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Abstract

The formation and propagation of laser-driven shock waves has been observed by optical shadowgraphy in fused quartz, α-quartz and sodium chloride. Target materials were irradiated with a 0.53 µm , ~ 2.5 ns FWHM laser pulse at intensities ranging between 0.2 — 2 x 10¹³ W/cm², producing peak pressures varying from 0.3 — 3 Mbar at the shock front. Observations in both varieties of quartz reveal transient, high-speed shock propagation followed by deceleration towards a steady asymptotic shock speed. Similar high-speed transients were not seen in sodium chloride. The results in quartz were found to be in significant disagreement with both one-dimensional and two-dimensional hydrodynamic calculations based on equilibrium equations of state. The non-steady shock propagation is interpreted as being due to a relaxation process in the phase transformation of quartz into the high-pressure stishovite phase which occurs at the shock front. The effects of such a relaxation process on the shock dynamics and shock compression process are considered for the case of a direct relaxation from quartz into stishovite, as well as for an indirect relaxation process in which the -transformation of quartz into stishovite is preceded by shock-induced amorphization of the quartz. It is shown that either scenario would result in higher shock speeds and less compressible shock states than those obtained under equilibrium conditions.