Chapter 3: MHD stability, operational limits and disruptions

Abstract

The present physics understandings of magnetohydrodynamic (MHD)stability of tokamak plasmas, the threshold conditions for onset of MHD instability,and the resulting operational limits on attainable plasma pressure (beta limit) anddensity (density limit), and the consequences of plasma disruption and disruptionrelated effects are reviewed and assessed in the context of their application to afuture DT burning reactor prototype tokamak experiment such as ITER. The principalconsiderations covered within the MHD stability and beta limit assessments are (i)magnetostatic equilibrium, ideal MHD stability and the resulting ideal MHD beta limit;(ii) sawtooth oscillations and the coupling of sawtooth activity to other types of MHDinstability; (iii) neoclassical island resistive tearing modes and the correspondinglimits on beta and energy confinement; (iv) wall stabilization of ideal MHDinstabilities and resistive wall instabilities; (v) mode locking effects ofnon-axisymmetric error fields; (vi) edge localized MHD instabilities (ELMs, etc.); and(vii) MHD instabilities and beta/pressure gradient limits in plasmas with activelymodified current and magnetic shear profiles. The principal considerations coveredwithin the density limit assessments are (i) empirical density limits; (ii) edge powerbalance/radiative density limits in ohmic and L-mode plasmas; and (iii) edge parameterrelated density limits in H-mode plasmas. The principal considerations covered in thedisruption assessments are (i) disruption causes, frequency and MHD instability onset;(ii) disruption thermal and current quench characteristics; (iii) verticalinstabilities (VDEs), both before and after disruption, and plasma and in-vessel halocurrents; (iv) after disruption runaway electron formation, confinement and loss; (v)fast plasma shutdown (rapid externally initiated dissipation of plasma thermal andmagnetic energies); (vi) means for disruption avoidance and disruption effectmitigation; and (vii) `integrated' modelling of disruptions and fast shutdown and ofthe ensuing effects. In each instance, the presentation within a given topical areaprogresses from a summary of present experimental and theoretical understanding to howthis understanding projects or extrapolates to an ITER class reactor regime tokamak.Examples of extrapolations to the specific ITER design concept developed during thecourse of the ITER EDA are given, and assessments of the degree of adequacy of presentunderstanding are also provided. In areas where present understanding is identified tobe less than fully adequate, areas in which continuing or new research is needed areidentified.