Description
The SOLPS-ITER code is used to assess the performance of a tightly-baffled, long-legged divertor (TBLLD) under conditions of increased detachment challenge. The TBLLD concept may offer a heat exhaust solution for future reactors, and is currently considered for the SPARC and ARC devices [1]. The Tokamak à Configuration Variable (TCV) is implementing a proof-of-principle TBLLD as its next divertor upgrade, with SOLPS-ITER simulations predicting an order-of-magnitude increase in power exhaust capability, compared to the unbaffled TCV divertor [2].
In this work, the TCV TBLLD configuration – with graphite tiles and tight baffling of the outer divertor– is extrapolated towards more reactor-like SOL conditions. Divertor performance is evaluated across a range of the Lengyel metric $(P_{sep} B_T/R)/n_{sep}^2$ – characterizing the detachment challenge – up to values overlapping with conditions expected in SPARC. This is achieved by scaling up the separatrix exhaust power. The impact of introducing a tungsten wall is assessed by isolating the contribution of carbon impurities, the wall reflection model and pumping localization. Fast particle and energy reflection coefficients, which are material-specific, are found to have a strong effect on the divertor plasma properties. Compared to the carbon case, the larger fraction of particles fast-reflected off a W surface decreases the cooling from plasma-neutral interactions, and degrades divertor neutral confinement. Moreover, large asymmetries in the power sharing between the unbaffled inner and tightly baffled outer divertor arise when the outer divertor detaches. This effect is enhanced at larger exhaust powers. Thermoelectric currents, driven by temperature asymmetries between the inner and outer target, are identified as a driver of this power redistribution. Such asymmetries are found to be more pronounced in a W TBLLD compared to C. These results underscore that a viable exhaust solution must address both targets, as baffling one divertor leg may degrade the performance of the other. Furthermore, they point out that wall material affects exhaust performance not only through impurities: intrinsic surface properties strongly affect neutral dynamics and therefore divertor properties.
References
[1] M. R. K. Wigram et al., Nucl. Fusion. 59, 106052 (2019).
[2] G. Sun et al., Nucl. Fusion. 63, 096011 (2023).