Anomalous plasma temperature at supercritical phase of pressurized CO2 after pulsed breakdown followed by large short-circuit current

The relation between breakdown characteristics and plasma temperature of a pulsed arc discharge in highly pressurized CO2 was investigated up to a supercritical phase. A transient arc discharge was generated by applying nanosecond pulsed voltage with a rising rate of 0.7 kV/ns to a point-to-plane gap of 1 mm. The breakdown voltage, arc current, and consumption energy increased with the CO2 density in the gas phase. However, they were constant at the CO2 densities in the supercritical phase. The plasma temperature determined from the blackbody radiation ranged from 8000 to 12000 K and its dependence with respect to the CO2 density was all similar to the breakdown voltage, arc current, and consumption energy characteristics except under the critical density of CO2 (469 kg/m3). Although the breakdown voltage, arc current, and consumption energy in supercritical phase were also constant against the CO2 density, the plasma temperature demonstrated a local decrease around the critical density. The anomaly of the plasma temperature is consistent with the calculated result of the isochoric specific heat of CO2 which has a local maximum around the critical density