Laser pulse transmission through the water breakdown plasma in laser shock peening

Laser shock peening (LSP) under a water confinement regime can produce plasma pressures on the target surface four times higher and 2-3 times longer than that under direct regime configurations. However, when the laser power density is above some threshold, a breakdown plasma occurs in water, which screens a significant amount of the incident laser pulse and therefore limits the magnitude and duration of the pressure induced on the target surface. A self-closed numerical model that can simulate the laser pulse transmission through the breakdown plasma generated in water during LSP has rarely been reported in literature. In this work, the breakdown plasma is simulated by solving an electron rate equation coupled with a Maxwell's wave equation. The peak irradiance and duration of the laser pulse transmitted through the breakdown plasma predicted from the model can be correlated reasonably well with experimental data for 25 ns-1064 nm laser pulses. This model is then coupled with a previously developed thermal model for LSP to calculate the pressure pulse induced on the target surface. The trend of the pressure saturation and the pressure pulse duration decrease beyond some threshold laser irradiance is captured successfully by the model, and good agreements with experimental data have been obtained under a variety of LSP conditions. (c) 2006 American Institute of Physics.