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Abstract. Global mode analyses of ideal MHD instability are performed under both fixed and free boundary conditions for L/M = 2/10 heliotron configurations, where L and M are the polarity and toroidal field period of helical coils, respectively. Under the fixed boundary condition, dangerous low-n ballooning modes with n < M (n is a typical toroidal mode number) are not destabilized, when the pressure gradient is quite weak near the boundary. However, such ballooning modes and/or peeling modes are destabilized under the free boundary condition even for the currentless condition, when the pressure gradient becomes strong there. Effects of the net toroidal current on the external modes are also discussed.
IAEA 2001
