Atomic processes in XFEL-driven plasmas

Over the past two decades, X-ray free-electron lasers (XFELs) have made significant progress, achieving peak brightness X-ray regions that were previously only attainable in the infrared. This has opened new possibilities in high-energy density science, enabling the creation of solid-density plasmas with larger volumes and better uniformity. XFEL heating proceeds through photoionisation and thermalisation, initially creating non-thermal electron distributions, offering a new platform for generating plasmas with tailored properties.


We investigate the effects of inelastic thermalisation in iron under intense X-ray irradiation using the atomic model BigBarT, which evolves the electron distribution self-consistently. Our study focuses on collisional M-shell ionisation, identified as the most efficient relaxation mechanism for non-thermal electrons.

This could impact high-energy electron transport and points to the relevance of including atomic processes in kinetic models. Comparing calculated spectra with experimental data may also help refine collisional cross sections, which are challenging to compute due to screening and continuum effects.

We also report the observation of the shake-off process in solid-density XFEL-produced plasmas. Titanium foils were irradiated with XFEL pulses at 5.1 and 6 keV. Emission spectra show satellite features only at the higher energy, consistent with the threshold for simultaneous K- and L-shell ionisation. These signatures are absent at lower energies and cannot be explained by standard collisional or Auger ionisation mechanisms.