In this work, the initiation process of an electrical discharge in pressurized carbon dioxide up to the liquid and supercritical phases was investigated using Schlieren and photomultiplier techniques. A pulsed positive voltage with a rise time of about 40 ns and half-width of 150 ns was applied to a point-to-plane gap. The experimental results showed that the discharge started with a primary streamer consisting of a burst primary streamer and a successive primary streamer, and a back discharge followed them. It was predicted from an analysis of the experimental results that the initiation criterion of a burst primary streamer was electron multiplication on the order of 108 independent of the medium conditions. That is, a direct ionization process without bubble formation is supported as an initiation mechanism of nanosecond positive discharges in pressurized carbon dioxide. The medium state in the burst primary streamer channel was a gas or pseudo-gas of 50–200 kg m−3 density.
A positive pulsed prebreakdown in a needle-to-plane gap at CO 2 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 (SCCO 2 ) 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 CO 2 were estimated to around D = 1.47, 1.60, and 1.73 with a coefficient of determination R 2 > 0.99; and 3) the fractal dimension D in SCCO 2 stayed constant at around D = 1.7 during the streamer growth phase.Index Terms-Fractal analysis, nanosecond pulsed discharge, positive streamer, supercritical carbon dioxide (SCCO 2 ).
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