Model and diagnostics of the high-pressure mercury discharge with sodium iodide additives

Spectroscopic data from a high-pressure mercury arc with sodium iodide additives are compared with model calculations. The fused silica arc tube was oriented horizontally, and rotated about its own axis at sufficiently high speed to center the arc. This mode of operation suppresses convective effects, creating a greater degree of axial symmetry and longitudinal homogeneity than is normally present in a metal halide arc. High-resolution line-of-sight spectra were recorded for all of the principal spectral features and for a series of offsets from the arc axis. Several independent measurements of the arc temperature are extracted from the spectra using standard techniques. The arc model assumes local thermodynamic equilibrium, axial symmetry, longitudinal homogeneity, absence of convection, and time independence. Equations associated with energy conservation, radiation transfer, chemical equilibrium, and multicomponent diffusion are solved simultaneously to determine a self-consistent arc temperature profile, radiation field, and distribution of chemical species. Boundary conditions on these calculations include the input power per unit length, the arc tube radius, the amount of mercury per unit length, and the sodium iodide vapor pressure at the wall. Direct comparisons between the data and the model are made for the high-resolution spectra at various offsets, and the arc temperature. Comparisons are also made for a similar arc without sodium iodide. The model is used to study the response of the discharge to independent variations in power loading and sodium iodide pressure.