Effects of voltage polarity, electric current, external resistance, number of sparkings, supply frequency, and addition of hydrogen and air on electrical breakdown in vacuum

The breakdown potential of a vacuum gap is measured using parallel planar electrodes made of sterling silver as a function of gap separations, number of sparkings, external resistance, and variation of hydrogen and air pressures using dc and ac (50 Hz) applied voltages. It is found that the dc and ac vacuum breakdown voltages become equal after full conditioning by repeated sparkings of both electrodes either with ac or with both polarities of the dc applied voltage. The vacuum breakdown voltage is found to be independent of supply frequency up to 250 Hz. The vacuum breakdown voltage decreases with decreasing the external circuit resistance in series with the gap in the range below 103 Ω. This decrease in the breakdown potential is attributed to the very short discharge time of the stray capacitive energy of the connecting leads into the gap compared to the time necessary for the current growth leading to breakdown. The minimum energy necessary to decondition (degrade) a vacuum gap is measured and the degree of deconditioning is determined as a function of energy discharged into the gap. The dc dielectric strength of the vacuum gap always recovers to its original value after a few sparkings.