Effect of the background gas pressure on the effectiveness of laser-induced material removal from deep cavities in irradiated targets

Abstract

Two-dimensional expansion of a plume, induced by short-pulse laser irradiation of a bottom of a cylindrical cavity in a copper target, into argon background gas at pressure ranging from 0 to 1 bar is studied numerically based on a hybrid computational model that includes a heat conduction equation for the irradiated target and a kinetic model of the gas mixture flow. It is found that an increasing pressure of non-reactive background gas can induce an order-of-magnitude increase in the effectiveness of removal of the laser ablation products from the cavity as compared to ablation under vacuum conditions. The effectiveness of the material removal, measured as the ratio of the mass of the ablated products leaving the cavity to the evaporated mass, is determined by the interplay between the confinement and focusing effects of the background gas, which decelerate the plume expansion and concentrate the ablation products far from the cavity wall, correspondingly. At small background gas pressure, the plume expansion is dominated by the focusing effect, which enhances the efficiency of the material removal. At large pressure, the confinement effect dominates and suppresses the material removal. As a result, there is an optimum background gas pressure that maximizes the efficiency of the material removal from the cavity. This optimum background gas pressure depends on the characteristic pressure in the plume and strongly increases with an increase in the absorbed intensity of laser radiation.