Marginal stability diagrams for infinite-n ballooning modes in quasi-symmetric stellarators

The influence of the pressure and rotational transform gradients on the local shear, which in turn influences the ballooning stability, is described. Stability diagram for the quasi-axially symmetric NCSX, a quasi-poloidally symmetric configuration and the quasi-helically symmetric HSX are compared. Regions of stability are observed in both NCSX and the quasi-helically symmetric device. The results show that the ballooning stability results when the local shear is small in regions of bad curvature with sufficient pressure gradient.