Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
2001-11-08
2003-05-13
Lefkowitz, Edward (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S322000
Reexamination Certificate
active
06563316
ABSTRACT:
This application claims Paris Convention priority of DE 100 60 284.3 filed Dec. 5, 2000 the complete disclosure of which is hereby incorporated by reference
BACKGROUND OF THE INVENTION
The invention concerns a magnet arrangement comprising an actively shielded superconducting magnet coil system for generating a magnetic field in the direction of a z axis in a working volume disposed on the z axis about z=0, and with a plurality of protective elements for protecting the superconducting magnet coil system in the case of a breakdown in the superconducting state thereof (=quench), wherein the superconducting magnet coil system comprises a radially inner partial coil system and a radially outer partial coil system which are electrically connected in series, arranged coaxially with respect to one another, and which each produce a magnetic field in the working volume, of opposite directions along the z axis, wherein the superconducting current path of the magnet coil system is divided into several sections which are each electrically connected in parallel with at least one of the protective elements, wherein at least one of these sections comprises windings of the radially inner partial coil system and windings of the radially outer partial coil system.
U.S. Pat. No. 5,644,233 discloses a magnet arrangement comprising an actively shielded magnet coil system with a radially inner and a radially outer partial coil system, wherein the magnet coil system is divided into several sections each comprising windings of the inner partial coil system and windings of the outer partial coil system, such that the dipole moment of each of these sections is approximately zero, wherein each of the sections is connected in parallel with a protective element.
Superconducting magnets are used for different applications including, in particular, high field applications, e.g. for magnetic resonance methods. High field magnets of this type generally produce a considerable stray field which can be dangerous for the surroundings of the magnet. This problem can be solved by providing the magnet with an active shielding, i.e. with an additional superconducting coil which is connected in series with the main coil of the magnet, however, which produces a field of opposite polarity.
In addition to strong stray fields, another problem with superconducting high field magnets is the risk of a sudden breakdown of the superconducting state (=quench). To protect the superconducting wire of the magnet from overheating and from being destroyed in case of a quench, superconducting magnets usually comprise a device which diverts the magnetic current from the coil sections which have become resistive during a quench and through protective elements, e.g. resistances. For actively shielded magnets, such a quench protection device can cause the following problem: Should e.g. the shielding coil quench, the current flows away from the shielding via the protective elements connected in parallel while substantially the same original magnet current continues to flow in the main coil. This causes an extremely large short-term increase in the stray field of the magnet arrangement compared to the stray field during normal operation.
In an arrangement in accordance with U.S. Pat. No. 5,644,233, the problem of a possibly excessive stray field during a quench is solved in that each coil section, connected in parallel with its own common protective element, is composed of parts of the main coil and the shielding coil such that the dipole moment of each section during operation is small. If a quench occurs in such a section thereby locally reducing the current, the dipole moment of the magnet arrangement will, at most, change slightly. For this reason, the stray field cannot increase considerably.
However, it is not always possible to provide all coil sections bridged by protective elements with negligible dipole moments, in particular, for high field magnets. There are two main reasons therefore. Firstly, for each section, a superconducting connection must be provided between the parts of the main coil and the shielding portions. This is difficult to realize when the magnet is finely divided into many sections. Secondly, the dipole moments of the main coil and shielding portions cannot be exactly matched to each other since the beginning and end of the sections cannot lie within a winding packet but only at the end of an entire coil layer. Therefore, typical high field magnets unavoidably have at least some of the sections bridged with protective elements which have a dipole moment which is considerably different from zero. This can cause a considerable change in the stray field of the magnet arrangement during a quench in one of these sections.
In contrast thereto, it is the object of the present invention to considerably reduce the danger of an excess stray field during a quench in a magnet arrangement having at least one section which is bridged with a protective element and which has a non-negligible dipole moment.
SUMMARY OF THE INVENTION
This object is achieved in accordance with the invention in that at least one additional closed current path is provided which has a non-vanishing areal winding number and a non-zero inductive coupling L
Ai⇄C3
to at least one section A
i
, in particular, such that the coupling coefficient
K
i
=
L
Ai
↔
C3
L
Ai
L
C3
is larger than 0.01, wherein L
Ai
and L
C3
characterize the self-inductances of the section Ai and of the additional current path C
3
, respectively.
The dipole moments of the sections bridged by a protective element are reduced as much as possible through combination of coil portions from the main coil and the shielding coil. Moreover, an additional current path with a non-vanishing areal winding number is also provided which is separate from the magnet coil system and which is inductively coupled to at least one of those sections whose dipole moment clearly differs from zero. In case of a quench in one of these sections, a current is induced in the additional current path which compensates for the change in the dipole moment of the magnet arrangement occurring during a quench, thereby keeping the stray field largely constant, or even reducing it, during the quench.
The advantage of an inventive arrangement consists in that, when forming the sections bridged with a protective element, the dipole moments of the sections must not necessarily be completely negligible, good protection from excess stray fields during a quench of the magnet arrangement is nevertheless still ensured. This permits a more flexible design of the quench protection device to facilitate production thereof or it can be dimensioned to provide optimum protection for the coil with regard to overheating during a quench.
One embodiment of the inventive magnet arrangement is particularly advantageous wherein the radially inner and outer partial coil systems have approximately equal magnetic dipole moments of opposite sign. Such a magnet arrangement has optimum magnetic shielding (i.e. a very small stray field) due to the substantially negligible magnetic dipole moment of the overall system. In such an arrangement, it would be particularly difficult to maintain the low stray field values during a quench, solely through suitable selection of the sections bridged by a protective element.
In one advantageous embodiment of the inventive magnet arrangement, the magnet arrangement is part of an apparatus for high-resolution magnetic resonance spectroscopy. In such magnet arrangements, the radially inner partial coil system generally has a very large dipole moment due to the required high field strengths, and therefore, the use of actively shielded magnet systems is particularly beneficial. The inventive arrangement guarantees that there are no excess stray fields in case of a magnet coil system quench, which would be particularly unfavorable in this case due to the generally large dipole moment of the main coil.
In a particularly preferred embodiment of the inventive magnet arrang
Amann Andreas
Bovier Pierre-Alain
Schauwecker Robert
Bruker Biospin AG
Lefkowitz Edward
Shrivastav Brij B.
Vincent Paul
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