Stability conditions of argon and helium gas mixtures in an atmospheric pressure plasma jet

Non-equilibrium plasmas can be generated by atmospheric pressure glow discharges, amongst others by atmospheric pressure plasma jets (APPJ), which feature a capacitive radio-frequency discharge between bare metallic electrodes.We investigated the stability conditions for discharges in an APPJ operated with helium–argon mixtures. Uniform α-discharges can be sustained in mixtures ranging from pure helium to pure argon. The ignition voltage increases drastically with argon content. There is also an upper limit for the existence of the α-mode, where α-sheath breakdown occurs. Critical plasma parameters for the α-mode were determined by equivalent circuit models and discussed in respect to dependences on the different models. A critical electron density of 2.4×1011 cm−3 was obtained for pure helium. It increases steadily with argon content and reaches a value of 1.2×1012 cm−3 for pure argon. Sheath thicknesses for α-sheath breakdown were calculated in the range of 0.17–0.29 mm for helium–argon mixtures.