Figures of Merit for Plasma-Coil Separation of Stellarators

Stellarator reactor size is limited by the separation between the outer boundary of the plasma and the external coils that produce a confining magnetic field. This is because at least 1 meter of additional components, such as breeding blankets and shielding, are required in between the plasma and the coils. Understanding the nature of this plasma-coil separation may unlock a deeper insight into why some stellarator configurations prove difficult to create coil shapes for. Here, we explore the hypothesis that largest distance between the coils and the plasma is bounded around the shortest scale length of the magnetic field, as expressed by the ∇ B tensor. We explore the relationship between the plasma-coil separation and the ∇ B scale length using the regularized current potential method REGCOIL. Compared to other coil optimization tools, REGCOIL’s sheet current approach means the objective function guiding the coil shapes can be minimized without local minima. Further development of the ∇ B scale length as a heuristic for the plasma-coil separation may allow for this separation to be maximized within plasma equilibrium design to create configurations with larger plasma-coil separations.