Reinforcement of a superconducting magnet coil

Abstract

A superconducting magnet assembly with a reinforced coil region (3) having a layered conductor coil assembly (10) forming cylindrical conductor layers (11, . . . ), each having plural circular conductor turns (12) centered around and aligned along the axis of cylindrical symmetry (z). The reinforced coil region further includes a layered corset coil assembly (20) having an inner radius bigger than an outer radius of the layered conductor coil assembly (10), and a corset sheet assembly (30) including a foil element forming a corset sheet (31, . . . ). A cross section of the corset sheet with any plane perpendicular to the z-axis forms a segmented circle centered around the z-axis, the radius of which is bigger than that of one of the conductor layers and smaller than that of another of the conductor layers. In addition, the segmented circle covers at least 90% of a full circle but has at most four segments. The assembly provides mechanical reinforcement against radial magnetic forces.

Claims

1. A superconducting magnet assembly comprising a cylindrical magnet coil assembly with an axis of cylindrical symmetry (z) and with a reinforced coil region, wherein the reinforced coil region comprises: a layered conductor coil assembly comprising a conductor tape or a conductor wire, forming at least four cylindrical conductor layers, each having a plurality of circular conductor turns centered around and aligned along the axis of cylindrical symmetry (z), the conductor tape or conductor wire comprising at least one superconducting element, forming a continuous path along the circular conductor turns of at least one of the cylindrical conductor layers and configured to carry an electrical current Icond, the conductor tape or conductor wire having an elastic modulus Econd in a circumferential direction of the circular conductor turns, a layered corset coil assembly comprising a corset tape or a corset wire, forming at least one cylindrical corset layer, having a plurality of circular corset turns centered around and aligned along the axis of cylindrical symmetry (z), the corset tape or the corset wire having an elastic modulus Ecors in the circumferential direction of the circular corset turns, with:
Ecors>1.5*Econd, the layered corset coil assembly having an inner radius bigger than an outer radius of the layered conductor coil assembly, a corset sheet assembly comprising a corset foil element forming at least one corset sheet, a cross section of the at least one corset sheet with any plane (A) perpendicular to the axis of cylindrical symmetry (z) forming a segmented circle having a radius and centered around the axis of cylindrical symmetry (z), the radius of the segmented circle of the at least one corset sheet being bigger than a radius of a first of the cylindrical conductor layers and smaller than a radius of a second of the cylindrical conductor layers, wherein the segmented circle formed by the cross section of the at least one corset sheet with the plane (A) perpendicular to the axis of cylindrical symmetry (z) covers at least 90% of a full circle and has no more than four segments; wherein the corset foil element has an in-plane elastic modulus Efoil, with
Efoil>1.5*Econd; and wherein the layered conductor coil assembly has at least one of the cylindrical conductor layers with at least one of the circular conductor turns with:
Icond>0.005*Econd*Acond/(Bz*r), where Icond is the electrical current in the circular conductor turn, Acond is the cross-sectional area of the conductor tape or conductor wire of the circular conductor turn, Bz is the component of the magnetic field along the axis of cylindrical symmetry (z) in the circular conductor turn, and r is the radius of the circular conductor turn.

2. A superconducting magnet assembly according to claim 1, wherein the segmented circle has no more than two segments.

3. A superconducting magnet assembly according to claim 2, wherein the segmented circle has exactly one segment.

4. A superconducting magnet assembly according to claim 1, wherein at least two of the corset sheets have respective cross sections with the plane perpendicular to the axis of cylindrical symmetry (z), wherein each of the cross sections forms a respective segmented circle, and wherein at least one segment of at least one of the segmented circles overlaps circumferentially with at least one gap of another of the segmented circles.

5. A superconducting magnet assembly according to claim 4, wherein the at least one corset sheet has a thickness <0.2 mm.

6. A superconducting magnet assembly according to claim 1, wherein the at least one corset sheet has a thickness <0.5 mm.

7. A superconducting magnet assembly according to claim 5, wherein the fraction of the volume of the reinforced coil region is empty of material.

8. A superconducting magnet assembly according to claim 1, wherein a fraction of at least 0.5% of a volume of the reinforced coil region is not filled with a solid material.

