X-ray Free Electron Laser Facility Research Articles

Schematic design of fully coherent X-ray generation using self-seeded harmonic lasing free-electron lasers

This paper studies the self-seeded harmonic lasing scheme and analyzes the feasibility of its application in the SHINE project. The scheme is used to improve the longitudinal coherence and extend the operating spectral range of the X-ray free electron lasers (FELs). Combining the benefits of traditional self-seeding and harmonic lasing schemes, the scheme would be compatible with existing facilities without any device modification. We have compared the harmonic performances under different monochromator types with numerical simulations. And considering the sensitivity to energy spread of the scheme, we also propose several possible solutions. The numerical simulation results show that the third harmonic radiation with GW-level peak power and quasi-full coherence can be achieved. The scheme will be helpful for improving the performance of the SHINE and other X-ray FEL facilities.

XFEL Microcrystallography of Self-Assembling Silver n-Alkanethiolates.

New synthetic hybrid materials and their increasing complexity have placed growing demands on crystal growth for single-crystal X-ray diffraction analysis. Unfortunately, not all chemical systems are conducive to the isolation of single crystals for traditional characterization. Here, small-molecule serial femtosecond crystallography (smSFX) at atomic resolution (0.833 Å) is employed to characterize microcrystalline silver n-alkanethiolates with various alkyl chain lengths at X-ray free electron laser facilities, resolving long-standing controversies regarding the atomic connectivity and odd-even effects of layer stacking. smSFX provides high-quality crystal structures directly from the powder of the true unknowns, a capability that is particularly useful for systems having notoriously small or defective crystals. We present crystal structures of silver n-butanethiolate (C4), silver n-hexanethiolate (C6), and silver n-nonanethiolate (C9). We show that an odd-even effect originates from the orientation of the terminal methyl group and its role in packing efficiency. We also propose a secondary odd-even effect involving multiple mosaic blocks in the crystals containing even-numbered chains, identified by selected-area electron diffraction measurements. We conclude with a discussion of the merits of the synthetic preparation for the preparation of microdiffraction specimens and compare the long-range order in these crystals to that of self-assembled monolayers.

Active Q-Switched X-Ray Regenerative Amplifier Free-Electron Laser.

Despite tremendous progress in x-ray free-electron laser (FEL) science over the last decade, future applications still demand fully coherent, stable x rays that have not been demonstrated in existing x-ray FEL facilities. In this Letter, we describe an active Q-switched x-ray regenerative amplifier FEL scheme to produce fully coherent, high-brightness, hard x rays at a high-repetition rate. By using simple electron-beam phase space manipulation, we show this scheme is flexible in controlling the x-ray cavity quality factor Q and hence the output radiation. We report both theoretical and numerical studies on this scheme with a wide range of accelerator, x-ray cavity, and undulator parameters.

Pulse-resolved beam position measurements of high energy X-ray pulses at MHz rate with a diamond sensor

The European X-ray Free Electron Laser facility produces extremely intense and short X-ray pulses. A diamond sensor proposed for non-invasive diagnostics of hard X-rays enables pulse-resolved beam position measurements within less than 1% uncertainty at 2.25MHz.

INVESTIGATION OF THE ULTRAFAST MAGNETIC DYNAMICS IN Co/Pt MULTILAYER STRUCTURES AND EXAMPLES OF OTHER STUDIES AT THE EUROPEAN XFEL FACILITY

The European X-ray Free-Electron Laser (EuXFEL) Facility is the leading international scientific center for studying the structure and properties of materials using coherent X-rays with high temporal and spatial resolution. The results of the collaboration of the EuXFEL experts and the researchers of the ITMO University in 2015–2022 are briefly described. The unique possibilities of the EuXFEL are demonstrated by an example of studying the ultrafast magnetic dynamics by the researchers of the ITMO University in 2019.

Superconducting undulator activities at the European X-ray Free-Electron Laser Facility

For more than 5 years, superconducting undulators (SCUs) have been successfully delivering X-rays in storage rings. The European X-Ray Free-Electron Laser Facility (XFEL) plans to demonstrate the operation of SCUs in X-ray free-electron lasers (FELs). For the same geometry, SCUs can reach a higher peak field on the axis with respect to all other available technologies, offering a larger photon energy tunability range. The application of short-period SCUs in a high electron beam energy FEL > 11 GeV will enable lasing at very hard X-rays > 40 keV. The large tunability range of SCUs will allow covering the complete photon energy range of the soft X-ray experiments at the European XFEL without changing electron beam energy, as currently needed with the installed permanent magnet undulators. For a possible continuous-wave (CW) upgrade under discussion at the European XFEL with a lower electron beam energy of approximately 7–8 GeV, SCUs can provide the same photon energy range as available at present with the permanent magnet undulators and electron energies. This paper will describe the potential of SCUs for X-ray FELs. In particular, it will focus on the different activities ongoing at the European XFEL and in collaboration with DESY to allow the implementation of SCUs in the European XFEL in the upcoming years.

