Introduction

Near-axis expansion

Near-axis expansion (NAE) can find quasi-symmetric (QS) stellarator equilibria without costly optimization. It directly constructs an equilibrium by solving a system of governing equations encoding the QS conditions, existence of nested flux surface and force balance by expanding them as a power series in effective minor radius from a known magnetic axis.

Compared to optimization, NAE: - Is fast and non-iterative. - Is reasonably accurate at order $n=2$. (Ref.6) - Has exact control over the parameters of the resulting equilibria. - Enforces QS as a constraint, rather than objective. - Has lower dimensionality, because QS is enforced.

This makes it uniquely suited for rapid exploration over the space of QS configurations.