Measurement of Ba-densities close to the electrode in an HPS-lamp by broadband absorption spectroscopy with a UHP-lamp
Michael Westermeier, J. Reinelt, G. Luijks, Jürgen Mentel, Peter Awakowicz
Journal of Physics D: Applied Physics 43 (2010)
Within this work, the emitter-effect of barium is investigated in a high-pressure-sodium (HPS) lamp. The emitter-effect reduces the thermal stress of the lamp electrode by a reduction in the effective work-function of the tungsten material. The investigated HPS-lamp has a special design of the Ba-emitter storage: it is deposited during lamp production in a tungsten coil around the electrode rod with a certain distance to the electrode tip. During lamp operation the Ba-emitter diffuses along the electrode surface to the electrode tip.
To optimize the Ba-diffusion along the electrode, a measurement of the Ba-vapour density close to the electrode is executed by means of a broadband absorption spectroscopy (BBAS) at a Ba resonance line. An ultra-high-pressure (UHP) lamp is used as an absorption backlight source. It provides a high radiance at the measuring wavelength and operates very stably over time. An approximately linear decrease in the Ba-vapour density close to the electrode rod from 1.5 × 1012 to 0.5 × 1012 cm?3 is measured between the coil and the tip. The UHP-broadband absorption measurement at the Ba resonance line yields results which are independent of the plasma temperature. It can be applied to any arbitrary position within the HPS-lamp. Thus, the presented BBAS by means of a UHP-lamp is a very flexible but simple alternative to laser-absorption spectroscopy or to emission spectroscopy.