Spark to glow discharge transition in cavities

The occurrence of spark, pseudoglow, and glow discharge in short gaps is discussed. The effect of gap spacing and overvoltage is examined in terms of both experimental observations and theoretical calculations. It is found that gaps or cavity diameters are more likely to undergo spark-type discharge, since the larger gas volume is more conductive to an uninterrupted exponential growth of the electron and ion concentration required for spark channel formation. In small gaps, the proximity of the electrode boundaries impedes the exponential growth of free electrons, permitting a smaller charge density and thus resulting in diffused glow-type discharge. A larger applied voltage will provide a greater energy input to the gap. This will result in a more rapid growth of the electron concentration, favoring the formation of spark discharges. It is noted that the high probability that all three forms of discharge tend to occur simultaneously should permit the continuous use of pulse detection methods on rotating machines