Vacuum breakdown and its effect on subsequent current emission

The breakdown potential of ultrahigh-vacuum (10¿9 torr) gaps has been measured using aluminium electrodes. The breakdown voltage increases linearily from 2.8 to 41.4 kV in the gap range 0.07¿0.76 mm. The enhanced critical breakdown field is found to be (5.7 ± 1.7) × 109 V/m, in reasonable agreement with the recent theoretical prediction of Chatterton. For gap separations larger than, 0.89 mm, a large increase in the prebreakdown current, at a particular applied voltage (ignition), is observed. The enhanced electric field at the cathode during the ignition is found to be (2.4 ± 1.0) × 109 V/m over the gap separation 0.89¿3.04 mm. The current before the onset of the breakdown is found to increase with increasing number of sparkings, in qualitative agreement with theory. The effects of repeated sparkings on both the ignition voltage and the enhancement-field factor are also studied over a wide range of electrode separations.