Exhaust pumping of DT fusion devices: Current state-of-the-art and a potential roadmap to a fusion power plant

The main functions of the exhaust pumping system of a DT fusion device are to pump out the helium ash and to control the divertor neutral gas density. This requires the handling of large gas throughputs at high pumping speeds (but at relatively moderate vacua). The pumped exhaust gas is then usually transported to the tritium plant for cleaning, which involves impurity removal and separation of the pure hydrogenic species for re-injection as fuel. In view of a fusion power plant, a systematic technical review of primary and roughing pump technologies is conducted in order to identify potential exhaust pumping concepts which eliminate some of the disadvantages that eventually result from simple scale-up of the ITER solutions that are based on batch-wise operating cryogenic pumps. This paper also illustrates the methodology applied to come to unbiased results and describes the final configuration which is based on a vapor diffusion pump as primary pump together with a metal foil pump for hydrogen separation, and a liquid metal ring pump as roughing pump. All pumps are working continuously and do not require cryogenic temperatures. The new concept will reduce the tritium inventories of a power plant, firstly because of the continuous pumping characteristics of the pumps involved, and secondly because the metal foil pump allows for internal recycling of the unburnt fuel species directly from the divertor to the fuelling systems, bypassing the tritium plant.