The distribution of F-containing species in atmospheric nanosecond He/CF4plasma with downstream dielectric material

Abstract

A two-dimensional self-consistent model has been established in order to investigate the distribution of F-containing species in He/CF4 atmospheric pressure plasma jet and the effect of CF4 concentration on the reactive F-containing species. A portion of electron energy is consumed whenever an F is ionized from the CF4, resulting that the average densities of CF4+, CF2+ and CF+ decrease sequentially. The density of CF3+ is greater than CF4+ density due to Penning ionization of He∗ with CF4 and He+ + CF4 → CF3+ + F + He. In the case of He + 0.5% CF4 at 200 ns, the electron impact reactions mainly concentrate on the streamer head on the dielectric surface. The Penning ionization of He∗ with CF4 dominates in the region of 0 < r < 2.2 mm benefits from long-lived He∗ of high density and the high concentration CF4 in this region. As gas flow gradually mixes with ambient air (r > 2.2 mm), the Penning ionization of metastable He with N2 and O2 dominates in this area. As the CF4 concentration increases from 0.1% to 1%, the densities of F-containing positive ions all increase significantly. CF3, F and F2 also show the upward trend while there is little change in the spatial densities of CF2 and CF. The F-containing neutral species flux benefits from two factors: the diffusion and the conversion of positive ions. In the case of 0.1% CF4, due to the low density of CF2+, CF+ and F+ and the diffusion of F, the F flux for is consistent with CF2 and is greater than CF. However, when CF4 concentration exceeds 0.5%, the concentration of CF3+, CF2+ and CF+ are much higher than F+. The quenching of positive ions dominates on the dielectric surface in the case of 0.5% and 1% CF4, resulting that the flux of F is lower than that of CF2 and CF.