A study on positive streamer channels in Marcol Oil

The streamer channels in Marcol oil have been studied under the positive “step” voltages. Experiments were performed with a point-plane gap of 8 cm. Three kinds of streamer channels were observed around breakdown voltage. These are dark, partly illuminating, and fully illuminating (reillumination) channels. No current could be detected for the dark channels. The partly illuminating channels correlate with small current pulses in some hundreds of milli-amperes. However, the reilluminations are associated with much higher current pulses of some amperes. All kinds of channels occur simultaneously and propagate almost equally fast. Streamers with reilluminations almost always terminate with breakdown. On the other hand, streamers with dark or partly illuminating channels have much lower probability for breakdown. Reilluminations do not have significant effects on the propagation of non-breakdown and breakdown streamers as well as the time to cross the gap and breakdown. In the 2^nd mode streamers, 30-50% of their branches are able to become illuminated, and the number of reilluminations as well as illuminating branches is proportional to the applied voltage. The effects of vacuum on streamers were also investigated. With vacuum, streamers travel with much lower voltage drop along the streamer channel. Therefore stopping length greatly increases, and the breakdown voltage drastically drops. All crossing streamers result in breakdown. The number of reilluminations in vacuum is lower than that in ambient pressure. The physics of the above streamer channels is discussed.