Observations of strong areal mass oscillations in a rippled target hit by a short pulse on the nike laser

When a multi-Mbar shock wave propagates into a laser target from a rough or non-uniformly irradiated surface, the shock front after a few rapidly decaying oscillations becomes planar. Much stronger oscillations of the shock front and the shocked mass have been theoretically predicted for the case when the shock front is unsupported. For example, after a short (sub-ns) laser pulse deposits finite energy in a target, the shock wave launched into it is immediately followed by a rarefaction wave. If the irradiated surface is rippled, theory and simulations predict strong oscillations of the areal mass perturbation amplitude in the target. We report the first experiments designed to observe this effect. They have become possible by adding short-driving-pulse capability to the Nike laser. The laser operated at peak intensity of 10¹⁴ W/cm², with Gaussian pulse FWHM 0.35 ns and focal spot flat top diameter 400 μm. The targets were planar plastic foils, 53 to 100 μm thick, rippled from the irradiated side at the wavelength 30 and 45 μm and peak-to-valley amplitude 4 to 6 μm. We have observed the predicted strong oscillations with the monochromatic x-ray imaging diagnostics fielded on Nike. The distribution of mass in the target is recorded by an x-ray camera providing spatial resolution in one dimension, perpendicular to the initial ripples, and continuously covering the whole time interval of shock propagation through the target. While the driving pulse is on, the areal mass perturbation amplitude grows by a factor ~2 due to the ablative Richtmyer-Meshkov instability. It then decreases, reverses phase, and reaches another maximum, noticeably larger than its initial value, before the shock breakout at the rear target surface.