The three-dimensional stability of magnetic equilibria with islands is investigated with use of reduced magnetohydrodynamics. It is found numerically that tokamak equilibria with large m/n=2 islands can be ideally unstable. The instability is explained by the strongly modified q profile in the helical equilibrium, where the q=3/2,5/3, resonant surfaces are all close to the inner separatrix of the islands. An analytic treatment of the three-dimensional coalescence instability is given to illustrate this effect.
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A. Bondeson (1983) studied this question.
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