Investigation of barium evaporation and diffusion in fluorescent lamps

Summary form only given. The lifetime of fluorescent lamps is typically restricted by the lifetime of their electrodes. During normal operation, the work function lowering emitter material (primarily Ba) is released from the cathode due to evaporation and sputtering. For lifetime prognosis the understanding and quantification of the barium transport in the electrode region is essential. Therefore, the transport of barium atoms and ions has been investigated experimentally and theoretically. In the experimental part the density of ´ground-state´ barium atoms and ions was measured time-resolved by laser induced fluorescence (LIF) in a short distance from the hot spot of a clear-ended T8 lamp driven at 25 kHz. Furthermore, corresponding resonance lines were recorded by optical emission spectroscopy. As an example the ground state density of barium atoms measured by LIF at a distance of 0.5 mm from the hot sport during ´normal´ operation is shown in Fig. la. The density strongly decays in the cathode phase due to ionization. During fast shorting of the lamp voltage for about 4 ms the atomic barium density escalates and saturates after a few msec, as shown in Fig. lb. This so-called blanking method was first introduced by R. Garner [R.C. Garner, ICOPS 2008] for electron density measurements.In contrast to density measurements on ´normal´ operating lamps, the blanking method reduces the influences of the surrounding plasma in the cathode region on barium atoms (excitation and ionization) and stops sputtering. This is used to separate and quantify the effect of barium evaporation.