Characterisation of surface charge density and net electric field during parallel-plate dielectric barrier discharge generated in atmospheric-pressure air

We measured the surface charge density in a parallel-plate dielectric barrier discharge (DBD) using the Pockels effect, and the axial and lateral components of the net electric field were evaluated for different gap lengths and applied voltages. The DBD was driven by a 5 kHz sinusoidal voltage in gap lengths of 0.5, 0.3, and 0.1 mm at atmospheric pressure with Bi4Ge3O12 crystal as a dielectric. Typical microdischarges (MDs) were observed for every gap length, and the number of MDs increased with the increasing applied voltage. As the gap length decreased, the spot radius of a single MD decreased, whereas the peaks of charge density were almost the same, indicating that the amount of charge transported per unit MD is limited for short gaps. The spatial memory effect, in which MDs occur at the same position in successive cycles, was not always observed under the experimental conditions, and it was evaluated considering the surface charge density distributions. Whether the memory effect works is determined by the surface charge density, and it is likely to occur under a balance between positive and negative charges across successive half-cycles. The net electric field was estimated from surface charge measurements. The axial component of the net electric field strength increased up to 150-170 Td before the MD occurred, and it decreased below 100 Td by the surface charge deposited by MD, being approximately 50%-60% of the external field amplitude. The maximum value of the lateral component of the electric field was approximately 150 Td at the edge of the MD spot with a gap length of 0.5 mm, and it decreased as the gap length decreased. As a negative surface charge generated a weaker lateral field than a positive one, the electric field distribution differed between anodic and cathodic dielectrics.