Michael Westermeier

Investigation of the gas-phase emitter effect of dysprosium in ceramic metal halide lamps

The behaviour of electrodes operated with ac-currents in ceramic metal halide lamps containing Hg + NaTlDy iodide has been investigated experimentally. Using transparent YAG lamp tubes with the so-called Bochum model lamp as an outer bulb phase resolved measurements were performed of the electrode temperature and Dy density in dependence on the cold-spot temperature of the salt filling. The electrode tip temperature and electrode power loss are deduced from the temperature profile measured along the electrode axis. The Dy density in front of the electrode is determined by spatially resolved spectroscopic measurements of absolute line intensities. It is found that doping of a mercury lamp only with Dy iodide generates at low operation frequencies a pronounced emitter effect at the cathode but it declines with increasing frequency. In a lamp doped with NaTlDy iodide the formation and movement of Dy ions are hampered by Na ions accumulated in front of the cathode due to cataphoresis. As a consequence the lowering of the power loss by Dy is in part diminished. It is shown that a gas-phase emitter effect of Dy is effective for standard operation conditions of lamps in spite of the counteracting effects of a long time constant of the emitter effect and Na accumulation.

Investigating the gas phase emitter effect of caesium and cerium in ceramic metal halide lamps in dependence on the operating frequency

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 Ttip(), plasma temperature Tpl() and particle densities N() 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 CeI3. In the YAG lamp seeded with CsI and CeI3 as well as in a YAG lamp seeded with DyI3 (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 Ttip() with increasing frequency. This different behaviour of the averaged values of Ttip() for increasing frequency is caused by the differing adsorption energies Ea of the respective emitter materials. In spite of the influence of Ea on the coverage of the electrode with emitter atoms, the cathodic gas phase emitter effect produces in the YAG lamps seeded with CsI, CeI3 and DyI3 a general reduction in the electrode tip temperature Ttip() in comparison with a YAG lamp with Hg filling only.

Combining emission and absorption spectroscopy at rare earth spectral lines: plasma temperature measurements in ceramic metal halide lamps

Presently, most high intensity discharge (HID) lamps contain mercury to generate a high pressure buffer gas and thereby an appropriate power input into the arc. Due to its toxicity, the replacement of Hg is of particular interest in recent research on HID lamps. Up to now, the emission coefficient of an atomic Hg double line is widely used to determine the plasma temperature Tpl in HID lamps. Tpl is needed to calculate the total density of atoms and ions of elements inside these lamps. A combination of optical emission and broadband absorption spectroscopy allows us to evaluate Tpl independently of Hg emission lines. The method is required for a determination of Tpl if the Hg line intensity within the investigated lamp is too low, is superimposed by other lines or if environmental-friendly Hg-free lamps are developed.Within this work, phase-resolved plasma temperatures are determined in front of the electrode of Hg-containing MH lamps by emission spectroscopy at atomic Hg lines. Above all, temperatures are measured by a combination of emission and absorption spectroscopy at atomic rare earth lines, namely Dy and Tm. A comparison of Tpl determined by both methods agree within an error margin of <10%. Total phase-resolved rare earth atom densities are obtained by means of the measured ground state densities and Tpl. The combination of emission and absorption spectroscopy is also applied to the bulk plasma of lamps where the intensity of the Hg emission lines is too low for plasma temperature measurements or Hg is absent. It provides the partial rare earth pressure and by comparison with thermodynamic data cold spot temperatures within the lamps.

Investigating the dependence of the temperature of high-intensity discharge (HID) lamp electrodes on the operating frequency by pyrometric measurements

Phase-resolved temperature distributions are determined along a rod-shaped tungsten electrode, by which an ac arc is operated within a model lamp filled with argon. Switched dc and sinusoidal currents are applied with amplitudes of several amperes and operating frequencies being varied between 10 Hz and 10 kHz. The temperature is deduced from the grey body radiation of the electrode being recorded with a spectroscopic measuring system. Phase-resolved values of the electrode tip temperature Ttip and of the power input Pin are determined comparing the measured temperature distributions with the integral of the one-dimensional heat balance with these parameters as integration constants. They are supplemented by phase-resolved measurements of the sum of cathode and anode fall called the electrode sheath voltage. If a switched dc current is applied it is found that both quantities are within the cathodic phase only marginally higher than for a cathode being operated with a dc current. Ttip and Pin start to decrease for low currents and to increase for high currents at the beginning of the anodic phase. But with increasing operating frequency the deviations from the cathodic phase are reduced until they cannot be resolved for frequencies of several kHz. A more pronounced modulation, but the same tendencies, is observed with a sinusoidal current waveform. For 10 kHz a diffuse arc attachment with an almost phase-independent electrode tip temperature, which deviates only marginally from that of a dc cathode, and an electrode sheath voltage proportional to the arc current is established with both current waveforms.

