On the effect of passive feedback in the toroidal Extrap configuration

The ideal MHD stability properties of the axisymmetric free-boundary m = 1 mode of toroidal Extrap equilibria with large aspect ratio and weakly non-circular cross-section are investigated. The effect of passive feedback due to induced currents in the external conductors, represented as ideal current loops, and in a surrounding conducting wall, is taken into account in an approximate fashion by using the inductances of the circuits. The dependence of the effect of the feedback on different parameters such as the plasma radius, the radius of the wall and the horizontal position of the equilibrium plasma is studied. It is found that the effect of feedback becomes significant in the Extrap configuration when the plasma radius is not too small. The axisymmetric mode is thereby stabilized over a parameter regime which is roughly corresponding to the experimental situation of the Extrap T1 device.