Effect of CF4 ratio on the interaction of atmospheric nanosecond pulsed He plasma jet with a dielectric material

Abstract

In this paper, a two dimensional (2D) axisymmetric fluid model is built to study the effect of the ratio of CF4 admixture on the plasma dynamics and F-containing species concentration in He atmospheric pressure plasma jet. The steady mole fraction distribution of He and CF4 is first studied, which presents that the mole fractions of CF4 show peaks at 5 mm from the symmetry axis on the dielectric surface due to the dual influences of the boundary layer effect and air mixing. The CF4 admixture enhances the propagation speed of ionization wave, and the axial velocity reaches the peak value in the case of He + 1.5% CF4. The conversion from the ring-shaped plasma–surface interaction to a solid-disk one results from the addition of CF4. The Penning ionization of CF4 accelerates the plasma jet propagation within 1.5% CF4. However, the excitation energy loss and electron attachment caused by the addition of CF4 also quench the propagation of plasma jet, which become obvious in the case of 2% CF4. F-containing groups (CF4+, CF3+, CF3−, F−, CF3, and F), mainly produced by the Penning ionization reaction, electron attachment reaction, and He+ + CF4 → He + CF3+ + F, also show peaks for He + 1.5% CF4. On the dielectric surface, as the radial distance increases, the ratio of F-containing neutral species (CF3 and F) to O atom at 200 ns decreases due to the increase of O atom concentration and then increases at the streamer head because the surface flux of CF3 reaches the maximum value. The maximum surface flux radial distribution distance for ratio of F-containing species to O atom, CF3 and F appear in the case of 1.5% CF4.

The distribution of F-containing species in atmospheric nanosecond He/CF4 plasma with downstream dielectric material

“…At 62 ns, the ionization wave approaches the dielectric surface. It can be clearly seen a solid-disk pattern on plasmasubstrate interface due to the addition of CF 4 [18,34]. Subsequently, the ionization wave propagates radially outward and stops when the calculation ends at 200 ns.…”

“…The geometric model of the simulation is shown in figure 1, which is exactly the same as our previous work [18]. The model consists of a gas flow module and a plasma dynamic module.…”

“…The model consists of a gas flow module and a plasma dynamic module. The outputs of the gas flow module, namely the mole fraction of He, CF 4 and air, have been shown in [18] and are used as the inputs of the plasma module. C.…”

“…C. The added reactions related to F-containing species and some reactions that needed to be discussed in this paper are listed in table 1 while the rest reactions are the same to [18]. The generation and propagation of APPJ require the seed electron.…”

“…What's more, the enhanced energy width of ion energy distribution can be achieved by increasing the voltage or diminishing the frequency of the low frequency source. Our previous work [18] firstly presented a two-dimensional (2D) self-consistent fluid model of He/CF 4 APPJ impinging on the dielectric surface. We proposed that the addition of CF 4 had dual effects on APPJ propagation: on the positive side, the admixture of CF 4 accelerates the propagation speed of the ionization wave due to the Penning ionization between He * and CF 4 and the velocity reaches the maximum value at 1.5% CF 4 .…”