MHD instabilities in current carrying toroidal heliotron/torsatron plasmas

Current driven ideal MHD instability with (m,n) = (1,1) becomes unstable in a current carrying toroidal heliotron plasma at zero pressure, where m(n) is the poloidal (toroidal) mode number. This result is consistent with the cylindrical plasma approximation under the stellarator expansion. For finite beta current carrying plasmas of Heliotron E with two (sic) = 1 resonant surfaces or with a highly peaked profile, the current driven m = 1 internal kink mode destabilized at the inner (sic) = 1 surface is gradually changed into the pressure driven m = 1 interchange mode localized at the outer (sic) = 1 surface with an increase of pressure, where a is the total rotational transform. The growth rate is first increased, then decreased with the increase of pressure, which is correlated with the formation of a magnetic well. In the hollow current case of the Large Helical Device (LHD), there is no purely current driven instability and a specific hollow current profile has a tendency to stabilize the pressure driven interchange modes compared with the currentless case. It is shown that a combination of a positive hollow current with a negative peaked current is particularly favourable to the MHD stability of LHD.