Free-electron Laser Beamlines Research Articles

Spatiotemporal response of concave VLS grating to ultra-short X-ray pulses.

In soft X-ray free-electron laser (FEL) beamlines, variable-line-spacing (VLS) gratings are often used as dispersive components of monochromators and spectrometers due to their combined dispersion and focusing properties. X-ray FEL pulses passing through the VLS grating can result in not only transverse focusing but also spatiotemporal coupling effects, such as pulse front tilt, pulse front rotation, and pulse stretching. In this paper, we present a theoretical study of the spatiotemporal response of concave VLS gratings to ultra-short X-ray pulses. The theoretical analysis indicates that the tilt angle of the non-zero diffraction orders varies with the propagation distance, and disappears at the focus, where the focal lengths and pulse stretching differ for different diffraction orders. The model demonstrates the pulse duration after the concave VLS grating is the convolution of the initial pulse duration and the stretching term induced by dispersion, while the beam size at the focus in x dimension is the convolution of the geometric scaling beam size and the dispersion term. This work provides a mathematical explanation for the spatiotemporal response of concave VLS grating to ultra-short X-ray pulses and offers valuable insights into the design of FEL grating monochromators, spectrometers, pulse compressors, and pulse stretchers.

Characterization in the extreme ultraviolet (XUV) domain of microchannel plate based device using synchrotron radiation

Innovative, low-loss, and compact optical systems are essential to meet the experimental requirements of emerging novel radiation sources. The microchannel plate (MCP), a metamaterial-based optical device, shows promising potential for shaping, condensing, and focusing soft x-ray radiation at synchrotron radiation (SR) facilities. This study highlights the impact of MCP optical devices on SR beam condensing capability and their sensitivity to the degree of coherence by investigating the profile of transmitted beams through single and double MCP optical devices. Transmitted diffraction patterns of soft x-ray SR radiation change with energy and radiation modes. At 92 eV, the double MCP-based device affects the beam divergence and degree of coherence more than the single MCP. Moreover, the double MCP device shows potential as a condensing optics at shorter wavelengths, i.e., 480 eV. Experiments were performed at the available end-station of the Circular Polarization beamline at the Elettra synchrotron facility in Trieste, using a high-vacuum chamber with a hexapod system, providing the precise movement necessary to align these diffractive optics. The findings contribute to the development of innovative optical systems for SR and free-electron laser beamlines, paving the way for advanced experiments in spectroscopy, microscopy, and imaging in a wide energy range.

Real-time machine-learning-driven control system of a deformable mirror for achieving aberration-free X-ray wavefronts.

A neural-network machine learning model is developed to control a bimorph adaptive mirror to achieve and preserve aberration-free coherent X-ray wavefronts at synchrotron radiation and free electron laser beamlines. The controller is trained on a mirror actuator response directly measured at a beamline with a real-time single-shot wavefront sensor, which uses a coded mask and wavelet-transform analysis. The system has been successfully tested on a bimorph deformable mirror at the 28-ID IDEA beamline of the Advanced Photon Source at Argonne National Laboratory. It achieved a response time of a few seconds and maintained desired wavefront shapes (e.g., a spherical wavefront) with sub-wavelength accuracy at 20 keV of X-ray energy. This result is significantly better than what can be obtained using a linear model of the mirror's response. The developed system has not been tailored to a specific mirror and can be applied, in principle, to different kinds of bending mechanisms and actuators.

Status and future of the soft X-ray free-electron laser beamline at the SHINE

The Shanghai High repetition rate XFEL and Extreme light facility (SHINE) is under construction and aims at generating X-rays between 0.4 and 25 keV with three FEL beamlines at repetition rates of up to 1 MHz. The soft X-ray FEL beamline, FEL-II, will be ready for commissioning in 2025. It is designed to cover the photon energy from 0.4 to 3 keV, in which the baselines of the FEL operation modes are self-amplified spontaneous emission (SASE), self-seeding, echo-enabled harmonic generation (EEHG), and polarization control. Therefore, a high repetition-rate external seed laser, large period length modulator, soft X-ray monochromator, planar undulator, and elliptically polarized undulator have been adopted in the FEL-II beamline. Several potentials such as an ultra-short pulse mode and a multi-color mode are also foreseeable without significant equipment changes in the follow-up operation. A dual-period undulator design is suggested for the echo-enabled harmonic generation (EEHG) commissioning, and it has great potential to break through the unreachable energy of the fully coherent X-ray in the future. The FEL-II beamline will deliver SASE radiation and fully coherent radiation in all the wavelengths of interest.

