On the Mechanism of the Generation of Runaway Electrons after a Breakdown of a Gap

Data that explain the generation of runaway electrons after the breakdown of a gap with a sharply inhomogeneous distribution of the electric field strength are presented. Using a special technique for measuring a displacement current caused by the appearance and propagation of a streamer, the voltage and current waveforms of runaway electrons have been synchronized to each other, as well as to the dynamics of the streamer formation, which has been recorded using a four-channel ICCD camera. It is shown that the first runaway electron beam is generated near the pointed cathode at the time when the streamer appears. The second runaway electron beam is generated at the time when a backward ionization wave reaches the pointed cathode. It is assumed that the generation of the second runaway electron beam occurs in the cathode layer. This is confirmed by the fact that the second runaway electron beam does not appear in those discharge implementations at which the cathode spot is observed before the gap is bridged by the plasma.