NONLINEAR COUPLING EFFECTS ON THE RELAXATION PROCESS IN THE REVERSED FIELD PINCH

The relaxation process in the reversed field pinch has been studied extensively with three-dimensional magnetohydrodynamic simulations. It is found that the non-linear coupling between multiple helicity modes plays a leading role in the relaxation process. Specifically, the (m; n) equals (0; 1) island is excited as a result of non-linear coupling between the linearly unstable m equals 1 modes, with a toroidal mode number differnce of one. The m equals 0 island is then subject to axisymmetric non-linear reconnection whereby reversed flux is effectively generated in the outer region. It should be noted that although the axisymmetric non-linear reconnection of m equals 0 island is the dominant relaxation process, helical non-linear reconnection of linearly unstable m equals 1 modes also plays some role in the relaxation of the whole system.