Influence of nitrogen and oxygen admixture on the development of helium atmospheric-pressure plasma jet

This paper presents the experimental and numerical results about the influence of nitrogen (N-2) and oxygen (O-2) admixture on the development of a helium (He) atmospheric-pressure plasma jet (APPJ) in a long dielectric tube. The results revealed that the jet length and the propagation velocity are strongly affected by introducing N-2 or O-2 into the He flow. Specifically, it was observed that a higher N-2/O-2 admixture led to the decrease in the density of both energetic and relative low-energy electrons outside the grounded electrode, which corresponds to the shortening of the jet length. In the He/O-2 mixture, the electrons are easily captured by O-2/O in the region of the plasma bulk. In the He/N-2 mixture, the jet propagation characteristics will change since N-2 has many low-level excitation states that consumed a large number of energetic electrons. The simulation shows that the magnitude of the axial electric field in the jet head depends strongly on the amount of N-2 and/or O-2 in the gas flow. In both cases, the peak electric field is on the order of 5kV/cm, which is significantly higher than that in pure helium of 3kV/cm even if the admixture is low (less than 4% N-2 or 2% O-2 in this work). Positive charges of higher density in the jet head are needed to induce a stronger electric field for the jet propagating in N-2(x%)/He and O-2(x%)/He mixtures compared with that in pure He.