Photographic investigation of formative stage of electric breakdown in diverging fields

A photographic investigation of luminescence in point-plane specimens of solid dielectric is described. Tests were performed with impulse voltages of both polarities on specimens having ratios of thickness t to pin radius r extending from 20 to 250. When t/r < 100, collapse of voltage accompanied luminescence, whose spatial and temporal development was recorded using a streak camera. In those specimens for which t/r > 100, treeing occurred. Light emitted during the inital period of tree development proved to be extremely faint and of short duration, and was recorded using a specially sensitive technique. When (t/r)<100, prebreakdown luminescence began at the specimen cathode irrespective of whether this was the point or the plane electrode. With t/r > 100, luminescence was observed initially at the point electrode and led to the formation of a tree-like structure of microcracks extending into the dielectric from a microscopic tubule at the tip of the point. With the application of many impulses the cracks developed into channels capable of supporting gas discharges, the presence of which marked the establishment of a conventional electrical tree.