Nonlinear Destabilization of Double Tearing Modes in Reversed Magnetic Shear Plasmas

A new nonlinear destabilization of double tearing modes (DTM) is observed for nonmonotonic 4-profiles and the mechanism is investigated in the single helicity reduced MHD simulation in cylindrical geometry. This nonlinear destabilization of DTM is caused by the generation of the higher harmonics from the main harmonics (3/1 in the present case) by the nonlinear mode coupling. Hence, there appears no anomalous resistivity caused by the turblence. In this nonlinear destabilization mechanism, the triangular deformation of the islands causes the rapid growth of DTM after going through the Rutherford-type regime by forming the steep current around the outer X-points, that is the structure driven mode.