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The divertor problem is one of the biggest ongoing challenges in the fusion energy sector. The magnetic geometry and topology of the divertor region in tokamaks significantly influences plasma edge dynamics as well as heat and particle exhaust.[1] Advanced divertor configurations, like the snowflake (SF) divertor, offer advantages in mitigating heat loads and enhancing plasma performance.[2] However, simulating these geometries can be extremely challenging due to the challenging meshing constraints and the complex physics of the edge region.
BOUT++ is a widely used open-source framework for simulating plasmas at the edge regions. [3] However, its capability to handle complex divertor geometries and topologies was limited by the previous mesh, which was unsuitable for non-standard divertor geometries. To address this, we have upgraded BOUT++'s mesh, thus giving it the ability to simulate complex divertor configurations, including the snowflake divertor. The upgraded mesh allows users to define complex divertor geometries (up to two X-Points) with flexibility and precision. We have also expanded INGRID[4] (a meshing code which can already handle complex divertor topologies) to produce BOUT++ grid files, which gives us a new tool that can already create grids for most SF geometries. Hermes-3 is one of the leading plasma edge codes; it is a physics model implemented on the BOUT++ framework. It has the ability to create 1, 2, and 3D simulations, making it the ideal candidate for understanding the SF. Ideal SFs, however, are hard to maintain experimentally, and they can easily evolve into another category of SF (SF+, and SF-) depending on where the secondary X-Point is. So we have started a study on the sensitivity of heat and particle fluxes at the divertor target to the X-point separation; this distinguishes the topology from ideal, SF+, and SF- configurations by implementing the upgraded BOUT++ mesh into Hermes-3.
The expanded capability to simulate complex divertor geometries within BOUT++ and Hermes-3 opens new ways of investigating advanced divertor concepts and optimizing divertor design for future fusion reactors. This development provides a valuable resource for theoretical and computational exploration of plasma behavior in complex divertor configurations. Understanding the behaviour and advantages of the advanced divertor geometries will be crucial for any fusion powerplant in the future.
Bibliography:
[1] A. Loarte, “Effects of divertor geometry on tokamak plasmas,” Plasma Phys. Control. Fusion, vol. 43, no. 6, p. R183, Jun. 2001, doi: 10.1088/0741-3335/43/6/201.
[2] D. D. Ryutov and V. A. Soukhanovskii, “The snowflake divertor,” Phys. Plasmas, vol. 22, no. 11, p. 110901, Nov. 2015, doi: 10.1063/1.4935115.
[3] B. Dudson et al., BOUT++. (Oct. 10, 2025). Zenodo. doi: 10.5281/zenodo.17313945.
[4] B. Garcia, M. Umansky, J. Watkins, J. Guterl, and O. Izacard, INGRID: an interactive grid generator for 2D edge plasma modeling. 2021. doi: 10.48550/arXiv.2102.07040.