Gold L-shell Spectroscopy of Laser-Heated Hohlraums

Ablation of wall material in laser-heated hohlraums produces a high-temperature non-LTE plasma consisting of a high-Z material, usually gold, that expands against a low-Z gas fill. At electron temperatures of a few keV, gold becomes ionized into the M shell, as evidenced by soft X-ray line emission arising from ion-electron collisional processes. In the photon energy range 8-14 keV, gold L-shell spectra comprise three dominant features, each of which is an unresolved blend of inner-shell n=3 to n=2 emission lines simultaneously representing several charge states. Variations in electron temperature are accompanied by variations in the charge state distribution and the energy centroids of the emission-line blends. To first order, the energy centroids track the electron temperature, thereby providing a temperature diagnostic signature that is based on a “natural” hohlraum constituent. We present results of spectroscopic measurements and analyses of gold L-shell spectra acquired at the National Ignition Facility, including an evaluation of the level of consistency with K-shell tracers, sensitivity to electron density and radiation temperature, spectral model comparisons, and potential implications for radiation hydrodynamics simulations of hohlraums associated with ICF research.