9. A superconducting magnet assembly according to claim 1, wherein the reinforced coil region comprises a solid filler material containing an acyclic saturated hydrocarbon compound or a resin compound.

10. A superconducting magnet assembly according to claim 1, wherein the cylindrical magnet coil assembly further comprises at least a second reinforced coil region, wherein the layered corset coil assembly of the first reinforced coil region has an outer radius equal to or smaller than an inner radius of a layered conductor coil assembly of the second reinforced coil region, wherein the first reinforced coil region has an extension along the axis of cylindrical symmetry (z) overlapping at least partially with an extension along the axis of cylindrical symmetry (z) of the second reinforced coil region.

11. A superconducting magnet assembly according to claim 1, wherein the cylindrical magnet coil assembly further comprises at least a second reinforced coil region, wherein each of the layered conductor coil assemblies of each of the reinforced coil regions has fewer than 14 of the cylindrical conductor layers.

12. A superconducting magnet assembly according to claim 1, wherein the superconducting element of the conductor tape or the conductor wire of the layered conductor coil assembly comprises either niobium and tin, or bismuth and strontium and calcium and copper oxide, or a rare earth element and barium and copper oxide.

13. A superconducting magnet assembly according to claim 1, wherein the corset foil element of the corset sheet assembly comprises a steel alloy or a nickel alloy or a fiber glass compound.

14. A superconducting magnet assembly according to claim 1, wherein the corset tape or the corset wire of the layered corset coil assembly comprises a steel alloy or a nickel alloy or a fiber glass compound.

15. A nuclear magnetic resonance (NMR) apparatus comprising the superconducting magnet assembly according to claim 1.

16. A method for mechanical reinforcement of a cylindrical magnet coil assembly of a superconducting magnet assembly according to claim 1, comprising: inducing bonding between the circular conductor turns and the at least one corset sheet of the reinforced coil region by heat treating the reinforced coil region at a temperature >500° C. for at least 24 hours.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Various exemplary embodiments of the invention are shown in the drawing.

(2) FIG. 1 shows a reinforced coil region of a cylindrical magnet coil assembly of a superconducting magnet assembly according to the invention in a cross-sectional view in a half-plane containing the axis of cylindrical symmetry of the cylindrical magnet coil assembly, with a cylindrical conductor layer and a corset sheet shown in a spatial representation and as enlarged cross-sectional cutout;

(3) FIG. 2 shows a spatial representation of a corset sheet assembly, and its cross-section with a plane perpendicular to the axis of cylindrical symmetry;

(4) FIG. 3 shows two adjacent reinforced coil regions of a cylindrical magnet coil assembly of a superconducting magnet assembly according to the invention in a cross-sectional view in a half-plane containing the axis of cylindrical symmetry of the cylindrical magnet coil assembly; and

(5) FIG. 4 shows a superconducting magnet assembly with a cylindrical magnet coil assembly in a semi-spatial representation, and an enlarged cross-sectional cutout of a cylindrical conductor layer of a reinforced coil region, according to prior art.

DETAILED DESCRIPTION

(6) FIG. 1 shows a reinforced coil region 3 in a cross-sectional view in a half-plane containing the axis of cylindrical symmetry z, and a spatial representation of a cylindrical conductor layer 11 and of a corset sheet 31, and an enlarged cross-sectional cutout thereof. The reinforced coil region 3 comprises a layered conductor coil assembly 10 with four cylindrical conductor layers 11, 11′, 11″, 11′″, and a layered corset coil assembly 20 with two cylindrical corset layers 21, 21′, and a corset sheet assembly 30 with three corset sheets 31, 31′, 31″.

(7) Each cylindrical conductor layer 11, 11′, 11″, 11′″ comprises a plurality of circular conductor turns 12, centered around and aligned along the axis of cylindrical symmetry z, each conductor turn 12 being wound with a round conductor wire and having a radius r from the axis of cylindrical symmetry z. The conductor wire can have a rectangular cross-sectional shape instead, or a conductor tape can be used in place of a wire (not shown in FIG. 1). The conductor wire has a cross-sectional area Acond and comprises a superconducting element 13, forming a continuous path along the circular conductor turns 12 of at least one cylindrical conductor layer 11, 11′, 11″, 11′″ and carrying an electrical current Icond. The superconducting element 13 can comprise several strands of superconducting material embedded in a metal matrix (typical for Nb based low temperature superconductors or Bi based high temperature superconductors), or a single layer deposited on a metal substrate (typical for rare earth based high temperature superconductors; not shown in FIG. 1).

