EP0221921B1 - Ironless solenoidal magnet - Google Patents

Abstract

Structure for connecting discs of a Bitter coil. According to the invention, the discs comprise each a slot-shaped cutting (15), which is waved for example so as to define from one disc to the other turn lapping zones (16) distributed into two groups and the electric connections are provided for by means of junctions between said overlapping zones of one group or the other, alternatingly from one disc to the other. Application to NMR imaging.

Description

The invention, due to the collaboration of the National Service of the Intensive Fields of the CNRS (Director M. AUBERT), relates to a solenoidal magnet, without iron, comprising one or more coils whose technological structure is similar to that of a Bitter coil classic; the invention more particularly relates to improvements making it possible to simplify the manufacture of the coil (s) and 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 discs (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, kerosene or oil.

Document FR-A-1 600 511 describes a tubular coil, in particular a power inductor, constituted by the assembly of a plurality of split flat turns, or each of said turns, molded with its junction elements to the adjacent turns, constitutes a individual module and, that the assembly of a plurality of said modules by their junction elements constitutes said coil.

The invention provides a magnet consisting 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, the center of which coincides with the center of symmetry of this magnet or 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 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 magnet or by aligning several coils of Bitter 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 both to a magnet with a single coil and to a magnet with several coils aligned, contiguous or spaced.

There may indeed remain other structural causes of inhomogeneity of the generated magnetic field or causes of disturbance of this magnetic field.

Thus, according to an currently widely used 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 includes a cut in sector form 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 coil (s) of this kind 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 an intrinsic cause of non-uniformity.

The invention firstly proposes a new type of assembly of the discs making it possible to solve this problem.

In this spirit, the invention therefore essentially relates to a solenoid magnet of the type comprising at least one coil consisting of a stack, with interposition of insulator, of annular conducting discs, each disc comprising a cut-out ie transforming into a turn and said turns being connected end to end, in that said discs extend in respective parallel planes perpendicular to the longitudinal axis of said coil, in that said cutout of each disc and a slot, in that the arrangement and shape of these slots define several overlapping zones of turns between the successive disks, characterized in that these zones being divided into two groups, and in that the electrical contact between any two adjacent disks is achieved by their junction on a group of said overlapping zones while that the electrical contacts from disc to disc are made by junctions on one or the other group of said zones of cheva alternately.

According to a possible embodiment, the aforementioned slots are festoon slots (wavy or sawtooth or the like) and they are reversed from one disc to another with respect to a plane passing through the axis of the coil , to define said overlapping areas.

The structure defined above has little effect on the configuration of the current density at the rods between neighboring turns. In particular, the increase in the thickness of the conductor at the junctions between turns is offset by the decrease in the surface of these same junctions. We can therefore consider that the current density does not vary over a full revolution. On the other hand, the radial distribution of current naturally undergoes a disturbance at the level of each junction between two adjacent turns, but these disturbances are compensated for from one turn to another. All these particularities mean that a coil thus constructed presents few intrinsic causes of inhomogeneity of the generated axial magnetic field. In addition, the axial component of the current, due in previous systems, to the helical shape of the turns, does not exist because each turn extends in a plane. On the contrary, a very “localized” longitudinal current component is created, parallel to a generator of the coil in the vicinity of the junction zones between discs. This disturbance can be easily compensated locally by means of longitudinal conductors traversed by currents flowing in the opposite direction. In this spirit, the invention makes it possible to solve another problem, namely the need to take into consideration the way in which the current is applied to the magnet. Indeed, 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, magnetic field disturbances generated by these conductors can degrade the homogeneity of the field in the aforementioned sphere of interest. If the current is brought back by means of longitudinal conductors judiciously placed in the vicinity of the junctions between discs, the desired compensation is achieved on the one hand and the current is brought back towards the axial end of the magnet on which it has been injected, without creating a loop capable of disturbing the homogeneity of the magnetic field delivered. This makes it possible to connect the current source to the same axial end of the magnet, by means of conductors with coaxial structure.

In this spirit, the invention also relates to a solenoid magnet according to the above definition, characterized in that the or each coil has at least one conduit parallel to said axis, defined by the superimposition of holes made in or in the vicinity of overlapping zones longitudinally superimposed, each conduit housing a current return conductor connected between the last turn of an axial end of the magnet and opening at its other axial end to be connected to a terminal of a DC power source. If the magnet has several spaced coils, the current return conductors can pass through the spaces between coils inside respective metal tubes, themselves connected to connect said coils in series. This type of coaxial connection structure does not create any field in these spaces.

• la figure 1 est une vue partielle montrant un disque d'une bobine constituant l'aimant, le disque étant pourvu d'une fente en festons selon la définition qui précède ;
• la figure 2 est une coupe partielle II-II de la figure 1 ;
• la figure 3 est une coupe partielle III-III de la figure 1 ;
• la figure 4 est une vue analogue à la figure 1, illustrant une variante ;
• la figure 5 est une vue partielle illustrant l'extrémité d'une bobine et son raccordement à une bobine voisine.

