Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Superconductive type
Reexamination Certificate
2001-08-17
2002-11-05
Barrera, Ramon M. (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Superconductive type
C335S301000, C324S319000, C324S320000
Reexamination Certificate
active
06476700
ABSTRACT:
This application claims Paris Convention priority of DE 100 41 677.2 filed Aug. 24, 2000 the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The invention concerns a superconducting magnet system for generating a magnetic field in the direction of a z axis in a working volume disposed about z=0, with at least one current-carrying magnet coil and at least one additional, superconductingly closed current path, which can react inductively to changes of the magnetic flux through the area enclosed by same, wherein the magnetic fields generated in the z direction in the working volume by these additional current paths during operation due to induced currents do not exceed 0.1 Tesla. The invention also concerns a method for dimensioning these additional current paths.
A device of this type is disclosed e.g. in U.S. Pat. No. 4,974,113-A.
Superconducting magnet arrangements of this type comprising actively shielded magnets are disclosed e.g. in U.S. Pat. No. 5,329,266 or U.S. Pat. No. 4,926,289.
Superconducting magnets are used for different applications, in particular, magnetic resonance methods, wherein the stability of the magnetic field over time is usually important. The most demanding applications are high-resolution nuclear magnetic resonance spectroscopy (NMR spectroscopy). Field fluctuations with time can be caused by the superconducting magnet itself and also by its surroundings. While modern magnet and conductor technology can produce fields which are very constant with time, there is still need for development in the field of suppression of external magnetic disturbances. We will describe means for counteracting these disturbances. The main focus thereby is disturbance compensation with superconducting solenoid magnets having active stray field shielding.
U.S. Pat. No. 4,974,113 describes i.a. a compensating superconducting solenoid magnet, however, without active shielding. At least two independent superconducting current paths are constructed using two coaxial superconducting solenoid coils and calculated such that external magnetic field disturbances occurring inside the arrangement are suppressed to a residual value in long-term behavior of not more than 20% of the original disturbance, thereby taking into consideration conservation of total magnetic flux for each closed superconducting current path. U.S. Pat. No. 4,974,113 further describes a method for calculating the disturbance behavior for such arrangements which is based on the principle of conservation of magnetic flux through a closed superconducting loop.
U.S. Pat. No. 5,329,266 describes an application of this idea to an actively shielded magnet system. A plurality of shielding, structured compensation coils are connected in superconducting series and have a current carrying capacity which is low compared to that of the main coils (on the order of at most one ampere) to ensure that, in case of a superconducting breakdown (=quench), the disturbance field outwardly radiated by the magnet arrangement remains as small as possible.
U.S. Pat. No. 4,926,289 shows an alternative approach which describes an actively shielded superconducting magnet system with a radially inner and a radially outer superconductingly short-circuited coil system, wherein a superconducting short-circuit with limited current carrying capacity is provided between the inner and the outer coil system, such that the current difference between the two coil systems is limited. To compensate for external disturbances, the superconducting current limiter between the two coil systems can produce a shift in the current distribution between the radially inner and the radially outer superconducting current path. In case of a quench, the small current carrying capacity of the current limiter ensures that the external stray field produced by the magnet arrangement remains small.
If additional current paths are dimensioned according to the above-mentioned teaching, the desired compensation effect is difficult to obtain in certain cases. With actively shielded magnets having only one individual superconductingly short-circuited current path, the observed disturbance behavior differs considerably from that calculated according to the above cited prior art. The reason therefor is that, in conventional methods for calculation of the disturbance behavior of a superconducting magnet arrangement, the superconductor is treated as non-magnetic material. The present invention also takes into consideration the fact that the superconductor mainly behaves as a diamagnetic material with respect to field fluctuations of less than 0.1 Tesla and thereby largely expels small field fluctuations from its volume. This results in a redistribution of the magnetic flux of the field fluctuations in the magnet arrangement which then influences the reaction of the superconducting magnet and additional superconductingly closed current paths to an external disturbance, since this reaction is determined by the principle of conservation of the magnetic flux through a closed superconducting loop.
In contrast thereto, it is the object of the present invention to modify a magnet arrangement of the above mentioned type with as easy and simple means as possible such that the disturbance behavior of the magnet system is corrected to an optimum degree by taking into consideration the diamagnetism of the superconductor. The object of the present invention is thereby not limited to modifying a magnet arrangement of the above mentioned type such that external field fluctuations in the working volume of the magnet arrangement are largely suppressed. Arrangements can also be designed which either amplify or weaken external field fluctuations to a certain degree. Such applications are desired e.g. when the external field fluctuation is generated by field modulation coils whose effect in the working volume should be as strong as possible.
SUMMARY OF THE INVENTION
This object is achieved in accordance with the invention in that the magnet coil(s) and the additional current path(s) are designed such that, in response to an additional disturbance coil which generates a substantially homogeneous disturbance field in the magnetic volume, the value &bgr; (that factor by which the disturbance is increased or weakened by the reaction of the magnet) is calculated according to
β
=
1
-
g
T
·
(
(
L
cl
-
α
L
cor
)
-
1
(
L
←
D
cl
-
α
L
←
D
cor
)
g
D
)
if and only if this value differs by more than 0.1 from a value
β
0
=
1
-
g
T
·
(
(
L
cl
)
-
1
L
←
D
cl
g
D
)
which would result if &agr;=0.
The above variables have the following definitions:
−&agr;: average magnetic susceptibility in the volume of the magnet coil(s) with respect to field fluctuations which do not exceed a magnitude of 0.1 T, wherein 0<&agr;≦1,
g
T
=(g
M
, g
P1
, . . . , g
Pj
, . . . g
Pn
),
g
Pj
: field per ampere of the current path Pj in the working volume without the field contributions of the current paths Pi for i≠j and the magnet coil(s),
g
M
: field per ampere of the magnet coil(s) in the working volume without the field contributions of the current paths,
g
D
: field per ampere of the disturbing coil in the working volume without the field contributions of the current paths and the magnet coil(s),
L
cl
: matrix of the inductive couplings between the magnet coil(s) and the current paths and among the current paths,
L
cor
: correction for inductance matrix L
cl
, which would result with complete diamagnetic expulsion of disturbance fields from the volume of the magnet coil(s);
L
←D
cl
: vector of inductive couplings between the disturbance coil and the magnet coil(s) and current paths;
L
←D
cor
: correction for the coupling vector L
←D
cl
, which would result with complete diamagnetic expulsion of disturbance fields from the volume of the magnet coil(s).
To improve the disturbance behavior of the magnet, additional current
Amann Andreas
Bovier Pierre-Alain
Schauwecker Robert
Tschopp Werner
Barrera Ramon M.
Bruker AG
Vincent Paul
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