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

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.