Characteristics of confinement and stability in large helical device edge plasmas

Recent progress in the heating capability in the large helical device [O. Motojima et al., Phys. Plasmas 6, 1843 (1999)] has allowed the highest average β value (4.1%) obtained in the helical devices, and enables exploration of magnetohydrodynamics (MHD) stability in this β region. MHD activities in the periphery are found to become stable spontaneously from the inner region to the outer region when the averaged β value exceeds a threshold, and then a flattening of the electron temperature profile is observed around the resonant surface. Such a flattening can be formed externally by producing an m∕n=1∕1 magnetic island, and the complete stabilization of the m∕n=1∕1 mode is demonstrated by the moderate island width. In addition, attempts to control peripheral plasmas are also performed by using a limiter and a local island divertor utilizing the m∕n=1∕1 island, to improve plasma confinement and, especially, to stabilize pressure-driven modes in the present study. The stabilization of peripheral MHD modes is obtained with both approaches, and this indicates that these are available to the production of higher-β plasmas without edge MHD activities.