Parametric conditions for glow-to-arc transition in a diffuse nonthermal atmospheric plasma

Summary form only given. Nonthermal atmospheric pressure gas discharges are characterized by their low gas temperature, spatial uniformity, and temporal stability. These features may them particular attractive to a wide range of many materials processing applications including surface modification, etching, deposition, and decontamination. To facilitate an efficient surface modification, it is usually desirable to enhance the concentrations of charged particles and reactive species generated. This can be most conveniently approached by increasing the plasma-generating power, and inevitably over a critical input power level the generated plasma will undergo glow-to-arc transition (GTA) and evolve into an arc discharge. Therefore it is of interest to know when GTA would occur so that the maximum particle densities can be estimated to aid an evaluation of the scope of nonthermal atmospheric plasmas as a material processing technology. This could then provide a base to study possible GTA mechanisms so as to develop effective control techniques. Although these topics can be studied experimentally, the range of relevant system parameters to be considered would make an experimentally based approach expensive. As an alternative, we propose to consider glow-to-arc transition numerically. For nonthermal atmospheric plasmas, there have been very few numerical studies reported and to our knowledge they are all one-dimensional. One key assumption of these 1-D studies is that the generated plasma uniformly fills up the space between the two electrodes. This is not appropriate as plasmas near GAT conditions are unlikely to be uniform. To this end, we develop a two-dimensional computer code for nonthermal atmospheric helium discharges. Based on this code, we calculate plasma voltage and current as well as densities of charged particles and reactive species under various plasma-generating conditions. Plasma voltage and current near the GAT conditions are used to see whether the ele- trical signature offers any distinct indication of an imminent occurrence of GAT. If so this would be a simple indicator in practice. On the other hand, calculated concentrations of electrons and that of reactive species immediately before GAT would suggest their maximum values. This can then be used to assess more quantitatively the scope of nonthermal atmospheric plasmas for specific surface modification applications.