Relaxation pathways in X-ray Free Electron Laser heated Iron

TL;DR

This paper models the relaxation pathways of iron under intense X-ray irradiation, highlighting the role of collisional M-shell ionization and its impact on satellite intensities, with implications for understanding non-thermal plasmas.

Contribution

It introduces the atomic model BigBarT for self-consistent evolution of electron distributions in X-ray heated iron, emphasizing the importance of collisional M-shell ionization.

Findings

M-shell satellite intensities are sensitive to non-thermal ionization.

Collisional M-shell ionization dominates relaxation processes.

The model can help refine collisional cross sections.

Abstract

Non-thermal photo-ionized plasmas are now established in the laboratory, and require models that treat the atomic processes and electron distribution self-consistently. We investigate the effects of inelastic thermalization in iron under intense X-ray irradiation using the atomic model BigBarT, suited for the self-consistent evolution of the electron continuum, including degeneracy effects. Our study focuses particularly on collisional $M$-shell ionization, which we identify as the dominant relaxation process of the non-thermal electrons. We show that $M$-shell satellite intensities are sensitive to non-thermal ionization, providing a potential method to refine collisional cross sections that are otherwise difficult to compute due to their proximity to the continuum and the associated plasma screening effects.