A mode transition in self-suppressing double tearing modes via Alfvén resonance in rotating tokamak plasmas

Abstract

The rotation profile effects on self-suppressing double tearing modes (DTMs) via Alfvén resonance in rotating tokamak plasmas with reversed magnetic shear are numerically investigated using a reduced magnetohydrodynamic model. The synergetic effects of Alfvén resonance and flow shear on suppressing the DTMs are addressed. It is found that the Alfvén resonances on both sides of the inner rational surface r s 1 decouple the strongly coupled DTMs, and simultaneously the flow shear further stabilizes the tearing instability on the outer rational surface r s 2 . When the tearing instability on r s 2 is stabilized so significantly that it becomes less unstable than the original one on r s 1 , a new mode transition occurs, in which the tearing instability excitation switches from r s 2 to r s 1 ; meanwhile, the Alfvén resonances switch from both sides of r s 1 to both sides of r s 2 . Moreover, the characteristics of the two eigenmode patterns of the DTM-induced Alfvén resonances are analysed in detail. In addition, it is observed that the critical rotation frequency of the mode transition is almost independent of resistivity.