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Description
When using a fluid model to simulate the plasma in the Scrape-Off Layer, boundary conditions are imposed at the entrance of the boundary layer next to the solid targets, where the model assumptions fail. This boundary layer reflects most electrons away from the target. With finite ion temperature, the largest length scale in the boundary layer is the collisional mean free path (along the magnetic field), associated with the collisional layer [1]. In the cold-ion limit, the largest length scale is instead the ion sound gyroradius, associated with the magnetic presheath [2]. Here, the ion polarisation drift grows significantly as the electric field strongly increases towards the wall, until the ion flow is bent across the magnetic field towards the target.
In the absence of gradients tangential to the target, the Bohm-Chodura condition requires that cold ions enter the magnetic presheath flowing at the sound speed parallel to the magnetic field [2,3]. However, due to the small angle between the magnetic field and the target, the small transport in the direction normal to the target caused by tangential gradients perpendicular to the magnetic field competes with the small projection of the ion parallel streaming [4]. This effect is included in some fluid codes via perturbative corrections to the sonic parallel flow [5]. Here, we explore the cold-ion flow boundary condition when cross-field transport causes large corrections, as is often the case.
[1] M. Abazorius, University of Oxford, PhD thesis (2025).
[2] R. Chodura, Phys. Fluids 25(9), 1628-1633 (1982).
[3] K.-U. Riemann, Physics of plasmas 1(3), 552-558 (1994).
[4] P. C. Stangeby, A. V. Chankin, Physics of Plasmas 2(3), 707-715 (1995).
[5] J. Loizu, P. Ricci, F. D. Halpern, S. Jolliet, Physics of plasmas 19, 12307 (2012).