Experimental results from the COMPASS-C tokamak reveal a sharp threshold in amplitude above which externally applied static resonant magnetic perturbations (RMPs) induce stationary magnetic islands. Such islands (in particular, m = 2, n = 1 islands) give rise to a significant degradation in energy and particle confinement, suppression of the sawtooth oscillation and a large change in the impurity ion toroidal velocity. The observed threshold for inducing stationary (2,l) islands is consistent with a phenomenological resistive MHD model which takes into account plasma rotation (including poloidal flow damping) and externally applied resonant fields. Broadly similar results are found for applied fields other than m = 2, n = 1. Other results from RMP experiments are also discussed, such as the stabilization of rotating MHD activity, stimulated disruptions and extensions to the disruptive density limit. Finally, the likely effect of field errors on large tokamaks is briefly examined in the light of the COMPASS-C results.

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It is demonstrated that tearing-mode-stable diffuse-pinch configurations of the reverse-field pinch type exist at zero β for a current-carrying column surrounded by a small vacuum region. The conducting wall plays a vital role in this stability both to m = 0 and m = 1 modes. The core of the plasma, which has to satisfy a resistive-stability criterion, is of the form given by Taylor for a minimum-energy state but the outer region is quite different. Values of the pinch configuration parameter θ up to 3 are possible, permitting strong Ohmic heating with zero or low current densities near the wall. Such configurations can be stable to ideal hydro magnetic modes for central values of β of up to 17% (average β 35%).

The structure of turbulent electric and magnetic field fluctuations is described, and the internal dynamics of the associated velocity fluctuations are compared with those of fluid turbulence. It is suggested that the plasma turbulence is qualitatively similar to fluid turbulence but with the turbulent elements elongated along the mean magnetic field to form rolls. This suggests that an appropriate comparison might be with the hypothetical two-dimensional limit of fluid turbulence, in which energy is expected to be transferred toward long wavelengths rather than to short wavelengths as for isotropic turbulence. In the plasma case, the direction of energy flow is inferred to be toward short wavelengths but the measured form of the triple correlation is close to that expected for two-dimensional turbulence; we cannot, therefore, make a clear choice between the two alternatives. Effects arising from the particle structure of the plasma do not appear to be important, except that at low pressures an increased damping occurs which may be ion Landau damping. The source of the turbulent energy is not primarily convection due to the pressure gradient but involves some mechanism of direct coupling with the plasma current associated with the “excess resistance” of the discharge.

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The paper presents linear and non-linear MHD calculations to examine the effect of a finite conductivity (resistive) wall on plasma stability. At a limiting safety factor qψ of approximately 2 in the tokamak and generally in the RFP, ideal modes are found, with a growth rate that varies inversely with the wall time constant. Resistive tearing modes can also be destabilized by a finite conductivity wall, but sufficiently fast plasma rotation can in turn stabilize these instabilities. It is shown that, non-linearly, the eddy currents driven in the resistive wall, by rotating MHD activity, produce a torque which opposes and slows the plasma rotation. This effect can be particularly strong in the RFP and leads to mode lock times which are shorter than in the tokamak.