Abstract

Calculations were made with an electronic computer for comparison with the measurements by Tholl of midgap breakdown in a N2–CH4 mixture. Tholl used a 3-cm gap, 400-Torr pressure, and an overvoltage of 20.7%. For the calculations, a one-dimensional Townsend avalanche model was used, which includes space charge but omits photoionization in the gas. The input data were adjusted to give agreement with Tholl's measured avalanche velocity and current-growth time constant in the absence of space charge. The initial effect of space charge was calculated to be a reduced current growth rate occurring concurrently with an increased avalanche velocity toward the anode, as noted by Tholl. The calculated values of these quantities for this current regime differ by a factor of ∼2 from the measured values, but this discrepancy could be due to the discharge channel expansion and to energy considerations. The later stage of current buildup is marked by a very rapid current buildup occurring at the time that the ``streamer'' propagates toward the cathode. Good agreement is obtained in this time regime. The cathode-directed streamer is shown to result from a secondary avalanche from the cathode. The moving junction between the primary and secondary avalanches is marked by a reduced channel diameter in Tholl's photographs. It is shown that this constriction of the discharge channel could not be caused by radial fields.

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