A study of electrode temperature lowering in Dy-containing ceramic metal halide lamps: II. An investigation of the converse effect of Tl and/or Na additives

The lowering of the gas phase emitter effect of Dy in ceramic metal halide lamps by the admixture of TlI and NaI to the rare earth iodide salt DyI ₃ is investigated at lamps with different additives. The arcs are operated in an Hg buffer gas atmosphere of 2 MPa between rod-shaped pure tungsten electrodes within transparent YAG lamp tubes with a switched-dc current at operating frequencies from 1 Hz to 1 kHz. The atomic ground state density of Dy is measured phase resolved half way between the electrodes and in front of an electrode by broad band absorption spectroscopy, the Dy ion density in front of an electrode by emission spectroscopy and the electrode tip temperature pyrometrically within lamps seeded with differently composed fillings. The measurements confirm that a strong reduction in the electrode tip temperature is correlated with a high Dy ion density in front of the electrode within the cathodic half period. The Dy ion density is depressed predominantly and with it the reduction in the electrode tip temperature by a competing ionization of Tl, and in addition by a lowering of the Dy vapour pressure above the pool of molten salt by TlI. The influence of Na is of minor importance.