Prebreakdown conduction measurments in vacuum gaps with alumina insulators

Predischarge current measurements, for a solid insulator bridged vacuum gap excited by 60 Hz AC, are reported here. To our knowledge it is for the first time that such results are published in the literature. The predischarge current in a plane vacuum gap is known to follow the Fowler-Nordheim type emission when subjected to DC voltages [1,2]. The studies are rather limited for AC voltages and the current is believed to follow a modified ion field emission [3,4]. The results presented in this work indicate that the plane vacuum gap predischarge current is modified by the presence of the insulating spacer. The modification of the current characteristics is believed to be caused by the charging of the insulator surface due to secondary electron emission from it. It is shown that the steady predischarge current can be reduced to a small value or eliminated, and the breakdown voltage increased, by reducing the stress at the triple junction either (a) by interposing a high permittivity material such as barium titanate between the spaeer and each of the electrodes, or (b) by shielding the triple junctions within recessed electrodes. The purpose of these experiments is to see if there is a correlation between prebreakdown currents and (a) the insulator surface charging, (b) the breakdown strength. The insulator shape and the secondary emission yield from its surface also affect the predischarge current characteristics, and these results will be published in the future.