Prebreakdown and breakdown characteristics of stainless steel electrodes in vacuum

Measurements of prebreakdown current as a function of applied electric field have been made for polished stainless steel electrodes in vacuum (similar 10⁻⁸ mm Hg) for gap separations 0·02<d<0·1 cm. Experimental data were obtained (a) before the electrodes were outgassed, (b) after outgassing at 300 °C, (c) after conditioning the electrodes be repeated sparking, (d) after reversing the polarity of the electrodes, and (e) after restoring the polarity to its original mode. The data were analysed on the basis of the Fowler-Nordheim field emission theory. The breakdown voltage, the current at initiation of the discharge, the field enhancement factor, and the current density and emitting area at breakdown are given as a function of the number of breakdowns. The geometry and dimensions of the cathode microprotrusions are deduced. Results indicate that outgassing is necessary to achieve a stable surface, and that repeated sparking only conditions one electrode. Reversing the polarity destroys all conditioning effects. The variation of V_s with d would seem to indicate that the discharge was initiated by electron beams from the cathode producing electrode vapour at the anode.