Experimental Determination of the Generation Moment of Runaway Electrons

The results of an experimental study of the generation of runaway electrons (REs) during the formation of a negative streamer in an 8.5-mm-long point-to-plane gap filled with air are presented. Negative nanosecond voltage pulses (18 kV, τ
_0.5
≈ 10 ns, and τ
_0.2-0.9
≈ 4 ns) were applied across the gap. In this paper, we attempted to determine when REs are generated relative to the appearance of the streamer. In our previous studies of a positive streamer, we have shown that the formation of the streamer is accompanied by a dynamic displacement current (DDC) caused by the redistribution of electric field strength in the gap. This current can be easily measured with a collector placed behind a grounded grid electrode. The collector is typically used to measure the current of REs at negative polarity. In this paper, we were able to measure together and separately the DDC caused by a negative streamer and the RE beam current. This allowed us to accurately determine the moment of generation of REs.