High speed camera imaging and other diagnostics of dielectric barrier discharges at atmospheric pressure

Summary form only given, as follows. Synchronous, High Speed Photography (HSP, using a Princeton Instruments PI-MAX 512RB Digital ICCD Camera System) and real-time dual detection (optical-electrical) diagnostics have been carried out for the study of dielectric barrier discharges (DBD) in flowing gases such as He, Ne, Ar, Kr, air and N/sub 2/. A phase-resolved synchronizing circuit was used to trigger the ICCD camera\´s shutter for durations varying from 2 ns up to 10 μs. The high voltage electrode was either bare or coated with a dielectric, while ITO coated glass was used as the ground electrode for HPS studies of discharges. Experiments have been performed for different inter-electrode distances (d), frequencies (f), and amplitudes of the electric field, in order to investigate their influence on the DBD characteristics in each gas. The DBD in He, generally characterized by a single short and intense light pulse per half-period, but under some conditions accompanied by additional pulses of equal spacing, comprises multiple luminous spots (columns, 2-9 mm in diameter), which reoccur in the same position over many thousands of cycles of the applied field. At high overvoltage, these multiple columns give way to a uniform glow over the entire electrode surface, that is, a single "column", the so-called atmospheric pressure glow discharge (APGD). Similar observations are made in other noble gases, but here the spots (columns) are smaller, extremely uniform in diameter and more luminous than in He. In all noble gases they can form stable "hexagonal lattices" ("2-D Coulomb crystals") in the HSP image, indicative of strong electrostatic (repulsive) interactions between columns, as also reported by Muller et al. (1999) for the case of lower pressure (100 Torr) He discharges. The numbers of columns vary with changes in f and d, within certain ranges. In the case of DBD in N/sub 2/, the HSP study clearly shows the transition from a filamentary regime, like that in- air, to a true glow (APGD), provided that the gas is very pure (< a few hundred PPM of O/sub 2/, for example). The observations described above are explained in terms of long-lived metastable species in the noble gases and in N/sub 2/.