European X-ray Free-Electron Laser Facility Research Articles

Diamond sensors for hard X-ray energy and position resolving measurements at the European XFEL.

The diagnostics of X-ray beam properties has a critical importance at the European X-ray Free-Electron Laser facility. Besides existing diagnostic components, utilization of a diamond sensor was proposed to achieve radiation-hard, non-invasive beam position and pulse energy measurements for hard X-rays. In particular, with very hard X-rays, diamond-based sensors become a useful complement to gas-based devices which lose sensitivity due to significantly reduced gas cross-sections. The measurements presented in this work were performed with diamond sensors consisting of an electronic-grade single-crystal chemical-vapor-deposition diamond with position-sensitive resistive electrodes in a duo-lateral configuration. The results show that the diamond sensor delivers pulse-resolved X-ray beam position data at 2.25 MHz with an uncertainty of less than 1% of the beam size. To our knowledge this is the first demonstration of pulse-resolved position measurements at the MHz rate using a transmissive diamond sensor at a free-electron laser facility. It can therefore be a valuable tool for X-ray free-electron lasers, especially for high-repetition-rate machines, enabling applications such as beam-based alignment and intra-pulse-train position feedback.

Undulator linear taper control at the European X-Ray Free-Electron Laser facility

Undulator tapering controls the resonance properties of the free-electron laser (FEL) amplification process. Wakefield energy losses in an undulator’s vacuum chamber are one of the factors that determine the undulator’s linear taper. While another contribution to energy losses, namely the losses due to spontaneous radiation, can be calculated analytically, estimating wakefield energy losses requires detailed knowledge of the chamber geometry and the electron beam current profile. We introduce a method for the automatic estimation of wakefield energy losses, which leverages noninvasive THz diagnostics, a current profile reconstruction algorithm enhanced with machine learning, and a recently developed analytical wakefield function for the European XFEL’s undulator beamline. The correctness of this method was validated by directly measuring wakefield-induced electron beam energy losses in the undulator section. This, in turn, enables the prediction of the optimal linear taper in the undulator. Published by the American Physical Society 2024

X-ray beam diagnostics at the MID instrument of the European X-ray Free-Electron Laser Facility.

The Materials Imaging and Dynamics (MID) instrument at the European X-ray Free-Electron Laser Facility (EuXFEL) is equipped with a multipurpose diagnostic end-station (DES) at the end of the instrument. The imager unit in DES is a key tool for aligning the beam to a standard trajectory and for adjusting optical elements such as focusing lenses or the split-and-delay line. Furthermore, the DES features a bent-diamond-crystal spectrometer to disperse the spectrum of the direct beam to a line detector. This enables pulse-resolved characterization of the EuXFEL spectrum to provide X-ray energy calibration, and the spectrometer is particularly useful in commissioning special modes of the accelerator. Together with diamond-based intensity monitors, the imager and spectrometer form the DES unit which also contains a heavy-duty beamstop at the end of the MID instrument. Here, we describe the setup in detail and provide exemplary beam diagnostic results.

Direct observation of 890 ns dynamics of carbon black and polybutadiene in rubber materials using diffracted x-ray blinking

Dynamic behavior in soft matter physics, biology, and nanoscience frequently occurs on submicrosecond timescales. Diffracted x-ray blinking (DXB) is a unique method that can provide a broad range of spatial scale information and is becoming an attractive tool for use at high repetition rate x-ray facilities. In this study, we performed DXB experiments with 890 ns time resolution at the European X-ray Free-Electron Laser Facility to obtain dynamic information about rubber samples that are typically used in automobile tires. Time-resolved scattering was simultaneously recorded for two samples that mainly consisted of carbon black (CB) and polybutadiene (PB). These samples contained either graphitized or non-graphitized CB and displayed significantly different dynamics. A clear interaction between CB and PB was observed, indicating that the mobility of PB was changed by the introduction of CB. Restricted polymer motion was observed in the q-range of 0.78–1.58 Å−1 regions. Our results suggest that the particle network can be flexibly controlled without impairing the mechanical strength of the rubber.

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.

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.

Effects of radiation damage and inelastic scattering on single-particle imaging of hydrated proteins with an X-ray Free-Electron Laser

We present a computational case study of X-ray single-particle imaging of hydrated proteins on an example of 2-Nitrogenase–Iron protein covered with water layers of various thickness, using a start-to-end simulation platform and experimental parameters of the SPB/SFX instrument at the European X-ray Free-Electron Laser facility. The simulations identify an optimal thickness of the water layer at which the effective resolution for imaging the hydrated sample becomes significantly higher than for the non-hydrated sample. This effect is lost when the water layer becomes too thick. Even though the detailed results presented pertain to the specific sample studied, the trends which we identify should also hold in a general case. We expect these findings will guide future single-particle imaging experiments using hydrated proteins.

