Effect of toroidal current and profile on ballooning instability mechanisms in a quasiaxisymmetric stellarator

Finite toroidal plasma current reduces the magnitude of the normal magnetic field line curvature near midvolume in a quasiaxisymmetric stellarator. This improves the ideal ballooning stability conditions. Near the edge of the plasma, a hollow toroidal current profile deteriorates the ballooning stability properties by enhancing the normal curvature, while a parabolic current profile improves this stability because the normal curvature decreases. The magnetic field line bending term can also impact the stability properties when the normal curvatures are comparable. The local magnetic shear and the geodesic curvature vanish where the normal curvature is most destabilizing. The quasiperiodic ballooning curvature contribution, unlike the normal curvature, fails to constitute a useful indicator about the localization of ballooning structures.