Free Electron Laser Research Articles

Greater than 1000-fold gain in a free-electron laser driven by a laser-plasma accelerator with high reliability

Greater than 1000-fold gain in a free-electron laser driven by a laser-plasma accelerator with high reliability

Design and start-to-end beam dynamics simulation of the first super-radiant THz free-electron laser source in Thailand

Design and start-to-end beam dynamics simulation of the first super-radiant THz free-electron laser source in Thailand

Relaxation pathways in x-ray free electron laser heated iron

Relaxation pathways in x-ray free electron laser heated iron

An injector testbed based on a direct current gun and an interchangeable very high frequency gun for superconducting continuous-wave free-electron lasers.

The continuous-wave free-electron laser (CW-FEL), based on superconducting radiofrequency (SRF) technology with an electron bunch repetition rate of up to MHz levels, is one of the most advanced light sources, providing exceptionally high average and peak-brightness FEL pulses. Among the new CW-FEL facilities worldwide, the recently proposed Dalian Advanced Light Source (DALS) occupies a unique position as an extreme ultraviolet (EUV) facility primarily designed for chemical physics research. Since the beam emittance requirement for DALS is not as stringent as that for X-ray CW-FEL facilities, a direct current (DC) gun is considered as the primary electron source, with a very high frequency (VHF) gun also planned. To demonstrate key technologies and characterize the electron beam performance, a superconducting CW injector testbed, named the Electron Source Test Facility (ESTF), has been designed and is currently under construction. The testbed is uniquely designed to accommodate both guns with minimal switching effort, where the rest of the beamline layout remains unchanged except for the swapped guns. The two-gun switching scheme for the testbed is shown to be a feasible and cost-effective approach. Furthermore, the injector performance with both guns has been evaluated through a start-to-end simulation based on the DALS configuration, including the production of electron beam in the ESTF injector, the following beam acceleration and compression in a superconducting linear accelerator, and finally the beam lasing performance in the undulator section. The evaluation confirms that the DC gun is a promising electron source for CW-FEL facilities, especially for EUV applications, even though all currently constructed CWfacilities have employed the VHF gun. This paper provides a comprehensive description of the injector design and the corresponding performance evaluation.

Beam dynamics in a storage ring with a free-electron laser

In this paper, we study the dynamics of the electron beam in a diffraction limited storage ring with a high-gain free-electron laser. We present in detail the design, including lattice, dynamic aperture, intrabeam scattering, and coherent synchrotron radiation. The simulation of the electron storage ring combined with the free-electron laser is carried out until the electron beam reaches its equilibrium. We find that the initial exponential radiation power is reduced approximately to linear with respect to the undulator length due to the large increase of the energy spread from the free-electron laser. Published by the American Physical Society 2025

Efficient Generation of Transversely and Longitudinally Truncated Chirped Gaussian Laser Pulses for Application in High-Brightness Photoinjectors

The optimization of photoinjector brightness is crucial for achieving the highest performance at X-ray free-electron lasers. To this end, photocathode laser pulse shaping has been identified as a key technology for enhancing photon flux and lasing efficiency at short wavelengths. Supported by beam dynamics simulations, we identify transversely and longitudinally truncated Gaussian electron bunches as a beneficial bunch shape in terms of the projected emittance and 5D brightness. The realization of such pulses from chirped Gaussian pulses is studied for 514 nm and 257 nm wavelengths by inserting an amplitude mask in the symmetry plane of the pulse stretcher to achieve longitudinal shaping and an aperture for transverse beam shaping. Using this scheme, transversely and longitudinally truncated Gaussian pulses can be generated and later used for the production of up to 3 nC electron bunches in the photoinjector. The 3D pulse shape at a wavelength of 514 nm is characterized via imaging spectroscopy, and second-harmonic generation frequency-resolved optical gating (SHG FROG) measurements are also performed to analyze the shaping scheme’s efficacy. Furthermore, this pulse-shaping scheme is transferred to a UV stretcher, allowing for direct application of the shaped pulses to cesium telluride photocathodes.

