Negative DC Breakdown Characteristics of C3F7CN / CO2 Gas Mixture for Application in High Voltage Accelerators

Abstract

This study investigates the technical feasibility of retro-filling a C<inf>3</inf>F<inf>7</inf>CN/CO<inf>2</inf> gas mixture for existing SF<inf>6</inf>-insulated accelerators. Negative DC breakdown characteristics of 20% C<inf>3</inf>F<inf>7</inf>CN / 80% CO<inf>2</inf> gas mixture and SF<inf>6</inf> for varying pressure, gap distance and field uniformity are analyzed experimentally using four distinctly different electrode configurations. The C<inf>3</inf>F<inf>7</inf>CN/CO<inf>2</inf> gas mixture shows a higher breakdown strength than SF<inf>6</inf> for configurations with high field uniformity, whereas SF<inf>6</inf> outperforms C<inf>3</inf>F<inf>7</inf>CN/CO<inf>2</inf> gas mixture under the non-uniform field. Importantly, 20% C<inf>3</inf>F<inf>7</inf>CN / 80% CO<inf>2</inf> gas mixture exhibits comparable insulation capability to SF<inf>6</inf> in a 10/30 mm coaxial geometry with similar field uniformity as found in the practical gas-insulated equipment. The results of this work contribute to the development of a potential retro-fill solution using 20% C<inf>3</inf>F<inf>7</inf>CN / 80% CO<inf>2</inf> gas mixture for accelerators.