Ultrashort X-ray Free Electron Laser Pulse Manipulation by Optical Matrix

Free electron laser (FEL) is capable of producing ultra-short X-ray pulses. The estimation of X-ray pulse propagation is the key process of X-ray FEL beamline design. By using the Kostenbauder matrix approach, the evolution of an ultra-short pulse in a beamline system can be calculated. Therefore, it is of significant importance to investigate the Kostenbauder matrices of different kinds of X-ray optics. In this work, we derive a unified 6 × 6 optical matrix to describe various kinds of X-ray optical elements, including varied-line-spacing (VLS) toroidal grating, VLS spherical grating, VLS cylindrical grating, VLS plane grating, toroidal grating, spherical grating, cylindrical grating, plane grating, toroidal mirror, spherical mirror, cylindrical mirror, and plane mirror. These optics are usually adopted in soft X-ray regime. We apply this method to describe the transverse focusing, pulse front tilt, and pulse stretching after an X-ray pulse going through a VLS plane grating monochromator (VLS-PGM). We also use this approach to simulate a grating compressor which can be used to compress chirped soft X-ray pulse. This work is helpful in the design and optimization of X-ray beamline systems.

Frequency and spatially chirped free-electron laser pulses

We have produced hard x-ray free-electron laser (FEL) pulses, which are chirped both in photon energy and in spatial position. The experiments have been carried out at the hard x-ray beamline Aramis at SwissFEL, located at the Paul Scherrer Institute in Switzerland. The FEL beamline was operated without any external focusing and with a tilted, energy-chirped electron bunch, whose properties are then transferred to the photon beam. The resulting FEL pulses were used for a single-shot absorption x-ray spectroscopy experiment at the Alvra endstation. But, as will be outlined, applications of this type of tailored FEL pulse go beyond such experiments, allowing for x-ray pulse compression and control of the transverse coherence within the pulse.Received 13 May 2022Accepted 22 February 2023DOI:https://doi.org/10.1103/PhysRevResearch.5.L022009Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Physical SystemsFree-electron lasersAccelerators & Beams

Data-driven modeling and control of an X-ray bimorph adaptive mirror.

Adaptive X-ray mirrors are being adopted on high-coherent-flux synchrotron and X-ray free-electron laser beamlines where dynamic phase control and aberration compensation are necessary to preserve wavefront quality from source to sample, yet challenging to achieve. Additional difficulties arise from the inability to continuously probe the wavefront in this context, which demands methods of control that require little to no feedback. In this work, a data-driven approach to the control of adaptive X-ray optics with piezo-bimorph actuators is demonstrated. This approach approximates the non-linear system dynamics with a discrete-time model using random mirror shapes and interferometric measurements as training data. For mirrors of this type, prior states and voltage inputs affect the shape-change trajectory, and therefore must be included in the model. Without the need for assumed physical models of the mirror's behavior, the generality of the neural network structure accommodates drift, creep and hysteresis, and enables a control algorithm that achieves shape control and stability below 2 nm RMS. Using a prototype mirror and ex situ metrology, it is shown that the accuracy of our trained model enables open-loop shape control across a diverse set of states and that the control algorithm achieves shape error magnitudes that fall within diffraction-limited performance.

Simulation studies of superconducting afterburner operation for the European XFEL

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

Cation-π Interactions between a Noble Metal and a Polyfunctional Aromatic Ligand: Ag+ (benzylamine).

The structure of an isolated Ag+(benzylamine) complex is investigated by infrared multiple photon dissociation (IRMPD) spectroscopy complemented with quantum chemical calculations of candidate geometries and their vibrational spectra, aiming to ascertain the role of competing cation‐N and cation‐π interactions potentially offered by the polyfunctional ligand. The IRMPD spectrum has been recorded in the 800–1800 cm−1 fingerprint range using the IR free electron laser beamline coupled with an FT‐ICR mass spectrometer at the Centre Laser Infrarouge d'Orsay (CLIO). The resulting IRMPD pattern points toward a chelate coordination (N‐Ag+‐π) involving both the amino nitrogen atom and the aromatic π‐system of the phenyl ring. The gas‐phase reactivity of Ag+(benzylamine) with a neutral molecular ligand (L) possessing either an amino/aza functionality or an aryl group confirms N‐ and π‐binding affinity and suggests an augmented silver coordination in the product adduct ion Ag+(benzylamine)(L) .

Synchronized beamline at FLASH2 based on high-order harmonic generation for two-color dynamics studies.

We present the design, integration, and operation of the novel vacuum ultraviolet (VUV) beamline installed at the free-electron laser (FEL) FLASH. The VUV source is based on high-order harmonic generation (HHG) in gas and is driven by an optical laser system synchronized with the timing structure of the FEL. Ultrashort pulses in the spectral range from 10 to 40eV are coupled with the FEL in the beamline FL26, which features a reaction microscope (REMI) permanent endstation for time-resolved studies of ultrafast dynamics in atomic and molecular targets. The connection of the high-pressure gas HHG source to the ultra-high vacuum FEL beamline requires a compact and reliable system, able to encounter the challenging vacuum requirements and coupling conditions. First commissioning results show the successful operation of the beamline, reaching a VUV focused beam size of about 20 µm at the REMI endstation. Proof-of-principle photo-electron momentum measurements in argon indicate the source capabilities for future two-color pump-probe experiments.

