Rotation dependence of tearing mode excitation by external perturbation fields on TEXTOR

The experimental study of the influence of plasma rotation on the 2/1 tearing mode excitation by external perturbation fields on TEXTOR shows that a large plasma rotation in either co- or counter-current direction has a stabilizing effect. However, the dependence of the mode onset threshold on the plasma angular frequency shows an asymmetry, i.e. counter-rotation is always stabilizing whereas co-injection of momentum first lowers the threshold before stabilization sets in. A possible explanation is the influence of the ion polarization current which can be stabilizing and destabilizing, depending on the mode frequency with respect to ion and electron diamagnetic frequencies.