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 su bmitted to UV laser irradiation is studied. An easy model based on the hypothesis of thermal equilibrium between ejected vapour and heated s urface, and of a local thermodynamic equilibrium state of the vapour, is used to characterize the metallic vapour at the end of the laser pu lse, Then the efficiency of the different elementary mechanisms liable to sustain or to prevent the ionization process in this medium is dis cussed depending on the laser power density, In this work, the calcula tions are applied to the case of the interaction between an excimer Xe Cl laser beam (lambda = 308 nm, tau(1) = 28 ns) and titanium target. I t is shown that the thermal heated metallic vapour is partially ionize d and contains excited and singly ionized species al high densities (1 0(19)-10(20) atoms cm(-2)), The electron temperature in this medium is found to be around 1 eV. The study of the ionization rise in the vapo ur 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 cm(-2) (laser fluence of 2 J cm(-2)) the ionization level is found to increase before the laser pulse end, and a thermal evaporation regime is reached, as the laser power density e xceeds 500 MW cm(-2) (fluence of 10 J cm(-2)), an avalanche breakdown is liable to occur in the vapour before the pulse end and the plasma g overns the evaporation mode. The results presented here are in goad ag reement with experimental observations and with results from more comp lex models reported in the literature.