Sample Delivery Systems for Serial Femtosecond Crystallography at the PAL-XFEL

Serial femtosecond crystallography (SFX) using an X-ray free electron laser (XFEL) enables the determination of room-temperature structures without causing radiation damage. Using an optical pump-probe or mix-and-injection, SFX enables the intermediate state visualization of a molecular reaction. In SFX experiments, serial and stable microcrystal delivery to the X-ray interaction point is vital for reasonable data collection and efficient beam time. The Pohang Accelerator Laboratory X-ray Free Electron Laser (PAL-XFEL) facility established SFX instruments at a nanocrystallography and coherent imaging (NCI) experimental station. Various sample delivery methods, including injection, fixed-target scanning, and hybrid methods, have been developed and applied to collect XFEL diffraction data. Herein, we report the currently available sample delivery methods for SFX at the NCI experimental station at the PAL-XFEL. This article will help PAL-XFEL users access the SFX system for their experiments.

A von Hámos spectrometer for diamond anvil cell experiments at the High Energy Density Instrument of the European X-ray Free-Electron Laser.

A von Hámos spectrometer has been implemented in the vacuum interaction chamber 1 of the High Energy Density instrument at the European X-ray Free-Electron Laser facility. This setup is dedicated, but not necessarily limited, to X-ray spectroscopy measurements of samples exposed to static compression using a diamond anvil cell. Si and Ge analyser crystals with different orientations are available for this setup, covering the hard X-ray energy regime with a sub-eV energy resolution. The setup was commissioned by measuring various emission spectra of free-standing metal foils and oxide samples in the energy range between 6 and 11 keV as well as low momentum-transfer inelastic X-ray scattering from a diamond sample. Its capabilities to study samples at extreme pressures and temperatures have been demonstrated by measuring the electronic spin-state changes of (Fe0.5Mg0.5)O, contained in a diamond anvil cell and pressurized to 100 GPa, via monitoring the Fe Kβ fluorescence with a set of four Si(531) analyser crystals at close to melting temperatures. The efficiency and signal-to-noise ratio of the spectrometer enables valence-to-core emission signals to be studied and single pulse X-ray emission from samples in a diamond anvil cell to be measured, opening new perspectives for spectroscopy in extreme conditions research.

Toward using collective x-ray Thomson scattering to study C–H demixing and hydrogen metallization in warm dense matter conditions

The insulator–metal transition in liquid hydrogen is an important phenomenon to understand the interiors of gas giants, such as Jupiter and Saturn, as well as the physical and chemical behavior of materials at high pressures and temperatures. Here, the path toward an experimental approach is detailed based on spectrally resolved x-ray scattering, tailored to observe and characterize hydrogen metallization in dynamically compressed hydrocarbons in the regime of carbon–hydrogen phase separation. With the help of time-dependent density functional theory calculations and scattering spectra from undriven carbon samples collected at the European x-ray Free-Electron Laser Facility (EuXFEL), we demonstrate sufficient data quality for observing C–H demixing and investigating the presence of liquid metallic hydrogen in future experiments using the reprated drive laser systems at EuXFEL.

Fourth-generation light sources.

New fourth-generation synchrotron radiation facilities bring large gains in X-ray source brightness, but also challenges in making full use of their potentials. Some of these challenges have been faced at X-ray free-electron laser facilities.

Sub-femtosecond time-resolved measurements of electron bunches with a C-band radio-frequency deflector in x-ray free-electron lasers

Time-resolved diagnostics are fundamental for x-ray free-electron lasers (FELs). Radio-frequency (RF) transverse deflector structures (TDSs) are typically employed to characterize the temporal properties of the electron beams driving FELs. In this article, we present time-resolved measurements with a resolution below one femtosecond using a C-band RF TDS at SwissFEL, the x-ray FEL facility at the Paul Scherrer Institute in Switzerland. The sub-femtosecond resolution is partially achieved due to an optimized optics setup and fits the expected values, showing a good understanding of our models. Measurements with a sub-femtosecond resolution are of crucial importance for ultra-fast x-ray FEL applications.

Single-pulse characterization of the focal spot size of X-ray free-electron lasers using coherent diffraction imaging.

