Thermal design of the ZBO cryostat for a 1.5T whole-body magnet system

All modern superconducting MRI needs a state of the art zero boil off (ZBO) technology for the warm bore helium cryostat. The MRI cryostat not only provide the necessary cooling for the NbTi magnet, it also provide a patient bore for imaging by precise positioning of the magnet. The static structural load of the cold mass of the magnet needs to be taken care by the cryostat withstanding the transportation and handling load. The NbTi magnet is dipped inside a helium bath. We have designed a warm bore of diameter of 900 mm helium cryostat to house the actively shielded 1.5T MRI magnet. A double stage 4K GM cryocooler having capacity of 1.25W at 4.2K has been used to achieve the zero-boil-off (ZBO) condition. A fin based heat exchanger has been developed for in-situ recondensation of the helium vapour inside the cryostat to make it " zero loss" thereby minimizing the helium refilling. There is an intermediate thermal shield maintained at 40 K using the refrigeration capacity of first stage of the cryocooler. The drop-in cryocooler is thermally connected to the thermal shield through a compressive contact. A cryocooler based test rig has been developed to study the performance of such compressive contact of the cryocooler in the temperature range of 30-40K. The ZBO condition can only be achieved when the 4K thermal budget of the cryostat will be within the available refrigeration capacity of the cryocooler. An extensive thermal simulation has been done to minimize heat budget at the 4K without compromising with the structural load capacity. A thorough thermal analysis for each component has been performed to achieve ∼ 0.8W of heat at 4.2K. This paper briefly discusses the detailed thermal analysis for the heat budgets at 40 K and 4K to locate the operating point in the cryocooler load map. This paper also describes the cryocooler based rig for testing the performance of the compression contact at 40K and the test results. (author)