Plasma-mediated surface evaporation of an aluminium target in vacuum under UV laser irradiation

Mathematical simulation is employed to investigate the dynamics of evaporation and condensation on the surface of a metal target under the conditions of plasma production in the vaporised material exposed to the 0.248-μm UV radiation of a KrF laser with the intensity G₀= 2×10⁸—10⁹ W cm^-2, and a pulse duration τ= 20 ns. A transient two-dimensional mathematical model is used, which includes, for the condensed medium, the heat conduction equation with the Stefan boundary condition and additional kinetic conditions at the evaporation surface and, for the vapour, the equations of radiative gas dynamics and laser radiation transfer supplemented with tabular data for the parameters of the equations of state and absorption coefficients. The target evaporation in vacuum induced by the UV radiation was found to occur during the laser pulse and is divided into two characteristic stages: initial evaporation with a sound velocity and subsonic evaporation after the plasma production. At the subsonic evaporation stage, one part of the laser radiation passes through the plasma and is absorbed by the target surface and another part is absorbed in a thin plasma layer near the surface to produce a high pressure, which significantly moderates the vapour ejection. After completion of the pulse, a part of the vaporised material is condensed on the surface, both in the evaporation region and some distance away from it due to the lateral expansion of the plasma cloud.