AVS 49th International Symposium
    Vacuum Technology Tuesday Sessions
       Session VT-TuM

Paper VT-TuM2
High Throughput Continuous Cryopump, with Gas Dynamic Compression of the Helium Minority Stream, for Pumping Fusion Reactors

Tuesday, November 5, 2002, 8:40 am, Room C-104

Session: Novel Vacuum Materials and Pumps, Including Getters
Presenter: C.A. Foster, Cryogenic Applications F, Inc.
Authors: C.A. Foster, Cryogenic Applications F, Inc.
S. Willms, Los Alamos National Laboratory
S. Letzring, Los Alamos National Laboratory
D. Schechter, Cryogenic Applications F, Inc.
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An analysis of the flow of gases in a large cryopumping system designed to pump a magnetic plasma fusion reactor is presented. The pumping system for a 3gigawatt thermal reactor must handle a throughput of 2 Pa-m3/s of He and 200Pa-m3/s of D/T and maintain a pressure in the diverter at about 0.5Pa. A cryopump which removes the cryo-ice during operation with a regenerating head or "snail" is capable of pumping the D/T stream. A set of twelve 500mm bore pumps would each pump 16.7 Pa-m3/s of the D/T stream with the He being pumped by a set of turbo pumps downstream to the cryopumps. In designing the entrance baffle and analyzing the gas flow in the pump, it was determined that whereas the gases were close to free molecular flow conditions in a room temperature design, at 30K they would be in the viscous flow regime. The viscous flow conductance of the entrance duct at room temperature was not drastically different from that calculated using the free molecular flow equations. However, Poiseuille flow has a strong temperature dependance, so that the pressure drop across a pipe in viscous flow at 30 K is dramatically lower than at 300K. Since a cold baffle is typically used to precool the gases entering a cryopump, it was decided to replace it and the room temperature ducts with a refrigerated duct operating in the viscous flow regime. An analysis of long cold ducts replacing the conventional vacuum pipes between the reactor diverter chamber and the cryopumps allowed the pipe size to be reduced by a factor of four in area. This is especially advantageous in a reactor since the vacuum ducts have to pass between the coils and through the neutron shielding blankets. An analysis of the fluid flow of the D/T and He in the cold ducts showed a gas dynamic drag compression of the He minority species(10X to 30X) by the D/T stream as was utilized in the Gaede diffusion pump. This pre-compression allows conventional turbo-molecular pumps to be used as the compound helium pumps.