Single-Stage Optimization for Reactor-Relevant Designs

A stellarator fusion reactor must be optimized for a variety of objectives to be commercially viable, including good particle confinement, low turbulent transport, stable dynamics, and feasible coil geometry. The traditional "two-stage" approach of optimizing a fixed-boundary equilibrium first and then optimizing the external coils afterwards is time-intensive and can lead to sub-optimal results. Simultaneously optimizing both the plasma boundary and the coil parameters in a "single-stage" approach has been shown to better satisfy all the design criteria for the resulting free-boundary solution. We present a new single-stage optimization algorithm that has been developed in the DESC optimization suite along with examples of reactor-relevant configurations. In contrast to previous single-stage approaches, this method works for finite plasma pressure and uses exact derivatives of the MHD equilibrium to improve speed and accuracy. Linear stability from TERPSICHORE is also efficiently included to provide a multi-objective optimization problem with realistic considerations for a commercial power plant. The example configurations presented are shown to have planar coil geometries that are consistent with Thea Energy's approach to simplified stellarator design.