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.

X-ray induced fluorescence measurement of segregation in a DyI3–Hg metal-halide lamp

“…Ambipolar diffusion [12], in particular, causes ions to diffuse out of the core faster than neutral atoms or molecules diffuse inwards. A steady state can only be achieved when the elemental density of Dy is lower in the core than it would be in the absence of the difference in diffusion velocities [3].…”

“…λ {α} is called the segregation parameter for the element α. Since densities, and not partial pressures, are measured experimentally, it is useful to reformulate equation (3) in terms of the mixing ratio, n {α} /n {Hg} . Dividing equation 3by the total lamp pressure p 0 ≈ n {Hg} (r, z)kT (r, z), which is not a function of position, gives…”

“…Use of x-ray induced fluorescence (XRF) to obtain spatiallyresolved, absolute densities of additives in metal-halide lamps has been described by Curry et al [3]. This method measures elemental density, the density of a particular element summed over all atomic and molecular species.…”

“…It can lead to deficiencies in colour uniformity (see figure 1) and a reduction in luminous efficacy. In order to gain a better understanding of the segregation phenomena, we have used x-ray induced fluorescence [3,4] to measure the spatial distributions of both the additives and the Hg in a test lamp specifically designed to enhance segregation.…”

X-ray absorption imaging of high-pressure lighting plasmas

“…This situation has motivated us to explore the use of X-ray methods to study metal-halide high-intensity discharge lamps. X-ray induced fluorescence [2] and X-ray absorption imaging [1] have both proven to be effective for this application. The latter technique, because of its imaging nature, returns a large quantity of data that is difficult to present, both in its entirety and in a quantitative way, in a publication.…”