Electricity: measuring and testing – Particle precession resonance – Spectrometer components
Reexamination Certificate
1999-07-02
2001-10-23
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Particle precession resonance
Spectrometer components
C324S309000, C324S322000
Reexamination Certificate
active
06307370
ABSTRACT:
The invention relates to a superconducting magnet arrangement with at least two current paths, which are short-circuited during operation via superconducting switches, which can carry different currents, wherein the magnet arrangement comprises at least one actively shielded superconducting magnet having a radially inner and a radially outer coil system which carry approximately the same current and have dipole moments of approximately the same magnitude, but opposite in direction, such that an external magnetic field disturbance in a working volume is suppressed in the centre of the magnet arrangement in its long-term behavior to a remaining value of less than 20% of the external magnetic field disturbance.
Actively shielded superconducting magnet arrangements of this type are known e.g. from U.S. Pat. Nos. 5,329,266 or 4,926,289.
The field of use of superconducting magnets comprises various fields of application, in particular magnetic resonance methods, in which in general the stability with time of the magnetic field is important. One of the most demanding applications comprises high-resolution nuclear magnetic resonance spectroscopy (NMR spectroscopy). Field fluctuations with time may be caused by the superconducting magnet itself as well as by its surroundings. While it is possible with the current magnet and conductor technology to generate fields which are very stable with time, there is still the need for further development in the field of suppressing external magnetic disturbances. In the following, methods are shown about how these disturbances can be counteracted. The main focus lies in the field of compensating disturbances with superconducting solenoid magnets with active shielding of the stray field.
U.S. Pat. No. 4,974,113 describes i.a. a compensating, superconducting solenoid magnet, wherein there is no mention of any active shielding of the magnet system. The idea there consists in that at least two independent superconducting current paths are realized by means of two coaxially arranged superconducting solenoid coils and are calculated such that occurring external magnetic field disturbances are suppressed to at least 80% in the inside of the arrangement by taking into consideration the invariance of the entire magnetic flux through each current path with closed superconducting loop.
U.S. Pat. No. 5,329,266 describes an application of this idea to an actively shielded magnet system. It provides a plurality of shielding structured compensation coils which are connected in series in a superconducting manner and comprise each individually a current limiter with a maximum current which is small in relation to that of the main coils, in the range of a maximum of one ampere, to ensure that in case of a breakdown of the superconductivity (=Quench), the stray field generated by the magnet arrangement on the outside, remains as small as possible.
U.S. Pat. No. 4,926,289 shows an alternative approach, in which an actively shielded superconducting magnet system having a radially inner and a radially outer coil system which are connected in a superconducting manner, is described, wherein a superconducting short-circuit with a current limiter is provided between the inner and the outer coil system in order to keep the difference current in a reasonable range. Compensation of external disturbances is made possible in that the superconducting current limiter enables a shift of the current distribution between the radially inner and the radially outer superconducting current path between the two coil systems. In case of a Quench, the small current carrying capacity of the differential current limiter is to ensure that the external stray field generated by the magnet arrangement remains small.
The problem of compensation of external magnetic field disturbances with actively shielded magnet systems is thus solved by the above-described prior art either by means of one or several compensation coils provided in addition to the field coils or by dividing the coil system into superconductingly short-circuited sections, in such a manner that homogeneous outer magnetic field disturbances in the working volume of the respective magnet system are reduced to less than 20% in the long-term behavior.
In contrast thereto, it is the object of the present invention to improve also the short-term behavior of such a magnet system in case external disturbances occur and to demonstrate new approaches for optimizing the dynamic disturbance behavior which require as little technical expense as possible, in particular with respect to the equipment.
According to the invention, this object is achieved in that sections of the current paths short-circuited via superconducting switches are bridged via ohmic resistors and that said sections and the ohmic resistors are chosen so that the magnetic field disturbance in the working volume is reduced already immediately after its occurrence to a value smaller than the absolute remaining value in the long-term behavior of less than 20% of the external magnetic field disturbance, and that subsequently the magnetic field disturbance in the working volume approaches monotonously the remaining value of less than 20% of the external magnetic field disturbance with a time constant &tgr;≧2 sec, preferably &tgr;≧5 sec, in particular &tgr;≈20 sec.
By means of a finer division of the current paths into many sections, the short-term behavior of the compensation of disturbances is considerably improved as compared with known magnet systems. Furthermore, the bridging of sections of the superconducting current paths by means of resistors in most known magnets is already an integral component as protection against thermal damages in case of a quench. In order to optimize the disturbance behavior of real magnets, however, it is imperative that the influence of said resistors be taken into account. It is pointless to have a magnet compensating disturbances perfectly after a long time but massively overcompensating or undercompensating said disturbances directly after they occur.
Quite often field disturbances occur only sporadically and disappear again after a short time. With such disturbances the long-term behavior of the disturbance compensation has only little importance, however, the short-term behavior is important. Owing to its optimized short-term behavior, the inventive disturbance compensation has in exactly this case, considerable advantages with respect to prior art.
The inventive compensation of disturbances is particularly advantageous if it is operated together with an NMR lock. The NMR lock is an extremely accurate field stabilizer which is, unfortunately, very sluggish and thus cannot react to quick field disturbances. At this point the compensation of disturbances comes into action through its capability of compensating just these and thus presents an ideal supplement to the NM R lock.
The fact that the sections are not short-circuited in a superconducting manner, like it is partly the case in prior art, but with ohmic resistors, has the advantage that it is easier to realize technically. In the case of a magnet with e.g. 20 additional superconducting switches instead of resistors, much too much heat, e.g. during charging, if all switches are open, would be generated in the heaters. The quench behavior would also be a problem if e.g. only the shielding would quench, which would result in an excessive stray field, and the field drift of the magnet could be increased. Furthermore, one can expect large cost savings by using resistors instead of superconducting switches.
The above-mentioned advantages of the invention are useful in particular with sensitive systems. For this reason, in a preferred embodiment, the magnet arrangement according to the invention is part of an apparatus for high-resolution magnetic resonance spectroscopy, e.g. in the field of NMR, ICR or MRI.
NMR apparatus comprise normally a means for stabilizing the magnetic field generated in the working volume. This device utilizes a lock coil for correction as well as an NMR
Bovier Pierre-Alain
Eckert Daniel M.
Schauwecker Robert
Bruker AG
Patidar Jay
Shrivastav Brij B.
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