The ideal magnetohydrodynamic instability of a symmetric q = 1 magnetic island is studied by using an initial value method for a pressureless cylindrical tokamak plasma. It is shown that the island is unstable to a half-coalescence motion which destroys the up-down symmetry of the magnetic configuration. The mode characteristics are qualitatively similar to those predicted by Bussac et al. [Phys. Lett. 105A, 51 (1984)].
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