Performance of a linear compressor driven JT cryocooler at liquid helium temperature

Abstract

The JT cryocooler at liquid helium temperature driven by large pressure ratio linear compressor(s) is one of the core components in the deep space detectors. Also, future quantum detection technology needs to work at or below the liquid helium temperature zones. The JT cryogenics system for obtaining the liquid helium temperature zone is mainly composed of a regenerative precooling unit and a JT cooling unit. The relative technology of the regenerative precooling unit is more mature than that of the JT unit, and related research reports are also numerous. A valved linear compressor (VLC) capable of providing a large pressure ratio is a key technology in the JT cryocooler. At present, the deep space exploration projects such as JWST and SPICA have adopted the VLC. In order to study the working characteristics of VLC, a test system of a closed JT cryocooler operating at liquid helium temperature is built. The cryocooler is precooled by a GM cooler and the precooling temperature of 10 K is selected. A single-stage VLC is used in the study and the output performance is tested independently. The experimental results show that when the charging pressure is 0.15 MPa, a pressure ratio of 3.7 with the mass flow of 4.5 mg/s can be obtained. While raising the charging pressure to a value of 0.21 MPa, the pressure ratio can increase to 4.5. Based on this performance of the single-stage VLC, the theoretical cooling capacity of JT cycle is analyzed. The coupling performance test of the VLC with a JT unit is developed. After about 30 h later, the evaporator temperature can be cooled down to the liquid helium temperature zone. By increasing the piston displacement of the VLC, the dead volume of the compression chamber can be reduced, thereby improving the output performance. With the displacement increased from 6.2 to 7.5 mm by supplementing the input power of 15 W, the cooling temperature can be reduced from about 4.4 to 3.94 K. A minimum temperature of 3.91 K is achieved by further adjusting the piston displacement and the VLC operating frequency. The cooling performance at liquid helium temperature is mentioned in the paper. A maximum cooling capacity of 10.8 mW at 4.09 K can be obtained. The study can provide a reliable research basis for multi-stage compression and lower temperature zones systems.