X-ray Free-electron Laser Beam Research Articles

On the thermal stability of multilayer optics for use with high X-ray intensities

High-intensity X-ray free electron laser (XFEL) beams require optics made of materials with minimal radiation absorption, high diffraction efficiency, and high radiation hardness. Multilayer Laue lenses (MLLs) are diffraction-based X-ray optics that can focus XFEL beams, as already demonstrated with tungsten carbide/silicon carbide (WC/SiC)-based MLLs. However, high atomic number materials such as tungsten strongly absorb X-rays, resulting in high heat loads. Numerical simulations predict much lower heat loads in MLLs consisting of low atomic number Z materials, although such MLLs have narrower rocking curve widths. In this paper, we first screen various multilayer candidates and then focus on Mo2C/SiC multilayer due to its high diffraction efficiency. According to numerical simulations, the maximum temperature in this multilayer should remain below 300°C if the MLL made out of this multilayer is exposed to an XFEL beam of 17.5 keV photon energy, 1 mJ energy per pulse and 10 kHz pulse repetition rate. To understand the thermal stability of the Mo2C/SiC multilayer, we performed a study on the multilayers of three different periods (1.5, 5, and 12 nm) and different Mo2C to SiC ratios. We monitored their periods, crystallinity, and stress as a function of annealing temperature for two different heating rates. The results presented in this paper indicate that Mo2C/SiC-based MLLs are viable for focusing XFEL beams without being damaged under these conditions.

Numerical simulations of temperature loads on multilayer Laue lenses

We present numerical simulations of the heat loads on novel diffractive X-ray optics, known as multilayer Laue lenses, exposed to high-intensity X-ray beams produced by an X-ray free electron laser (XFEL). These lenses can be used to focus XFEL beams to nanometer spots. The temperature distribution within the lens and temperature evolution as a function of incident pulse frequencies were calculated for two different lens geometries and several material pairs and material ratios of the MLLs. Simulations considered the special pulse structure of European XFEL with X-rays being delivered in pulse trains. After defining the geometric model, computational grid, material properties, and boundary conditions, a grid sensitivity study was carried out. We solved the transient heat energy transport equation in solids for mixed boundary conditions. The results of these simulations will help select materials and lens geometry for future XFEL experiments.

Molecular Simulations of Liquid Jet Explosions and Shock Waves Induced by X-Ray Free-Electron Lasers.

X-ray free-electron lasers (XFELs) produce x-ray pulses with high brilliance and short pulse duration. These properties enable structural investigations of biomolecular nanocrystals, and they allow one to resolve the dynamics of biomolecules down to the femtosecond timescale. Liquid jets are widely used to deliver samples into the XFEL beam. The impact of the x-ray pulse leads to vaporization and explosion of the liquid jet, while the expanding gas triggers the formation of shock wave trains traveling along the jet, which may affect biomolecular samples before they have been probed. Here, we used molecular dynamics simulations to reveal the structural dynamics of shock waves after an x-ray impact. Analysis of the density and temperature in the jet revealed shock waves that form close to the explosion center, travel along the jet with supersonic velocities, and decay exponentially with an attenuation length proportional to the jet diameter. A trailing shock wave formed after the first shock wave, similar to the shock wave trains in experiments. High shock wave velocities in our simulations are compatible with the phenomenon of "fast sound," as emerging at large sound frequencies. Although using purely classical models in the simulations, the resulting explosion geometry and shock wave dynamics closely resemble experimental findings, and they highlight the importance of atomistic details for modeling shock wave attenuation.

Gain dynamics of inner-shell vacancy states pumped by high-intensity XFEL in Mg, Al and Si.

High-intensity X-ray free-electron laser (XFEL) beams create transient and non-equilibrium dense states of matter in solid-density targets. These states can be used to develop atomic X-ray lasers with narrow bandwidth and excellent longitudinal coherence, which is not possible with current XFEL pulses. An atomic kinetics model is used to simulate the population dynamics of atomic inner-shell vacancy states in Mg, Al, and Si, revealing the feasibility of population inversion between K-shell and L-shell vacancy states. We also discuss the gain characteristics of these states implying the possibility of atomic X-ray lasers based on inner-shell vacancy states in the 1.5 keV region. The development of atomic X-ray lasers could have applications in high-resolution spectroscopy and nonlinear optics in the X-ray region.

