Nonlinear stability of externally induced magnetic islands in multi-helicity helical systems

Self-healing (spontaneous shrinkage) of externally induced magnetic islands is a critical issue in helical systems, where helical ripple-induced neoclassical viscous torques play essential roles. In this study, effective helical ripple rates of magnetic fields in multi-helicity helical systems are revisited. In a typical parameter regime of the Large Helical Device, effective helical ripple rates are sensitive to magnetic axis positions. An extended theory of the self-healing taking into account effective helical ripple rates is firstly developed. It is newly found that self-healing thresholds considerably depend on magnetic axis positions, which is due to neoclassical viscous torques depending on effective helical ripple rates. © 2015 The Japan Society of Plasma Science and Nuclear Fusion Research.