Experimental observations of an n = 1 helical core accompanied by a saturated m/n = 2/1 tearing mode with low mode frequencies in JT-60U

Abstract

Experimental finding of n = 1 helical cores (HCs) accompanied with saturated m/n = 2/1 tearing modes (TMs) having low mode frequencies in JT-60U is reported. The HCs accompanied with TMs 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. Here, fm/n=1/1(HC) is the mode frequency of HCs and fm/n=2/1(TM) is the mode 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 reconstructions of MHD equilibria with Motional Stark Effect measurement and the MEUDAS code. Our observation on m/n = 2/1 TMs having HCs would contribute to the understanding of the excitation mechanism of HCs in tokamak plasmas.