XCC: an X-ray FEL-based γγ Compton collider Higgs factory

This report describes the conceptual design of a γγ Higgs factory in which 62.8 GeV electron beams collide with 1 keV X-ray free electron laser (XFEL) beams to produce colliding beams of 62.5 GeV photons. The Higgs boson production rate is 80,000 Higgs bosons per 107 second year, roughly the same as the ILC Higgs rate at √s = 250 GeV. The electron accelerator is based on cold copper distributed coupling (C3) accelerator technology. Unlike the center-of-mass energy spectra of previous optical wavelength γγ collider designs, the sharply peaked γγ center-of-mass energy spectrum of XCC produces model independent Higgs coupling measurements with precision on par with e + e - colliders. For the triple Higgs coupling measurement, the XCC center-of-mass energy can be upgraded to 380 GeV, where the cross section for γγ → HH is twice that of e + e - → ZHH at √s = 500 GeV. Design challenges are discussed, along with the R&D to address them, including demonstrators.

Towards a wavefront-preservation X-ray crystal monochromator for high-repetition-rate FELs.

The wavefront preservation of coherent X-ray free-electron laser beams is pushing the requirement on the quality and performance of X-ray optics to an unprecedented level. The Strehl ratio can be used to quantify this requirement. In this paper, the criteria for thermal deformation of the X-ray optics are formulated, especially for crystal monochromators. To preserve the X-ray wavefront, the standard deviation of the height error should be sub-nm for mirrors and less than 25 pm for crystal monochromators. Cryocooled silicon crystals combined with two techniques can be used to achieve this level of performance for monochromator crystals: (1) using a focusing element to compensate the second-order component of the thermal deformation; (2)introducing a cooling pad between the cooling block and silicon crystal and optimizing the effective cooling temperature. Each of these techniques allows the thermal deformation in standard deviation of the height error to be reduced by an order of magnitude. As an example, for the LCLS-II-HE Dynamic X-ray Scattering instrument, the criteria on thermal deformation of a high-heat-load monochromator crystal can be achieved for a 100 W SASE FEL beam. Wavefront propagation simulations confirm that the reflected beam intensity profile is satisfactory on both the peak power density and focused beam size.

Ultrafast time-resolved 2D imaging of laser-driven fast electron transport in solid density matter using an x-ray free electron laser.

High-power, short-pulse laser-driven fast electrons can rapidly heat and ionize a high-density target before it hydrodynamically expands. The transport of such electrons within a solid target has been studied using two-dimensional (2D) imaging of electron-induced Kα radiation. However, it is currently limited to no or picosecond scale temporal resolutions. Here, we demonstrate femtosecond time-resolved 2D imaging of fast electron transport in a solid copper foil using the SACLA x-ray free electron laser (XFEL). An unfocused collimated x-ray beam produced transmission images with sub-micron and ∼10fs resolutions. The XFEL beam, tuned to its photon energy slightly above the Cu K-edge, enabled 2D imaging of transmission changes induced by electron isochoric heating. Time-resolved measurements obtained by varying the time delay between the x-ray probe and the optical laser show that the signature of the electron-heated region expands at ∼25% of the speed of light in a picosecond duration. Time-integrated Cu Kα images support the electron energy and propagation distance observed with the transmission imaging. The x-ray near-edge transmission imaging with a tunable XFEL beam could be broadly applicable for imaging isochorically heated targets by laser-driven relativistic electrons, energetic protons, or an intense x-ray beam.

Coherent diffractive imaging of lipid vesicles and synaptic vesicles by femtosecond x-ray FEL pulses.

The structure of biological vesicles, in particular synaptic vesicles (SVs), as well as synthetic lipid model systems, in particular small unilamellar lipid vesicles (LVs) has been subject of abiding interest. Given the small size of both LVs and SVs (R≈20 nm), high spatial resolution is required to identify the distribution of lipids and protein constituents. Cryogenic electron microscopy studies of synaptic vesicles have revealed the outer and inner layer of proteins around the lipid bilayer. However, the samples have to be cryogenically fixed for this technique and throughput is limited. Another standard technique for structural characterization is solution small angle x-ray scattering (SAXS), which enables the measurement of vesicles in a quasi-physiological environment combined with a high spatial resolution. However, due to the average over an extremely large ensemble, SAXS yields information only about the average structure (size and electron density profile). The distribution function of structural parameters is not accessible, and many structural details are lost or screened by polydispersity, as well as by powder averaging. To overcome these limitations, we now have performed coherent diffractive imaging experiments on single vesicles using single femtosecond x-ray free electron laser (XFEL) pulses. For these measurements, single vesicles surrounded by a thin buffer layer are delivered into a nano-focused XFEL beam by an aerosol injector. By the ‘diffract-before-destroy’ principle, the individual vesicles can be probed without radiation damage. This approach leads to the measurement of thousands of diffraction patterns that can now be analyzed and reconstructed. The (preliminary) results of this analysis will be presented.

