MHD Stability of Low-n Ideal External Mode in Large-Helical-Device Plasma

The characteristics of low-n ideal external modes in the Large-Helical-Device are studied. The low-n analysis is carried out with the TERPSICHORE code [W.A. Cooper, Plasma Phys. Control. Fusion 34, 1011 (1992)]. Under fixed boundary conditions, the low-n instability region is limited within the Mercier unstable region and the external mode does not appear. On the other hand, with free boundary conditions, the low-n unstable region spreads and the external modes become unstable. The effects of a conducting wall similar in shape to that of the plasma show that stability to external modes of higher growth rate can be obtained even with the wall further away from the plasma. Keywords: ideal MHD, LHD, external mode, conducting wall, TERPSICHOR