The Mechanism of the Positive Point-to-Plane Corona in Air at Atmospheric Pressure

Abstract

Attention is called to the fluctuating character of the current in the positive point-to-plane corona in air at atmospheric pressure. It is shown that in the self-sustaining corona the current is due to a large number of individual current-pulses or bursts which are distributed over the surface of the point in regions of adequate field strength. The bursts of current are quite random in time, space, and intensity, and in general are composed of many successive avalanches of electron ionization which flow into regions of high field strength caused by space charges left by preceding avalanches. These bursts propagate outward across the gaps until extinguished by space charge accumulations, diffusion, self-repulsion, and ion migration in the field as well as by infelicitous coincidences of avalanche and the space charge distribution in the gap. The succeeding electron avalanches are initiated in air by photoelectrons produced in the gas ahead of the point. Evidence of an electrical, visual, and photographic character is presented confirming the existence of bursts and giving orders of magnitude of various features under operating conditions. The ionization produced is most intense at the point in the early stages and the number and intensity of the bursts increase with field strength. The development of a burst at one spot is exceedingly rapid (about ${10}^{\ensuremath{-}8}$ sec.) and is followed by a longer period (of about ${10}^{\ensuremath{-}6}$ sec.) as regards that spot, in which the field clears the space charge. In air it appears that the self-sustaining corona requires a high enough potential to overcome the space charge, an adequate current to furnish photoelectrons, and in addition the formation of negative ions to insure that fluctuations shall not break off the discharge. As both the field at the point and clearing field increase, the bursts propagate further into the gap and ultimately form streamers from 6 to 11 mm long. When these catch up with previous extinct space charge streamers further out in the gap, they project themselves further and further towards the cathode much in the manner of the leader-strokes in lightning discharge. If such a streamer striking the cathode creates an active cathode spot the return discharge produces a spark or an arc.