Cryostat with external refrigerator for super-conducting nmr spectrometer

Abstract

An improved cryostat for the superconducting magnet of an NMR spectrometer comprises a nested structure of isothermal shells surrounding a thermally conductive central reservoir of substantially spherical shape containing liquid helium in which the superconducting solenoid remains operational when only partially immersed. A radiation shield surrounding the central reservoir is cooled by the boil-off of escaping helium vapor. The radiation shield is enclosed within a isothermal shell maintained at the temperature of liquid nitrogen by thermal contact with a liquid nitrogen reservoir disposed above the region of the central reservoir and shielded therefrom by a wall of the isothermal shell. An outer radiation shield surrounds the liquid nitrogen reservoir and associated isothermal shell and the outer radiation shield is maintained at a temperatures of the order of 180.degree. K. by an external refrigeration facility. A hermitically sealed containment vessel forms the outer wall of the cryostat and provides a port for evacuating to a very low common pressure all the interior spaces between adjacent nested structures. The spacing between adjacent nested surfaces is maintained by a novel internal mechanical bracing system of low thermal conductivity comprising polyester cords disposed under tension between adjacent shells.