We have investigated dynamics of self-organizing toroidal plasmas such as spheromak and low-q spherical tokamak produced in the HIST devise [1] by decreasing the external toroidal field and reversing its sign in time. Consequently, we have discovered that the helicity-driven toroidal plasma relaxes towards flipped state. Surprisingly, it has been observed that not only toroidal flux but also poloidal flux reverses sign spontaneously during the relaxation process. The self-reversal of the magnetic fields is attributed to the non-linear growth of the n=1 kink instability of the central open flux [2]. Three-dimensional MHD numerical simulations have succeeded in demonstrating the formation of the flipped ST and substantially enabled us to understand the non-linear dynamic field-reversal mechanisms [3]. The simulation results indicate that a large helical distortion of the open flux and the following magnetic reconnection between open and closed field lines play a major role in the self-reversal process.
From the viewpoint of coaxial helicity injection (CHI) current drive, it is conceivable that the flipped ST (F-ST), which consists of only closed flux surfaces, compares favorably with the normal ST. We have investigated how the F-ST plasma is sustained, although it is isolated from a helicity source. The helicity-driven relaxed theory shows that there exist the mixed states of ST and F-ST in a boundary. Helicity is transferred to F-ST through the ST with coupling with gun electrodes. It has been found that magnetic reconnection of the toroidal magnetic field at the outer edge region takes place during the current sustainment.

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