Single-stage stellarator coil optimisation for quasi-symmetry under uncertainty*

When designing a new stellarator, it is crucial to design coils that properly contain the plasma. Once designed, building these coils is extremely expensive and tight manufacturing tolerances only increase the cost further. The classical way of designing coils is a two-stage approach in which first a magnetic configuration with desirable physics properties is found, and then coils that aim at realizing this magnetic configuration are designed. Recently, a single-stage approach for vacuum fields was introduced that directly optimises coils for their physics properties. We combine this approach with a model for coil manufacturing errors to find coil designs that are robust with respect to small perturbations of the coils.

As opposed to existing approaches, the new method

1) is gradient based, enabling the use of faster converging optimisation algorithms;

2) separates the discretisation of the coils and the model for the perturbations; and

3) directly targets quasi symmetry of the magnetic field.

We show that the resulting stochastic optimisation problem is better conditioned than the original deterministic problem, and that the coil configurations obtained from the stochastic problem perform better as measured in terms of quasi symmetry and particle confinement.