Start-to-end simulation for a compact terahertz free electron laser with a beam chopper system

Start-to-end simulation for a compact terahertz free electron laser with a beam chopper system

First steps on plasma beam prebuncher for free electron lasers through more suitable relativistic reference-frame-based particle-in-cell tools

First steps on plasma beam prebuncher for free electron lasers through more suitable relativistic reference-frame-based particle-in-cell tools

Analysis strategy for ultrafast X-ray photon correlation spectroscopy

Abstract We explain an analysis strategy for ultrafast X-ray photon correlation spectroscopy, a technique enabled by X-ray free electron lasers to probe nano- and atomic-scale dynamics in complex systems on nanosecond timescales or faster. Central to the technique is the methodology for extracting contrast in coherent X-ray diffraction patterns, known as speckle patterns. Guided by simulations, we analyze common challenges and examine errors encountered in contrast extraction. A method for accurately determining contrast amid shot noise across a wide range of count rates is presented. Graphical abstract

Boosting the efficiency of narrowband THz radiators via three-dimensional emission collection

The demand for high-power, narrowband, and tunable THz radiation sources is steadily increasing. Smith-Purcell radiation offers a compact method for resonantly generating narrowband THz sources with notable advantages over other approaches, though the power output still requires improvement. This work introduces a novel 3D emission-collection concept that enhances the efficiency of THz radiators, achieving an enhancement of over an order of magnitude in coherent radiation power compared to the 2D approach. Systematic simulations reveal that the single-layer helically rotated structure, optimized through inverse design, emits stronger THz radiation than other reported circumferentially closed gratings, achieving millijoule radiated energy. Helix gratings exhibit significant potential for developing accelerator-based high-power and tunable THz radiators and are easy to manufacture. Some methods concerning optimizing beam-grating properties are employed to improve the radiated THz intensity. These results provide new insights into developing high-power THz radiators, which may provide an alternative THz source for free-electron laser facilities. Published by the American Physical Society 2025

Single-shot measurements of shaped XUV FEL pulses with frequency-resolved optical gating

The ability to control the amplitude and phase of extreme ultraviolet (XUV) and X-ray free-electron laser (FEL) pulses can allow for the extension of optical techniques, such as multidimensional spectroscopy or coherent control, to higher photon energies, for probing and controlling core electronic transitions. However, this requires the ability to make single-shot, and complete, electric field measurements of potentially complex FEL pulses, in order to develop, and verify, pulse shaping strategies. Here, we present direct, single-shot measurements of XUV pulses generated under special operating configurations for producing specific pulse shapes from a laser-seeded XUV FEL. To do this, we built upon our past work using transient grating (TG) cross correlation frequency-resolved optical gating (FROG), where an optical reference pulse is diffracted from an XUV TG produced by a pair of interfering FEL pulses. The resulting nonlinear signal versus frequency and delay, i.e., the FROG trace, contains the electric field of the FEL pulse. The FEL pulse electric field is reconstructed from the FROG trace using a phase retrieval algorithm. Here we confirmed three different pulse shaping strategies for generating chirped, double and multiple FEL pulses by tuning the seed laser and FEL parameters and measuring the resulting shaped FEL pulses with TG XFROG. This work paves the way for generating on-demand pulse shapes with a seeded FEL by improving the characterization of FEL pulses, for transform-limited to more complex shapes.

Femtosecond laser-induced optical breakdown and cavitation dynamics in water imaged with an X-ray free-electron laser

Femtosecond laser-induced optical breakdown and cavitation dynamics in water imaged with an X-ray free-electron laser

RF design of a 1.3 GHz 3-cell superconducting cavity for high-current beam acceleration

Superconducting radio-frequency (SRF) cavities are widely used in particle accelerator facilities. The SRF-based energy recovery linac (ERL) equipped with special SRF cavities offers feasibility for application in high-power free electron lasers (FELs). This study aims to design and optimize a 3-cell cavity that serves as the accelerating structure for a 10 mA class injector of high-brightness ERL-FEL. We apply the middle-cell shape of the mature TESLA cavity, and the end-groups are optimized for high-current beam operation. The cavity is designed with two fundamental power coupler (FPC) ports, which enable high-power input by two FPCs. In addition, the beam pipe is enlarged to damp potentially strong higher-order modes (HOMs) induced by the high current beams. A multi-objective genetic algorithm is utilized to optimize the cavity geometry. The damping of HOMs, multipacting, and mechanical issues are also investigated to verify the rationality of the cavity design.

