US3493904A

US3493904A - Device for producing an intense and uniform magnetic field within a volume of revolution such as a sphere or ellipsoid

Info

Publication number: US3493904A
Application number: US686251A
Authority: US
Inventors: Jacques Favereau
Original assignee: Compagnie Electro Mecanique SA
Current assignee: Compagnie Electro Mecanique SA
Priority date: 1966-12-16
Filing date: 1967-11-28
Publication date: 1970-02-03
Anticipated expiration: 1987-02-03
Also published as: GB1207402A; DE1589670A1; FR1516849A

Description

3,493,904 NETIC 2 Sheets-Sheet 1 4 W); PMLW JQCQUQS FGUQVEQU J. FAV'EREAU N'G Feb. 3, 1910 DEVICE FOR PRODUCI AN INTENSE AND UNIFORM MAG FIELD WITHIN A VOLUME 0F REVOLUTION SUCH AS A SPHERE OR ELLIPSOID Filed Nov. 28, 1967 3, 1970 J. FAVERE AU 3,493, 0

DEVICE FOR PRODUCING AN INTENSE AND UNIFORM MAGNETIC FIELD WITHIN A VOLUME OF REVOLUTION SUCH AS A SPHERE 0R ELLIPSOID Filed Nov. 28, 1967 2 Sheets-Sheet 2 Jac ues Favereau United States Patent Int. (:1. HOlf 7/22 US. Cl. 335216 10 Claims ABSTRACT OF THE DISCLOSURE A device for the production of an intense and uniform magnetic field which comprises concentric inductance and shield windings in the form of a volume enclosing surface of revolution such as a sphere or ellipsoid, the two windings being made of superconductor material placed in a cryostat and being connected in series so that the inductance and shielding effects are proportional.

The present invention relates to a device for producing an intense and uniform magnetic field in a bounded volume, by avoiding, through the presence of a shield winding around the inductor element any scattering of the field in the surrounding space.

It concerns, more particularly, a device of this type which consists of superconductor elements placed in a very-low-temperature cryostat.

The use of superconductors makes it possible to obtainvery high inductances without the continuous consumption of electric power (resistance null) with a reduced volume of materials to channelize the current, due to the very high density of the current that can be admitted into superconductors of the niobium-zirconium, niobium-titanium, niobium-tin type, for example.

The present invention has for its object:

(1) A device for the production of an intense and uniform magnetic field, of the type comprising an inductor winding and a shield winding disposed around the former, these two windings being formed of superconductors placed in a cryostat, characterized by the fact that the two windings constitute two concentric volume enclosing convex surfaces of revolution, and by the fact that the two windings are electrically connected in series so that the induction and shield functions are proportional.

(2) A device in accordance with paragraph 1, in which an access channel to the magnetic field inside of the induction windings is provided near the axis of generation of the enclosed volume.

(3) A device according to one of the preceding paragraphs, in which the enclosed volumes are ellipsoids or spheres.

By way of illustration, the invention is described hereinafter with reference to the annexed drawing, in which:

FIG. 1 is a section along the axis of revolution of a device for the production of an intense and uniform magnetic field according to the invention, which is spherical in form.

FIG. 2 is a plan section of FIG. 1, showing the configuration of the conductors used.

By referring to the drawing, it is possible to see at 1 the outer spherical envelope of the cryostat; at 2, its inner spherical envelope; at 3 and 4, the two cylindrical chan nels with a radius R allowing access to the inner spherical zone 5 in which the magnetic field is maximum, uniform,

3,493,994 Patented Feb. 3, 1970 and oriented in parallel to the axis of revolution 6 of the system, along the induction line 10.

The induction winding for producing the magnetic field within the spherical inner zone 5 consists of coaxial circular spirals concentric with the generation axis 6, appropriately distributed over a sphere 7 with a radius R The shield winding also consists of coaxial circular spirals concentric with the axis 6, appropriately distributed over a sphere 8 with a radius R concentric to the sphere 7.

The two windings are arranged in series, with the fluxes in the inductor ind the shield being magnetically in opposition. The distribution of the ampere-turns on the spheres 7 and 8 is similar.

The direction of the fluxes has been shown in FIG. 1 by crosses and dots on the spirals of each winding. The inner volume 9 of the cryostat defined by the space between the inner and outer spherical envelopes 2, 1 contains the liquid helium needed to cool the inductor and shield windings. In this figure are not shown the conventional therman insulation of the inner and outer surfaces (1, 2, 3, 4) of the cryostat, the intakes and outputs of helium in the liquid and gaseous phases, or the crossovers for the two terminals of the inductor and shield windings, which are in series.

When a device of this type is designed to store a magnetic force which is to be used elsewhere in a pulsed manner, the transfer of the force can be accomplished by the previously-mentioned winding or a secondary winding, by a transformer effect. If this winding is also a superconductor, it should be placed in the cryostat. If it is not a superconductor, it can be placed inside of the surface 2 and outside of surface 1. In both cases, however, it should represent a distribution similar to that of the inductor and shield windings, in order to avoid the apparition of a magnetic field in the surrounding medium during the transfer of force.

In certain applications (storage and transfer of magnetic force, for example), the cryostat can be reduced to its outer envelope 1, a situation which would result in the elimination of

surfaces

2, 3 and 4 of FIG. 1.

