Recent Developments in Analysis of the Mechanisms of Positive and Negative Coronas in Air

Abstract

The nature of the detailed mechanism of the negative Trichel corona pulses in air at atmospheric pressures is reanalyzed on the basis of recent data of English giving the time duration of the pulse and details of its structure. The onset of corona from a negative point is shown to consist of pulses of about 4 μsec. duration with a rise time of about 1 μsec. It is shown to initiate according to the classical Townsend equation, building up a positive space charge of increasing intensity at the point surface, and an electronic and beyond this, an ionic space charge further out in the gap. The intense positive space charge leads to an auto‐accelerative ionizing mechanism with Morton‐Johnson ionization near the point. The magnitude of these space charge fields account for the sputtering previously observed. On the basis of the field distributions which can be deduced, the course of the growth of the visual and electrical phenomena can be traced to the point where the space charge distortion by negative ion formation out in the gap causes the pulse to choke off. New pulses will then start as soon as the field has cleared away the inhibiting negative ion space charge, if triggering electrons are present. The rate of removal of the negative ions depends on the clearing time for negative ions across the gap, and the rate increases as the applied potential increases. The duration of the pulse depends on the rate of electron production by the auto‐accelerative process and the negative ion accumulation appears to require nearly the same total number of negative ions to choke itself off as does the positive corona. The axial space charge gradients account for the constrictions and flaring characteristics of the discharge. These findings are correlated with the coronas in very pure N₂ and H₂. The observed near equality of the thresholds for positive burst pulse corona onset and Trichel pulses is analyzed and shown to be caused only by a fortuitous and exceptionally high work function of the metal point for electron emission by positive ion bombardment in the presence of O₂. The difference in positive and negative corona currents at the same potential above onset in the linear regime is ascribable to differences in ion mobility, since both signs of pulses give about the same number of ions. The character of the positive point corona is briefly analyzed by means of the same criteria of field distortion used for the negative corona and the transient character of the streamer pulses is described through a diagram of successive field distortions. In comparing positive and negative pre‐onset coronas, it is emphasized that despite similarities introduced by the choking off of the discharge through space charge accumulations, the positive point removes electrons quite effectively, giving a field distorted by only one sign of space charge, while the negative point has a much more complicated distortion caused by accumulations of positive and negative ions. The negative point has also a much smaller sensitive volume than the positive point, which introduces differences caused by lack of triggering electrons, making the negative point useless as a Geiger counter.