Demonstration of a compact x-ray free-electron laser using the optical klystron effect

We demonstrate the operation of a compact x-ray free-electron laser (FEL) using the optical klystron mechanism. This effect speeds up the FEL process using the dispersion of magnetic chicanes placed between the undulator modules of the FEL beamline. The demonstration was performed at the soft x-ray beamline of SwissFEL, called Athos, which is, as far as we are aware, the only x-ray FEL to date with magnetic chicanes between every two undulator modules. In our measurements, we show that, compared to standard operation without chicanes, the required undulator length to achieve FEL saturation is reduced between 15% and 30% for radiation wavelengths between 1 and 2 nm. Fully exploiting the optical klystron effect represents an important step toward more compact FEL designs, rendering this key technology more affordable and hence accessible to a larger science community.

Design and analysis of stress-free clamping of mirrors used in free-electron laser beamlines

The predecessor of China Optics was China Optics and Applied Optics Digest, which was founded in 1985. At that time, it was the only retrieval journal in the field of optics in China. At the end of 2008, China Optics and Applied Optics Digest was renamed

The SwissFEL soft X-ray free-electron laser beamline: Athos.

The SwissFEL soft X-ray free-electron laser (FEL) beamline Athos will be ready for user operation in 2021. Its design includes a novel layout of alternating magnetic chicanes and short undulator segments. Together with the APPLE X architecture of undulators, the Athos branch can be operated in different modes producing FEL beams with unique characteristics ranging from attosecond pulse length to high-power modes. Further space has been reserved for upgrades including modulators and an external seeding laser for better timing control. All of these schemes rely on state-of-the-art technologies described in this overview. The optical transport line distributing the FEL beam to the experimental stations was designed with the whole range of beam parameters in mind. Currently two experimental stations, one for condensed matter and quantum materials research and a second one for atomic, molecular and optical physics, chemical sciences and ultrafast single-particle imaging, are being laid out such that they can profit from the unique soft X-ray pulses produced in the Athos branch in an optimal way.

Scientific instruments for soft X-ray photon-in/photon-out spectroscopy on the PAL-XFEL.

An overview is given of the soft X-ray photon-in/photon-out instruments on the free-electron laser (FEL) beamline at the Pohang Accelerator Laboratory, and selected commissioning results are presented. The FEL beamline provides a photon energy of 270 to 1200 eV, with an energy bandwidth of 0.44%, an energy of 200 µJ per pulse and a pulse width of <50 fs (full width at half-maximum). The estimated total time resolution between optical laser and X-ray pulses is <100 fs. Instruments for X-ray absorption spectroscopy (XAS) and resonant inelastic X-ray scattering (RIXS) have been set up. X-ray magnetic circular dichroism spectra for a Co/Pt multilayer film and RIXS spectra for α-Fe2O3(100) have been obtained and the performance of the spectrometer has been evaluated.

Laser systems for time-resolved experiments at the Pohang Accelerator Laboratory X-ray Free-Electron Laser beamlines.

Optical laser systems for ultrafast X-ray sciences have been established at the Pohang Accelerator Laboratory X-ray Free-Electron Laser (PAL-XFEL) beamlines. Three Ti:sapphire regenerative amplifier systems are synchronized to the XFEL with femtosecond precision, and the low temporal jitter of the PAL-XFEL results in an experimental time resolution below 150 fs (full width at half-maximum). A fundamental wave and its harmonics are currently provided for all beamlines, and tunable sources from ultraviolet to near-infrared are available for one beamline. The position stability of the optical laser extracted from the intensity-based center of mass at the sample position is less than 3% (r.m.s.) of the spot size.

Arrival timing diagnostics at a soft X-ray free-electron laser beamline of SACLA BL1.

An arrival timing monitor for the soft X-ray free-electron laser (XFEL) beamline of SACLA BL1 has been developed. A small portion of the soft XFEL pulse is branched using the wavefront-splitting method. The branched FEL pulse is one-dimensionally focused onto a GaAs wafer to induce a transient reflectivity change. The beam branching method enables the simultaneous operation of the arrival timing diagnostics and experiments. The temporal resolution evaluated from the imaging system is ∼22 fs in full width at half-maximum, which is sufficient considering the temporal durations of the soft XFEL and the optical laser pulses.

Dynamic adaptive X-ray optics. Part I. Time-resolved optical metrology investigation of the bending behaviour of piezoelectric bimorph deformable X-ray mirrors.

