Revealing Non-Equilibrium Effects in Laser-Induced Plasma Plumes Using Combined Direct Simulation Monte-Carlo and Collision-Radiation Model

Laser material processing has applications in pulsed laser ablation, laser-induced breakdown spectroscopy (LIBS), and laser powder bed fusion (L-PBF). The absorption of incident laser radiation in the expanding plume of ablation products results in the partial shielding of the target. Pulsed-laser ablation experiments show that laser parameters can affect the efficiency of material removal. Single-track L-PBF experiments show that laser absorption by the vapor plume can negatively impact print quality at high ambient pressures. The prediction of plasma formation and shielding is a complex problem as the plasmas which are formed can be non-homogenous ranging from low to high degrees of ionization. The goal of the present work is to investigate the influence of laser pulse duration and fluence on the degree of non-equilibrium in laser ablation plasma plumes. Simulations are performed using an in-house hybrid computational model for plasma flows that combines a collision-radiation model
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of ionization and absorption with the direct simulation Monte Carlo (DSMC) method.
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The collision-radiation model accounts for the non-equilibrium distribution of ions over electron energy states and includes a two-temperature model for describing the energy exchange between the heavy particles and free electron gas through elastic collisions. The simulations are performed for the ablation of a bulk copper target into argon atmospheres at a pressure ranging from vacuum to 1 bar and for pulse durations ranging from 0.1 ns to 100 ns. The results show the degree of non-equilibrium is stronger for short pulse durations, but thermal non-equilibrium can exist for longer pulses if the degree of ionization is low.