Free Electron Laser Research Articles

Compensation for the source drift of a free-electron laser beamline by adjusting the fixed-focus constant of the grating monochromator.

For large-scale free-electron laser facilities based on linear accelerators, the laser saturation point is not frequently fixed in the undulator, which will cause a longitudinal source point drift of the beamline. The longitudinal source point drift will cause an instability in the performance of the beamline, especially affecting the energy-resolving power of variable-line-spacing grating monochromators for soft X-ray beamlines. A method of adjusting the fixed-focus constant to compensate for this longitudinal source point drift is introduced in this work. Simulation results indicate that this method can effectively recover the energy-resolving power of the grating monochromator.

Characterization results of the first full-scale readout HYLITE chip for STARLIGHT

Shanghai HIgh repetition rate XFEL aNd Extreme light facility (SHINE) is a free electron laser facility with a high pulse repetition rate of up to 1 MHz. STARLIGHT (SemiconducTor Array detectoR with Large dynamIc ranGe and cHarge inTegrating readout) is a pixel array detector with a high frame rate (≥ 10 kHz) and a large dynamic range (up to 104 photons of 12.4 keV), which is being developed for SHINE. The readout ASIC utilized by STARLIGHT in the front-end module is HYLITE (High dYmamic range free electron Laser Imaging deTEctor). HYLITE200F with 200 μm × 200 μm pixel size and 64 × 64 pixels is the first full-scale chip of the HYLITE series.To facilitate the improvement of the detector, the performances of the single chip and a detector prototype module with 2 × 2 ASICs are carefully evaluated, especially in terms of gain, noise, and dynamic range. The test results show that the gains of the HYLITE200F chip are uniform with a single-photon resolution. Meanwhile, the chip maintains a good linearity at 104 photons of 12 keV. After bump-bonding with the sensor, the performance of HYLITE200F still fulfills the requirements.

Theoretical studies on M-shell relocalization of K-hole Mg ions produced by x-ray free-electron laser.

In hot dense plasmas, orbital delocalization and relocalization considerably influence the ionization balance, equationof state and radiative properties of matter. A self-consistent plasma screening potential is applied in atomic structure calculations for solid-density Mg plasmas, generated by an x-ray free electron laser (XFEL). As the charge state increases, the M-shell orbitals of K-hole Mg ions gradually relocalize, beginning with Mg^{6+} at solid density. The level population distributions are determined by solving a time-dependent rate equationusing fine-structure level accounting approximations. The predicted K_{α} emission spectra align well with XFEL experiments only when orbital relocalization is carefully accounted for, revealing that the 3d orbital of Mg^{7+} is delocalized, consistent with density functional theory calculations.

Infrared Ion Spectroscopy of Gaseous [Cu(2,2'-Bipyridine)3]2+: Investigation of Jahn-Teller Elongation Versus Compression.

Symmetry breaking is ubiquitous in chemical transformations and affects various physicochemical properties of materials and molecules; Jahn-Teller (JT) distortion of hexa-coordinated transition-metal-ligand complexes falls within this paradigm. An uneven occupancy of degenerate 3d-orbitals forces the complex to adopt an axially elongated or compressed geometry, lowering the symmetry of the system and lifting the degeneracy. Coordination complexes of Cu2+ are known to exhibit axial elongation, while compression is far less common, although this may be due to the lack of rigorous experimental verification. Here, we present the gas-phase vibrational spectrum of the archetypal [Cu(2,2'-bipyridine)3]2+ ionic complex, obtained by infrared multiple-photon dissociation (IRMPD) spectroscopy using the widely tunable IR free-electron laser FELIX. Predicted vibrational spectra at the density functional level of theory are nearly─but not entirely─identical for the two JT-distorted geometries. We compare experimental and theoretical spectra and address the question of an axially elongated or compressed geometry of the complex, or a mixture thereof, in the gaseous ion population.

