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.