Use of Cataphoresis to Determine Discharge Parameters

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Abstract
The balance between diffusion and ion migration in dc discharges in certain mixtures of metal vapors and rare gases produces a very sharp line of demarcation between the region containing only the rare gas and the region containing both the rare gas and metal vapor. Although the formation of such a sharp boundary depends in a complex way upon the elastic and inelastic cross sections for electron‐atom collisions, it can be used in a simple way for the determination of certain discharge parameters. If a cold spot with a well‐defined location is provided near the cathode end of the positive column, the distance from the cold spot to the line of demarcation and the potential drop across the tube are, at a given current and rare gas pressure, accurately reproducible functions of the cold‐spot temperature. If these functions are determined, and if the neutral‐atom diffusion coefficient and positive ion mobility are known for the metal vapor in the rare gas, a simple theory yields values for electron concentration, electron mobility, and the ratio of electron to ion current as a function of pressure of the metal vapor. Detailed application has been made to a hot‐cathode neon‐mercury discharge over a range of mercury pressure from 2.5 to 25μ and for a neon pressure of 3 Torr. For a 500‐mA discharge n− is about 3.5×1011 cm−3, μ− ranges from 1.4 to 2.5×106 cm2/V sec, and μ−/μ+, from 800 to 1800.