Sensitivity of a low-shear heliotron configuration to localised ferrite-steel perturbations
The influence of ferritic steel on low-shear stellarator/heliotron magnetic configurations is investigated for the Heliotron J device using a point dipole magnetisation model. By numerically evaluating ferritic steel plates assumed at several locations inside the Heliotron J vacuum vessel, the changes in the rotational transform and magnetic island width are shown to be sensitive to the installation location. This location sensitivity arises from the toroidal variation of poloidal mode coupling between the background nonaxisymmetric field and ferritic-steel perturbation, rather than being determined solely by the perturbation amplitude. The resulting mode coupling can enhance the resonant vacuum magnetic perturbation at specific locations. Ferritic steel plates placed on the outer side of a straight section produce the most significant changes in the magnetic topology and exhibit the highest sensitivity to violations of the $M=4$ toroidal periodicity. Additionally, we show that appropriate arrangements of passive magnetic dipoles can reduce the effective helical ripple while preserving the vacuum magnetic well depth in Heliotron J, and can induce a stellarator-asymmetric boundary perturbation in low-field experiments.