  As tokamaks are designed and built with increasing levels of stored energy in
the plasma, disruptions become increasingly dangerous. It has been reported
that 95% of the disruptions in the Joint European Torus (JET) tokamak with the
ITER-like wall are preceded by the growth of large locked islands, and these
large islands are mostly produced by off-normal events other than neoclassical
tearing modes. This paper discusses the use of RF current drive to stabilize
large islands, focusing on nonlinear effects that appear when relatively high
powers are used to stabilize large islands. An RF current condensation effect
can concentrate the RF driven current near the center of the island, increasing
the efficiency of the stabilization. A nonlinear shadowing effect can hinder
the stabilization of islands if the aiming of the ray trajectories does not
properly consider the nonlinear effects.
