Dynamic Balancing between Thermal -Field Emission Currents of Series-connected Vacuum Gaps

The objective of this paper is to study thermal- field emission during prebreakdown process of series-connected vacuum gaps. Thermal-field emission of a microtip on cathode surface of on single vacuum gap was calculated via the MurphyGood theory considering the Nottingham effect, space charge and nano-scale curvature radius. Then through the bidirectional coupling of the aforementioned model and circuit model, thermal-field emission current of series-connected vacuum gaps was calculated. Results showed that a selfbalancing process that emission currents of series-connected vacuum gaps tend to be consistent existed during the prebreakdown process. This explains the astonishing synchronicity on breakdown instant of the series-connected vacuum gaps that was sometimes observed. However, time to reach a balanced emission current could be prolonged by adopting grading capacitors in parallel with the vacuum gaps. This explains the high probability of asynchronous breakdowns observed for series-connected vacuum gaps under lighting impulse voltage.