Dependence of the threshold for perturbation field generated m/n = 2/1 tearing modes on the plasma fluid rotation

The dynamic ergodic divertor (DED) on the TEXTOR tokamak allows for the creation of static and rotating helical magnetic perturbation fields. In the 3/1 configuration the strong m/n = 2/1 sideband excites a locked 2/1 tearing mode above a critical perturbation field strength. The mode onset threshold depends strongly on the plasma fluid rotation with respect to the mode. Rotation in plasma current direction destabilizes the mode in a certain range of rotation frequencies, whereas counter-rotation has a stabilizing influence. The threshold shows a minimum when the frequency of the external perturbation equals the MHD frequency of the mode. (letter)