An experimental investigation into rotating MHD modes has been performed in the TEXTOR tokamak. The effects on the stability of the MHD tearing modes of coupling between m/n = 2/1 and 1/1 modes and of the slowing down of the mode rotation by wall friction have been studied. Tangential neutral beam injection (NBI) has been used to change the toroidal rotation to observe the influence of plasma rotation on the development of MHD modes. Two phases in the rotation frequency behaviour of the modes can be distinguished: a slow reversible decrease and a fast exponential decay, which starts with a short transient increase in frequency. The last phase finally results in mode locking, which mostly leads to a disruption. By injecting low power NBI in the last phase of the slowing-down process, mode locking could be prevented. The mode amplitude and rotation frequency have been stabilized, without major changes to the pressure profile. By decreasing the central plasma velocity, using NBI, a coupling of the two modes has been forced. The coupling of the modes had a destabilizing effect on the MHD activity.
Spontaneous mode rotation in RFX
Study of slowing down mechanism of locked-mode-like instability in helical plasmas