Observation of pre-breakdown emission sites and breakdowns of vacuum gaps under short-pulsed testing

Emission activity of the stainless steel cathode under 200 kV, 50 to 200 ns voltage pulses was studied with the use of a 4-channel, 12-bit ICCD camera. The gap was formed by a bulk cathode and a thin-foil titanium anode. Both the cathode and anode surfaces were cleaned and polished preliminary by means of electron-beam treatment in the mode of short-time surface melting in vacuum. Observations gave a ground to tell about prebreakdown emission sites of two kinds that are stable emission sites (SES´s) and unstable ones (UES´s), which are different in behavior and in ability to initiate breakdowns. SES´s were recognized on the cathode at relatively low electric field strengths (at 0.55 MV/cm in average). Close to 1 MV/cm, UES´s start to be recognized in images. An UES´s pattern is changed from shot to shot without breakdowns. Often, breakdowns occur in spatial locations different from UES´s recorded in the previous shot. The results obtained could be of interest in study of pre-breakdown and breakdown phenomena at extremely high electric fields in vacuum.