Reduction of islands in full-pressure stellarator equilibria

The control of magnetic islands is a crucial issue in designing stellarator s. Islands are associated with resonant radial magnetic fields at rational rotational-transform surfaces and can lead to chaos and poor plasma confine ment. In this article it is shown that variations in the resonant fields of a full pressure stellarator equilibrium can be related to variations in th e boundary via a coupling matrix, and that inversion of this matrix determi nes a boundary modification for which the island content is significantly r educed. The numerical procedure is described and the results of island opti mization are presented. Equilibria with islands are computed using the Prin ceton Iterative Equilibrium Solver, and resonant radial fields are calculat ed via construction of quadratic-flux-minimizing surfaces. A design candida te for the National Compact Stellarator Experiment [A. Reimann, L. Ku, D. M onticello , Phys. Plasmas 8, 2083 (2001)], which has a large island, is use d to illustrate the technique. Small variations in the boundary shape are u sed to reduce island size and to reverse the phase of a major island chain. (C) 2001 American Institute of Physics.