An absolute immersion-type electrical plasma conductivity probe

The feasibility of measuring the spatial distribution of the electrical conductivity in plasmas by means of an immersed plasma probe has been examined. By solving the Maxwell-Heimholtz equations for the case of a coil inside a cylinder immersed in an electrically conducting medium, the change in coil impedance due to the presence of the medium is predicted. The exact solution is numerical but for certain limiting cases analytical and empirical design formulas are obtained. A bridge and phase-sensitive demodulator allows separation of the resistive and reactive signal components; experiments with electrolytes and with shock-tube plasmas show that the theoretical predictions are borne out in practice to a good degree of accuracy, thereby providing an absolute method for which no known electrical conductivity standards are required. The plasma cooling effect of the probe is considered and an estimate for corrections is suggested.