Stabilization of neoclassical tearing modes by electron cyclotron current drive in JT-60U

Isayama, Akihiko; Oyama, Naoyuki; Urano, Hajime; Suzuki, Takahiro; Takechi, Manabu; Hayashi, Nobuhiko; Nagasaki, Kazunobu*; Kamada, Yutaka; Ide, Shunsuke; Ozeki, Takahisa

Results from active control of neoclassical tearing modes (NTMs) by electron cyclotron current drive (ECCD) in are described. Evolution of an NTM with the poloidal mode number and the toroidal mode number has been suppressed by ECCD inside the sawtooth inversion radius in the co-direction, showing the possibility of the coexistence of sawtooth oscillations and a small-amplitude NTM without large confinement degradation. Stabilization of an NTM by ECCD at the mode rational surface has been demonstrated with a small value of the ratio of the current density driven by electron cyclotron (EC) wave to the local bootstrap current density (). Also, dependence of the stabilization effect on ECCD location has been investigated in detail, and it has been clarified that an NTM can be completely stabilized with the misalignment of the ECCD location less than about half of the island width, and that the NTM is destabilized with the misalignment comparable to the island width. Time-dependent, self-consistent simulation of magnetic island evolution using the TOPICS code has shown that the above stabilization and destabilization of an NTM are well reproduced with the same set of coefficients of the modified Rutherford equation. The TOPICS simulation has also clarified that EC wave power required for complete stabilization can be significantly reduced by narrowing the ECCD width.