High-resolution inelastic x-ray scattering in warm dense matter

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The project aims at determining low-frequency structural dynamics in warm dense matter by highly resolved inelastic x-ray scattering. We jointly develop (a) the experimental platform at the High-Energy Density instrument at the European XFEL GmbH in Schenefeld (EuXFEL) and (b) the required theoretical framework by using density functional theory molecular dynamics simulations (DFT-MD) at the U Rostock. We treat Al and Fe in this project. The behavior of both materials under extreme conditions is of high interest but specific for our purposes. First, we investigate the simple metal Al which is experimentally and theoretically well manageable so that many data on the structural, thermodynamic, transport an optcial properties exist both for the solid and liquid state. This data set will be extended to the warm dense matter region in this project. Based on this benchmark study for Al we will treat Fe under such extreme conditions which are relevant for geophysics (solid inner and liquid outer core of Earth) as well as forplanetary physics (super-Earths). Warm dense matter is characterized by temperatures of few eV, high densities (condensed matter and beyond) and pressures (several Mbar). Such plasma parameters are generated by, e.g., irradiating the target with intense optical laser pulses for which the DiPOLE-100X laser will be availbale at EuXFEL (< 100 J, < 15 ns) from 2022 onwards. These highly transient excited states will then be probed by bright, ultrashort pulses with few meV spectra bandwidth from the EuXFEL. Since the experiments will allow for repetition rates of several Hz, the quality of the expected data goes far beyond current capabilities so that new insight in the structure and dynamics of warm dense matter, in particular of Al and Fe, will be gained. Simultaneously, the x-ray spectra will be determined by calculating the dynamic structure factor of warm dense matter (Al and Fe) by using DFT-MD simulations. Comparing the experimental data with a generalized hydrodynamic model finally allows us for the first time to directly extract the ion temperature, sound speed, thermal conductivity and viscosity for the extreme conditions of warm dense matter. In particular, the results for Fe will be essential for geophysics and planetary physics and will allow new insight in processes in the solid inner and liquid outer core of Earth. Furthermore, the new data will also allow to benchmark current models for the interior structure and evolution of super-Earths, besides mini-Neptunes the most abundant class of exoplanets.