A mechanism is proposed for a nonlinear self-sustainment of magnetic islands that relies on the presence of a chaotic region. Particles diffusing in the stochastic magnetic field produce a local increase of current at X points on a rational surface, which in turn sustains the original perturbation if the equilibrium profile is concave. It is shown that the new equilibrium is stable. In a tokamak the mechanism is operative in the region outside the q=2 surface, in agreement with the observation of strong electron confinement degradation in this region.
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White et al. (1989) studied this question.
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