Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
2000-08-25
2002-05-28
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C335S216000, C505S892000
Reexamination Certificate
active
06396377
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention pertains to a superconducting magnet having two separate magnet coils.
Superconductivity is a phenomena whereby certain materials lose all electrical resistance when the temperature of the material is lowered below a certain critical point. Although superconductivity was discovered at the turn of the last century, it was not until the advent of superconducting alloys such as niobium titanium (Nb Ti) in the early days of the decade of the 1960's that the superconductivity effect was put to practical use, particularly in the production of very high-field superconducting magnets. Low temperature superconductors require refrigeration at temperatures close to absolute zero (0° K or −273° C.) before the critical superconducting limits are reached. At these low temperatures, the only refrigerant medium that can be used is helium. The normal boiling point of liquid helium at one atmosphere is 4.2° K.
In the 1980's, high temperature superconductors (HTS) were discovered. Unlike the low temperature superconducting materials which tend to be metallic alloys and compounds, the new high temperature superconducting materials are ceramic based. Because of the brittle nature of these ceramic-like materials, high temperature superconductors are currently confined mainly for use as straight sectioned current leads.
U.S. Pat. No. 5,442,929 discloses and claims a compact conduction-cooled superconducting magnet wherein the magnet is cooled by liquid helium with the evaporating liquid helium re-condensed by a closed cycle refrigerator. All of the main coils are wired in series and powered together.
U.S. Pat. No. 5,448,214 discloses and claims an actively-shielded open MRI high-temperature niobium tin superconducting magnet, conduction cooled by a high-temperature cryo cooler. The niobium-tin superconductor is cooled to 10° K using a closed cycle refrigerator to cool both halves of the magnet coil. The second coil is conduction cooled to the first coil which is then in turn conduction cooled by the closed cycle refrigerator.
U.S. Pat. No. 5.448,214 discloses and claims an actively-shielded open MRI high-temperature niobium tin superconducting magnet, conduction cooled by a high-temperature cryo cooler. The niobium-tin superconductor is cooled to 10° K using a closed cycle refrigerator to cool both halves of the magnet coil. The second coil is conduction cooled to the first coil which is then, in turn, conduction cooled by the closed cycle refrigerator.
U.S. Pat, No. 5,412,363 discloses and claims an open MRI superconducting magnet cooled by a single closed cycle refrigerator. The coils of the magnet according to patentee are energized in series and powered up to produce a homogenous high central field. The liquid helium returns to the closed cycle refrigerator for recondensing. According to patentees, the first magnet coil is cooled by first circulating liquid helium around the heat exchange circuit associated with the first magnet coil. After circulating around the first heat exchange circuit. the helium is returned to the refrigerator for re-cooling before it passes around the heat exchanger associated with the second magnet coil. The MRI superconducting magnet is operated in persistence made where the current is perpetually circuited internally, separate from the power supply.
U.S. Pat. No. 5,934,082 discloses indirect cooling of a magnetic device in order to reduce vibration by using low-vibration thermal coupling. Patentees describe using a 10° K (high-temperature) cryo cooler with no cooling system described in detail.
SUMMARY OF THE INVENTION
The present invention pertains to a superconducting magnet having at least two magnet coils separated from one another. The magnetic coils are so constructed and arranged to provide a central magnetic field that is accessible along an X, Y, and Z axis with the axis generation point being at the center of the magnetic field. The magnet according to the invention has full open access to the central magnetic field.
According to the present invention, each of the magnet coils is contained in a separate vacuum jacket with an intermediate radiation shield disposed between the magnet and the vacuum jacket. A two stage closed cycle cryogenic refrigerator is directly coupled to the magnet coil and the radiation shield. The first stage of the cryogenic refrigerator is in direct thermal contact with the radiation shield and the second or colder stage of the closed cycle refrigerator is in direct thermal contact with the magnet. With a system according to the present invention, the magnet can be cooled to cryogenic temperatures utilizing only the closed cycle refrigerator. Inclusion of a coil of conductive tubular material around each magnet coil through which a liquid cryogen (e.g. liquid nitrogen or liquid helium) can be circulated, can significantly reduce the cool-down time for the magnet coil. By circulating the liquid cryogen through the tubing, more rapid cool-down of the magnet coils can be effected.
Therefore, in one aspect, the present invention is a superconducting magnet assembly comprising, in combination, two separate generally toroidally shaped magnet coils positioned in spaced apart relationship to define access to a magnetic field along X, Y, and Z axes originally from the center of the magnetic field; each of the magnetic coils contain within, and spaced apart from an outer vacuum jacket, at least one radiation shield disposed within each of the vacuum jackets between each of the magnet coils and its vacuum jacket, and a separate two stage closed cycle refrigerator adapted to cool each of the magnets to a temperature of about 4° K and each of the radiation shields to a temperature of about 50° K.
In another aspect, the present invention is a superconducting magnet assembly comprising, in combination, two separate generally toroidally shaped magnet coils positioned in spaced apart relationship to define access to a magnetic field along X, Y, and Z axes originally from the center of the magnetic field; each of the magnetic coils contain within, and spaced apart from an outer vacuum jacket, at least one radiation shield disposed within each of the vacuum jackets between each of the magnet coils and its vacuum jacket, a separate two stage closed cycle refrigerator adapted to cool each of the magnets to a temperature of about 4° K and each of the radiation shields to a temperature of about 50° K, and auxiliary means to cool the magnet coils by circulating a liquid cryogen around the magnet coils.
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Donovan Lincoln
Everson Electric Company
Ratner & Prestia
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