MODELING OF THE NONLINEAR GROWTH OF NEOCLASSICAL TEARING MODES

The nonlinear growth of neoclassical tearing modes is studied numerica lly. In the present computational model the perturbed bootstrap curren t, as well as the nonlinear coupling between different harmonics of th e same helicity, are included, and the large aspect-ratio tokamak appr oximation is used. This work is focused on the nonlinear growth time a nd 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, t he island width at saturation is not linearly proportional to the frac tion of the bootstrap current. The formation of the island leads to a near constant decrease in plasma pressure in the region between the ma gnetic axis and the island. The simulated island growth time, as well as the saturated island width, agree very well with experimental obser vations. (C) 1998 American Institute of Physics. [S1070-664X(98)00111- 6].