The spherical surfaces shown are a particular case of a volume enclosing surface according to the invention, i.e. one generated by rotation of a wholly convex line about an axis passed through the ends of the line. This geometrical form, either spherical or ellipsoidal,- presents several advantages in practice and, in particular, it ensures a perfect magnetic shield, contrary to the case of a cylindrical winding, in which there is a scattering of the field along the axis, on each side of the Winding; and it is especially well suited to maintain the conductors against the Laplace forces. Furthermore, the cryostats in which the superconductor windings are located, and operating at a temperature close to that of liquid helium, are simple in form and present a low ratio of the outer surface to the inner volume; this is advantageous in their thermal insulation. Finally, in the present case of sinusoidal distribution of the surface current density in the inductor and shield windings, the current is null on the axis of generation 6 (axis of the field); this makes it possible to provide, in the vicinity of this axis, for an access channel with a radius R to the internal uniform magnetic field, without actually disturbing this field.

In the theoretical case of current layers of negligible thickness, with this sinusoidal distribution of the surface current density, with the form: a =A sin 0 amp/meter-for the inductor with the radius R a =-A sin 6 amp/meter-for the shield, with the radius R where 0 is the angle with respect to the axis 6 defining the position of a spiral (FIG. 1), and a (1 are the current densities in amp/meter, it is possible to obtain a uniform field in the inner sphere, while preventing its scattering outside of the external sphere, on the condition that the following relation is satisfied:

A1R13:A2R23 In addition, if the relation:

R2 :4R13 Or R1 is satisfied, the uniform internal induction is maximal, the radius R is minimal for a given radius R and the induction has the same value in the equatorial plane (defined by in part and other of the coiled inductor (radius R In setting the induction at B=5 Wb/rn. and magnetic force stored in the entire field of W =1MJ, the radii R =23 cm. and R :36.5 cm.

Taking an external diameter of 80 cm. for the cryostat, a megajoule of magnetic force is stored in a volume of the order of the quarter of a cubic meter, which corresponds to an apparent density of magnetic force of The inductor and shield windings, connected in series, are excited by the same current. They present the same true current density in the conductors, or apparent density, if account is taken of the necessary separation between conductors for the electrical insulations and the circulation of helium.

In noting that the quantity A=a/ sin is constant, it is seen that it is possible to make a spherical winding with a sinusoidal distribution, by means of a series of coils with a constant a thickness (FIG. 2).

The apparent density d is defined by d=A /e.

In reverting to the previous numerical example, with A =8- l0 A/m. for the inductor winding, and d=4-10 A/cm. it is found that e =2 cm. for R =23 cm.

With the same apparent current density a and A =2-10 A/m. for the shield winding, it is found that 2 cm. for R =36.5 cm. (FIG. 2).

The thicknesses e and 2 are small as compared to the radii R and R due to the heavy flux densities that can be admitted into the superconductors; this is not too far apart from the theoretical theory of current layers of negligible thickness.

These coils of constant thickness and of decreasing radius as they approach the axis 6, can be given a constant height (il and I1 i.e. a constant section as shown in FIG. 2. In this figure, the thicknesses e and 2 have been greatly enlarged in order to show more clearly the geometrical form proposed for the inductor and shield windings.

It is obvious that the invention is not limited to the example of the embodiment described, nor to the form of implementation shown.

I claim:

1. In a device for producing an intense and uniform magnetic field, an inductor winding of superconductor material having a configuration corresponding to a volume enclosing surface generated by rotation of a wholly convex line about an axis passed through the ends of the line, and a shield winding of superconductor material of the same configuration as said inductor winding, said shield winding being concentric with and surrounding said inductor winding, andvsaid inductor and shield windings being connected in series.

2. A device as defined in claim 1 wherein the fluxes produced by said windings are magnetically in opposition.

3. A device as defined in claim 1 wherein said windings hav a spherical configuration.

4. A device as defined in claim 1 wherein said windings have an ellipsoidal configuration.

5. A device as defined in claim 1 wherein said windings consist of parallel fiat spiral coils the respective planes of which are normal to said axis of generation.

6. A device as defined in claim 5 where adjacent spiral coils of said windings are essentially contiguous.

7. A device as defined in claim 5 wherein the conductors forming said spiral coils are of uniform cross-section.

8. A device as defined in claim 1 and which further includes inner and outer concentric spaced surfaces having a configuration corresponding to the configuration of said inductor and shield windings, said inductor and shield windings being located in the space between said surfaces, and said space being filled with a cooling medium for said superconductor windings.

9. A device as defined in claim 5 wherein the surface current density in the coils is substantially sinusoidal and an access channel to the magnetic field is provided close to the axis of generation.

10. A device as defined in claim 8 wherein each of said inductor and shield windings consists of flat contiguous spiral coils having a uniform cross section and the respective planes of said coils are normal to said axis of generation.

References Cited UNITED STATES PATENTS 3,098,181 7/1963 Cioffi 335-216 3,210,610 10/1965 Fraser 3352l6 XR FOREIGN PATENTS 765,937 8/ 1967 Canada.

G. HARRIS, Primary Examiner U.S. Cl. X.R. 335299