Axial and radial development of microdischarges of barrier discharges in N2/O2 mixtures at atmospheric pressure

Microdischarges (MDs) of filamentary barrier discharges (BDs) in air and N-2/O-2 gas mixtures at atmospheric pressure were investigated using the techniques of spatially resolved cross-correlation spectroscopy (CCS) and short exposure time photography (ICCD-camera). The BDs were generated in symmetric discharge cells (configuration of the type 'glass-glass') with two semi-spherical electrodes in order to localize the repetitive MDs at a fixed position. In the case of CCS measurements, the MD development was imaged through spatio-temporal distributions of the radiation intensity of the (0-0) transition of the 2nd positive system of molecular nitrogen (lambda = 337 nm). Two-dimensional optical scanning of the MD channel (in the axial and radial directions of the MD) was carried out for a BD operated in a gas mixture consisting of 6 vol% O-2 and 94 vol% N-2. This gas composition had been found to provide the extremely high stability of the discharge that was necessary for the time-consuming scanning procedure. In the middle of the gap, the MD channel diameter was found to be about 0.3 turn and to expand towards both electrodes. With dielectrics, outward propagating discharges were observed. Short exposure time photos of the MDs taken using an ICCD-camera under the same experimental conditions as for the CCS measurements revealed a branched structure of these discharges on the surfaces, not only on the cathode but on the anode as well. The influence of voltage amplitude on the spatio-temporal distribution of individual MDs of a BD in air was investigated using the CCS-instrument operated in a so-called 'direct start-stop' mode which enabled statistical analysis of the MD sequences within a time range of about 30 mu s.