Laser Absorption Wave Formation

Abstract

The interaction region over an opaque solid target that is vaporizing under irradiation by an intense laser beam is modeled by a layer of expanding vapor and an air shock layer. It is shown that the formation of a subsonic laser-absorption wave in the air is triggered by laser-induced heating of the gas, and this heating occurs first in the vapor rather than in the shocked air. The role of nonequilibriu m phenomena is reviewed, and a scaling law is given that determines the laser intensity-wavelength regime over which such phenomena can occur. A numerical solution of the interaction problem is developed that accounts for gasdynamics and radiative transport in the gas layers, and for heat conduction within the target. The computed results agree with experiments in which subsonic absorption waves have been observed to form over metal targets irradiated by a CO2 gas laser at intensities of 0.1-1 Mw/cm2. This agreement confirms the validity of the theoretical analysis as a useful tool for investigating the laser-target interaction and subsonic absorption wave formation phenomena for laser beam and target parameters other than those of the experiments.