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 beta value (4.1%) obtained in the helical devices, and enables exploration of magnetohydrodynamics (MHD) stability in this beta region. MHD activities in the periphery are found to become stable spontaneously from the inner region to the outer region when the averaged beta 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-beta plasmas without edge MHD activities