Assessment of the impact of fuelling puff location on divertor impurity compression and enrichment in STEP

Abstract

In order to achieve a compatible solution between the divertors and the core, SOLPS-ITER simulations were performed in a Spherical Tokamak for Energy Production (STEP) connected double-null geometry to investigate the possibility of using deuterium ( $\mathrm{D}_2$ ) puff locations as an actuator for divertor argon (Ar) compression/enrichment. It was found that puffing ${\mathrm{D}}_2$ from the inner-midplane (IMP) ${\mathrm{D}}_2$ puff enhanced Ar compression and enrichment on the high-field-side (HFS) and lowered the required upstream Ar fraction to achieve acceptable target conditions. An interesting link was found between argon compression and the number of stagnation points of the middle-charge-state Ar ions in the HFS SOL, $N_{\mathrm{st}}$ ; significantly improved compression and enrichment were obtained for $N_{\mathrm{st}} = 1$ (corresponding mostly to the cases with IMP puff) compared to $N_{\mathrm{st}} = 3$ (corresponding mostly to the cases without IMP puff). An intermediate compression and enrichment was obtained when $N_{\mathrm{st}} = 5$ . The change of $N_{\mathrm{st}}$ from 3 to 1 (or 5) was achieved by a combination of $\mathrm{D}^+$ outflow, high collision frequency, and flipped temperature gradients around the IMP. Given a possible drawback of ${\Gamma}_{\text{D}}^{{\text{OMP}}+{\text{IMP}}}$ , that is a direct effect on the upstream main plasma density and temperature, we propose ${\Gamma}_{\text{D}}^{{\text{PFR}}+{\text{IMP}}}$ as the best solution, balancing the negative and positive effects of private-flux-region and IMP ${\mathrm{D}}_2$ puffs. Further studies will be carried out both experimentally and numerically.

Keywords

• puff location
• STEP
• SOLPS-ITER
• divertor design
• impurity compression