Manipulation of islands in a heliac vacuum field

To confine plasma adequately in toroidal magnetic fields for nuclear fusion experiments, it is essential to control the magnetic islands caused by perturbations and non-axisymmetry inherent to stellarators. A method is introduced which enables the perturbation harmonics of the field line Hamiltonian (which determines the size of islands and the degree of chaos) to be approximated quickly. The method is based upon quadratic-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 methods to achieve desirable configurations. A major island chain in the H-1NF heliac vacuum field has been made to both disappear and to reappear with opposite phase. These results have significance for self-healing phenomena and the method is generally applicable to all stellarators and 1 1/2-dimensional Hamiltonian dynamical systems.