Initial images of complete breakdown of liquid nitrogen/polymer film

This paper describes an experimental study,on the prebreakdown phenomena in a liquid-nitrogen/polymer-film composite subjected to dc ramp and step voltages under point-plane electrode system. In order to clarify the formative process of the complete breakdown of the composite, photographic observation was carried out using a long image guide scope and an image intensifier with high speed gate in the nanosecond region. When a negative-plane electrode was covered by a polymer film (N-cover arrangement), a luminous region on the initial image of the complete breakdown was located near the negative-plane under both the ramp and the step voltages. These results suggested that the complete breakdown of the composite in the N-cover arrangement was caused by the film breakdown due to positive charges deposition and subsequent electron emission from the film breakdown spot. When a positive-plane electrode was covered by the film (P-cover arrangement), film breakdown could not trigger the complete breakdown under the ramp voltage because the film breakdown occurred at a low applied voltage. However, intense light near the positive-plane was observed at the initial stage of the complete breakdown under the relatively high step voltage. This was because the film breakdown under the high applied voltage induced the positive streamer propagation from the film breakdown spot.