Positive Point-to-Plane Corona Studies in Air

Formative time lags for the development of the positive point-to-plane corona in dry air were measured oscillographically at pressures ranging from atmospheric to a few centimeters of Hg. Studies with a photomultiplier tube show that the observed formative lags are associated with a filamentary streamer type of corona. These corona formative lags are of the order of 10^-7 sec even near threshold and vary much more slowly with overvoltage than do uniform field formative lags in air. The results indicate that no long buildup process is associated with the formation of the filamentary streamer type of corona in air, and in particular rule out any cathode secondary mechanism from playing a role in the formation. Near atmospheric pressure, with the experimental conditions used, the corona formative time lags were often too short to be resolved from the statistical scatter; when resolvable they were found to be too long to be ascribed solely to a single transit time of the initiating electron avalanche across the high field region of the gap. The results therefore do not preclude a fast buildup process in the gas preceding streamer formation.Threshold measurements on both impulse and dc corona indicate that the steady glow type of corona has a different threshold than the streamer type. No formative lag data on the steady glow corona were obtained.