Quasi-symmetry is a guiding principle to modern stellarator optimization for improved plasma confinement. However, the robustness of optimized configurations, which can be crucial for maintaining performance under diverse engineering constraints and practical limitations, has received relatively little attention. Here we present various case studies on this robustness, by investigating variations in neoclassical transport when plasma configurations are smoothly altered across distinct optimized targets. These targets, optimized from different families—quasi-axisymmetric, quasi-helical, and quasi-isodynamic—are approximately matched in major radius as part of a flexible stellarator design. Our study shows that an optimized target does not always represent a local minimum in transport and that the robustness of a local minimum when present can vary significantly. Furthermore, there are configurations which belong to no established families but have transport levels as low as those of optimized targets. These results highlight the importance of conducting extended searches with key parametric variations around optimized configurations, to ensure its robustness as well as flexibility if desired.
This paper investigates the robustness of optimized stellarator configurations, which is crucial for maintaining performance under practical constraints. The study shows that an optimized target does not always represent a local minimum in transport, and the robustness of a local minimum can vary significantly. Additionally, there are configurations that belong to no established families but have transport levels as low as optimized targets. These findings highlight the importance of extended parametric searches around optimized configurations to ensure robustness and flexibility.