Electrical Insulation Behavior of High Vacuum under Variable Frequency Alternating Excitations

This paper presents results leading towards a superior understanding of the insulating properties of high vacuum by focusing mainly on the pre-breakdown and breakdown behavior of unbridged vacuum gaps under variable frequency alternating excitations. The supply frequency variation is from 30-300 Hz for a fixed inter-electrode spacing of 0.5 mm. The pre-breakdown (PBD) current magnitudes and the breakdown voltages are inversely proportional. The behavior is unpredictable over a specific range of frequencies. A pronounced breakdown voltage peak occurs at 130 Hz with stainless steel electrode pair, whereas this peak is less prominent with copper and aluminum electrodes. Fowler-Nordheim (F-N) emission, ion assisted field (I-F) emission and thermal interplay due to quasi-continuous shower of anode micro-particles predominantly neutral, lend support to the experimental observations. It is surmised that the apparent cause for time delay between voltage and current peaks is not only due to the presence of neutral molecules in the gap, but also principally due to their transit time.