Impulse Breakdown in the10−9-Sec. Range of Air at Atmospheric Pressure

Abstract

The formative lag of spark breakdown has been measured over the range from 0.5 to 50×${10}^{-9\mathrm{}}$ sec. using transmission line circuits in conjunction with the micro-oscillograph. It is found to be a function only of the applied field (independent of gap-width) for the shorter times (high fields), but to increase for decreasing gapwidths for the longer times (low fields). A calculation of the formative lag is presented which is based on the assumption that it consists mainly of the time for a single electron avalanche to build up a space-charge field comparable with the applied field. This predicts the observed formative lags within the experimental accuracy of the measurements over the entire range used. The increasing times for decreasing gap-widths for the longer times is interpreted as the transition from a single avalanche to a multiple avalanche mechanism of breakdown. The critical field where this transition takes place for a given gap-width is computed and found to predict the observed critical fields within the experimental accuracy. The good agreement between theory and experiment enables a more reliable prediction than has previously been possible of the critical gap-width above which the threshold field is determined by a single avalanche mechanism.