O Langenscheidt

Different modes of arc attachment at HID cathodes: simulation and comparison with measurements

Based on a model for the plasma boundary layer of high intensity discharge cathodes, simulations are performed and compared with experimental results. To solve the power balance of the cathode body different methods are used, namely a 1D integral solution as well as 1D, 2D and 3D finite-element calculations. The simulations are done for cylindrical tungsten cathodes operated in different pure noble gas discharges (0.1–1.0 MPa) and with currents between 0.5 and 10 A. Under these conditions different modes of arc attachment are found, both in simulations and experiments. For the diffuse mode of arc attachment an excellent quantitative agreement between measurements and the simulations is obtained, reflecting an improved accuracy of measurements and simulation. In addition, different spot modes are found. At least one of these modes is also observed in the experiment. Also for this spot mode the agreement between measurements and simulation for the integral quantities is good. There are still some open questions concerning the spot mode of cathodic arc attachment. Varying the geometric dimensions of the cathode, the proper simulation of the heat conduction problem of the cathode body is shown. Variations of the plasma properties, like gas type and pressure, prove the conceptional capability of the boundary layer model for the simulation of different modes of arc attachment. Evaluating the cathode fall characteristics, regions of existence for the different modes are found, which are similar to the experiments.

Determination of HID electrode falls in a model lamp II: Langmuir-probe measurements

A special measuring method with Langmuir probes was established which yields the plasma potential in high-pressure plasmas to determine separate cathode and anode falls of HID electrodes. The probes consist of two tungsten wires which are inserted via side arms into the discharge tube of a model lamp. Their bended ends form together a full circle which surrounds the arc at its boundary. One probe is operated at floating potential with the other one a current-voltage characteristic of the probe is recorded. Its shape corresponds to that taken with a plane probe in a low-pressure plasma. Taking into account the radial voltage drop between the axis and the boundary of the arc, it was shown that the inflection point between the retarded electron current and electron saturation current represents the plasma potential in the arc axis. The electrode falls were determined by an extrapolation of the plasma potential which was measured along the arc axis towards the electrodes. Cathode and anode fall measurements at tungsten electrodes of different diameters are presented which were performed with an arc in argon at 0.26 MPa at currents between 1.5 and 6 A.

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.

Spectroscopic investigation of the plasma boundary layer in front of HID-electrodes

To investigate the properties of the plasma boundary layers of an argon arc in front of high intensity discharge (HID) electrodes a special spectroscopic measuring system was set-up in front of a model lamp. It is characterized by a high spatial resolution and a high degree of automation of the measuring procedure. Measurements were performed at the boundary layers in front of anodes and cathodes made of pure and thoriated tungsten with diameters from 0.6 to 2 mm operated with currents from 0.5 to 10 A at pressures from 0.1 to 0.3 MPa. The electron temperatures Te were deduced from Boltzmann-plots of population densities of a couple of excited states just below the ionization limit, which were obtained from measured line intensities. Electron densities ne were determined from measured continuum intensities. In front of the anodes, maxima of Te and ne were found with Te of the order of 12 000 K and ne of the order of 5 × 1021 m−3, indicating strong deviations from local thermal equilibrium. In the diffuse mode of cathodic arc attachment, steep gradients are measured with values of Te of the order of 12 000 K and of ne of the order of 5 × 1022 m−3. The measured temperatures in the boundary of the spot mode of cathodic arc attachment exceed 22 000 K. The electron temperatures in front of the cathode are in accordance with the results of modelling.

Arc attachment at HID anodes: measurements and interpretation

Anodes for high intensity discharge lamps made of cylindrical tungsten rods and the plasma in front of them are investigated in a special lamp filled with argon and other noble gases at pressures of 0.1–1 MPa. The arc attachment on these anodes takes place in a constricted mode. The temperature is measured pyrometrically along the electrode axis and the anode fall electrically. The electron temperature, Te, and the electron density, ne, within the anodic boundary layer are determined spectroscopically with high spatial resolution. It is found that the power input into the anode increases nearly linearly with the arc current. The proportionality constant is mainly determined by the work function of the electrode material and Te but is independent of the electrically measured anode fall and scarcely dependent on the electrode dimensions. The constriction is more pronounced in cold anodes, with maxima of Te and ne in front of the electrode surface, than on hot anodes with thermionic electron emission and vaporization of the electrode material. The distances of the Te- and ne-maxima from the anode surface are increased and Te is reduced in front of the anode with increasing anode temperature. The experimental findings may be explained by a model of the anodic boundary layer consisting of a thin sheath in front of the surface and a more extended constriction zone. The current and voltage are anti-parallel within the sheath. The power which is needed to sustain the sheath is supplied by an enhanced electrical power input into the constriction zone.

The boundary layers of ac-arcs at HID-electrodes: phase resolved electrical measurements and optical observations

Arcs are operated in noble gases with sinusoidal and switched dc currents of different frequencies in a model lamp between tungsten electrodes with properties appropriate for high intensity discharge lamps. The sum of the cathode and anode fall, called electrode sheath voltage ESV(t) = uc(t) + ua(t), is determined by a variation of the arc length and the anode fall ua(t) by probe measurements along the arc axis and an extrapolation procedure. It is found that ua(t) adopts low, approximately constant values within a half cycle. Therefore the time variation of the ESV reflects that of uc. It was observed by high speed photography that a high value of uc after current zero crossing (CZC) may initiate a transition of the initially diffuse mode of cathodic arc attachment into a spot mode. It is accompanied by a breakdown of uc causing the formation of a voltage peak starting at current zero crossing. It is a prominent example of the so-called commutation peak (CP). By an adjustment of the operation conditions to a low uc the cathodic arc attachment remains diffuse during the whole half-cycle. It is also indicated by high speed photography that the arc constriction in front of the anode is relieved with increasing operation frequency. It is illustrated by measurements that commutation peaks may be removed by an increase in the arc current, a decrease in the electrode cooling, a reduction of the filling gas pressure and an increase in the operation frequency. Moreover it is demonstrated, that the formation of a CP is sensitively dependent on the electrode surface structure.

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.