Generation of ultrafast user defined multi-micro-pulse burst structure from a femtosecond mode-locked fiber oscillator by using semiconductor optical amplifier for photoinjector.

We demonstrate and present detailed technical insights into the generation of a user-defined multi-micro-pulse burst structure from an in-house developed femtosecond mode-locked fiber oscillator using a standard semiconductor optical amplifier (SOA), which acts as an excellent candidate for an ultrafast fiber-based pulse-picker. An in-house developed 130MHz mode-locked fiber oscillator followed by a 500m long optical fiber as the pulse stretcher along with polarization control units was used to achieve a highly stable multi-micro-pulse structure at variable repetition rates by using and adapting a fiber coupled SOA. The timing system with a synchronous trigger setup has been explained in detail to achieve the multi-micro-pulse structure from the fiber laser system, which is being used to generate multi-micro-electron bunches in a photoinjector based free electron laser facility at IUAC named as Delhi Light Source. The technical insights from the detailed experimentation and results help bring out various advantages and challenges in the use of an SOA as an ultrafast pulse-picker for generating a multi-micro-pulse structure, which will be useful for widespread applications of the modern compact femtosecond lasers and in the field of photoinjector based systems for higher electron currents.

First experiments with ultrashort, circularly polarized soft x-ray pulses at FLASH2.

Time-resolved absorption spectroscopy and magnetic circular dichroism with circularly polarized soft x-rays (XAS and XMCD) are powerful tools to probe electronic and magnetic dynamics in magnetic materials element- and site-selectively. By employing these methods, groundbreaking results have been obtained, for instance, for magnetic alloys, which helped to fundamentally advance the field of ultrafast magnetization dynamics. At the free-electron laser facility FLASH, key capabilities for ultrafast XAS and XMCD experiments have recently improved. In an upgrade, an APPLE-III helical afterburner undulator was installed at FLASH2 in September 2023. This installation allows for the generation of circularly polarized soft x-ray pulses with a duration of a few tens of femtoseconds covering the -edges of the important 3d transition metal elements with pulse energies of several J. Here, we present first experimental results with such ultrashort x-ray pulses from the FL23 beamline employing XMCD at the -edges of the 3d metals, Co, Fe, and Ni. We obtain significant dichroic difference signals indicating a degree of circular polarization close to 100%. With the pulse-length preserving monochromator at beamline FL23 and an improved pump-laser setup, FLASH can offer important and efficient experimental instrumentation for ultrafast demagnetization studies and other investigations of ultrafast spin dynamics in 3d transition metals, multilayers, and alloys.

Redox-State-Dependent Structural Changes within a Prokaryotic 6-4 Photolyase.

Photolyases repair UV damage to DNA by using absorbed blue light. Within the photolyase/cryptochrome superfamily (PCSf), a major subgroup consists of prokaryotic (6-4) photolyases. These enzymes rely on flavin adenine dinucleotide (FAD) as a catalytic cofactor, besides an ancillary antenna chromophore, and a [4Fe-4S] cluster with yet unknown function. For the prokaryotic 6-4 photolyase of Caulobacter crescentus, we investigated structural changes associated with its different redox states by damage-free crystallography using X-ray free-electron lasers. EPR and optical spectroscopy confirmed redox-dependent structural transitions, including the formation of an oxidized [4Fe-4S]3+ cluster with the dynamic cleavage of a single iron-sulfur bond. Photoreduction to the catalytic FADH- state alters the flavin binding site at the proximal aromatic pair Y390/F394 that is part of the electron transport pathway. Upon oxidation, the observable structural transitions of the protein matrix around the [4Fe-4S] cluster may affect DNA binding and are consistent with the much-debated role of the iron-sulfur cluster in DNA-binding proteins for quenching electron holes.

