Ml Mark Beks

Demixing in a metal halide lamp, results from modelling

Convection and diffusion in the discharge region of a metal halide lamp is studied using a computer model built with the plasma modelling package Plasimo. A model lamp containing mercury and sodium iodide is studied. The effects of the total lamp pressure on the degree of segregation of the light emitting species are examined and compared to a simpler model with a fixed temperature profile. Significant differences are observed, justifying the use of the more complete approach.

A model for additive transport in metal halide lamps containing mercury and dysprosium tri-iodide

The distribution of additives in a metal halide lamp is examined through numerical modelling. A model for a lamp containing sodium iodide additives has been modified to study a discharge containing dysprosium tri-iodide salts. To study the complex chemistry the method of Gibbs minimization is used to decide which species have to be taken into account and to fill lookup tables with the chemical composition at different combinations of elemental abundance, lamp pressure and temperature. The results from the model with dysprosium additives were compared with earlier results from the lamp containing sodium additives and a simulation of a pure mercury lamp. It was found that radial segregation creates the conditions required for axial segregation. Radial segregation occurs due to the unequal diffusion of atoms and molecules. Under the right conditions convection currents in the lamp can cause axial demixing. These conditions depend on the ratio of axial convection and radial diffusion as expressed by the Peclet number. At a Peclet number of unity axial segregation is most pronounced. At low Peclet numbers radial segregation is at its worst, while axial segregation is not present. At large Peclet numbers the discharge becomes homogeneously mixed. The degree of axial segregation at a Peclet number of unity depends on the temperature at which the additive under consideration fully dissociates. If the molecules dissociate very close to the walls no molecules are transported by the convective currents in the lamp, and hence axial segregation is limited. If they dissociate further away from the walls in the area where the downward convective currents are strongest, more axial segregation is observed.

An elemental diffusion description for LTE plasma models

A novel method to describe diffusive processes in plasmas in local thermodynamic equilibrium (LTE) was developed, based on the transport of elements instead of individual species. This method combines the elegance of the LTE description of a chemical composition with the flexibility of explicit transport for each element. A simple model of a metal halide lamp containing Hg dosed with NaI is used to illustrate the method.

Competition between convection and diffusion in a metal halide lamp, investigated by numerical simulations and imaging laser absorption spectroscopy

The effect of the competition between convection and diffusion on the distribution of metal halide additives in a high pressure mercury lamp has been examined by placing COST reference lamps with mercury fillings of 5 and 10 mg in a centrifuge. By subjecting them to different accelerational conditions the convection speed of the mercury buffer gas is affected. The resulting distribution of the additives, in this case dysprosium iodide, has been studied by numerical simulations and measurements of the density of dysprosium atoms in the ground state using imaging laser spectroscopy. The competition between axial convection and radial diffusion determines the degree of axial segregation of the dysprosium additives.

2-D Images of the Metal-Halide Lamp Obtained by Experiment and Model

The metal-halide lamp shows color segregation caused by diffusion and convection. Two-dimensional imaging of the arc discharge under varying gravity conditions aids in the understanding of the flow phenomena. In this paper, we show results obtained by experiments and by numerical simulations in PLASIMO.

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

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%.

Semi-empirical model for axial segregation in metal-halide lamps

Diffusive and convective processes in the metal-halide lamp cause an unwanted non-uniform distribution of the radiating metal additive (Dy in our case), which results in colour separation. The axial segregation has been described by Fischer (1976 J. Appl. Phys. 47 2954) for infinitely long lamps with a constant axis temperature. However, for our lamps this is not valid. We propose a semi-empirical extended model. The density inhomogeneity gives a measure for the non-uniformity of the Dy density distribution in the lamp. As an example, this parameter is calculated for some measurements obtained by imaging laser absorption spectroscopy.

A Study on the Effects of Geometry on Demixing in Metal–Halide Lamps

Convection and diffusion in the discharge region of a metal-halide lamp are studied using a computer model built with the plasma-modeling package Plasimo. A model lamp containing mercury and sodium iodide is studied. Recently, the underlying program architecture in Plasimo has been overhauled to allow nonrectangular computational domains. We used this new feature to model the effects of the electrodes protruding into the plasma. The effects of the total lamp pressure on the degree of segregation of the light-emitting species are examined and compared to the earlier model with flat electrodes. Significant differences are observed, justifying the use of the more complete approach.

Phase-resolved response of a metal-halide lamp

The metal-halide (MH) lamp sometimes shows unwanted colour segregation, caused by a combination of convection and diffusion. In the past we investigated the lamp, running on a switched dc ballast of 120 Hz, using a dc approximation for the distribution of the radiating species. Here we present phase-resolved intensity measurements to verify this approximation. The MH lamp contains Hg as buffer gas and DyI3 as salt additive; we measure the light emitted by Dy and by Hg atoms. An intensity fluctuation of ~25% close to the electrodes is found only. The observed fluctuations are explained by the cataphoresis effect and temperature fluctuations; the time scales are in the same order. Furthermore, measurements at higher gravity in a centrifuge (up to 10g) show that the effect becomes smaller at increasing gravity levels. From these results it is concluded that a dc approximation, which is generally assumed by lamp developers, is allowed for this MH lamp.