Dependence of Compressed Ablator Conditions on the Shell Adiabat in NIF Implosions

The x-ray continuum emitted from the hot spot of an inertial confinement fusion implosion around stagnation provides a probe to diagnose the $\rho R$, $n_{\mathrm{e}}$, and $T_{\mathrm{e}}$ of the compressed, Ge-doped CH ablator using the Ge K edge, 1$s$--2$p$ and 1$s$--3$p$ absorption features. Measured x-ray absorption spectra from a low-adiabat ($\alpha =$ $P_{\mathrm{ablator}}$/$P_{\mathrm{Fermi}})$, indirect-drive implosion and a high-$\alpha $, polar-drive implosion are compared. Using the Ge opacity calculations of the \textit{VISTA} code, the inferred quantities are $\rho R\sim $ 0.5 g/cm$^{\mathrm{2}}$, $n_{\mathrm{e}}\sim $ 0.5 $\times $ 10$^{\mathrm{26}}$ cm$^{\mathrm{-3}}$ and $T_{\mathrm{e}}\sim $ 200 eV for the low-$\alpha $ case, while lower compression and higher temperatures are inferred for the high-$\alpha $ case, consistent with radiation--hydrodynamics simulations. This material is based upon work supported by the Department of Energy National Nuclear Security Administration under Award Number DE-NA0001944.