Sensitivity of quasisymmetry in flexible stellarator optimization

Quasisymmetry, or broadly omnigeneity, is a guiding principle of modern stellarator optimization with its favorable implication on neoclassical transport. However, its sensitivity to perturbations can vary significantly, as recent studies of flexible stellarator designs have shown [1]. Variations in plasma boundary configurations—such as QA, QH, or QI—using DESC and NEO codes reveal that quasisymmetric optimization does not always represent a local or robust minimum in transport. In addition, FORTEC-3D global particle simulations indicate that small additions of ambipolar electric fields or finite orbit widths can markedly alter predictions based solely on effective helical ripples. It will also be important to clarify the role of non-ideal MHD as initial M3D-C1 code applications show nontrivial island-chain formation when the toroidal beta increases. To incorporate these effects implied from high-fidelity modeling, a self-consistent perturbed equilibrium approach which was successfully applied in tokamaks [2] will be discussed as a potentially useful and efficient scheme. These efforts are part of Korea's emerging stellarator R&D program, which will also be introduced in this presentation. [1] T. G. Jeong et al., Plasma Phys. Control. Fusion (2025), To be published. [2] J.-K. Park et al., Phys. Rev. Lett. 126, 125001 (2021).