The distribution of F-containing species in atmospheric nanosecond He/CF4 plasma 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/CF 4 atmospheric pressure plasma jet and the effect of CF 4 concentration on the reactive F-containing species. A portion of electron energy is consumed whenever an F is ionized from the CF 4 , resulting that the average densities of CF 4 + , CF 2 + and CF + decrease sequentially. The density of CF 3 + is greater than CF 4 + density due to Penning ionization of He* with CF 4 and He + + CF 4 → CF 3 + + F + He. In the case of He + 0.5% CF 4 at 200 ns, the electron impact reactions mainly concentrate on the streamer head on the dielectric surface. The Penning ionization of He* with CF 4 dominates in the region of 0 < r < 2.2 mm benefits from long-lived He* of high density and the high concentration CF 4 in this region. As gas flow gradually mixes with ambient air ( r > 2.2 mm), the Penning ionization of metastable He with N 2 and O 2 dominates in this area. As the CF 4 concentration increases from 0.1% to 1%, the densities of F-containing positive ions all increase significantly. CF 3 , F and F 2 also show the upward trend while there is little change in the spatial densities of CF 2 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% CF 4 , due to the low density of CF 2 + , CF + and F + and the diffusion of F, the F flux for is consistent with CF 2 and is greater than CF. However, when CF 4 concentration exceeds 0.5%, the concentration of CF 3 + , CF 2 + 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% CF 4 , resulting that the flux of F is lower than that of CF 2 and CF.