EP0203952A1

EP0203952A1 - Ironless solenoidal magnet.

Info

Publication number: EP0203952A1
Application number: EP85905842A
Authority: EP
Inventors: Guy Aubert
Original assignee: Thomson CGR
Current assignee: General Electric CGR SA
Priority date: 1984-12-14
Filing date: 1985-11-29
Publication date: 1986-12-10
Also published as: DE3574073D1; EP0203952B1; WO1986003881A1; FR2574982B1; FR2574982A1; US4808956A

Abstract

Aimant de Bitter à homogénéïté de champ améliorée. Selon l'invention, on utilise les tirants (119) de l'empilement de disques (113) pour ramener le courant vers l'une des extrémités axiales de l'aimant sans créer de champ parasite. Application à l'imagerie par RMN.Bitter magnet with improved field homogeneity. According to the invention, the tie rods (119) of the stack of discs (113) are used to bring the current to one of the axial ends of the magnet without creating a parasitic field. Application to NMR imaging.

Description

SOLENOIDAL MAGNET WITHOUT IRON

The invention relates to a solenoidal magnet, without iron, comprising one or more coils whose technological structure is derived from that of a conventional Bitter coil; the invention more particularly relates to improvements making it possible to improve the homogeneity of the magnetic field generated by such a type of magnet.

Bitter coils are well known for the production of strong magnetic fields. In theory, the structure proposed by Bitter is a coil made up of metallic annular discs, split to form as many turns and connected to define a substantially helical winding with flat turns. The stacking of the discs is maintained by a plurality of tie rods. This structure is advantageous because it allows efficient cooling of the magnet by making holes in the rings (and in the insulators separating these discs), these holes being arranged in the same configuration from one disc to another to materialize a set of channels parallel to the axis of the coil, in which circulates a cooling fluid, for example deionized water, eerozene or oil. The invention proposes to perfect such a type of magnet so that the magnetic field generated in a sphere of interest with a prescribed radius, the center of which coincides with the center of symmetry of this magnet, is of very good homogeneity. A preferred field of application of the invention is indeed that of nuclear magnetic resonance imaging (NMR) 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 Bitter coil, a certain homogeneity can be obtained around the center of symmetry of this coil. This homogeneity will be more easily achieved and with a more compact structure, either by varying the thickness of the discs along the axis of the coils or by aligning several Bitter coils along a common axis, the lengths of the coils and their spacings being chosen to achieve the required homogeneity. These solutions are the subject of other patent applications filed by the Applicant. The improvements according to the invention apply as well to a magnet with a single coil as with a magnet with several aligned coils.

There may indeed remain other structural causes of inhomogeneity of the magnetic field generated or causes of disturbance of this magnetic field. Among these, particular consideration must be given to the way in which the current is applied to the magnet. In fact, if the connection between the power source and the magnet is conventionally established by means of two conductors respectively connected to the axial ends of the magnet, field disturbances generated by these conductors can degrade the homogeneity of the field in the sphere of interest. The invention aims in the first place to solve this problem.

In this spirit, the invention therefore relates to a coiled magnet comprising at least one Bitter type coil, essentially consisting of a substantially helical winding materialized by a stack, with interposition of insulator, of annular discs, each comprising a cut materializing a turn, said turns being connected to each other, characterized in that it comprises one or more conductors ensuring the return of the current to one of the axial ends of the magnet, shaped (s) and / or disposed (s) for distributing the flow of said current longitudinally over a cylindrical surface coaxial with said coil.

This way of "bringing the current" parallel to the axis of the magnet and all around it does not create any field disturbance in the internal space of the magnet. In addition, thanks to the arrangement defined above, the current leads are located at the same axial end of the magnet. The supply wires can therefore be easily arranged in parallel in pairs to avoid creating any disturbance. Cables with a coaxial structure are preferably used. The realization of the concept stated above can result in a single tubular conductor surrounding the coil and connected to one of the axial ends thereof to bring the current. This arrangement also has the advantage of forming a sort of Faraday cage which, in the case of application to NMR imaging, protects the radiofrequency antennas from external disturbances. It is also possible to approximate this tubular casing by carrying out the current return by means of several longitudinal rods regularly distributed over said cylindrical surface, these rods being connected in parallel with one another and in series as a whole with said coil so as to be traversed by substantially equal fractions of the total current flowing through said coil. In this second possible embodiment, these rods are preferably the tie rods (or some of them) whose primary function is to maintain the stack of discs of the Bitter coil. -

