Plasma Parameters in Short-Pulse-Heated Buried Tracer Layers via Fits of High-Resolution X-ray Spectra

A quartet of high-resolution x-ray crystal spectrometers was deployed at the Titan laser to measure thermal self-emission of heated Ti and Mn layers. Solid targets were produced with thin (0.1--1 $\mu$m) layers of mid-Z tracer elements sandwiched between Al foil and a thin Al tamp (0--4 $\mu$m). When exposed to the relativistic-intensity laser pulse ($>$100 J in 1 ps), targets heat comparably to undoped Al foils, but only the thin tracer layer emits fine structure x-rays visible to spectrometers. By shooting a set of targets with varied tracer layer (Ti, MnAl, or both), tracer thickness, and tamp thickness, the time-integrated x-ray flux can be measured at many localized depths in the target. The fine structure spectra of He- and Li-like Ti and Mn is gathered by spherically-curved crystals in the focusing Johann geometry. The spectra, composed of both isolated and overlapping line emission, are fit to a multi-Gaussian model by a genetic algorithm, extracting line widths, heights and positions. These parameters are compared to atomic physics calculations, populations of excited electronic states from collisional-radiative models, and line width predictions from Stark, Doppler and opacity broadening, all of which are used to infer plasma conditions in the buried layer region.