Abstract
A Dc positive sharp- to –plane Corona phenomena have been studied in electronegative gases (air ,and SF6 ) for pressure up to (3.2bar and gap length from 0.1 to 0.5 cm), with high degree of non-uniformity electric field. The corona –breakdown phases can exhibit anomalous characteristics. The exact physical process of such phenomena is not yet fully understood complete. To better understand this process precise knowledge about corona and spark discharge is necessary. Hence the dependency of corona inception and spark breakdown voltage on the pressure, non-uniformity field factor and the type of applied voltage ,also the type of materials.
 The steady region of corona discharge is observed in the ( sharp- to- plane ), for applied voltage between the threshold value voltage and onset value , we observe an effect that attaching gases. A Townsend type of discharge develops in the region close to point. At voltages above the onset value , the burst pulses are replaced by a Dc current ,and in some cases 'pre-breakdown ' streamer are also observed.
Highlights
In a uniform electric field spark gap, when the technical conditions have been fulfilled for breakdown a highly ionized conducting channel propagates rapidly across the gap until it bridges the electrodes, and breakdown ensure
In non-uniform field gap, the conditions for breakdown may be fulfilled within a region where the applied electric field strength is high, but the ionized channel may not be able to propagate through the region where the field is lower
In first stage of the study, a great deal of the efforts were stressed on the characteristics of very non uniform field gaps, whose breakdown voltage has unusual negative slope upon gas pressure [4, 5, 6]
Summary
In a uniform electric field spark gap, when the technical conditions have been fulfilled for breakdown a highly ionized conducting channel propagates rapidly across the gap until it bridges the electrodes , and breakdown ensure. For the corona inception to occur, the triggering electron must exist at a position where the electric field is high enough to produce effective ionization and lead to the formation of an avalanche of critical size.
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