Avoidance of tearing modes wall-locking in a reversed field pinch with active feedback coils

A general analysis of the feedback on the dynamo tearing modes (TMs) in a cylindrical force-free reversed field pinch equipped with an active coils grid is shown. A single resistive shell with thickness much smaller than its minor radius is considered. Nevertheless, the radial field variation across the shell is taken into account. The active coils are placed outside the shell. The model describes the evolution of several TMs at the same time, taking into account the viscous torque due to the fluid motion and the electromagnetic torque developed by the image currents induced onto the shell, by the feedback currents and by the non-linear interaction between different TMs. The feedback is shown to prevent the TMs wall-locking: regardless of the amplitudes at the resonant surfaces, TMs rotate with edge values very close to those they would have in the presence of an ideal shell in the place of the resistive shell. The feedback performance and efficiency dependence on the shell characteristics will be discussed.