Streamer Mechanism and Main Stroke in the Filamentary Spark Breakdown in Air as Revealed by Photomultipliers and Fast Oscilloscopic Techniques

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Abstract
A study has been made of the development of the luminosity in the transition from a corona or a Townsend predischarge to a filamentary spark in atmospheric air for a wide range of gap geometry extending from a positive needle point-to-plane gap to large sphere-to-plane gaps very close to the parallel plate case. Two photomultipliers viewed thin slices of the gap perpendicular to its axis; the one fixed near the anode triggered the Tektronix 517 oscilloscope and the other, which could be moved parallel to the gap axis, provided the signal for display. From the oscillograms, cross plots of the developing spatial distribution of luminosity across the gap were obtained for a number of gaps. It was found that, in general, a primary and a secondary dendrite, and even a tertiary, in some cases, develop out from the anode to the cathode at high speeds and prepare the way for the growth of the main stroke. In divergent fields the primary dendrite consists of a number of simultaneous streamer filaments which, in all cases, cross to the cathode, while the branch streamers of the secondary dendrite slow down in midgap and even fail to reach the cathode in the longer gaps. There are several sets of primary-secondary dendrites before each main stroke, in longer point-to-plane gaps, with about 200 μsec between sets, and about 1 μsec between the last set and the main stroke. While in the case of large sphere-to-plane gaps it was not possible to tell whether a dendrite consists of one or of several streamers, a primary (close to the cathode) and a secondary pulse can be observed in the longer gaps. With short gaps which are very close to the parallel plate case, only a secondary can be seen, and below about 1.0 to 1.3 cm a dendrite or streamer could not be resolved from the main stroke rise with the present equipment. In a few sphere-to-plane gaps, oscillograms and cross plots were obtained which show the development of the main stroke as its toe crosses from the anode to within about 0.5 cm of the cathode, at which position the luminosity distribution of the fully developed main stroke shows a trough, with a maximum near the anode and a rise toward the cathode.