Opportunities for gas-phase science at short-wavelength free-electron lasers with undulator-based polarization control

Free-electron lasers (FELs) are the world's most brilliant light sources with rapidly evolving technological capabilities in terms of ultrabright and ultrashort pulses over a large range of photon energies. Their revolutionary and innovative developments have opened new fields of science regarding nonlinear light-matter interaction, the investigation of ultrafast processes from specific observer sites, and approaches to imaging matter with atomic resolution. A core aspect of FEL science is the study of isolated and prototypical systems in the gas phase with the possibility of addressing well-defined electronic transitions or particular atomic sites in molecules. Notably for polarization-controlled short-wavelength FELs, the gas phase offers new avenues for investigations of nonlinear and ultrafast phenomena in spin-orientated systems, for decoding the function of the chiral building blocks of life as well as steering reactions and particle emission dynamics in otherwise inaccessible ways. This roadmap comprises descriptions of technological capabilities of facilities worldwide, innovative diagnostics and instrumentation, as well as recent scientific highlights, novel methodology, and mathematical modeling. The experimental and theoretical landscape of using polarization controllable FELs for dichroic light-matter interaction in the gas phase will be discussed and comprehensively outlined to stimulate and strengthen global collaborative efforts of all disciplines. Published by the American Physical Society 2025

Parallel Bayesian optimization of free-electron lasers based on laser wakefield accelerators

Abstract In this paper, the parallel Bayesian optimization algorithm is investigated for the simulation optimization of compact free electron lasers (FELs) driven by laser wakefield accelerators (LWFAs). The radiation energy serves as the objective function, which can be optimized to 25 μJ after several tens of iterations, initiating from random input samplings in soft x-ray regimes. The analysis of the FEL gain process has revealed that the localized properties of the electron beam (e beam) can be concurrently optimized, ensuring a high efficiency of energy extraction from the e beam. Our proposed scheme not only presents a viable design for compact FELs driven by LWFAs in the soft x-ray regime utilizing the start-to-end model, but also highlights the great potential of parallel Bayesian optimization algorithms for tackling the challenges associated with costly and complex black-box optimization problems.

Pulsed laser deposition assisted epitaxial growth of cesium telluride photocathodes for high brightness electron sources

The development of high-brightness electron sources is critical to state-of-the-art electron accelerator applications like X-ray free electron laser (XFEL) and ultra-fast electron microscopy. Cesium telluride is chosen as the electron source material for multiple cutting-edge XFEL facilities worldwide. This manuscript presents the first demonstration of the growth of highly crystalized and epitaxial cesium telluride thin films on 4H-SiC and graphene/4H-SiC substrates with ultrasmooth film surfaces. The ordering of the film was characterized by in situ reflection high energy electron diffraction and multiple X-ray diagnostics. The results of the quantum efficiency performance for epitaxial cesium telluride photocathodes are also reported.

Improved robustness of sequentially deposited potassium cesium antimonide photocathodes achieved by increasing the potassium content towards theoretical stoichiometry

Alkali antimonide semiconductor photocathodes are promising candidates for high-brightness electron sources for advanced accelerators, including free-electron lasers (FEL), due to their high quantum efficiency (QE), low emittance, and high temporal resolution. Two challenges with these photocathodes are (1) the lack of a universal deposition recipe to achieve crystal stoichiometries and (2) their high susceptibility to vacuum contamination, which restricts their operation pressure to ultrahigh vacuums and leads to a short lifetime and low extraction charge. To resolve these issues, it is essential to understand the elemental compositions of deposited photocathodes and correlate them to robustness. Here, we report depth profiles for potassium cesium antimonide photocathodes, which were investigated using synchrotron radiation x-ray photoelectron spectroscopy, and the robustness of those photocathodes. We prepared two types of photocathodes with different potassium contents via sequential thermal evaporation. Depth profiles revealed that the photocathodes with a potassium deficit had excess cesium at the surface, while the ratio of potassium and cesium to antimony decreased rapidly within the film. In contrast, the photocathodes with sufficient potassium had close to the theoretical stoichiometry of K2CsSb at the surface and maintained that stoichiometry for over half the entire film thickness. Both photocathode types had a similar maximum QE at 532 nm; however, exposure to oxygen revealed that the photocathode with a crystalline stoichiometry of K2CsSb maintained QE at one order of magnitude higher pressure compared to its potassium-deficit counterpart. These results highlight the importance of synthesizing potassium cesium antimonide photocathodes with sufficient potassium to achieve the theoretical crystalline stoichiometry for both high QE and improved robustness.

Technique for forming substrates for grazing incidence X-ray mirrors with cylindrical surface profiles

The paper demonstrates the possibility of forming flat substrates for grazing incidence X-ray mirrors with an accuracy of RMS ∼0.7nm, and also implements a technique for manufacturing grazing incidence X-ray optical elements with cylindrical surface profiles, using ion beam etching. The technique assumes the linear movement of the workpiece behind a slit diaphragm with a speed that varies depending on the coordinate. The result of the work was the creation of a pair of X-ray mirror substrates for a Kirkpatrick-Baez collimating system, having surface shape deviations from the ideal plane of HM1=1.05µm and HM2=1.13µm, and from the calculated parabolic cylinders of RMSM1=3.0nm and RMSM2=2.9nm. The described technique makes it possible to produce substrates for mirrors in the form of elliptical and parabolic cylinders used in commercial diffractometers, synchrotrons, and free electron lasers for collimation and focusing of X-ray radiation, as well as in the telescopes of space observatories.