Thermus thermophilus polyploid cells directly imaged by X-ray laser diffraction.

Thermus thermophilus is reportedly polyploid and carries four to five identical genome copies per cell, based on molecular biological experiments. To directly detect polyploidy in this bacterium, we performed live cell imaging by X-ray free-electron laser (XFEL) diffraction and observed its internal structures. The use of femtosecond XFEL pulses enables snapshots of live, undamaged cells. For successful XFEL imaging, we developed a bacterial culture method using a starch- and casein-rich medium that produces a predominance of rod-shaped cells shorter than the focused XFEL beam size, which is slightly smaller than 2 µm. When cultured in the developed medium, the length of T. thermophilus cells, which is typically ~4 µm, was less than half its usual length. We placed living cells in a micro-liquid enclosure array and successively exposed each enclosure to a single XFEL pulse. A cell image was successfully obtained by the coherent diffractive imaging technique with iterative phase retrieval calculations. The reconstructed cell image revealed five peaks, which are most likely to be nucleoids, arranged in a row in the polyploid cell without gaps. This study demonstrates that XFELs offer a novel approach for visualizing the internal nanostructures of living, micrometer-sized, polyploid bacterial cells.

Damage-Free Femtosecond X-Ray Laser Snapshot Imaging of Catalyst Layer Nano-Structures of Polymer Electrolyte Fuel Cells

The realization of the hydrogen society, where hydrogen is used as clean and renewable energy, is desired to reduce carbon dioxide emissions and achieve sustainable development. The polymer electrolyte fuel cell (PEFC) is a key technology for automobiles in the hydrogen society as an alternative to conventional internal combustion engines. However, further improvements in PEFC performances are required for the full-scale spread of fuel cell vehicles. This study aims to image the nanostructures of the catalyst layer, which is particularly important in determining the performance of PEFCs, and correlate the observed structures with mass transfer in the catalyst layer. The catalyst layer contains catalyst-loaded carbon supports thinly covered with an ionomer responsible for proton conduction. We are especially interested in observing the support structure and ionomer coverage.We performed coherent X-ray imaging studies on the catalyst particles of PEFCs using an X-ray free-electron laser (XFEL) facility: SACLA (SPring-8 Angstrom Compact free electron LAser). Femtosecond XFEL allows us to capture radiation-damage-free snapshot images by outrunning major radiation damage processes. Damage-free imaging is of great value because radiation damage often causes a problem in high-resolution electron microscopy observation of ionomers. We used the coherent diffractive imaging (CDI) technique to observe catalyst particles of PEFCs. In CDI, sample images are reconstructed from measured coherent diffraction patterns using phase-retrieval calculation instead of objective lenses. CDI enables quantitative phase imaging and allows high-contrast imaging of the catalyst particles, which are almost transparent to X-rays.The measurement was carried out for catalyst particles in the dried condition and in solution. Our study showed that the image contrast can be adjusted by changing the electron density of the volume surrounding the sample1). The dried sample was prepared by dropping catalyst ink onto a silicon nitride membrane and air-dried. The solution sample was enclosed in micro-liquid enclosure arrays2,3) developed at Hokkaido University. The coherent diffraction patterns were measured by irradiating the sample catalyst particles with the 100-nm-focused XFEL beam with a photon energy of 4 keV using MAXIC-S (Multiple Application X-ray Imaging Chamber-S)4). The phase-retrieval calculation reconstructed the sample images. Fig. 1 shows a reconstructed image of a catalyst particle in water. The reconstructed image shows many white spots, which can be interpreted as platinum-cobalt catalysts. The pixel size of the reconstructed image is 1.3 nm, demonstrating among the highest spatial resolution in X-ray imaging. We will continue our study to observe the support structures and their ionomer coverage to understand the mass transfer in the catalyst layer.This work was supported by the FC-Platform of NEDO (New Energy and Industrial Technology Development Organization). The XFEL experiments were performed at SACLA with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal Nos. 2020A8106, 2021A8010, and 2021B8019).

