The gas-phase emitter effect and the formation of an emitter-spot-attachment in ceramic metal halide lamps

Summary form only given. The behavior of electrodes operated with alternating currents in ceramic metal halide lamps containing Hg+NaTlDy-iodide has been investigated experimentally. Transparent YAG lamp tubes have been used to perform phase resolved measurements of the electrode temperature and Dy-densities in dependence on the cold-spot temperature of the salt filling. The electrode temperature is measured by pyrometry along its axis executed with a spectrograph. It is used to determine the power loss of the electrode. The Dy-density in front of the electrode is measured spectroscopically using absolute Dy-line intensities and taking into account the plasma temperature, which is deduced from Hg-line measurements assuming LTE. It was found that a doping of a mercury lamp with Dy-iodide generates a so called "gas-phase emitter effect" for direct current operation, which reduces the effective work function and thereby lowers the power losses of the cathode even in combination with NaI and TlI salt fillings. Recently it was shown that the emitter effect of Dy is in part diminished for switched-DC operation with f>10 Hz in NaTlDy-lamps. It is caused by a cathaphoretic accumulation of Na which hampers the Dy-emitter as well as by the long time constants for the formation of a Dy-monolayer which is required for the emitter effect. While operating lamps with different salt mixtures and changing the Dy-particle densities by influencing externally the cold-spot temperature, apparently a different type of arc attachment on the electrode, a so called "emitter-spot", was observed. It is less constricted than the well known spot, which is mainly defined by the electrode-geometry and current density. The emitter spot is induced by high densities of the Dy-emitter which forms a localized monolayer on the electrode surface. The formation and the effect of this emitter-spot will be further investigated for dysprosium and also for other rare earth elements.