Production Of Magnetic Coils For The Creation Of Intense Fields

Abstract

A magnetic coil designed so as to permit the creation of intense magnetic fields under pulsating or continuous working conditions is constituted by a stack of conducting metal sheets and insulating sheets said metal sheets making local contacts in such manner that they form a conducting spiral, said metal sheets comprising more than two locally cut-out portions staggered from one metal sheet to the next following metal sheet, and means delimiting a circuit for a cooling fluid against said cut-out portions so that the fluid is in contact with the lateral faces of the metal sheets. By this means, the copper-water thermal exchange surface is considerably increased ; the insulating sheets may project between the turns thus improving the quality of the insulation ; and the stack can be tightly clamped by studs reinforcing the electrical contacts and improving the resistance to electro-dynamic forces.

Description

The present invention has for its object a magnetic coil designed so as to permit the creation of intense fields under pulsating or continuous conditions.

Coils are already known, generally by the name of Bitter coils, each constituted by a stack of conducting sheets of copper alternating with insulating sheets. The latter, of small thickness (50 microns for example) are cut-out in such manner that they permit local contact between the copper sheets and in consequence there is produced a coil the turns of which are formed by these sheets, by clamping the edges of the stack by means of jaws. The whole unit is cooled by de-ionized water circulating through the coil by means of channels pierced perpendicularly to the stack.

The invention is directed to the design of a coil which is especially suitable, in a very particular example, to the production of an installation operating by impulses, in which the intensity of induction along a circumference of 2 m. in diameter will reach 6T and the stored magnetic energy will attain 40 MJ, the heat energy dissipated in an assembly of 24 coils during the course of one impulse will be 150 MJ and the frequency of operation will be one impulse every 4 minutes. As the duration of the impulse is very short, the temperature rise will therefore be adiabatic. The intense magnetic field necessary should be created by a toric solanoid of 24 coils. These coils may also be utilized to create a rectilinear field having an intensity which may reach 6T and even more.

Coils of this kind would appear to be particularly well adapted to resist high mechanical stresses. It has however been found that these coils such as they are formed, could not be suitable as a solution of the problem set for many reasons, and in particular:

The copper-water heat-exchange surface is inadequate since it is reduced to the section of the copper sheets. This makes it necessary to execute a very large number of cooling channels which are difficult to machine and involve very high cost.

The leakage lines between turns are too short, the thickness of the insulation material being of the order of 50 to 100 microns, and the slightest impurity in the cooling water results in breakdown.

The present invention proposes to modify the formation of coils of this kind in order to eliminate the preceding drawbacks and in particular to permit the use of these coils for the installation referred to above.

It has for its object a magnetic coil constituted by a stack of conducting sheets and insulating sheets, the said sheets having local contacts such that they form a conducting spiral, characterized in that the said sheets comprise locally cut-out portions greater than two in number displaced from one sheet to the next following, and means delimiting a circuit for a cooling fluid against the said cut-out portions in such manner that the fluid is in contact with the lateral faces of the said sheets.

The cooling is thus much more effective, since water for example circulating in the cooling circuit can pass over the surface of the sheets at the level of the displaced cut-out portions, and not only along their edges. The stack can then advantageously be clamped by means of studs passing through the metal sheets and the insulating sheets.

According to another characteristic feature of the invention, the conducting sheets are separated in pairs by the thickness of two insulating sheets, these latter sheets being arranged to overlap in such manner as to increase each leakage line by the length of the overlap.

According to another particular feature of the invention, concerning more especially operation by impulses, the edges of each conducting sheet are cut-out in castellations so as to form cooling fins, the castellations being arranged in staggered relation between the various metal sheets, and a cooling fluid circulates in the channel formed by these fins around the stack and over the entire height of the stack.

According to another special feature of the invention, relating more particularly to operation by continuous conditions, the metal sheets comprise openings for the passage of the cooling fluid into the mass of the stack, of round or oblong shape, which overlap from one metal sheet to the next following, the insulation extending beyond the metal sheets in each opening.

