Nonequilibrium mechanism for optical breakdown of rare gases near a refractory target

An analytical and numerical investigation is made of "steady-state" optical breakdown in rare gases near refractory metals irradiated with a cw CO₂ laser. Surface temperatures are considered at which there is no ionization equilibrium in the mixture of the gas and the metal vapor, and the electron distribution departs from Maxwellian (for tungsten, 3.5 ≤ T_m ≤ 4.5 kK). In the proposed model of optical breakdown we shall take into account both direct ionization and multistage ionization involving the excitation of the metastable levels in the gas and in the metal vapor. An analytical breakdown condition is deduced for the case when the system has a plane geometry. The threshold power density is found to depend on the heat of vaporization of a metal ion from the surface. The optical breakdown threshold in argon near a tungsten target is calculated numerically for a cone of radiation.