Stepwise Expansion Pattern of Surface Streamer Propagation in C₄F₇N/CO₂ Mixture Under Impulse Voltage

C<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub>F<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub>N/CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> mixture is one of the most promising environmentally friendly alternatives to SF<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> gas. However, the research on the development characteristics of surface discharge for C<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub>F<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub>N/CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> mixture is still insufficient. In this study, the surface streamer propagation pattern in 0.1 MPa 10%C<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub>F<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">7</sub>N/90%CO<sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> mixture under impulse voltage with a needle-plate electrode configuration is investigated. From the discharge morphology, it can be observed that surface discharge exhibits a distinct stepwise expansion pattern. The first-step streamer develops outward from the needle tip and appears in a circular shape. Subsequent streamers grow out from the head of the previous streamer and extend outward in a fan-shaped pattern. Compared to positive polarity, surface streamers are more difficult to develop under negative polarity voltage. This is due to the more significant corona stabilization effect observed in negative polarity streamer discharges. Each stepwise expansion of surface streamers corresponds to a streamer-to-leader transition. The surface streamers transition to leaders through the precursor mechanism. Only when the electric field at the head of the streamer reaches the critical electric field, the streamer-to-leader transition can be triggered, resulting in the stepwise expansion pattern. In this study, the threshold potential at the head of the streamer for the transition from primary streamer to primary leader is approximately +5.44 kV for positive polarity and −7.78 kV for negative polarity. Due to the lack of relevant parameters, it is not possible to calculate the critical electric field at the head of the primary streamer in this study.