Abstract-This paper reports on dependence of voltage rise rate on positive streamer branching and shock waves in supercritical carbon dioxide. Voltages with rise rates of 0.21 and 4.5 kV/ns were applied to a needle-to-plane electrode. Pre-breakdown phenomena involving streamer growth and shock wave propagation were observed by means of a shadowgraph method. Results show that the spread angle of the streamer at the needle tip under 4.5 kV/ns was nearly twice as large as that under 0.21 kV/ns. The discharge may initiate without density reduction due to electrostrictive force. While shock wave Mach number was little affected by the voltage rise rate, it increased by increasing the negative voltage peak. Consequently, the velocity of shock wave is presumed to be influenced by population of the vibrational state of carbon dioxide.Index Terms-Nanosecond pulsed discharge, positive streamer, shock waves, supercritical carbon dioxide, shadowgraph method, electrostrictive force.
I. INTRODUCTIONISCHARGE plasma in supercritical (SC) fluid displays excellent controllability of both physical properties and chemical reactivity and is thus anticipated in new chemistry synthesis fields such as phenol polymerization [1], metallic oxide nanomaterials [2], carbon nanomaterials [3] and nanodiamonds [4]. Recently, the study of a pulsed power switch using SC-CO2 and SC-N2 has been launched as a novel application [5]- [7]. Understanding of streamer discharge and shock waves in SC fluids is of importance in the development of a SC reactor.Several studies related to direct current (DC) breakdown characteristics have been reported. For example, in SC-He, DC breakdown voltage was seen to drop in the vicinity of the critical point under the quite narrow gap of 3 m under a quasi-uniform electric field [8], [9], while in SC-CO2, thisThe manuscript was submitted in March 2016 "This work was supported by Grant-in-Aid for Research Activity Start-up (No. 26889048) and by TEPCO Memorial Foundation".T. Furusato, T. Fujishima, and T. Yamashita are with the Graduate School of Engineering, Nagasaki University, Nagasaki 852-8521, Japan (t-furusato@nagasaki-u.ac.jp).M. Ota, T. Sakugawa, S. Katsuki and H. Akiyama are with the Graduate School of Science and Technology, Kumamoto University, . critical anomaly did not occur under gaps ranging from 80 to 200 m under a non-uniform electric field [10], [11]. Despite these great discrepancies, little attention has been given to the study of pre-breakdown phenomena in SC fluids.No unified pre-breakdown theory in dense medium yet exists due to its complex mechanism, with both the direct impact ionization model and bubble theory independently proposed through many studies [12], [13]. However, recent research has suggested a new pre-breakdown mechanism: ultra-short pulsed voltage in dielectric liquids [14]-[16], which is that nano-sized pores created by the electrostrictive force in the vicinity of the needle tip affects the pre-breakdown mechanism in dielectric liquids with pulse widths ranging from sub-nanosec...