Accessing Ultra-High Pressure, Quasi-Isentropic States of Matter

Summary form only given. A new approach to materials science at extreme pressures has been developed on the OMEGA laser, using a ramped plasma piston drive. The laser drives a shock through a solid plastic reservoir that unloads at the rear free surface, expands across a vacuum gap, and stagnates on the metal sample under study. This produces a gently increasing ram pressure, compressing the sample nearly isentropically. The peak pressure on the sample, diagnosed with VISAR measurements, can be varied by adjusting the laser energy and pulse length, gap size, and reservoir density, and obeys a simple scaling relation. This has been demonstrated at OMEGA at pressures of P = 0.1-2.0 Mbar in Al foils. In an important application, using in-flight X-ray radiography, the material strength of solid-state samples at high pressure can be inferred by measuring the reductions in the growth rates (stabilization) of Rayleigh-Taylor (RT) unstable interfaces. The material strength is predicted to be as much as an order of magnitude higher at P ~ 1 Mbar than at ambient pressures. Initial RT measurements testing this prediction in foils of Al and V will be shown. We also use TEM microscopy of recovered targets to show that the samples never melted, and the presence of pressure-induced structural defects. Experimental designs based on this drive have been developed for the NIF laser, predicting that solid-state samples can be quasi-isentropically driven to pressures an order of magnitude higher than on Omega - accessing new regimes of dense, high-pressure matter