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Introduction of cross-field drifts in SOLPS-ITER [1] simulations of STEP [2, 3] with fully tracked Ar impurities was found to re-distribute the impurities in the simulation volume differently from the main ions, improving the representation of Ar in the plasma from the first drift simulations for STEP [4], where Ar was proxied as a fixed fraction of the main ion content, and complementing earlier drift-free simulations with evolving Ar [5—7]. The drift terms were activated in a connected double-null configuration with Ar seeded into the outer divertors for detached initial conditions.
The Ar content and the radiated power inside the separatrix were found to increase by approximately 50% and 30%, respectively, with the drifts active. At the separatrix, the Ar ion concentration increased from 3% to 4% of the electron density at the outer midplane, posing a risk for potential degradation of the upstream conditions from the point of view of core-edge compatibility. Strong accumulation of Ar by almost an order of magnitude was observed also in the inner lower divertor.
The activation of drifts resulted in an up-down asymmetry in the power entering the divertor volumes with the upper divertors receiving more power according to a 53:47 ratio. The direction of the asymmetry is opposite to that in [4] but follows the earlier experimental observations in DIII-D [8], MAST [9] and Alcator C-Mod [10], being favourable for the foreseen easier maintenance access into the upper divertor. Due to the notable increase in the power entering the outer upper divertor leg, the 2—5-eV temperature fronts were found to shift significantly closer to the divertor target, but at the target detached conditions were preserved by simultaneous increase in the Ar content and radiation in the divertor volume.
[1] S. Wiesen et al., Journal of Nuclear Materials 463 (2015) 480—484
[2] H. Wilson et al., in Commercialising Fusion Energy, IOP Publishing (2020) 8-1—8-18
[3] H. Meyer, 29th IAEA Fusion Energy Conference, London, UK, 16.—21.10.2023
[4] J. Karhunen et al., Nuclear Fusion 64 (2024) 096021
[5] R.T. Osawa et al., Nuclear Fusion 63 (2023) 076032
[6] R.T. Osawa et al., Nuclear Fusion 64 (2024) 106007
[7] S.L. Newton et al., Nuclear Fusion 65 (2025) 096026
[8] C.J. Lasnier et al., Nuclear Fusion 38 (1998) 1225
[9] G.F. Counsell et al., Plasma Physics and Controlled Fusion 44 (2002) B23—B37
[10] D. Brunner et al., Nuclear Fusion 58 (2018) 076010