Chapter 3: MHD stability, operational limits and disruptions

Abstract

Progress in the area of MHD stability and disruptions, since the\npublication of the 1999 ITER Physics Basis document (1999 Nucl.\nFusion 39 2137-2664), is reviewed. Recent theoretical and\nexperimental research has made important advances in both understanding\nand control of MHD stability in tokamak plasmas. Sawteeth are\nanticipated in the ITER baseline ELMy H-mode scenario, but the tools\nexist to avoid or control them through localized current drive or fast\nion generation. Active control of other MHD instabilities will most\nlikely be also required in ITER. Extrapolation from existing experiments\nindicates that stabilization of neoclassical tearing modes by highly\nlocalized feedback-controlled current drive should be possible in ITER.\nResistive wall modes are a key issue for advanced scenarios, but again,\nexisting experiments indicate that these modes can be stabilized by a\ncombination of plasma rotation and direct feedback control with\nnon-axisymmetric coils. Reduction of error fields is a requirement for\navoiding non-rotating magnetic island formation and for maintaining\nplasma rotation to help stabilize resistive wall modes. Recent\nexperiments have shown the feasibility of reducing error fields to an\nacceptable level by means of non-axisymmetric coils, possibly controlled\nby feedback. The MHD stability limits associated with advanced scenarios\nare becoming well understood theoretically, and can be extended by\ntailoring of the pressure and current density profiles as well as by\nother techniques mentioned here. There have been significant advances\nalso in the control of disruptions, most notably by injection of massive\nquantities of gas, leading to reduced halo current fractions and a\nlarger fraction of the total thermal and magnetic energy dissipated by\nradiation. These advances in disruption control are supported by the\ndevelopment of means to predict impending disruption, most notably using\nneural networks. In addition to these advances in means to control or\nameliorate the consequences of MHD instabilities, there has been\nsignificant progress in improving physics understanding and modelling.\nThis progress has been in areas including the mechanisms governing NTM\ngrowth and seeding, in understanding the damping controlling RWM\nstability and in modelling RWM feedback schemes. For disruptions there\nhas been continued progress on the instability mechanisms that underlie\nvarious classes of disruption, on the detailed modelling of halo\ncurrents and forces and in refining predictions of quench rates and\ndisruption power loads. Overall the studies reviewed in this chapter\ndemonstrate that MHD instabilities can be controlled, avoided or\nameliorated to the extent that they should not compromise ITER\noperation, though they will necessarily impose a range of constraints.