Active shielded superconducting assembly with compensation...

Electrical transmission or interconnection systems – Anti-induction or coupling to other systems – Magnetic or electrostatic field control

Reexamination Certificate

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C335S216000, C335S299000, C335S301000, C361S019000

Reexamination Certificate

active

06369464

ABSTRACT:

The invention relates to an actively shielded superconducting magnet arrangement for the generation of a magnetic field in the direction of a z-axis in a working volume, arranged about z=0, having two coil systems, a radially inner and a radially outer one, wherein both coil systems carry approximately the same current and have dipole moments of approximately the same magnitude, but opposite in direction, wherein at least parts of the radially outer coil system are electrically connected in series with the radially inner coil system, and wherein a first superconducting switch is provided which can cause, in the operating state, a superconducting short-circuit of a first current path of the magnet arrangement.
An actively shielded superconducting magnet arrangement of this type is known from U.S. Pat. No. 5,426,366.
The field of use of superconducting magnets comprises various fields of application, in particular magnet resonance methods, in which in general the stability with time of the magnetic field is important. The most demanding applications comprise 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 in the case of 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 closed superconducting current path.
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 magnetic 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 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 their long-term behaviour.
The initially cited U.S. Pat. No. 5,426,366 describes a further improvement with respect to the magnet system according to U.S. Pat. No. 4,926,289. According to this paper, the differential current limiter is to be designed as partial short-circuit through a section of the radially inner coil system. The fact that the connection points of the differential current limiter do not extend between the two ends of the radially inner coil system, as is the case with U.S. Pat. No. 4,926,289, but that connecting points can be placed at a distance from the coil ends over a partial section of the radially inner coil system, achieves additional flexibility for the optimization of the compensation of disturbances of the system. Further flexibility may be gained by dividing the differential current limiter into several partial sections above the radially inner coil system.
However, a disadvantage of the magnet system according to U.S. Pat. No. 5,426,366 consists in that the partial section of the radially inner coil system is inductively coupled with the rest of the magnet system. This coupling can lead to undesired side effects like an increase of the magnet drift or uncontrolled charging of the section (leading to distortions of homogeneity with time, possible increase of the stray field, charging up to quenching). As preventive measures for these problems, a current limiter is introduced. However, the current limiter has the disadvantage that it can reduce to zero flowing current. The decay of this current is a disturbance that the magnet cannot compensate.
In contrast thereto, it is the object of the present invention to modify a magnet arrangement of the initially mentioned type by simple measures such that the above-mentioned disadvantages are avoided.
According to the invention, this object is achieved in that a section of the radially outer coil system, which during operation can be short-circuited in a superconducting manner via a further superconducting switch, is arranged symmetrically to the plane z=0 and that the further superconducting current path formed by the superconductingly short-circuited section of the radially outer coil system is essentially inductively decoupled from the other parts of the magnet arrangement which are connected in series.
In case there are several inductively coupled superconducting current paths present in a superconducting magnet system, undesired side effects may occur like an increase of the magnet drift, uncontrolled charging of a current path and resulting distortion with time of the homogeneity, a possible increase of the stray field, charging up to quenching, dissipation of energy in switches when charging the magnet system, large excess currents in a current path in case of a quench of a different current path etc. These problems are solved according to the invention by the mutual inductive decoupling of the superconducting current paths. A section inductively decoupled from the rest of the magnet is typically located in the innermost layers of the outer coil system. Thus, the undesired side effects of the coupling are eliminated and the current limiter provided according to prior art is no longer required.
In reality one has always to take into account a remaining coupling due to production inaccuracies. Therefore, in case of a quench of one of the superconducting current paths, it is nevertheless possible to induce a current into a different current path. In order to avoid high electromagnetic forces in case of a quench, the section has to have the same plane and axis of symmetry as the entire magnet.
The superconductingly short-circuited section of the radially outer coil system may also be divided into several subsections. By means of a finer division of the current paths into many sections, the behaviour of the compensation of disturbances is furthe

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