Electrode temperature diagnostics and a two-dimensional electrode model have been developed to improve our understanding of electrode behaviour in ceramic metal halide lamps. Using transparent YAG arc tubes in dc and ac operation, anode and cathode characteristics, like the effective work function, anode fall and electrode input power, could be derived from the measured temperature profiles. It is found that Dy-iodide in the metal halide lamp filling has a strong so-called gas-phase emitter effect. In order to improve our understanding of the observed phenomena and to help design electrodes for future lamps, a rotational symmetric two-dimensional electrode model has been created. The model is completely phase resolved so that time-dependent effects can be studied (both ac and dc). Furthermore, it contains various options for calculating the power input distribution, including a complex cathode sheath model as well as a simple anode model. The model has been shown to predict spot/diffuse transitions (in Hg-lamps with heavy electrodes/low currents) very similar to the behaviour seen in real lamps.

To cite this version:J Reinelt, M Westermeier, C Ruhrmann, A Bergner, G M J F Luijks, et al.. Investigating the influence of the operating frequency on the gas phase emitter effect of dysprosium in ceramic metal halide lamps. Journal of Physics D: Applied Physics, IOP Publishing, 2011, 44 (22) Abstract. The dependence of the gas phase emitter effect of Dy on a variation of the operating frequency between some Hz and 2 kHz is investigated in a HID lamp. The buffer gas of the lamp consisting of Ar, Kr and predominantly Hg is seeded with DyI 3 , its burner vessel is formed from transparent YAG 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 set up. 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 T tip and input power P in 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 of both, atoms and ions, is observed in front of the cathode. In contrast, an increase of 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 T lI. It is found that an up rating of the Dy ion density is correlated with a decline of T tip and P in . 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 on the order of several hundred Kelvin compared to 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.Submitted to: J. Phys. D: Appl. Phys.Investigating the influence of the operating frequency on the gas phase emitter effect of Dy2

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HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Abstract. 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 (ϕ), plasma temperature T pl (ϕ) and particle densities N (ϕ) 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 (ϕ) with increasing frequency. This different behaviour of the averaged values of T tip (ϕ) 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 of the electrode tip temperature T tip (ϕ) in comparison to a YAG lamp with Hg filling only.

A diagnostic technique is presented to determine the electrode work function in ac-operated metal halide lamps. The heart of the experimental set-up is a high-speed photodiode array detector, which is able to follow real-time variations of electrode tip temperature and near-electrode plasma emissions in ac-operated experimental YAG lamps, enabling discrimination between the anode and cathode effects. Electrode tip temperature ripples have been measured for 100 Hz square wave operation and simulated with an existing electrode model. By using the electrode work function as main fit parameter for the simulations it is found that the measured cooling effect of the electrode tip in a NaTlDy-iodide lamp is caused by a gas-phase emitter effect of Dy. It is concluded that Dy coverage of the electrode tip causes an effective work function reduction of 0.3 eV at 100 Hz square wave operation, considerably less than the 1.0 eV reduction measured earlier for dc operation.