LASER-INDUCED BREAKDOWN OF METAL VAPOR

The interaction of laser radiation in the intensity range G = 10(6) di vided 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 describin g the kinetics of nonequilibrium ionization and recombination. It is s hown that the interaction of laser radiation with vapor can proceed in two qualitatively different ways, namely, prebreakdown and optical br eakdown regimes. If the radiation intensity is insufficient to induce avalanche ionization, the system transfers to a stationary state chara cterized 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 pa rtially ionized vapor to a fully ionized plasma where Coulomb collisio ns are predominant. Simulations confirm experimental results that opti cal breakdown refers to a distinct threshold in the radiation intensit y. In the macroscopic description of breakdown, the threshold intensit y values depend on the ionization potential and the electronic state d istribution of neutral atoms as well as on the initial temperature of the evaporated material and the laser pulse width. (C) 1994 Academic P ress, Inc.