Anomalously strong effects of plume contraction and material redeposition in nanosecond pulsed laser vaporization

Abstract

Long-term dynamics of vapor plumes induced by irradiation of a copper target in argon background gas by nanosecond laser pulses is studied numerically based on a one-dimensional hybrid computational model. The model includes a thermal model of the irradiated target and a kinetic model of the gaseous plume flow. The latter is implemented in the form of the direct simulation Monte Carlo method. The simulations, performed at moderate laser fluences in the range from 1.5 to 4 Jcm−2 and background gas pressures between 0.01 and 1 bar, show that the long-term plume dynamics can be divided into three major stages: initial inertial expansion, plume contraction, and subsequent diffusive expansion. The simulations unexpectedly predict extremely strong effects of plume contraction, when the plume size can exhibit a two-fold reduction, and delayed material redeposition back to the irradiated surface, when ∼90% of vaporized material returns to the irradiated surface. Both plume contraction and delayed redeposition originate from the strong backward flow induced by the internal (secondary) shock wave that propagates from the mixing layer towards the irradiated surface. The major part of vaporized material condenses at the surface with a long time delay with respect to the laser pulse after the plume stopping and before the onset of diffusive expansion. As a result, only a marginal part of vapor, which can be as small as ∼5%, is retained in the plume by the beginning of the diffusive expansion. The obtained simulation results suggest that the plume contraction and delayed material redeposition are common phenomena for nanosecond laser vaporization and may affect the efficiency and quality of laser surface modification in the ablation regime as well.