Investigation of the blast pressure following laser ablation at a solid-fluid interface using shock waves dynamics in air and in water

Simple and universal relations between the laser pulse energy of the third harmonic of a Nd:YAG laser source (5 ns) and the pressure at the ablation point are investigated for the ablation of a solid target in air and in water. Shockwaves propagation are reconstructed using time-resolved shadowgraph imaging. The propagation in air is well-described by Taylor-von Neumann-Sedov's theory for over-pressure at the shock front larger than ten times the atmospheric pressure, but the energy distribution appears to be anisotropic and mostly directed in the normal direction to the target's surface. We compare the pressure at the shock front deduced from its velocity using Rankine-Hugoniot's relation with the pressure deduced from Taylor's model only related to the laser pulse energy. Rankine-Hugoniot's relation appears more appropriate to evaluate the early pressure in the plasma plume. The shockwaves velocity in water is hyper-sonic during 50 ns and then reaches the sound velocity. Pressure values at the shock front are compared using two different approaches accounting for different state's equations, namely the Rice and Walsh's equation and Tait's equation. In both cases, the ablation pressure is found out to reach several GPa and increases as the square root of the laser intensity.