Effect of electrode surface finish on electrical breakdown in vacuum

Measurements of prebreakdown current as a function of applied electric field have been made for molybdenum electrodes in ultra-high vacuum (10 −9 Torr), for a fixed gap separation d =0·05 cm. Four sets of electrodes were used, each set having a different surface finish. The prebreakdown currents are found to be consistent with the Fowler-Nordheim field emission theory. The effects of the electrode surface finish on the field enhancement factor, the enhanced breakdown field, and on the current, current density and emitting area at breakdown are shown. The nature of the cathode microgeometry is deduced from the measurements of electron emission, and the change in the microgeometry with continued sparking is discussed.