Magnetohydrodynamics stability of compact stellarators

Recent stability results of external kink modes and vertical modes in compa ct stellarators are presented. The vertical mode is found to be stabilized by externally generated poloidal flux. A simple stability criterion is deri ved in the limit of large aspect ratio and constant current density. For a wall at infinite distance from the plasma, the amount of external flux need ed for stabilization is given by F-i = (kappa(2) - kappa)/(kappa(2) + 1), w here kappa is the axisymmetric elongation and F-i is the fraction of the ex ternal rotational transform. A systematic parameter study shows that the ex ternal kink mode in a quasiaxisymmetric stellarator (QAS) can be stabilized at high beta (similar to 5%) without a conducting wall by magnetic shear v ia three-dimensional (3D) shaping. It is found that external kinks are driv en by both parallel current and pressure gradient. The pressure contributes significantly to the overall drive through the curvature term and the Pfir sch-Schluter current. (C) 2000 American Institute of Physics. [S1070-664X(0 0)91005-X].