Onset Studies of Positive Point-to-Plane Corona in Air at Atmospheric Pressure

Positive point-to-plane corona studies in air have been continued with special emphasis on pre-onset region. Oscillographic and photographic studies show two kinds of current pulses occurring below onset voltages. Both types of pulses can be observed by inductive effects on a plate placed near the gap and by the ion currents produced. At lowest voltages for counting action, pulses lasting for about 0.003 second are recorded on the oscillograph and are accompanied by a glow around the point as described by Trichel for steady corona. These correspond to the regular corona process continuing for several thousand bursts after which space charge modification of the field causes extreme likelihood of extinction. At voltages approaching corona onset, streamers are observed which extend far out into the gap and which can propagate only into a space-chargefree gap. The steady corona process is normally initiated by one of these streamers. The mechanism for both bursts and streamers is shown to involve space charge intensification of the field. Streamer propagation and stable burst corona both depend on photoelectric ionization in the gas. Streamer length is found to depend primarily on point size and for very fine needle points no distinction can be made between bursts and streamers. By the use of a shielding screen to cut out the electrostatic pulse produced by a streamer or a group of bursts, it was possible to determine the number of ions produced. This was found to be approximately 5×${10}^9$ ions per cm of visible streamer, and about 2×${10}^9$ ions in an average burst pulse. The time necessary for positive ions produced by a streamer to cross the gap was measured and found to be of the order of ${10}^{-3\mathrm{}}$ second, and to vary with gap geometry in a way consistent with the minimum time between two consecutive streamers. Considerable photoelectric ionization was found to occur at the plate as much as 2.15 cm ahead of the visible streamers. Streamers occurring just prior to spark breakdown of the gap are associated with the pre-onset streamers. In this case the higher voltage near breakdown gives the high field necessary for streamer propagation despite space charge modification of the field by the stable corona process.