Shock waves from a water-confined laser-generated plasma

TLDR

The study aims to explain the limit of high‑amplitude shock‑wave generation by laser plasma in water confinement and to explore breakdown plasma effects at the water surface. High‑amplitude shock waves were generated by a 25–30 ns, 40 J, 1.064 µm pulsed laser in a water‑confined regime, with temporal and spatial profiles measured by a velocimetry interferometer, and plasma absorption reducing power density and shock‑wave duration. Above a 10 GW cm⁻² threshold, peak pressure saturates and pulse duration shortens due to parasitic plasma that forms mainly at the water surface, limiting process efficiency and explaining the observed generation limits.

Abstract

Generation of a high amplitude shock wave by laser plasma in a water confinement regime has been investigated for an incident 25–30 ns/40 J/λ=1.064 μm pulsed laser beam. Experimental measurements of temporal and spatial profiles of induced shock waves for this regime of laser shock processing of materials were performed using a velocimetry interferometer system for any reflector system. Above a 10 GW/cm2 laser intensity threshold, a saturation of the peak pressure is shown to occur while the pressure pulse duration is reduced by parasitic plasma occurring in the confining water. The observation of the interaction zone with a fast camera system shows that this breakdown plasma, which mainly occurs at the very surface of the water rather than within the water volume, limits the efficiency of the process. This plasma absorbs the incident laser energy, and the power density reaching the target gradually decreases with increasing power densities while the shock-wave duration is correspondingly reduced. Both pressure measurements and plasma observations allow explaining the current limit of high amplitude shock-waves generation by laser plasma in the water-confinement mode and open new research areas for the understanding of breakdown plasma effects at the surface of the confining water.