Investigation of helium barrier discharges with small admixtures of oxygen

Barrier discharges in helium and in helium with small admixtures of oxygen were investigated by electrical measurements, the spatiotemporally resolved optical emission spectroscopy and surface charge diagnostics via the electro-optic Pockels effect. As already known, in pure helium a diffuse discharge is typically formed because of the significant role of the metastable species. However, even a very small oxygen admixture (0.025 vol.%) causes the transition to a filamentary mode as a result of the effective quenching of helium metastables by oxygen molecules. This effect was indicated by a significant decrease of N-2(+) the first negative system emission. The transition region was characterized by several Townsend-like discharge breakdowns becoming more and more unstable with an increasing O-2 admixture. The formation of the atmospheric pressure Townsend-like discharge was confirmed by the spatiotemporally resolved emission. The development of the surface charges agrees qualitatively and quantitatively well with the transported charge during the discharge breakdown calculated from the discharge current.