Non-linear tearing modes in tokamaks

Numerical codes have been developed for the description of the non-linear evolution of tearing modes in tokamaks. The m=1 mode is observed to continue rapid growth in the non-linear 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 non-linear 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>=2, on the other hand, cease exponential growth upon entering the non-linear regime and saturate shortly thereafter. A model for the saturation processes is presented. Modifications due to thermal transport are discussed. (author)