Abstract
The initial phase of a spark discharge in the gap between the pin(cathode) and a plane 1.5 mm long in atmospheric pressure air underconditions of preliminary photoionization by an auxiliary discharge wasinvestigated by the method of shadow photography. In the absence ofpreionization, the discharge from the first nanoseconds after breakdown isan aggregate of a large number of micron-diameter channels. It was foundthat the electron concentration resulting from preionization, estimated at108-109cm-3, increases the degree of uniformity of the discharge channel inthe near-cathode region; however, in the near-anode region, the channelremains microstructured. Within the framework of the mechanism ofmicrostructure formation due to the instability of the ionization wavefront, a criterion for the formation of a uniform discharge is obtained andan explanation of the results obtained is presented. Keywords: spark discharge, preionization, photoionization, instability of the ionization front, microstructure, shadow photography method\
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