The invention will be better understood and other advantages of it will appear better in the light of the following description of several embodiments implementing its principle, given only by way of example and made with reference to the drawings annexed in which:

• Figure 1 is a partial view showing a disc of a coil constituting the magnet, the disc being provided with a scalloped slot according to the preceding definition;
• Figure 2 is a partial section II-II of Figure 1;
• Figure 3 is a partial section III-III of Figure 1;
• Figure 4 is a view similar to Figure 1, illustrating a variant;
• Figure 5 is a partial view illustrating the end of a coil and its connection to a neighboring coil.

Referring to the drawings, there is partially shown annular discs 11 constituting a coil forming part of the magnet. These are metallic annular discs (typically made of copper or aluminum) stacked with the interposition of insulating sheets 12 of the same shape and connected end to end to constitute said coil. The sections of Figures 2 and 3 show five annular discs 11 a, 11b, 11c, 11d, 11e, mounted in this way. As mentioned above, several coils of this kind are used, axially aligned, placed side by side or spaced from one another, to produce a magnet delivering a highly homogeneous magnetic field in a given internal volume. According to the example described, the discs have a structure in accordance with that proposed by Bitter, that is to say that they in particular have holes 13 according to the same configuration from one disc to another to overlap and define channels through which a coolant flows. Optionally, the discs also have holes 14 of larger diameter, overlapping in a similar manner to allow the passage of isolated tie rods 17. The main function of the tie rods (which can also be outside the discs) is to hold the discs 11 and the insulating sheets 12 in a tight stack.

According to the invention, the discs are not deformed to become more or less helical portions, but on the contrary extend parallel to each other, in their respective planes, perpendicular to the longitudinal axis of the coil and each disc has a slot 15 which (in the examples described) is a festoon slot, extending from its outer edge to its inner edge. Furthermore, all the festoon slots are all grouped in the same longitudinal portion of the coil but inverted from one disc to another. Thus, in Figures 1 and 4, there is shown one of the slots 15, in solid line while the slot 15 _j of the adjacent disc is sketched by a dashed line. It can be seen that, by the nature of the slots on the one hand and their inversion from one disc to the other on the other hand, zones of overlapping of turns 16 are defined between two juxtaposed slots, the number of overlapping zones here depends on the number of undulations of the scalloped slot. Thus in the example of FIG. 1, four overlapping zones are defined between any two adjacent discs whereas in the example of FIG. 4, there are only three. It is clear that the overlapping zones between turns constitute as many possible contact areas for connecting the turns end to end in order to define the coil. According to another important characteristic of the invention, the electrical contact between any two adjacent discs is made by their junction on a group (a part) of said overlapping zones while the electrical contacts from disc to disc are made by junctions on the either group of said overlapping zones, alternately.

As such, in the example of FIGS. 1 to 3 where the overlapping zones are in even number (four), said zones are alternately used (radially) to make the above-mentioned electrical contacts. In other words, the overlapping zones at the level of each disc are divided into two groups, zones 16 ₁ , 16 ₃ on the one hand and 16 ₂ , 16 ₄ on the other hand, which will always be used together to make the electrical contacts between two neighboring discs. In the particular example of Figure 4 wherein the areas of overlap are three in number 16a, _{16b, 16c} for each disk (zone 16 _b being located between the zones 16a and _16e), the electrical contacts between disks will be done alternately by their junction on a zone 16 _b then by their junction on two zones 16a, 16 _c and so on.

As shown in FIGS. 2 and 3 relating to the embodiment of FIG. 1, the junctions are established through windows 20 made in the insulating sheets 12. The windows are arranged opposite the overlapping zones selected to establish contact between two discs considered. Advantageously, the reliability of the contact is improved by a weld 21 with filler metal, said weld having substantially the same thickness as the insulating sheet. Preferably use an indium solder. If the insulating sheet is of sufficiently low thickness, the addition of indium may be carried out beforehand by electrolytic deposition on the selected overlapping zones, the welding then consisting in locally reheating the turns during assembly.

