Experimental test of whether electrostatically charged micro-organisms and their spores contribute to the onset of arcs across vacuum gaps

Abstract

Recently it was proposed [L. R. Grisham et al. Phys. Plasmas 19, 023107 (2012)] that one of the initiators of vacuum voltage breakdown between conducting electrodes might be micro-organisms and their spores, previously deposited during exposure to air, which then become electrostatically charged when an electric potential is applied across the vacuum gap. This note describes a simple experiment to compare the number of voltage-conditioning pulses required to reach the nominal maximum operating voltage across a gap between two metallic conductors in a vacuum, comparing cases in which biological cleaning was done just prior to pump-down with cases where this was not done, with each case preceded by exposure to ambient air for three days. Based upon these results, it does not appear that air-deposited microbes and their spores constitute a major pathway for arc initiation, at least for exposure periods of a few days, and for vacuum gaps of a few millimeters, in the regime where voltage holding is usually observed to vary linearly with gap distance. V C 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4858895]Although the voltage gradient which can be reliably sustained across a vacuum gap between two metallic conductors without arcing is the most significant constraint in the design of electrostatic accelerators and in related applications, the physical mechanisms responsible for the onset of electrical arcs between electrodes have remained unclear. Proposed mechanisms have included field emission of electrons from surface imperfections, the growth of whiskers that enhance field emission of electrons, emission of ions from surfaces, and the acceleration of electrically charged clumps, presumably of oxide, from one electrode to the other. 2-16 The clump model yields the best agreement with the generally observed scaling of voltage holding with the vacuum gap distance, namely, but suffers from the fact that oxides are tightly bound to their surfaces, and thus not easily detachable for acceleration.Recently, it was proposed 1 that some or all of the "clumps" of the clump model might be microbes such as bacteria, bacterial spores, fungi, and fungal spores. They are ubiquitous in air, and thus constantly being deposited upon exposed surfaces, they are appropriately sized, 17,18 with dimensions of microns, they are in most cases only weakly attached, if attached at all, to the surfaces on which they land, and they readily acquire an electrostatic charge, especially as spores.If microbes and their spores deposited from air, as hypothesized, play a significant role in initiating at least some electrical arcs between metallic electrodes in vacuum, then one way to test this hypothesis would be to explore whether changes in the timing of the final cleaning of the electrodes comprising a high voltage vacuum gap relative to when the electrodes are put under vacuum is associated with changes in the number of high voltage conditioning shots required to reach the nominal operating voltage for the vacuum gap. The results of thi...