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Abstract: Positive streamers are thought to propagate by photo-ionization whoseparameters depend on the nitrogen:oxygen ratio. Therefore we study streamers innitrogen with 20%, 0.2% and 0.01% oxygen and in pure nitrogen, as well as inpure oxygen and argon. Our new experimental set-up guarantees contamination ofthe pure gases to be well below 1 ppm. Streamers in oxygen are difficult tomeasure as they emit considerably less light in the sensitivity range of ourfast ICCD camera than the other gasses. Streamers in pure nitrogen and in allnitrogen-oxygen mixtures look generally similar, but become somewhat thinnerand branch more with decreasing oxygen content. In pure nitrogen the streamerscan branch so much that they resemble feathers. This feature is even morepronounced in pure argon, with approximately 10^2 hair tips-cm^3 in thefeathers at 200 mbar; this density could be interpreted as the free electrondensity creating avalanches towards the streamer stem. It is remarkable thatthe streamer velocity is essentially the same for similar voltage and pressurein all nitrogen-oxygen mixtures as well as in pure nitrogen, while the oxygenconcentration and therefore the photo-ionization lengths vary by more than fiveorders of magnitude. Streamers in argon have essentially the same velocity aswell. The physical similarity of streamers at different pressures is confirmedin all gases; the minimal diameters are smaller than in earlier measurements.



Autor: S Nijdam, F M J H van de Wetering, R Blanc, E M van Veldhuizen, U Ebert

Fuente: https://arxiv.org/







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