Abstract: A liquid nitrogen Dewar includes a spun can for the liquid nitrogen, which spun can is surrounded by an exterior shell. Between the exterior shell and the can is an evacuated space containing a second shell that functions as a radiation shield. The can and both shells are formed of an aluminum alloy having a very high percentage of aluminum. The exterior can surface, interior surface of the exterior shell, and both surfaces of the second shell have clean, non-pitted, smooth, matted, smut free and etched appearances free from discoloration and stains so that these surfaces have low radiation emissivity properties and there is a minimum amount of radiant energy transmitted from the exterior shell to the can and reduced heating of the liquid nitrogen in the can. Each of the surfaces is initially polished, either mechanically, electrochemically or chemically, and then chemically treated with an etchant of nitric acid and hydrofluoric acid for a sufficient duration to achieve the desired appearance.

Abstract: An NMR instrument incorporating a cryogenic magnet includes a constant pressure device to maintain the pressure of cryogenic vapors at a constant value whereby a stable magnetic environment is maintained.

Abstract: Vapor from a boiling cryogenic liquid contained in a cryostat is warmed prior to discharge by passing said vapor through a heat exchanger for transferring heat at ambient temperature to the escaping vapor whereby condensation of atmospheric moisture on the cryostat is minimized.

Abstract: Component coils for providing a desired magnetic field distribution, which coils are subject to interaction via mutual inductance thereof, are connected each to respective current regulated power supplies whereby the interaction is compensated. Circuits are provided to control selected pluralities of interacting coils via persistence switches with minimum number of control leads in order to reduce the thermal communication to the interior of a cryostat housing the magnet and persistence switches.

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.

Abstract: Apparatus and method for reduction of noise in NMR signals during high power heteronuclear decoupling experiments. A material is introduced into the evacuated area of the variable temperature probe to decrease collisional probability of ions generated in the interwall space of the evacuated envelop.

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 an 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 235.degree. K. by vapor cooling from the boil-off of escaping nitrogen and helium.

Abstract: An NMR spectrometer includes a superconducting, solenoid coil formed of superconducting wire having a rectangular cross-section. The wire is formed of a non superconducting metal surrounding superconducting material having a small enough cross sectional area that a magnetic field resulting from current flowing in the superconductor material has a relatively constant position relative to the cross section of the wire. In one embodiment, the wire has a monofilament configuration including a core of the superconducting material surrounded by a sheath of the metal. In a second embodiment, the wire includes a copper block of the metal through which a multiplicity of superconductive strands extend.

Abstract: Thermal isolation of a cryostat is improved for a superconducting solenoid having a plurality of separately energizable coils or winding sections by reducing the number of conductors required to selectively energize the persistence switch controlling a particular winding section. As a consequence, the dimensions of the vent and fill tube openings of the cryostat which accommodate these conductors may be reduced. In one embodiment a diode is incorporated in series with heating element of each switch. The switches are connected to form a steering network to control as many as p.ltoreq.n(n-1) separate coils using n conductors to actuate p persistence switches. In another embodiment n(n-1)/2 coils are similarly controlled with n conductors using the time-current response of persistence switches without diodes. Still another embodiment utilizes a binary to hexadecimal converter within the cryostat to permit n externally generated signal conductor to select as many as p.ltoreq.2.sup.n -1 persistence switches.

Abstract: A persistence switch comprises a diode and superconductive wire in thermal contact with the diode. The heat evolved by current through the forward biased diode raises the temperature of the superconductor above the transition temperature. The diode characteristic of the switch permits a plurality of such switches to be arranged in a network for selection by a relatively few signal conductors.

Abstract: An NMR spectrometer includes a superconducting, solenoid coil formed of superconducting wire having a rectangular cross-section. The wire is formed of a non superconducting metal surrounding superconducting material having a small enough cross sectional area that a magnetic field resulting from current flowing in the superconductor material has a relatively constant position relative to the cross section of the wire. In one embodiment, the wire has a monofilament configuration including a core of the superconducting material surrounded by a sheath of the metal. In a second embodiment, the wire includes a copper block of the metal through which a multiplicity of superconductive strands extend.