According to still another feature of the invention, each coil can be constructed either for operation under pulsating conditions or for operation under continuous conditions, or for both these conditions of operation.

It will be assumed in the text which follows that each coil is of quadrangular form with a central opening of circular section, but it is clear that such a form of the coil with its central opening is in no way exclusive and that the invention is applicable to any other form.

A coil of this kind according to the invention will now be described below in full detail, this description serving to bring out other particular features of the invention, reference being made to the accompanying drawings showing one non-limitative example of construction of this coil.

In the drawings:

FIG. 1 is a view looking on the top of the coil assumed to be for operation by pulsation;

FIG. 2 is a view of a copper sheet of the stack;

FIG. 3 is a view in cross-section along the plane III--III of FIG. 1;

FIG. 4 is a view in cross-section taken along the plane IV--IV of FIG. 1;

FIG. 5 is a enlarged view in cross-section of a portion of turns of the coil taken along the plane V--V of FIG. 1;

FIG. 6 is a view in cross-section of a portion of the stack at the level of the cooling channels, in an alternative form provided for operation with continuous condition;

FIG. 7 illustrates an alternative form of construction of FIG. 6;

FIGS. 8 and 9 show diagrammatically two other methods of construction of the stack, as an alternative to FIG. 5.

Before commencing the actual description of the structure of the coil, a few preliminary remarks must be made on the design of such coils in the case of an installation with an intense magnetic field operating under pulsating conditions, such as that referred to at the beginning of the present text.

These coils must withstand large stresses:

of a mechanical order due to magnetic pressure; there are concerned on the one hand static stresses due to the field intensity and on the other hand fatigue stresses due to the continuous repetition of the forces;

and of a thermal order which are characterized by cycles of rapid heating followed by slow cooling.

These necessities thus make it essential to choose preferably as the conducting material, copper slightly alloyed with zirconium and phosphorus, or copper alloyed with silver which, in addition to excellent electrical conductivity, has remarkable mechanical properties, in particular a high elastic limit equal to at least 38 kg/sq.m., up to high temperatures. As regards the insulating sheets, they are made of sheets of material known under the name of Kapton.

It will however be understood that the invention is in no way limited by the choice of these materials, and that the coil as described below may be made with other materials without thereby departing from the scope of the patent.

Furthermore, in the case of operation under pulsating condition, the period of rest between two consecutive impulses leaves the heat time to become diffused through the metal, so that it is advantageous to be satisfied by eliminating the heat by peripheral cooling of the coils. Since however the thickness of each of the conducting metal sheets is of the order of 0.7 mm., there can be no question, as in conventional systems, of welding a fluid cooling pipe to the periphery of the conducting sheets.

The arrangements advocated by the Applicants with a view to arriving at the same result are original, efficient and not evident, as will be brought out during the course of the following description of the coil.

As shown in FIGS. 1, 3, 4 and 5, the stack of conducting metal sheets and insulating sheets of the coil is gripped between quadrangular armatures; each armature is made-up of two sectors, a sector 10 of 270.degree. and the other 11 of 90.degree.. On each of the upper and lower faces of the coil, the sector 11 overlaps one of the electrodes 12 of the coil with the connection tab 13. Studs 14 (or screws with countersunk heads) co-operating with nuts 16 and locking washers 17, ensure a powerful clamping action between the two armatures of the stack and with the electrodes. The heads of the studs and screws and the nuts are housed in recesses 18 formed in the armatures with the interposition of an insulating layer or washer 19 under the heads. The shaft of each stud or screw is sheathed over its entire height by an insulating tube 20.

