One-dimensional kinetic simulations of plume expansion induced by multi-pulse laser irradiation in the burst mode at 266 nm wavelength

Expansion of a plume into an argon background gas induced by irradiation of a copper target by a burst of nanosecond laser pulses is studied based on a one-dimensional hybrid computational model, including a thermal model of the irradiated target and a kinetic model of neutral gas flows. The kinetic model is implemented in the form of the Direct Simulation Monte Carlo method. The simulations are performed for a laser with 266 nm wavelength, producing bursts of 1, 3, and 7 pulses of 10 ns duration with the peak-to-peak separation varying from zero to 60 ns, in the range of argon pressure from zero to 1 bar. It is found that multi-pulse irradiation induces plumes with complicated internal structures, including cascades of primary and secondary shock waves. The mechanism of interaction between shock waves depends on the background gas pressure. The distributions of temperature and density in the plumes are strongly affected by the inter-pulse separation and number of pulses, which determine the conditions of plasma ignition. Simulations performed based on a model, where ionization and absorption in the plume are accounted for, suggest that tuning the parameters of the burst allows one to reduce the effect of plasma shielding.