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 T-p and its tip temperature T-t due to the influence of the cataphoresis. If T-p is reduced at the cathode the T-t value has also lowered, whereas T-p at the anode is raised together with its T-t data. This correlation disappears at ballast frequencies above 100 Hz, whereas the cataphoresis influence on T-p 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 and Tt is caused 4:1 T-p 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 T-t depends on the ballast type. For vertical operation the strong influence of convection on T-t has also to be taken into account. Above 100 Hz, where only convection plays a role, the upper electrode T-t exceeds the T-t 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.