Ideal interchange instabilities in stellarators with a magnetic hill

The prediction of the Mercier (Suydam) criterion for localized interchange modes in stellarators shows stability in cases in which global modes are unstable. One case is non-resonant pressure driven instabilities with low mode numbers which become unstable even if the mode resonant surface does not exist inside the plasma column. The other case is interchange instabilities when the pressure gradient vanishes at the mode resonant surface. If the pressure becomes flat in a narrow region around the mode resonant surface, high mode number instabilities are suppressed and the beta limit at the particular resonant surface increases. The radial mode structure at nearly marginal beta also changes significantly. The properties of the non-resonant mode and the transition from a resonant to a non-resonant mode are clarified with a cylindrical plasma model for a low shear stellarator with a magnetic hill.