ASYMPTOTIC MEASUREMENTS OF FREE-SURFACE INSTABILITIES IN LASER-INDUCED SHOCK-WAVES

An experimental technique based on optical scattering to detect meltin g in release of strongly shocked materials is presented. This method i s used to study the asymptotic behavior of the free surface of shock-l oaded materials. After reflection of a shock wave from a metallic samp le free surface, occurrence of a solid to liquid transition will induc e a dynamic behavior - such as mass ejection and development of instab ilities. A study of the mass ejection due to laser-induced shock waves in aluminium, copper, and tin targets is presented. Shock waves of or der of hundreds of kilobars to more tl-lan one megabar are produced by a Nd:YAG laser system with a wavelength of 1.06 mu m, pulse width of 7 ns FWHM focused to spot of 200 mu m. The velocities, size, and topol ogical structure of the ejected particles are measured. The radii of t he ejecta are in the range 0.5-7 mu m. The size distribution of the ej ected particles, moving ahead of the free surface, fit well to a power scaling law N(r) similar to r(-b), characteristic of percolation theo ries. The experimental values for b are in the range 3-4, depending on the material. Calculations of the threshold pressure for melting, bas ed on realistic equations of state (EOS), predict that in the experime nts reported here the Sn samples melt during the laser-induced shock w ave, while the Al and Cu samples melt during the release (rarefaction wave) following the shock wave. Two topological patterns of the ejecta clouds were observed: a shell-like pattern in Al and Cu and a jet-lik e pattern in Sn.