Metal working – Method of mechanical manufacture – Electrical device making
Reexamination Certificate
2000-09-25
2004-05-18
Vo, Peter (Department: 3729)
Metal working
Method of mechanical manufacture
Electrical device making
C029S605000, C029S890036, C174S125100, C324S248000, C335S216000, C505S430000
Reexamination Certificate
active
06735848
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates generally to a high field magnet and, particularly, to a high field superconducting magnet having a wide bore for use in a nuclear magnetic resonance (NMR) spectrometer.
The technique of NMR has proven to be a powerful and unique tool for the study of complex molecular structures. High current density superconducting magnets are particularly well suited to provide the magnetic field uniformity and persistence required for NMR. As a result, a relationship exists between the available range of application of NMR spectroscopy, in field and sample volume, and the state of the technology of high current density superconducting magnets. Traditionally, increased field strength in high resolution NMR magnets has been sought for the study of the structure of molecules of increasing size. The number of spectral lines associated with larger molecules requires the increased line separation and sensitivity afforded by higher fields. Recently, unexpected benefits of high fields have been realized due to mechanisms of line width minimization at fields being approached in available spectrometer magnets. As a result, the motivation for increased field strength in NMR magnets is greater than ever. There are currently a number of programs under way with the objective of NMR at 1 GHz, corresponding to 23.5 T, and above. The possibility of these high fields depends, as a necessary condition, on the availability of a superconductor and associated coil technology for that field.
Given the scientific and commercial importance of NMR and the associated spectrometer magnets, there is motivation to address the technology of high current density superconducting magnets. More specifically, very high field NMR magnet technology is desired for instrumentation to support high field NMR research and to provide a wide bore 900 MHz spectrometer magnet. Such a magnet is also desired because the technology development activities directed toward the requirements of the 900 MHz magnet specifically are also applicable generally to high field NMR magnets, to high field, high current density superconducting magnets, and more generally to many aspects of magnet technology regardless of the type of conductor and construction being employed.
Moreover, a high field magnet with a larger bore than presently available is desired. Those skilled in the art believe that such a wide bore or large bore magnet will serve as an essential stepping stone to 1 GHz or higher frequency systems. Due to the high stored energy of the 900 MHz system and the associated large magnetic forces, however, the production of a successful system is challenging.
In general, a magnet of this type employs Nb
3
Sn and NbTi conductors in a set of epoxy impregnated long solenoids plus compensation coils for uniformity. The high field and large bore result in large mechanical stress in the coils and large magnetic stored energy. Therefore, reinforcement of the windings and an active protection system is desired.
Moreover, magnetic field uniformity is critical to NMR. Ferromagnetic welds cause field inhomogeneity. Historically, magnet designs have avoided ferrous structural alloys to prevent potential field distortions from welds. This strategy is problematic in fabricating high field magnets because austenitic stainless steel is the preferred heat treatment material for bore tubes in Nb
3
Sn coils. Early high field NMR designs employed the removal of the bore tubes after heat treatment and epoxy impregnation. Bore tube removal is dangerous due to the risk of damaging the reacted Nb
3
Sn conductor and leads. A more practicable fabrication approach is to leave the stainless steel bore tubes in place. For this reason, a weld metal on the coil form that avoids magnetic fields is desired.
A major obstacle to producing a wide bore, high field magnet involves the relatively large mechanical stresses caused by the magnetic fields in the magnet. Energizing a wound coil with an electric current produces a magnetic field accompanied by an associated mechanical stress in the coil. As the strength of the magnetic field produced by the coil increases, the magnitude of the mechanical stress increases as well. In this instance, a magnetic coil wound with superconductor produces a very high field and, thus, mechanical stresses become an important design factor.
In general, superconductors are composite materials in the form of flat tapes or wires (round or rectangular). The composite conductor typically includes copper or silver for protection and stabilization in addition to a superconducting alloy or compound. The composite conductor may also have substantial fractions of other materials (e.g., bronze). Unfortunately, the materials normally found in high field superconductors are generally of low strength and the high temperature heat treatment and annealing to which such conductors are subject diminishes their strength even further. For this reason, a magnetic coil structure providing sufficient strength to withstand the high mechanical stresses that appear in the windings of a high field magnetic coil is desired. There have been some attempts at high strength versions of superconductors, but even these materials would benefit from additional high strength supporting materials in high field magnet applications.
Magnetic coils used for the production of high magnetic fields are often cylindrical in form. In a cylindrical coil, there are two main components to the mechanical forces in the windings. First, a force in a radially outward direction generally tends to expand the diameter of the coil. Second, an axial force at each end of the coil toward the center results in a pressure at the midplane of the coil and tends to make the coil shorter. Both of these forces can produce excess mechanical stress on the conductor. Therefore, magnet reinforcement is desired for containing the radial component of the force to limit the radial expansion of the windings as well as containing the axial component of the force to reduce the pressure on the conductor at the center of the coil about the midplane.
Those skilled in the art are familiar with reinforcing a cylindrical magnetic coil by applying structural material to the outside surface of the coil. The added material forms a secondary cylindrical structure in contact with the cylindrical structure of the coil windings. For example, high strength wire wound into place over the magnetic coil provides reinforcement for the conductor in the coil. This construction has strength in the radial direction, against the expansion of the hoops formed by the reinforcement winding, but can be weak in the axial direction, where any spaces between the turns in the reinforcement winding reduce the stiffness in the axial direction. External reinforcement of this type may be applied without additional bonding material, relying on winding tension alone to hold the reinforcement winding in position, but is commonly applied along with a bonding material such as epoxy. The epoxy serves to fill any gaps between the turns in the reinforcement winding and to increase the stiffness of the reinforcement in the axial direction.
Those skilled in the art recognize that the forces or stresses on the magnet increase as the strength of the magnetic field increases. The so-called Al5 high field superconductors, including Nb
3
Sn, are used to produce coils with the highest fields and forces but also tend to be the most brittle and subject to damage from mechanical stress. Unfortunately, the fabrication process for this type of coil places restrictions on the manner in which the reinforcement can be included in the design. One method of fabricating a high field superconducting coil, commonly referred to as “wind and react,” begins with winding the coil with an intermediate stage of conductor. The coil is then heat treated in a furnace at high temperature allowing the components of the intermediate stage conductor to react to form the final superconducting compound. The coil may then be finished by impregnation with a
Dixon Iain R.
Markiewicz W. Denis
Marshall W. Scott
Painter Thomas
Swenson Charles A.
FSU Research Foundation, Inc.
Nguyen Donghai
Senniger Powers Leavitt & Roedel
Vo Peter
LandOfFree
Method of manufacturing a superconducting magnet does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method of manufacturing a superconducting magnet, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method of manufacturing a superconducting magnet will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3226990