Design of a closed divertor with mid-leg pumping for DIII-D

A new baffling and pumping structure has been designed and modelled for the upper divertor in DIII-D to test the concept of pumping along the divertor leg upstream of the target as a mechanism to passively stabilize the detachment front, maintaining a hot X-point (T_e,Xpt ≈ T_e,sep at the outer midplane) simultaneously with a detached divertor target (T_e,targ < ~5 eV). This “chimney” design employs a closed baffling structure along an extended outer divertor leg with a pump duct opening positioned vertically upstream from the target on the common flux side. Maintaining a volume of trapped neutrals near the target enables dissipation processes from recycled neutrals to cool the plasma before it encounters the material surface. Moving upstream, most neutral hydrogenic atoms are then either pumped or re-ionized by the incoming plasma flux before they reach the main chamber, minimizing core fueling in the detached state and stalling upstream propagation of the ionization front. Boundary plasma and neutral simulations using both SOLPS-ITER [J. Yu et al., Nucl. Mater. Energy 2024] and UEDGE [A. Holm et al., Nucl. Mater. Energy 2024] find that the positioning of pumping part way up the divertor leg in this way reduces the upstream density at the onset of detachment and stabilizes the poloidal position of the ionization front near the pump duct over an experimentally attractive range of injected power and particle flux. Design considerations and comprehensive diagnostic plans will be presented.