and diagnostics of X-ray and THz OAM beams has been successfully established at the Shanghai Soft X-ray Free-Electron Laser (SXFEL) facility. This work effectively bridges the gap between advanced orbital angular momentum (OAM) physics and the stringent beam quality requirements of a high-performance XFEL facility. The ability to produce soft X-ray vortex radiation with GW-level peak power and THz OAM pulses reaching 385 μJ represents a significant technical milestone. However, the research is further distinguished by its remarkable flexibility in manipulating topological charge numbers and the development of a robust multi-pinhole interference diagnostic system for precise beam characterization. These advancements, combined with the innovative use of wavefront-tilt reflectors, offer a practical and versatile path toward high-brightness structured light sources for frontier scientific applications.

Speaker: Yin Kang (Shanghai Advanced Research Institute, Chinese Academy of Sciences)

190 Tunable Phase-Locked Hard-X-Ray Pulse Pairs from a Cavity-Based Free-Electron Laser

We propose a novel and simple scheme for generating tunable, phase-locked hard-X-ray pulse pairs in cavity-based free electron lasers. A short electron bunch first amplifies one temporal slice of a long coherent cavity field; after a magnetic delay, the same bunch overlaps a fresh seed slice in a second undulator section, producing a second pulse locked to the same optical reference. The relative phase between the two pulses is controlled by modest detuning of the second undulator. Theoretical analysis and simulations indicate independently tunable pulse delay and multi-radian relative phase control with preserved spectral quality. This mode would enable phase-stable hard-X-ray pump–probe measurements, coherent wave-packet control, nonlinear X-ray spectroscopy, and interferometric studies of ultrafast structural and electronic dynamics.

Speaker: Jingyi Tang (Stanford University)