Metal-halide lamps in micro-gravity: experiment and model

Abstract

The results from optical emission spectroscopy experiments of metal-halide lamps under the micro-gravity conditions on board the international space station are compared with the results of a numerical LTE model constructed with the platform Plasimo. At micro-gravity there is no convection which allows for easier modelling and for a separate study of the diffusion-induced radial segregation effect, undisturbed by convection. The plasma parameters that were experimentally determined and compared with the model were the Dy atom and ion density, the Hg ion density and the temperature.The model and experiments applied to a reference lamp burning on a plasma mixture of DyI3 and Hg were found to be in reasonable agreement with each other. The cross-section for electron-Hg collisions was studied, it was found that the Rockwood values give the correct results. Experimental results guided a sensitivity analysis of the model for the Langevin cross-sections. The ratio of the ion densities Hg+/Dy+ was found to be extremely sensitive for the cross-section of the elastic interaction σ(Hg, Dy+) between the Dy ion and the Hg atom. The sensitivity analysis suggests that equating σ(Hg, Dy+) to a value that is 10% higher than the Langevin cross-section is the best choice. We also found deviations from LTE in the outer regions of the plasma for relative radial positions of r/R > 50%.