Probing photo-ionization: simulations of positive streamers in varying N2:O2 mixtures

TL;DR

This study uses simulations to investigate how photo-ionization influences positive streamer propagation in various N2:O2 mixtures, revealing its dominance in air and the impact of background ionization, and explaining observed feather-like structures.

Contribution

The paper provides new insights into the role of photo-ionization and background ionization in streamer propagation across different gas mixtures, with detailed simulation analysis.

Findings

Photo-ionization dominates in air at ≥1 kHz repetition frequency.

Background ionization significantly affects streamer behavior in pure nitrogen.

Feather-like structures are explained by photo-ionization effects in high-purity nitrogen.

Abstract

Photo-ionization is the accepted mechanism for the propagation of positive streamers in air though the parameters are not very well known; the efficiency of this mechanism largely depends on the presence of both nitrogen and oxygen. But experiments show that streamer propagation is amazingly robust against changes of the gas composition; even for pure nitrogen with impurity levels below 1 ppm streamers propagate essentially with the same velocity as in air, but their minimal diameter is smaller, and they branch more frequently. Additionally, they move more in a zigzag fashion and sometimes exhibit a feathery structure. In our simulations, we test the relative importance of photo-ionization and of the background ionization from pulsed repetitive discharges, in air as well as in nitrogen with 1 ppm O2 . We also test reasonable parameter changes of the photo-ionization model. We find that photo- ionization dominates streamer propagation in air for repetition frequencies of at least 1 kHz, while in nitrogen with 1 ppm O2 the effect of the repetition frequency has to be included above 1 Hz. Finally, we explain the feather-like structures around streamer channels that are observed in experiments in nitrogen with high purity, but not in air.