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

Abstract

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 O 2 . We also test reasonable parameter changes of the photo-ionization model. We find that photoionization dominates streamer propagation in air for repetition frequencies of at least 1 kHz, while in nitrogen with 1 ppm O 2 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.

Spatiotemporally resolved imaging of streamer discharges in air generated in a wire-cylinder reactor with (sub)nanosecond voltage pulses

“…It is generally found and understood that a higher applied electric field increases the streamer propagation velocity and that the velocities are in the range of 10 5 -10 7 m·s −1 [2,5,6,9,3,11,4,47,12]. Furthermore, various experiments show that negative streamers generally have a lower propagation velocity than positive streamers [12,48,47,45,40,10,6], which is surprising because negative streamers propagate through electron drift and positive streamers propagate against the electron drift through photoionisation (and to a lesser extent background ionisation) [9,38,49]. Therefore, for identical field enhancement at the streamer head, and a similar electron distribution, a negative streamer is expected to propagate faster [50].…”

Numerical simulation of back discharge: Influence of pinhole geometry on the regime transitions

“…[17]. The detachment process [26,27], as well as photoionization [26,28], are simulated by assuming an initial background of electrons of n e ¼ 10 10 cm À3 at the pinhole region to ensure the possible propagation of a positive streamer.…”

Positive streamers in air of varying density: experiments on the scaling of the excitation density

“…The production of various excited species and the increase of the electron density due to the passage of a streamer were simulated in several papers [3,[23][24][25][26][27][28][29][30][31][32][33], to name but a few, most of which agree that the electron density on the axis of the streamer is on the order of 10 13 − 10 14 cm − 3 . In general, the density of the excited species on the streamers axis will depend both on the neutral gas density and on the maximal electric field at the streamer tip.…”

“…We do not treat photo-ionization here, as several works demonstrated that this effect can be neglected at ground pressure. Photo-ionization may result in the breaking of scaling when laboratory streamers are compared with sprites [25,30,40].…”