Philip Heimann

The reliable and rapid detection of uncontrolled X-ray beam propagation is critical for machine and personnel protection at high-power X-ray free-electron lasers. We present the design, operation, and performance of the X-Ray Flux Capacitor (XRFC) burn-through monitor (BTM), a parallel-plate capacitor device implemented as a printed circuit board, intended to provide an inexpensive, scalable, and fail-safe beam containment monitoring solution when used with appropriate readout electronics. The capacitor BTM operates on the principle that focused high-power X-rays degrade the FR-4 dielectric between the capacitor layers, resulting in a high-voltage electrical short that is easily detectable. Ninety-two high-voltage prototype capacitor BTMs were tested at various beam parameters at the Linac Coherent Light Source XCS and XPP endstations, using X-ray energies between 7.17 keV and 13.1 keV. All capacitor BTMs shorted when exposed to beam conditions with sufficient energy density, with shorting thresholds ranging from 0.7 eV atom ⁻¹ pulse ⁻¹ to 5.8 eV atom ⁻¹ pulse ⁻¹ (in copper), depending on X-ray energy and beam focus. In all cases, a high-voltage short was detected before full burn-through of the capacitor BTM occurred. These results suggest that the XRFC BTM concept can be used as a reliable, fast-response diagnostic for beam integrity monitoring in high-flux environments.

The chemRIXS instrument at the Linac Coherent Light Source (LCLS) offers new opportunities for studying solution-phase systems with time-resolved soft x-ray spectroscopic methods such as resonant inelastic x-ray scattering (RIXS) using the recently commissioned high-repetition-rate LCLS-II x-ray free-electron laser. The orders-of-magnitude x-ray flux improvement provided by the superconducting accelerator, combined with corresponding advances in the optical laser system and the liquid-jet recirculation system, enables studies on dilute systems with high signal-to-noise compared to what was possible with the LCLS-I copper accelerator. These capabilities open up time-resolved x-ray absorption spectroscopy and RIXS to entirely new classes of samples, as well as enable the development of new soft x-ray spectroscopic methods on liquid samples. An overview of the beamline components and the first LCLS-II commissioning results is presented.

The Linac Coherent Light Source II, a high-repetition-rate X-ray free-electron laser (FEL), produces high average power as well as high peak power. Air is a key radiation safety element, helping to contain the FEL beams from reaching accessible areas. At high average power, air absorption can produce a high-temperature and low-density channel along the X-ray beam path. In this case, the X-ray attenuation no longer follows the Beer–Lambert equation, exp(−μ x ). Air attenuation measurements were performed with X-rays focused into a gas cell at the LCLS time-resolved AMO instrument. In the measurements, several parameters were varied: the repetition rate from 102 Hz to 33.2 kHz, the pulse energy from 18 to 340 µJ and the photon energy at 316 and 346 eV. The air transmission was observed to significantly increase with X-ray power. Simulations were also performed, which included the thermal processes related to the high average power. In comparison with the measurements, the calculated air transmission is higher, confirming that the simulations are conservative and suitable for radiation safety analyses.

Intense x-ray pulses can cause the non-thermal structural transformation of diamond. At the SACLA XFEL facility, pump x-ray pulses triggered this phase transition, and probe x-ray pulses produced diffraction patterns. Time delays were observed from 0 to 250 fs, and the x-ray dose varied from 0.9 to 8.0 eV/atom. The intensity of the (111), (220), and (311) diffraction peaks decreased with time, indicating a disordering of the crystal lattice. From a Debye–Waller analysis, the rms atomic displacements perpendicular to the (111) planes were observed to be significantly larger than those perpendicular to the (220) or (311) planes. At a long time delay of 33 ms, graphite (002) diffraction indicates that graphitization did occur above a threshold dose of 1.2 eV/atom. These experimental results are in qualitative agreement with XTANT+ simulations using a hybrid model based on density-functional tight-binding molecular dynamics.