The photon beamline vacuum system of the European XFEL

The photon beamline vacuum system of the European X-ray Free-Electron Laser Facility (European XFEL) is described. The ultra-large, in total more than 3 km-long, fan-like vacuum system, consisting of three photon beamlines is anessential part of the photon beam transport. It is located between the accelerator vacuum system and the scientific instruments. The main focus of the design was on the efficiency, reliability and robustness of the entire system to ensure the retention of beam properties and the operation of the X-ray optics and X-ray photon diagnostics components. Installation started in late 2014, the first of the three beamline vacuum systems was commissioned in spring 2017, and the last one was operational in mid-2018. The present state and experience from the first years of operation are outlined.

Materials Imaging and Dynamics (MID) instrument at the European X-ray Free-Electron Laser Facility.

The Materials Imaging and Dynamics (MID) instrument at the European X-ray Free-Electron Laser (EuXFEL) facility is described. EuXFEL is the first hard X-ray free-electron laser operating in the MHz repetition range which provides novel science opportunities. The aim of MID is to enable studies of nano-structured materials, liquids, and soft- and hard-condensed matter using the bright X-ray beams generated by EuXFEL. Particular emphasis is on studies of structure and dynamics in materials by coherent scattering and imaging using hard X-rays. Commission of MID started at the end of 2018 and first experiments were performed in 2019.

Shock Damage Analysis in Serial Femtosecond Crystallography Data Collected at MHz X-ray Free-Electron Lasers

Serial femtosecond crystallography (SFX) data were recorded at the European X-ray free-electron laser facility (EuXFEL) with protein microcrystals delivered via a microscopic liquid jet. An XFEL beam striking such a jet may launch supersonic shock waves up the jet, compromising the oncoming sample. To investigate this efficiently, we employed a novel XFEL pulse pattern to nominally expose the sample to between zero and four shock waves before being probed. Analyzing hit rate, indexing rate, and resolution for diffraction data recorded at MHz pulse rates, we found no evidence of damage. Notably, however, this conclusion could only be drawn after careful identification and assimilation of numerous interrelated experimental factors, which we describe in detail. Failure to do so would have led to an erroneous conclusion. Femtosecond photography of the sample-carrying jet revealed critically different jet behavior from that of all homogeneous liquid jets studied to date in this manner.

Mirror to measure small angle x-ray scattering signal in high energy density experiments.

Small angle x-ray scattering (SAXS) is a well established technique to detect nanometer scale structures in matter. In a typical setup, this diagnostic uses a detector with a direct line of sight to the scattering target. However, in the harsh environment of high intensity laser interaction, intense secondary radiation and high-energy particles are generated. Such a setup would therefore suffer a significant increase of noise due to this background, which could eventually prevent such measurements. In this paper, we present a novel tool consisting of a mosaic graphite crystal that works as a mirror for the SAXS signal and allows us to position the detector behind appropriate shielding. This paper studies the performance of this mirror both by experiment at the European XFEL (X-Ray Free-Electron Laser Facility) laboratory and by simulations.

Hard x-ray single-shot spectrometer at the European X-ray Free-Electron Laser.

The European X-ray Free-Electron Laser Facility in Germany delivers x-ray pulses with femtosecond pulse duration at a repetition rate of up to 4.5 MHz. The free-electron laser radiation is created by the self-amplified spontaneous emission (SASE) process, whose stochastic nature gives rise to shot-to-shot fluctuations in most beam properties, including spectrum, pulse energy, spatial profile, wavefront, and temporal profile. Each spectrum consisting of many spikes varies in width and amplitude that appear differently within the envelope of the SASE spectrum. In order to measure and study the SASE spectrum, the HIgh REsolution hard X-ray single-shot (HIREX) spectrometer was installed in the photon tunnel of the SASE1 undulator beamline. It is based on diamond gratings, bent crystals as a dispersive element, and a MHz-repetition-rate strip detector. It covers a photon energy range of 3 keV-25 keV and a bandwidth of 0.5% of the SASE beam. The SASE spikes are resolved with 0.15 eV separation using the Si 440 reflection, providing a resolving power of 60 000 at a photon energy of 9.3 keV. The measured SASE bandwidth is 25 eV. In this paper, we discuss the design specifications, installation, and commissioning of the HIREX spectrometer. The spectral results using Si (110), Si (111), and C (110) crystals are presented.

MHz data collection of a microcrystalline mixture of different jack bean proteins

We provide a detailed description of a serial femtosecond crystallography (SFX) dataset collected at the European X-ray free-electron laser facility (EuXFEL). The EuXFEL is the first high repetition rate XFEL delivering MHz X-ray pulse trains at 10 Hz. The short spacing (<1 µs) between pulses requires fast flowing microjets for sample injection and high frame rate detectors. A data set was recorded of a microcrystalline mixture of at least three different jack bean proteins (urease, concanavalin A, concanavalin B). A one megapixel Adaptive Gain Integrating Pixel Detector (AGIPD) was used which has not only a high frame rate but also a large dynamic range. This dataset is publicly available through the Coherent X-ray Imaging Data Bank (CXIDB) as a resource for algorithm development and for data analysis training for prospective XFEL users.

Pump-probe laser system at the FXE and SPB/SFX instruments of the European X-ray Free-Electron Laser Facility.

