  It was recently shown in [Wechsung et. al., Proc. Natl. Acad. Sci. USA, 2022]
that there exist electromagnetic coils that generate magnetic fields which are
excellent approximations to quasi-symmetric fields and have very good particle
confinement properties. Using a Gaussian process based model for coil
perturbations, we investigate the impact of manufacturing errors on the
performance of these coils. We show that even fairly small errors result in
noticeable performance degradation. While stochastic optimization yields minor
improvements, it is not able to mitigate these errors significantly. As an
alternative to stochastic optimization, we then formulate a new optimization
problem for computing optimal adjustments of the coil positions and currents
without changing the shapes of the coil. These a-posteriori adjustments are
able to reduce the impact of coil errors by an order of magnitude, providing a
new perspective for dealing with manufacturing tolerances in stellarator
design.
