Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Patent
1991-06-25
1992-11-10
Tolin, Gerald P.
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
324318, G01V 300, H01F 100, H01F 300, H01F 700
Patent
active
051627680
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a magnetic field generating assembly.
A wide variety of magnetic field generating assemblies have been designed in the past for use in different applications, all of which require the generation of a magnetic field in a working volume within which tasks are to be performed. Examples of such applications include magnetic resonance imaging (MRI) and nuclear magnetic resonance spectroscopy. These particular applications are complicated further by the fact that in order to achieve the high field strength required, superconducting magnets must be used with the consequent need for bulky cryogenic equipment and the like. In the past, the working volume has been provided within the bore of a superconducting coil although more recently certain proposals have been made for projecting at least a homogeneous region of the magnetic field outwardly of the bore to increase the ease of access. However, these constructions are still bulky and expensive.
GB-A-2174248 described a magnetohydrodynamic device. This is, however, unsuitable for performing a nuclear magnetic resonance experiment on the human body.
In accordance with the present invention, the magnetic field generating assembly comprises a magnetic field generator positioned between and spaced from opposed pole pieces which are mounted in a wall of magnetic material surrounding the magnetic field generator, the wall providing a substantially closed magnetic flux path, and the arrangement being such that forces on the generator due to magnetic flux in the wall are substantially balanced, characterized in that the magnetic field generator comprises at least two counter-running, nested coils; in that the gap between the magnetic filed generator and at least one of the pole pieces is sufficient to accommodate a human body; and in that the opposed pole pieces are non-planar, the arrangement being such that when working currents flow in the coils a magnetic field is generated in a working region situated in the gap between the magnetic field generator and the at least one pole piece having a homogeneity suitable for performing a nuclear magnetic resonance experiment.
This design leads to a new concept in magnetic field generating assemblies since by causing the wall of magnetic material to have a significant influence, a single, short magnetic field generator can be used to provide a source of magnetomotive force leading to the use of a very cheap cryostat. Furthermore, the net force on the cryostat will be substantially zero due to the balancing affect of the wall. In addition, the wall provides magnetic shielding externally of the assembly.
Preferably, the balancing of forces is achieved by constructing the assembly in a symmetrical form with the magnetic field generator symmetrically positioned within the wall. Alternatively, it may be possible to achieve the same balancing affect with a non-symmetrical arrangement by creating a pseudo-pole on one side of the generator relatively closer to the generator than the other pole.
Typically, the wall will be made from iron or some other ferro-magnetic material.
In some arrangements, the homogeneity of the magnetic field in the working region between the magnetic field generator and at least one of the pole pieces will be sufficient for the purpose to which the assembly is to be put. The homogeneity can be controlled during the design of the assembly by considering the effect of the wall and the magnetic field generator together using finite element methods. In particular, the wall itself may be contoured to achieve homogeneity within the working region. For example, the wall is preferably rectangular or square with the magnetic field generator placed substantially parallel with opposed sides of the wall. In this case, preferably the internal surfaces of the sides of the wall extending from the generator to the opposed walls taper towards the generator. In some cases, however, the homogeneity produced by the generator and wall alone may not be high enough in which case additional magnetic shims could
REFERENCES:
patent: 4647887 (1987-03-01), Leupold
Armstrong Alan G. A. M.
Begg Michael C.
Longmore Donald
McDougall Ian L.
Barrera Raymond
Oxford Medical Limited
Tolin Gerald P.
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