First-principles study of structural, elastic, electronic, transport properties, and dielectric breakdown of Cs2Te photocathode

The pursuit to operate photocathodes at high accelerating gradients to increase brightness of electron beams is gaining interests within the accelerator community, particularly for applications such as free electron lasers (FEL) and compact accelerators. Cesium telluride (Cs2Te) is a widely used photocathode material and it is presumed to offer resilience to higher gradients because of its wider band gap compared to other semiconductors. Despite its advantages, crucial material properties of Cs2Te remain largely unknown both in theory and experiments. In this study, we employ first-principles calculations to provide detailed structural, elastic, electronic and transport properties of Cs2Te. It is found that Cs2Te has an intrinsic mobility of 20 cm2/Vs for electrons and 2.0 cm2/Vs for holes at room temperature. The low mobility is primarily limited by the strong polar optical phonon scattering. Cs2Te also exhibits ultralow lattice thermal conductivity of 0.2 W/(m*K) at room temperature. Based on the energy gain/loss balance under external field and electron–phonon scattering, we predict that Cs2Te has a dielectric breakdown field in the range from ~ 60 to ~ 132 MV/m at room temperature dependent on the doping level of Cs2Te. Our results are crucial to advance the understanding of applicability of Cs2Te photocathodes for high-gradient operation.

Observation of the Smallest Three‐Dimensional Neutral Boron Cluster

Abstract Despite major progress in the investigation of boron cluster anions, direct experimental study of neutral boron clusters remains a significant challenge because of the difficulty in size selection. Here we report a size‐specific study of the neutral B9 cluster using threshold photoionization with a tunable vacuum ultraviolet free electron laser. The ionization potential of B9 is measured to be 8.45±0.02 eV and it is found to have a heptagonal bipyramid D7h structure, quite different from the planar molecular wheel of the B9‐ anionic cluster. Chemical bonding analyses reveal superior stability of the bipyramidal structure arising from delocalized σ and π bonding interactions within the B7 ring and between the B7 ring and the capping atoms. Photoionization of B9 breaks the single‐electron B‐B bond of the capping atoms, which undergo off‐axis distortion to enhance interactions with the B7 ring in the singlet ground state of B9+. The single‐electron B‐B bond of the capping atoms appears to be crucial in stabilizing the D7h structure of B9. This work opens avenues for direct size‐dependent experimental studies of a large variety of neutral boron clusters to explore the stepwise development of network structures.

Observation of the Smallest Three-Dimensional Neutral Boron Cluster.

Despite major progress in the investigation of boron cluster anions, direct experimental study of neutral boron clusters remains a significant challenge because of the difficulty in size selection. Here we report a size-specific study of the neutral B9 cluster using threshold photoionization with a tunable vacuum ultraviolet free electron laser. The ionization potential of B9 is measured to be 8.45±0.02 eV and it is found to have a heptagonal bipyramid D7h structure, quite different from the planar molecular wheel of the B9 - anionic cluster. Chemical bonding analyses reveal superior stability of the bipyramidal structure arising from delocalized σ and π bonding interactions within the B7 ring and between the B7 ring and the capping atoms. Photoionization of B9 breaks the single-electron B-B bond of the capping atoms, which undergo off-axis distortion to enhance interactions with the B7 ring in the singlet ground state of B9 +. The single-electron B-B bond of the capping atoms appears to be crucial in stabilizing the D7h structure of B9. This work opens avenues for direct size-dependent experimental studies of a large variety of neutral boron clusters to explore the stepwise development of network structures.

