Electrode-Gap Effects on the Discharge Structure in Atmospheric Radio-Frequency Microplasmas With He/O2 Mixtures

The electrode-gap effects on the discharge characteristics have attracted increasing attention to the design of atmospheric discharge sources. In this paper, a 1-D fluid model is used to explore the discharge structures in atmospheric radiofrequency microplasmas with He/O₂ mixture as working gas as well as to discuss the generation of reactive oxygen species (ROS) with the electrode gap varied. As oxygen admixtures are introduced into the helium microplasmas, the heavy anions of O^- and O₂^- become the dominative negative species instead of electrons, and then establish the bulk plasma region together with the positive ions. Thus, even at very small gaps, a normal glow-discharge discharge structure can be formed, and a sheath-dominated discharge structure, which is usually formed in pure helium microplasmas in a small size, cannot be observed any more. The simulation results also indicate that the production of main ROS is more effective in small dimensions.