Near-Axis Constrained Stellarator Equilibria with DESC

We present novel results of near-axis constrained equilibria in DESC, connecting near-axis expansion theory to global 3D ideal MHD. Near-axis expansion (NAE) theory can provide analytic insight into stellarator equilibria, but the accuracy of the theory degrades far from axis. Conventional approaches to connect global MHD codes to NAE theory evaluate the NAE at a finite radius and use that as the boundary condition, which uses the NAE information where it is least accurate. In DESC, boundary conditions constraining only the near-axis behavior to match near-axis theory have been implemented and shown to result in optimized configurations with near-axis properties matching the NAE and overall lower force balance. This allows DESC to use NAE theory where it is most reliable (near the axis) and turn to the global MHD equations to satisfy force balance far from the axis.

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