Nonlinear tearing modes in tokamaks

Numerical codes have been developed for the description of the nonlinear evolution of tearing modes in tokamaks. The m = 1 mode is observed to continue rapid growth in the nonlinear regime, producing significant changes on a time scale consistent with the experimentally observed minor disruption. The double tearing mode associated with a hollow current profile also continues rapid growth in the nonlinear regime, and thus may play a significant role in the anomalous current penetration during the early stages of a tokamak discharge. Single magnetic islands with m greater than or equal to 2, on the other hand, cease exponential growth upon entering the nonlinear regime and saturate shortly thereafter. A model for the saturation processes is presented. Modifications due to thermal transport are discussed