Self organized formation and sustainment of helicity-driven spherical high beta torus plasmas

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In the Helicity Injected Spherical Torus device, internal magnetic field and current density structures of spherical torus (ST) plasmas sustained by coaxial helicity injection (CHI) have been revealed via intensive internal magnetic measurements. The internal magnetic configuration of the ST plasma formed by CHI is in good agreement with the results of numerical equilibrium fitting calculations. The generation of closed poloidal flux of ST has been verified by varying the external toroidal field strength in the same device. In order to understand self-organization in helicity-driven systems, we have investigated the dynamics of low-aspect-ratio toroidal plasmas by decreasing the external toroidal field and reversing its sign in time. Consequently, we have discovered that the helicity-driven toroidal plasma relaxes towards the flipped state. Surprisingly, it has been observed that not only toroidal flux but also poloidal flux reverses sign spontaneously during the relaxation process. Th … More e self-reversal of the magnetic fields is attributed to the nonlinear growth of the n 1 kink instability of the central open flux. The dynamics of helicity-driven spheromak plasmas has been investigated using three-dimensional magnetohydrodynamic (MHD) numerical simulations. It is found that the toroidal current is driven by continuous plasmoid injection accompanied by the n=1 oscillation of central open flux column. Also, we propose that the pulse injection of the helicity is effective for suppressing the n=1 fluctuation.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 the sustainment mechanism of the F-ST plasma. The helicity-driven relaxed theory shows that there exist the mixed states of ST and F-ST in the flux conserver. Helicity is transferred to F-ST through the ST with coupling with gun electrodes. It has been found that magnetic reconnection between the toroidal magnetic field plays important role in the sustainment of the F-ST. The magnetic field in the outer edge region shows regular oscillations which have a large amplitude of the n=1 mode. The core region of the F-ST seems to be relatively stable. Less