Electron temperature and density in a metal-halide discharge lamp

Abstract

Summary form only given. The utilization of self-reversed line spectroscopy (SRLS) as a diagnostics method independent of the LTE assumption for studying metal halide discharges is discussed. In a previous paper, electron and ion densities had been deduced in Hg-NaI discharges from a chemical equilibrium model using optically thin lines and assuming LTE (Karabourniotis et al., 1994). Recently, the electron temperature, T/sub e/, was obtained in a pure Hg discharge using SRLS (Karabourniotis, 2001). This work is aimed at applying SRLS for determining the electron temperature in mercury-metal halide discharges. Since this method is based on relative intensity measurements, it might be used in discharge lamps with ceramic envelopes. Another motive is the determination of the electron and ion densities. Sodium iodide is a very usual additive in metal halide lamps. To avoid cataphoretic effects they are operated in alternating current. A Hg-NaI discharge lamp operated in AC (50 Hz, 75 W/cm/sup 3/) was studied at a pressure of 2.8 bar. Sodium is an easily ionized element so that in AC operation electron and ions densities are expected to vary strongly within the current period. It shows that only time-resolved measurements will have a clear physical meaning. High-precision side-on-phase resolved radiation intensity measurements were performed by means of an automated experimental set-up. The local values of T/sub e/ were determined from the Hg lines 5461 and 4047 A using SRLS. Since the ionization energy E/sub x//sup +/ of the high-lying levels x=Hg 6/sup 3/D/sub 2/ and Na 5/sup 2/S/sub 1/2/ is sufficiently small with E/sub x//sup +/<2(kT/sub e/), they are in equilibrium with the continuum. This implies that their population densities, n/sub x/, satisfy the Saha equation. By Abel-inverting the lines Hg 5770 and Na 6161 A, the radial density profiles of n/sub x/ were obtained. Since the radial profile of the sodium line 6161 A is nonmonotonic, high-accuracy Abel-inversion technique is required. Thus, having determined T/sub e/ and n/sub x/ the densities of the mercury and sodium ions as well as the electron density were deduced. Assuming complete LTE, the plasma temperature and the electron density were also obtained from the line Hg 5770 A and the known mercury quantity for comparison.