A five-watt G-M/J-T refrigerator for LHe target at BNL

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 Synchrotron (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 length, which is made of Mylar film of 0.35 nun 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 cryocooler for precooling the Joule-Thomson circuit that consists of five Linde-type heat exchangers. A mass flow rate of 0.8similar to1.0 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 a thermosyphon. This paper presents the system design as well as the test results including the control of the thermal oscillation.