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.