Laser interaction in a water tank configuration: Higher confinement breakdown threshold and greater generated pressures for laser shock peening

The authors present a new configuration for laser-induced plasmas in confined regimes for a 10 ns-range laser pulse in the green wavelength (532 nm) that repulses the breakdown threshold above 20 GW/ cm2 compared to 8 GW/ cm2 as generally indicated in previous works. Using this new configuration, pressures above 12 GPa have been reached for the first time in confined regimes. This can enlarge the range of applications of laser shock such as the range of treatable materials (very high strength materials) or facilities’ costs since neither vacuum nor heavy laser systems will be needed to reach these levels of pressure. The proposed configuration mainly consists of the usage of a water tank. Hence, a great thickness of water is used instead of the extensively used thin water layer. Therefore, the water breakdown plasma will not initiate at the surface of the water, as the laser beam is still not focused there. Instead, it will occur in the depth of the water. In that case, the breakdown threshold value is increased as either the avalanche breakdown or the multiphoton ionization may start at higher laser intensities than at the air/water interface. The authors experimentally demonstrated this new breakdown threshold with the measurement of transmitted intensity, transmitted pulse duration, and the indirect measurement of the plasma pressure. Multiple shots (laser shock treatment) were also performed, and the specimen surface deformation was measured, leading to the same conclusion.