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.

CF4 is an important source of fluorine groups in atmospheric pressure plasma jet (APPJ). In order to obtain reactive fluorine species under atmospheric pressure, noble gas (Ar or He) and CF4 are usually mixed and used as the working gas of APPJ. In this paper, the differences in the discharge dynamics on He/CF4 APPJ and Ar/CF4 APPJ are investigated experimentally. Meanwhile, combined with simulation, the effects of downstream targets with different relative permittivity on the radial propagation range of the plasma plume and the distribution of F-containing reactive species are studied. It is discovered that the addition of a small amount of CF4 (20 sccm) will increase the intensity of He/CF4 APPJ due to the contribution of Penning ionization of metastable He with CF4. Differently, the addition of CF4 will continuously lead to a significant decrease in the intensity of Ar/CF4 APPJ. The radial propagation range of He/CF4 APPJ on the target surface decreases with the increase in the relative permittivity of the downstream target. The smaller relative permittivity inhibits the axial propagation speed of APPJ, but it increases the radial propagation range of reactive species. The larger relative permittivity promotes the production of F-containing reactive species and their flux intensity on the target surface.

O2 impurity in the working gas of an He/CF4 atmospheric pressure plasma jet (APPJ) can affect the discharge dynamics and the density of reactive species. Therefore, a two-dimensional (2D) fluid model is built in order to explore the influence of an O2 admixture on the propagation and F-containing species distribution of He/CF4 APPJ. The addition of 0.1% O2 accelerates the ionization rates of APPJ due to the increase of Penning ionization reactions of O2, resulting in the increases of axial speed and F-containing reactive species (CF4+, CF3+, CF2+, CF+, F+, CF3, F, CF3−) when APPJ approaches the dielectric surface. The addition of O2 has the inhibitory effect on the rise of some F-containing reactive species (CF3+ and F). As O2 concentration increases to 2%, the concentration of F-containing reactive species shows a downward trend due to the increase of excitation energy loss and an electron attachment reaction of O2. Different from the axial speed, the radial speed decreases continuously with the increase of O2 because of the high O2 concentration on the dielectric surface when APPJ propagates radially. This also results in a reduced distribution of reactive species fluxes. The excitation energy loss and electronegativity of O2 and CF4 in the case of He + 0.5% CF4 + 0.5% O2 have been presented in this paper. It is discovered that excitation energy loss of O2 is stronger than that of CF4, but the electronegativity of CF4 is stronger than that of O2.

A two dimensional (2D) axisymmetric fluid model is built to investigate the effect of different O2 and H2O admixture on the plasma dynamics and the distribution of reactive species in He atmospheric pressure plasma jet (APPJ). The increase of O2: H2O ratio slows down both the intensity and the propagation speed of ionization wave. Due to the decrease of both H2O ionization rate and H2O Penning ionization as well as the stronger electronegativity of O2, the increase of O2: H2O ratio results in a significant reduction of electron density in the APPJ, which restricts the occurrence of electron collision ionization reactions and inhibits the propagation of plasma. The excitation energy loss of O2 is not the reason for the weakening of the plasma ionization wave. The densities of O2+, O- and O2- increase with the rise of O2 admixture while H2O+ decreases due to the decrease of electron density and H2O concentration. OH- density is affected by both the increase of O- and the decrease of H2O so it shows peak in the case of O2: H2O=7:3. O is mainly produced by the excitation reactions and the electron recombination reaction (e + O2+ → 2O), which is directly related to the O2 concentration. OH is mainly produced by e + H2O → e + H + OH so the OH density decreases due to the decrease of electron density and H2O concentration with the increase of O2: H2O ratio. On the dielectric surface when the propagation of streamer extinguishes, O flux shows an upward trend while the OH flux decreases, and the propagation distance of O and OH decreases with the increase of O2: H2O ratio.

A two dimensional self-consistent model has been established in order to investigate the distribution of F-containing species in He/CF4 APPJ 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.