Vacuum breakdown between outgassed molybdenum electrodes

Measurement of prebreakdown currents as a function of applied electric field have been made for polished molybdenum electrodes in ultra-high vacuum (¿ 10¿8 torr). The prebreakdown currents have been measured (a) before and (b) after outgassing the electrodes separately at 600°C and then at 1000°C. The experimental data were analysed on the basis of the Fowler-Nordheim field-emission theory, and the breakdown voltage, the field enhancement factor, the onset current, the current density and the emitting area at breakdown are given as a function of the number of breakdowns. Results indicate that the breakdown voltage is increased by outgassing the electrodes and that anode outgassing is more important than cathode outgassing in obtaining a conditioned discharge gap with high breakdown voltage and low breakdown current. Exposing the electrodes to the atmosphere destroys the conditioned state of the electrode surface.