Probing photo-ionization: Experiments on positive streamers in pure gasses and mixtures

TL;DR

This study investigates how photo-ionization influences positive streamer propagation across various gases and mixtures, revealing consistent velocities despite differing photo-ionization lengths and detailed streamer morphology variations.

Contribution

It provides new experimental data on streamer behavior in pure and mixed gases with contamination control, highlighting the effects of oxygen content on streamer morphology and confirming physical similarity across pressures.

Findings

Streamer velocity is consistent across different gases and oxygen concentrations.

Streamer morphology varies with oxygen content, becoming thinner and more branched at lower oxygen levels.

Pure nitrogen and argon streamers exhibit significant branching, resembling feathers.

Abstract

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