Investigation of Local Thermodynamic Equilibrium in HID lamps

Abstract

Summary form only given. High intensity discharge (HID) lamps have a small volume, high pressure and high luminance. This makes them very useful for illumination of large surface areas such as streets and stadiums. A numerical model is being set up in order to fully understand the energy balance and transport processes in such lamps. The model can be greatly simplified if the discharge of the HID lamp is in Local Thermodynamic Equilibrium (LTE). LTE was experimentally investigated by determining the temperature of a model HID mercury lamp. This lamp has an arc length of 39 mm and an 18 mm diameter. It contains pure mercury and has no outer jacket. The validation of the LTE assumption requires the measurements of different temperatures. This was done using three different techniques. X-ray absorption (XRA) measures the heavy-particle temperature, Thomson scattering (TS) the electron temperature, absolute line intensity (ALI) measurements determine the excitation temperature. By comparing the different temperatures the LTE assumption can be validated. XRA uses an x-ray source, produced by a molybdenum anode, for the plane-wave illumination of the entire lamp. This is done for both lamp-on and off measurements. The Hg density distribution follows from the ratio of these, from which the heavy particle temperature can be determined using the ideal gas law. TS is the scattering of (laser) light on the electrons in the plasma. From the scattered photons the electron temperature and density can be determined. Emission spectroscopy (ALI) is used to determine the state density of mercury from the different optically thin lines. An atomic state distribution function (ASDF) then gives the excitation temperature. The TS temperature profile shows a relatively constant radial profile, whereas the XRA profile is parabolic. This is a clear indication of departure from LTE in the outer regions of the discharge.