Modeling of the nonlinear growth of neoclassical tearing modes

The nonlinear growth of neoclassical tearing modes is studied numerically. In the present computational model the perturbed bootstrap current, as well as the nonlinear coupling between different harmonics of the same helicity, are included, and the large aspect-ratio tokamak approximation is used. This work is focused on the nonlinear growth time and the saturated width of the magnetic island. The perturbed bootstrap current is found to play a destabilizing role in the nonlinear island as in previous analytical theories. For sufficiently large islands, the island width at saturation is not linearly proportional to the fraction of the bootstrap current. The formation of the island leads to a near constant decrease in plasma pressure in the region between the magnetic axis and the island. The simulated island growth time, as well as the saturated island width, agree very well with experimental observations.