Suppression and control of magnetic islands in toroidal plasmas

Some recent theoretical developments on the formation and dynamics of magnetic islands in toroidal plasmas are reported. It is shown that an energetic ion population can have a significant effect on the non-linear stability of a single helicity tearing mode. A dynamical equation for the magnetic island width is derived from kinetic theory. It is shown that island growth can be suppressed in tokamak plasmas by injecting an energetic ion beam with a density profile which peaks just outside the rational surface. This technique can be used to suppress major disruptions caused by the non-linear growth of m=2, n=1 islands in a tokamak. The formation of magnetic islands as a result of plasma pressure in three-dimensional equilibria is also studied. The width of the equilibrium islands is shown to depend on the resistive interchange properties of the plasma. This analytical theory is applied to the Heliotron-E configuration. Equilibrium beta limits due to the criterion of island overlap are investigated. It is suggested that experimental observations of internal disruptions on Heliotron-E can be interpreted as a loss of equilibrium due to island overlap. Energetic ions can also be used to modify magnetic islands in three-dimensional equilibria. It is shown that energetic ions can be used to reduce the size of equilibrium islands in a stellarator due to vacuum field errors. (author). 19 refs, 2 figs