Polarity effect of density change streamer structure in liquid helium

Abstract

The prebreakdown density change streamers and the current pulses in liquid helium were investigated. A pointplane electrode system, consisting of a tungsten needle whose tip radius is 1 pm and a stainless steel plane was used. The streamers under impulse voltage conditions were observed using high-speed photography for both negative and positive polarities. For a negative streamer, the structure was only filamentary and propagates with very high speed. On the other hand, for a positive streamer, the structure was mainly spherical or hemispherical, and filamentary structure similar to the negative one was observed for a rare case. Comparing the propagation speed of filamentary streamer, the negative streamer had two orders of magnitude higher than the positive one.