X-ray Thomson Scattering from Dense Non-Equilibrium Hydrogen Plasmas*

Understanding the behavior of hydrogen in extreme environments is pivotal for unraveling astrophysical phenomena, such as the interiors of giant planets and stellar interiors, as well as the energy transport properties in short-lived fusion platforms like self-constricted plasmas in Z-pinches and inertial confinement fusion (ICF) experiments. The combination of the European XFEL’s ultrabright properties, a 5-10 Hz fs-laser, and a novel H₂ microjet has enabled the direct X-ray probing of isochorically heated hydrogen on ps-timescales. High repetition rate data collection has allowed for the accumulation and averaging of photon counts over many scattering events, resulting in a precise, background-limited dataset with a high signal-to-noise ratio.  Non-collective inelastic scattering provides a direct measurement of the thermal velocity distribution of free electrons in the plasma, via Doppler broadening. Under collective (plasmon) scattering, the intensity ratio between the upshifted (energy-gain) and the downshifted (energy-loss) plasmon signals is directly related to the temperature of the plasma, via detailed balance. Here we show, the utilization of high repetition rate data collection has enabled measurements of non-equilibrium hydrogen plasmas over a range of temperature states approaching T_e=400 eV, irradiated with fs-laser intensities of 1.5x10¹⁸ W/cm².