EFFECTS OF NET TOROIDAL CURRENT ON THE MERCIER CRITERION IN THE LARGEHELICAL DEVICE

The effects of the net toroidal current on the local ideal MHD stabili ty or the Mercier criterion are investigated for a plasma in the helio tron/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 tra nsform improves the stability, while an additive current that increase s the rotational transform reduces the stability. The change of the ro tational transform at the magnetic axis due to the net toroidal curren t is the essential mechanism for the change of stability. Mercier stab ility diagrams are given for the configurations of the Large Helical D evice in which the plasma position is shifted inward or outward.