(8) Each cylindrical corset layer 21, 21′ comprises a plurality of circular corset turns 22, centered around and aligned along the axis of cylindrical symmetry z, each corset turn 22 being wound with a round corset wire. The corset wire can have a rectangular cross-sectional shape instead, or a corset tape can be used in place of a wire (not shown in FIG. 1).

(9) Each corset sheet 31, 31′, 31″ comprises a corset foil element thinner than 0.5 mm, preferably thinner than 0.2 mm. Each corset sheet 31, 31′, 31″ is assembled between two radially adjacent cylindrical conductor layers 11, 11′, 11″, 11′″, thus taking the shape of a segmented cylindrical surface, interrupted circumferentially by at least one gap, as shown for corset sheet 31 with exactly one segment and one gap.

(10) Typically, during operation a reinforced coil region 3 is exposed to a magnetic field with a strong axial component Bz. If the electrical current Icond carried by the superconducting element 13 of a circular conductor turn 12 itself creates a magnetic field with an axial component parallel to the background field then the magnetic force acting on the circular conductor turn 12 has a strong component pointing radially outwards, i.e. away from the axis of cylindrical symmetry z. This radial magnetic force creates a radial magnetic pressure inside the layered conductor coil assembly 10, pushing on the layered corset coil assembly 20 and on the corset sheet assembly 30 of the reinforced coil region 3, thereby straining the reinforced coil region 3 circumferentially. In a reinforced coil region 3 with negligible corseting effect from a weak corset sheet assembly 30 the radial magnetic pressure is accumulated throughout the whole reinforced coil region 3 and is highest at the interface between the layered conductor coil assembly 10 and the layered corset coil assembly 20. If the corset sheet assembly 30 is stronger, then the corset sheets 31, 31′, 31″ can intercept the transmission of radial magnetic pressure between cylindrical conductor layers 11, 11′, 11″, 11′″ efficiently, leading to a more uniform distribution of radial magnetic pressure throughout the reinforced coil region 3.

(11) As long as no radial gaps are present in a reinforced coil region 3 and neglecting the radial compressibility of the reinforced coil region 3, the radial expansion due to the radial magnetic pressure and thereby also the circumferential strain can be assumed to be the same in all mechanical components of the reinforced coil region 3. With a certain amount of corseting material contained in a layered corset coil assembly 20 and in a corset sheet assembly 30 the strain of the reinforced coil region 3 can be reduced most efficiently if the elastic modulus Ecors, Efoil is large in both the corset wire and the corset foil element. In case of anisotropic material, only the elastic modulus relating strain and stress in circumferential direction has to be taken into account. Typically, Ecors and Efoil should be at least 50% larger than the elastic modulus Econd of the conductor tape or conductor wire. The elastic modulus of a material may depend on the strain of the material. Therefore, the relation between Ecors or Efoil and Econd is valid only under the assumption of similar strain in all corset and conductor materials of a reinforced coil region 3.

(12) In an exemplary embodiment of the reinforced coil region 3 of FIG. 1 a circular conductor turn 12 has a radius of 50 mm and it carries an electrical current Icond of 470 A in a background magnetic field with an axial component Bz of 15 T. The round conductor wire has a diameter of 1 mm, a cross-sectional area Acond of 0.785 mm.sup.2, and an elastic modulus Econd of 50 GPa. Without external reinforcement, this wire would be strained by 0.9%, which would damage its superconducting element 13. In the exemplary embodiment of the reinforced coil region 3 the circumferential strain is reduced by a factor of 3 to 0.3% by a layered corset coil assembly 20 and a corset sheet assembly 30, each having an elastic modulus Ecors and Efoil respectively of 200 GPa. In order to achieve the desired reduction of circumferential strain, the cross-sectional area of the combined layered corset coil assembly 20 and corset sheet assembly 30 in a plane containing the axis of cylindrical symmetry z must be half of the cross-sectional area of the layered conductor coil assembly 10. In general, the ratio of corseting and conducting cross-sectional area is the strain reduction factor minus 1, times the quotient of Econd and Ecors.

