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<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">%</sup>02/90% CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>, 30% O<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>/ 70% CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> and 5% C4-FN / 50% O<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>/ 90% CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf>. 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 CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> /O<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> mixtures, a scaling constant of 12 - 20 Pa m for positive streamers and 20 - 35 Pa <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$m$</tex> for negative streamers is found. In the C4-FN/ CO<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> /O<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> mixture, a value of 13 - 18 Pa m for positive streamers and 18 - 25 Pa <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$m$</tex> 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.