Theory of Thomson scattering in inhomogeneous media

TL;DR

This paper develops a theoretical framework to understand how Thomson scattering spectra are affected by inhomogeneities and fluctuations in plasma properties, which are common in laser-produced high-energy-density plasmas, impacting diagnostic accuracy.

Contribution

It introduces a modified cross section formula for Thomson scattering in inhomogeneous and dynamic media, highlighting the need for careful spectral interpretation in such conditions.

Findings

Thomson scattering spectra are significantly altered in inhomogeneous media.

Inhomogeneities can cause violations of detailed balance in spectra.

Accurate plasma diagnostics require accounting for spatial and temporal variations.

Abstract

Thomson scattering of laser light is one of the most fundamental diagnostics of plasma density, temperature and magnetic fields. It relies on the assumption that the properties in the probed volume are homogeneous and constant during the probing time. On the other hand, laboratory plasmas are seldom uniform and homogeneous on the temporal and spatial dimensions over which data is collected. This is partic- ularly true for laser-produced high-energy-density matter, which often exhibits steep gradients in temperature, density and pressure, on a scale determined by the laser focus. Here, we discuss the modification of the cross section for Thomson scattering in fully-ionized media exhibiting steep spatial inhomogeneities and/or fast temporal fluctuations. We show that the predicted Thomson scattering spectra are greatly altered compared to the uniform case, and may even lead to violations of detailed balance. Therefore, careful interpretation of the spectra is necessary for spatially or temporally inhomogeneous systems.