Phenomena associated with prebreakdown conduction in vacuum gaps under 50Hz alternating voltages

Previous work [1–6] has shown that prebreakdown (pbd) currents in vacuum gaps under 50Hz a.c. excitations are significantly higher than under comparable d.c. stresses. It has also been shown that the current peak occured later than the voltage peak and the current waveform was asymmetrical about its own peak causing the Fowler-Nordheim (F-N) plots to be branched. This has been attributed [7] to anode vapourisation due to electron beam impact and consequent ion-assisted field emission (I-F emission) of electrons at the cathode as proposed by Porotnikov and Rodnevich[8]. Srinivasa et al [7] also proposed that the fine beam of primary electrons generates at the anode spot a diffuse positive ion beam causing secondary electron emission over an extended area at the cathode. In the present work it has been attempted to verify this assumption using a needle-collector combination as one elctrode and the other electrode is a plane of 30mm dia. The currents in the needle and the concentric collector electrode (when the needle is the main emitter i.e., when they are negative) are of interest. The influence of the anode material (plane electrode), magnitudes and waveforms of the needle and collector currents, F-N plots, Ρ-factors for the needle are studied.