Sensitivity of a low-shear heliotron configuration to localised ferrite perturbations

Abstract

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 sensitivity arises from the coupling between the background nonaxisymmetric field and ferrite perturbation, rather than being determined solely by the perturbation amplitude. 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 stellaratorasymmetric boundary perturbation in low-field experiments near the ferrite saturation threshold.