Investigating the gas phase emitter effect of caesium and cerium in ceramic metal halide lamps in dependence on the operating frequency
Cornelia Ruhrmann, Michael Westermeier, Andre Bergner, Gerard Luijks, Peter Awakowicz, Jürgen Mentel
JOURNAL OF PHYSICS D-APPLIED PHYSICS, Volume: 44, Issue: 35, Article Number: 355202, DOI: 10.1088/0022-3727/44/35/355202, Published: SEP 7 2011
The work function and with it the temperature of tungsten electrodes in HID lamps can be lowered and the lifetime of lamps increased by the gas phase emitter effect. A determination of the emitter effect of Cs and Ce is performed by phase resolved measurements of the electrode tip temperature T(tip)(phi), plasma temperature T(pl)(phi) and particle densities N(phi) by means of pyrometric, optical emission and broadband absorption spectroscopy in dependence on the operating frequency. The investigated HID lamps are ceramic metal halide lamps with transparent discharge vessels made of YAG, filled with a buffer gas consisting of Ar, Kr and predominantly Hg and seeded with CsI or CeI(3). In the YAG lamp seeded with CsI and CeI(3) as well as in a YAG lamp seeded with DyI(3) (corresponding results can be found in a preceding paper) a gas phase emitter effect is observed in the cathodic phase due to a Cs, Ce or Dy ion current. In the YAG lamp seeded with CsI the phase averaged coverage of the electrode surface with emitter atoms decreases and the electrode temperature rises with increasing frequency, whereas the emitter effect of Ce and Dy is extended to the anodic phase, which leads to a decreased average temperature T(tip)(phi) with increasing frequency. This different behaviour of the averaged values of T(tip)(phi) for increasing frequency is caused by the differing adsorption energies E(a) of the respective emitter materials. In spite of the influence of E(a) on the coverage of the electrode with emitter atoms, the cathodic gas phase emitter effect produces in the YAG lamps seeded with CsI, CeI(3) and DyI(3) a general reduction in the electrode tip temperature T(tip)(phi) in comparison with a YAG lamp with Hg filling only.