Radial Electron Temperature Profiles in Laser-Heated, Solid-Density Plasmas from X-ray Spectra Ensembles

Spectroscopic line ratios are a key means of measuring electron temperature in plasmas, even when plasma density is too high for most other diagnostics. However, laboratory plasmas at solid density are typically transient and contain extreme spatial gradients, such that line-ratio measurements are time- and volume-integrated over a range of plasma conditions. We investigate these temperature gradients with distributions of x-ray spectra, recorded from high-intensity laser interactions with solid targets, where the laser focal position varies with respect to embedded tracer "micro-stripes." Given the high repetition rate of the ALEPH laser facility, high-resolution x-ray spectra were collected in statistical ensembles: identical plasma conditions overlay the tracer micro-stripes with random radial offsets in each shot, so that a comprehensive range of stripe offsets is mapped over many shots. X-ray line ratios and thus x-ray-weighted electron temperatures change based on the relative stripe position. Collectively, ensembles of x-ray spectra are fit to radial profiles of electron temperature, allowing detailed study of electron heat transport in solid-density plasmas.