Magnetic helicity injection using coaxial electrodes (Coaxial Helicity Injection: CHI) has successfully demonstrated the non-inductive current generation on HIT-II and NSTX spherical tokamak (ST) devices, however, the issue of large amount of fluctuations in closed flux is serious because the closed flux is surrounded by open flux linking the helicity source. On the other hand, in the HIST experiment [1], another novel ST configuration in CHI-driven systems, a "flipped" ST, has been discovered to be formed by reversing the external toroidal magnetic field in a normal ST plasma. The configuration is characterized by the field polarity opposite to the normal ST in closed flux surfaces. In the flipped state, since the closed flux is isolated from open flux, the smaller fluctuations have been observed than the normal CHI-driven ST plasmas. In order to clarify the formation mechanism of such a novel ST configuration, 3-dimensional numerical simulations based on resistive MHD model are executed.
Our numerical simulations have demonstrated, for the first time, that reversing the direction of external toroidal magnetic field makes a ST plasma relax towards a flipped ST accompanied by a spontaneous reversal of the magnetic polarity. The relaxation is shown to be triggered by the n=1 mode growing in the central open flux and to be caused by the reconnection process that follows the growth of the mode. The force-free parameter λ is calculated to discuss the origin of the unstable mode, and it is found that the enhancement of λ due to the reversal of the external field is responsible for the growth of the mode, which is in agreement with the stability analysis. It is concluded that the transition form a ST to a flipped ST corresponds to the relaxation of high λ plasma in the central open flux towards a low λ state.

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