Effects of Net Toroidal Current Profile on Mercier Criterion in Heliotron Plasma

The effects of the variation in the net toroidal current on the Mercier criterion are studied in a Heliotron configuration under the free boundary condition. The plasma column and the magnetic axis are shifted horizontally by the net toroidal current because the effective vertical field is changed. The direction of the shift depends on the direction of the current. In the case of the subtractive current which decreases the rotational transform, the equilibrium is more stable than the no net current equilibrium, although the plasma axis is shifted inward by the current itself. This is because the Shafranov shift is superior to the axis shift by the current and the magnetic well is enhanced. In the case of the additive current which increases the rotational transform, there is a tendency that the stability is improved as the peak position of the current density varies from the axis to the peripheral region. This improvement is due to the enhancement of the magnetic shear at low beta and the magnetic well at high beta. When the subtractive current peaked at the axis and the additive hollow current flow simultaneously so that the total current should be zero or slightly additive, the interchange mode can be stabilized even if the corresponding equilibrium with no net current is strongly Mercier unstable