Stellarator divertor design by optimizing coils for surfaces with sharp corners

TL;DR

This paper introduces a novel coil optimization method for stellarator divertors with sharp corners, achieving cleaner separatrices and lower chaos than previous designs, and demonstrating a new approach to divertor configuration in stellarators.

Contribution

It presents the first LHD-like helical divertor design using optimized modular coils, advancing stellarator divertor engineering with new optimization strategies.

Findings

Achieved low-chaos separatrices with optimized coils.

Demonstrated a new coil optimization approach for sharp-corner divertors.

Reduced chaos compared to traditional LHD designs.

Abstract

In stellarators, achieving effective divertor configurations is challenging due to the three-dimensional nature of the magnetic fields, which often leads to chaotic field lines and fuzzy separatrices. This work presents a novel approach to directly optimize modular stellarator coils for a sharp X-point divertor topology akin to the Large Helical Device's (LHD) helical divertor using a target plasma surface with sharp corners. By minimizing the normal magnetic field component on this surface, we target a clean separatrix with minimal chaos. Notably, this approach demonstrates the first LHD-like helical divertor design using optimized modular coils instead of helical coils. Separatrices are produced with significantly lower chaos than in LHD, demonstrating that a wide chaotic layer is not intrinsic to the helical divertor. Additional optimization methods are implemented to improve engineering feasibility of the coils and reduce chaos, including weighted quadrature and manifold optimization, a method which does not rely on normal field minimization. The results outline several new strategies for divertor design in stellarators, though it remains to achieve these edge divertor features at the same time as internal field qualities like quasisymmetry.