Measurement of Ba-densities close to the electrode in an HPS-lamp by broadband absorption spectroscopy with a UHP-lamp

Within this work, the emitter-effect of barium is investigated in a high-pressure-sodium (HPS) lamp. The emitter-effect reduces the thermal stress of the lamp electrode by a reduction in the effective work-function of the tungsten material. The investigated HPS-lamp has a special design of the Ba-emitter storage: it is deposited during lamp production in a tungsten coil around the electrode rod with a certain distance to the electrode tip. During lamp operation the Ba-emitter diffuses along the electrode surface to the electrode tip.To optimize the Ba-diffusion along the electrode, a measurement of the Ba-vapour density close to the electrode is executed by means of a broadband absorption spectroscopy (BBAS) at a Ba resonance line. An ultra-high-pressure (UHP) lamp is used as an absorption backlight source. It provides a high radiance at the measuring wavelength and operates very stably over time. An approximately linear decrease in the Ba-vapour density close to the electrode rod from 1.5 ? 1012 to 0.5 ? 1012?cm?3 is measured between the coil and the tip. The UHP-broadband absorption measurement at the Ba resonance line yields results which are independent of the plasma temperature. It can be applied to any arbitrary position within the HPS-lamp. Thus, the presented BBAS by means of a UHP-lamp is a very flexible but simple alternative to laser-absorption spectroscopy or to emission spectroscopy.

Development of the arc attachment at HID lamp electrodes in the range from low to RF-frequencies

At cathodes of high intensity discharges two different modes of arc attachment are observed: a diffuse mode characterized by a low current density and a uniformly distributed high global temperature, and a spot mode with a high current density and a high local but low global surface temperature. For dc and low frequency operation an investigation of these different modes has already been presented. Different modes of arc attachment are shown in the case of RF operation (up to 1 MHz operation frequency) of the Bochum model lamp. Images of the arc attachment demonstrate that with increasing frequencies the occurrence of the spot mode is reduced. But it cannot be excluded that it arises accidentally at high frequencies. It is found that with increasing frequency the differences between the electrode tip temperature in the cathodic and the anodic half period are levelled and that the course of the electrode sheath voltage (ESV(t)) adjusts to the sinusoidal current waveform resulting in an increase in the average power input into the electrodes with proceeding adjustment. Moreover, it indicates a reshaping of the electrode boundary layers.

Visualization of the Gas-Phase Emitter Effect of Dysprosium in Ceramic Metal Halide Lamps

The gas-phase emitter effect of dysprosium is demonstrated by high-speed photography, spectroscopic, and pyrometric measurements at a high-pressure mercury lamp doped with Nal/Tll/Dyl3, which is operated in a transparent yttrium-aluminia-garnet tube with an f = 100 Hz switched dc current.

Power balance of HID lamp electrodes under HF operation

Operating HID lamps with high frequencies is not yet a commonly used method. It is assumed that the efficacy of the whole lamp system including the lamp driver can be improved with higher operating frequencies as it has already been done for low pressure lamps. Indeed the increasing frequency will lead to changes in the electrode physics which may cause a different lamp behavior Thus the Bochum model lamp is used to investigate the changes in the lamp electrode behavior while increasing the frequency. The model lamp is an easy system which allows a fundamental research of the electrodes and the near electrode region without the occurrence of acoustic resonances. For the investigation phase resolved pyrometry and spectroscopy combined with U/I measurements (ESV) are used. It can be shown that for an operation of the lamp with high frequencies the behavior during the anodic and cathodic phase changes. The presented results describe changes in the electrode temperature and the electrode power balance depending on frequency. With increasing frequency the course of voltage as well as the course of the electrode sheath voltage (ESV(t)) adjust to the sinusoidal current waveform. This causes a rise in the average input power into the discharge and the power input into the electrode boundary layers. Also very remarkable is the sinusoidal waveform of the ESV(t). It indicates a change of the mechanism of current transfer between the electrode and the bulk plasma in the cathodic phase. It means that the present model of the stationary boundary layer has to be revised. For a better understanding a comparison between the power input into the cathodic and anodic boundary layer is made. This is possible since ESV(t) = Ucathode(t) + Uanode(t) can be measured and Vcathode(t) can be estimated.