The effects of the net toroidal current on the local ideal MHD stability or the Mercier criterion are investigated for a plasma in the heliotron/torsatron configuration by taking the Large Helical Device as an example. The three dimensional equilibrium code VMEC is used to study the local stability of equilibria with given net toroidal currents. It is found that a subtractive current that decreases the rotational transform improves the stability, while an additive current that increases the rotational transform reduces the stability. The change of the rotational transform at the magnetic axis due to the net toroidal current is the essential mechanism for the change of stability. Mercier stability diagrams are given for the configurations of the Large Helical Device in which the plasma position is shifted inward or outward
A numerical study of double-helix current drive for tokamaks