Modeling of nanosecond laser ablation with vapor plasma formation

A thermal model for nanosecond pulsed laser ablation is developed, where the heat conduction equation in the target and the gas dynamic equations in the vapor and ambient gas phase are coupled through the Knudsen layer (KL) relations for evaporation/recondensation at the target-vapor interface. The plasma formation and laser-plasma interactions are simulated in the model, which are found to have a significant effect on the laser-induced evaporation process. The shielding effect of the plasma reduces the laser energy reaching the target surface and therefore decreases the surface temperature, and the laser energy deposition in the plasma contributes to the increase of the vapor pressure above the KL. All of these will make the transition earlier from sonic evaporation stage to the subsonic evaporation and then to the recondensation stage, and therefore decrease the laser-induced evaporation depth. The simulation results are compared with experimental data for the plasma transmissivity, plasma front locations and velocities, laser ablation depth, and average plasma temperatures, and reasonably good agreements are obtained. This model is valid when the phase explosion does not occur, that is, when the target surface temperature does not reach or exceed the target material critical temperature.