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Helical cores (HCs) in plasmas are one of the important phenomena because the helical structure affects impurity transport and induces loss of confined energetic particles or loss of toroidal momentum. Substantial studies on experimental observation of HCs were reported, because HCs would be excited in hybrid scenarios in ITER or DEMO. In addition to the effect on transport by HCs, recent studies suggested that MagnetoHydroDynamics (MHD) dynamo accompanied with HCs redistributes the current profile in the core and realizes the experimentally observed sawtooth-free plasmas of hybrid scenarios in DIII-D, in which the minimum value of the safety factor profile, qmin, is kept slightly above unity by "flux pumping". Several theoretical models to explain the excitation mechanism of HCs have been proposed, such as the generation of the magnetic island by the excitation of tearing modes (TMs) or HC equilibria induced by internal kink modes. Though substantial experimental observations were reported in previous studies, the experimental observations focusing on the detailed relationship between the mode structures of the HC and other MHD modes have not been reported well.
In this paper, we report experimental finding of n = 1 HCs accompanied with saturated m/n = 2/1 TMs having low rotation frequencies in JT-60U with various diagnostics. Here, m is the poloidal mode number and n is the toroidal mode number. The TMs with HCs are observed after an increase of the mode amplitude and a decrease of the mode frequency of m/n = 2/1 precursors having the tearing parity. The decreased mode frequency is lower than 20 Hz typically. With various diagnostics, the coupling of n = 1 helical cores and m/n = 2/1 TMs have been clearly observed. Because the coherent oscillations in the ion temperature are observed in the core region and in the edge region, the flux surfaces including the m/n = 2/1 magnetic island appear to have m = 1 helical deformation. It has also been suggested that the m/n = 2/1 TM and the HC rotate in the electron diamagnetic direction keeping fm/n=1/1(HC) = 2fm/n=2/1(TM) in several plasmas even with different bulk rotation speed. Here, the fm/n=1/1(HC) is the rotation frequencies of HCs and fm/n=2/1(TM) is the rotation frequency of TMs. In addition, the core seems to be shifted to the high field side when the “O” points of the m/n = 2/1 magnetic island line up in the midplane, which is confirmed by the reconstruction of MHD equilibria with Motional Stark Effect measurement and MEUDAS code. Our observations on m/n = 2/1 TMs having HCs would contribute to the understanding of the excitation mechanism of HCs in tokamak plasmas.
会議概要（会議名, 開催地, 会期, 主催者等）
The 28th International Toki Conference on Plasma and Fusion Research
Experimental observations of n=1 helical cores accompanied with saturated m/n = 2/1 tearing modes having low rotation frequencies in JT-60U