The characterization of X-ray focal spots is of great significance for the diagnosis and performance optimization of focusing systems. X-ray free-electron lasers (XFELs) are the latest generation of X-ray sources with ultrahigh brilliance, ultrashort pulse duration and nearly full transverse coherence. Because each XFEL pulse is unique and has an ultrahigh peak intensity, it is difficult to characterize its focal spot size individually with full power. Herein, a method for characterizing the spot size at the focus position is proposed based on coherent diffraction imaging. A numerical simulation was conducted to verify the feasibility of the proposed method. The focal spot size of the Coherent Scattering and Imaging endstation at the Shanghai Soft X-ray Free Electron Laser Facility was characterized using the method. The full width at half-maxima of the focal spot intensity and spot size in the horizontal and vertical directions were calculated to be 2.10 ± 0.24 µm and 2.00 ± 0.20 µm, respectively. An ablation imprint on the silicon frame was used to validate the results of the proposed method.

Few-femtosecond X-ray pulse generation and pulse duration control in a seeded free-electron laser

With the development of ultrafast science, free-electron lasers (FELs) with ultrashort pulses have become a state-of-the-art tool in ultrafast phenomena studies. In an externally seeded FEL, the output pulse duration is usually determined both by the seed laser pulse duration and FEL amplification process, which can hardly reach the timescale of a few femtoseconds. In this study, through a simple method of changing the relative time delay and correspondingly the pulse energy of the two seed lasers employed in a seeded FEL, we demonstrated the possibility of generating few-femtosecond soft X-ray pulses and controlling the final FEL pulse durations. Based on theoretical calculations and practical experiments, we conducted a detailed study on the capabilities and limitations to this method with the parameters of the Shanghai Soft X-ray FEL Facility. Start-to-end simulations indicate that we can achieve ultrashort soft X-ray FEL pulses with the pulse duration down to 5.2 fs, and the final pulse durations can also be controlled in terms of relative time delays.

Exploring the Ultrafast Charge-Transfer and Redox Dynamics in Layered Transition Metal Oxides

The rapid development and broad deployment of rechargeable batteries have fundamentally transformed modern society by revolutionizing the sectors of consumer electronics, transportation, and grid energy storage. Redox reactions in active battery cathode materials are ubiquitous, complicated, and functionally very important. While a lot of effort has been devoted to investigating redox heterogeneity and its progressive evolution upon prolonged battery cycling, the ultrafast dynamics in these systems are largely unexplored. In this article, we discuss the potential significance of understanding redox dynamics in battery cathodes in the ultrafast time regime. Here, we outline a conceptual experimental design for investigating the ultrafast electron transport in an industry-relevant layered transition metal oxide battery cathode using a plasma-acceleration-based X-ray free-electron laser (FEL) facility. Going beyond the proposed experiment, we provide our perspectives on the use of compact FEL sources for applied research, which, in our view, is an area of tremendous potential.

Damage threshold in pre-heated optical materials exposed to intense X-rays

Materials exposed to ultrashort intense x-ray irradiation experience various damaging conditions depending on the irradiation temperature. A pre-heated target exposed to intense x-rays plays a crucial role in numerous physical-technical systems, ranging from the heavily and repeatedly radiation-loaded optics at x-ray free-electron laser facilities to the inner-most wall of prospective inertial fusion reactors. We study the temperature dependence of damage thresholds in different classes of materials theoretically: an insulator (diamond), a semiconductor (silicon), a metal (tungsten), and an organic polymer (PMMA). The numerical techniques used here enable us to trace the evolution of both the electronic state and the atomic dynamics of the materials. It includes damage mechanisms such as thermal damage, induced by an increased irradiation temperature due to energy transfer from x-ray-excited electrons, and nonthermal phase transitions, induced by rapid interatomic potential changes due to the excitation of electrons. We demonstrate that in the pre-heated materials, the thermal damage threshold tends to stay the same or lowers with the increase of the irradiation temperature, whereas nonthermal damage thresholds may be lowered or raised, depending on the particular material and specifics of the damage kinetics.

Cavity based x-ray free electron laser demonstrator at the European X-ray Free Electron Laser facility

In this article, the concept of a recently funded R project for the installation of a proof-of-concept cavity-based x-ray free electron laser (CBXFEL) demonstrator experiment at the European XFEL facility is presented, with the first results expected in 2024. It is composed of an x-ray cavity design in backscattering geometry with a 133 m round trip length using cryogenically cooled diamond crystals. It employs the concept of retroreflection to reduce the sensitivity to vibrations. The FEL radiation is produced in four undulator segments of 20 m total length. Simulations at 16 GeV beam energy and 250 pC bunch charge show that the expected x-ray pulses in saturation surpass state-of-the-art x-ray sources considering spectral flux and three-dimensional coherence. However, the stability of the proof of concept setup is severely challenged by the finite thermal transport in the diamond crystals. Therefore, suitable measures such as cooling the crystals to 70 K are explained in this paper and additional ones will have to be developed in the course of this project.

