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A five-watts G-M/J-T refrigerator was built and installed for the high-energy physics research at Brookhaven National Laboratory in 2001. A liquid helium target of 8.25 liters was required for an experiment in the proton beam line at the Alternating Gradient Synchrotrons (AGS) of BNL. The large radiation heat load towards the target requires a five-watts refrigerator at 4.2 K to support a liquid helium flask of 0.2 meter in diameter and 0.3 meter in lengtk which is made of Mylar film of 0.35 mm in thickness; The liquid helium flask is thermally exposed to the vacuum windows that are also made of 0.35 mm thickness Mylar film at room temperature. The refrigerator uses a two-stage Gifford-McMahon c~ocooler for precooking the Joule-Thomson circuit that consists of five Lindetype heat exchangers. A mass flow rate of 0.8-l .O-grams per second at 17.7 atm is applied to the refrigerator cold box. The two-phase helium flows between the liquid target and liquid/'gas separator by means of thermosyphon. The paper presents the system design as well as the test results including the control of thermal oscillation.

The g-2 muon storage ring magnet system consists of four large superconducting solenoids that are up to 15.1 m in diameter[I,2]. In addition there is a 1.8 meter long actively shielded inflectordipole that is to guide the beam into the storage ring. The g-2 superconductingmagnets will be cooled using forced two-phase helium in tubes that is provided fromthe J-Tcircuit of a 625 W refrigerator. The two-phase helium flows from the refrigeratorJ-T circuit through a heat exchanger in a storage dewar that acts as a phase separatorand a buffer for helium returning from the magnets. The g-2 magnet cooling system consists of three parallel two-phase helium flow circuits that provide cooling to; the four large superconductingsolenoids, the current interconnectsbetween the solenoids with the 5300 A solenoid gas cooled electrical leads, and the inflector dipole with its 2850 A gas cooled electrical leads.

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