Target Concavity as a Design Parameter for Closed Divertors Facilitating Detachment

Exploration of different divertor targets with SOLPS has suggested target concavity as being potentially important for closed slot divertors to facilitate detachment at lower n_sep/n_div via neutral trapping. Concave targets in SOLPS better position the neutral distribution with respect to the ion/electron distribution, increasing n_div at the strike point by a factor of 2, and similarly reducing the heat flux, compared to similar slots with vertical or horizontal targets, with upstream Q_|| = 450 MW/m². Assume a divertor target, spanning 0 < ψ_n < 2λ_q of the SOL heat footprint, is divided into two conjoined segments oriented at angles θ₁ and θ₂ with respect to the separatrix normal. Concavity is approximated here simply as the difference θ₁-θ₂ < 0, where more negative values concentrate the neutral population more sharply. SOLPS modeling is presented for a divertor target which utilizes concavity to (1) maximize divertor volumetric dissipation, and (2) better confine neutrals to the divertor volume at detachment. Target probe J_sat and T_e profiles from recent DIII-D experiments with the Small-Angle Slot (SAS) divertor qualitatively support the value of target concavity.