Self-annihilation of magnetic islands in helical plasmas

The evolution of the magnetic island which is induced by the resonant deformation by external currents in helical systems (such as LHD) is analyzed. The defect of the bootstrap current, caused by the magnetic island, has a parity which reduces the size of the magnetic island, if the bootstrap current enhances the vacuum rotational transform. The width of the externally-driven magnetic island can be suppressed to the level of ion banana width if the pressure gradient exceeds a threshold value. This island annihilation is self-sustained. That is, the annihilation continues, for fixed beta-value, until the external drive for island generation exceeds a threshold. Effects of the reversal of the direction of the bootstrap current and of the sign of radial electric field are also investigated. The possibility of the neoclassical tearing mode in the LHD-like plasma is discussed. (author)