Optimizing particle transport for enhanced confinement in quasi-isodynamic stellarators.

Despite substantial advances in mitigating turbulent heat losses, including those achieved in the stable quasi-isodynamic design family [J. Plasma Phys. 89, 905890504 (2023)0022-377810.1017/S002237782300065X; PRX Energy 3, 023010 (2024)2768-560810.1103/PRXEnergy.3.023010], particle confinement remains a principal performance bottleneck in modern quasi-isodynamic stellarators, a challenge not fully addressed in previous optimization efforts. Using gyrokinetic simulations within the gene-Tango framework, we identify suppressed inward thermodiffusion, caused by unfavorable magnetic geometry, as the primary cause. To overcome this limitation, we design a new configuration with a reduced mirror ratio, which enhances the contribution of passing electrons to the inward particle flux. This facilitates the formation of strongly peaked density profiles, suppresses turbulence, and leads to a substantial improvement in confinement. Our optimized configuration achieves nearly a twofold increase in energy confinement compared to Stellaris, highlighting the crucial role of optimizing particle transport in next-generation stellarator designs.