Robust and Simple Coils for a Low-Error Quasi-Helical Stellarator

Recently, new stellarator designs have been found with record-low quasisymmetry error. Specifically, the quasihelical magnetic geometry described in (Landreman & Paul, Phys. Rev. Lett. 128 (2022) 035001) has ε_eff^3/2 ≤ 10^-6. For this configuration to be of practical interest, magnetic coils must be found which can recreate the boundary while satisfying simplicity criteria. In this work, we use the SIMSOPT framework to optimize a set of coils to accurately recreate the targeted magnetic boundary using a least squares minimization method, while imposing constraints on length, coil to coil distance, coil to surface distance, maximum curvature, and mean squared curvature. We introduce a linking number penalty to the optimization to prevent interlocking coils, and analyze how perturbations of the coils affect their adherence to the targeted magnetic boundary. We evaluate the quality of the coils through fast particle tracing using the guiding-center code SIMPLE.