Effects of Phase Changes in Multi-Phase Plasma Plumes Induced by Irradiation of a Metal Target with Short Laser Pulses: Simulations Based on a Hybrid Computational Model

Pulsed laser ablation is a material removal process induced by irradiation of a material target by short pulses of intense laser radiation. The multi-phase plasma plume of the ablation products includes both atomic and molecular species as well as clusters and droplets that are formed during the target material fragmentation and condensation inside the plume. The droplets and nanoparticles in the plume can provide a significant contribution to the absorption and scattering of incident laser radiation, thus, affecting the efficiency of the material processing. In the present work, a hybrid computational model for simulations of multi-phase laser-induced plasma plumes is developed. The model includes a thermal model of the irradiated target, model of fragmentation of the target material into atomic and droplet components, kinetic model of expansion of low-temperature plasma plume into a background gas, model of motion and evaporation of droplets, as well model of radiation absorption by atoms and ions in multiphoton ionization and inverse bremsstrahlung processes, and radiation scattering on nanoparticles. The kinetic model of plasma plume expansion is based on the direct simulations Monte Carlo method for gas flows combined with the equilibrium or non-equilibrium ionization models.
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All components of the hybrid computational model are fully coupled with each other. The simulations of laser ablation of a Cu target by nanosecond laser pulses based on the developed model predict the strong effect of droplets on both dynamic properties of the plume and degree of plasma shielding. This effect is found to be even stronger in the case of burst laser ablation, when a target is irradiated by a burst of laser pulses with the intra-burst repetition rate in the GHz range 2.