INFLUENCE OF IRRADIATION CONDITIONS ON PLASMA EVOLUTION IN LASER-SURFACE INTERACTION

The plasma plume induced by pulsed CO2 laser irradiation of a Ti target at power densities up to 4 X 10(8) W cm-2 was studied by emission spectroscopy. Time- and space-resolved measurements were performed by varying laser intensity, laser temporal pulse shape, ambient gas pressure, and the nature of the ambient gas. Experimental results are discussed by comparison with usual models. We show that shock wave and plasma propagation depend critically on the ratio I(vap)/I(i), I(vap) being the intensity threshold for surface vaporization and I(i) the plasma ignition threshold of the ambient gas. Spectroscopic diagnostics of the helium breakdown plasma show maximum values of electron temperature and electron density in the order of kT(e) is similar to 10 eV and n(e)=10(18) cm-3, respectively. The plasma cannot be described by local thermodynamic equilibrium modeling. Nevertheless, excited metal atoms appear to be in equilibrium with electrons, hence, they can be used like a probe to measure the electron temperature. In order to get information on the role of the plasma in the laser-surface interaction, Ti surfaces were investigated by microscopy after irradiation. Thus an enhanced momentum transfer from the plasma to the target due to the recoil pressure of the breakdown plasma could be evidenced.