Distributions of prebreakdown emission sites on broad area rounded shaped copper cathode of a vacuum gap

The distribution of prebreakdown emission sites distributions on a rounded shaped copper cathode were investigated by using a transparent anode. Oxygen-free copper was used as the cathode material. Experiments revealed that prebreakdown sites were not always located at the position where the field value was strongest, that is, at the center of the cathode. Prebreakdown sites far from the center of the cathode were sometimes observed. Breakdown sites were found not only at the center but also in the surrounding region, where the field strength is much less than that at the center.