Extending the near-axis description of quasi-isodynamic stellarators*

Quasi-isodynamic (QI) stellarators represent one of the leading concepts of optimised stellarator design. The meticulous optimisation demanded by such fields shows a particular sensitivity to the initial search point, as compared with other symmetry classes, especially when typical approaches focus on finding local optima.

A simpler construction of fields that can provide such initial search points is therefore highly desirable. The asymptotic description of fields near their centre, known as the near-axis description, serves this job.  While these methods have achieved significant maturity in quasisymmetric stellarators, another class of optimised fields, the complexity of QI fields has so far limited the available tools within this framework. 

In our work, we advance the near-axis description to the second order in the distance from the magnetic axis, which provides improved control and understanding on the QI fields. We present the technical advancements necessary to address these intricate fields and show applications of this approach to exploring and mapping the space of QI stellarators.