The effects of variations in net toroidal currenton the Mercier criterion are studied in a heliotron configurationunder the free boundary condition.The plasma column and the magnetic axis are shifted horizontally by the net toroidal current because the free boundary condition changes the effective vertical field.The direction of the shift depends on the direction of the current. In the case of a subtractive current density peaked at the axis, which decreases the rotational transform,the equilibrium is more stable than the no net current equilibrium,although the plasma axis is shifted inwards by the current itself. This is because the Shafranov shift is greater than the inward axis shift caused by the current itself, and the magnetic well is enhanced.In the case of additive currents which increase the rotational transform,there is a tendency for the stability to be improvedas the peak position of the current densityvaries from the axis to the peripheral region.This improvement is due to the enhancement of the magnetic shear at low betaand the magnetic well at high beta. As in the no net current case,the equilibrium with an additive hollow currentunder the free boundary condition is more stable than that under the fixed boundary conditionbecause magnetic well stabilization is enhanced. When the subtractive current peaked at the axis and the additive hollow current flow simultaneously so that the total current is zero or slightly additive,the interchange mode can be stabilized even if the corresponding equilibrium with no net currentis strongly Mercier unstable.
MHD instabilities in current carrying toroidal heliotron/torsatron plasmas