Fractal Analysis of Positive Pulsed Streamer Patterns in Supercritical Carbon Dioxide

A positive pulsed prebreakdown in a needle-to-plane gap at CO₂ phases up to supercritical (SC) condition was observed by means of Schlieren imaging method. Our observations confirmed that the pattern of positive prebreakdown discharge was a treelike streamer independent of medium phase and that the SC phase led to streamer branches of higher complexity than those in the gas and liquid phases. Both streamer branches´ complexity and fractal dimension variation of the developing streamer were quantitatively assessed by means of fractal analysis. Experimental results are summarized as follows: 1) SC carbon dioxide (SCCO₂) led to the greatest complexity of streamer branching in comparison with gas and liquid phases; 2) the fractal dimensions of gas, liquid, and SC phases of CO₂ were estimated to around D = 1.47, 1.60, and 1.73 with a coefficient of determination R² >; 0.99; and 3) the fractal dimension D in SCCO₂ stayed constant at around D = 1.7 during the streamer growth phase.