Superfluid cooling system for high field NMR and MRI magnets

Abstract

In cryogenic systems which are using high field superconducting magnets such as NMR and MRI, it is important to maintain superconductivity of superconducting magnets. To maintain its superconductivity, the superconducting magnets are generally cooled by subcooled superfluid (liquid helium which has a lower temperature than 2.17 K). The superfluid in a saturated state can be made by subatmospheric cooling system as the simplest method (by maintaining the liquid helium pressure at 0.03 bar), then the saturated superfluid cools down the liquid helium to superfluid at atmospheric pressure to make it subcooled state. Since the cooling system uses pressure difference, the cooling efficiency can be enhanced if a heat exchange process and a JT (Joule-Thomson) expansion process are used. This cooling method is widely used to reject heats from high field superconducting magnet. This paper describes the cooling method of high field superconducting magnets which are used in NMR and MRI to understand the subatmospheric cooling system and we expect the new idea which can design more efficient system by sharing about such the cooling system.