A geometric approach to constructing quasi-isodynamic fields

The near-axis theory for quasi-isodynamic stellarator equilibria is reformulated in terms of geometric inputs to allow greater control of the ‘direct construction’ of quasi-isodynamic configurations and to facilitate understanding of the space of such equilibria. This includes a method to construct suitable magnetic axis curves by solving Frenet–Serret equations and an approach to controlling magnetic surface shaping at first order (plasma elongation), which previously has required careful parameter selection or additional optimisation steps. The approach is suitable for studying different classes of quasi-isodynamic stellarators including different axis ‘helicities’ and topologies (e.g. knotted solutions), and as the basis for future systematic surveys using higher order near-axis theory. As an example application, we explore a family of configurations with per-field-period axis helicity equal to one half, demonstrating an approximate scaling symmetry relating different field period numbers.