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.

In this paper, the dielectric breakdown properties in SF6/CO2 mixed gas, the development of the streamer in SF6/CO2 mixed gas, and the distribution of each component with time were studied. First, the electron transport parameters (mean energy, longitudinal diffusion coefficients, Townsend coefficient, critical reduced electric field coefficients, and electron energy distribution function) in SF6/CO2 mixtures with different ratios in the E/N range of over 50–1000 Td were obtained by two-term Boltzmann equation analysis. Then, coupled with the Boltzmann drift–diffusion equation and Poisson equation, the hydrodynamic model of discharge of SF6/CO2 mixtures in a strongly non-uniform electric field was established. Many different influence factors are considered, such as the gas mixture ratio, applied voltage, space temperature, space pressure, and electrode structures. The results indicate that the increase in SF6 content in the mixed gas will reduce the ionization rate of the total mixed gas, and therefore, it takes a longer time for breakdown. The higher the pressure, the more concentrated the form of the streamer. As the temperature increases, the shape of the streamer head becomes more scattered, and it loses its contoured shape at about 3000 K; in addition, the existence of the maximum electron number density value appears at the tip of the rod electrode rather than at the streamer head. The simulation also revealed that the dielectric strength of SF6/CO2 mixtures is stronger than that of SF6/N2 mixtures and reached a turning point at an SF6 ratio of 60% under extremely non-uniform electric fields, which agreed well with experiments.