Possible new mechanism involved in non-uniform field breakdown in gaseous dielectrics

摘要

The electrical breakdown of gases under uniform field conditions is fairly well understood in terms of the Townsend's breakdown theory. In most cases involving uniform fields, the breakdown voltage can be estimated via this theory using basic electron impact parameters for molecules in their ground electronic states. In contrast, a consistent model of gaseous breakdown under nonuniform fields is not available at present although substantial progress has been made recently. We point out the possibility that electron impact processes involving high-lying electronically-excited states may play a significant role under non-uniform field conditions. Thus, such processes may need to be included in order to obtain a better understanding of non-uniform field breakdown phenomena. The general, breakdown characteristics of highly non-uniform field gaps can be illustrated by that for a point-plane geometry. It has been found that the breakdown voltage for such a gap can be calculated by a simple streamer criterion if the pressure P, is above a critical value, P(sub c); for P < P(sub c), the estimated breakdown voltage is found to coincide with the corona inception voltage, with the actual breakdown occurring at a higher voltage, corona discharges occur only for P < P(sub c). In other words, the presence of corona in the pressure region below P(sub c) seems to prevent the breakdown from occurring at the predicted value. This has led to the term ''corona stabilization'' to describe the enhancement in the breakdown voltage for pressures below P(sub c). Non-uniform field breakdown measurements in gases will be discussed. We will discuss the possibility that the ''corona stabilization'' is due to the prevention of avalanche progression by attachment of free electrons to molecules in their high-lying electronically-excited states. Information on electron attachment to electronically-excited states of molecules was not available up until the late 1980's.