Use of the M & S Configuration in Theta Pinches

The M & S configuration ** gives equilibrium for a toroidal plasma without net current in the azimuthal direction and without rotational transform. Explicit solutions are obtained only for the restricted cases ß = 0 , and ß 1 in the surface current model, but configurations with continuous pressure profile are proved to exist. The use of the M & S configuration to confine a toroidal theta pinch plasma leads to problems in the establishment of the equilibrium and in particular in its stability. The stability of the M & S configuration is investigated for the simplified case of the corrugated linear theta pinch, LIMPUS. This configuration is theoretically unstable with respect to m 1 flutes. Mechanisms to suppress these modes are based on finite gyroradii stabilisation for m 2. and on the conducting wall for m = \ , the latter case being limited to very high ß . No m 2 instabilities are observed experimentally, although m = l instabilities with growth times agreeing with MHD theory are detected. An exrapolation of these results to an M & S configuration gives tm & s /td ~ (£o/rp)1/2’ where rp is the average minor radius, f 0 an initial deviation from the M & S equilibrium position, and r M&S and t d are the M & S confinement time and the toroidal drift time, respectively. M & S experiments with theta pinch plasmas near 10 eV, tm & S is extended by a factor 3 — 4 beyond td , which is in agreement with the above relation. This suggest firstly, that the M & S configurations investigated are an adequate approximation to the required equi­ librium, and, secondly, that the MHD instabilities substantially limit the confinement time. Simple models show that oscillating axial currents, a steady axial motion of the LIMPUS mag­ netic field configuration or a standing wave like oscillation can suppress the instabilities mentioned. Experiments on such dynamic stabilisation are in progress.