ABSTRACT
INTRODUCTION The starting process of a fluorescent lamp is particularly deleterious to electrode operation because it may cause heavy sputtering of electrode material, including tungsten and the workfunction-lowering emitter material. Cold start conditions, in which a fluorescent lamp is switched on without electrode preheating, is especially bad because the lamp starts in a glow mode. In this mode the lamp voltage is high because there is several hundred volts of sheath potential (cathode fall), which is necessary to sustain the discharge, near the cathode, via secondary electron emission. Ions from the discharge are accelerated collisionlessly toward the electrode by the electric field of the sheath and bombard the electrode with energies corresponding to the cathode fall. The high cathode falls present during the glow mode lead to very high sputtering yields. The bombarding ions also heat the electrode, which ultimately leads to a transition to an arc discharge mode. In this mode the cathode emits thermionically and the cathode fall therefore drops to a few volts greater than the 10.4 V ionization potential of mercury. At this point, the heavy sputtering damage is greatly reduced, if not eliminated. The glow mode of a cold start lamp lasts typically in the range of 20 to 100 msec.
