The development of dielectric barrier discharges in gas gaps and on surfaces

Dielectric barrier discharges (DBDs) occur in configurations which are characterized by a dielectric layer between conducting electrodes. Two basic configurations can be distinguished: a volume discharge (VD) arrangement with a gas gap; and a surface discharge (SD) arrangement with surface electrode(s) on a dielectric layer and an extensive counter electrode on its reverse side. At atmospheric pressure the DBD consists of numerous microdischarges (VD) and discharge steps (SD), respectively, their number being proportional to the amplitude of the voltage. These events have a short duration in the range of some 10 ns transferring a certain amount of charge within the discharge region. The total transferred charge determines the current and hence the volt-ampere characteristic of each arrangement. The microdischarges (discharge steps) have a complicated spatial structure. The discharge patterns on the dielectric surface depend on the polarity and amplitude of the applied voltage as well as on the specific capacity of the dielectric. Experimental findings on DBDs in air and oxygen are presented and discussed. On the basis of a self-consistent two-dimensional modelling the temporal and spatial development of a microdischarge and discharge step are investigated numerically. The results lead to an understanding of the dynamics of DBDs. Although in VD arrangements cathode-directed streamers appear especially in electronegative gases, their appearance is rather unlikely in SD arrangements. The application of DBDs for plasma-chemical reactions is determined by the productivity, with which the energy of the electric field can be converted into internal states of atoms and/or molecules. Depending on the desired product it could be both the generation of internal electronic states of molecules or atoms and dissociation products of molecules. The discharge current and current density of DBDs in both the SD and VD arrangements as well as the energy release and energy density distribution in the discharge region are presented. As an example the effectiveness of the energy conversion into ozone production is detailed. Some peculiarities of the discharge parameters, for instance the correlation between discharge patterns (microdischarges or discharge steps) and surface charge density, are discussed.