In addition to their effect on the linear stability of tearing modes, energetic particles can influence the nonlinear evolution of a magnetic island through an uncompensated cross field current due to the effect of charge separation when the orbit width of an energetic particle is much larger than the island width. The corresponding return parallel current may compensate the loss of bootstrap current in the magnetic island. This nonlinear effect depends on the island's propagation frequency (the rotation frequency of the island relative to the plasma), the density gradient of energetic ions and magnetic shear. If the island's propagation frequency is positive, the effect of the uncompensated current plays a stable role on neoclassical tearing modes. When the magnetic shear is sufficiently small, this effect becomes significant and can partially cancel or even overcome the destabilizing effect of the perturbed bootstrap current. In ITER this provides a possibility of using energetic ions to suppress the neoclassical tearing mode for the steady state and hybrid scenarios with weak magnetic shear.
Nonlinear evolution of tearing modes in inhomogeneous plasmas