Kinetic simulations of laser-induced plume expansion into a background gas under conditions of spatial confinement

Expansion of vapor plume induced by short-pulse laser irradiation of a bottom of a cylindrical cavity or planar trench in a copper target into helium, argon, and xenon background gases at pressure varying from zero to 1 bar is studied numerically. The simulations are performed based on a hybrid computational model that includes an unsteady heat conduction equation, Hertz-Knudsen model of evaporation, and a kinetic model of two-component gas flow implemented in the form of the Direct Simulation Monte Carlo method. It is found that increasing background pressure results in the formation of a complex structure of shock waves, which drastically changes the flow compared to the case of expansion into a vacuum. The moving shocks strongly affect the deposition of the ablated material inside the cavity. In particular, they induce the formation of a near-surface layer with a reduced fraction of vapor, which protects the cavity wall from vapor redeposition. The overall effect of the increasing background gas pressure on the flow structure and redeposition of vapor inside the cavity can be described as a trade-off between the confinement effect, i.e. a reduction in the propagation speed of the primary shock and overall rate of the plume expansion, and the focusing effect of moving shock waves that induce the radial flow towards the cavity axis, increase vapor density around the axis of symmetry, and reduce vapor density at the cavity wall. The efficiency of vapor removal out of the cavity can increase or decrease with increasing molar mass of the background gas depending on whether the flow is dominated by the focusing or confinement effect. These findings suggest that tuning the background gas parameters and geometrical parameters of spatial confinement can be used to effectively control the laser-induced plume expansion process in applications of laser ablation. (C) 2018 Elsevier Ltd. All rights reserved.