A statistical analysis of spontaneous mode rotations in the reversed field experiment (RFX) reversed field pinch is presented. These rotations involve only m = 1 toroidal magnetic field fluctuations with a velocity that is found to be smaller than that of the fluid motion and in the opposite direction. In RFX magnetic fluctuations are mainly present as tearing modes and are usually phase and wall locked. However, a rotation can start when a mode decouples from the interaction with all the other harmonics and its amplitude reduces to a sufficiently small value so that the torque due to the resistive shell is unable to prevent its motion. Both single and multiple simultaneous mode rotations are characterized in terms of the shape of the m = 1 mode spectrum. In the presence of a dominant helicity it is more likely that multiple rotations will be observed. A qualitative torque analysis is presented assuming a three-waves coupling. Also, m = 0 mode rotations are observed and some preliminary results are presented: no m = 1 rotation is seen during these events. An interpretation of the possible mechanisms for mode rotation is provided: m = 0 modes are subject to plasma drag flow at the edge, while secondary m = 1 modes are subject to the electron diamagnetic drift.
Physics of tearing mode rotation and slow-down in the RFX-mod tokamak