Stabilization of external kink and axisymmetric modes in a highly elongated tokamak

Various schemes for stabilizing the external kink and axisymmetric modes are examined for a highly elongated tokamak equilibrium by using a new initial value code. The axisymmetric mode is more strongly affected by placing the conducting wall at the inboard-outboard position rather than at the top-bottom position. Using only a one-sided wall is substantially inferior to using two walls. A warm mantle in the vacuum region weakens the unstable mode in a rather insignificant way. These results are similar for the external kink mode. Optimum coverage of the plasma boundary with conducting material was also considered for both external kink and axisymmetric modes. Only about 20% coverage of the total circumference area is sufficient to significantly reduce the growth rates of both modes.