X-RAY-EMISSION AND PREBREAKDOWN CURRENTS IN PLAIN AND DIELECTRIC BRIDGED VACUUM GAPS UNDER DC EXCITATION

X-ray activity in both plain and solid insulator bridged-vacuum gaps was investigated under DC stresses. The gap conditions under which X-ray emission takes place, and the correlation between X-ray activity and predischarge current were studied. Steady X-ray activity was observed for a plain gap with electrodes subjected to repeated breakdowns, and for gaps bridged with insulators having low secondary emission yields. The hysteresis in the V-I characteristics of both plain and bridged vacuum gaps is attributed to a regenerative feedback process between X-ray and photoemission processes. The nonresistive component of predischarged current for bridged vacuum gaps obeys the Fowler-Nordheim theory only in the high-field region ( greater than 60 kV/cm), and only in the absence of X-ray activity. Chromium oxide coatings on Wesgo AL-300 alumina ceramic gave a substantial improvement ( greater than 100%) in the voltage hold-off, but at the expense of increased prebreakdown current and X-ray activity. It has been shown that chromium oxide coatings do not improve the voltage hold-off unless a dense, low porosity, surface is produced by the coating.