Vapour and Plasma Ignition Thresholds for Laser Ablation of Metallic Targets in Vacuum

摘要

The coupling of 0.532 mm, 6-ns laser pulses to aluminium (Al) targets irradiated in vacuum is studied. The expressions of the vapour and plasma ignition times for the Al target are obtained, which are substantially different from the expressions reported in the literature. Two cases for vapour breakdown are considered. The first case is that 40 generations of new electrons are born in vapour generation time before plasma formation as assumed in the literature. The second case is that 1 generation of new electrons is born in vapour generation time. The vapour and plasma ignition times as a function of the incident laser pulse intensity are calculated and compared with those from the reported expressions. The plasma ignition threshold estimated in the second case is noted to be in good agreement with the reported experimental result.