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) 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 experiment such as ITER. The principal considerations 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 MHD instability; (iii) neoclassical island resistive tearing modes and the corresponding limits on beta and energy confinement; (iv) wall stabilization of ideal MHD instabilities 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 plasmas with actively modified current and magnetic shear profiles. The principal considerations covered within the density limit assessments are (i) empirical 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 assessments 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-vessel halo currents; (iv) after disruption runaway electron formation, confinement and loss; (v) fast plasma shutdown (rapid externally initiated dissipation of plasma thermal and magnetic energies); (vi) means for disruption avoidance and disruption effect mitigation; and (vii) 'integrated' modelling of disruptions and fast shutdown and of the ensuing effects. In each instance, the presentation within a given topical area progresses from a summary of present experimental and theoretical understanding to how this understanding projects or extrapolates to an ITER class reactor regime tokamak. Examples of extrapolations to the specific ITER design concept developed during the course of the ITER EDA are given, and assessments of the degree of adequacy 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.

Three-dimensional non-linear magnetohydrodynamic modeling of massive gas injection triggered disruptions in JET

“…A tokamak disruption [1][2][3] is a violent loss of plasma confinement due to the development of a global instability. It comprises two consecutive phases: the Thermal Quench (TQ) when the thermal energy is lost and the Current Quench (CQ) when the plasma current is lost due to the very large resistivity of the cold post-TQ plasma.…”

Rotational stabilization of the resistive wall modes in tokamaks with a ferritic wall

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] In theory, this was predicted for an ideal wall, but experiments demonstrate 1-15 that the gain can be quite large even in long-pulse discharges when the wall must behave as resistive. This is a valuable asset that is expected to provide access to b (the ratio of the plasma pressure to the magnetic field pressure) essentially above the "no-wall" stability limit in the ITER 13,14 and JT-60SA (Refs. [16][17][18] tokamaks.…”

Error-field penetration in reversed magnetic shear configurations

“…The locked modes sometimes produce seed islands for neo-classical tearing modes (NTMs) that may cause severe damage to plasma confinement, such as triggering plasma disruptions. Three main sources are used to generate error-fields in experiments: [1][2][3] (i) toroidal field-coil misalignments, (ii) ferritic materials (assembled in the inner wall), and (iii) magnetohydrodynamic (MHD) instabilities.…”

“…Such locked modes induced by the error-field were extensively observed in tokamak experiments. 1,2,[4][5][6] Theoretical studies on locked modes have been conducted since the 1980s, 7,8 in which the forces operating on both the moving plasmas and the magnetic islands were analyzed in terms of the tearing mode stability parameter D 0 . Many important theoretical results of early research were based on constant-w or non-constant-w assumptions in various types of fluid models.…”