Initiation of electrical breakdown in vacuum

Abstract

Measurements of prebreakdown current as a function of applied electric field have been made for molybdenum electrodes of unlike surface finish in ultra-high vacuum (10−9 Torr), for a fixed gap separation d = 0·05 cm. Two sets of electrodes were used: unpolished anode, polished cathode; and polished anode, unpolished cathode. The prebreakdown currents are found to be consistent with the Fowler-Nordheim field emission theory. The breakdown voltage, the field enhancement factor, the current, and the current density and emitting area at breakdown are given as a function of the number of breakdowns. Also, the critical field enhancement factor, the temperature of the anode hot spot and the cathode microgeometry are calculated. Results indicate that the state of the anode surface governs the breakdown voltage and the microgeometry of the cathode surface. Also, in a conditioned gap both anode and cathode are involved in the discharge initiation. Anode initiation is only obtained with a certain set of electrodes.