On the relationship between streamer branching and propagation in liquids: influence of pyrene in cyclohexane

This paper presents an experimental study on the influence of an aromatic additive (pyrene) on the propagation of positive filamentary streamers and breakdown in liquid cyclohexane. Experiments are carried out in point-plane gaps up to 5 cm over a wide voltage range. With pyrene, the propagation is facilitated, and thus the breakdown voltage is lowered in point-plane geometry. Streamers also become very branched, and a much larger number of filaments propagate than in pure cyclohexane. Correlated to this, a large increase of the inception voltage of fast streamers (acceleration voltage) is observed at high voltage. This fact can be explained by macroscopic electrostatic properties of streamers, which depend on their geometrical structure. When more and more filaments propagate at high voltage they shield each other, which in turn limits their tip field (and thus velocity) at a nearly constant value over a large voltage range. The propagation velocity is thus not only determined by microscopic processes at the filament heads. Consequently, it does not always constitute an adequate parameter to directly appreciate and compare microscopic propagation processes. A discussion about streamer mechanisms is also presented.