MHD Stability and Disruptions
Magnetohydrodynamic stability and the avoidance of plasma disruption — rapid loss of plasma energy and abrupt termination of the plasma current caused by global MHD instability growth — are key considerations to the attainment of burning plasma conditions in ITER and to the further ITER goal of extending burning plasma operation to the high-performance steady-state regime needed for a future reactor. In either case, the corresponding MHD stability limit on attainable plasma pressure — usually measured in terms of beta, the ratio of plasma pressure to toroidal magnetic field pressure — determines the toroidal magnetic field strength and physical size of the ITER device. Realization of adequate MHD stability and careful tailoring and control of the plasma operation to avoid undue occurrence of disruption (disruption avoidance) will be critical ITER operation issues.  ITER be a unique test bed for the detail study and validation of the MHD stability aspects of future magnetic fusion reactors, tokamak and otherwise.

Present understanding of MHD stability is already adequate to accurately calculate the ideal (infinite plasma conductivity) stability characteristics and the corresponding ideal MHD beta limit in ITER plasmas. Extension of this understanding to non-ideal cases, eg., with a resistive plasma and/or a nearby resistive wall and/or the presence of energetic alpha particles, have also indicated the ITER conditions where a more-restrictive (lower) beta limit may (with some present uncertainty) be expected and where design provisions to provide active mitigation of the onset of non-ideal MHD instability are prudent or mandatory. Demonstration of the effectiveness of such active means in optimizing the attainable beta in ITER will be an important element of the ITER science experimentation and fusion technology program. ITER operation will also provide a unique opportunity to assess disruption characteristics and effects in reactor-regime plasmas and to develop robust means for disruption avoidance and effect mitigation.