Study by a Monte Carlo simulation of the influence of a background gas on the expansion dynamics of a laser-induced plasma plume

The expansion of a laser-induced plasma plume into a non-reactive background gas is investigated theoretically using a Monte Carlo simulation. The method allows the description of the collective motion of both laser-ablated and background gas particles. The influence of the background gas pressure on the plume shape and on the ambient gas particles distribution within the plume of ejected matter is studied. The expansion dynamics under various ambient gas pressures and nature is also investigated. The compression of the ambient gas particles by the ejected particles in the leading edge of the Flume (snowplow effect) is clearly observed for a residual argon pressure greater than 50 Pa. The narrowing, or focusing, of the laser-induced plasma plume is also observed with increasing the ambient gas pressure. The ambient gas particles are then preferentially located at the edge of the plume of ejected matter, whereas the high-density region in the plume appears to be deficient in background gas particles. The dynamics of the expansion of the plume under a high residual pressure appears to be different from low-pressure cases, with a stronger thermalization of the plume and an evidence of the oscillatory behavior of the plume expansion. The Monte Carlo simulation also predicts vortical flow formation at the plume periphery.