J Mentel

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.

Observation of different modes of cathodic arc attachment to HID electrodes in a model lamp

Electrodes made of pure and doped tungsten are operated in a special model lamp with a DC current of 1-10 A in argon or xenon atmosphere within the pressure range of 0.1-1 MPa. Cylindrical electrodes with different designs are investigated with regard to the mode of cathodic arc attachment. Three modes are observed: a diffuse mode, a spot mode and a super-spot mode. The major difference between the diffuse and the spot mode is the current density, which is low in the case of the diffuse mode and high in the spot mode. The diffuse mode is favoured by high current, low pressure and weak cooling of the electrode, the spot mode by the opposite conditions. In a transition region the cathode changes statistically between these modes. Whereas the global electrode temperature and the cathode fall of the diffuse and the spot mode differ slightly, the super-spot mode is associated with a significantly decreased global electrode temperature and cathode fall at similar parameters. SEM pictures show that the surface structure of the electrodes has wide influence on the mode of cathodic arc attachment. Due to the significant difference between the super-spot mode and the two other modes this paper is concentrated on the comparison between the spot and the diffuse mode.

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.

Observation of an extremely constricted cathodic arc attachment to electrodes of high intensity discharge lamps

Usually two modes of arc attachment to cathodes for high intensity discharge (HID) lamps are observed, the spot mode for high pressure, low currents and large electrode diameters and the diffuse mode for the opposite conditions. Recently, a very constricted attachment to cathodes was observed both in real HID lamps and in Bochum’s model lamp. In the model lamp, the extremely constricted arc attachment was observed in argon and xenon atmosphere at a pressure p = 0.26 MPa and for currents I = 1–6 A. Whereas the global electrode temperature and the cathode fall of the diffuse and the spot mode differ only slightly, the extremely constricted arc attachment (so-called ‘super spot mode’) is associated with a significantly decreased global electrode temperature at similar parameters. The reduction in electrode temperature implies low cathode falls and low electrode losses, respectively. Scanning electron micrographs show that the surface structure of the electrodes has substantial influence on the mode of cathodic arc attachment.

Determination of HID electrode falls in a model lamp III: Results and comparison with theory

Electrodes of pure and doped tungsten for high intensity discharge lamps are investigated in a special model lamp. Our intention is to obtain experimental data to verify models describing the near-electrode region. For the DC case, we have two totally independent methods to determine the cathode fall separately from the anode fall. The first method is based on the spatially resolved measurement of the cathode surface temperature and a treatment of the cathodic power balance. The second method uses Langmuir-probe measurements to determine the plasma potential at known distances from the cathode. The comparison of both methods shows good agreement which justifies the assumptions and simplifications introduced. Parameter studies yield a strong dependency of the cathode fall on the cathode diameter and the material. The cathode length and the fill gas pressure seem to have only minor influence. Comparing the results of our experiment with outcomes of models by several authors, the values for the cathode fall show good agreement, too. However, concerning the total power loss of the cathode, differences between experiment and theory were found.

The gas phase emitter effect at the anode in a high pressure sodium vapour discharge

To allow spectroscopic measurements within a high pressure sodium lamp, the translucent tube of the lamp is substituted by a transparent sapphire tube. Moreover, the sodium pressure in the lamp is adjusted by an external control of its cold spot temperature. The temperature distribution along the anode of a sodium dc discharge is determined pyrometrically with a special set-up for spectroscopic measurements in the near infrared in dependence on the arc current and the sodium vapour pressure. The power input from the arc plasma into the anode was determined by comparison with temperature distributions obtained by numerical simulation. A physical analysis of the power input yields a lowering of the work function of the tungsten anode to 2.6 eV in accordance with the reduced work function, which was found for the cathode of the sodium lamp.

Optical investigation of arc spot formation on cold cathodes in air

The column of a high current arc was magnetically blown against a cathode positioned perpendicularly to the arc axis. A linearly increasing voltage was applied to this electrode. It was shown by high-speed photography that the arc spot ignition is initiated by an intensified light emission on the cathode within an area of approximately 40- mu m diam. SEM micrographs from the traces produced during the ignition phase revealed that craters with diameters of some micrometers are formed within a burning spot, which corresponds in size to the optical spot. They are similar in size and shape to the craters formed by vacuum arcs on cathodes of the same material. >

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.

Measurement of arc spot formation delay times at cold cathodes in air

The column of a high current arc is magnetically blown against a cathode positioned perpendicularly to the arc axis. A linearly increasing voltage is applied to this electrode. The delay time of the arc spot formation which initiates the arc commutation was measured for smooth electrodes made of Cu, steel, graphite, Mg, and Ce. The cumulative frequency distribution of this delay time was found to be characteristic function of the electrode material and to be related to its work function. Experiments with copper electrodes covered with oxide layers of different thicknesses show that the thickness of the layer has only a weak influence on the delay time. >