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 lase r 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 v apor against the rock surface with a thin water layer. The high pressure ge nerated during expansion of the ablated material produced a shock wave whic h 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 thi n rock layer to be spalled. Examination of the structure within the interio r of the rock after laser exposure revealed extensive grain disruption near to the laser spot, due to the shock-wave passage. Rock specimens of thickn ess 50 mm were tested via Vickers hardness microindenter. The laser exposur e 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 empiri cal model accounting for shock-wave formation and propagation was developed and compared to the experimental results. (C) 1999 American Institute of P hysics. [S0021-8979(99)07222-9].