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Breakdown characteristics of non-planar plasma sheaths

This paper addresses the problem of breakdown of the negative space-charge sheath which surrounds highly-stressed positive electrodes. The "primary" electrons originating in the plasma cross the sheath boundary at a radius r/sub s/ wth their thermal current density and on the way to the anode they generate secondary electrons and ions through impact ionization. Due to the large difference in mass, the residence time of the secondary ions in the sheath is longer than that of the electrons. As a consequence, the sheath becomes less negative and the sheath radius r/sub s/ must increase in order to compensate for the additional positive charge. This, however, results in an increase in the number of primaries entering the sheath and, under proper conditions, in a runaway of the sheath radius. The runaway of the sheath radius is identified with breakdown. The case in which magnetic field effects are negligible is presented.