Electricity: magnetically operated switches – magnets – and electr – Magnets and electromagnets – Magnet structure or material
Patent
1987-01-06
1988-05-17
Harris, George
Electricity: magnetically operated switches, magnets, and electr
Magnets and electromagnets
Magnet structure or material
335300, H01F 500
Patent
active
047453877
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention, due to the collaboration of the Service National des Champs Intenses of the CNRS (Director M. AUBERT), relates to an ironless solenoidal magnet having one or more coils whose technological structure is closely related to that of a conventional Bitter coil, the invention relates more particularly to improvements for simplifying the manufacture of the coil or coils and improving the homogeneity of the magnetic field generated by such a type of magnet.
Bitter coils are well known for producing intense magnetic fields. In theory, the structure proposed by Bitter is a coil formed of annular metal disks split so as to form as many turns and connected together so as to define a substantially helical winding with flat turns. The stack of disks is held in position by a plurality of tie rods. This structure is advantageous for it allows efficient cooling of the magnet by forming holes in the disks (and in the insulators separating these disks), these holes being disposed in the same configuration from one disk to another so as to materialize an assembly of channels parallel to the axis of the coil, in which flows a cooling fluid, for example deionized water, kerosene or oil.
The invention provides a magnet formed of at least one coil derived from this concept and more particularly designed so that the magnetic field generated in a sphere of interest of prescribed radius, whose center merges with the center of symmetry of this magnet, has a very good homogeneity. A privileged field of application of the invention is in fact that of NMR (nuclear magnetic resonance) image formation where it is necessary to have a relatively high magnetic field (0.15 to 1.5 teslas) with a very high homogeneity, of the order of 1 to 10 parts per million (ppm). With a sufficiently long coil, a certain homogeneity can be obtained about the center of symmetry of this coil. Such homogeneity will be more easily attained and with a more compact structure either by varying the thickness of the disks along the axis of the magnet or by aligning several Bitter coils along a common axis, the lengths of the coils and the spacing therebetween being chosen so as to provide the required homogeneity. These solutions are described in other patent applications filed by the applicant. The improvements of the invention apply not only to a magnet having a single coil but also to a magnet having several aligned coils, disposed side by side or spaced apart.
There may in fact exist other structural causes of inhomogeneity of the magnetic field generated or causes of disturbance of this magnetic field.
2. Description of the Prior Art
Thus, in one widely used embodiment of the Bitter coil, the connection of two adjacent turns is simply obtained by forming each insulating disk, inserted between the two conducting rings, so that it has a cut out in the form of a sector and by clamping the stack of conducting disks and insulating disks between two end plates, by means of the above mentioned tie rods. The electric contact between two adjacent turns is thus established through the corresponding cut out under the effect of the clamping, the construction of the magnet being greatly facilitated thereby. However, the fact of facing up to the problem of obtaining a very uniform field from coils of this kind leads to recognizing in such an arrangement another cause of disturbance of the magnetic field. In fact, the variation of current density in each turn in the contact sector is an intrinsic cause of inhomogeneity.
SUMMARY OF THE INVENTION
The invention provides firstly a new type of assembly of disks for overcoming this problem.
To this end, the invention relates then essentially to a solenoidal magnet of the type having at least one coil formed of a stack of annular conducting disks with interpositioning of insulators, each disk having a cut out transforming it into a turn and said turns being connected end to end, wherein said disks extend in respective parallel planes perpendicular to the longitu
REFERENCES:
patent: 4591818 (1986-05-01), Butson
patent: 4623864 (1986-11-01), Inoue et al.
Centre National de la Recherche Scientifique
Harris George
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