Global Mode Analysis of Ideal MHD Modes in L = 2 Heliotron/Torsatron Systems

"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 eqililibria, 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 gg 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.