Simulations of a DIII-D dissipative divertor design using mid-leg particle pumping to enhance divertor neutral compression

Simulations show strategic placement of the particle pump duct poloidally upstream of the divertor target but below the X-point serves to decouple power and particle exhaust. Such a mid-leg divertor pump duct operates by allowing recycled neutrals to create a dissipative neutral gas cushion near the target, while particles are removed through the pump duct located on the common flux region side of the divertor baffle. With mid-leg pumping in the outer divertor, SOLPS-ITER simulations without drifts in upper single null magnetic configurations and up to 17.5 MW flowing into the divertors predict a factor of ~5 reduction of the peak heat flux on the divertor target and ~25% lower outer midplane separatrix density at detachment, compared to a pump located at the divertor target in the outer corner of the divertor. Modeling kinetic neutrals in a pump duct located approximately half way up the outer divertor baffle predicts that the detachment front forms between the divertor target and the X-point and is relatively stable near the pump entrance, without a strong dependence on gas puff rate. This indicates that the mid-leg pump acts as a type of relief valve and has potential benefits for core-edge integration. The benefit of enhanced divertor dissipation using mid-leg pumping comes at the cost of a higher outer midplane separatrix density for a given gas puff rate; however, the requirement on midplane density needed for detachment is reduced.