Efficiency of current-potential patches for toroidal ripple reduction

Determine whether placing current-potential patches on the outboard-side corner regions of each stellarator period—locations previously used to produce non-axisymmetric fields in the HSX stellarator—can efficiently reduce the toroidal ripple that arises when the number of poloidally encircling toroidal-field or modular coils is reduced.

Background

The paper proposes using current-potential patches (a discretized representation of the single-valued current potential via surface-normal dipoles) to study and realize coil configurations that increase access to the plasma chamber and potentially reduce the number of poloidally encircling toroidal-field or modular coils. An analytic solenoid model is developed to quantify ripple and show how adding single-valued current potential between encircling coils can cancel low-n Fourier components of the ripple, enabling larger coil spacing.

Todd Elder’s study on HSX showed that patches for non-axisymmetric field production are concentrated on the outboard side at the corners of periods, where ripple concerns are strongest. The authors highlight an unresolved question about whether patches in those same regions can also effectively mitigate toroidal ripple when the coil count is reduced, which would influence the placement strategy for toroidal-field or modular coils.