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.