(13) If an isolated corset sheet 31 were exposed to radial pressure against its inner cylindrical surface it would not be strained circumferentially but the gap interrupting the corset sheet 31 on its circumference would widen. Inside of a reinforced coil region 3, widening of the gap can be avoided by bonding the edge regions alongside a gap of the corset sheet 31 to the adjacent cylindrical conductor layers 11, 11′ by friction. The circumferential extension of one bonded edge region of the corset sheet 31 is the product of circumferential strain and of the elastic modulus Efoil and radial thickness of the corset foil element, divided by the radial magnetic pressure from the underlaying cylindrical conductor layer 11, and divided by the coefficient of friction between the corset foil element and the conductor tape or conductor wire, and divided by two. The factor of two is due to frictional bonding on the inner as well as on the outer surface of the corset sheet 31.

(14) The radial magnetic pressure of an individual circular conductor turn 12 against a hypothetical completely rigid corseting element would be the product of the axial component of the magnetic field Bz, the current density Icond/Acond, and the width of the conductor wire in the radial direction of the circular conductor turn 12. For the radial magnetic pressure on a realistic corseting element, this product has to be multiplied with the actual strain reduction factor achieved by corseting, compared to an uncorseted circular conductor turn 12 under the same operating conditions. For the exemplary embodiment of the reinforced coil region 3 of FIG. 1 with Bz of 15 T, Icond of 470 A, Acond of 0.785 mm.sup.2, with a radial conductor width of 1 mm, and with a strain reduction factor of (0.9%-0.3%)/0.9%=2/3, the radial magnetic pressure on a corset sheet 31 is 6 MPa. The effective radial pressure on a corset sheet 31 in a reinforced coil region 3 may deviate from this value because the calculation neglects mechanical deformation of conductor or corset elements due to the winding process or due to cooling of the reinforced coil region 3 below the critical temperature of the superconducting element 13. Moreover, it neglects that, in general, radial magnetic pressure can accumulate throughout the reinforced coil region 3 over several cylindrical conductor layers 11, 11′, 11″, 11′″.

(15) With this approximation of a radial magnetic pressure of 6 MPa on a corset sheet 31, the circumferential extension of one bonded edge region of the corset sheet 31 can now be calculated to have a value of 25 mm for the exemplary embodiment, further assuming an elastic modulus of the corset foil element Efoil of 200 GPa, a radial thickness of the corset foil element of 0.1 mm, and a coefficient of friction between the corset foil element and the conductor wire of 0.2. Each gap in a corset sheet 31 creates two bonded edge regions, i.e. the total circumferential extension of the bonded edge regions of corset sheet with one gap is 50 mm in the exemplary embodiment. This accounts for 16% of the area covered by the corset sheet 31, assuming a radius of 50 mm for the underlaying cylindrical conductor layer 11.

(16) In the bonded edge regions a corset sheet 31 cannot act as a circumferential reinforcement for cylindrical conductor turns 12 of an underlaying cylindrical conductor layer 11, it even tends to strain them further. Therefore, the circumferential extension of the bonded edge regions alongside a gap of the corset sheet 31 must be minimized, as well as the gap itself, which should account for less than 10% of the area covered by the corset sheet 31. Reduction of the circumferential extension of the bonded edge regions can be achieved by increasing the friction coefficient between the corset sheet 31 and adjacent cylindrical conductor layers 11, 11′. The choice of corset sheet material and of conductor tape or conductor wire material is determined by other criteria than by a large friction coefficient, mainly by good mechanical or superconducting properties respectively. Nevertheless, the coefficient of friction can be increased e.g. by introduction of a solid filler material in the reinforced coil region 3, typically containing an acyclic saturated hydrocarbon compound (e.g. paraffin) or a resin compound. A method to greatly enhance friction is induction of thermal bonding by a heat treatment step of the reinforced coil region 3, typically at a temperature >500° C. for at least 24 hours.

(17) The exemplary embodiment of the reinforced coil region 3 of FIG. 1 shows that circumferential segmentation of the corset sheet 31 has to be avoided because each additional gap introduces two additional bonded edge regions where the corset sheet 31 is not effective as a mechanical reinforcement for the underlaying cylindrical conductor layer 11.

