Helicity of the magnetic axes of quasi-isodynamic stellarators

TL;DR

This paper investigates how the helicity of the magnetic axis affects the design of quasi-isodynamic stellarators, revealing new axis shapes compatible with near-axis methods that can achieve high-quality configurations without boundary optimization.

Contribution

It identifies a new class of magnetic axes with half-helicity, expanding the near-axis formalism to construct stellarators with multiple field periods and low effective ripple.

Findings

Half-helicity axes are compatible with near-axis formalism.

Constructed stellarators have up to 5 field periods and low effective ripple.

Configurations achieve similar rotational transform without boundary optimization.

Abstract

In this study, we explore the influence of the helicity of the magnetic axis-defined as the self-linking number of the curve-on the quality of quasi-isodynamic stellarator-symmetric configurations constructed using the near-axis expansion method (Camacho Mata et al. 2022; Plunk et al. 2019). A class of magnetic axes previously unexplored within this formalism is identified when analyzing the axis shape of the QIPC configuration (Subbotin et al. 2006): the case of half-helicity (per field period). We show these shapes are compatible with the near-axis formalism and how they can be used to construct near-axis stellarators with up-to 5 field-periods, $\epsilon_{eff} \approx$ 1.3%, and similar rotational transform as existing conventionally optimized designs, without the need of a plasma boundary optimization.