Calculations of Electrical Breakdown in Air at Near-Atmospheric Pressure

A computer program based on the Townsend avalanche model, but explicitly including space charge, has been used to calculate electrical breakdown in air at near-atmospheric pressure. A good fit to Bandel's experimental temporal-current-growth curves may be obtained by using a secondary ionization coefficient that includes 10% ion and 90% photon portions. Only a poorer fit could be obtained when using a 100% photon effect, with either a fixed photon delay independent of overvoltage or a constant reduced electron mobility to simulate an attachment-detachment process. The same agreement and disagreement, respectively, were obtained by comparison of the results of calculations with the breakdown-time-versus-overvoltage measurements of Fisher and Bederson. The effect of variations of the number of initiating electrons from the cathode and of the approach voltage is detailed. The calculations indicate that a luminous front propagates from the anode to the cathode at a velocity of up to ∼3×${10}^7$ cm/sec in the final calculable stages of breakdown.