Barium transport in fluorescent lamps

Summary form only given. Using the atomic absorption diagnostic, we determine diffusion coefficients of barium in the gas within the lamp. Specifically, we measure the loss time or barium during the afterglow of a DC glow. The loss time, together with a diffusion model, gives the diffusion coefficient. We report on results for different gas pressures and different gas types. In addition, we compare diffusion of neutral and singly ionized barium. In a second type of experiment, we measure the loss time of barium just after the glow-to-arc transition (i.e., the transition from cold to hot cathode operation) of a lamp operating with an electronic instant start ballast. In these experiments, barium is detectable after the source (i.e., sputtering by high energy ions during the glow phase) is terminated within the lamp, Since, during this time, a discharge is still present, the measurements yield, for neural barium, an "effective" diffusion coefficient (i.e., one that is augmented by the additional loss of neutral barium via electron impact ionization). For barium ions the measurements are used with a drift-diffusion model to provide insight into the diffusion coefficient as well as the mobility. Results are used to infer barium mass loss rates both during starting (using the absorption diagnostic) and during steady-state (using the temperature diagnostic).