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This paper deals with the effect of the polarity and gap geometry on DC breakdown voltage characteristics of a point-to-plane gap in supercritical carbon dioxide (SC CO2) that is required to design a plasma reactor. In the experiments, the gap width d and the curvature radius of the point tip r were changed in the range of 80 to 300 mum and 50 to 170 mum respectively, and the CO2 state was controlled within the gas, liquid, and SC phases. The experimental results showed that a remarkable polarity effect appeared under certain gap conditions. As a result, it was found that negative polarity and a higher stressed electrode are desired conditions for the dc plasma reactor since an active corona supplying rich chemical radicals appears in SC CO2 under such conditions.