ROLE OF GAS, SAMPLE AND LASER BEAM PARAMETERS IN LASER GENERATION OF A LOW THRESHOLD BREAKDOWN PLASMA IN A GAS IN FRONT OF A SOLID TARGET

We studied the ignition process of the plasma which is created when a pulsed CO2 laser beam irradiates a (Ti) metal sample in presence of an ambient gas. It is pointed out that the first step is due to target surface micro-defect vaporization. Then thermal ionization of the metal vapour occurs producing electrons which are heated by the inverse bremsstrahlung process. Finally, we observe an erosion plasma (which is dominated by the metal vapour) or a gas breakdown plasma, depending on the laser intensity and on the ambient gas properties. This can be described by the energy balance of plasma electrons and by the evaluation of material thermal heating