The effect of bound states on X-ray Thomson scattering for partially ionized plasmas

TL;DR

This paper enhances X-ray Thomson scattering models by incorporating bound electron contributions in partially ionized plasmas, crucial for accurate plasma diagnostics with new X-FEL sources in high energy density physics.

Contribution

It introduces an improved scattering model that includes bound electrons, validated with experimental data, and predicts spectral features for higher Z plasmas affecting plasma parameter measurements.

Findings

Bound electrons significantly alter scattering spectra.

Additional spectral peaks predicted for high Z materials.

Bound contributions impact plasma temperature and density inference.

Abstract

X-ray Thomson scattering is being developed as a method to measure the temperature, electron density, and ionization state of high energy density plasmas such as those used in inertial confinement fusion. X-ray laser sources have always been of interest because of the need to have a bright monochromatic x-ray source to overcome plasma emission and eliminate other lines in the background that complicate the analysis. With the advent of the xray free electron laser (X-FEL) at the SLAC Linac Coherent Light Source (LCLS) and other facilities coming online worldwide, we now have such a source available in the keV regime. Most Thomson scattering codes used to model experimental data greatly simplify or neglect the contributions of the bound electrons to the scattered intensity. In this work we take the existing models of Thomson scattering that include elastic ion-ion scattering and inelastic electron-electron scattering and add the contribution of bound electrons in the partially ionized plasmas. To date, most experiments have studied hydrogen or beryllium plasmas. We first analyze existing experimental data for beryllium to validate the code. We then consider several higher Z materials such as Cr and predict the existence of additional peaks in the scattering spectrum that requires new computational tools to understand. For a Sn plasma, we show that bound contributions change the shape of the scattered spectrum in a way that would change the plasma temperature and density inferred from experiment.