Fast plasma production by intense femtosecond extreme ultraviolet and x-ray pulses

We develop a semiempirical model to describe the creation of an electron plasma in matter at extreme conditions, as excited by high-intensity, extreme ultraviolet (EUV) or x-ray radiation pulses currently available at free-electron laser (FEL) facilities. In typical FEL experiments, EUV and x-ray pulses are used to investigate highly excited states of matter produced by optical pump pulses. The pump pulses, promoting electrons from delocalized valence band states, excite the system often through multiphoton absorption processes and/or shock wave propagation. Instead, the use of FEL radiation as a pump allows triggering electron transitions from bound, core states to nearly free levels above the Fermi energy, promptly generating a highly excited state, toward the warm dense matter regime. We model such an excitation process starting from the optical properties of the analyzed materials and first-principle considerations, with the aim of motivating further studies in this scheme. We apply our model to predict the differences in the plasma creation process in insulators and metals, at varying exciting pulse wavelength and fluence, and we interpret the results in terms of the different nature of the investigated systems. Furthermore, we explore the effect of the finite core hole lifetime on the plasma formation and relaxation dynamics. We show the model is suited for the interpretation of real experimental data obtained from a prototypical insulating crystal (LiF) in a EUV FEL pump–optical transmission probe experiment. Published by the American Physical Society 2025

Revealing the reaction path of UVC bond rupture in cyclic disulfides with ultrafast x-ray scattering.

Disulfide bonds are ubiquitous molecular motifs that influence the tertiary structure and biological functions of many proteins. Yet, it is well known that the disulfide bond is photolabile when exposed to ultraviolet C (UVC) radiation. The deep-UV-induced S─S bond fragmentation kinetics on very fast timescales are especially pivotal to fully understand the photostability and photodamage repair mechanisms in proteins. In 1,2-dithiane, the smallest saturated cyclic molecule that mimics biologically active species with S─S bonds, we investigate the photochemistry upon 200-nm excitation by femtosecond time-resolved x-ray scattering in the gas phase using an x-ray free electron laser. In the femtosecond time domain, we find a very fast reaction that generates molecular fragments with one and two sulfur atoms. On picosecond and nanosecond timescales, a complex network of reactions unfolds that, ultimately, completes the sulfur dissociation from the parent molecule.

Transmission spectroscopy of CF4 molecules in intense x-ray fields

The nonlinear interaction of x rays with matter is at the heart of understanding and controlling ultrafast molecular dynamics from an atom-specific viewpoint, providing new scientific and analytical opportunities to explore the structure and dynamics of small quantum systems. At increasingly high x-ray intensity, the sensitivity of ultrashort x-ray pulses to specific electronic states and emerging short-lived transient intermediates is of particular relevance for our understanding of fundamental multiphoton absorption processes. In this work, intense x-ray free-electron laser (XFEL) pulses at the European XFEL are combined with a gas cell and grating spectrometer for a high-intensity transmission spectroscopy study of multiphoton-induced ultrafast molecular fragmentation dynamics in CF4. This approach unlocks the direct intrapulse observation of transient fragments, including neutral atoms, by their characteristic absorption lines in the transmitted broadband x-ray spectrum. The dynamics with and without initially producing fluorine K-shell holes are studied by tuning the central photon energy. The absorption spectra are measured at different FEL intensities to observe nonlinear effects. Transient isolated fluorine atoms and ions are spectroscopically recorded within the ultrashort pulse duration of a few tens of femtoseconds. An isosbestic point that signifies the correlated transition between intact neutral CF4 molecules and charged atomic fragments is observed near the fluorine K edge. The dissociation dynamics and the multiphoton absorption-induced dynamics encoded in the spectra are theoretically interpreted. Overall, this study demonstrates the potential of high-intensity x-ray transmission spectroscopy to study ultrafast molecular dynamics with sensitivity to specific intermediate species and their electronic structure. Published by the American Physical Society 2025

A System Control and Waveform Acquisition Framework for the Kicker System at SHINE

A lumped kicker magnet system is being developed as part of the electron beam switchyard of the Shanghai High Repetition Rate X-Ray Free-Electron Laser (XFEL) and Extreme Light (SHINE) facility at Shanghai Advanced Research Institute (SARI), Chinese Academy of Sciences (CAS). SHINE is a linac-based facility with an energy of 8 GeV and a repetition rate of 1 MHz for multiple users. The kicker system is a critical element of the deflection system and determines the stability performance of the beamlines. To ensure autonomous and reliable operation in a harsh environment, a dedicated control system—consisting of a custom-designed programmable logic controller (PLC) and waveform acquisition system—has been developed based on the Experimental Physics and Industrial Control System (EPICS). The PLCsystem enables status acquisition, control implantations, and alarm handling, allowing for automated operation. The waveform acquisition system provides an intuitive demonstration and enables the real-time diagnosis of kicker waveforms. The results demonstrate a half quasi-sinuous waveform with a peak current of 11.5 A and a full width at half maximum of 218 ns at a repetition rate of 10 kHz. The detailed design, overall concept, achieved performance, and initial test results of two modular controls are provided for further demonstration.