User operation at the European X-ray Free-Electron Laser Facility started at the SASE1 undulator beamline in fall 2017. The majority of the experiments utilize optical lasers (mostly ultrafast) for pump-probe-type measurements in combination with X-ray pulses. This manuscript describes the purpose-developed pump-probe laser system as installed at SASE1, implemented features and plans for further upgrades.

Starting User Operation at the European XFEL

In the early fall of 2017, the European X-ray Free-Electron Laser facility (European XFEL) receives first users to perform experiments and take data. The European XFEL is an international user faci...

First results on DEPFET Active Pixel Sensors fabricated in a CMOS foundry—a promising approach for new detector development and scientific instrumentation

DEPFET Active Pixel Sensors (APS) have been introduced as focal plane detectors for X-ray astronomy already in 1996. Fabricated on high resistivity, fully depleted silicon and back-illuminated they can provide high quantum efficiency and low noise operation even at very high read rates. In 2009 a new type of DEPFET APS, the DSSC (DEPFET Sensor with Signal Compression) was developed, which is dedicated to high-speed X-ray imaging at the European X-ray free electron laser facility (EuXFEL) in Hamburg. In order to resolve the enormous contrasts occurring in Free Electron Laser (FEL) experiments, this new DSSC-DEPFET sensor has the capability of nonlinear amplification, that is, high gain for low intensities in order to obtain single-photon detection capability, and reduced gain for high intensities to achieve high dynamic range for several thousand photons per pixel and frame. We call this property “signal compression”.Starting in 2015, we have been fabricating DEPFET sensors in an industrial scale CMOS foundry maintaining the outstanding proven DEPFET properties and adding new capabilities due to the industrial-scale CMOS process. We will highlight these additional features and describe the progress achieved so far. In a first attempt on double-sided polished 725 μm thick 200 mm high resistivity float zone silicon wafers all relevant device related properties have been measured, such as leakage current, depletion voltage, transistor characteristics, noise and energy resolution for X-rays and the nonlinear response. The smaller feature size provided by the new technology allows for an advanced design and significant improvements in device performance. A brief summary of the present status will be given as well as an outlook on next steps and future perspectives.

X-ray spectrometer based on a bent diamond crystal for high repetition rate free-electron laser applications.

A precise spectral characterization of every single pulse is required in many x-ray free-electron laser (XFEL) experiments due to the fluctuating spectral content of self-amplified spontaneous emission (SASE) beams. Bent single-crystal spectrometers can provide sufficient spectral resolution to resolve the SASE spikes while also covering the full SASE bandwidth. To better withstand the high heat load induced by the 4.5 MHz repetition rate of pulses at the forthcoming European XFEL facility, a spectrometer based on single-crystal diamond has been developed. We report a direct comparison of the diamond spectrometer with its Si counterpart in experiments performed at the Linac Coherent Light Source.

Versatile optical laser system for experiments at the European X-ray free-electron laser facility.

We present the main features of the final prototype of a pulsed optical laser, developed for pump-probe and other experiments in conjunction with the femtosecond x-ray beams at the European X-ray free-electron laser facility. Adapted to the temporal x-ray emission pattern of the facility, the laser provides 10 Hz bursts of up to 600 µs duration with intra-burst pulse frequencies as high as 4.5 MHz. In this mode, we have generated pulses as short as 12 fs at 350 W average power during the burst and with beam qualities close to the diffraction limit. This is, to the best of our knowledge, the highest power to date of a few-cycle laser operating at a center wavelength of 800 nm. Important for experimental flexibility, the laser can be configured in various unique ways, enabling, e.g., energy scaling to >3 mJ per pulse through a frequency change down to 100 kHz and the generation of nearly transform limited pulses between 12 fs and 300 fs. In addition to the 800 nm femtosecond beam line, a synchronized long pulse (0.8 ps or 400 ps) 1030 nm beam can be utilized, offering up to 4 kW burst average power, i.e. up to 40 mJ per pulse at 100 kHz. Efficient nonlinear wavelength conversion and tuning through intrinsic and external means further enhance the capabilities of the laser.

High power burst-mode optical parametric amplifier with arbitrary pulse selection

We present results from a unique burst-mode femtosecond non-collinear optical parametric amplifier (NOPA) under development for the optical - x-ray pump-probe experiments at the European X-Ray Free-Electron Laser Facility. The NOPA operates at a burst rate of 10 Hz, a duty cycle of 2.5% and an intra-burst repetition rate of up to 4.5 MHz, producing high fidelity 15 fs pulses at a center wavelength of 810 nm. Using dispersive amplification filtering of the super-continuum seed pulses allows for selectable pulse duration up to 75 fs, combined with a tuning range in excess of 100 nm whilst remaining nearly transform limited. At an intra-burst rate of 188 kHz the single pulse energy from two sequential NOPA stages reached 180 µJ, corresponding to an average power of 34W during the burst. Acousto- and electro-optic switching techniques enable the generation of transient free bursts of required length and the selection of arbitrary pulse sequences inside the burst.