Direct construction of stellarator-symmetric quasi-isodynamic magnetic configurations

TL;DR

This paper develops a formalism for constructing stellarator-symmetric quasi-isodynamic magnetic configurations using near-axis expansion, demonstrating the importance of axis shape control for low-ripple, high-aspect-ratio designs.

Contribution

It introduces a method to construct axis shapes with favorable curvature and torsion, enabling the design of low-ripple, high-aspect-ratio QI stellarators with specific configurations.

Findings

Constructed a three-field-period configuration at aspect ratio 20

Achieved effective ripple below 1%

Showed high elongation is not necessary for QI stellarators

Abstract

We develop the formalism of the first order near-axis expansion of the MHD equilibrium equations described in Garren & Boozer (1991), Plunk et al. (2019) and Plunk et al. (2021), for the case of a quasi-isodynamic, N-field period, stellarator symmetric, single-well magnetic field equilibrium. The importance of the magnetic axis shape is investigated, and we conclude that control of the curvature and torsion is crucial to obtain omnigenous configurations with finite aspect ratio and low effective ripple, especially for a higher number of field periods. For this reason a method is derived to construct classes of axis shapes with favourable curvature and torsion. Solutions are presented, including a three-field-period configuration constructed at an aspect ratio of A=20, with a maximum elongation of e=3.2 and an effective ripple under 1%, which demonstrates that high elongation is not a necessary feature of QI stellarators.