On the other hand, according to a currently widespread embodiment of the Bitter magnet, the connection of two adjacent turns is simply obtained by shaping each disc of insulation, interposed between the two conducting rings, so that it comprises a sector-shaped cut and by clamping the stack of conductive discs and insulating discs between two end plates, by means of the tie rods mentioned above. The electrical contact between two adjacent turns is thus established through the corresponding cutout under the effect of tightening, the construction of the magnet being greatly facilitated. However, the fact of posing the problem of obtaining a very uniform field from Bitter coil (s) leads to recognize in this arrangement another cause of disturbance of the magnetic field. Indeed, the variation of current density at each turn in the contact sector is another cause of non-uniformity. In this spirit, the invention also relates to a magnet according to the definition above, characterized in that each end of the turn has a recess and that two adjacent turns are connected end to end by welding such recesses of complementary shapes. The invention will be better understood and other advantages of it will appear better in the light of the description which follows, given solely by way of example, and made with reference to the appended drawings in which:

- Figure 1 is a perspective view of a conventional Bitter coil;

- Figure 2 is a detail view of two adjacent turns illustrating one of the modifications of the Bitter coil;

- Figure 3 is a schematic view in longitudinal half-section of a magnet according to the invention, and. - The figure is a partial view of the magnet of Figure 3 according to section IV-IV thereof

FIG. 1 schematically represents an exploded perspective view of a conventional Bitter magnet 11 essentially used to produce an intense magnetic field inside the central hole 12 defined by a stack of annular conductive discs 13 (typically made of copper or aluminum) comprising each a cut 14, here a radial slot, transforming each disc 13 into a turn. The magnetic field generated is oriented along the axis zz '. Thin insulating discs 15 similar to discs 13 are interposed therebetween to isolate the turns. Instead of a fine cut, they each have a large sector-shaped cutout 17, to allow the turns to be connected by simply clamping the stack between two end plates such as 18, by means of a plurality tie rods 19 regularly distributed over a cylindrical surface coaxial with the axis zz '. The conductive discs 13 and the insulating discs 15 are pierced with holes 20 made in the same configuration from one disc to another, so as to define a plurality of channels parallel to the axis zz ', in which a fluid circulates. cooling. The concentration of holes 20 is more important towards the center of the disc because, in a Bitter coil, the current density at a point of a flat turn is inversely proportional to the distance from the point considered to the axis zz ¹ . The heating is therefore more important in the heart of the

- mass of conductors, hence the need to increase the number of cooling fluid channels in the vicinity of hole 12.

The magnet in FIGS. 2 to •> is derived from the conventional structure described with reference to FIG. 1. The similar structural elements are identified by the same reference numerals

IQ increased by 100. The coil of Bitter 111 therefore has a central hole 112, conductive discs 113 and insulating discs 115. The discs 113 and 115 have the same configuration of holes 120 and the stack of discs 113 and 115 is tight by tie rods

119 between two annular end plates 118a, 118b. The holes ι r 120 made in the discs 113 and 115 are in concordance to define a plurality of longitudinal channels 121, parallel to the axis zz '. In the example of FIG. 3, the general proportions of the coil have been adapted for application to NMR imaging. The external diameter and especially the internal diameter of the discs are

2 _Q increased to release sufficient volume to accommodate a lying human.

According to one of the modifications of the conventional structure described above, the adjacent turns are no longer connected by contact of the faces of the discs in the vicinity of the cuts but by welds

25 of the turns ends. The ends of each turn comprise for this purpose recesses of complementary shapes 122, 123 and two adjacent turns (see FIG. 2) are connected end to end by welding two recesses of complementary shapes. In Figure 2 an insulating disc 115 is shown between the two turns

30 adjacent connected end to end. In practice, the insulating disc is for example cut from a thin dielectric film, but it can be envisaged to remove it if the conductive discs are made of aluminum and the insulator is produced by anodizing these discs. When using cut-out insulating discs in a dielectric material, these are cut radially and connected end to end as and when the turns are welded.

Furthermore, according to an important characteristic of the invention, the magnet comprises one or more conductors ensuring the return of the current to one of the axial ends of the magnet, shaped (s) and / or arranged (s) to distribute the flow of said current longitudinally on a cylindrical surface coaxial with the coil or coils constituting the magnet. The configuration which, in theory, best meets this definition is a cylindrical tubular envelope 0, external to the coil, coaxial with the latter and connected by one of its axial ends to one of the end plates, for example the tray 118b. It is shown that the flow of current in this tubular envelope does not create a disturbing magnetic field in the central hole 112. The cylindrical surface

, ₅ coaxial above is in this case that of the tubular casing itself. However, this tubular casing can be replaced by a sufficient number of longitudinal rods, regularly distributed over a fictitious cylindrical surface 12Ψ, these rods being connected together so as to define a kind of cage

_2Q of squirrel and this cage being connected in series, as a whole, with said coil. Thus, these rods are traversed by substantially equal fractions of the total current flowing through the coil or coils. In the embodiment of figs 2 to 4, the tie rods 119 are used as current return rods. These tie rods

