Analysis of plasma-controlled laser evaporation of Al target in vacuum

摘要

The plasma-controlled evaporation of the Al target induced by the laser pulse with intensity of 8x10~8 W/cm~2 and wavelength of 1.06μm is analysed with account for the two-dimensional effects. The self consistent model is applied, consisting of the heat transfer equation in condensed medium, the system of radiation gas dynamics in evaporated substance, and the Knudsen layer model at the two media boundary. It is established that the phase transition of the target surface is controlled by the two factors: the surface temperature that depends on the transmitted radiation intensity and the plasma pressure, governed by the expansion regime. The process comes through three characteristics stages ― the sonic evaporation at the beginning, the condensation during the period of plasma formation and initial expansion and, finally, the recommence of evaporation in subsonic regime after the partial brightening of the plasma. During the subsonic evaporation stage the vapour flow and the mass removal rate is much higher near the beam boundaries than in the centre due to smaller plasma counter-pressure. The vapour plasma pattern is characterised by the dense hot zone near the surface where the deposition of laser energy occurs, and rapid decrease of density outside the zone due to three-dimensional expansion. The application of the laser beam of smaller radius at the same intensity leads to the formation of more rarefied and more transparent plasma, that allows to improve the mass removal efficiency.