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N. Nakajima1 2, J. Chen2,
K. Ichiguchi1 2, M. Okamoto1 2
1 National Institute for Fusion Science, Toki 509-5292,
Japan
2 Graduate University for Advanced Studies, Japan
Abstract. By means of a global mode analysis of ideal MHD modes for
Mercier-unstable equilibria in a planar axis L = 2/M = 10 heliotron/torsatron
system with an inherently large Shafranov shift, the conjecture from local
mode analysis for Mercier-unstable equilibria has been confirmed and the
properties of pressure-driven modes have been clarified. According to the
degree of the decrease in the local magnetic shear by the Shafranov shift,
the Mercier-unstable equilibria are categorized into toroidicity-dominant
(strong reduction) and helicity-dominant (weak reduction) equilibria. In
both types of equilibria, interchange modes are destabilized for low
toroidal mode numbers n < M, where M is the toroidal field period
of the equilibria, and both poloidally and toroidally localized ballooning
modes purely inherent to three-dimensional systems are destabilized for
fairly high toroidal mode numbers n > > M. For moderate toroidal mode numbers
n 
M, tokamak-like poloidally localized ballooning modes with a weak
toroidal mode coupling are destabilized in toroidicity-dominant equilibria,
and in contrast, in the helicity-dominant equilibria, interchange modes are
destabilized. The interchange modes are localized on the inner side of the
torus, because the Shafranov shift enhances the unfavorable magnetic
curvature there rather than on the outer side of the torus. A continuous or
quasi-point unstable spectrum is briefly discussed.
IAEA 2001
