Persistent superconducting switch for cryogen-free MR magnets

A persistent superconducting switch which allows a cryogen-free MR magnet to be ramped up to its designed magnetic field and parked in a persistent state has been designed, built, and installed in a magnet. The switch uses niobium-tin tape superconductor which is laminated with stainless steel foil for structural rigidity. Due to its high electrical resistance, the switch is well suited for high inductance magnets. Moreover, the switch is designed such that its ramping heat load into the magnet is minimal and within the capability of the refrigerating cryocoolers which are used to cool the magnet. The compact design of this switch permits a quick conversion between the normal and the superconducting states. In this paper, the design criteria for such a switch and its relationship to the cryogen-free superconducting magnet is discussed. Furthermore, the stability tests performed on a sample switch is presented. Finally, detail design of a switch which has been used in a high inductance, cryogen-free, open MR magnet is discussed along with its electrical and thermal performances.<>