Island divertor shape optimization for stellarator design

The W7-AS and W7-X experiments have successfully demonstrated the island divertor as a solution for heat and particle exhaust for stellarators [1]. However, divertor design is left out of the stellarator optimization process as validation with edge-boundary codes like EMC3-EIRENE is computationally expensive. To remedy this, we add a computationally lightweight island divertor optimization suite to the open-source stellarator optimization code SIMSOPT [2] by coupling it to FLARE, a field line analysis code that quickly computes proxies for particle and heat loads with a field line diffusion model [3]. The shape of the first wall of an island divertor is optimized given a prescribed magnetic geometry. Metrics considered in the optimization are the field line pitch, heat flux magnitude, and toroidal spread. We present optimized divertor geometries where we minimize peak heat flux by targeting specific field line strike angles in an optimized quasi-isodynamic stellarator equilibrium. This work opens up avenues for including divertor design as a part of the full-device optimization process.

[1] Feng, Y. & W7-X-team. (2022). Plasma Physics and Controlled Fusion, 64(12), 125012.

[2] Landreman, M., et al. (2021). Journal of Open Source Software, 6(65), 3525.

[3] Frerichs, H. (2024). Nuclear Fusion, 64(10), 106034.