The surfaces of the different overlapping zones of the same disc are not equal, they depend both on their even or odd number (thus in the example of FIG. 4, the zone _16b is necessarily larger) and on the value of the current density in the vicinity of said zones. This feature is especially important when the current feedback system which will be described below is implemented in order to compensate for local field disturbances due to the existence of longitudinal current components (see the arrows in Figures 2 and 3 symbolizing the current path) at the passages between adjacent discs. In fact, according to another aspect of the invention, holes 25 are made in the overlap zones (FIG. 1) or in the vicinity of these (FIG. 4), these holes being superimposed to define one or more parallel conduits housing each a current return conductor 26, connected between the last turn of an axial end of the magnet and opening out at the other axial end to which the DC power source is connected. Each conductor 26 is of course isolated inside the conduit which encloses it. The fact of bringing the current to this axial end of the magnet facilitates the connection to the two poles of the supply, this connection being able to be carried out from conductors with coaxial structure not creating disturbance of magnetic field. Furthermore, as mentioned above, the current return conductors in the magnet itself can, if they are judiciously arranged, provide compensation for local disturbances created at the junctions between discs. Compensation is ensured by taking into account the following parameters: the number of overlapping zones in each disc, their respective surfaces, the number of current return conductors and their locations with respect to the overlapping zones. The general principle to be observed for fixing these various parameters is that each current return conductor must be traversed by a current substantially equal to the current which crosses the overlapping zone or zones (or the fractions of zones) which it influences. However, for reasons of simplicity, all the current return conductors are connected in parallel to the axial end of the magnet opposite to that where the power source is connected; they are therefore traversed by substantially equal fractions of the total current flowing in the magnet. It is therefore necessary that the homologous overlapping zones (where the fractions of such zones) "compensated by the current return conductors are traversed by equal currents. However, it is recalled that in a coil with annular discs, the current density in the width of the annular part is not uniform radially, but varies in 1 / R, R being the distance from a point considered to the axis longitudinal of the coil. To take this fact into account, the surfaces of the overlapping zones can be varied accordingly. The example in FIG. 1 shows how the different parameters have been chosen in the case of an even number of overlapping zones (four in this case). Because the overlapping zones are even in number, the electrical contacts are ensured by half of the zones with each passage from one disc to the other. It is therefore sufficient to choose the surface of these zones, taking essentially into account the variation of the current density in 1 / R in the annular disc so that the currents flowing through these contact zones are substantially equal. This is why, in FIG. 1, the surfaces of the zones 16 ₁ , 16 ₂ , 16 ₃ , 16 ₄ decrease from the outside towards the inside of the annular disc. As soon as the surfaces are correctly chosen, the current is equally distributed in each pair of junctions, from disc to disc and compensation can be obtained from as many conductors 26 as there are areas of overlap (four in the example), each conductor passing substantially through the center of all the overlapping zones longitudinally superimposed. In the example of FIG. 4, corresponding to an odd number of overlapping zones, one must take into account both the current density in 1 / R, and the number of overlapping zones brought into play alternately to ensure the passage from one disc to another, since the groups of abovementioned overlapping zones necessarily include different numbers of such zones. Thus, in the specific case of FIG. 4, if the current density were constant radially, the area 16 _b would have an area twice that of each of the areas 16a or 16 _c . To reflect the current density 1 / R, the surface of the region 16 _b is greater than that of each zone 16a or _16b, but it is less than twice the surface of the area 16a and the double the area of area 16 _c . In this case, there may be provided two current return conductors 26a, 26 _b, respectively traversed by substantially one half of the reverse current and provided between the areas of overlap. The driver 26a thus ensures the current compensation for all the overlapping areas 16a and part of the overlapping areas 16 _b while the conductor 26 _b ensures the compensation current for the entire overlapping areas _16c and the other part of the areas superimposed 16 _b . The determination of the areas of the overlapping zones, that is to say of the shape of the scalloped slots which delimit them, is within the reach of a person skilled in the art by applying the principles set out above, to the in light of the examples described.

FIG. 5 illustrates the connection structure between two coils of the magnet when the latter consists of a number of coils spaced axially from each other. The four conductors 26 emerge from the last turn of the coil and each of them crosses the space between the two coils inside a metal tube 30 welded at each of its ends to the end turn of the corresponding coil. All of these tubes thus ensure the series connection of the coils. Substantially equal currents thus flow in opposite directions in the tubes 30 and the current return conductors 25 and these connection structures do not create a magnetic field in the spaces between the coils.

Claims (6)

1. A solenoidal magnet of the type comprising at least one coil constituted by a stack, with the interposition of insulation, of annular conductor disks, each disk (11) comprising an interruption converting it into a turn, and the said turns being connected end to end, in that the said disks extend in respective planes which are perpendicular to the longitudinal axis of the said coil, in that the said interruption of each disk is a gap (15), in that the disposition and the form of these gaps define several zones of overlap of the turns (16) between the successive disks, characterized in that, these zones being divided up into two groups (16₁, 1₃-16₂ and 16₄) and in that the electrical contact between any two adjacent disks is in the form of their junction (21) on a group of the said overlap zones, while the electrical contacts between one disk and another are in the form of junctions with one or the other group of these said zones of overlap, altematively.

2. The solenoidal magnet as claimed in claim 1, characterized in that the said junctions (21) are produced through windows (20) made in insulators (21) placed between the said disks, the said windows being placed opposite selected overlap zones.

3. The solenoidal magnet as claimed in claim 1 or claims 2, characterized in that the junctions (21) of the said selected overlap zones between disks are soldered junctions.

4. The solenoidal magnet as claimed in claim 1 or claim 2, characterized in that the junctions (21) of the said selected zones of overlap between disks are soldered junctions made with the addition of indium.

5. The solenoidal magnet as claimed in any one of the preceding claims, characterized in that the or each coil comprises at least one conductor parallel to the said axis, defined by the superposition of the holes (25) made in or near the longitudinally superposed overlap zones, each duct enclosing a return current conductor (26) connected between the last turn of one axial end of the magnet and opening at its other axial end in order to be connected with a terminal of a source of supply DC.

6. The solenoidal magnet as claimed in any one of the preceding claims, characterized in that, in an inherently known manner, the coil or coils are Bitter coils comprising more especially channels for the circulation of cooling fluid, extending longitudinally and gripping tie rods of the stack of the said disks.

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