Characterization of α and γ modes in radio-frequency nonthermal atmospheric plasmas

Nonthermal atmospheric gas discharges generated at radio frequencies have recently commanded much interest because of their immense potential for numerous applications such as etching, deposition, surface modification and sterilization. Through several detailed studies, much has been gained in the understanding of their basic electrical characteristics, their optical emission and their reaction chemistry^1-2. These studies are typically performed at a limited number of discrete points of input RF power, and as such the general perception of RF nonthermal atmospheric plasmas is that their behaviour remains largely unchanged as long as they remain diffuse and glow. Recently we reported an experimental observation that within the diffuse and glow state RF nonthermal atmospheric plasmas can behave very differently at different levels of input power³. By comparison with RF glow discharges at intermediate pressure (10 - 100 Torr), we established that there are at least two modes, α and γ modes, in RF nonthermal atmospheric plasmas and that their current-voltage characteristics are very different. In this contribution, we report theoretical identification and characterization of these two modes based on a nonequilibrium and self-consistent model that removes the usual hydrodynamic assumption of electrons being in equilibrium with the local electric field. Through numerical examples, it is shown that RF nonthermal atmospheric plasmas in the γ mode are sustained predominately through ionization within the electrode sheath region and that secondary electrons are particularly important. On the other hand, RF nonthermal atmospheric plasmas in the a mode are sustained by volume ionization both in the sheath region and in the quasi-neutral plasma bulk region. Similar to our experimental observations, current and voltage characteristics are very different in the two modes. A detailed comparison of the two modes is provided in terms of densities of charged particles, densities of excited species, mean electron energy, and electric field. These findings will be useful for control and optimization of RF nonthermal atmospheric plasmas tailored for intended applications.