Ideal plasma response to externally applied perturbations in sheared stellarators

The application of perturbations at the edge of the plasma can trigger an ideal plasma response with a large nonresonant m=1,n=1 component in a 10-field period Heliotron. The magnetic axis displacement associated with the ideal internal interchange structure in the stellarator is weaker than the helically kinked m/n=1/1 cores of tokamaks mainly because the plasma current provides a stronger source of free energy to drive low order modes than the pressure gradient. The interchange-like distortions are driven by the edge deformations' Fourier components that are immediate sidebands of the equilibrium field period number Nper, namely, (m,n=ℓNper±1), where ℓ is any integer. The magnitude of the plasma edge corrugations that cause an amplified ideal plasma response can be smaller than 1 mm. The core m=1,n=1 structure develops in the region of weak magnetic shear with rotational transform 1/3<ι<1/2 with peak values around ι∼1/2.