Visualization of positive pulsed streamer in supercritical carbon dioxide by Schlieren Method

This paper deals with visualization of positive pulsed streamers and arc discharge under a needle-to-plane gap in supercritical carbon dioxide (SCCO2) by the Schlieren Method. In addition, differences in streamer branches in gas, liquid and supercritical phases of CO2 are evaluated by means of fractal dimension. The experimental results are summarized as follows: 1) a tree-like streamer growing from the tip of needle was successfully visualized. The field strength for streamer initiation and the mean velocity of the streamer are estimated as approximately 9 MV/cm and 60 km/s, respectively. Pre-breakdown phenomena in SCCO2 are divided into two processes: streamer growth and shockwave expansion. 2) Streamer branches in SC phase were found to be more complicated than in other phase of CO2. 3) Bubble formation in the streamer initiation process was not recognized in SCCO2. For dielectric breakdown, the cylindrical shockwave grows along the streamer channel. A trace of light emission is recognized near the center of the arc column with a residual image of the streamer.