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The paper describes results of experimental studies to evaluate the effects of phase-change and/or density variation at the post-breakdown on the dielectric recovery characteristics of liquid and supercritical CO2. The study aims to explore the possibility of using supercritical CO2 to develop high-repetition-rate pulsed-power switches. The experimental setup consisted of a sphere-to-sphere 10 mm radius electrodes configuration with 50 mu m gap distance. Three phases of pressurized CO2; subcritical liquid state (SLS), liquid with pressure higher than critical point (LHCP), and supercritical phase (SC), were used for the experiments. Experimental results revealed that SC CO2 has a very short recovery time making it an appropriate fluid for MHz range highly-repetitive switches. Post-breakdown stage in SLS consisted of gas phase and liquid phase at the plasma channel between the electrodes, with the alternative expansion-contraction motion behavior of gas bubble determining dielectric recovery characteristics. Post-breakdown stages in SC were similar to those in LHCP, generating a low density region at the post-breakdown, where the state of density variation determined dielectric recovery characteristic. The low density region could be compressed by the surrounding medium, resulting in a considerably shorter recovery time.
