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.