A Transmission Line Bridge for the Diagnostics of Plasma Channels

Abstract : This paper describes a device for measuring the average electrical conductivity of a long, narrow, cylindrical plasma. Interest in plasmas of this shape originates from two areas of study: the study of electron beam propagation through neutral gas, and the study of long straight electrical discharges for use as a communication antenna. Propagation of an electron beam through a neutral gas produces an ionized channel whose properties and evolution are very important to the propagation of the beam itself. Long straight electrical discharges are created in a channel that is ionized by a pulsed laser beam focused with a long focal length lens, and the properties of the plasma in this channel are important to the propagation of the electrical discharge along with channel. The properties of these plasmas are inferred from changes in the transmission properties of a transmission line placed close to and parallel to the plasma. In this paper an analysis is made of the relation between the bridge output voltage and the conductivity of the perturbing plasma for the case in which the conductivity is very low. In this case all perturbations on the bridge parameters are small, and linear approximations are appropriate. The usefulness of the bridge, however, extends to plasma well above this low conductivity range, although a different set of approximations may be necessary to make the analysis of the experiment tractable. A transmission line bridge is a device for measuring very small changes in the properties of transmission lines.