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The intrinsic breakdown by large area laser beams of gases containing the attaching species SF6 mixed with clean dry Air has been analyzed in detail. The results show that SF6 is equally effective in inhibiting avalanche breakdown of these gases for near infrared pulses longer than 100 nanoseconds as for DC fields. The breakdown intensities were obtained for various pulse durations at 10.6 μm by numerically solving the Boltzmann equation for the energy distribution n(ε) of electrons in the generated plasma. Published electron collision cross sections considered the best available for our application were used. The calculated breakdown intensity for very long pulses of constant intensity in air at 1 atm and 300K is I* = 2.7 × 109 w/cm2, which agrees within experimental uncertainty with the measured value. At any given intensity, the power absorbed by the plasma and the power losses in the various electron collision processes (elastic, inelastic excitation of vibrational and electronic levels, ionization, and attachment) are evaluated in our calculations. Discrete photon effects in power absorption and photoelectric processes are expected to be small for wavelengths larger than 1 μm, so the frequency scaling (I*/ω2) of classical field theory should hold for our results over the near infrared region.
attaching species, avalanche ionization, breakdown threshold, electron attachment, electronegative gas, gas breakdown, laser beam, laser-induced breakdown, laser-produced plasmas, plasma production
University of Dayton Research Institute Air Force Weapons Laboratory Kirtland Air Force Base, NM