Interactions between neoclassical tearing modes

Neoclassical tearing modes (NTMs) with different mode numbers have been obs erved in tokamak plasmas. These NTMs, however, never occur simultaneously i n ASDEX Upgrade. As a mode with a different helicity starts to grow, this u sually leads to suppression of the first NTM. Both analytical and numerical modelling have been carried out to investigate the interaction between NTM s of different helicities. It is found that once the two magnetic islands a re close, the more unstable magnetic island survives and suppresses the les s unstable one, in agreement with the experimental results. The mechanism r esponsible for this suppression is the decreased fundamental harmonic press ure perturbations of the NTM in the presence of magnetic perturbations of d ifferent helicities.