FIG. 2 shows one of the conducting metal sheets 21 of the stack. Each sheet is slit at 22 alternately in the stack along the axis AA.sub.1 or the axis AA.sub.2. There can be seen in the drawing the various openings for the passage of the studs or the clamping screws. Each side of the sheet is castellated in order to provide projections forming fins 23 alternating with hollow spaces 24, and the stack is made in such manner that the projections and the hollows of one sheet are respectively facing the hollows and projections of an adjacent metal sheet, following a staggered arrangement. It should be noted that in fact all these metal sheets are identical since, as shown in the drawing, each sheet is of partial symmetry with respect to the central axis B.sub.1, B.sub.2, so that it is only necessary to turn it through 180 degrees around this axis so as to obtain the next following metal sheet of the stack.

With regard to the insulating sheets, they are uniformly square but their dimensions are such with respect to those of the conducting metal sheets that they uniformly extend about 1 mm. beyond, on the one hand the marginal hollows of the metal sheets and on the other hand the circular internal border 25 of these metal sheets, which improves the quality of insulation as compared with conventional Bitter coils. In the drawings, this extension has been exaggerated in order to show it more clearly. As has been previously seen, there exist two insulating sheets 26 for the separation of two adjacent conducting metal sheets, these being slit so as to enable them to be placed in position, as will be described later.

It is necessary to recall here that the slots of the conducting metal sheets are alternately superimposed along two lines only. Nevertheless, in order to permit the production of a conducting spiral, they are in contact in pairs, without intercalated isolating sheets. For two pairs of metal sheets separated from each other by insulating sheets over 270.degree. from one slot to the other, the contact is made over 90.degree. between the two slots by one metal sheet only of each pair, the insulating sheets passing into the slots so as to be displaced by one thickness of metal sheet at the first slot and by a second thickness at the second slot (see FIG. 5).

In order to form the peripheral circulation channels for the water ensuring the cooling of the castellated edges of the conducting metal sheets, especially for the pulsating condition, the four lateral faces of the coil are each closed by a first insulating plate 27 bearing against the projections of the castellated edges of the conducting sheets, and made fluid-tight on the edges by a joint 28 with a base of an elastomer which polymerizes in the cold state. Against this first plate is applied a second insulating plate 29, fixed by screws 30 on the lateral edges of the armatures 10 and 11. In FIG. 1, there are shown at 31 the cooling water supply tubes and at 32 the tubes for evacuating this water.

In view of the above-described arrangement of the edges of the conducting metal sheets, the cooling water coming in through the tubes 31 will sweep over all the lateral faces of the coil internally of the plates 27, following paths in tiers along the cooling fins 23 and passing from one turn to another by the effect of the hollow spaces 24, wider than the fins. In view of the large contact surface between the water and the fins, cooling is effected in a rapid and efficient manner, using means of moderate cost.

The duplication of the insulating sheets 26 intercalated between the conducting metal sheets 21 enables the leakage line between the two copper sheets to be elongated by the overlap of these insulating sheets, as shown in FIG. 5. In fact, each sheet is provided in known manner with a slot enabling it to be put in position in the stack by passing it into the slot of a conducting metal sheet and sliding it underneath this sheet. As shown in the drawing, the slots of two adjacent insulating sheets are staggered, in such manner that the path of the leakage lines between the two conducting sheets which they separate is extended by the length of the overlap.

If advantage is taken of the fact that the conducting metal sheets are doubled as well as the insulating sheets, it is possible to effect a particularly simple assembly by causing the slots of the insulating sheets to coincide with those of the conducting metal sheets, in accordance with the alternative form shown in FIG. 8; each sheet is then only staggered once by one thickness of metal sheet and it is coupled to an adjacent sheet over 270.degree. and with the other adjacent sheet over 90.degree..

Following another alternative form illustrated by FIG. 9, the coil is formed by a simple stack of elements each composed of two conducting metal sheets and two insulating sheets deformed so as to pass over each other over 90.degree. , as shown in the drawing.

In the case where the coil is to operate on a continuous condition, it is preferable to utilize an alternative form of the invention in which cooling channels are provided in the whole mass of the stack and from one face of the coil to the other. An arrangement increasing the cooling surface area per sheet, as for peripheral cooling, is then employed. The essential part of this arrangement is shown in cross-section in FIGS. 6 and 7 for two alternative forms of construction.

