Measurement of Streamer Radii in SF₆ Alternatives: Mixtures of CO₂/O₂ and C4F7N in CO₂/O₂

Streamer discharges are imaged in three gas mixtures of interest for high voltage insulation equipment - 10%02/90% CO₂, 30% O₂/ 70% CO₂ and 5% C4-FN / 50% O₂/ 90% CO₂. The radii of streamer channels are evaluated, and the scaling constant for the inverse law relationship between the streamer radius and pressure is determined. In CO₂ /O₂ mixtures, a scaling constant of 12 - 20 Pa m for positive streamers and 20 - 35 Pa $m$ for negative streamers is found. In the C4-FN/ CO₂ /O₂ mixture, a value of 13 - 18 Pa m for positive streamers and 18 - 25 Pa $m$ for negative streamers fits the evaluated radii. The scaling constant is a crucial parameter when modelling the breakdown process in these mixtures and the radii values can be utilized for validating numerical streamer simulation codes.