Effect of the radial beam mode on expansion of laser-induced plasma plumes

Expansion of plasma plumes induced by a single and multiple short-pulse laser irradiation of a copper target into argon background gas is numerically studied using a two-dimensional hybrid computational model which includes a thermal model that describes the thermal state of the target and a kinetic model that describes non-equilibrium expansion of laser-induced plasma plume. The kinetic model is based on the Boltzmann kinetic equations for the gas mixture and is implemented in the form of a generalized direct simulation Monte Carlo method, which accounts for absorption of laser ablation as well as ionization and recombination processes. The radial beam mode is assumed to be either Gaussian TEM₀₀ mode or TEM_*01 mode with a donut-type intensity distribution. The results of simulations show that the plume structures induced by individual Gaussian and donut-type beams are qualitatively different. The simulations reveal a more complex plume structure created with the donut-type beam which contains multiple shock waves and high temperature regions near the axis of symmetry. It is shown that the radial beam mode also affects the degree of plasma shielding of the target from incident multi-pulse laser radiation.