It consists in providing for each cooling channel a succession of openings 33 circular, or better still of oblong shape, which overlap each other from one metal sheet to that next following in the case of FIG. 6. The insulating sheets comprise corresponding openings which are however longer, so as to permit communication between the staggered openings of two successive metal sheets. They project however by 1 mm. for example beyond the edges of these openings. The path of cooling in tiers, shown diagrammatically by a broken line, follows essentially the horizontal overlapping surfaces of the metal sheets, so that the projections of the insulating material do not substantially reduce the contact cooling surface.

Similarly, the influence of the degree of clamping of this form of cooling becomes insignificant. As the circulation of the water thus takes place obliquely in the stack, it can be effected from the lateral edges of the stack, which leaves the lower and upper faces available for placing in position the studs for clamping the stack. In the case of FIG. 7, the openings are staggered between two adjacent conducting metal sheets and the circulation takes place substantially parallel to the plates.

To sum-up, it can be seen that, by virtue of the invention:

The copper-water thermal exchange surface has been considerably increased;

The insulating sheets can be allowed to project between the turns, which improves the quality of the insulation;

The stack can be clamped by studs, which reinforces the electrical contacts and improves the resistance to electro-dynamic forces.

There can be produced separately coils which are more particularly suitable either to operation by impulses or to operation under continuous conditions. It is obviously not excluded to produce also coils which combine the characteristics of these two alternative forms so that they may be utilized at will and with the same advantages for either of these operating conditions. The two corresponding cooling circuits furthermore make it possible to act in either of the operating conditions on the speed of elimination of the heat.

By way of indication, there are given below a few figured characteristics of the structure of a coil according to the invention:

Number of turns 71 Thickness of a copper sheet 0.7mm. Thickness of the insulating material 0.075.times.2=0.15mm. Thickness of one turn 1.4mm+0.15=1.55mm. Diameter of the interior bore 600mm.

Claims

What we claim is:

1. A magnetic coil formed by a stack of conducting metal sheets and insulating sheets, said metal sheets making local contacts such that they form a conducting spiral, in which said metal sheets comprise locally cut-out portions, more than two in number, staggered from one metal sheet to the next following metal sheet, and means delimiting a circuit for a cooling fluid against said cut-out portions so that said fluid is in contact with the lateral faces of said metal sheets.

2. A coil as claimed in claim 1, in which each conducting metal sheet has its edges cut-out in castellations so as to form cooling fins in contact with said fluid, said castellations being arranged in staggered relation from one metal sheet to the other and being in contact by their external edge with a casing defining a cooling circuit around said stack.

3. A coil as claimed in claim 1, in which said metal sheets are provided with openings for the passage of the cooling fluid into the mass of said stack, said openings, especially of oblong shape, overlapping from one metal sheet to that next following, so that each metal sheet is swept over by said fluid over a portion of its lateral surface which is not covered by an adjacent metal sheet.

4. A coil as claimed in claim 1, in which the circuit of said cooling fluid is supplied over the edges of the stack, and in which said stack is kept tight by mechanical means passing through the conducting metal sheets and the insulating sheets.

5. A coil as claimed in claim 1, in which said insulating sheets are dimensioned in such manner as to project over all their edges from at least one of the conducting metal sheets directly adjacent.

6. A coil as claimed in claim 1, in which said conducting metal sheets are provided with slots located alternately in two planes radial to the said stack and are in contact in pairs, each alternately with the preceding metal sheet and with the following metal sheet, and in that two successive conducting metal sheets are separated by two adjacent insulating sheets having staggered slots.

7. A coil as claimed in claim 6, in which the successive pairs of conducting metal sheets are separated by two adjacent insulating sheets which pass successively through the slots of two consecutive metal sheets and are provided, beyond said slots of said conducting metal sheets, with slots staggered one with respect to the other.