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High energy density physics 32, 63 - 69 (2019). doi:10.1016/j.hedp.2019.05.002, We present results from the SPring-8 Angstrom Compact free electron LAser (SACLA) X-ray free electron laser (XFEL) facility, using an X-ray pump, X-ray probe scheme to observe ultrafast changes in the structure of heated graphite. The 9.8 keV XFEL beam was focused to give an intensity on the order of ∼ 10$^{19}$ W/cm$^2$, and the evolution of the diffraction pattern observed up to delays of 300 fs. The interplanar diffraction peaks weaken significantly within 10s of femtoseconds, but in-plane diffraction orders i.e. those with Miller Index $(hk0)$, persist up to 300 fs, with the observed signal increasing. We interpret this as nonthermal damage through the breaking of interplanar bonds, which at longer timescales leads to ablation by removal of intact graphite sheets. Post-experiment examination of the graphite samples shows damage which is comparable in size to the range of the excited photoelectrons. These results highlight the challenges of accurately modelling X-ray driven heating, as it becomes a routine approach to generating high energy density states., Published by Elsevier, Amsterdam [u.a.]

In Uranus and Neptune methane and other hydrocarbons are highly abundant. Their planetary interior conditions can be mimicked using high intensity lasers in the laboratory on a nanosecond timescale. Nanodiamond formation from shock-compressed polystyrene (~150GPa, ~5000K) was demonstrated via in situ X-ray diffraction with a XFEL. The lower size estimate is 4nm. 60% of the carbon atoms in the plastic are transferred to a diamond lattice. However, in total a maximum of ~16μg of nanodiamonds are expected from a 125nm CH foil and a 500μm focal spot. In order to understand the underlying hydrocarbon separation mechanism the physical recovery of nanodiamonds is pursued to learn from their shape, size, surface modifications and defects.

The accurate description of electrons at extreme density and temperature is of paramount importance for, e.g., the understanding of astrophysical objects and inertial confinement fusion. In this context, the dynamic structure factor S(q,w) constitutes a key quantity as it is directly measured in X-ray Thomson (XRTS) scattering experiments and governs transport properties like the dynamic conductivity. In this work, we present the first ab initio results for S(q,w) by carrying out extensive path integral Monte Carlo simulations and developing a new method for the required analytic continuation, which is based on the stochastic sampling of the dynamic local field correction G(q,w). In addition, we find that the so-called static approximation constitutes a promising opportunity to obtain high-quality data for S(q; !) over substantial parts of the warm dense matter regime.

Mixtures of carbon and hydrogen at megabar conditions and at temperatures below one electronvolt can be found in a variety of planets of our solar system like Uranus and Neptune and are believed to feature prominently in extrasolar planets. The equation of state, transport and mixing properties of carbo-hydrates thus strongly influence the inner structure and evolution of these astrophysical objects. We present results of density-functional molecular dynamics simulations for the structure and mixing properties of carbo-hydrates at such conditions in order to explain recent experimental results showing the demixing of carbon and hydrogen and the formation of nano-diamonds at double shock conditions.

© 2017 The Author(s). The effects of hydrocarbon reactions and diamond precipitation on the internal structure and evolution of icy giant planets such as Neptune and Uranus have been discussed for more than three decades1. Inside these celestial bodies, simple hydrocarbons such as methane, which are highly abundant in the atmospheres2, are believed to undergo structural transitions3,4that release hydrogen from deeper layers and may lead to compact stratified cores5-7. Indeed, from the surface towards the core, the isentropes of Uranus and Neptune intersect a temperature-pressure regime in which methane first transforms into a mixture of hydrocarbon polymers8, whereas, in deeper layers, a phase separation into diamond and hydrogen may be possible. Here we show experimental evidence for this phase separation process obtained by in situ X-ray diffraction from polystyrene (C8H8)nsamples dynamically compressed to conditions around 150 GPa and 5,000 K; these conditions resemble the environment around 10,000 km below the surfaces of Neptune and Uranus9. Our findings demonstrate the necessity of high pressures for initiating carbon-hydrogen separation3and imply that diamond precipitation may require pressures about ten times as high as previously indicated by static compression experiments4,8,10. Our results will inform mass-radius relationships of carbon-bearing exoplanets11, provide constraints for their internal layer structure and improve evolutionary models of Uranus and Neptune, in which carbon-hydrogen separation could influence the convective heat transport7.

Powerful femtosecond pump lasers and femtosecond x-ray probe beams created by free electron lasers make it possible to create and diagnose strongly correlated states of matter in which the electron and ion Wigner distribution functions are of a strongly non-equilibrium type. We therefore present a quantum theory for the dynamic structure factors in just such non-equilibrium, correlated, two-component systems. To make our theory applicable for x-ray scattering as in equilibrium, a generalized Chihara decomposition for the total electron structure factor in non-equilibrium is derived. Examples are given and the influence of correlations and exchange on the structure and the x-ray scattering spectrum are discussed for a model non-equilibrium distribution, as often encountered during laser heating of materials, as well as for two-temperature systems.

