Time of appearance of a plasma due to action of laser radiation of different wavelengths on an aluminum target in air

The problem of appearance of a plasma in air above an aluminum target irradiated with neodymium or $CO_2$ laser pulses is solved numerically. The published experimental data are shown to agree with the results of calculations carried out assuming evaporation of the target, reduction in the thermal conductivity of the surface layer, and “flash” absorption in the vapor. A rectangular pulse and thermally insulated layers $1\mu m$0 thick are assumed in the derivation of the dependence of the time of the onset of evaporation and of the time of appearance of a flash on the density of the radiation flux in the range of 0.5-100 MW/cm${}^2$. It follows from these calculations that the threshold of plasma formation is not greatly affected by variation of the laser radiation wavelength from 1.06 to 10.6$\mu m$.