A contribution to the understanding of the plasma ignition mechanism above a metal target under UV laser irradiation

In this paper, the plasma ignition process above a metallic surface submitted to UV laser irradiation is studied. An easy model based on the hypothesis of thermal equilibrium between ejected vapour and heated surface, and of a local thermodynamic equilibrium state of the vapour, is used to characterize the metallic vapour at the end of the laser pulse. Then the efficiency of the different elementary mechanisms liable to sustain or to prevent the ionization process in this medium is discussed depending on the laser power density. In this work, the calculations are applied to the case of the interaction between an excimer XeCl laser beam ( nm, ns) and titanium target. It is shown that the thermal heated metallic vapour is partially ionized and contains excited and singly ionized species at high densities ( atoms ). The electron temperature in this medium is found to be around 1 eV. The study of the ionization rise in the vapour evidences the important role played by the single-photon ionization process and the electron/ion inverse bremsstrahlung effect. For laser power densities above 100 MW (laser fluence of 2 J cm), the ionization level is found to increase before the laser pulse end, and a thermal evaporation regime is reached. As the laser power density exceeds 500 MW (fluence of 10 J ), an avalanche breakdown is liable to occur in the vapour before the pulse end and the plasma governs the evaporation mode. The results presented here are in good agreement with experimental observations and with results from more complex models reported in the literature.