Geodesic torsion of the magnetic field line and other predictors of coil non-planarity in QI stellarators

Abstract

Stellarator magnetic fields are often realized with non-planar modular coils with their out-of-plane bending being one of the main engineering challenges. We ask whether the non planarity of the fialment can be predicted from the equilibrium alone. We look at a dataset of coils for stellarator equilibira and find that the geodesic torsion of the magnetic field line is a strong and robust predictor in a wide range of coil feasibility requirements. We compare the performance of the geodesic torsion against other equilibrium and surface geometry properties and find it to be a superior predictorv hardly replaceable by other quantities we considered in this dataset. The ranking it induces also reproduces the ordering of four published configurations --- precise QA, precise QI at nfp 2 and 3, and W7-X. Our conclusions suggest that geodesic torsion of the magnetic field line may be considered as a cheap and robust candidate for a stage-one non-planarity proxy.