Investigations on emitter depletion of cathodes during ignition of 50 Hz low-pressure Hg discharge lamps

Summary form only given. Cathodes in low-pressure Hg discharge lamps (fluorescent lamps) consist of a tungsten coil filled with a mixture of BaO, SrO and CaO*. The active part of the cathode/emitter system is formed by filling the W-coil with a suspension of the appropriate carbonates. During an activation process in lamp production, the oxides are formed from decomposition of the carbonates. Besides, reduction reactions of the oxides with W give rise to the formation of tungstates (in particular of the type Ba/sub 3/WO/sub 6/) and atomic Ba. The Ba is essential for the low work function for thermionic emission. To study these effects in a defined way, a number of lamp burning experiments has been set up, where the glow-switch functionality was replaced by a computer-controlled switch. Two plasma modes are found to be operative: an arc discharge and a glow discharge. The arc essentially is a narrow spot attachment on the cathode. The glow involves the well-known sputtering due to Ar-ion bombardment. In order to study these processes, special lamps have been made using a radioactive Ba-enriched emitter mass. Ba depletion related to defined electrical ignition conditions will be shown in combination with photomicrographs and dynamic heat-up measurements of depleted cathodes.