The mechanism of the electrical breakdown of air in uniform fields at voltages up to 400 kv

Abstract

Measurements of pre-breakdown ionization currents in uniform fields in dry air at pressures of about 1600 torr were carried out over a range of voltages from 165 kv to over 400 kv, corresponding to a range of values of the parameter p20d (p20 is the pressure reduced to 20°c and d is the electrode separation) from 500 to 12 600 torr cm. Over the whole range investigated the spatial growth of ionization was found to be in accordance with the well-known generalized Townsend equation. Moreover, at a value of p20ds of 12 200 torr cm (ds is the sparking distance), the sparking potential of 408 ± 9 kv predicted by using the Townsend breakdown criterion was in good agreement with the experimentally measured value of 4034 ± 05 kv. In these experiments the gas pressure and electrode separation were chosen so that the maximum value of the field used was less than about 60 kv cm-1. At this value field emission was negligible compared with the externally generated initial current I0 (similar 10-12 A), and values of I0 of this magnitude were shown, experimentally, not to give rise to significant space-charge effects in the measurement of pre-breakdown ionization. The values of the secondary ionization coefficient ω/ά were found to be markedly dependent on the state of the cathode surface showing that, just as at lower pressures and voltages, the cathode plays an important role in the breakdown process. At higher pressures (similar 5000 torr) and higher pre-breakdown fields (similar 100 kv cm-1) field emission currents of about 10-10 A were observed, which can be accounted for on the basis of the Fowler-Nordheim equation modified to take account of the presence of a thin insulating surface film: this work is currently being extended to the investigation of breakdown in these conditions, when space-charge distortion of the applied field due to high field emission from small cathode areas may become important.