Finite-β Magnetic Field Calculations for Non-Resonant Stellarator Divertor Optimization

Commercially relevant magnetically confined plasmas will require divertors to exhaust particles and heat while preventing impurities in the main plasma. In stellarators, divertor design presents unique challenges due to the absence of axisymmetry, requiring careful analysis of proposed stellarator equilibria to ensure the magnetic field geometry can facilitate a realistic divertor. We present a comparative analysis of numerical methods for computing the magnetic field in the vacuum region outside the plasma volume. The assumption of nested flux surfaces is avoided and therefore allows for the presence of divertor-relevant magnetic field topologies such as poloidal field nulls (X-points), magnetic islands, and chaos. We assess these numerical methods across a variety of metrics including adherence to the divergence-free condition, accuracy benchmarks against other codes, and computational performance. The vacuum region magnetic field calculation is implemented in the DESC optimization suite and used in novel objective functions to demonstrate stellarator designs optimized for both desirable plasma physics properties and an effective divertor solution.