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.

New experiments on COMPASS-D, DIII-D and JET have identified the critical scalings of error field sensitivity and harmonic content effects, enabling predictions of the requirements for larger devices such as ITER. Thresholds are lowest at low density, a regime proposed for H mode access on ITER. Results suggest a moderate error field sensitivity (δB/B~10-4) for ITER, comparable with the size of its intrinsic error, although there are uncertainties in scaling behaviour. Other studies on COMPASS-D and DIII-D show that sideband harmonics to the (2,1) component play an important role. Thus a correction system for ITER will be important, with flexibility to correct sidebands desirable, possibly assisted by beam rotation. Such a system has been designed and is capable of reducing multiple harmonic error levels to ~2×10-5 .

Recent DIII-D experiments with reduced neutral beam torque and minimum nonaxisymmetric perturbations of the magnetic field show a significant reduction of the toroidal plasma rotation required for the stabilization of the resistive-wall mode (RWM) below the threshold values observed in experiments that apply nonaxisymmetric magnetic fields to slow the plasma rotation. A toroidal rotation frequency of less than 10 krad/s at the q=2 surface (measured with charge exchange recombination spectroscopy using C VI) corresponding to 0.3% of the inverse of the toroidal Alfvén time is sufficient to sustain the plasma pressure above the ideal MHD no-wall stability limit. The low-rotation threshold is found to be consistent with predictions by a kinetic model of RWM damping.

͑2000͔͒ reveal the commonalities of resistive wall mode ͑RWM͒ stabilization by sufficiently fast toroidal plasma rotation in devices of different size and aspect ratio. In each device the weakly damped n = 1 RWM manifests itself by resonant field amplification ͑RFA͒ of externally applied n = 1 magnetic fields, which increases with the plasma pressure. Probing DIII-D and JET plasmas with similar ideal magnetohydrodynamic ͑MHD͒ stability properties with externally applied magnetic n = 1 fields, shows that the resulting RFA is independent of the machine size. In each device the drag resulting from RFA slows the toroidal plasma rotation and can lead to the onset of an unstable RWM. The critical plasma rotation required for stable operation in the plasma center decreases with increasing q 95 , which is explained by the inward shift of q surfaces where the critical rotation remains constant. The quantitative agreement of the critical rotation normalized to the inverse Alfvén time at the q = 2 surface in similar DIII-D and JET plasmas supports the independence of the RWM stabilization mechanism of machine size and indicates the importance of the q = 2 surface. At low aspect ratio the required fraction of the Alfvén velocity increases significantly. The ratio of the critical rotation in similar NSTX and DIII-D plasmas can be explained by trapped particles not contributing to the RWM stabilization, which is consistent with stabilization mechanisms that are based on ion Landau damping. Alternatively, the ratio of the required rotation to the sound wave velocity remains independent of aspect ratio.

Error field optimization on DIII-D tokamak [Luxon, J.L., Nucl. Fusion 42 (2002) 8141 plasma discharges has routinely been done for the last ten years with the use of the external " n = 1 coil" or the "C-coil". The optimum level of correction coil current is determined by the ability to avoid the locked mode instability and access previously unstable parameter space at low densities. The locked mode typically has toroidal and poloidal mode numbers n = 1 and m = 2, respectively, and it is this component that initially determined the correction coil current and phase. Realization of the importance of nearby n = 1 mode components m = 1 and in = 3 has led to a revision of the error field correction algorithm. Viscous and toroidal mode coupling effects suggested the need for additional terms in the expression for the radial "penetration" field B that can induce a locked mode. To incorporate these effects, the low density locked mode threshold database was expanded. A database of discharges at various toroidal fields, plasma currents, and safety factors was supplemented with data from an experiment in which the fields of the n = 1 coil and C-coil were combined, allowing the poloidal mode spectrum of the error field to be varied. A multivariate regression analysis of this new low density locked mode database was done to determine the low density locked mode threshold scaling relationship ne B7°'01q,"~79B,,e, and the coefficients of the poloidal mode components in the expression for B,,,, . Improved plasma performance is achieved by optimizing B by Pen varying the applied correction coil currents.Pen GENERAL