Shock-wave propagation in supercritical CO₂ induced by nanosecond-pulsed arc plasma

Shock waves generated by arc plasma in supercritical (SC) CO₂ have the potential to create novel reaction fields. However, there have been few studies of shock-wave characteristics in SC-CO₂. This study provides the results of visualization of shock-wave propagation in SC-CO₂ generated by nanosecond-pulsed arc plasma. A propagating cylindrical shock wave originating from a discharge channel was observed using time-resolved shadowgraph imaging. The shock wave separated from the cylindrical dark-region induced by pulsed arc plasma within 64 nanoseconds. The Mach numbers of the shock waves were investigated against the medium density of the CO₂ ranging from a high-pressure gas phase to the SC phase. The Mach number reached a local maximum at the critical CO₂ density level. The anomaly in the Mach number can be explained by the local maximum of the specific heat capacity ratio of the CO₂ at close to the critical condition of SC-CO₂. (letter)