Finite-β Single-Stage Optimization for Commercially Relevant Stellarator Designs

Stellarator design involves weighing many different and often competing objectives to find an optimal configuration. Desirable plasma physics performance including good particle confinement, low turbulent transport, and stable dynamics must be balanced with engineering constraints such as coil feasibility and an accessible maintenance scheme. The traditional "two-stage" approach to stellarator optimization finds a promising plasma geometry first and then designs coils to reproduce it afterwards, but this can result in sub-optimal free-boundary equilibrium solutions overall. We present advances in the "single-stage" approach implemented in the DESC optimization suite, where the plasma and coil geometries are optimized simultaneously. This is shown to produce accurate free-boundary equilibria using less high temperature superconducting tape, while preserving desirable plasma physics properties. Recent developments have made it computationally tractable to include high fidelity models within this optimization loop, such as linear MHD stability and finite-build coils. We show examples of the planar coil stellarators that were optimized with this approach to achieve commercially relevant qualities, including a divertor solution and engineering feasible coils.