Particle-in-Cell (PIC) codes are the the working horses of computational modeling in plasma physics, as they describe even the most turbulent kinetic processes from first-principles. But great applicability comes with great computational demand, requiring leadership-scale HPC systems in order to model full 3D geometries with solid-density. This poster shows the open software architecture of the world's fastest PIC code PIConGPU which is addressing these demands for the community. With a blazingly short time-to-solution, GPU-powered high-performance computing opens unique opportunities for studying transient plasma processes, e.g. by including XFEL photon distributions from simex_platform. Trading speed for enhanced predictive capabilities, we including collisional-radiative non-LTE models from SCFLY into the electro-magnetic PIC cycle.

The state and the evolution of planetary cores, drown dwarfs and neutron star crusts is determined by microscopic properties, such as viscosity and thermal conductivity, of the dense and compressed matter of which such system are made of. Due to the inherent diffculties in modelling strongly coupled plasmas, where classical long-range Coulomb forces dominate interactions between ions while electrons are partially to fully degenerate, current predictions of transport coeffcients differ by many orders of magnitude. This not only affects our ability to understand the evolution of planets and evolved stars, but also impacts our ability to accurately predict the implosion material characteristics in inertial confinement fusion experiments. The response of the compressed matter to density perturbations gives rise to collective modes, either electrostatic or acoustic waves, that serve as an important tool to validate theoretical predictions. Until recently, only electron modes could be measured experimentally. With the recent advances in free electron laser technology, x-rays with small enough bandwidth have become available, allowing the investigation of the low-frequency ion modes in dense matter as well. Here, we present numerical predictions for these ion modes and demonstrate signifcant changes to their strength and dispersion if dissipative processes are included by Langevin dynamics. In particular, a strong diffusive mode around zero frequency arises which is not present, or much weaker, in standard simulations. Our results have profound consequences in the interpretation of transport coefficients in dense plasmas.

The dynamic and static structure factors contain a wealth of information about the state and properties of a physical system. The structure of a system relates directly to its equation of state, collective modes, stopping power and relaxation processes. Since x-ray and particle scattering are extremely important experimental tools to diagnose warm dense matter, theoretical predictions of the structure have become one of the best modes of comparison between theory and experiment in the warm dense matter regime. We give an overview of state of the art theoretical methods to calculate the structure of warm dense matter in equilibrium and non-equilibrium. Results are given for such states as created in recent experiments. Limits of the current theories and possible future developments are discussed.

This talk shows the progress of PIConGPU on modeling transient high-energy density plasmas for the EUCALL annual workshop. We report on new features in PIConGPU, challenges in HPC-scale I/O for PIC simulations and how we interact with simex_platform and our approach for collisional-radiative non-LTE modeling within the scope of particle-in-cell.

PIConGPU reportedly is the fastest particle-in-cell code in the world with respect to sustained Flop/s. Written in performance-portable, single-source C++ we constantly push the envelope towards Exascale laser-plasma modeling. However, solving previously week-long simulation tasks in a few hours with a speedy framework is only the beginning. This talk will present the architecture and recent additions driving PIConGPU. As we speak, we run on the fastest machines and the community approaches a new generation of TOP10 clusters. Within those, many-core computing architectures and severe limitations in available I/O bandwidth demand fundamental rethinking of established modeling workflows towards in situ-processing. We present our ready-to-use open-source solutions and address scientific repeatability, data-reduction in I/O, predictability and new atomic modeling for XFEL pump-probe experiments.

We present a quantum theory for the dynamic structure factors in non-equilibrium, non-ideal, two-component warm dense matter. This general framework allows the derivation of expressions for the local field corrections in non-equilibrium. Based on a perturbation expansion of the non-equilibrium polarization function in terms of the interaction strength and taking the Wigner function as input quantity, we calculate the dynamic structure for a variety of typical scenarios and demonstrate typical effects. Example situations include laser heated matter or shock produced warm dense matter. We provide a generalized Chihara decomposition of the total dynamic electron structure factor. The formula features the free electrons, the non-equilibrium ion structure, the generalized non-equilibrium screening cloud and an additional term arising due to not invoking the Born-Oppenheimer approximation. This puts the theory of x-ray scattering in non-equilibrium on a sound theoretical basis and makes x-ray scattering a possible diagnostics for non-equilibrium warm dense matter on all time scales, in particular on femtosecond electronic time scales. We give examples for the expected x-ray scattering signals in laser heated systems and in two-temperature systems. We discuss the possibility to resolve temperature relaxation using x-ray scattering and point out possible problems in the current models used for the description of such relaxation processes.

Calculating Debye-Scherrer diffraction patterns from polycrystalline materials under dynamic compression has been done in the Voigt (iso-strain) limit or in the Reuss (iso-stress) limit for small deviatoric stresses. These methods are appropriate for materials with low yield strength, where only small deviations from the hydrostat can be supported by the crystal structure or materials with low elastic anisotropy where the Voigt limit is valid. Here we present a method to calculate Debye-Scherrer diffraction patterns from polycrystalline samples under an arbitrary stress field. This method can calculate the strain tensor for each crystallite and can account for arbitrary sample texture and probe x-ray sources.

GSI Scientific Report 2014 - GSI Report 2015-1