Amortized Bayesian Inference of GISAXS Data with Normalizing Flows

Grazing-Incidence Small-Angle X-ray Scattering (GISAXS) is a modern imaging technique used in material research to study nanoscale materials. Reconstruction of the parameters of an imaged object imposes an ill-posed inverse problem that is further complicated when only an in-plane GISAXS signal is available. Traditionally used inference algorithms such as Approximate Bayesian Computation (ABC) rely on computationally expensive scattering simulation software, rendering analysis highly time-consuming. We propose a simulation-based framework that combines variational auto-encoders and normalizing flows to estimate the posterior distribution of object parameters given its GISAXS data. We apply the inference pipeline to experimental data and demonstrate that our method reduces the inference cost by orders of magnitude while producing consistent results with ABC

Observation of ultrafast solid-density plasma dynamics using femtosecond X-ray pulses from a free-electron laser

The complex physics of the interaction between short pulse high intensity lasers and solids is so far hardly accessible by experiments. As a result of missing experimental capabilities to probe the complex electron dynamics and competing instabilities, this impedes the development of compact laser-based next generation secondary radiation sources, e.g. for tumor therapy [Bulanov2002,ledingham2007], laboratory-astrophysics [Remington1999,Bulanov2015], and fusion [Tabak2014]. At present, the fundamental plasma dynamics that occur at the nanometer and femtosecond scales during the laser-solid interaction can only be elucidated by simulations. Here we show experimentally that small angle X-ray scattering of femtosecond X-ray free-electron laser pulses facilitates new capabilities for direct in-situ characterization of intense short-pulse laser plasma interaction at solid density that allows simultaneous nanometer spatial and femtosecond temporal resolution, directly verifying numerical simulations of the electron density dynamics during the short pulse high intensity laser irradiation of a solid density target. For laser-driven grating targets, we measure the solid density plasma expansion and observe the generation of a transient grating structure in front of the pre-inscribed grating, due to plasma expansion, which is an hitherto unknown effect. We expect that our results will pave the way for novel time-resolved studies, guiding the development of future laser-driven particle and photon sources from solid targets

Ultra-fast yttrium hydride chemistry at high pressures via non-equilibrium states induced by x-ray free electron laser

Controlling the formation and stoichiometric content of desired phases of materials has become a central interest for the study of a variety of fields, notably high temperature superconductivity under extreme pressures. The further possibility of accessing metastable states by initiating reactions by x-ray triggered mechanisms over ultra-short timescales is enabled with the development of x-ray free electron lasers (XFEL). Utilizing the exceptionally high brilliance x-ray pulses from the EuXFEL, we report the synthesis of a previously unobserved yttrium hydride under high pressure, along with non-stoichiometric changes in hydrogen content as probed at a repetition rate of 4.5\,MHz using time-resolved x-ray diffraction. Exploiting non-equilibrium pathways we synthesize and characterize a hydride with yttrium cations in an \textit{A}15 structure type at 125\,GPa, predicted using crystal structure searches, with a hydrogen content between 4.0--5.75 hydrogens per cation, that is enthalpically metastable on the convex hull. We demonstrate a tailored approach to changing hydrogen content using changes in x-ray fluence that is not accessible using conventional synthesis methods, and reveals a new paradigm in metastable chemical physics

Observation of Ultrafast Solid-Density Plasma Dynamics Using Femtosecond X-Ray Pulses from a Free-Electron Laser

The complex physics of the interaction between short-pulse ultrahigh-intensity lasers and solids is so far difficult to access experimentally, and the development of compact laser-based next-generation secondary radiation sources, e.g., for tumor therapy, laboratory astrophysics, and fusion, is hindered by the lack of diagnostic capabilities to probe the complex electron dynamics and competing instabilities. At present, the fundamental plasma dynamics that occur at the nanometer and femtosecond scales during the laser-solid interaction can only be elucidated by simulations. Here we show experimentally that small-angle x-ray scattering of femtosecond x-ray free-electron laser pulses facilitates new capabilities for direct in situ characterization of intense short-pulse laser-plasma interactions at solid density that allows simultaneous nanometer spatial and femtosecond temporal resolution, directly verifying numerical simulations of the electron density dynamics during the short-pulse high-intensity laser irradiation of a solid density target. For laser-driven grating targets, we measure the solid density plasma expansion and observe the generation of a transient grating structure in front of the preinscribed grating, due to plasma expansion. The density maxima are interleaved, forming a double frequency grating in x-ray free-electron laser projection for a short time, which is a hitherto unknown effect. We expect that our results will pave the way for novel time-resolved studies, guiding the development of future laser-driven particle and photon sources from solid targets

Visualizing Ultrafast Kinetic Instabilities in Laser-Driven Solids using X-ray Scattering

