Near-axis measures of quasi-isodynamic configurations

TL;DR

This paper develops measures and techniques to characterize and construct quasi-isodynamic stellarators using near-axis models, enabling efficient optimization without global equilibrium computations.

Contribution

It introduces new measures and methods for analyzing and designing quasi-isodynamic stellarators within the near-axis framework, facilitating practical optimization.

Findings

Effective measures for omnigeneity and ripple wells

Tools for assessing MHD stability and sensitivity to pressure

Second-order expansions are crucial for accurate modeling

Abstract

We present a number of measures and techniques to characterise and effectively construct quasi-isodynamic stellarators within the near-axis framework, without the need to resort to the computation of global equilibria. These include measures of the reliability of the model (including aspect-ratio limits and the appearance of ripple wells), quantification of omnigeneity through $\epsilon_\mathrm{eff}$, measure and construction of MHD stabilised fields, and the sensitivity of the field to the pressure gradient. The paper presents, discusses and gives examples of all of these, for which expansions to second order are crucial. This opens the door to the exploration of how key underlying choices of the field design govern the interaction of desired properties (``trade-offs''), and provides a practical toolkit to perform efficient optimisation directly within the space of near-axis QI configurations.