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.

Otherwise stable discharges can become nonlinearly unstable to disruptive locked modes when subjected to a resonant m =2, n= 1 error field from irregular poloidal field coils, as in DIII-D [Nucl. Fusion 31, 875 (1991)], or from resonant magnetic perturbation coils as in Experiments in Ohmically heated deuterium discharges with q-3.5, n z 2 x 1019 me3 and BT z 1.2 T show that a much larger relative error field (Br21/BT =: 1 X 10e3) is required to produce a locked mode in the small, rapidly rotating plasma of COMPASS-C (Re = 0.56 m, f z 13 kHz) than in the medium-sized plasmas of DIII-D (Re = 1.67 m, f z 1.6 kHz), where the critical relative error field is Brzl/BT =: 2 X 10m4. This dependence of the threshold for instability is explained by a nonlinear tearing theory of the interaction of resonant magnetic perturbations with rotating plasmas that predicts the critical error field scales as (fRo/BT)4'3i?'3. Extrapolating from existing devices, the predicted critical field for locked modes in Ohmic discharges on the International Thermonuclear Experimental Reactor (ITER) [Nucl. Fusion 30, 1183] (f=O.17 kHz, Rc = 6.0 m, BT = 4.9 T, ii = 2 X 1019 mm3) is &I/BT = 2 X lo-'. Such error fields could be produced by shifts and/or tilts of only one of the larger poloidal field coils of as little as 0.6 cm with respect to the toroidal field. A means to increase the rotation frequency would obviate the sensitivity to error fields and increase allowable tolerances on coil construction.

The poloidal distribution of the halo current density on the top dump plate in JET can now be measured thanks to a new set of Rogowskii coils. These are the first measurements in JET able to offer an insight in the width of the halo current interaction with the wall. Therefore they offer both validation of the assumption made for JET disruption design criteria and one additional point in the extrapolation of the expected halo current width, and hence halo current density (and related local electro-mechanical loads on in-vessel components) for ITER. During upward events, the measured current density is consistent with the measured total poloidal halo current. The halo footprint extends over most of the upper dump plate, converting to a halo current flux tube width of ∼100 mm. A set of four toridal field pick-up coils installed 90° apart now allows a more accurate measurement of the poloidal halo current, in particular its toroidal peaking factor, and direct comparison between halo and plasma asymmetries.

Simple scaling laws for the ideal-MHD /3-limit (]3 C ) as a function of plasma shape parameters (inverse aspect ratio e, elongation K, and triangularity 5) in the range covered by recent high-0 (< 4.5%) experiments in Doublet III are derived. /3-limits are obtained by optimizing the current profile. A large class of profiles is considered. /3-limits are presented for a single Gaussian profile as they do not significantly improve with more elaborate profiles. Excluding the region dominated by sawtooth activity, experimental values do not exceed the n = °° ballooning mode /3-limit, but do exceed the n = 1 kink mode limit if no wall stabilization is assumed, even in the presence of a cool mantle. If wall stabilization is assumed, the kink limit is above the ballooning limit when the safety factor at the plasma surface (q s ) is greater than two. Even with a fairly close wall, the kink is still unstable when q s < 2. Kinetic effects are not found to significantly improve the ballooning limit in this work. A unified limit curve is postulated which combines the commonly observed hard q s = 2 kink limit with the n -°° ballooning limit for q s > 2: |3 C (%) = 27 qj 1-1 e 1<3 K 1 2 (1 + 1.55) for q s >2 and 0 C = 0 for q s < 2. This expression shows that triangularity is almost as important as elongation to reach high /3-values.