X-ray Absorption Spectroscopy Experiments of Radiatively Heated Argon gas

We report absorption spectroscopy results from argon gas-filled targets at the Omega-60 Laser Facility that are heated by an x-ray source created by a laser irradiated gold foil. We measured the foil temperature to be 80 eV and used Helios-CR to model the propagation of the radiative heat front through argon gas at 3 atm. To diagnose the experimental heat front, we use a capsule backlighter to measure changes in the absorption of the argon gas in the 2.7 to 3.7 keV range. The absorption analysis is complicated by significant structure in the backlighter spectrum, which arises from K-shell transitions in a sulfur contaminant and higher-order lines presented in the data. However, this structure offers independent energy fiducials that increase our confidence in the measured shift of the heated argon K-edge, which is shifted from the cold K-edge of argon by about 50 eV due to ionization. We compare the measured absorption spectrum to two independent atomic models, PrismSpect and SCRAM, and show that the observed K-edge shift is consistent with ionization to Ar IV, 3 times ionized, and temperatures of about 10 eV at 1.15 mm from the foil using a 5 ns drive.