X-ray spectroscopy of buried layer foils irradiated at laser intensities in excess of 10²⁰ W/ cm²

Observations of a rapid decrease in thermal temperature as a function of depth of solid targets irradiated with a short pulse, ultrahigh-intensity laser are reported. This phenomenon is investigated using the Titan short pulse laser with intensities greater than 1020 W/ cm² interacting with buried layer targets. The longitudinal temperature profile is determined by measuring K -shell spectra from a 0.4 μm copper tracer layer placed at various depths (i.e., 0-1.5 μm) within the 2.4 μm thick target. It is observed that the line ratios (He-like K -shell lines) as a function of temperature require a consideration of at least three parameters to analyze the K -shell spectra: hot electron population, time-dependent plasma conditions, and opacity. Here, the study of the effect of these three parameters on measured spectra in the short pulse high intensity laser-matter interactions using the atomic model FLYCHK [H.-K. Chung, High Energy Density Phys. 1, 3 (2005)] is presented.