Nonlinear interaction of tearing modes in highly resistive tokamaks

A mechanism for the major disruption in tokamaks is proposed involving the nonlinear destabilization of tearing modes by the m=2, n=1 tearing mode. A three-dimensional cylindrical nonlinear code based on a set of equations valid in the limit of low β and large ratio of the toroidal and poloidal magnetic fields has been constructed. The essential result is that for safety factor profiles flat in the plasma core the 2/1 mode significantly destabilizes other modes, particularly odd modes such as the 3/2 mode, before the 2/1 island in the single-pitch limit has expanded to its maximum width. Many magnetic islands of different pitch are produced and the corresponding deformation of the toroidal current density is more severe than in the two-dimensional case. The magnetic islands generated can extend across the plasma cross section; presumably, the corresponding ergodic magnetic fields can result in the escape of particles and heat from the plasma core. An analytic model in agreement with these results is also presented.