Prebreakdown light in glass-plate/polymer-film composite subjected to dc ramped voltages

Breakdown processes of a dielectric double layer under point-sphere electrode system subjected to dc ramped voltages were studied. The double layer consisted of a 1 mm-thick slide-glass-plate and a 50 μm-thick polymer film. In order to clarify the formative process of the complete breakdown, photographic observation was carried out using a long image guide scope and a high-speed-gated image intensifier. When a dc ramped voltage with 1.0 kV/s was applied to the composite, the initial discharge light of the complete breakdown was always located at the tip of the point electrode. However, the initial discharge light was observed on the lower-sphere electrode in the case of 0.1 kV/s. This was because a microdischarge due to film breakdown caused by charges accumulation at the glass-film interface induced the streamer propagation from the sphere to the point electrode in both the polarities.