Hydrodynamic splitting of laser-induced plasma plumes: two-dimensional kinetic simulations

The two-dimensional axisymmetric plasma plume expansion induced by irradiation of a copper target in argon background gas with a nanosecond laser pulse is studied based on a combined computational model. The model includes a thermal model of the irradiated target, the Hertz-Knudsen model of material removal, and a kinetic model of the multicomponent plasma plume. The kinetic model is implemented in the form of the direct simulation Monte Carlo method combined with a special approach to simulate equilibrium ionization in the plume, absorption of laser radiation through photoionization and inverse bremsstrahlung, as well as emission of bremsstrahlung radiation in free-free transitions. The simulations show that, at moderate peak laser fluence and background gas pressure, the bimodal distributions of radiation emission intensity appear due to the snow-plow effect that induces formation of a high-density and high-temperature zone at the plume edge between the secondary and primary shock waves. As a result, the transient splitting of the plume into the slow component, which corresponds to the plume core, and fast component, which corresponds to the plume edge, is observed. With increasing expansion time, the slow component disappears due to a relatively fast drop of density and temperature in the plume core. The simulations also show that an increase in the laser spot size is favorable for observing splitting in laser-induced plumes due to the snow-plow effect.