Thomas Dr. Hartmann

Ignition of mercury-free high intensity discharge lamps

To achieve a better understanding of the ignition behaviour of D4 lamps for automotive headlights the ignition of mercury-free metal iodide test lamps characterized by a high xenon pressure, a small electrode distance and small electrode–wall distances is investigated. The ignition of these lamps is dominated by a high voltage requirement. Nevertheless lamps are found that show a surprisingly low ignition voltage. Electrical measurements and simultaneous optical observations of the ultra-fast streamer processes show that the breakdown takes place in two different modes. One of the ignition modes which requires a high ignition voltage is characterized by a breakdown in the volume between the electrode tips. The other mode is characterized by streamer discharges along the wall. In this case the cathode, its base and the wall around is involved in the ignition process and the lamp breaks down at low voltages.

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.

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.

Sodium monolayers on thermionic cathodes

Under certain conditions alkali vapours form dipole monolayers on metallic electrodes that can lower the work function of the bulk material. In this case, the power balance of the electrode, the electrode fall voltage and the electrode loss power can change considerably. To verify this effect a pyrometric technique was adapted and optimized for the diagnostics of tungsten electrodes in high pressure sodium discharges. Using an already verified model of thermally emitting cathodes the effect was observed in a Na DC discharge and the range of existence was investigated. An interpretation of the results is given using a Langmuir description of forming the Na monolayers and first-principles electronic structure calculations using a pseudopotential plane wave method to solve the Kohn-Sham equations of density-functional theory.