Interactions between neoclassical tearing modes

Neoclassical tearing modes (NTMs) with different mode numbers have been observed in tokamak plasmas. These NTMs, however, never occur simultaneously in ASDEX Upgrade. As a mode with a different helicity starts to grow, this usually leads to suppression of the first NTM. Both analytical and numerical modelling have been carried out to investigate the interaction between NTMs of different helicities. It is found that once the two magnetic islands are close, the more unstable magnetic island survives and suppresses the less unstable one, in agreement with the experimental results. The mechanism responsible for this suppression is the decreased fundamental harmonic pressure perturbations of the NTM in the presence of magnetic perturbations of different helicities.