Chapter 3: MHD stability, operational limits and disruptions

The present physics understandings of magnetohydrodynamic (MHD) stability o f tokamak plasmas, the threshold conditions for onset of MHD instability, a nd the resulting operational limits on attainable plasma pressure (beta lim it) and density (density limit), and the consequences of plasma disruption and disruption related effects are reviewed and assessed in the context of their application to a future DT burning reactor prototype tokamak experime nt such as ITER. The principal considerations covered within the MHD stabil ity and beta limit assessments are (i) magnetostatic equilibrium, ideal MHD stability and the resulting ideal MHD beta limit; (ii) sawtooth oscillatio ns and the coupling of sawtooth activity to other types of MHD instability; (iii) neoclassical island resistive tearing modes and the corresponding li mits on beta and energy confinement; (iv) wall stabilization of ideal MHD i nstabilities and resistive wall instabilities; (v) mode locking effects of non-axisymmetric error fields; (vi) edge localized MHD instabilities (ELMs, etc.); and (vii) MHD instabilities and beta/pressure gradient limits in pl asmas with actively modified current and magnetic shear profiles. The princ ipal considerations covered within the density limit assessments are (i) em pirical density limits; (ii) edge power balance/radiative density limits in ohmic and L-mode plasmas; and (iii) edge parameter related density limits in H-mode plasmas. The principal considerations covered in the disruption a ssessments are (i) disruption causes, frequency and MHD instability onset; (ii) disruption thermal and current quench characteristics; (iii) vertical instabilities (VDEs), both before and after disruption, and plasma and in-v essel halo currents, (iv) after disruption runaway electron formation, conf inement and loss; (v) fast plasma shutdown (rapid externally initiated diss ipation of plasma thermal and magnetic energies); (vi) means for disruption avoidance and disruption effect mitigation and (vii) 'integrated' modellin g of disruptions and fast shutdown and of the ensuing effects. In each inst ance, the presentation within a given topical area progresses from a summar y of present experimental and theoretical understanding to how this underst anding projects or extrapolates to an ITER class reactor regime tokamak. Ex amples of extrapolations to the specific ITER design concept developed duri ng the course of the ITER EDA are given, and assessments of the degree of a dequacy of present understanding are also provided. In areas where present understanding is identified to be less than fully adequate, areas in which continuing or new research is needed are identified.