Control of Growth of Neoclassical Tearing Mode by Central Co-ECCD in JT-60U

Abstract

Growth of a neoclassical tearing mode (NTM) with a poloidal mode number m = 3 and a toroidal mode number n = 2 has been controlled by electron cyclotron current drive (ECCD) in the same direction as the plasma current (co-ECCD) in the central region away from the NTM island. By using ECCD at a fraction of about 10% of the plasma current, the amplitude of the m/n = 3/2 NTM is limited to about 1/5 of that without ECCD. The frequency spectrum of magnetic perturbations shows that the frequency of the m/n = 3/2 NTM is modulated by a sawtooth crash. The beta value, which is the ratio of the plasma pressure to the magnetic pressure, is higher by 6% than that without ECCD at the same input power. This operational scenario is quite different from that for conventional NTM stabilization in which the ECCD location is accurately adjusted at the mode rational surface and is thus off-axis. This result shows the compatibility of sawtooth oscillations and a small-amplitude 3/2 NTM, and suggests the possibility of a new and easier operational scenario for NTM control.