Chapter 3: MHD stability, operational limits and disruptions

Abstract

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.

Validation metrics for turbulent plasma transport

“…In the context of MFE plasmas, these tests arise most frequently in tests of predicted global mode stability made by ideal or resistive magnetohydrodynamic (MHD) calculations. In these cases, the test is whether or not the plasma exhibits a specific behavior (e.g., onset of a specific global mode or disruption 168,169 ) at the time or condition predicted by a given model. In MFE studies, such tests are generally plotted in terms of parameter space visualizations which indicate the predicted regions of (in)stability, combined with data points indicating where the measurements of whether the plasma is observed to be (un)stable.…”

Conversion of the dominantly ideal perturbations into a tearing mode after a sawtooth crash

“…In this paper, only the mechanism of the seed island formation by strong internal drive due to sawteeth is investigated in detail. This type of tearing mode formation is considered to be one of the most dangerous for future fusion reactors like ITER [8], because large sawteeth provide the strongest internal magnetic perturbations compared to other possible triggers and are able to trigger the mode already at very small normalized pressure values [3,4]. Previous observations from different tokamaks, for example from JET [9] or TCV [10], report large island widths directly after the crash based on analysis of magnetic and SXR measurements.…”

Post-disruptive runaway electron beams in the COMPASS tokamak

“…One of the remaining tasks is control or mitigation of Runaway Electrons (RE) in ITER after the disruption. Estimations from codes predict RE with several tens of MeV to carry up to 70% of predisruptive plasma current (Hender et al 2007, p. S178). As deposition of runaway electron beam can be highly localised, it could severely damage plasma facing components and blanket modules of ITER.…”