Structure in Radiative Shock Experiments
University of Michigan
The basic radiative shock experiment is a shock launched into a gas of high-atomic-number material at high velocities, which fulfill the conditions for radiative losses to collapse the post-shock material to high densities (over 20 times the initial gas density). This has been accomplished using the OMEGA Laser Facility by illuminating a beryllium ablator for 1 ns with a total of 4 kJ, launching the requisite shock, faster than 100 km/sec, into a polyimide shock tube filled with xenon. The experiments have lateral dimensions of approximately six hundred microns and axial dimensions of two to three millimeters, and are diagnosed by x-ray point-projection backlighting. Repeatable structure beyond the one-dimensional picture of a shock as a planar discontinuity was discovered in the experimental data. One form this took was that of two-dimensional, radial boundary effects near the tube walls, extended approximately seventy microns into the system. The cause of this effect -- low density wall material which is heated by radiation transport ahead of the shock, launching a new converging shock ahead of the main shock -- is apparently unique to high-energy-density experiments. Another form of structure is the appearance of small-scale perturbations in the post-shock layer, modulating the shock and material interfaces and creating regions of enhanced and diminished aerial density within the layer. The authors have applied an instability theory, a variation of the Vishniac instability of decelerating shocks, to describe the growth of these perturbations. This instability mirrors effects believed to be present in astronomically scaled systems involving decelerating, diverging blast waves.