Stellarators close to quasisymmetry

TL;DR

This paper examines the degree of quasisymmetry needed in stellarators to enable high rotation speeds, linking magnetic field deviations to rotation and turbulence effects.

Contribution

It establishes a quantitative criterion for how close a stellarator must be to quasisymmetry to achieve high rotation velocities.

Findings

Rotation speed depends on the deviation from quasisymmetry.

A threshold for deviation, b1 < b5^{1/2}, is identified.

Turbulent transport influences the rotation profile.

Abstract

Rotation is favorable for confinement, but a stellarator can rotate at high speeds if and only if it is sufficiently close to quasisymmetry. This article investigates how close it needs to be. For a magnetic field $\mathbf{B} = \mathbf{B}_0 + \alpha \mathbf{B}_1$, where $\mathbf{B}_0$ is quasisymmetric, $\alpha\mathbf{B}_1$ is a deviation from quasisymmetry, and $\alpha\ll 1$, the stellarator can rotate at high velocities if $\alpha < \epsilon^{1/2}$, with $\epsilon$ the ion Larmor radius over the characteristic variation length of $\mathbf{B}_0$. The cases in which this result may break down are discussed. If the stellarator is sufficiently quasisymmetric in the above sense, the rotation profile, and equivalently, the long-wavelength radial electric field, are not set neoclassically; instead, they can be affected by turbulent transport. Their computation requires the $O(\epsilon^2)$ pieces of both the turbulent and the long-wavelength components of the distribution function. This article contains the first step towards a formulation to calculate the rotation profile by providing the equations determining the long-wavelength components of the $O(\epsilon^2)$ pieces.