Spot Mode Operation of a Helium-Xenon Discharge for Lighting

Summary form only given. Low-pressure discharges in He/Xe mixtures have been recently studied and suggested for the construction of Hg-free fluorescence lamps. Rapid starting and temperature independent radiation output promote outdoor application e.g. for advertising. However, applying conventional design with cup electrodes, gas adsorption mainly at the cathode leads to clean up of xenon and limits the lifetime of the lamp. Therefore, new oxide electrode configurations are under investigation. Experimental and theoretical studies focus on the cathode with a hot spot and the surrounding plasma region. The spatial profiles of the gas and the cathode surface temperature in the vicinity of the spot have been measured spectroscopically in dependence on the discharge current. In addition, the density profiles of metastable (ls5) xenon atoms were determined by laser atom absorption spectroscopy. An example is given in the Figure. Improved understanding of the processes in the cathode region which influences the gas adsorption has been obtained from theoretical investigations. A two-dimensional fluid model was applied including particle balance equations for electrons, ions and metastable atoms, the energy balance equation for electrons and the Poisson equation. The required electron transport and collision rate coefficients were treated as functions of the mean electron energy. These functions were generated by solving the steady-state spatially homogeneous electron Boltzmann equation. The profile of the electron thermionic emission flux was given in the model in correspondence to the measured cathode temperature at the spot. The broadening of the discharge from the narrow spot towards the anode connected with large spatial changes of the electric field in axial and radial direction has been obtained from the model. The measured density profiles of the metastable atoms have been used to evaluate the model results.