Onset and Evolution of m/n = 2/1 Neoclassical Tearing Modes in High-βp Mode Discharges in JT-60U

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The onset and evolution of an m/n = 2/1 neoclassical tearing mode (NTM) in JT-60U under nearly constant discharge conditions are investigated (m and n are the poloidal and toroidal mode numbers, respectively.). In about 80% of the discharges, the 2/1 NTM appears from a small amplitude without a noticeable triggering event, and in the rest of the discharges, the 2/1 NTM is triggered by a collapse localized at the mode location. Although the former grows with regular oscillations of the magnetic perturbations, the latter grows with irregular oscillations from the beginning. In addition, although a collapse causing a temperature change of ∼20% triggers a 2/1 NTM, a smaller-amplitude collapse, e.g., a temperature change of ∼7%, does not trigger an NTM. This suggests the existence of a threshold amplitude that triggers an NTM. The characteristics of the localized collapse are similar to those of a previously observed barrier localized mode.