X-ray induced fluorescence measurement of density distributions in a metal–halide lighting arc

Abstract

The use of x-ray induced fluorescence to measure elemental densities in a metal–halide lighting arc is described. High-energy synchrotron radiation generated on the Sector 1 Insertion Device beam line at the Advanced Photon Source induces K-shell fluorescence in a high-pressure plasma arc. The detected fluorescence is spectrally resolved, so that multiple elemental species are observed simultaneously. Absolute calibration of the measured densities is straightforward and robust. The penetrating nature of high-energy photons allows these measurements to be made in situ, with the arc contained by an optically translucent polycrystalline alumina (Al2O3) arc tube and a glass vacuum jacket. Spatial distributions extending from one end of the arc tube to the other and from the arc core all the way to the wall have been obtained for all the principal elements in the arc. A volume element measuring 1 mm × 1 mm × 1 mm is resolved in the present work, with significantly better spatial resolution possible. Densities as low as 2×1016 cm−3 have been observed. X-ray induced fluorescence is useful for the observation of many important high-pressure plasma lighting chemistries including those containing Hg, Tl, Dy, Tm, Ho, Cs, Sn, I, and Xe.