Growth of the spark current at the breakdown of short vacuum gaps at steady voltage

Abstract

The growth time of the spark current at the time of breakdown in vacuum at the constant voltage and pressure existing during this period was investigated with high time resolution. It was shown that, as in the case of a pulsed breakdown: 1) the growth time tc of the current increases in proportion to the length d of the gap and the ratio d/tc is ∼2.5·106 cm/sec; 2) the transition to a rapid rate of current growth (di/dt > 108 A/sec) takes place in 1–3 nsec; 3) during the period of current growth, x-ray radiation appears and a transfer of anode material to the cathode is observed. These results serve as evidence that the origin of current growth is connected with the appearance of efficient electron sources on the cathode. These, as in the case of pulsed breakdowns, are evidently cathode flares, the formation of which has an explosive character and can be identified with the act of breakdown initiation. In essence this event does not change with the rate at which voltage is supplied across the gap and consists of the explosive disintegration of emitting microspikes on the cathode. The appearance of cathode flares indicates the start of an irreversible breakdown of vacuum insulation. The emergence of a burst of x-rays and the erosion of the anode are explained by the action on the anode of a powerful stream of electrons that are emitted from the cathode flares, providing the spark current.