Investigation of Shock Heating and Heat-Front Penetration in Direct-Drive Targets Using Absorption Spectroscopy

Time-resolved Al 1$s$--2$p$ absorption spectroscopy was used to diagnose direct-drive, shock-heated, and compressed planar targets having nearly Fermi-degenerate predicted plasma conditions ($T_{e} \quad \sim $ 10 to 30 eV, $n_{e} \quad \sim $ 1 to 6 $\times $ 10$^{23}$ cm$^{-3})$. A 50-\textit{$\mu $}m-thick CH foil with a buried Al tracer layer was irradiated with 10$^{14}$ to 10$^{15 }$W/cm$^{2}$, and $\sim $1.5 keV x~rays from a point source Sm backlighter were transmitted through the drive foil. The measured absorption spectra were modeled with the atomic physics code PrismSPECT to infer $T_{e}$ and $n_{e}$. The shock heating and heat-front penetration were simulated with the 1-D hydrocode \textit{LILAC}, using a flux-limited or nonlocal transport model. Shock-heating observations are consistent with \textit{LILAC} for the lower drive intensity, but there is evidence of preheat for the higher one. The timing of the heat-front penetration is consistent with a time-dependent flux limiter. This work was supported by the U.S. Department of Energy Office of Inertial Confinement Fusion under Cooperative Agreement DE-FC52-92SF19460.