Piezoelectric bimorph deformable mirrors (`bimorphs') are routinely used on many synchrotron and free-electron laser beamlines to provide active variation in the size and shape of the X-ray beam. However, the time-domain potential of such optics has never been fully exploited. For the first time, the fast dynamic bending response of bimorphs is investigated here using Fizeau interferometry. Automated scripts for acquisition and analysis were developed to collect Fizeau data at a rate of 0.1 Hz to record dynamic changes in the optical surface as voltages were applied to the electrodes of the piezoelectric actuators. It is demonstrated that residual drift in the tangential radius of curvature of a bimorph can be significantly reduced using enhanced opto-mechanical holders and a fast programmable high-voltage power supply. Further improvements are achieved by applying small opposing voltages to compensate for piezoelectric creep. The present study shows that bimorphs can truly be used as high-speed adaptive optics for the X-ray domain, even without closed-loop feedback correction. This opens the possibility for relatively simple real-time tuning of the profile of X-ray bimorphs. Part II of this study [Alcock, Nistea, Signorato, Owen, Axford, Sutter, Foster & Sawhney (2019), J. Synchrotron Rad. 26, 45-51] builds upon these results and demonstrates how bimorphs can rapidly provide customisable sizes and shapes of synchrotron X-ray beams, specifically tailored to suit the experimental samples being investigated.

Towards chemically neutral carbon cleaning processes: plasma cleaning of Ni, Rh and Al reflective optical coatings and thin Al filters for free-electron lasers and synchrotron beamline applications

The choice of a reflective optical coating or filter material has to be adapted to the intended field of application. This is mainly determined by the required photon energy range or by the required reflection angle. Among various materials, nickel and rhodium are common materials used as reflective coatings for (soft) X-ray mirrors. Similarly, aluminium is one of the most commonly used materials for extreme ultraviolet and soft X-ray transmission filters. However, both of these types of optics are subject to carbon contamination, which can be increasingly problematic for the operation of the high-performance free-electron laser and synchrotron beamlines. As an attempt to remove this type of contamination, an inductively coupled plasma source has been used in conjunction with N2/O2/H2 and N2/H2 feedstock gas plasmas. Results from the chemical surface analysis of the above materials before and after plasma treatment using X-ray photoelectron spectroscopy are reported. It is concluded that a favorable combination of an N2/H2 plasma feedstock gas mixture leads to the best chemical surface preservation of Ni, Rh and Al while removing the carbon contamination. However, this feedstock gas mixture does not remove C contamination as rapidly as, for example, an N2/O2/H2 plasma which induces the surface formation of NiO and NiOOH in Ni and RhOOH in Rh foils. As an applied case, the successful carbon removal from ultrathin Al filters previously used at the FERMI FEL1 using an N2/H2 plasma is demonstrated.

Proposal of a femtosecond megahertz repetition-rate electron diffraction instrument based on the Chinese Academy of Engineering Physics terahertz free electron laser beamline.

We report the design and optimization of a femtosecond electron diffraction instrument with megahertz repetition-rate. The proposed instrument is mainly based on the photoinjector of the Chinese Academy of Engineering Physics Terahertz free electron laser facility, which merges a 320 kV DC photocathode gun with two 4-cell super-conducting linac operating at 1.3 GHz. Simulations show that high quality electron probes with fC charge, ultralow emittance, and femtosecond scale bunch duration can be generated. Together with the capability of operating at continuous wave mode, this instrument is well matched with the demand of future electron diffraction.

Non-perturbing THz generation at the Tsinghua University Accelerator Laboratory 31 MeV electron beamline.

In recent experiments at Tsinghua University Accelerator Laboratory, the 31 MeV electron beam, which has been compressed to subpicosecond pulse durations, has been used to generate high peak power, narrow band Terahertz (THz) radiation by transit through different slow wave structures, specifically quartz capillaries metallized on the outside. Despite the high peak powers that have been produced, the THz pulse energy is negligible compared to the energy of the electron beam. Therefore, the THz generation process can be complementary to other beamline applications like plasma wakefield acceleration studies and Compton x-ray free electron lasers. This approach can be used at x-ray free electron laser beamlines, where THz radiation can be generated without disturbing the x-ray generation process. In the experiment reported here, a high peak current electron beam generated strong narrow band (∼1% bandwidth) THz signals in the form of a mixture of TM01 and TM02 modes. Each slow wave structure is completed with a mode converter at the end of the structure that allows for efficient (>90%) power extraction into free space. In the experiment, both modes in these two dielectric-loaded waveguides TM01 (0.3 THz/0.5 THz) and TM02 (0.9 THz/1.3 THz) were explicitly measured with an interferometer. The THz pulse energy was measured with a calibrated Golay cell at a few μJ.