# Curl-free magnetic fields for stellarator optimization

## TL;DR

This paper introduces a new method for defining curl-free magnetic fields in stellarator design, enabling more efficient optimization and potentially improving plasma confinement without relying on plasma self-confinement.

## Contribution

A novel and efficient approach to defining curl-free magnetic fields for stellarator optimization, facilitating better initial states and improved plasma confinement strategies.

## Key findings

- Codes can follow optimized curl-free states to full-pressure equilibrium.
- Designs with rapid transport in the core and low transport in the surrounding annulus are beneficial.
- Stellarators uniquely do not rely on plasma for confinement, enabling reliable computational design.

## Abstract

This paper describes a new and efficient method of defining an annular region of a curl-free magnetic field with specific physics and coil properties that can be used in stellarator design. Three statements define the importance: (1) Codes can follow an optimized curl-free initial state to a final full-pressure equilibrium. The large size of the optimization space of stellarators, approximately fifty externally-produced distributions of magnetic field, makes success in finding a global optimum largely determined by the starting point. (2) The design of a stellarator is actually improved when the central region of the plasma has rapid transport with the confinement provided by a surrounding annulus of magnetic surfaces with low transport. (3) The stellarator is unique among all fusion concepts, inertial as well as magnetic, in not using the plasma itself to provide an essential part of its confinement concept. This permits reliable computational design, which opens a path to faster, cheaper, and more certain achievement of fusion energy.