The development of a pulse tube cooler with more than 1 W at 4.2 K

A pulse tube cooler (PTR) for shield cooling and for re-condensation of Helium in MRI magnets has been developed successfully by co-operation of Forschungszentrum Karlsruhe (FZK) and Siemens Magnet Technology Ltd. (SMT, formerly Oxford Magnet Technology, OMT). Refrigeration power of 1.1 W at 4.2 K and 40 W at 45 K were achieved with less than 8 kW of compressor input power. The two-stage PTR is of a '4-valve type on each stage. A special non-wear rotary valve has been developed at SMT. The design of the cooler is based on investigations carried out at FZK on a test rig, which was versatile enough to allow many different experiments to be carried out. They have been accompanied by numeric studies. The code FZKPTR based on the thermoacoustic theory has proved a very helpful tool. This development work at FZK and SMT lead to a commercial PTR solution for MRI magnets. The industrial partner, SMT, defined the needs for his application, and has modified the FZK design according to these requirements and those of industrial production. Some of the intermediate development steps performed during several years of co-operation are mentioned. Prototype units have been successfully installed at clinical sites. A preliminary version; a 10 K shield cooler has been running continuously for two years. More than two months of recondensing performance on the 4K unit fitted to an MRI magnet, with a margin sufficient to prevent boil off of liquid He during imaging, has been obtained.