Description
W7-X has shown that magnetic optimization can drastically reduce neoclassical transport; however, turbulent transport now dominates and limits overall confinement. This paradigm shift emphasizes the need to consider turbulent transport in stellarator optimization alongside neoclassical effects. Modern quasi-isodynamic (QI) stellarator designs, such as Stellaris [1], are developed to minimize turbulent heat flux while preserving good neoclassical and energetic-particle confinement. Despite this, particle transport in these advanced QI configurations remains insufficiently explored. Through high-fidelity gyrokinetic simulations using the GENE–Tango framework, we find that inward thermodiffusion is strongly suppressed in these configurations due to unfavorable magnetic geometry. This reduces the fraction of passing electrons contributing to inward particle flux, leading to suboptimal particle confinement [2].
To address this limitation, we propose a density-optimized configuration featuring a lower magnetic mirror ratio, which increases the fraction of passing electrons driving inward particle transport. This adjustment results in strongly peaked density profiles that suppress ion temperature gradient (ITG) turbulence and lower heat flux. The optimized design achieves nearly twice the calculated energy confinement time compared to non-optimized configurations, underscoring the importance of controlling particle transport in combination with heat flux.
These findings indicate that enhancing inward thermodiffusion and actively shaping density profiles are critical for robust confinement in next-generation stellarators. They also suggest that future stellarator optimization should treat particle transport on the same level of importance as turbulent heat flux.
References
[1] J. Lion et al., Fusion Engineering and Design 214, 114868 (2025).
[2] A. B. Navarro et al., arXiv preprint arXiv:2507.21003 (2025).