Measurements of super-sonic to sub-sonic transition within SiO2 foams at OMEGA

A comparison of simulated shock interactions with x-ray ablation fronts to experimental data has shown some disagreement. Specifically, the measured shock front history produced does not show the sharp increase in velocity predicted by a reflected shock interacting with an x-ray ablation front. To study the interaction of strong shocks interacting with diffuse pressure fronts, we developed a platform to study the transition of a supersonic shock into a subsonic shock within silica aerogel on the OMEGA laser facility. The target package consists of a halfraum that is driven by 21 Omega beams to create an ~110 eV radiation drive, and a cylinder that contains a low density (32 mg/cc) silica (SiO2) aerogel. The radiation from the halfraum induces a supersonic radiation transport within the aerogel that travels along the cylinder. Reduction of the energy density in the radiation front along the cylinder transitions radiation transport from the supersonic to the subsonic regime and creates a pressure front which creates a pressure-induced density perturbation within the aerogel. The time history of the density change is studied by using X-ray radiography and a time-gated , 4-strip, x-ray detector. Using hydrodynamic simulations of the entire experiment, we can match the experimental results with simulations.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.