Magnetohydrodynamic simulation on Co- and counter-helicity merging of spheromaks and driven magnetic reconnection

A magnetohydrodynamic relaxation process of spheromak merging is studied by means of an axi-symmetric numerical simulation. As a result of counter-helicity merging, a field-reversed configuration is obtained in the final state, while a larger spheromak is formed after co-helicity merging. In the counter-helicity case, a clear pressure profile of which iso-surfaces coincide with flux surfaces is generated by thermal transport of a poloidal flow that is induced by driven reconnection. It is also found that a sharp pressure gradient formed at the edge of a current sheet causes a bouncing motion of spheromaks. According to the bounce motion, the reconnection rate changes repeatedly. As is shown by the TS-3 experiments, furthermore, strong acceleration of a toroidal flow and reversal of a toroidal field are observed in the counter-helicity merging. (author)