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 CO
₂
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 CO
₂
density in the gas phase. However, they were constant at the CO
₂
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 CO
₂
density was all similar to the breakdown voltage, arc current, and consumption energy characteristics except under the critical density of CO
₂
(469 kg/m
³
). Although the breakdown voltage, arc current, and consumption energy in supercritical phase were also constant against the CO
₂
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 CO
₂
which has a local maximum around the critical density.