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

摘要

Photo-ionization is the accepted mechanism for the propagation of positivestreamers in air though the parameters are not very well known; the efficiencyof this mechanism largely depends on the presence of both nitrogen and oxygen.But experiments show that streamer propagation is amazingly robust againstchanges of the gas composition; even for pure nitrogen with impurity levelsbelow 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 afeathery structure. In our simulations, we test the relative importance ofphoto-ionization and of the background ionization from pulsed repetitivedischarges, in air as well as in nitrogen with 1 ppm O2 . We also testreasonable parameter changes of the photo-ionization model. We find that photo-ionization dominates streamer propagation in air for repetition frequencies ofat least 1 kHz, while in nitrogen with 1 ppm O2 the effect of the repetitionfrequency has to be included above 1 Hz. Finally, we explain the feather-likestructures around streamer channels that are observed in experiments innitrogen with high purity, but not in air.