Preheat in x-ray driven equation-of-state experiments at the National Ignition Facility

The high power and energy of the National Ignition Facility enables equation-of-state (EOS) experiments at pressures exceeding 100 Mbar. With indirect drive experiments, soft x rays at energies up to 1 keV are created in a laser-irradiated hohlraum and used to drive high pressure shock waves into EOS samples. The shock wave velocities are measured and used to extract EOS data. However, hard x rays are also created and can preheat the targets. This can alter the pre-shock conditions of the samples and compromise the EOS measurements. One such effect is the expansion of a gold layer placed in the target package to protect the sample material from preheat. This launches an elastic wave, which can perturb the sample ahead of the main shock. The role of preheat is studied with computational radiation/hydrodynamic simulations. The simulation results are used to provide a) quantitative assessment of the errors introduced by the preheat effects, b) methods to correct for the preheat effects in the analysis of the data, and c) target designs to minimize the effects.