Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Reexamination Certificate
2003-06-09
2004-08-24
Nguyen, Tuyen T. (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
C335S216000, C335S301000
Reexamination Certificate
active
06781494
ABSTRACT:
This application claims Paris Convention priority of DE 102 27 877.6 filed Jun. 22, 2002 the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The invention concerns a magnet assembly for generating a magnetic field in the direction of a z axis in a working volume disposed on the z axis about z=0 with an actively shielded superconducting magnet coil system and at least one current path which is superconductingly closed in the operating state, wherein the actively shielded superconducting magnet coil system comprises a radially inner and a radially outer partial coil system which are disposed coaxially to each other and whose magnetic dipole moments have opposite signs in the operating state which differ by an amount of &Dgr;
m
with |&Dgr;
m
|<2.5% of the magnetic dipole moment magnitude of the radially inner partial coil system, wherein the magnetic field of the magnet assembly can be expanded along the z axis about z=0 in a polynomial in z having the coefficients Hn and wherein the magnet assembly has an external device for charging the superconductingly closed current paths with an operating current in the operating state and wherein the overall contribution of the superconductingly closed current paths to the magnetic field of the magnet assembly in the working volume of the magnet assembly in the operating state is smaller than 5% of the field contribution of the actively shielded superconducting magnet coil system.
A magnet assembly of this type comprising an actively shielded magnet coil system and at least one additional superconductingly closed current path is disclosed in the patent document U.S. Pat. No. 6,265,960. In this magnet assembly, an additional, superconductingly closed current path acts as superconducting shim device to improve the field homogeneity in the working volume of the magnet assembly.
Patent document WO 00/52490 discloses a further magnet assembly comprising an actively shielded magnet coil system and at least one additional superconductingly closed current path. This magnet assembly comprises an additional superconductingly closed current path for compensating external electromagnetic disturbances, for compensating a field drift caused by the magnet coil system itself, or for fine adjustment of the magnetic field strength in the working volume.
Superconducting magnets have various fields of application which include high-field applications, e.g. for magnetic resonance methods. Such high-field magnets also typically generate a large fringe field. This fringe field can represent a danger for the surroundings of the magnet. This problem can be solved when the magnet comprises an active shielding, i.e. an additional superconducting coil which is connected in series with the main coil of the magnet but which generates a field of opposite polarity.
In particular magnets with highly efficient fringe field shielding pose the problem that deviations from the design specifications for the magnet coils may cause considerable changes in the fringe field generated by the magnet assembly such that required fringe field specifications are not met. Small deviations from the design specifications due to production tolerances are unavoidable. For example, the wire diameters may have tolerances of up to one percent. Such small deviations can dramatically deteriorate the fringe field values since large field contributions with different signs are mutually superposed for compensating the fringe field outside of the magnet assembly. At a location where a fringe field of 0.5 mT should result, the mutually compensating amounts of main coil and fringe field shielding are e.g. in an order of magnitude of 100 mT. A deviation of one of these two field contributions from its desired value by approximately 1% caused by the production inaccuracies in the coil system produces a deviation of the fringe field strength from the desired value of approximately 1 mT at the location of the 0.5 mT contour surface. The required fringe field limit at this location could thereby be exceeded; in this case by multiple factors.
It is the underlying purpose of the present invention to improve a conventional magnet assembly such that its fringe field boundary values are maintained even when individual parameters of the coil arrangement differ from the desired values in consequence of production inaccuracies. In particular, fringe field effects from deviations in the winding data from their desired values and due to geometrical deviations in the coil formers carrying the individual coil systems from their desired geometry, shall be compensated for.
SUMMARY OF THE INVENTION
This object is achieved in accordance with the invention in that the magnet assembly comprises at least one additional superconducting current path, which can be charged independently of the actively shielded magnet coil system. To permit improvement of the fringe field of the magnet assembly by the additional current paths without negative side effects on the function of the magnet assembly, the dimensioning of these current paths must meet various requirements. The field contribution generated by the additional current paths in the working volume of the magnet assembly should not exceed 5% of the field contribution value of the actively shielded superconducting magnet coil system. Moreover, it should be ensured that the currents flowing in the additional current paths do not substantially deteriorate the homogeneity of the magnetic field of the magnet assembly in the working volume. Since the homogeneity of the magnet assembly is mainly determined by the coefficient of second order in the polynomial expansion of the magnetic field about the center, the contribution of the additional current paths to the coefficient of second order of the overall field of the magnet assembly should be small compared with the corresponding coefficient of the actively shielded superconducting magnet coil system and/or not exceed an absolute maximum value of 0.25T/m
2
. These conditions are also at least partially met by additional superconductingly closed current paths in accordance with the cited prior art.
In one inventive embodiment of the additional, superconductingly closed current path, one ensures that the area enclosed by its windings is large enough that the currents flowing therein generate a sufficiently large fringe field contribution, in particular a sufficiently large magnetic dipole moment. In this manner, suitable selection of the currents flowing in the individual additional current paths in the operating state of the configuration can at least partly compensate for a deviation of the magnetic dipole moment of the actively shielded magnet coil system from its desired value caused by production tolerances via the dipole moment of the currents flowing in the additional superconductingly short-circuited current paths without substantially deteriorating other properties of the magnetic field of the overall configuration by field contributions of the additional superconductingly short-circuited current path or paths.
The inventive configuration is advantageous in that even for very well actively shielded superconducting magnet systems whose fringe field reacts excessively to deviations in the design parameters of the magnet coil arrangement from their desired values, the theoretically achievable fringe field limits can be kept without having to take expensive and demanding measures during production to prevent such deviations from the design specifications. This permits more economical manufacture requiring no additional measures for precise production tolerances in the winding data in accordance with design specifications, such that wires and coil formers having precise tolerances are not required. One particular advantage of the inventive configuration is that it can prevent excessive fringe field values caused by deviations of various design parameters of the actively shielded magnet coil system from their desired values without having to calculate and
Amann Andreas
Bovier Pierre-Alain
Schauwecker Robert
Bruker Biospin AG
Nguyen Tuyen T.
Vincent Paul
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