Tearing modes transition from slow to fast rotation branch in the presence of magnetic feedback

The fast-slow-fast rotation frequency transitions of tearing modes under magnetic feedback boundary conditions have been studied in some recent RFX-mod experiments performed at low current, with tearing mode amplitudes close to the wall-locking threshold. The avoidance of strict wall-locked conditions for amplitudes above the threshold thanks to the feedback-induced slow rotations is a well-established result of RFX-mod. The present experiments, well reproduced by simulations performed with a validated MHD model, demonstrate another benefit of the feedback: the possibility of recovering the fast rotation with no hysteresis when reducing the amplitude. This evidence changes the standard paradigm valid in the absence of feedback, which considers the transition from fast rotation to wall-locked condition a substantially irreversible process.