An efficient method is given to reconstruct the current profile parameters, the plasma shape, and a current profile consistent with the magnetohydrodynamic equilibrium constraint from external magnetic measurements, based on a Picard iteration approach which approximately conserves the measurements. Computational efforts are reduced by parametrizing the current profile linearly in terms of a number of physical parameters. Results of detailed comparative calculations and a sensitivity study are described. Illustrative calculations to reconstruct the current profiles and plasma shapes in ohmically and auxiliarily heated Doublet III plasmas are given which show many interesting features of the current profiles.

Progress in the area of MHD stability and disruptions, since the publication of the 1999 ITER Physics Basis document Nucl. Fusion 39 2137-2664, is reviewed. Recent theoretical and experimental research has made important advances in both understanding and control of MHD stability in tokamak plasmas. Sawteeth are anticipated in the ITER baseline ELMy H-mode scenario, but the tools exist to avoid or control them through localized current drive or fast ion generation. Active control of other MHD instabilities will most likely be also required in ITER. Extrapolation from existing experiments indicates that stabilization of neoclassical tearing modes by highly localized feedback-controlled current drive should be possible in ITER. Resistive wall modes are a key issue for S128 Chapter 3: MHD stability, operational limits and disruptions advanced scenarios, but again, existing experiments indicate that these modes can be stabilized by a combination of plasma rotation and direct feedback control with non-axisymmetric coils. Reduction of error fields is a requirement for avoiding non-rotating magnetic island formation and for maintaining plasma rotation to help stabilize resistive wall modes. Recent experiments have shown the feasibility of reducing error fields to an acceptable level by means of non-axisymmetric coils, possibly controlled by feedback. The MHD stability limits associated with advanced scenarios are becoming well understood theoretically, and can be extended by tailoring of the pressure and current density profiles as well as by other techniques mentioned here. There have been significant advances also in the control of disruptions, most notably by injection of massive quantities of gas, leading to reduced halo current fractions and a larger fraction of the total thermal and magnetic energy dissipated by radiation. These advances in disruption control are supported by the development of means to predict impending disruption, most notably using neural networks. In addition to these advances in means to control or ameliorate the consequences of MHD instabilities, there has been significant progress in improving physics understanding and modelling. This progress has been in areas including the mechanisms governing NTM growth and seeding, in understanding the damping controlling RWM stability and in modelling RWM feedback schemes. For disruptions there has been continued progress on the instability mechanisms that underlie various classes of disruption, on the detailed modelling of halo currents and forces and in refining predictions of quench rates and disruption power loads. Overall the studies reviewed in this chapter demonstrate that MHD instabilities can be controlled, avoided or ameliorated to the extent that they should not compromise ITER operation, though they will necessarily impose a range of constraints.

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Locked n = 1 tearing modes are observed over a wide range of parameter space in DIII-D and other tokamaks. Much of the difficulty with low density operation is attributed to locked modes and they are also observed as precursors of density limit disruptions. From observations of a consistent locked mode toroidal phase, it appears that the modes are locking to a small field perturbation caused by slight irregularities in the location of one or more of the vertical field coils with respect to the toroidal field coil. By intentionally producing an n = 1 field with an external coil, it was possible to influence the onset of locked modes in low q, low density plasmas. The result is a significantly expanded or reduced stable operating parameter space, depending on the polarity and magnitude of the external perturbation applied and whether or not intrinsic field errors are reduced or increased. A simple theoretical model of the non-linear stability of a tearing mode under the effect of externally applied resonant field perturbations yields a critical island width for the onset of a locked mode. The island widths, computed by field line tracing, for combinations of intrinsic field errors and ‘n = 1 coil’ fields are in qualitative agreement with the critical island width for instability.

Discharges which can satisfy the high gain goals of burning plasma experiments have been demonstrated in the DIII-D tokamak under stationary conditions at relatively low plasma current (495 > 4). A figure of merit for fusion gain (P~&9/4&) has been maintained at values corresponding to Q = 10 operation in a burning plasma for >6 s or 36 ZE and 22R. The key element is the relaxation of the current profile to a stationary state with qrnin > 1. In the absence of sawteeth and fishbones, stable operation has been T. C. Luce, et al. STATIONARY HIGH-PERFORMANCE DISCHARGES IN THE DIII-D TOKAMAKissues which need to be addressed for projection to a burning plasma experiment will be discussed, followed by summary and conclusions. 2 GENERAL ATOMICS REPORT GA-A24 146 STATIONARY HIGH-PERFORMANCE DISCHARGES IN THE DIII-D TOKAMAK T. C. Luce, et al.