TOROIDAL COUPLING OF IDEAL MAGNETOHYDRODYNAMIC INSTABILITIES IN TOKAMAK PLASMAS

A theoretical framework is developed to describe the ideal magnetohydr odynamic (MHD) stability properties of axisymmetric toroidal plasmas. The mode structure is described by a set of poloidal harmonics in conf iguration space. The energy functional, SW, is then determined by a se t of matrix elements that are computed from the interaction integrals between these harmonics. In particular, the formalism may be used to s tudy the stability of finite-n ballooning modes. Using for illustratio n the s-alpha equilibrium, salient features of the n double right arro w infinity stability boundary can be deduced from an appropriate choic e of test function for these harmonics. The analysis can be extended t o include the toroidal coupling of a free-boundary kink eigenfunction to the finite-n ideal ballooning mode. A unified stability condition i s derived that describes the external kink mode, a finite-n ballooning mode, and their interaction. The interaction term plays a destabilizi ng role that lowers the instability threshold of the toroidally couple d mode. These modes may play a role in understanding plasma edge pheno mena, L-H physics and edge localized modes (ELMs). (C) 1996 American I nstitute of Physics.