Tearing Mode Rotation by External Field in a Reversed Field Pinch

We present results of the first experiments on resistive tearing mode control in a reversed field pinch by an external m = 0 magnetic perturbation. In this way the helical deformation of the plasma column produced by resonant m = 1 modes can be localized in a preselected toroidal position. We also show that a continuous toroidal rotation of this deformation can be induced by a rotating external field. From the study of such toroidal rotations the nonlinear origin of the torque acting on the internally resonant modes clearly emerges: in particular, we demonstrate the occurrence of phase conjugation and harmonic generation phenomena.