Shock waves in basalt rock generated with high-powered lasers in a confined geometry

An investigation was made into the interaction of Q-switched Nd: glass laser pulses, with energies of 5-100 J and intensities in the range of 1-15 GW cm^-2, with thin slab samples of basalt, in a geometry in which the laser-matter interaction was enhanced by confining the laser-induced plasma and vapor against the rock surface with a thin water layer. The high pressure generated during expansion of the ablated material produced a shock wave which traveled into the rock. On basalt samples of thicknesses of up to 18 mm, reflection of the shock wave at the rear surface of the sample caused a thin rock layer to be spalled. Examination of the structure within the interior of the rock after laser exposure revealed extensive grain disruption near to the laser spot, due to the shock-wave passage. Rock specimens of thickness 50 mm were tested via Vickers hardness microindenter. The laser exposure was shown to produce a hardness decrease within the rock which was up to 50%, and extended to depths of 25 mm beneath the target position. An empirical model accounting for shock-wave formation and propagation was developed and compared to the experimental results.