The charge spreading due to micro discharges on insulating films from an evaporated metal electrode subjected to a DC ramp voltage

The charge spreading phenomena on polymeric insulating films from a circular evaporated electrode were studied in the atmospheres of air and nitrogen gas atmospheres on the application of a slowly increasing dc ramp voltage. The flash over voltage between the circular and its opposing concentric ring electrodes on the film was also measured. The observation on very faint light using an image intensifier camera revealed that micro discharges occurred repeatedly at the surrounding region of the circular electrode as the voltage increased. When the positive voltage was applied to the circular electrode, several or more streamer-like micro discharges occurred in a row at the same time in both atmospheres. In the negative polarity, a single fan-shaped micro discharge occurred at a time in air and luminous regions were observed as one or a few small spots at the electrode edge in nitrogen gas. The electrons created by the negative micro discharges in nitrogen gas had a major role of the expansion of charging region. The flash over voltage in air was higher than that in nitrogen gas in both polarities; more remarkable difference in the flash over voltage was obtained in the negative polarity. The voltage for the negative polarity was much higher than that for the positive polarity in air, while the former was rather slightly lower than the latter in nitrogen gas