Stellarator coil optimization supporting multiple magnetic configurations

TL;DR

This paper introduces a coil design method for stellarators that enhances flexibility across multiple magnetic configurations while maintaining quasisymmetry and flux surface volume, using an optimization approach that balances these factors.

Contribution

The paper presents a novel coil optimization technique that explicitly incorporates configuration flexibility, improving upon previous methods by enhancing quasisymmetry and flux surface volume.

Findings

Achieved flexible stellarator configurations with multiple rotational transforms.

Improved quasisymmetry and flux surface volume through optimized coil placement.

Demonstrated a simple coil set can meet multiple magnetic configuration targets.

Abstract

We present a technique that can be used to design stellarators with a high degree of experimental flexibility. For our purposes, flexibility is defined by the range of values the rotational transform can take on the magnetic axis of the vacuum field while maintaining satisfactory quasisymmetry. We show that accounting for configuration flexibility during the modular coil design improves flexibility beyond that attained by previous methods. Careful placement of planar control coils and the incorporation of an integrability objective enhance the quasisymmetry and nested flux surface volume of each configuration. We show that it is possible to achieve flexibility, quasisymmetry, and nested flux surface volume to reasonable degrees with a relatively simple coil set through an NCSX-like example. This example coil design is optimized to achieve three rotational transform targets and nested flux surface volumes in each magnetic configuration larger than the NCSX design plasma volume. Our work suggests that there is a tradeoff between flexibility, quasisymmetry, and volume of nested flux surfaces.