Investigation of spatial correlations of pre-breakdown emission sites and breakdowns of vacuum gaps under short-pulsed testing

Abstract

Emission activity of the stainless steel surface under 200-kV, 100-ns dc voltage pulses was studied with the use of the 4-channel, 12-bit ICCD camera. The gap was formed by a bulk cathode and a thin-foil anode. Both the cathode and anode surfaces were cleaned and polished preliminary by means of electron-beam treatment in the mode of short-time melting in vacuum. Observations gave a ground to tell about pre-breakdown emission sites of two kinds that are stable emission sites (SES's) and unstable ones (UES's), which are different in behaviour and in ability to initiate breakdowns. SES's occurred on the cathode surface lead to breakdowns coinciding with SES's in location. Breakdown strength as high as 1 MV/cm and even higher is achieved only under condition of absence any SEC. Close to 1 MV/cm, UES's start to be recognized in images. An UES pattern is changed from shot to shot without breakdowns. As a rule, breakdowns occur in locations different from UES's, recorded in the previous frame just before the breakdown. The results obtained could be of interest in study of pre-breakdown phenomena at extremely high electric fields in vacuum.