Novel Radiation Facilities Based on Plasma Acceleration: The Future of Free Electron Lasers

Exploiting acceleration gradients that are up to three orders of magnitude higher than those achievable using conventional radiofrequency-based devices, plasma-based devices promise a revolution in particle acceleration, enabling particles to reach high energies over much shorter distances than existing accelerators [...]

MHz X-ray photon correlation spectroscopy using an acoustic levitator at the European XFEL.

The structural and dynamical properties of soft-matter systems play an important role in crystallization and nucleation theory. Despite their significance, thedynamical properties are still poorly understood because of experimental constraints and the requirement of performing measurements with high spatial and temporal resolution. Here, we demonstrate MHz X-ray photon correlation spectroscopy (XPCS) using a contactless sample holder at the European X-ray Free-Electron Laser. A millimetre-sized liquid sample droplet was levitated in air via acoustic waves with the solvent slowly evaporating. A colloidal suspension of silica nanospheres was used to track the structural evolutions using small-angle X-ray scattering, and the dynamical information was captured by time-resolved MHz XPCS as a function of evaporation time. This study outlines a new path towards the investigation of metastable structure and dynamics using X-ray speckle techniques, for instance, XPCS, X-ray speckle visibility spectroscopy and X-ray cross-correlation analysis.

Direct Observation of the ππ* to nπ* Transition in 2-Thiouracil via Time-Resolved NEXAFS Spectroscopy.

The photophysics of nucleobases has been the subject of both theoretical and experimental studies over the past decades due to the challenges posed by resolving the steps of their radiationless relaxation dynamics, which cannot be described in the framework of the Born-Oppenheimer approximation (BOA). In this context, the ultrafast dynamics of 2-thiouracil has been investigated with a time-resolved NEXAFS study at the Free Electron Laser FLASH. Near Edge X-ray Absorption Fine Structure spectroscopy (NEXAFS) can be used to observe electronic transitions in ultrafast molecular relaxation. We performed time-resolved UV-pump/X-ray probe absorption measurements at the sulfur 2s (L1) and 2p (L2/3) edges. We are able to identify absorption features corresponding to the S2 (ππ*) and S1 (nπ*) electronic states. We observe a delay of 102 ± 11 fs in the population of the nπ* state with respect to the initial optical excitation and interpret the delay as the time scale for the S2 → S1 internal conversion. We furthermore identify oscillations in the absorption signal that match a similar observation in our previous X-ray photoelectron spectroscopy study on the same molecule.

Experimental and Computational Insights into the Structural Dynamics of the Fc Fragment of IgG1 Subtype from Biosimilar VEGF‐Trap

The constant fragment (Fc) of the immunoglobulin G1 (IgG1) subtype is increasingly recognized as a crucial scaffold in the development of advanced therapeutics due to its enhanced specificity, efficacy, and extended half‐life. A prime example is VEGF‐Trap (Aflibercept), a recombinant fusion protein that merges the Fc region of the IgG1 subtype with the binding domains of vascular endothelial growth factor receptors (VEGFR)‐1 and VEGFR‐2. The Fc region's role in N‐glycosylation is particularly important, as it significantly influences protein stability. Herein, the first near‐physiological temperature structures of the N‐glycan‐bound Fc fragment of IgG1 subtype from a biosimilar VEGF‐Trap are presented, determined using the SPring‐8 Angstrom Compact free electron LAser (SACLA) and the Turkish Light Source (Turkish DeLight). Comparative analysis with cryogenic structures, including existing data, reveals alternate conformations within the glycan‐binding pocket. Furthermore, molecular dynamics simulations indicate the presence of a high degree of structural plasticity, explaining how the protein adapts its structure through conformational changes. The observed structural fluctuations/conformational changes demonstrate the effect of N‐glycans on protein stability. These findings offer new insights into the molecular basis of Fc‐mediated functions and provide valuable information for the design of next‐generation therapeutics.