Emission Decay of Oxide Cathode Induced by Electrode Contamination

Abstract

The emission decays of the oxide-coated cathode due to the contamination of the positive electrode have been studied with barium fluoride as the contaminant. All the experimental processes—positive electrode (anode) cleaning by electron bombardment, vacuum deposition of a known amount of the contaminant onto the anode, and dc and pulsed emission measurement—were carried out in a vacuum system using movable molybdenum anodes. As soon as an anode voltage higher than about 10 V was applied, the emission decayed to a minimum value, and then increased slowly. The minimum emission seemed to occur when the incoming rate of poisoning material to the cathode became equal to its liberation rate from the cathode. The emission decay was detected even with 5×10−8 g/cm2 of barium fluoride. The emission recovered gradually when the anode was replaced by a clean one. The decay-recovery process proved repeatable at least 15 times and the activation energy of the recovery was estimated to be about 36–45 kcal/mole, when the amount of the contaminant was 2×10−6 g/cm2. The decay and recovery processes are discussed.