Hexapole Field Effects on Currentless MHD Equilibrium and Stability of l=2 Heliotron/Torsatron

Abstract

Averaged flux surfaces with respect to the toroidal angle in l=2 heliotron/torsatrons are deformed in D-shape or inverse D-shape by the hexapole field. D-shape deformation of vacuum flux surfaces enhances magnetic well or reduces magnetic hill in Heliotron-E and Large Helical Device (LHD) configuration. However, complete elimination of the magnetic hill region is not obtained by the D-shape deformation without substantial reduction of plasma radius. The inverse D-shape deformation degrades MHD stability against ideal interchange modes in the central region of both devices for fairly peaked pressure profiles. In the LHD configuration, a broad pressure profile proportional to (1−ψ²)² degrades the MHD stability and the hexapole effect is almost irrelevant, where ψ is a poloidal flux function; however, second stability appears against both the local and non-local interchange modes.