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

This study investigates the technical feasibility of retro-filling a C3F7CN/CO2 gas mixture for existing SF6-insulated accelerators. Negative DC breakdown characteristics of 20% C3F7CN / 80% CO2 gas mixture and SF6 for varying pressure, gap distance and field uniformity are experimentally analyzed using four distinctly different electrode configurations. The C3F7CN/CO2 gas mixture shows a higher breakdown strength than SF6 for configurations with high field uniformity, whereas SF6 outperforms C3F7CN/CO2 gas mixture under the non-uniform field. Importantly, 20% C3F7CN / 80% CO2 gas mixture exhibits comparable insulation capability to SF6 in a 10/30 mm coaxial geometry with similar field uniformity as found in practical gas-insulated equipment. The results of this work contribute to the development of a potential retro-fill solution using 20% C3F7CN / 80% CO2 gas mixture for accelerators.