Abstract Details
Abstracts
Author: Dominic C. Power
Requested Type: Consider for Invited
Submitted: 2025-02-20 19:18:12
Co-authors: M.V. Umansky, V.A. Soukhanovskii
Contact Info:
Lawrence Livermore National Laboratory
7000 East Avenue
Livermore, CA 94550
United States
Abstract Text:
In a divertor tokamak, a separatrix with an X-point separates the confined plasma from the open field lines that connect to the divertor plates. To reduce heat loads to the plates, the snowflake divertor (SFD) has been proposed [1]. The SFD is formed by bringing a secondary X-point into the vicinity of the first, resulting in four divertor legs instead of two. Near the X-points, SFD features a large region with high poloidal beta β_p, and when the distance between the two X-points, d_xx, is comparable in size to the β_p≫1 region axisymmetric convective poloidal motion can occur (the “churning mode” (CM)), which can spread heat exhaust across divertor legs [2]. Extending an earlier model [3], simulations of CM are carried out using a 2D reduced-MHD model with field-aligned thermal conduction in MAST-U conditions. The simulations confirm predictions [1] that: a) CM turbulence is activated when d_xx is comparable to the scrape-off layer width λ_q projected to the X-points, d_SOL; b) the CM turbulence enhances plasma transport across the X-point region, consistent with SFD experiments [4]. The simulations show that the CM transport across the X-points scales with the local pressure gradient p_null/d_SOL, in agreement with the diffusive paradigm. A new finding is that with the CM physics steady-state solutions exist that are topologically distinct from those without the CM physics. When the initial equilibrium features a secondary X-point in the high-field side SOL at a nearby flux surface, the CM physics can force a mirroring of this configuration, with the secondary X-point moving to the low field side SOL. In SFD experiments this may be a source of MHD reconstruction error, and would result in a secondary strike point receiving more power than predicted using MHD equilibria from Grad-Shafranov codes.
[1] Ryutov et al, PPCF 54 (2012)
[2] Ryutov et al, Phys. Scr. 89 (2014)
[3] Umansky & Ryutov, PoP 23 (2016)
[4] Soukhanovskii et al, Nucl. Fusion 58 (2018)
Characterization: 2.0
Comments: