Effects of Net Toroidal Current on Mercier Criterion in the Large Helical Device

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 current. It is found that a subtractive current which decreases the rotational transform improves the stability while an additive current which increases the rotational transform deteriorates 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 the stability property. Stability diagrams are drawn against the Mercier criterion for the configuration of the Large Helical Device shifting the plasma position inward or outward