Speaker
Description
We present a novel edge-SOL coupling scheme linking SOLPS-ITER SOL transport simulations with EPED pedestal predictions and validation efforts through MAST-U dedicated experiments. This scheme employs a flux-gradient-driven density pedestal model [1], taking as inputs the separatrix conditions from a converged SOLPS solution and predicting the density pedestal top through empirically informed transport settings. This top density is used as an input to the EPED model as in [2], along with self-consistent shaping from the SOLPS equilibrium, to predict pedestal dimensions limited by ideal P-B and KBM stability constraints.
SOLPS-EPED validation efforts were conducted on MAST-U through experiments manipulating the role of neutral sourcing vs transport on the pedestal parameters through 1) modifying divertor closure and 2) adding divertor gas puffing. Reducing the divertor closure leads to considerably higher pedestal density at roughly constant pedestal pressure, which is reproduced by the SOLPS-EPED modeling. However, the strong localized HFS gas fueling required for reliable type I ELMy H-mode operation poses a challenge in characterizing the poloidal source distribution with divertor closure changes due to small variation in the inner gap in close proximity to the HFS fueling valve.
Conversely, additional divertor puffing at fixed divertor geometry and HFS fueling is shown to have a negligible effect on the density pedestal due to inefficient fueling in the conventional divertor configuration, but a significant effect on degrading the temperature pedestal. The density profile does not incur any obvious outward profile shift that could explain a reduction in P-B stability as in similar JET experiments [3]. Based on the EPED ideal-MHD and KBM constraints, the SOLPS-EPED coupled model is unable to capture this pedestal degradation, which may arise due to additional transport mechanisms, or the local HFS fueling interaction. Further experiments with varied plasma current will facilitate additional validation opportunities by moderating the natural edge density as well as isolating the effect of the HFS localized gas puff.
[1] S. Saarelma et al 2024 Nucl. Fusion 64 076025
[2] R.S. Wilcox et al Nucl. Fusion [In Review]
[3] L. Frassinetti et al 2021 Nucl. Fusion 61 126054
*Work supported by US DOE Contracts DE-SC0014664, DE-AC05-00OR2272, DE-SC0023289 and EPSRC, UK Energy Programme grant EP/W006839/1. For the MAST-U Team, see the author list of J. Harrison et. al. 2024 Nucl. Fusion 64 112017.