Effect of dielectric barriers in RF atmospheric pressure glow discharges

Abstract

Summary form only given. Atmospheric pressure glow discharges (APGD) generated at radio frequencies generally employ naked electrodes, since the rapid oscillation of the applied voltage suppress very effectively a sustained energy supply to an unrestricted current growth. Consequently, RF APGD are typically stable and it is usually unnecessary to insulate electrodes with dielectric barriers - a common and often essential technique for low frequency APGD in the kilohertz range. As more and more material processing procedures favor high-density plasmas for a superior application efficiency, it is highly desirable that RF APGD can be operated at increasingly high current densities thus imposing greater demands on reliable control of plasma stability. Inevitably, the rapid oscillation of RF voltages alone becomes no longer adequate to ensure plasma stability, and new and additional stability control strategies become important. In this contribution, we present a computational study of how electrode insulation may be used to enhance plasma stability in RF APGD. Temporal characteristics of the discharge current, the applied voltage and the gas voltage are computed over a full range of the current density, from pre-breakdown to arcing. Relationships of the current density with both the applied and the gas voltages are studied in details. These are used to demonstrate that RF atmospheric dielectric-barrier discharges (DBD) acquire a negative differential conductivity at high current densities and as such acquire a similar level of plasma stability to that in conventional RF APGD. However with the dielectric barriers, the differential conductivity of the plasma enclosing electrode unit is always positive in the external circuit. As a result, any plasma instability is suppressed easily and conveniently by capping the power output of the RF power source. In other words, RF atmospheric DBD are more stable than conventional RF APGD even though their plasma characteristics are very similar. Results reported here are likely to form a basis of a stability control strategy for RF APGD