Prebreakdown light emission and density change streamer in liquid helium

The investigations of two prebreakdown phenomena, the light emission and the density change streamer in liquid helium are reported. Concerning the observation of light emission phenomena, the light pulses and the current pulses were measured simultaneously. The activity of the light emission was detected using a photomultiplier tube. Two structures of light emission, filamentary and bush-like, were found for both positive and negative polarities. Most of the negative light emission was a filamentary structure, while it was a bush-like one for the positive. Light emission appeared for only a few microseconds for a negative needle, while it lasts much longer than 25 /spl mu/s for a positive needle. The pulse frequency of the light emission differed between the two polarities. For the density change streamer, only the bush-like streamer was observed in positive polarity. Similar to the light emission, the positive density change streamer lasted longer than the negative one. The density change streamer and the current pulses were observed simultaneously and their relation was discussed.