Electrical-Insulation Properties of Sterling-Silver Electrodes in Ultrahigh Vacuum

Prebreakdown currents flowing between a pair of planeparallel sterling-silver electrodes are measured as a function of gap length in ultrahigh vacuum (~10-9torr). The emitted currents are found to follow the Fowler-Nordheim (FN) field-emission theory. The electric-field enhancement factor (ß) at the cathode is determined both before and after subjecting the electrodes to repeated breakdowns. Prior to sparking the electrodes ß is found to increase from 160 to 320 over the gap length 0.38-3.04 mm. After sparking the electrodes ß increases from 136 to 220 with increasing electrode separation in the range d=0.25-2.54 mm. The effect of sparking is to decrease the value of ß at a fixed gap length. It is also observed that repeated sparkings of the electrodes result in a considerable decrease in the emitted current (by up to three orders of magnitude) at a fixed gap length and a fixed applied voltage. The vacuum breakdown potential is measured in the range d=0.25-3.04 mm. It is found to increase linearly with increasing gap length up to d = 1.5 mm at which a transition to the Cranberg-clump mechanism of vacuum breakdown ensues. For d=1.5 mm the breakdown voltage follows a dependence law on distance to the power of 0.41. The critical electric field at breakdown is determined to be (7.7 ± 0.7) × 10-9 Vm-1 in the range d=0.25-2.54 mm.