Coaxial helicity injection and n=1 relaxation activity in the HIST spherical torus

In order to understand comprehensively the role of the n=1 instability and relaxation on current generation processes in helicity-driven spherical systems, we have investigated dynamics of ST plasmas produced in the HIST device by decreasing the external toroidal field (TF) and reversing its sign in time. In result, we have discovered that the ST relaxes towards flipped ST configurations through formation of reversed-field pinches (RFPs)-like magnetic field profiles. Surprisingly, it has been observed that not only toroidal flux but also poloidal flux reverses sign spontaneously during the relaxation process. The dynamics associated to self-reversal of the magnetic fields is presently investigated by using three-dimensional magnetohydrodynamic (MHD) numerical simulations. Furthermore, we have first demonstrated that a flipped ST plasma can be successfully sustained by CHI. The n=1 relaxation activity is found to be essential in the current sustainment of the flipped ST as well as the spheromak and the unflipped ST. (author)