<title>Gas dynamics of laser ablation: two-dimensional expansion of the vapor in an ambient atmosphere</title>

Abstract

A 2D gas-dynamic model of laser ablation an ambient gas atmosphere is proposed. To obtain the boundary conditions at the evaporated target surface, a nonlinear heat transfer problem in the target including the dynamics of the melt and evaporation fronts is considered. Back condensation of the vapor at the target is taken into account. At later stages, compete absorption of the vapor and back condensation thereof with a local sound speed are assumed. The gas- dynamic problem in divided into the initial 1D and final 2D stages. The 1D stage describes the ablation plume formation under the action of laser pulse. The 2D stage is responsible for the formation of the energy and angular distributions of the ablated material. A considerable compression of the ambient gas around the expanding plume of the laser- evaporated material and a shock front propagating through the undisturbed ambient one. Once the laser pulse is over, the vapor pressure eventually drops down to the value comparable to the compressed ambient gas pressure. From this time on, the gas considerably suppresses the vapor expansion. There is a noticeable difference between the vapor distribution in vacuum and the one in the ambient atmosphere: the vapor fills the entire plume volume in vacuum while in the presence of ambient atmosphere it is accumulated near the plume boundary and tends to form a thin shell. The angular and energy distributions of the ablated material are especially sensitive to the nature and pressure of the ambient gas. Both the kinetic energy of the ablated atoms and the width of their angular distribution decrease with the ambient pressure.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.