The role of divertor pumping in plasma detachment and particle exhaust in a closed divertor

The impact of pumping on divertor power dissipation and particle exhaust in a closed divertor with flat target configuration is examined using SOLPS modeling. A closed divertor can increase neutral pressure and enhance radiative dissipation; accordingly it has been proposed as a direction for the design of advanced divertors to achieve detachment at as low an upstream plasma density as possible. However, the necessity to pump the closed divertor results in a reduction of the high density and pressure of neutrals near the target. The quantitative effect of this reduction on the achievement of detachment is assessed here. By independently varying both the pumping speed S using different pump opening surface areas, together with the upstream plasma density at the outside midplane, n(e,sep)(OMP), it is established quantitatively how the pump exhaust rate, Q(exh) (particles/s), depends on these two quantities. As expected, pumping increases the detachment onset density, n(e,sep)(OMP,onset); however, for S = 40 m(3) s(-1), it is shown that Q(exh) = 1 kA and detachment onset-as defined by a peak T-e at the outer target of similar to 5 eV-can be obtained simultaneously, which is consistent with DIII-D requirements for particle and energy exhaust. By placing the pump surface at different distances from the target, it is established how the pump location affects Q(exh), which in turn affects the divertor plasma conditions, including achievement of detachment. High pumping speed reduces neutral density and radiated power, thus increasing T-e and the heat flux to the target. The essential difference between the various pump locations is the ratio of Q(exh) to the total number of neutral deuterium particles plus carbon particles (atoms and ions) in the outer divertor, f(exh) = Q(exh)/Ntot-OD. It was found that the best pump location is near the target at the CFR (common flux region) side of the closed divertor slot. Pumping at the PFR (private flux region) side near the target gave similar results, indicating flexibility with regard to pump location in closed divertors with a flat target plate.