EP0215043B1 - Method for making a coil of a solenoidal magnet - Google Patents

Abstract

Method for making a coil of the Bitter-type by indium weldings. According to the invention, Bitter discs or parts thereof (12) are welded with indium, preferably deposited by electrolysis on the portions (14, 15) to be assembled. Application to NMR.

Description

The invention relates to a method of manufacturing a Bitter type coil, more particularly forming a large magnet capable of being used in a nuclear magnetic resonance (NMR) imaging installation; the invention also relates to a solenoidal magnet comprising at least one coil of the Bitter type constructed by implementing this method.

It is known that NMR imaging installations require a large magnet capable of generating a uniform magnetic field in a determined 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 at least 40 cm in diameter.

Furthermore, Bitter coils are well known for the production of strong magnetic fields. The structure proposed by Bitter is a coil made up of metallic annular discs (generally made of copper or aluminum), split to form as many turns and connected to define a substantially helical winding with flat turns. The stack of discs is maintained by a plurality of tie rods. This structure is advantageous because it allows efficient cooling of the magnet, by making turns in the discs (and in the insulators separating these discs), these holes being arranged in the same configuration from one disc to another to materialize as a set of channels parallel to the axis of the coil, in which circulates a cooling fluid, for example deionized water, kerosene or oil.

It is possible to calculate a magnet delivering a magnetic field of homogeneity required in a certain volume in the vicinity of its center of symmetry and consisting of a number of such Bitter coils arranged along a common longitudinal axis. Methods of calculating such magnets are for example explained in other patent applications of the Applicant. It is also known to weld metal parts together by low temperature welds. But the problem to be solved here amounts to avoiding the numerous short-circuits caused by an overall soldering of an entire coil at once. Application FR-A-2 175 530 uses, for example, inter alia, indium to perform welds.

The invention mainly relates to a method of manufacturing a Bitter type coil consisting in forming a helical winding of turns in the form of split flat metal annular discs, in producing a contiguous stack of such discs, and in connecting end to end at the other ends of such discs or of parts of such discs by welding with addition of indium, characterized in that the addition of indium is carried out prior to welding by electrolytic deposition on portions to be assembled.

The indium welding can be carried out at relatively low temperature (of the order of 200 ° C.) to avoid deformation of the discs or portions of discs, which makes it possible to obtain a particularly regular winding with contiguous turns. In addition, indium has the additional advantage of excellent electrical conductivity.

The welding then takes place by heating said portions in the assembly position, for example by means of an "HF turn". According to a possible embodiment, the portions to be assembled are provided with dimples of complementary shapes and dimensions and the deposition of indium is carried out on these dimples.

• - la figure 1 représente une partie de disque utilisée pour la réalisation d'une bobine de type Bitter;
• - la figure 2 est une vue en détail ilustrant l'assemblage de deux parties de disque semblables, après le dépôt d'indium et avant la soudure desdites parties.

The invention will be better understood and other advantages thereof will appear better in the light of the description which follows, given solely by way of example and made with reference to the appended drawing in which:

• - Figure 1 shows a disc part used for the production of a Bitter type coil;
• - Figure 2 is a detail view illustrating the assembly of two similar disc parts, after the deposition of indium and before welding of said parts.

Referring to the drawings, there is shown the basic structure used according to the invention, for producing a large Bitter coil usable, in particular, in an NMR imaging installation to generate a uniform magnetic field of high homogeneity. Each coil consists of the end-to-end assembly of parts 12 of flat annular discs. Each disc part 12 comprises, in the example shown, opposite recesses 14, 15, of complementary shapes and dimensions, at each of its ends respectively. Thus, the recess 14 of a disc part can be assembled by welding to the recess 15 of another neighboring disc part and so on with the interposition of insulation between the turns, until a coil of Complete bitter. As shown, each disc part 12 comprises all 16 in a predetermined configuration which, with the corresponding holes in the disc parts of the other turns, reconstitute the coolant circulation channels parallel to the axis of the coil. Others all 17 of larger diameter are also provided in each disc part for the passage of the tie rods ensuring the winding with contiguous turns, according to the conventional technique defined by Bitter.

According to the invention the welds of the disc parts 12 are carried out with the addition of indium by heating to a relatively low temperature, which avoids deformations of the Bitter discs whose thickness is only around 2 mm for a diameter. outside of the order of a meter. We can for example interpose a thin indium plate between the dimples in the assembly position and heat to melt indium. O noted, however, that the positioning accuracy of the parts to be assembled and the quality of the weld were significantly improved if indium 18 was previously electrolytically deposited on the surfaces to be assembled, i.e. here on the surfaces of the recesses 14 and 15. The electrolytic deposition of indium on a piece of copper is within the reach of those skilled in the art. It will preferably be carried out after electrolytic polishing of the portions comprising said recesses. The pre-cutting of the disc parts is such that, as shown, each of them represents a fraction of turn of the coil, here close to a third of turn. More precisely, the portions of discs are all identical and each represents a not whole fraction of a complete turn (that is to say different from a half, a third, a quarter, ...) so that the zones welded from the stack are distributed in a helix, and not grouped along one or more generators of the coil.

Claims (8)

1. A method for the manufacture of a Bitter-type coil consisting of forming a helical winding of turns in the form of flat split annular metallic disks, of producing a joined stack of such discs and of connecting the ends of the disks, or parts (12) of such disks, end to end by welding with the application of indium, characterized in that the application of indium is effected prior to the welding by electrolytic deposition on the portions (14 and 15) to be assembled.

2. The method as claimed in claim 1, characterized in that the portions to be assembled each comprise step joint parts with complementary forms and dimensions, and the method consists in placing the indium on at least one of the said steps prior to effect welding by heating the said portions in the position of assembly.

3. The method as claimed in either of the preceding claims, characterized in that annular disk parts are successively assembled to represent less than one turn.

4. The method as claimed in claim 3, characterized in that equal parts of annular disks are assembled, each of the parts differing by a fraction of a complete turn, one entire fraction being a part corresponding to a complete turn divided a whole number of times so that the welded zones of the said stack are distributed in accordance with a helix.

5. A solenoid magnet of the Bitter type with flat annular disks, comprising a helical winding with turns in the form of annular, metallic, flat, split, stacked disks or portions of disks stacked with joins therebetween connected together at their ends by welding with the application of indium, characterized in that at each welded joint the indium is present in the form of a uniform layer resulting from the fusion of two electrolytic deposits (18) of indium on the assembled ends.

6. The solenoid magnet as claimed in claim 5, characterized in that the indium covers joint steps (14 and 15) with complementary forms and dimensions produced at the ends of the assembled parts.

7. The solenoid magnet as claimed in either claim 5 or claim 6, characterized in that it is produced by the end to end assembly of equal parts of annular flat disks, each representing less than one turn, and preferably about a third of a turn.

8. The solenoid magnet as claimed in claim 9, characterized in that each annular disk part represents a non-integral fraction of a complete turn, one non-entire fraction being opposite to an integral fraction which corresponds to a complete turn divided by a whole number so that the welded zones of the solenoid magnet are distributed in accordance with a helix.

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