Pulsed pre-breakdown phenomena in high pressurized carbon dioxide including supercritical state

Pulsed streamers in highly pressurized carbon dioxide up to and including the supercritical state were observed by means of Schlieren method. The fluid temperature was set to T = 305 K; pressures, to P = 7.5 and 8.0 MPa. The gap was needle-to-plane; the gap lengths in positive and negative were d = 7 and 1 mm, respectively. Pulse voltages of positive/negative polarity generated by a magnetic pulse compression generator were applied to the needle electrode. The main results obtained can be summarized as follows. 1) Positive streamers in supercritical state appeared as a branched tree associated with shock waves of spherical and cylindrical shapes. 2) The negative corona appearance in supercritical state differed greatly from the positive one, and was spherical without branches at the initial stage. The spherical shock wave started from the needle tip at the corona onset. Appearances of positive/negative discharge clearly differed. These results indicate a polarity effect on pulsed discharge phenomena.