Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Superconductive type
Reexamination Certificate
2001-08-27
2004-11-09
Donovan, Lincoln (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Superconductive type
C335S301000
Reexamination Certificate
active
06816046
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to superconducting electromagnet apparatus. Such apparatus can be used in various applications including nuclear magnetic resonance (NMR) spectroscopy and imaging and Fourier-transform mass spectrometry (FTMS).
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
In conventional superconducting electromagnet apparatus a main superconducting coil assembly of cylindrical form is used to produce a central magnetic field which varies very little over a specified working volume within the bore of the cylinder, that is the so-called homogeneity volume. In many applications of such apparatus, it is necessary for a precise value of the central magnetic field to be set. Furthermore it is often necessary or desirable to make a fine adjustment to this central magnetic field value, for example to reset the central magnetic field to the specified value after it has been altered by use of a passive or superconducting shim coil assembly (which is used to achieve the optimum degree of homogeneity within the homogeneity volume but which can sometimes lead to a slight change in the central magnetic field value). Ideally this fine adjustment should be made without any degradation of the magnetic field homogeneity within the homogeneity volume.
It is known to make use of a so-called B
0
shim coil assembly to provide fine adjustment of the central magnetic field value with minimal degradation of the magnetic field homogeneity within the homogeneity volume. Such a B
0
shim coil assembly comprises a plurality of superconducting coils connected in series to form a closed loop which is electrically separate from the main superconducting coil assembly but which couples magnetically with the main coil assembly. The B
0
shim coils are wound on an outer former which surrounds an inner former on which the main coils are wound and within which the homogeneity volume is located. The B
0
shim coil assembly also incorporates a superconducting switch within the loop and current input terminals for adjusting the amount of current passing through the shim coils and for enabling persistence of this current when the required current value is reached and the superconducting switch is closed, the required current value being zero or a positive or negative value. The geometries of the B
0
shim coils are chosen so as to have substantially no effect on the homogeneity of the magnetic field within the homogeneity volume. Furthermore the B
0
shim coils are constructed using materials such that the maximum required operating current is well below the critical current at which the coils are no longer superconducting and such that there is insignificant drift in the magnetic fields associated with these coils. Thus, in the persisted mode in which the uniform central magnetic field is maintained within the homogeneity volume, the current in the shim coils will remain substantially unchanged ignoring the effect of any inductive interactions with any other part of the electromagnet apparatus or with any external field source. Typically the shim coils are constructed using wire having a critical current of the same order of magnitude as the wire used for the main magnet coils.
In most circumstances the coupling of the B
0
shim coil assembly with other shim coil assemblies, which are provided to correct distortions in the magnetic field due to other disturbing influences, is minimal. This can be ensured by appropriate choice of the shim coil geometries. As is well known the superconducting switch of the B
0
shim coil assembly can be operated in the same manner as the superconducting switch which is provided for controlling the main coil assembly. In both cases, after the coils have been charged to the required current level from an external current source whilst the loop is open-circuited by opening of the switch, the switch is closed to allow the required constant current level to be maintained within the superconducting loop incorporating the coils. Where the main coil assembly is being initially energised, such switching of the switches associated with the main coil assembly and the B
0
shim coil assembly can be effected in series. However, when the main coil assembly is in the persisted mode, switching of the B
0
shim coil assembly must be effected independently of the main coil assembly to provide the required fine adjustment of the central magnetic field.
In addition to the ability to set the central magnetic field to a precise desired value, it is important in many applications that this set value remains stable with time. For example, in NMR spectroscopy, experiments conducted within the apparatus can last over several days and even small variations in the value of the central magnetic field, for example of the order of a few parts per billion, can lead to systematic differences in the spectral results obtained as a result of such experiments. There are essentially two ways in which the central magnetic field value can exhibit time-dependence, that is either as a result of a change in the ambient magnetic field due to external sources, or as a result of variation in the magnetic field generated by the main coil assembly itself.
WO 89/09475 discloses a superconducting electromagnet apparatus which makes use of an assembly of auxiliary coils which magnetically couple to the main coil assembly in order to reduce the effect of variation of the central magnetic field value due to changes in the ambient magnetic field. In such apparatus an assembly of superconducting shielding coils connected in series within a loop is arranged so that the effective areas and mutual and self inductances of the main coils and the shielding coils are such that any change in the ambient magnetic field causes changes in the currents of the main coils and the shielding coils in such a manner as to generate a change in the central magnetic field which opposes the change in the central magnetic field due to the change in the ambient magnetic field alone. The main coil assembly would itself usually partially shield the homogeneity volume from the effect of such changes in the ambient magnetic field even without the use of such shielding coils, but the shielding effect can be significantly increased by the use of the shielding coils.
Furthermore EP 0468425A discloses an active-shield superconducting electromagnet apparatus comprising a first superconducting coil assembly for generating a first magnetic field, and a second superconducting coil assembly for generating a second magnetic field, the second coil assembly being electrically connected in series with the first coil assembly and the two assemblies being arranged such that a resultant, uniform magnetic field is generated in the working volume and the second magnetic field opposes the first magnetic field externally of the apparatus so that the stray magnetic field outside the coil assemblies is very small. This enables personnel to work safely relatively close to the apparatus without requiring an excessive amount of cumbersome and expensive iron shielding.
However the automatic shielding arrangement of WO 89/09475 is no longer effective in relation to conventional active-shield superconducting electromagnet apparatus. Accordingly EP 0468425A proposes an arrangement in which screening coils connected within a closed loop are provided having a number of turns at least an order of magnitude less than the number of turns in the first and second coil assemblies so as to reduce the effect of disturbing influences on the central magnetic field whilst having an insignificant effect on the homogeneity of the central magnetic field. In this case the screening coils are wound from superconducting wire having a critical current such that they revert to the normal conducting state during quenching of the first and second coil assemblies. In this way the maximum contribution which the screening coils can make to the stray magnetic field is rendered insignificant. Furthermore the numbe
Donovan Lincoln
Magnex Scientific Limited
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