Destabilization and nonlinear interaction of tearing modes in tokamak plasmas with locally reversed shear

Linear destabilization and nonlinear dynamics of m/n = 2 tearing modes in tokamak plasmas with locally reversed shear have been investigated in a reduced magnetohydrodynamic model. It is found that a transformation from the single tearing mode structure to the global double tearing mode structure takes place depending on the local shear parameter. The resistivity scaling law of the mode growth rate also changes from similar to S-Hp(-3/5) to similar to S-Hp(-1/3) with q(min) decreasing, where S-Hp is the magnetic Reynolds number, and q(min) is the local minimum q value at position outsider q=2 surfaces. Nonlinear simulation shows that once q(min) < 2, the fast growing islands on the outer rational surfaces interact each other and trigger a plasmoid instability, which can couple with the inner tearing mode and generate a turbulent structure finally. This might provide an implication for understanding of relative nonlinear physics in ITER or astrophysical plasmas.