TOROIDALLY LOCALIZED AND NONLOCALIZED BALLOONING INSTABILITIES IN A STELLARATOR

It is shown that the coexistence of toroidally nonlocalized ideal-hydr omagnetic ballooning instabilities, with a quasidiscrete spectrum, and toroidally localized ballooning instabilities with a broad continuous spectrum, as predicted by Dewar and Glasser [Phys. Fluids 26, 3038 (1 983)] can be realized in a Mercier-unstable equilibrium case modeling the Large Helical Device (LHD) [A. Iiyoshi et al., Fusion Technol. 17, 148 (1990)] with a broad pressure profile. The quasidiscrete, interch ange branch corresponds to extended modes that can be understood on th e basis of a ripple-averaged ballooning equation, whereas the broad-co ntinuum, ballooning branch corresponds to modes localized along a flux tube, The physical origin of the two branches is discussed. (C) 1998 American Institute of Physics.