Small 3.7K Closed-Cycle Refrigerator without Auxiliary Vacuum Pumps

Abstract

The neutral beam injection (NBI) system is the equipment for heating the plasma within the fusion reactor to an even higher temperature and driving the current by means of high-rate injection of neutralized hydrogen into the plasma. There is a need for high performance cryopumps cooling at 3.7 K for exhausting the large amount of hydrogen gas that remains in the NBI chamber. This paper proposes to provide multiple small systems to cool individual cryopanels. A small helium refrigerator consisting of a Gifford-McMahon expander with a Joule-Thomson circuit with no auxiliary vacuum pumps was designed and tested to demonstrate the feasibility of applying this type of small closed-cycle refrigerator as an alternative to liquid helium supplied-cryopumps for operating temperatures below 4 K. The prototype system is adjustable to provide up to 6.6 watts of cooling at 3.7 K. Furthermore, the authors have achieved a no-load temperature of 2.2 K.