A framework for discrete optimization of stellarator coils

TL;DR

This paper introduces a novel framework for designing stellarator coils that optimizes magnetic fields while respecting spatial constraints, using two methods: RCLS and GSCO, demonstrated through practical examples.

Contribution

It presents a new coil design framework enabling sparse, constrained stellarator coil solutions with two innovative optimization methods, RCLS and GSCO.

Findings

Both methods produce high-accuracy magnetic fields.

Solutions respect spatial constraints for easier assembly.

The framework allows flexible coil placement and design.

Abstract

Designing magnets for three-dimensional plasma confinement is a key task for advancing the stellarator as a fusion reactor concept. Stellarator magnets must produce an accurate field while leaving adequate room for other components and being reasonably simple to construct and assemble. In this paper, a framework for coil design and optimization is introduced that enables the attainment of sparse magnet solutions with arbitrary restrictions on where coils may be located. The solution space is formulated as a "wireframe" consisting of a mesh of interconnected wire segments enclosing the plasma. Two methods are developed for optimizing the current distribution on a wireframe: Regularized Constrained Least Squares (RCLS), which uses a linear least-squares approach to optimize the currents in each segment, and Greedy Stellarator Coil Optimization (GSCO), a fully discrete procedure in which loops of current are added to the mesh one by one to achieve the desired magnetic field on the plasma boundary. Examples are presented of solutions obtainable with each method, some of which achieve high field accuracy while obeying spatial constraints that permit easy assembly.