Investigating the influence of the operating frequency on the gas phase emitter effect of dysprosium in ceramic metal halide lamps

Abstract

The dependence of the gas phase emitter effect of Dy on a variation of the operating frequency between a few Hz and 2 kHz is investigated in a high intensity discharge lamp. The buffer gas of the lamp consisting of Ar, Kr and predominantly Hg is seeded with DyI3, its burner vessel is formed from transparent yttrium–alumina–garnet material. Phase and spatial resolved emission spectroscopy in front of the lamp electrode and pyrometric temperature measurements along the tungsten electrode are performed with a spectroscopic setup. Dy atom and ion densities in front of the electrode are deduced from absolute intensities of optically thin Dy lines and a plasma temperature, derived from the absolute intensity of mercury lines. Phase resolved values of the electrode tip temperature Ttip and input power Pin are obtained from temperature distributions along the electrode. Distinctly higher Dy ion and atom densities are measured in front of the electrode within the cathodic phase. With increasing operating frequency a reduction in both atoms and ions is observed in front of the cathode. In contrast, an increase in the ion density in front of the anode is seen. Moreover, the Dy ion density is drastically reduced by an additional seeding of the lamp with TlI. It is found that an up rating of the Dy ion density is correlated with a decline of Ttip and Pin. At higher frequencies this effect takes place not only within the cathodic phase but also within the anodic phase. The reduction of the average electrode tip temperature of the order of several hundred kelvin compared with a YAG lamp with a pure mercury filling is explained by a Dy monolayer on the electrode surface which is sustained by a Dy ion current.