Effects of Mosaic Crystal Instrument Functions on X-ray Thomson Scattering Diagnostics

TL;DR

This paper analyzes how the instrument function of mosaic crystals affects X-ray Thomson scattering diagnostics, highlighting the importance of considering asymmetries in the instrument function to accurately infer plasma temperatures.

Contribution

It provides a detailed analysis of the instrument function of mosaic HAPG crystals and demonstrates the impact of asymmetries on temperature measurements in XRTS.

Findings

Asymmetry in the instrument function affects spectral broadening.

Ignoring asymmetry can lead to overestimation of plasma temperature.

Full consideration of the instrument function improves diagnostic accuracy.

Abstract

Mosaic crystals, with their high integrated reflectivities, are widely-employed in spectrometers used to diagnose high energy density systems. X-ray Thomson scattering (XRTS) has emerged as a powerful diagnostic tool of these systems, providing in principle direct access to important properties such as the temperature via detailed balance. However, the measured XRTS spectrum is broadened by the spectrometer instrument function (IF), and without careful consideration of the IF one risks misdiagnosing system conditions. Here, we consider in detail the IF of 40 $\mu$m and 100 $\mu$m mosaic HAPG crystals, and how the broadening varies across the spectrometer in an energy range of 6.7-8.6 keV. Notably, we find a strong asymmetry in the shape of the IF towards higher energies. As an example, we consider the effect of the asymmetry in the IF on the temperature inferred via XRTS for simulated 80 eV CH plasmas, and find that the temperature can be overestimated if an approximate symmetric IF is used. We therefore expect a detailed consideration of the full IF will have an important impact on system properties inferred via XRTS in both forward modelling and model-free approaches.