Curvature effects on the dynamics of tearing modes in tokamaks

The curvature effects on the dynamics of magnetic island evolution in tokam aks are investigated both theoretically and numerically. By taking into acc ount perpendicular and parallel heat diffusion, a new dispersion relation i s derived for tearing modes that match the linear and nonlinear results. Th is evolution equation allows a quantitative description over the whole rang e of island sizes. It predicts a nonlinear instability, i.e., growing magne tic islands in linearly stable magnetic configurations. All these predictio ns are in excellent agreement with full tridimensional linear and nonlinear magnetohydrodynamic (MHD) computations with the latest version of XTOR [K. Lerbinger and J. F. Luciani, J. Comput. Phys. 97, 444 (1991)]. These resul ts have important consequences on the onset of neoclassical tearing modes b ecause they predict a resistive MHD threshold. (C) 2001 American Institute of Physics.