High-Pressure Microdischarges: Sources of Ultraviolet Radiation

Spatially confined plasmas with dimensions in the submillimeter range have been found to be stable at atmospheric pressure. These microplasmas are nonequilibrium plasmas with an electron energy distribution which contains a significant fraction of high energy electrons. This favors, in combination with the high gas density, the formation of excimers. The possibility for operating these discharges in parallel, or expanding the nonequilibrium plasma two-dimensionally on a flat cathode allows for extended area light-sources, including excimer lamps. The spectral range of these lamps reaches from the visible into the vacuum ultraviolet, down to wavelengths of 75 nm for helium excimer radiation. Highest efficiencies of 6-9% were obtained for xenon excimers when the discharge was operated dc, and 20% when operated in a nanosecond pulsed mode. Besides excimer radiation, microdischarges have also been shown to emit intense line radiation in the vacuum ultraviolet when noble gases with small admixtures of hydrogen and oxygen were used. In this paper, we discuss basic properties of several types of high-pressure microplasmas, focusing on their dc and pulsed dc operation, followed by an overview of the experimental and modeling results relevant for their use as ultraviolet light sources. The prospect of developing microplasma lamps by forming arrays of microdischarges and possibly excimer lasers by operating microdischarges in series is briefly discussed.