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.

A self-consistent two-dimensional modelling of microdischarges in devices in which one of the electrodes is covered with a dielectric is presented. The discharge development can be divided into four phases, a Townsend. an ionization wave or streamer, a cathode layer formation, and a decay phase. While during the Townsend phase the initial field strength distribution is hardly distorted, an ionization wave propagates towards the cathode during the following phase. On the wave reaching the cathode, a cathode layer develops. Its radial extension is determined by the increase of the current. During the decay phase the distributions of, for example, field strength and charge carriers are nearly frozen. The discharge fades because of t h e slow decrease of the field strength within its column. Energy and temperature distributions of microdischarge channels in air at atmospheric pressure are given

Based on experimental results, numerical investigations of dielectric barrier discharges (DBDs) have been performed in three basic configurations: in the volume, coplanar and surface discharge arrangements. It is shown that the DBD dynamics is the same in all arrangements and it is determined by the development of a few principal constituents, i.e. cathode-and anode-directed streamers, discharge channel, cathode layer and surface charges. It is found that the anode-and cathode-directed streamers appear with a highly conductive channel in between. The interaction of the streamers with conductive and dielectric surfaces determines the filamentary or homogeneous appearance of the discharge and its properties. The cathode-directed streamer is a self-sustaining phenomenon, which moves in a gas gap or along an electrode driven by a positive loop-back between photoemission and electron multiplication. The anode-directed streamer plays a subsidiary role. Depending on the kind of gas (electronegative or electropositive) and/or the degree of development of the cathode-directed streamer, the field strength in the conductive channels changes significantly. When the cathode-directed streamer touches the electrode surface, a cathode layer appears with parameters close to those of normal glow discharges. In volume discharge arrangements the movement of the streamers results in the appearance of Lichtenberg figures on dielectric surfaces.

The behaviour of microdischarges in air-fed ozonizers is investigated experimentally and theoretically. High-speed streak and single-picture photographs as well as precise current measurements reveal details in the discharge development, which are modelled by a quasi two-dimensional numerical simulation. The modelling is based on the solution of a system of continuity equations in connection with the Poisson equation. Space charge effects have been taken into account. The results of the modelling are compared with those of experiments. Fundamental features of microdischarges, which influence the properties of air-fed ozonizers are described.

Abstract:The discharge structure, development and the transferred charge of dielectric barrier discharges (DBD) in arrangements with a gas gap (volume discharge, VD) and in such with pure surface discharges (SD) are compared. On the basis of the properties of DBD some parameters influencing efficiency and ozone production like field strength distribution and energy density are discussed. An explanation for observed unexpected low ozone yields in air-fed SD generators is proposed.

Abstract

Nonthermal gas discharges at atmospheric pressure, such as dielectric barrier discharges are currently investigated for low-temperature packaging sterilization in order to reach the conditions required for aseptic food packaging. In particular, understanding the basic sterilization mechanisms and the enhancement of the main bacterial reduction pathways are the goals of our investigations. For this purpose, germ reduction experiments were carried out with Bacillus Subtilis and Aspergillus Niger spores using different gas mixtures and plasma conditions with the direct and the indirect influence of barrier discharges. In order to analyze the contribution of UV radiation during plasma germ deactivation, experiments with different excimer UV lamps, also driven by barrier discharges in special UV-emitting gas mixtures, have been carried out. Results of germ reduction experiments using barrier discharges and prospects for atmospheric discharge systems, suitable for industrial packaging sterilization, are presented in this paper

The development of a discharge channel in coplanar dielectric barrier arrangements is investigated numerically. Its behaviour in oxygen, like the spatial and temporal distributions of the field strength, charged and neutral particles and energy density, is described in detail. It is found that the streamer development is mainly determined by photoemission. A cathode layer appears near the position where the cathode directed streamer touches the dielectric surface. Secondary electron emission by ion collisions becomes significant and the parameters of the cathode layer are near those of a normal glow discharge. The charge transfer and energy release happen in the conductive channel of the discharge, which appears on the dielectric surface as a result of the cathode streamer development. The field strength in the conductive channel is nearly constant and about 70–100 Td in oxygen and air.