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

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.

Figures (5)

• Figure 1: $B$ on the flux surface of a prototypical QA-pwO field. For $B_\mathrm{mid}=B_\mathrm{max}$, the field becomes QA.
• Figure 2: $B$ on the flux surface of a smooth nearly QA field with a pwO perturbation that reduces the bootstrap current.
• Figure 3: Radial ($D_{11}$) and parallel ($D_{31}$) neoclassical transport coefficients as a function of the collisionality.
• Figure 4: Change in the rotational transform due to the bootstrap current in a reactor scenario for a model QA-pwO field.
• Figure 5: Dependence of the radial charge current on the radial electric field $E_r$ for $n_{e,0}=4\times 10^{20}\,$m$^{-3}$ and $T_{e,0}=12\,$keV.