Ideal magnetohydrodynamic ballooning stability boundaries in three-dimensional equilibria

The impact of three-dimensional geometry on ideal magnetohydrodynamic ballooning mode stability is studied. By using a class of "local 3D equilibria" [C. C. Hegna, Phys. Plasmas 7, 3921 (2000)], the effects of plasma shaping, profile variations and symmetry on local plasma physics properties can be addressed. As an example, a local helical axis equilibrium case is constructed that models the magnetic field spectrum of a quasihelically symmetric stellarator. In this case, the magnetic harmonic structure of the local shear (which can be manipulated via changes in the magnetic geometry) has an important impact on the stability boundaries and eigenvalue properties of three-dimensional equilibria. The presence of symmetry breaking components in the local shear produces localized field-line-dependent ballooning instabilities in regions of small average shear. These effects lower first ballooning stability thresholds and can eliminate the second stability regime. A geometric interpretation of these results is given. (C) 2002 American Institute of Physics.