Morphology and Development of Underwater Subsonic Streamer Under Needle to Plane Electrodes

An integrated observation platform for underwater streamer discharge were established in order to study the mechanism of underwater subsonic streamer morphology and development. The polarity effect on the streamer morphology and development was analyzed from the perspective of microphysics. The initial stage of streamer development can be divided into: density fluctuation of liquid around electrode tip, micro-bubble initiation and streamer initiation from the micro-bubble. The difference between the positive and negative streamers lies in the streamer morphology and development process. The morphology of positive streamer goes a transition process. The positive streamer propagates in the shape of bubble cluster with a low speed (20~80m/s) at the most time. However, at the final stage, a tree-like luminous streamer (secondary streamer) is formed on the bubble cluster tip propagating in a very fast speed (> 30km/s). The negative streamer has a tree-like structure with numerous filaments. The negative streamer develops in a period branching mode with a speed of 100~350m/s. At the final stage of the negative streamer development, there is a remarkable increase of the streamer velocity. The distinct difference between the mobility of electron and ion, and the interface instability caused by deposited charges results in the polarity effect which leads to higher initiation voltage, propagation speed and divergent shape of the negative streamer.