Laser beam induced breakdown in helium and argon

Abstract

Measurements of the laser radiation power density P_th required to cause ionization growth and breakdown in the pressure ranges 1800 less-than-or-eq, slant p less-than-or-eq, slant 16 000 torr and 200 less-than-or-eq, slant p less-than-or-eq, slant 20 000 torr have been made in helium and argon, respectively. Three Q-switched lasers (ruby, Nd, and dye) were used to investigate the dependence of P_th upon pressure, radiation frequency ω and flash duration 2τ. Values of P_th lie between 10¹¹ W cm⁻² at low pressures and 10¹⁰ W cm⁻² at high pressures. The experimental results are compared with expressions based upon an extrapolation of classical microwave breakdown theory to optical frequencies. Reasonably good agreement between measured and calculated threshold intensities is obtained. The interaction between the laser radiation and the gas proceeds via inelastic electron-atom collisions and photoionization. Three pressure regimes may be specified: a low pressure regime in which the threshold intensity and ionization growth rate are governed principally by diffusion losses, an intermediate régime in which the laser flash duration is the dominant factor and a high pressure régime where electron-ion recombination controls the rate of ionization growth and the onset of breakdown. The ionization rate is shown to be proportional to the laser beam intensity and, in the intermediate régime, the threshold intensity P_th is proportional to ω²/τ at constant pressure. A value for the electron-atom momentum transfer collision frequency of about 3-4×10⁹p and of about 5 eV for the electron mean energy is derived for argon at the onset of breakdown.