Investigation of the connection between plasma temperature and electrode temperature in metal-halide lamps

Spatial profiles of electrode temperatures and plasma temperatures have been measured on 'real' HID lamps filled with a commercial metal-halide compound. The absolute accuracy of pyrometric determination of electrode tip temperatures was ±30 K, while the determination of plasma core temperatures, using a modified Bartels method, has an accuracy of ±100 K. We could deduce a close correlation between the plasma temperature in front of an electrode Tp and its tip temperature Tt due to the influence of the cataphoresis. If Tp is reduced at the cathode the Tt value has also lowered, whereas Tp at the anode is raised together with its Tt data. This correlation disappears at ballast frequencies above 100 Hz, whereas the cataphoresis influence on Tp continues up to 500 Hz. Based on the latter limit, a rough estimation of the cataphoresis velocity delivers 700 cm s-1. As a tentative interpretation, we suggest that the connection between Tp and Tt is caused by an increase of the ion part of the total current at the cathode due to Na accumulation before it. Thus, the cathode has to emit fewer electrons and works at a lower temperature. Further results are the temporal behaviour of Tt depends on the ballast type. For vertical operation the strong influence of convection on Tt has also to be taken into account. Above 100 Hz, where only convection plays a role, the upper electrode Tt exceeds the Tt value of the lower electrode by nearly 400 K. This discrepancy one may explain, tentatively, by convection heating of the upper electrode and convection cooling of the lower one. (author)