The possibility of noninvasive biochemical analysis provided by the measurement of spectra of phosphorous and other nuclei has motivated the increasing use of high field superconducting magnets in Magnetic Resonance Imaging (MRI) systems. A large fraction of the operating cost of a superconducting MRI system is associated with the cryostat design and the consumption of cryogens used to cool the magnet. Present cryostats operate in an open-cycle mode and spent cryogens are discarded to the atmosphere. There is a strong need for a simple, reliable closed-cycle cryocooler that would eliminate the consumption of cryogens and reduce the cost of the cryostat. The few existing laboratory cryocoolers of approximately the right capacity are positive displacement units based on the Gifford-McMahon cycle. Recent technological developments in the miniaturization of gas bearings and turbines indicate that a Claude cycle using turbomachines may be an attractive alternative to the Gifford-McMahon cryocooler. This proposal describes a Phase I project to evaluate the technical and economical feasibility of developing a MRI cryocooler system based on miniature cryogenic turbo-expanders. The objectives of Phase I are to establish cryocooler specifications and perform preliminary design and costing of key components. The effort will establish whether the proposed cryocooler is feasible and what technology developments are required to demonstrate a working prototype.
