Two Coil rf Determination of Plasma Conductivity

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Abstract
This paper describes a new method of measuring the electrical conductivity of a plasma by utilizing the dependence of the mutual inductance of two coils upon the conductivity of the medium lying between them. In addition to its simplicity, the method has several advantages over previous single coil measurements; these are (a) high speed response ∼0.02 μsec, (b) it gives a measure of the average conductivity in the plasma rather than that very close to the coil, (c) it can be used over a continuous and wide range of frequencies with the result that a very wide range of conductivity can be measured (to 106 mhos/m). Electrostatic effects can be eliminated by the use of a third coil which avoids any necessity for electrostatic screening. Measurements have been performed upon electrolytes and plasmas with conductivities from 10 to 104 mhos/m at frequencies from 0.1 to 50 MHz, and in every case agreement with theory is excellent.