Low-n stability calculations for three-dimensional stellarator configurations.

A set of reduced magnetohydrodynamic (MHD) equations to study linear stability in general toroidal configurations is presented. The equations are derived by applying the averaging method to an equilibrium represented in a straight-field-line coordinate system. They permit the calculation of the low-n stability of fully three-dimensional (3-D) equilibria for stellerator configurations with either a planar or a helical magnetic axis. Numerical solutions are shown to agree well with marginal stability boundaries given by the classical stellerator expansion when average stellarator equilibria with a planar magnetic axis are used as input. [ABSTRACT FROM AUTHOR]