Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Superconductive type
Reexamination Certificate
2001-08-17
2002-12-17
Barrera, Ramon M. (Department: 2832)
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Superconductive type
C335S301000, C324S319000, C324S320000
Reexamination Certificate
active
06496091
ABSTRACT:
This application claims Paris Convention priority of DE 100 41 683.7 filed Aug. 24, 2000 the complete disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
The invention concerns a superconducting magnet arrangement for generating a magnetic field in the direction of a z axis in a working volume disposed about z=0, with a magnet coil system having at least one current-carrying superconducting magnet coil, a shim device with at least one superconducting shim coil and one or several additional superconductingly closed current paths, wherein the magnetic fields in the z direction produced by induced currents through the additional current paths during operation do not exceed a magnitude of 0.1 Tesla in the working volume, and wherein the shim device generates a field which varies along the z axis with the k
th
power of z for an even power of k>0.
A superconducting magnet arrangement with a Z
2
shim device of this type is part of e.g. the NMR Magnet System 600 SB UltraShield™ (company leaflet dated May 15, 1999) distributed by the company Bruker Magnetics.
U.S. Pat. No. 5,329,266 describes a superconducting magnet arrangement with an actively shielded magnet and additional superconducting current paths without a Z
2
shim device.
Superconducting magnets are used for different applications, in particular for magnetic resonance methods, in which the local homogeneity of the magnetic field is usually important. One of the most demanding applications is high-resolution nuclear magnetic resonance spectroscopy (NMR spectroscopy). The basic homogeneity of the superconducting magnet can be optimized through the geometrical arrangement of the field-generating magnet coils. In demanding applications, the basic homogeneity of the magnet is usually insufficient due to deviations from the design caused by production tolerances. To compensate for residual inhomogeneities of the magnet, the magnet system is equipped with autonomous superconducting coils which compensate for field inhomogeneities having a certain geometrical symmetry in the working volume, so-called shim devices. Examples of such shim devices are so-called Z
n
shims which produce a field which varies along the magnet axis z as z
n
. The main focus of the invention is the dimensioning of superconducting Z
n
shims in magnet systems with active stray field shielding and additional superconducting current paths, e.g. to compensate for external field fluctuations.
The field contribution of a superconducting Z
n
shim in a working volume must be substantially zero in the working volume at z=0, irrespective of the current in the shim coils, thereby taking into consideration the field contributions of the shim coils themselves and also of the field change due to currents induced in the superconducting magnet and in further superconductingly closed current paths during charging of the shim device. Dimensioning of a Z
n
shim according to a conventional method will produce, in certain cases, an undesired shift of the magnetic field strength in the working volume at z=0 during charging of the shim device. In actively shielded magnets, this behavior is particularly clear with shim devices whose coils are distributed over different radii because the conventional methods for dimensioning superconducting shim devices treat the superconductor as a non-magnetic material. The present invention also takes into consideration that the superconductor substantially exhibits a diamagnetic material behavior in response to field fluctuations of less than 0.1 Tesla, which can e.g. occur in the magnet volume during charging of a superconducting shim device, and thereby largely expels small field fluctuations from its inner regions. This results in a redistribution of the magnetic flux of the field fluctuations in the magnet arrangement which, in turn, influences the reaction of the superconducting magnet and further superconductingly closed current paths to a current change in the shim device, since this reaction is determined by the principle of magnetic flux conservation through a closed superconducting loop.
In contrast thereto, it is the underlying purpose of the present invention to modify a superconducting Z
n
shim in a magnet arrangement of the above-mentioned type in as simple a manner as possible such that the shim device is correctly dimensioned while taking into consideration the diamagnetism of the superconductor such that, in particular, its field contribution in the working volume is substantially zero in the working volume at z=0, irrespective of the current in the shim coils.
SUMMARY OF THE INVENTION
This object is achieved in accordance with the invention by designing the shim device such that the variable g
S
eff=g
S
−g
T
·(L
cl
−&agr;L
cor
)
−1
·(L
←S
cl
−&agr;
L←S
cor
) is substantially zero if and only if the variable g
S
eff,cl
=g
S
−g
T
·(L
cl
)
−1
·L
←S
cl
, which would have resulted were &agr;=0, is larger than zero, in particular larger than 0.2 millitesla per ampere.
The above variables have the following definitions:
g
S
eff
: Field contribution per ampere current of the shim device in the working volume at z=0 thereby taking into consideration the field contributions of the shim coils themselves and of the field change due to currents induced in the superconducting magnet coil system and in the further superconductingly short-circuited current paths during charging of the shim device,
−&agr;: average magnetic susceptibility in the volume of the magnet coil system 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 a current path P
j
in the working volume without the field contributions of current paths Pi for i≠j and of the magnet coil system and without the field contributions of the shim device,
g
M
: Field per ampere of the magnet coil system in the working volume without the field contributions of the additional current paths and without the field contributions of the shim device,
g
S
: Field per ampere of the shim device in the working volume without the field contributions of the additional current paths and of the magnet coil system,
L
cl
: Matrix of inductive couplings between the magnet coil system and the additional current paths and among the additional current paths,
L
cor
: Correction for the inductance matrix L
cl
, which would result with complete diamagnetic expulsion of disturbing fields from the volume of the magnet coil system,
L
←S
cl
: Vector of the inductive couplings of the shim device with the magnet coil system and the additional current paths,
L
←S
cor
: Correction for the coupling vector L
←S
cl
, which would result for complete diamagnetic expulsion of disturbance fields from the volume of the magnet coil system.
According to prior art, correct dimensioning of a Z
n
shim entails correct calculation of the field efficiencies g
S
, g
p1
, . . . , g
Pn
and g
M
of the shim device, of the additional superconducting current paths and of the magnet (without taking into consideration the respective reactions of the other components) and the mutual inductive couplings among the shim device, the additional current paths and the magnet as well as all self-inductances, wherein the shim device is then designed such that the variable g
S
eff,cl
=g
S
−g
T
·(L
cl
)
−1
·L
←S
cl
is substantially zero. When dimensioning the shim device of an arrangement in accordance with the invention, in addition to the above-mentioned coil properties, the magnetic shielding behavior of the superconducting volume portion of the magnet coil system is also taken into consideration. For this reason, the shim device is dimensioned such that instead of the variable g
S
eff,cl
the variable g
S
eff
=g
S
−g
T
·(L
cl
−&agr;L
cor
)
−1
·(L
←S
cl
−&agr;L
←S
cor
) is substantial
Amann Andreas
Bovier Pierre-Alain
Schauwecker Robert
Tschopp Werner
Barrera Ramon M.
Bruker AG
Vincent Paul
LandOfFree
Dimensioning of a superconducting shim device in a... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Dimensioning of a superconducting shim device in a..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Dimensioning of a superconducting shim device in a... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2969247