Observing the Coupling of the Toroidal Plasma Rotation Due to m/n=2/1 and m/n=3/2 Neoclassical Tearing Modes by Uncorrected n=2 Error Field in DIII-D

Injection of electromagnetic torque by tearing mode rotation control feedback can sustain rotation of the 2/1 NTM, avoiding mode locking for several seconds after the mode appearance. This feedback process optimizes the phasing of rotating applied n=1 field relative to the mode, hence preventing the locking and simultaneously compensating the n=1 error field (EF). In high beta discharges, the large amplitude sustained 2/1 NTM reduces the local toroidal rotation to near zero at the q=3/2 surface and =1, implying the angular momentum is coupled between the two rational surfaces. The mode at the q=3/2 surface is identified as a m/n=3/2. The mode is presumably affected by n=2 EF as well as remaining uncorrected n=1 EF. A possible process of sustained NTM with velocity shear due to the $E_r$ buildup by large size magnetic islands will also be discussed