T Tanya Nimalasuriya

Axial segregation in metal–halide lamps under varying gravity conditions during parabolic flights

Metal–halide lamps have high efficiencies. These lamps often contain rare-earth additives (in our case dysprosium iodide) which radiate very efficiently in the visible spectrum. Colour separation is a problem in these lamps; this is caused by axial segregation of these additives as a result of diffusion and convection. To vary the effect of convection, parabolic flights were performed with micro-gravity (0g) and hyper-gravity (~1.8g) phases. During these flights, the atomic dysprosium density was measured by means of laser absorption spectroscopy. In addition, the lamp voltage, which is strongly influenced by the total amount of Dy in the lamp, was measured. The Dy density and axial segregation are dependent on the gravity. The dynamic lamp behaviour during the parabolas was investigated: the dysprosium density and lamp voltage followed the gravity variations. When entering the micro-gravity phase, the axial diffusion time constant is the slowest time constant; it is proportional to the mercury pressure in the lamp.

Axial segregation in high intensity discharge lamps measured by laser absorption spectroscopy

High intensity discharge lamps have a high efficiency. These lamps contain rare-earth additives (in our case dysprosium iodide) which radiate very efficiently. A problem is color separation in the lamp because of axial segregation of the rare-earth additives, caused by diffusion and convection. Here two-dimensional atomic dysprosium density profiles are measured by means of laser absorption spectroscopy; the order of magnitude of the density is 1022m−3. The radially resolved atomic density measurements show a hollow density profile. In the outer parts of the lamp molecules dominate, while the center is depleted of dysprosium atoms due to ionization. From the axial profiles the segregation parameter is determined. It is shown that the lamp operates on the right-hand side of the Fischer curve [J. Appl. Phys. 47, 2954 (1976)], i.e., a larger convection leads to less segregation.

Metal halide lamps in the international space station ISS

Optical emission spectroscopy was performed on a metal-halide lamp under micro-gravity conditions of the international space station. Several transitions of atomic and ionic Dy and atomic Hg have been measured at different lateral positions from which we obtained atomic and ionic Dy and atomic Hg intensity profiles. After Abel inversion, the calibrated radial intensity profile of Hg was used to calculate a radial temperature profile. By combining the radial temperature profile with the calibrated radial intensity profile of the additive, the absolute radial density profile of the total atomic and ionic density of Dy was obtained. The measurements showed a hollow density profile for the atoms and ions in the centre. In the outer parts of the lamp molecules were found to dominate. Lamps containing Dy showed contraction of the arc, which increased for higher powers. Measurements were duplicated at 1-g and showed less radial segregation than for 0-g. As the power was increased, the difference between 0-g and 1-g of the radial intensity, density and temperature profile were diminished.

Imaging Laser Absorption Spectroscopy of the metal-halide lamp in a centrifuge (1–10g)

Imaging Laser Absorption Spectroscopy (ILAS) was performed on a metal-halide lamp under hyper-gravity conditions in a centrifuge (acceleration ranging from 1 to 10g). Diffusive and convective processes in the arc discharge lamp cause an unwanted non-uniform distribution of the radiating metal additive, which results in colour separation. Convection is induced by gravity, and measuring under different apparent gravity conditions aids the understanding of the flow processes in the lamp. The centrifuge was built to investigate the lamp under varying apparent gravity conditions. The metal additive density distribution in the lamp is measured by ILAS. In this novel diagnostic technique the laser beam is expanded, so the absorption in the complete plasma volume is imaged simultaneously.

Optical emission spectroscopy of metal-halide lamps: Radially resolved atomic state distribution functions of Dy and Hg

Absolute line intensity measurements are performed on a metal-halide lamp. Several transitions of atomic and ionic Dy and atomic Hg are measured at different radial positions from which we obtain absolute atomic and ionic Dy intensity profiles. From these profiles we construct the radially resolved atomic state distribution function (ASDF) of the atomic and ionic Dy and the atomic Hg. From these ASDFs several quantities are determined as functions of radial position, such as the (excitation) temperature, the ion ratio Hg+∕Dy+, the electron density, the ground state, and the total density of Dy atoms and ions. Moreover, these ASDFs give us insight about the departure from equilibrium. The measurements show a hollow density profile for the atoms and the ionization of atoms in the center. In the outer parts of the lamp molecules dominate.

X-ray absorption tomography of a high-pressure metal-halide lamp with a bent arc due to Lorentz-forces

Due to its practical importance x-ray absorption (XRA) tomography is becoming a widely applied technique for reconstruction of gas temperature distribution in high intensity discharge lamps. Most of the previous papers have dealt with the cylindrically symmetric plasmas that normally occur when lamps are burned vertically. In this work, a XRA tomographic diagnostic is applied for the first time to a high-pressure metal-halide lamp with a bent arc. X-ray lamp-off and lamp-on measurements have been made for two orthogonal directions. The maximum entropy-based algorithm was applied for the reconstruction of local absorption coefficients which are directly related to the local values of mercury density and plasma temperature. For correct reconstruction of the temperature profile at the edges a new numerical strategy based on the adaptive regularization parameter is developed. This strategy allows use of the measured value of the wall temperature as a reference point to obtain a quantitative temperature distribution.

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.

Discharges for lighting

The most common man-made discharge is a lamp. Even though lamps are often considered a mature technology, the discharge physics is often poorly understood. Two recent initiatives discussed here show that plasma research can help to make significant improvements. First we discuss color separation in metal halide lamps, which is a problem that prevents these highly efficient lamps from being used in more applications. Secondly a novel lamp concept is presented that may replace the current mercury based fluorescent lamps.

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: Gravitational influence on color separation

Metal halide lamps are very efficient light sources based on a Hg plasma arc with metal halide salt additions. In spite of their high efficiency, the lamps suffer from several problems, such as color separation and instabilities, which currently hinder large-scale use. These phenomena are caused by a complex interaction of convection and diffusion flows in the plasma. In order to unravel the various contributions, experiments under microgravity have been performed where convection is absent. The experiments confirm the previously held qualitative views, but also provide absolute data on densities and temperatures that will be used to validate numerical models of these lamps.