2 are isolated from the conductive discs 113 and it suffices that the tie rods are connected to the end plate 118b if the coil 111 is single or to the corresponding plate of the coil closest to the axial end of the magnet to which no cable power supply is not connected. In this case, the plate 118b ensures the

30 distribution of the current between the tie rods. On the other hand, if we consider the first axial end cited to which the current supply cables are connected (that is to say the end materialized by the end plate 118a if the coil 111 is single or if c 'East the coil closest to said first end) the tie rods 119 pass through the plate 118a while being isolated from the latter while this plate 118a comprises as many connection terminals 125 as there are tie rods, disposed respectively in the vicinity of each of them for allow the magnet to be powered from a set of pairs of conductive wires. In each pair, the conductive wires are arranged parallel to one another so as not to produce a stray field and in the example described, each pair of conductive wires in question forms a cable 126 with a coaxial structure. Thus, any disturbing magnetic field (which could have been created by a "loop" including the magnet and its connection wires if these had been respectively connected to the axial ends of the magnet) is eliminated in the vicinity of the magnet. The fact of using the tie rods (or the tubular casing) to bring the current towards an axial end of the magnet has the additional advantage of compensating for the axial component of the current which circulates in the Bitter coil, due to the pitch d winding propeller. This component is weak and does not create any field along the axis zz '. It modifies little the module of the field but only its orientation. The compensation of this longitudinal component of current by the currents which circulate in the tie rods thus brings back the orientation of the magnetic field along the axis zz '.

In addition, the connection structure illustrated in Figure 3 can be used for the passage of current between the coils if the magnet has more than one. In this case, all the end plates of the coils (except the one located at the other axial end of the magnet) are similar to the plate il 8a and two neighboring coils are connected by as many coaxial cables as there , there are tie rods.

The invention is not limited to the embodiment which has just been described. In addition, it should be noted that if we defined certain structural elements (a tubular envelope, rods or tie rods) as ensuring "the return" of the current, it was for better highlight the new function of these structural elements but it is obvious that the arrangement described above is protected in the claims which follow regardless of the conventional direction of the current flowing in the coil or coils and the elements aforementioned structure. The invention also covers all the technical equivalents of the means used if these are within the scope of the claims which follow.

Claims

1. Solenoid magnet comprising at least one Bitter type coil, essentially consisting of a substantially helical winding materialized by a stack, with interposition of insulator, of conductive discs (113) each comprising a cut materializing a turn, said turns being connected to each other, characterized in that it comprises one or more conductors (119) ensuring the return of the current to one of the axial ends of the magnet, shaped (s) and / or arranged (s) to distribute the flow of said current longitudinally on a cylindrical surface (124) coaxial with said coil (111).

2. solenoid magnet according to claim 1, characterized in that it comprises a single tubular conductor surrounding said coil, connected in series with respect thereto.

3. solenoid magnet according to claim 1, characterized in that it comprises several longitudinal rods (119) regularly distributed over said cylindrical surface (124) and in that these rods are connected together so as to define a sort of cage d squirrel, this cage being connected in series, as a whole, with said coil, so that said rods are traversed by substantially equal fractions of the total current flowing through said coil.

4. Magnet according to claim 3, of the type in which said stack of discs (113) is held by means of insulated tie rods, characterized in that said longitudinal rods are constituted by at least some of said tie rods (119).

5. Magnet according to claim 3 or 4, of the type in which said stack of the or each coil is clamped between two conductive end plates, characterized in that the longitudinal rods adjacent to the end plate (118b) located at the other axial end of said magnet are electrically connected to this plate to ensure the distribution of current between said rods longitudinal (119).

6. Magnet according to claim 5, characterized in that the above-mentioned longitudinal rods pass through the end plate (118a) situated at the first cited axial end of the magnet while being electrically isolated from it and in that this end plate comprises as many connection terminals (125) as such rods (119) located respectively in the vicinity of each of them to allow the electrical supply of said magnet from an equal number of pairs of parallel conductive wires .

7. Magnet according to claim 6, characterized in that ^' each pair of conductive wires is a cable (126) with coaxial structure.

8. Magnet according to claim 6 or 7, comprising several coils aligned along a common axis, characterized in that two neighboring coils are connected by as many coaxial cables as there are aforementioned rods.

^• 9. Coiled magnet according to one of the preceding claims, characterized in that each end of the turn has a recess and that two adjacent turns are connected end to end by welding such recesses (122, 123) of complementary shapes.

10. Magnet wound according to one of the preceding claims, characterized in that said conductive discs (113) are made of aluminum and that the aforementioned insulation is produced by anodizing these rings.