Gas-dynamic effects of the interaction between a pulsed laser-ablation plume and the ambient gas: analogy with an underexpanded jet

Abstract

The expansion of a laser-ablation plume into different ambient gases is investigated theoretically using a two-fluid gas-dynamic model and experimentally with time-of-flight mass spectrometry. Both calculations and measurements, performed for laser ablation of Cu in oxygen or noble gases, reveal an oscillatory behaviour of plume expansion dynamics which is strongly dependent on the molecular weight of the ambient gas. Simple gas-dynamic considerations based on the analogy between an ablation plume and a supersonic underexpanded gaseous jet are found to explain a number of the effects of the interaction between the plume and the background gas. The effect of plume focusing observed previously at fairly high pressures of various ambient gases is reasonably described using the phenomenology of the underexpanded jet. The analogy also predicts vortex formation at the plume periphery.