Study of Breakdown/Arcing for High Power RF Antennas on Fusion Applications

A study into the fundamental causes of vacuum breakdown on metal electrodes exposed to high vacuum conditions was performed. The onset of emission current is identified as the culprit for initiating the chain of events that terminates on insulation failure of the vacuum gap. Ion bombardment caused by microdischarges destabilizes the emitters on the electrodes causing microdischarges to progressively increase in strength until the affected electrode suffers a breakdown. Several such breakdowns are required to finally trigger damaging arcs, which further destroy the electrode. The arc features observed are consistent with the model of vacuum breakdown by unipolar arc formation. CCl₄ and XeF₂ were used as sources of halogen ions to take advantage of the ion bombardment for breakdown amelioration by modification of the electrodes. It was found that CCl₄ stabilizes the emission current but increases the enhancement factor of copper electrodes. XeF₂ exhibits a self-extinguishing emission current behavior on copper and tungsten electrodes, which allows the electrodes to achieve higher hold-off voltages. To reveal the mechanism by which chemicals modify field emitters on high voltage electrodes a different system with ultra high vacuum, and in-situ diagnostics is required. Further study is recommended