Dimensioning of magnet arrangement comprising an additional...

Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Superconductive type

Reexamination Certificate

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C324S319000

Reexamination Certificate

active

06680662

ABSTRACT:

This application claims Paris Convention priority of DE 100 41 672.1 filed Aug. 24, 2000 the complete disclosure of which is hereby incorporated by reference.
BACKROUND OF THE INVENTION
The invention concerns a 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, and with one further current-carrying coil system which can be fed via an external current source to produce a magnetic field in the working volume which is substantially different from zero, in particular a magnetic field of an amount >0.2 millitesla per ampere current, and optionally with one or more additional superconductingly closed current paths, wherein the magnetic fields in the z direction produced by induced currents through the additional current paths during operation and the field of the current-carrying coil system in the working volume do not exceed 0.1 Tesla.
A magnet arrangement of this type comprising a superconducting magnet coil system and a further coil system fed via an external current source, however, without additional superconductingly closed current paths, is known e.g. from the EPR (Electron Paramagnetic Resonance) system ELEXSYS E 600/680, distributed since 1996 by the company Bruker Analytik GmbH, Silberstreifen, D-76287 Rheinstetten (company leaflet).
Superconducting magnets are used for different applications, in particular, different magnetic resonance methods. Some of these methods require modulation of the field strength in the working volume during an experiment. In particular, the use of a superconducting magnet has considerable disadvantages if the field modulation is produced through variation of the current in the main coil system. The main coil system typically has a high self-inductance and therefore permits only slow current and field changes.
Connection of current feed lines from the room temperature region to the cooled superconducting magnet during operation disadvantageously affects the cooling of the superconducting magnet coil system. If the region within which the magnetic field strength is to be modulated is not too large (in particular smaller than 0.1 Tesla), field modulation can be produced through varying the current in a coil system which supplements the main coil system.
A further field of use of field-generating additional coils in a superconducting magnet system are so-called superconducting Z
0
shim devices. A current change in such a device compensates for a drift in the main coil system over a certain period of time, without having to reset the current in the main coil.
The main focus of the invention is the dimensioning of magnet arrangements having an additional current-carrying coil system which can be fed via an external current source to produce a magnetic field in the working volume which is substantially different from zero, in particular, the dimensioning of magnet arrangements having a superconducting magnet with active stray field compensation and further superconducting current paths.
An additional field-producing coil system in a magnet arrangement must produce a relatively strong field while occupying as little space as possible. To achieve the required field strengths, an additional field-producing coil system must frequently be disposed close to the working volume of the magnet arrangement. This produces undesired “expansion” of the superconducting coil system and associated increased costs.
In contrast thereto, it is the underlying purpose of the present invention to modify a magnet arrangement of the above-mentioned type with as simple means as possible such that an additional field-producing coil system can be integrated in the magnet arrangement which “expands” the main coil system to a lesser extent while nevertheless maintaining the required functions.
SUMMARY OF THE INVENTION
This object is achieved in accordance with the invention in that the efficiency of the additional field-generating coil system is improved by utilizing the interaction between the additional field-generating coil system and the remaining magnet arrangement to produce the field. In addition to inductive couplings between the superconducting magnet coil system and further superconductingly closed current paths, an arrangement in accordance with the invention also uses the diamagnetic behavior of the superconducting material in the superconducting magnet coil system, which is characterized in that field changes of less than 0.1 Tesla, which occur e.g. during charging of an additional field-generating coil system, are expelled from the superconducting volume portion of the magnet coil system.
This manifests itself in a redistribution of the magnetic flux of the field changes in the magnet arrangement which effects the reaction of the superconducting magnet coil system and the additional superconductingly closed current paths to a current change in the additional field-generating coil system, since this reaction is determined by the principle of conservation of the magnetic flux through a closed superconducting loop. The present invention utilizes the interaction between the additional field-generating coil system and the residual magnet arrangement for generating a field such that the variable g
D
eff
=g
D
−g
T
·(L
cl
−&agr;L
cor
)
−1
·(L
←D
cl
−&agr;L
←D
cor
) is calculated and the magnet arrangement is optimized such that |g
D
eff
|>1.2·|g
D
eff,cl
|, wherein

g
D
eff,cl
=g
D
−g
T
·(
L
cl
)
−1
·L
←D
cl
.
These variables have the following definitions:
g
D
eff
: Field contribution per ampere current of the additional field-generating coil system in the working volume taking into consideration the field contributions of the additional field-generating coil system itself and the field change due to currents induced in the superconducting magnet coil system and additional superconductingly closed current paths during charging of the additional field-generating coil system and taking into consideration the diamagnetic expulsion of small field changes from the volume of the magnet coil system,
g
D
eff,cl
: Field contribution per ampere current of the additional field-generating coil system in the working volume taking into consideration the field contributions of the additional field-generating coil system itself and the field change due to currents induced in the superconducting magnet coil system and in additional superconductingly closed current paths during charging of the additional field-generating coil system while neglecting the diamagnetic expulsion of small field changes from the volume of the magnet coil system,
−&agr;: average magnetic susceptibility in the volume of the magnet coil system with respect to field changes which do not exceed the amount 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, which react inductively to flux changes, and the magnet coil system,
g
M
: Field per ampere of the magnet coil system in the working volume without the field contributions of additional current paths which inductively react to flux changes,
g
D
: Field per ampere of the additional field-generating coil system in the working volume without the field contributions of additional current paths, which react inductively to flux changes, and of the magnet coil system,
L
cl
: Matrix of the inductive couplings between the magnet coil system and additional current paths which react inductively to flux changes, and among these 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
←D
cl
: Vector of inductive couplings of the additional field-generating coil system with the magnet coil

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