Probing photo-ionization: experiments on positive streamers in pure gases and mixtures

Abstract

Positive streamers are thought to propagate by photo-ionization whose\nparameters depend on the nitrogen:oxygen ratio. Therefore we study streamers in\nnitrogen with 20%, 0.2% and 0.01% oxygen and in pure nitrogen, as well as in\npure oxygen and argon. Our new experimental set-up guarantees contamination of\nthe pure gases to be well below 1 ppm. Streamers in oxygen are difficult to\nmeasure as they emit considerably less light in the sensitivity range of our\nfast ICCD camera than the other gasses. Streamers in pure nitrogen and in all\nnitrogen/oxygen mixtures look generally similar, but become somewhat thinner\nand branch more with decreasing oxygen content. In pure nitrogen the streamers\ncan branch so much that they resemble feathers. This feature is even more\npronounced in pure argon, with approximately 10^2 hair tips/cm^3 in the\nfeathers at 200 mbar; this density could be interpreted as the free electron\ndensity creating avalanches towards the streamer stem. It is remarkable that\nthe streamer velocity is essentially the same for similar voltage and pressure\nin all nitrogen/oxygen mixtures as well as in pure nitrogen, while the oxygen\nconcentration and therefore the photo-ionization lengths vary by more than five\norders of magnitude. Streamers in argon have essentially the same velocity as\nwell. The physical similarity of streamers at different pressures is confirmed\nin all gases; the minimal diameters are smaller than in earlier measurements.\n