Speaker
Description
Magnetic flux expansion in the scrape-off layer (SOL) is a critical mechanism for spreading exhaust heat loads across divertor target surfaces, and its accurate modelling is essential for predicting performance in present and next-generation devices. This phenomenon is typically modelled using fluid codes, which incorporate flux expansion geometrically through area factors in the governing equations [1,2]. However, fluid models often rely on assumptions such as those in the Braginskii closure [3], which break down in the plasma edge. To accurately capture the non-local and non-equilibrium behaviour displayed by the divertor plasma during detached conditions and transient events like edgelocalised modes (ELMs), kinetic treatment of the SOL is required [4,5,6].
A finite volume discretisation of the kinetic electron equation that includes magnetic flux expansion has been developed and implemented within the ReMKiT1D framework, a reduced 1D2V Vlasov-Fokker-Planck (VFP) code [7]. Unlike higher-dimensional models, where these effects emerge naturally from the coordinate geometry, the 1D reduction requires their explicit reconstruction at the level of the governing terms and their spatial discretisation. This approach shares the mathematical structure of the cosmic ray transport formulation of Bell et al. [8]. However, while that work embeds the method within a full multi-physics model, here the contribution of the kinetic flux expansion terms is isolated and validated numerically via velocity-space moment analysis of the electron distribution function.
To clearly expose the role of kinetic electrons, the model intentionally employs simplified treatments of ions and neutrals so that any observed effects can be attributed to non-Maxwellian electron dynamics. Within this controlled framework, systematic parameter scans over flux expansion factor and input power for both steady-state and transient scenarios reveal how kinetic electron effects modify SOL profiles and target heat flux under conditions relevant to detachment and ELM-driven transients. Beyond characterising these effects, this implementation enables future work on the development of reduced kinetic–fluid coupling strategies for transient SOL modelling.
References
[1] Dudson, B. D. et al. PPCF 61(6) (2019)
[2] Derks, G. L. et al. PPCF 64 (2022)
[3] Braginskii, S. I. Rev. Plasma Phys. 1, 205 (1965)
[4] Tskhakaya, D. et al. Contrib. Plasma Phys., 48(1–3), 89–93 (2008)
[5] Chankin, A. V. et al. PPCF 60 (2018)
[6] Mijin, S. et al. PPCF 62(9) (2020)
[7] Mijin, S. et al. Comput. Phys. Commun., 300 (2024)
[8] Bell, A. R. et al. MNRAS 539, 1236–1247 (2025)
This work has been part-funded by the EPSRC Energy Programme [grant number EP/W006839/1] and used the ARCHER2 UK National Supercomputing Service as well as resources provided by the Cambridge Service for Data Driven Discovery (CSD3) operated by the University of Cambridge Research Computing Service.