Detecting of melting by changes of rear surface reflectivity in shocked compressed metals using an interferometric diagnostic method

When a high power laser (10(12) W/cm(2)) irradiates a target, it induces a shock wave, which reaches the (free) rear surface. The free surface is acce lerated and the-shock wave is back-reflected reflected as a rarefaction wav e. In the shock wave pressure regime involved here, melting of the target d uring the shock or during the rarefaction may occur. An optically recording velocity interferometric system (ORVIS) has been developed to measure the time evolution of the change in the reflectivity of the free surface. Shock waves of the order of hundreds of kilobars are produced in 50-125 mu m thi ck Sn and Al foils, by a Nd: YAG laser system with a wavelength of 1.06 mu m, pulse width of 7 ns (FNHM), and irradiance in the range (1.4-2.4) 10(13) W/cm(2) The changes in the reflectivity occur along two time scales: a slo w One, more than 17 ns in Al and more than 30 ns in Sn, and a rapid one, le ss than 2.5 ns, in both:materials. A possible explanation for the sharp dec reases in the time scale is that melting:occurs during the release of the-f ree surface.