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.