Free electron laser in the magnetically dominated regime: simulations with the ONEDFEL code

Using the ONEDFEL code we perform free electron laser simulations in the astrophysically important guide-field dominated regime. For wigglers’ (Alfvén waves) wavelengths of tens of kilometres and beam Lorentz factor ${\sim }10^3$ , the resulting coherently emitted waves are in the centimetre range. Our simulations show a growth of the wave intensity over fourteen orders of magnitude, over the astrophysically relevant scale of approximately a few kilometres. The signal grows from noise (unseeded). The resulting spectrum shows fine spectral substructures, reminiscent of those observed in fast radio bursts.

Assessment of the general clinical condition and functional properties of the eyes of rabbits after THz irradiation.

THz radiation is increasingly used for diagnostics in medicine. As technology utilizing THz radiation continues to develop rapidly, it is becoming increasingly important to consider its biological effects and establish safe exposure standards and parameters. The paper presents data on the clinical status and functional properties of the anterior and posterior structures of the eyes of rabbits after THz irradiation at the frequency of 2.3 THz. Terahertz radiation was generated at Novosibirsk Free Electron Laser (NovoFEL) at "Siberian Synchrotron and Terahertz Radiation Centre" (Budker Institute of Nuclear Physics of the Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia). The exposure durations used were 15 and 30 minutes. Intensity ranges were I1=0.012 mW/cm2, I2=0.018 mW/cm2, and I3=0.024 mW/cm2. The study investigated the effects of various time and power irradiation protocols on the California rabbit's eyes and after a period of one month, but no significant clinical or functional alterations were observed in response to the established intensity protocols. However, the study identified statistically significant changes in corneal hydration and endothelial cell density over time, particularly under protocols with 15- and 30-minute exposures. A negative correlation was found between endothelial cell density and corneal thickness (r=-0.36, p=0.042), suggesting that a reduction in the endothelial cell pool may be associated with increased corneal thickness. These changes were subclinical and did not lead to clinically significant pathological changes in the cornea. There were no signs of ASOCT (anterior segment-optical coherence tomography) hyperreflectivity. THz radiation with parameters listed above of 2.3 THz and an intensities of 0.012-0.024 mW/cm2 for 30 minutes has been shown to be conditionally safe for the structures of the rabbit eye. However, the detected subclinical corneal changes require further study to determine safe exposure limits.

Multiphoton emission of x-rays from cooperative resonant nuclei

The advent of x-ray free electron lasers (XFELs) has opened the door to exploring the x-ray-nuclei interaction in the multiexcitation regime. Here, we show the dependence of emission properties of x-ray photons from an ensemble of identical resonant nuclei on the number of excitations in the system, employing the theoretical methods we have introduced. Under the multiexcitation condition, the detection of x-ray quanta is emission-order selective. Our work presents an efficient approach to investigating the multiphoton emission of x-rays from cooperative resonant nuclei within and beyond weak-excitation regimes, offering valuable insights into nonlinear x-ray optics and many-body nuclear physics. Published by the American Physical Society 2025

An Upgrade of Radio Frequency Reference Generation and Distribution Modules for FLASH2020+

Free-Electron Laser in Hamburg (FLASH), first launched in 2005, was the first free-electron laser that provided ultrashort radiation pulses in extreme ultraviolet and soft X-ray spectral range. In 2017, it was decided to improve the existing FLASH facility within the FLASH2020+ project, which led to upgrading the existing linac with variable gap tunable undulators in the FLASH1 line and refurbishing two cryomodules to achieve a beam energy increase to 1.35 GeV. It was also a perfect opportunity to completely redesign and rebuild the radio frequency (RF) phase reference generation and distribution system. This paper presents the design and parameters of new, custom-made phase reference signal generation and distribution modules, successfully installed in FLASH. These are the main oscillator, the RF distribution module, and the frequency conversion modules. The new instrumentation presents a significant improvement in terms of RF reference signal parameters, state-of-the-art phase noise performance (an improvement in the total jitter of the 1.3 GHz RF signal from 55.9 fs to 10.7 fs in the integration range from 10 Hz to 1 MHz), module compactness (size reduction from three fully occupied rack cabinets to four 19″ modules only), and serviceability. The presented Main Oscillator system design is foreseen for easy modifications, making it suitable for applications in other accelerator facilities or hardware platforms.