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

Abstract

Quasi-axisymmetric stellarators are the stellarator analogue of axisymmetric tokamaks, retaining many of their favorable confinement properties, their compactness and their relative coil simplicity, while avoiding their 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 potentially achieves, simultaneously, simple coil geometries, tokamak-like confinement 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.