MANIPULATION OF ISLANDS IN A HELIAC VACUUM FIELD

To confine plasma adequately in toroidal magnetic fields for nuclear f usion experiments, it is essential to control the magnetic islands cau sed by perturbations and non-axisymmetry inherent to stellarators. A m ethod is introduced which enables the perturbation harmonics of the fi eld line Hamiltonian (which determines the size of islands and the deg ree of chaos) to be approximated quickly. The method is based upon qua dratic-flux minimizing surfaces which pass directly through both the X and O points of the island chains. Using a suitable measure of island width, it is possible to utilize standard numerical optimization meth ods to achieve desirable configurations. A major island chain in the H -1NF heliac vacuum field has been made to both disappear and to reappe ar with opposite phase. These results have significance for self-heali ng phenomena and the method is generally applicable to all stellarator s and 1 1/2-dimensional Hamiltonian dynamical systems.