Non-Resonant n = 1 Helical Core Induced by m/n = 2/1 Neoclassical Tearing Mode in JT-60U

Abstract

We have investigated the excitation mechanism of n = 1 helical cores (HCs) with m/n = 2/1 neoclassical tearing modes (NTMs) in JT-60U. It is found that the n = 1 HC is observed with the mode frequency from several Hz to 6 kHz. This indicates that the resistive wall and the plasma control system do not induce n = 1 HCs because the both time scales are different from the mode frequency. In addition, n = 1 HCs appear to be the non-resonant mode from the two observations: n = 1 HCs do not rotate with the plasma around the q = 1 surface in the core and an n = 1 HC is observed even when q_min. > 1. It is also observed that the fluctuation due to an n = 1 HC in the core region disappears with the stabilization of an m/n = 2/1 NTM, implying that n = 1 HCs are driven by m/n = 2/1 NTMs. We revisit a quasi-linear MHD model where the n = 1 HC is induced directly by the sideband of the current for the m/n = 2/1 NTM, which potentially excites the non-resonant m/n = 1/1 mode.