Secondary Leader Channels in the Impulse Breakdown of Air and Nitrogen

Abstract

In impulse gaseous breakdown, it has never been quite established how the transition from the low ion density of the primary streamers to the high carrier density of the pre-spark channel occurs. This phase of development was investigated for air and N2 by employing the Lichtenberg figure technique. Gas pressure varied between 50 and 760 Torr. It was surprising to detect an ionization process, termed a ``secondary leader,'' that is distinctly different from the primary streamer. During this process a relatively highly ionized channel is formed. Similar channels were previously observed only under static voltages and in very long gaps. The paths of the secondary channels were found to be narrow and to follow a radial pattern; they terminate in ``threads'' of low intensity that have an irregular circular pattern. Measurements were made of their onset voltage and their rate of growth as a function of the voltage and pressure. Their onset voltage in air lies between 4 and 12 kV for 100 and 400 Torr, respectively; in N2 it is about 15% lower. Their length is linearly dependent upon the voltage, though the rate of growth varies with pressure.