Control of the bootstrap current in approximately quasi-axisymmetric magnetic fields

TL;DR

This paper proposes a new approach to quasi-axisymmetric stellarators that enables high bootstrap current control and compatibility with divertor concepts, combining tokamak-like confinement with coil simplicity.

Contribution

It introduces approximately quasi-axisymmetric fields with omnigenous perturbations as a novel strategy for stellarator design, addressing bootstrap current and divertor challenges.

Findings

Proposes a new stellarator configuration with controlled bootstrap current.

Demonstrates compatibility with island divertor concepts.

Discusses implications for high bootstrap current in tokamaks.

Abstract

Quasi-axisymmetric stellarators are the stellarator analogue of the axisymmetric tokamak, retaining many of its favorable confinement properties, its compacity and its relative coil simplicity, while avoiding its principal limitation, the need for an inductively driven plasma current. Despite these attractive physics properties, the development of quasi-axisymmetric configurations has been severely constrained by the absence of an experimentally validated divertor concept compatible with their large bootstrap current. In this Letter, approximately quasi-axisymmetric fields, complemented with piecewise omnigenous perturbations, are proposed as the basis for a new strategy towards a stellarator reactor that simultaneously achieves simple coil geometries, tokamak-like confinement properties and, through tailoring of the bootstrap current, compatibility with an island divertor. Implications for attaining a high bootstrap current fraction in tokamak devices are also discussed.