Upgraded Combined Inject-and-Transfer System for Serial Femtosecond Crystallography

Serial femtosecond crystallography (SFX) using an X-ray free-electron laser (XFEL) can be applied to determine the room-temperature structure of target molecules while minimizing radiation damage and visualizing molecular dynamics. In SFX, a sample delivery system is required to deliver microcrystals to the XFEL beam path in a serial manner. We recently developed a sample delivery method, the combined inject-and-transfer system (BITS), which is a hybrid method based on the injector and fixed-target scanning approach. In this study, we introduced recently upgraded hardware to move the injection needle in the direction of the XYZ-axis and a graphic user interface for user motion control. Furthermore, we report that the viscous solution containing 10% (w/v) PEG 3350 or PEG 6000 that is widely used for protein crystallization can be stably deposited on polyimide film with a hydrophobic surface without any special treatment. Moreover, the development of an inject-and-diffuse method for time-resolved studies with liquid applications in the BITS and its preliminary results are reported. This study provides up-to-date instrument information to SFX users using BITS and provides insights to instrument developers for SFX.

Towards real-time analysis of liquid jet alignment in serial femtosecond crystallography.

Liquid sample delivery systems are used extensively for serial femtosecond crystallography at X-ray free-electron lasers (XFELs). However, misalignment of the liquid jet and the XFEL beam leads to the X-rays either partially or completely missing the sample, resulting in sample wastage and a loss of experiment time. Implemented here is an algorithm to analyse optical images using machine vision to determine whether there is overlap of the X-ray beam and liquid jet. The long-term goal is to use the output from this algorithm to implement an automated feedback mechanism to maintain constant alignment of the X-ray beam and liquid jet. The key elements of this jet alignment algorithm are discussed and its performance is characterized by comparing the results with a manual analysis of the optical image data. The success rate of the algorithm for correctly identifying hits is quantified via a similarity metric, the Dice coefficient. In total four different nozzle designs were used in this study, yielding an overall Dice coefficient of 0.98.

Generation of ultrahigh-brightness pre-bunched beams from a plasma cathode for X-ray free-electron lasers

The longitudinal coherence of X-ray free-electron lasers (XFELs) in the self-amplified spontaneous emission regime could be substantially improved if the high brightness electron beam could be pre-bunched on the radiated wavelength-scale. Here, we show that it is indeed possible to realize such current modulated electron beam at angstrom scale by exciting a nonlinear wake across a periodically modulated plasma-density downramp/plasma cathode. The density modulation turns on and off the injection of electrons in the wake while downramp provides a unique longitudinal mapping between the electrons’ initial injection positions and their final trapped positions inside the wake. The combined use of a downramp and periodic modulation of micrometers is shown to be able to produces a train of high peak current (17 kA) electron bunches with a modulation wavelength of 10’s of angstroms - orders of magnitude shorter than the plasma density modulation. The peak brightness of the nano-bunched beam can be O(1021A/m2/rad2) orders of magnitude higher than current XFEL beams. Such prebunched, high brightness electron beams hold the promise for compact and lower cost XEFLs that can produce nanometer radiation with hundreds of GW power in a 10s of centimeter long undulator.

Real-time spatial characterization of micrometer-sized X-ray free-electron laser beams focused by bendable mirrors.

A real-time and accurate characterization of the X-ray beam size is essential to enable a large variety of different experiments at free-electron laser facilities. Typically, ablative imprints are employed to determine shape and size of µm-focused X-ray beams. The high accuracy of this state-of-the-art method comes at the expense of the time required to perform an ex-situ image analysis. In contrast, diffraction at a curved grating with suitably varying period and orientation forms a magnified image of the X-ray beam, which can be recorded by a 2D pixelated detector providing beam size and pointing jitter in real time. In this manuscript, we compare results obtained with both techniques, address their advantages and limitations, and demonstrate their excellent agreement. We present an extensive characterization of the FEL beam focused to ≈1 µm by two Kirkpatrick-Baez (KB) mirrors, along with optical metrology slope profiles demonstrating their exceptionally high quality. This work provides a systematic and comprehensive study of the accuracy provided by curved gratings in real-time imaging of X-ray beams at a free-electron laser facility. It is applied here to soft X-rays and can be extended to the hard X-ray range. Furthermore, curved gratings, in combination with a suitable detector, can provide spatial properties of µm-focused X-ray beams at MHz repetition rate.

Effect of Optics Contamination on X-ray Free-Electron Laser Beam Quality

Effect of Optics Contamination on X-ray Free-Electron Laser Beam Quality

Effects of x-ray free-electron laser pulse intensity on the Mn K β 1,3 x-ray emission spectrum in photosystem II-A case study for metalloprotein crystals and solutions.

