Divertor topology and vacuum vessel design for stellarators

Abstract

We present stellarator optimization algorithms for designing the edge magnetic structure in vacuum fields, together with the vacuum vessel. First, we introduce a numerical method that robustly computes periodic-orbit fixed points of any type (elliptic, hyperbolic, or parabolic), which could form the basis of a divertor. To couple divertor and vacuum vessel design, we introduce parametric families of vacuum vessels for which point-to-vessel distances, and their derivatives, can be computed efficiently. The resulting algorithms use signed distance functions to enforce coil-vessel clearance while allowing coils to be placed on or off the vessel. Using these methods, we jointly optimize modular coils and the vacuum vessel to realize a wide range of magnetic topologies for diverting exhaust, including standard X-point divertors, and single- and double-null configurations. For the first time, we show that precise snowflake divertors can be achieved in stellarators. Using this framework, we generate a number of quasi-axisymmetric stellarator designs with compatible vacuum vessels and diverse divertor architectures, which we consider to be candidates for a next-generation STAR Lite prototype.