LASER-INDUCED BREAKDOWN OF METAL VAPOR

The interaction of laser radiation in the intensity range G = 10(6) divided by 10(10) W/cm(2) and at the wavelength lambda = 1.06 mu m with copper vapor is simulated, using a collision-radiation model describing the kinetics of nonequilibrium ionization and recombination. It is shown that the interaction of laser radiation with vapor can proceed in two qualitatively different ways, namely, prebreakdown and optical breakdown regimes. If the radiation intensity is insufficient to induce avalanche ionization, the system transfers to a stationary state characterized by one temperature, the equilibrium between the radiation and the vapor being absent. For higher Values of laser intensity, optical breakdown starts, i.e., the nonequilibrium transition state from a partially ionized vapor to a fully ionized plasma where Coulomb collisions are predominant. Simulations confirm experimental results that optical breakdown refers to a distinct threshold in the radiation intensity. In the macroscopic description of breakdown, the threshold intensity values depend on the ionization potential and the electronic state distribution of neutral atoms as well as on the initial temperature of the evaporated material and the laser pulse width. (C) 1994 Academic Press, Inc.