Barium transport in fluorescent lamps

Summary form only given. The lifetime of fluorescent lamps is mainly limited by the loss of emitter material which is released from the thermionic cathode due to sputtering and evaporation. To predict the lifetime of these lamps it is desirable to understand and quantify the transport of barium in the cathode region. For this purpose, the transport of barium atoms and ions in ground and excited states has been studied by a combination of experimental and theoretical parts. The densities of barium atoms and ions in ground state were measured by time- resolved laser induced fluorescence (LIF) in a short distance from the spot for a T8 lamp driven at 25 kHz. Furthermore, the intensities of transition lines were determined by optical emission spectroscopy. The time-dependent cathode fall voltage was approximately determined using an improved band method. The model comprises the solution of the time- dependent particle balance equations of barium atoms and ions in ground and excited states. The electron density and the rate coefficients of exciting and ionizing collisions of electrons with barium atoms and ions were determined by solving the space-dependent electron Boltzmann equation in spherical geometry using the measured cathode fall voltage and the discharge current as input. As an example the density of Ba atoms in ground state is shown in Fig. la and compared with the LIF result (circles) at 1 mm from the spot center. TheGood agreement between the measured and calculated density of barium atoms has been obtained. The results demonstrate the sensitive dependence of the Ba density profiles on the ionization which leads to a strong depletion of the Ba density in the cathode phase of the investigated electrode. The model yields the Ba flux from the cathode which limits the lifetime of the lamp.