Half‐Coalescence Ideal Magnetohydrodynamic Instability of the q=1 Magnetic Island in Tokamaks

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)].