X-ray laser-induced structural changes in silicon undergoing femtosecond melting have been investigated by using an x-ray pump-x-ray probe technique. The experimental results for different initial sample temperatures reveal that the onset time and the speed of the atomic disordering are independent of the initial temperature, suggesting that equilibrium atomic motion in the initial state does not play a pivotal role in the x-ray-induced ultrafast melting. By comparing the observed time-dependence of the atomic disordering and the dedicated theoretical simulations, we interpret that the energy transfer from the excited electrons to ions via electron-ion coupling (thermal effect) as well as a strong modification of the interatomic potential due to electron excitations (non-thermal effect) trigger the ultrafast atomic disordering. Our finding of the interplay of thermal and non-thermal effects in the x-ray-induced melting demonstrates that accurate modeling of intense x-ray interactions with matter is essential to ensure a correct interpretation of experiments using intense x-ray laser pulses.

Shock experiments give a unique insight into the behavior of matter subjected to extremely high pressures and temperatures. Understanding the behavior of materials under such extreme conditions is key to modeling material failure and deformation dynamics under impact. While studies on pure silica are extensive, the shock behavior of other commercial silicates that contain additional oxides has not been systematically investigated. To better understand the role of composition in the dynamic behavior of silicates, we performed laser-driven dynamic compression experiments on soda-lime glass (SLG) up to 315 GPa. Using the accurate pulse shaping offered by the long pulse laser system at the Matter in Extreme Conditions end-station at the Linac Coherent Light Source, SLG was shock compressed along the Hugoniot to multiple pressure-temperature points. Velocity Interferometer System for Any Reflector was used to measure the velocity and determine the pressure inside the SLG. The Us–up relationship obtained agrees well with the previous parallel plate impact studies. Within the error bars, no transformation to the crystalline phase was observed up to 70 GPa, which is in contrast to the behavior of pure silica under shock compression. Our studies show that the glass composition strongly influences the shock compression behavior of the silicate glasses.

Ronchi testing of a focused electromagnetic wave has in the last few years been used extensively at X-ray free-electron laser (FEL) facilities to qualitatively evaluate the wavefront of the beam. It is a quick and straightforward test, is easy to interpret on the fly, and can be used to align phase plates that correct the focus of aberrated beams. In general, a single Ronchigram is not sufficient to gain complete quantitative knowledge of the wavefront. However the compound refractive lenses that are commonly used at X-ray FELs exhibit a strong circular symmetry in their aberration, and this can be exploited. Here, a simple algorithm that uses a single recorded Ronchigram to recover the full wavefront of a nano-focused beam, assuming circular symmetry, is presented, and applied to experimental measurements at the Matter in Extreme Conditions instrument at the Linac Coherent Light Source.

For LCLS-II, a fluorescence intensity monitor for the non-invasive, pulse-by-pulse normalization of experiments has been developed. A prototype diagnostic was constructed with a microchannel plate assembly and two photodiodes. The diagnostic was then installed in the LCLS SXR instrument Kirkpatrick–Baez mirror chamber with the detectors located above the vertically reflecting mirror. The linearity, noise and position sensitivity of the detectors have been characterized. The photodiode responsivity is suitable for high pulse energies. The microchannel plate detector shows sufficient responsivity over a wide range of pulse energies. The relative signal from the two photodiodes provides a sensitive measure of the X-ray beam position. The fluorescence intensity monitor provides intensity normalization while being compatible with high incident power, a 0.93 MHz repetition rate and ultra-high vacuum.

For the LCLS-II X-ray instruments, laser power meters are being developed as compact X-ray power diagnostics to operate at soft and tender X-ray photon energies. These diagnostics can be installed at various locations along an X-ray free-electron laser (FEL) beamline in order to monitor the transmission of X-ray optics along the beam path. In addition, the power meters will be used to determine the absolute X-ray power at the endstations. Here, thermopile power meters, which measure average power, and have been chosen primarily for their compatibility with the high repetition rates at LCLS-II, are evaluated. A number of characteristics in the soft X-ray range are presented including linearity, calibrations conducted with a photodiode and a gas monitor detector as well as ultra-high-vacuum compatibility tests using residual gas analysis. The application of these power meters for LCLS-II and other X-ray FEL sources is discussed.