J. Plasma Physics (1982), vol. 28, part 2, pp. 325-333 325 Printed in Great Britain Magneto-acoustic oscillations as a plasma diagnostic technique for a current-carrying plasma column By M. H. BRENNANf, A. L. MCCARTHY AND M. L. SAWLEYI School of Physical Sciences, The Flinders University of South Australia, Bedford Park, S.A. 5042, Australia (Received 29 October 1981) Magneto-acoustic oscillations were excited in a non-uniform, current-carrying plasma column. Measurements of the oscillating axial magnetic flux at a number of excitation frequencies were used to determine the equilibrium plasma pro- perties. In agreement with previous theoretical studies, the radial profile of the plasma density is shown to be accurately determined, while it is not possible to obtain detailed information on the radial profile of the equilibrium magnetic field. 1. Introduction The application of magneto-acoustic oscillations to the diagnostic analysis of plasmas with no, or small, equilibrium currents has been reported by a number of workers. Recent experiments (Brennan, Jessup & Jones 1977, 1978; Blackwell & Cross 1979) have shown that measurements of the oscillations, made external to the plasma, provide sufficient sensitivity to permit accurate determination of the density profile in an afterglow plasma. An experiment has also been reported by Moser et al. (1979) in which magneto- acoustic oscillations were excited in a theta-pinch plasma in which substantial equilibrium currents wereflowing.Probe measurements of the radial dependence of the amplitude and phase of the oscillating axial magnetic field were obtained. The experimental data were compared with calculations for a non-uniform plasma column, although the plasma temperature and viscosity coefficient were assumed independent of radial position. Using the measured values of the equilibrium quantities, an estimate of the viscosity coefficient was obtained. However, the agreement between theory and experiment was rather poor, and an anomalously low viscosity coefficient was determined. Sawley (1979) has recently extended the theory of magneto-acoustic oscil- lations to cover the general case of oscillations in a non-uniform, current- carrying plasma column. The analysis, which includes the dissipation mechanisms of resistivity and viscosity, enables the oscillation fields to be determined for t Present address: Wills Plasma Physics Department, The University of Sydney, Sydney, Australia. % Present address: Centre de Recherches en Physique des Plasmas, 21 avenue des Bains, Lausanne, Switzerland.