The mechanism of breakdown of ambient air in long uniform-field gaps

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Abstract
Measurements of pre-breakdown ionization currents in uniform-field gaps in filtered atmospheric air have been undertaken for values of the parameter pd (pressure × gap length) up to 10 500 Torr cm, corresponding to a maximum breakdown voltage of 350 kV. Over the whole range investigated, the spatial growth of ionization in the gaps was found to be in accordance with the generalized Townsend equation, the main source of secondary electrons being due to the photoelectric effect at the cathode. For given values of Es/p, where Es is the breakdown field stress, there is good agreement between the values of ds (sparking distance) obtained by experiment and those predicted by the Townsend breakdown criterion.

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