Observation of tearing mode deceleration and locking due to eddy currents induced in a conducting shell

Growth to large amplitude of a single core-resonant tearing mode in the Madison Symmetric Torus @R. N. Dexter et al., Fusion Technol. 19, 131 ~1991!# reversed-field pinch is accompanied by braking and eventual cessation of mode rotation. There is also a concurrent deceleration of bulk plasma rotation. The mode deceleration is shown to be well described by a time-dependent version of a magnetohydrodynamical model @R. Fitzpatrick et al., Phys. Plasmas 6, 3878 ~1999!# in which a braking torque originates from eddy currents induced by the rotating mode in the conducting shell surrounding the plasma. According to the model, the electromagnetic braking torque is localized to the plasma in the immediate vicinity of the mode's resonant surface, but viscosity transfers the torque to the rest of the plasma. Parametrizing the plasma viscous momentum diffusivity in terms of the global momentum confinement time, the model is used to predict both the momentum confinement time and the time evolution of the decelerating mode velocity. In both respects, the model is quite consistent with experimental data. © 2004 American Institute of Physics.