Ultra-intense lasers that ionize and accelerate electrons in solids to near the speed of light can lead to kinetic instabilities that alter the laser absorption and subsequent electron transport, isochoric heating, and ion acceleration. These instabilities can be difficult to characterize, but a novel approach using X-ray scattering at keV energies allows for their visualization with femtosecond temporal resolution on the few nanometer mesoscale. Our experiments on laser-driven flat silicon membranes show the development of structure with a dominant scale of ~60\unit{nm} in the plane of the laser axis and laser polarization, and ~95\unit{nm} in the vertical direction with a growth rate faster than $0.1/\mathrm{fs}$. Combining the XFEL experiments with simulations provides a complete picture of the structural evolution of ultra-fast laser-induced instability development, indicating the excitation of surface plasmons and the growth of a new type of filamentation instability. These findings provide new insight into the ultra-fast instability processes in solids under extreme conditions at the nanometer level with important implications for inertial confinement fusion and laboratory astrophysics

Optimizing laser coupling, matter heating, and particle acceleration from solids using multiplexed ultraintense lasers

Realizing the full potential of ultrahigh-intensity lasers for particle and radiation generation will require multi-beam arrangements due to technology limitations. Here, we investigate how to optimize their coupling with solid targets. Experimentally, we show that overlapping two intense lasers in a mirror-like configuration onto a solid with a large preplasma can greatly improve the generation of hot electrons at the target front and ion acceleration at the target backside. The underlying mechanisms are analyzed through multidimensional particle-in-cell simulations, revealing that the self-induced magnetic fields driven by the two laser beams at the target front are susceptible to reconnection, which is one possible mechanism to boost electron energization. In addition, the resistive magnetic field generated during the transport of the hot electrons in the target bulk tends to improve their collimation. Our simulations also indicate that such effects can be further enhanced by overlapping more than two laser beams

Conceptual Design Report: Scientific Instrument High Energy Density Physics (HED)

High energy density (HED) matter is defined as having an energy density above $10^{11}$ $J•m^{-3}$, which is equivalent to 100 GPa (1 Mbar) pressure and 500 T magnetic pressure. The scientific objectives of the High Energy Density Physics instrument (HED instrument) at the European XFEL are to create and explore HED states using the particular features of X-ray free-electron laser (FEL) radiation. Strongly excited solid but below-HED states are also considered to be a key science area, as such transient states are precursors to the formation of HED states in laboratory experiments. This report describes the conceptual design of the HED instrument at the European XFEL. The HED instrument will be one of the six baseline instruments of the facility. It will provide to a wide community of condensed-matter, plasma, and high-power laser physicists a scientific instrument for experiments not possible anywhere else in the world.This report is based on the requirements emerging from the scientific objectives of the HED instrument and the anticipated X-ray techniques to be fielded at this instrument. Major subsystems of the HED instrument are described, including the beam transport elements, the instrument components, and the requirements for further subsystems and infrastructure. Furthermore, the interfaces of the HED instrument to other work packages of the European XFEL project are described with the goal of realizing a highly integrated project

The High Energy Density science instrument at the European XFEL, Hamburg, Germany: a new platform for shock compression research

Free-electron laser facilities enable new applications in the field of high-pressure research including geo- and planetary sciences. At the European X-ray Free Electron Laser (XFEL) in Hamburg, one of the six baseline instruments is dedicated to High Energy Density Science (HED). A 100 J optical laser with nanosecond pulse duration, high repetition rate and pulse shaping option will be integrated at this instrument, which will allow to shock compressing matter to very high internal pressures of up to 1 TPa and to off-Hugoniot states at lower pressures with ramped compression. This will enable to mimic conditions similar to the interior of Saturn, Neptune, Uranus, Earth and Venus. The extreme states of matter can then be probed with the FEL X-ray source. At European XFEL hard X-rays with photon energies of up to 25 keV will be available. At the HED instrument, several X-ray techniques will be realized to probe the samples such as X-ray diffraction, X-ray imaging and spectroscopy including XANES. The high intensity and time structure of the FEL beam will enable time-resolved pump-probe studies of the samples generated during dynamic compression. The envisaged instrument time resolution is in the range of a few 10s fs which is well below the time scales of phase transitions. In addition, the high brilliance and coherence of the FEL radiation will allow spatially resolved studies. The HED instrument is currently in its detailed design phase and first user operation is foreseen for late 2017. In this contribution, we will present the capabilities of the HED instrument with respect to geo- and planetary sciences, compare this to state-of-the-art in-situ techniques and will show planned first experiments of this instrument

Perspectives for studying planetary matter using intense X-ray pulses at the high energy density science instrument at the European XFEL

Free-electron laser facilities enable new applications in the field of high-pressure research including planetary materials. The European X-ray Free Electron Laser (European XFEL) in Hamburg, Germany will start user operation in 2017 and will provide photon energies of up to 25 keV. The high-energy density science instrument (HED) at the European XFEL is dedicated to the study of dense material at strong excitation in a temperature range from eV to keV and pressures > 100 GPa which is equivalent to an energy density > 100 J/mm$^{3}$. It will enable studying structural and electronic properties of excited states with hard X-rays at a repetition rate of up to 4.5 MHz. The instrument is currently in its technical design phase and first user experiments are foreseen for end of 2017. In this contribution, we present the X-ray instrumentation and the foreseen X-ray techniques at HED. In addition, we discuss prototype hard-condensed matter experiments in the field of planetary research as proposed during recent user consortium meetings for this instrument. These include optical laser induced quasiisentropic (ramped) compression and shock compression experiments and diamond anvil cell experiments