Direct construction of low-aspect-ratio advanced stellarators via asymptotic expansion

Abstract Advanced stellarators are promising candidates for fusion reactors due to their intrinsic steady-state operation capability and excellent plasma confinement properties. Near-axis expansion (NAE) provides an efficient framework for directly constructing advanced stellarator configurations with quasisymmetry or quasi-isodynamicity. However, conventional low-order NAE is fundamentally restricted to large-aspect-ratio configurations, limiting its applicability to more compact designs. To overcome this limitation, we employ recursive near-surface expansion to extend the flux surface generated by NAE, enabling the design of significantly more compact stellarator configurations. The method is applied to design both quasisymmetric and quasi-isodynamic configurations. Compared with results obtained from the first-order NAE, the new method shows substantially improved agreement with configurations computed using three-dimensional equilibrium codes. In addition, the new approach reduces magnetic-axis deviations and enhances core plasma confinement. The proposed method can also use the higher-order NAE result as the initial surface to construct the configuration. This work demonstrates that the new asymptotic expansion framework can design compact advanced stellarators and offer a fresh perspective on stellarator configuration design.