Transition from the Primary Streamer to the Arc in Positive Point-to-Plane Corona

Abstract

This paper reports for the first time measurements with multiple techniques delineating the complete sequence of events from the primary streamer to the thermalization of a highly ionized channel to the arc phase in the common spark transition for relatively small point‐to‐plane gaps. The observations cover a range of point diameters from 0.1 to 2 mm, for gaps in room air from 1 to 4 cm long, covering the whole range of potentials from streamer onset to in excess of 30% above the standard sparking threshold. It is shown that starting with the primary streamer, which occasionally at its start creates photoionization up to the midgap and at the cathode, there is produced a succession of ionizing waves of potential starting in many cases on the arrival of its tip at the cathode. These waves, observed by photomultipliers as well as by current pulses over a period of some microseconds, create what has been called a ``secondary streamer'' by Hudson and Loeb. Unless overvoltage exceeds 30%, the electron density and temperature in the resulting channel are not adequate to thermalize to an arc. Above this value, thermalization occurs in several hundred nanoseconds. At lower overvoltages, there is a dark phase lasting at low values for hundreds of microseconds. During the dark phase, there is a virtually nonluminous current involving ion motion. This regenerates the anode point field and leads to a series of one or more very heavy, very fast, luminous ionizing‐potential waves. The higher the overvoltage, the fewer the number of these pulses spaced in time on ion movement scales. After the last pulse the thermalization follows through gas heating time from 40 to 60 nsec.