Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Patent
1986-11-14
1988-09-27
Harris, George
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
336227, 324318, 335296, H01F 500
Patent
active
047744871
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention, due to the collaboration of the National Intense Field Service of the CNRS(director M. AUBERT) generally relates to a solenoidal magnet with high magnetic field homogeneity, formed of several coils spaced apart from each other; it relates more particularly to a structure for connection between the coils, avoiding the creation of parasite field components.
It is known that NMR image forming installations require a large sized magnet capable of generating a uniform magnetic field in a given region of space. Typically, it is necessary to generate a magnetic field of 0.15 to 0.5 teslas with a homogeneity of 1 to 10 parts per million (ppm) in a sphere of a diameter of 40 cm at least.
2. Description of the Prior Art
It is known to form such a magnet from an assembly of coils spaced apart from each other at chosen distances, along a common axis. In another patent application, the applicant describes a method for calculating the characteristics of such a magnet formed of coils all having the same inner and outer diameters and formed more particularly of Bitter type coils. In calculating such a magnet, it is assumed that no current flowing through the spaces between the coils is capable of creating a magnetic field. Now, the coils are connected together in series and connecting conductors necessarily pass through these spaces. The invention relates more specifically to a structure for connection between coils, arranged so as to avoid the formation of parasite field components between said coils.
SUMMARY OF THE INVENTION
In this spirit, the invention relates to a solenoidal magnet with high magnetic field homogeneity, formed of an assembly of coils of the same inner and outer diameters, spaced apart from each other by chosen distances along a common axis, said coils being connected in series, wherein two adjacent coils are connected together by groups of conductors symmetrical with respect to a plane passing through said axis and the ends of said adjacent coils so that the current components perpendicular to said axis in said groups of conductors create ampere turns in opposition, at every point of the space between said coils.
In a specific embodiment, two groups of above mentioned conductors are given material form by one or more circular conducting rings, each group thus having the ring part situated on each side of said plane. This or (preferably) these rings are arranged substantially transversely to the axis while being slightly deformed so as to define, each one, two substantially helicoidal and opposite half turns. Furthermore, this or these rings are connected to the ends of said coils on the one hand and/or together on the other by successively diametrically opposite junctions. In other words, if a single ring is used (case of relatively small spacing between two adjacent coils) it is connected to the two ends of the two adjacent coils at diametrically opposite points. If, on the other hand, several rings are used connected end to end for passing through the space between two coils, then the first ring is connected to one coil end at a first point and to an adjacent ring at a second diametrically opposite point and so on until connection of the last ring with the end of the other coil.
With the above described structure, it may be estimated as a first approximation that the magnetic field components generated by the currents which flow in the groups of conductors such as defined above cancel each other out mutually. There remains a longitudinal component of the current directed along the common axis of the coils, which depends more especially on the helical pitch of the connecting conductors. This current component is always small (especially if the number of rings connected end to end is increased) and creates little field. If the circumstances so require, it may also be offset by using the current return to the power supply source, that is to say by causing this return current to pass through at least each space between two coils, t
REFERENCES:
patent: 3227930 (1966-01-01), Hnilicka
Journal of Physics E: Scientific Instruments, vol. 6, No. 4, Apr. 1973, London, GB, C. B. Wheeler et al: "Multiply Crowbarred Solenoids for Plasma Research", pp. 332-338.
Harris George
Thomson-CGR
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