MHD INSTABILITIES IN CURRENT-CARRYING TOROIDAL HELIOTRON TORSATRON PLASMAS/

Current driven ideal MHD instability with (m,n) = (1,1) becomes unstab le in a current carrying toroidal heliotron plasma at zero pressure, w here m(n) is the poloidal (toroidal) mode number. This result is consi stent with the cylindrical plasma approximation under the stellarator expansion. For finite beta current carrying plasmas of Heliotron E wit h two (sic) = 1 resonant surfaces or with a highly peaked profile, the current driven m = 1 internal kink mode destabilized at the inner (si c) = 1 surface is gradually changed into the pressure driven m = 1 int erchange mode localized at the outer (sic) = 1 surface with an increas e of pressure, where a is the total rotational transform. The growth r ate is first increased, then decreased with the increase of pressure, which is correlated with the formation of a magnetic well. In the holl ow 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 positiv e hollow current with a negative peaked current is particularly favour able to the MHD stability of LHD.