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Description
The new ITER baseline in which tungsten (W) replaces beryllium as the chosen first wall (FW) material reinforces the need for plasma backgrounds for the analysis of W sources and transport. Crucial in this context are the FW particle/heat fluxes, which depend on the far scrape-off layer (SOL) radial transport profile. This is still very much uncertain, but there is strong evidence from current devices that broad SOL density profiles might be expected under ITER burning plasma conditions with a detached divertor. To explore this far-SOL plasma self-consistently requires numerical simulations on numerical grids extending to the FW. This is performed here with the SOLEDGE3X plasma boundary code, with standard burning plasma assumptions: 100 MW of power injected into the inner grid boundary, using neon (Ne) impurity seeding for divertor power dissipation and including helium ash. To account for uncertainties in the anomalous radial transport, two parameters are scanned: the amplitude of the far SOL transport $D_⊥^{FarSOL}$ and the distance Δr from the separatrix at which this transport enhancement begins.
We first compare the SOLEDGE3X solutions with those obtained for the same input parameters on the same magnetic equilibrium with the newly released wide-grid version of the SOLPS-ITER code. The two codes are found to yield comparable results on key metrics. The presence of shoulders in the density profile increases the FW $n_e$ and $T_e$ by up to 100x, 2x respectively compared with the standard cross-field transport used in the main ITER divertor simulation database for cases considered. This reduces the divertor target heat flux which is then partly redistributed along the FW.
In all the simulations, highly charged Ne ions (especially $Ne^{8+}$) remain the main contributors to W sputtering at the wall, consistent with earlier main chamber W source estimates supporting the ITER re-baseline [1]. The relative contribution of lower charge states tends to increase in the presence of density shoulders, due to the combined effect of the increase in T_e, changes parallel transport, and higher recombination rates from increased far-SOL density. These effects are more influenced by the reduction in Δr from 3 to 1 cm than by increasing $D_⊥^{FarSOL}$ from 1.5 to 3 $m^2s^{-1}$. The divertor is the main gross sputtering source of W throughout the far SOL transport scan. Including prompt redeposition, the outboard FW becomes the largest source of net sputtering in the presence of density shoulders.
[1] K. Schmid et al. 41 (2024) 101789.