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Cryotrapping Vacuum Pumping System Design for a Helium Neutral Beam Injector

M. A. Hoffman, A. S. Blum

Fusion Science and Technology

Volume 1 / Number 2 / April 1981 / Pages 275-284


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The conceptual design of a vacuum pumping system to handle a large gas flow on the order of 2.31 Pa m3/s (17.3 standard (std) Torr/s) of helium gas in the pressure range from ∼ 3.1 × 10−2 down to 4.0 × 10−4 Pa (2.3 × 10−4 down to 3 × 10−6Torr) is described. The neutral helium gas originates partly as leakage from the plasma ion source and partly as additional gas required in the neutralizer duct of the neutral beam injector. The vacuum pumping design is based on the recently demonstrated process of cryotrapping the helium in a frost layer of argon formed by spraying the argon onto a liquid-helium-cooled cryopanel surface. About 10.6 m2 of cryopanel area in the ducts and chambers of the injector is required for an allowed frost thickness of 1 mm. The design is based on preliminary experimental results that indicated that ∼15 atoms of argon were needed to pump and cryotrap each helium atom, and that the specific pumping speed of the fully baffled cryopanels would be ∼31.5 std m3/m2⋅s (3.15 std⋅FS./cm2⋅s). Preliminary estimates of costs indicate that this vacuum system can cost as much as 74% of the entire neutral beam injector and that the LHe cryo-refrigerator alone can cost 24% of the total direct cost. The design points up the problem areas of cryotrapping helium and the need for clever new design concepts and improved performance to reduce costs.

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