Computational study of the role of gas-phase oxidation in CW laser ablation of Al target in an external supersonic air flow

Vapor expansion from Al target irradiated by a continuous wave laser into a supersonic external air flow is investigated in kinetic simulations performed for values of pressure in the external flow ranging from 50 Pa to 10000 Pa. The direct simulation Monte Carlo method enhanced with a simplified description of Al vapor burning in air is used in the simulations. The results of the simulations reveal a significant effect of the external gas pressure on the flow structures and the mechanisms of the alumina and oxygen transport to the target surface. At small values of pressure, the transport of both alumina and oxygen is controlled by diffusion and the formation of alumina film on the surface of the laser spot is dominated by the direct sedimentation of alumina from the gas flow. In this regime, the flux of oxygen to the laser spot is by an order of magnitude smaller than that of alumina. At higher values of pressure, the diffusion cannot directly deliver oxygen and alumina to the surface of the laser spot, but the circulating flow generated upstream the spot can effectively trap alumina formed in the flame zone and deposit it to the target surface. In this regime, the likely mechanism of the alumina film formation within the spot is the heterogeneous oxidation and growth of the film from the upstream edge of the spot in the downstream direction.