Plume accumulation effect and interaction of plumes induced by irradiation of a copper target with a burst of nanosecond laser pulses near the ionization threshold

Abstract

The interaction between plumes induced by irradiation of a copper target in argon background gas with a burst of nanosecond laser pulses is studied numerically. The two-dimensional (2D) simulations are performed based on a model that includes a thermal model of the irradiated target and a kinetic model of plume expansion. The latter is implemented in the form of the direct simulation Monte Carlo method, where the equilibrium ionization and absorption of laser radiation are considered. For the laser spot diameter varying from 20 μm to 200 μm, the 2D plume structure is found to be different from the structure predicted by a one-dimensional model. The simulations reveal a strong plume accumulation effect when the plumes generated by preceding pulses in a burst change the conditions of propagation for plumes induced by subsequent pulses. The inter-plume interaction can result in merging shock waves induced by individual pulses and formation of a single plume. The degree of plume accumulation depends on the inter-pulse separation, laser spot diameter, background gas pressure, and number of pulses. When the irradiation conditions for a single pulse are below the ionization threshold, the conditions of plasma ignition can be reached by increasing the number of laser pulses due to the plume accumulation effect. This results in plasma shielding and reduction of the effectiveness of material removal by the subsequent pulses in the burst. The plume accumulation effect, thus, determines the optimum number of pulses in the burst in applications of laser ablation for material removal.