Numerical and experimental investigation of gas flow field variations in three-dimensional printed gas-dynamic virtual nozzles

Gas-dynamic virtual nozzles (GDVNs) play a vital role in delivering biomolecular samples during diffraction measurements at X-ray free-electron laser facilities. Recently, submicrometer resolution capabilities of two-photon polymerization 3D printing techniques opened the possibility to quickly fabricate gas-dynamic virtual nozzles with practically any geometry. In our previous work, we exploited this capability to print asymmetric gas-dynamic virtual nozzles that outperformed conventional symmetric designs, which naturally leads to the question of how to identify the optimal gas-dynamic virtual nozzle geometry. In this work, we develop a 3D computational fluid dynamics pipeline to investigate how the characteristics of microjets are affected by gas-dynamic virtual nozzle geometry, which will allow for further geometry optimizations and explorations. We used open-source software (OpenFOAM) and an efficient geometric volume-of-fluid method (isoAdvector) to affordably and accurately predict jet properties for different nozzle geometries. Computational resources were minimized by utilizing adaptive mesh refinement. The numerical simulation results showed acceptable agreement with the experimental data, with a relative error of about 10% for our test cases that compared bell- and cone-shaped sheath-gas cavities. In these test cases, we used a relatively low sheath gas flow rate (6 mg/min), but future work including the implementation of compressible flows will enable the investigation of higher flow rates and the study of asymmetric drip-to-jet transitions.

Generating High-Power, Frequency Tunable Coherent THz Pulse in an X-ray Free-Electron Laser for THz Pump and X-ray Probe Experiments

Precisely synchronized X-ray and strong-field coherent terahertz (THz) enable the coherent THz excitation of many fundamental modes (THz pump) and the capturing of X-ray dynamic images of matter (X-ray probe), while the generation of such a light source is still a challenge for most existing techniques. In this paper, a novel X-ray free-electron laser based light source is proposed to produce a synchronized high-powered X-ray pulse and strong field, widely frequency tunable coherent THz pulse simultaneously. The technique adopts a frequency beating laser modulated electron bunch with a Giga-electron-volt beam energy to generate an X-ray pulse and a THz pulse sequentially by passing two individual undulator sections with different magnetic periods. Theoretical analysis and numerical simulations are carried out using the beam parameters of the Shanghai soft X-ray free-electron laser facility. The results show that the technique can generate synchronized 4 nm X-ray radiation with a peak power of 1.89 GW, and narrow-band THz radiation with a pulse energy of 1.62 mJ, and the frequency of THz radiation can be continuously tuned from 0.1 to 40 THz. The proposed technique can be used for THz pump and X-ray probe experiments for dynamic research on the interaction between THz pulse and matter at a femtosecond time scale.

Electronic heat generation in semiconductors: Non-equilibrium excitation and evolution of zone-edge phonons via electron–phonon scattering in photo-excited germanium

We investigate experimentally and using first-principles theory the generation of phonons and the relaxation of carriers on picosecond timescales across the Brillouin zone of photo-excited Ge by inter-valley electron–phonon scattering. The phonons generated are typical of those generated in semiconductor devices, contributing to the accumulation of heat within the material. We simulate the time-evolution of phonon populations, based on first-principles band structure and electron–phonon and phonon–phonon matrix elements, and compare them to data from time-resolved x-ray diffuse scattering experiments, performed at the Linac Coherent Light Source x-ray free-electron laser facility, following photo-excitation by a 50 fs near-infrared optical pulse. We show that the intensity of the non-thermal x-ray diffuse scattering signal, which is observed to grow substantially near the L-point of the Brillouin zone over 3–5 ps, is due to phonons generated by scattering of carriers between the Δ and L valleys. These phonons have low group velocities, resulting in a heat bottleneck. With the inclusion of phonon decay through 3-phonon processes, the simulations also account for other non-thermal features observed in the x-ray diffuse scattering intensity, which are due to anharmonic phonon–phonon scattering of the phonons initially generated by electron–phonon scattering.

The MING proposal at SHINE: megahertz cavity enhanced X-ray generation

The cavity-based X-ray free-electron laser (XFEL) has promise in producing fully coherent pulses with a bandwidth of a few meV and very stable intensity, whereas the currently existing self-amplified spontaneous emission (SASE) XFEL is capable of generating ultra-short pulses with chaotic spectra. In general, a cavity-based XFEL can provide a spectral brightness three orders of magnitude higher than that of the SASE mode, thereby opening a new door for cutting-edge scientific research. With the development of superconducting MHz repetition-rate XFEL facilities such as FLASH, European-XFEL, LCLS-II, and SHINE, practical cavity-based XFEL operations are becoming increasingly achievable. In this study, megahertz cavity enhanced X-ray generation (MING) is proposed based on China’s first hard XFEL facility - SHINE, which we refer to as MING@SHINE.