In the last ten years, x-ray free-electron lasers (XFELs) have been successfully employed to characterize metalloproteins at room temperature using various techniques including x-ray diffraction, scattering, and spectroscopy. The approach has been to outrun the radiation damage by using femtosecond (fs) x-ray pulses. An example of an important and damage sensitive active metal center is the Mn4CaO5 cluster in photosystem II (PS II), the catalytic site of photosynthetic water oxidation. The combination of serial femtosecond x-ray crystallography and Kβ x-ray emission spectroscopy (XES) has proven to be a powerful multimodal approach for simultaneously probing the overall protein structure and the electronic state of the Mn4CaO5 cluster throughout the catalytic (Kok) cycle. As the observed spectral changes in the Mn4CaO5 cluster are very subtle, it is critical to consider the potential effects of the intense XFEL pulses on the Kβ XES signal. We report here a systematic study of the effects of XFEL peak power, beam focus, and dose on the Mn Kβ1,3 XES spectra in PS II over a wide range of pulse parameters collected over seven different experimental runs using both microcrystal and solution PS II samples. Our findings show that for beam intensities ranging from ∼5 × 1015 to 5 × 1017 W/cm2 at a pulse length of ∼35 fs, the spectral effects are small compared to those observed between S-states in the Kok cycle. Our results provide a benchmark for other XFEL-based XES studies on metalloproteins, confirming the viability of this approach.

Comparison of EMC and CM methods for orienting diffraction images in single-particle imaging experiments.

In single-particle imaging (SPI) experiments, diffraction patterns of identical particles are recorded. The particles are injected into the X-ray free-electron laser (XFEL) beam in random orientations. The crucial step of the data processing of SPI is finding the orientations of the recorded diffraction patterns in reciprocal space and reconstructing the 3D intensity distribution. Here, two orientation methods are compared: the expansion maximization compression (EMC) algorithm and the correlation maximization (CM) algorithm. To investigate the efficiency, reliability and accuracy of the methods at various XFEL pulse fluences, simulated diffraction patterns of biological molecules are used.

Resonant X-ray Emission Spectroscopy with a SASE Beam

Aqueous iron (III) oxide nanoparticles were irradiated with pure self-amplified spontaneous emission (SASE) X-ray free-electron laser (XFEL) pulses tuned to the energy around the Fe K-edge ionization threshold. For each XFEL shot, the incident X-ray pulse spectrum and Fe Kβ emission spectrum were measured synchronously with dedicated spectrometers and processed through a reconstruction algorithm allowing for the determination of Fe Kβ resonant X-ray emission spectroscopy (RXES) plane with high energy resolution. The influence of the number of X-ray shots employed in the experiment on the reconstructed data quality was evaluated, enabling the determination of thresholds for good data acquisition and experimental times essential for practical usage of scarce XFEL beam times.

Observability of Coulomb-assisted quantum vacuum birefringence

We consider the scattering of an x-ray free-electron laser (XFEL) beam on the superposition of a strong magnetic field $\bf{B}_{\rm ext}$ with the Coulomb field $\bf{E}_{\rm ext}$ of a nucleus with charge number $Z$. In contrast to pure Delbr\"uck scattering (Coulomb field only), the magnetic field $\bf{B}_{\rm ext}$ introduces an asymmetry (i.e., polarization dependence) and renders the effective interaction volume quite large, while the nuclear Coulomb field facilitates a significant momentum transfer $\Delta\bf k$. For a field strength of $B_{\rm ext}=10^6~\rm T$ (corresponding to an intensity of order $10^{22}~\rm W/cm^2$) and an XFEL frequency of 24 keV, we find a differential cross section $d\sigma/d\Omega\sim10^{-25}~Z^2/(\Delta{\bf k})^2$ in forward direction for one nucleus. Thus, this effect might be observable in the near future at facilities such as the Helmholtz International Beamline for Extreme Fields (HIBEF) at the European XFEL.

Effect of X-ray free-electron laser-induced shockwaves on haemoglobin microcrystals delivered in a liquid jet

X-ray free-electron lasers (XFELs) enable obtaining novel insights in structural biology. The recently available MHz repetition rate XFELs allow full data sets to be collected in shorter time and can also decrease sample consumption. However, the microsecond spacing of MHz XFEL pulses raises new challenges, including possible sample damage induced by shock waves that are launched by preceding pulses in the sample-carrying jet. We explored this matter with an X-ray-pump/X-ray-probe experiment employing haemoglobin microcrystals transported via a liquid jet into the XFEL beam. Diffraction data were collected using a shock-wave-free single-pulse scheme as well as the dual-pulse pump-probe scheme. The latter, relative to the former, reveals significant degradation of crystal hit rate, diffraction resolution and data quality. Crystal structures extracted from the two data sets also differ. Since our pump-probe attributes were chosen to emulate EuXFEL operation at its 4.5 MHz maximum pulse rate, this prompts concern about such data collection.