Demixing of additive species can lead to significant axial and radial nonuniformities in the electrical and radiative characteristics of a vertical high-pressure arc. An optical emission method has been developed to determine both the radial and axial distributions of temperature and additive density as well as the pressure in a rotationally symmetric, enclosed arc in local thermodynamic equilibrium. The method can be used if one knows the vaporized quantity of the major component of the gas mixture in the arc tube. The procedure has been demonstrated by applying it to an ac 175-W Na-Sc-Hg lamp arc, with mercury the major component and sodium the additive of interest. An optically thin Hg line and an optically thin portion of the broadened Na D line were used. The resulting distributions clearly show the effects of segregation; the upper portion of the arc and the axial regions are significantly depleted of sodium.
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Rogoff et al. (1987) studied this question.