Numerical study on nonlinear double tearing mode in ITER

The nonlinear dynamics of the m/n = 2/1 double tearing mode (DTM) in ITER are systematically studied using the three-dimensional toroidal magnetohydrodynamic code, CLT. We carefully investigate the effects of the radial locations and magnetic shear strengths of the inner and outer rational surfaces r1, r2, s1 and s2, as well as the safety factor at the magnetic axis q0 on DTM. It is found that the explosive burst takes place only with the moderate separation of the two rational surfaces or the stronger magnetic shear strength in which the strong interaction of magnetic islands in the two rational surfaces happens in the early nonlinear phase of the island development. The explosive burst can result from either the direct mutual driving associated with the fast growth island in the two rational surfaces or a strong nonlinear mode–mode coupling. For a large separation and a weak shear strength of the two rational surfaces, the magnetic islands saturate without strong interaction with each other, and (win + wout)/2 is always below the separation Δrs. For a small separation, the kinetic evolution of DTM only exhibits an oscillation with a very low level and then decreases.