Stellarator divertor design by optimising coils for surfaces with sharp corners

In stellarators, achieving effective divertor configurations is challenging due to the three-dimensional nature of the magnetic fields, which often leads to chaotic field lines and separatrices surrounded by a thick stochastic layer. This work presents a novel approach to directly optimise modular stellarator coils for a sharp X-point divertor topology akin to the Large Helical Device’s (LHD) helical divertor using a target plasma surface with sharp corners. By minimising the normal magnetic field component on this surface, we construct a clean separatrix with minimal chaos. Notably, this approach demonstrates the first LHD-like helical divertor design using optimised modular coils instead of helical coils. Separatrices are produced with significantly lower chaos than in LHD, demonstrating that a wide chaotic layer is not intrinsic to the helical divertor. Additional optimisation methods are implemented to improve engineering feasibility of the coils and reduce chaos, including weighted quadrature and manifold optimisation, a method which does not rely on normal field minimisation. The results outline several new strategies for divertor design in stellarators, although it remains to achieve these edge divertor features at the same time as internal field qualities like quasisymmetry.