Short Vacuum Gap Research Articles

Photographic observations of impulse breakdown in short vacuum gaps

An image intensifier and image converter with a maximum sweep speed of 1 ns mm-1 have been used to observe the development of impulse breakdown in short vacuum gaps of up to 1 mm. Electrodes of stainless steel, copper and aluminium have been used and it has been found that in each case metal vapour is always produced first at the cathode surface and later at the anode surface; the final arc discharge develops by propagation of the anode vapour. Time delays between voltage application and the appearance of the anode vapour have been measured in the range 10-50 ns and their dependence upon electrode separation and electrode material has been explained using a simple model involving anode heating by the emission current from a vaporised emitting site at the cathode.

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

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.

955. Current growth in sparks in the pulse breakdown of short vacuum gaps: G A Mesyats and D I Proskurovskiy,Izv VUZ Fiz, 11 (1), 1968, 81–85 ( in Russian)

955. Current growth in sparks in the pulse breakdown of short vacuum gaps: G A Mesyats and D I Proskurovskiy,Izv VUZ Fiz, 11 (1), 1968, 81–85 ( in Russian)

Current growth in pulse breakdown of a short vacuum gap

Current growth in pulse breakdown of a short vacuum gap

Vacuum arc recovery phenomena

The present experimental and theoretical study has been designed to uncover the mechanism underlying the rapid recovery of electrical strength of a short vacuum gap after arcing. In the experiment the contacts were of gas-free silver and the contact area and gap length were varied. Recovery strength was measured following the forced extinction of a 250 amp arc in 0.5 µsec. This rapid cutoff of current has revealed detail in vacuum arc recovery curves which is difficult to observe by other experimental techniques. The experimental results show that the gap recovery times range from 1 to 30 µsec depending on the gap proportions. Recovery is most rapid for large diameter contacts which are closely spaced. In the theoretical analysis, the recovery times for various gap proportions were calculated by following the decay of the metal vapor density in the gap from an initial density, present at arc extinction, to a final critical density characteristic of full gap recovery. The reasonable agreement between the measured and calculated recovery times lends support to a model in which condensation of metal vapor on the contacts plays a decisive role in the rapid recovery of electrical strength in a vacuum gap after arcing.