(18) Due to the loss of corseting efficiency in the bonded edge regions of a corset sheet 31, it is advisable to mount two corset sheets 31, 31′ with at least one segment of the first corset sheet 31 covering a gap of the other corset sheet 31′. Such two corset sheets 31, 31′ can be mounted radially adjacent to each other, or with a cylindrical conductor layer 11 in between. FIG. 2 shows such a corset sheet assembly 30. Moreover, FIG. 2 shows the geometrical meaning of a corset sheet 31, 31′ taking the shape of a segmented cylindrical surface, interrupted circumferentially by at least one gap: i.e. a corset sheet 31, 31′ has a cross section with at least one plane A perpendicular to the axis of cylindrical symmetry z comprising a segmented circle 32, 32′, covering more than 90% of a full circle and having exactly one segment as in the embodiments of FIG. 1 and FIG. 2, or at least not more than four segments (not shown in FIG. 2).

(19) FIG. 3 shows two reinforced coil regions 3, 3′ in a cross-sectional view in a half-plane containing the axis of cylindrical symmetry z. Each reinforced coil region 3, 3′ has its own layered conductor coil assembly 10, 10′, its own layered corset coil assembly 20, 20′, and its own corset sheet assembly 30, 30′. The layered corset coil assemblies 20, 20′ comprise a corset tape with a flat rectangular cross-sectional shape. The layered conductor coil assemblies 10, 10′ comprise a conductor wire with a round cross-sectional shape. The voids between the circular conductor turns 12 account for up to 21% of the volume of the layered conductor coil assemblies 10, 10′. A high void fraction makes the reinforced coil regions 3, 3′ inherently soft, leading to a reduction of radial pressure in the innermost cylindrical conductor layers 11, 11′, thereby increasing the circumferential extension of the bonded edge regions of the adjacent corset sheets 31, 31′. Subdivision of a large reinforced coil region 3, 3′ into smaller ones with preferably less than 14 cylindrical conductor layers 11, 11′, 11″, 11′″ is therefore advisable. Reduction of the void fraction by a solid filler material also helps to avoid loss of radial pressure within the reinforced coil region 3, 3′.

(20) In addition to the foil elements forming a corset sheet 31, 31′, 31″, additional foil elements, known per se from prior art, can be included into the reinforced coil region 3, 3′, e.g. between the layered conductor coil assembly 10, 10′ and the adjacent layered corset coil assembly 20, 20′, or between cylindrical corset layers 21, 21′ of a layered corset coil assembly 20, 20′, or underneath the first cylindrical conductor layer 11, 11′ of a layered conductor coil assembly 10, 10′. Such foil elements can facilitate over-winding of a cylindrical conductor layer 11, 11′, 11″, 11′″ or of a cylindrical corset layer 21, 21′, or they can serve as an electrical insulation.

(21) FIG. 4 shows a superconducting magnet assembly 1 with a cylindrical magnet coil assembly 2 with an axis of cylindrical symmetry z, in a semi-spatial representation. The superconducting magnet assembly 1 also comprises an electrical current Icond, flowing in the superconducting element 13 of a conductor wire with a cross-sectional area Acond. The cylindrical magnet coil assembly 2 comprises a reinforced coil region 3, as known from prior art. Reinforcing elements can be a layered corset coil assembly 20 on the outer radius of the reinforced coil region 3 (not shown in FIG. 4), and/or a rigid filler material (not shown in FIG. 4), and/or reinforcing strands co-wound with the conductor wire (not shown in FIG. 4), and/or axial stripes (not shown in FIG. 4) comprising a corset foil element for axial reinforcement of the reinforced coil region 3.

LIST OF REFERENCE SIGNS

(22) 1 Superconducting magnet assembly 2 Cylindrical magnet coil assembly 3, 3′ Reinforced coil region 10, 10′ Layered conductor coil assembly 11, 11′, 11″, 11′″ Cylindrical conductor layer 12 Circular conductor turns 13 Superconducting element 20, 20′ Layered corset coil assembly 21, 21′ Cylindrical corset layer 22 Circular corset turns 30, 30′ Corset sheet assembly 31, 31′, 31″ Corset sheet 32, 32′ Segmented circle z Axis of cylindrical symmetry A Plane perpendicular to the axis of cylindrical symmetry (z) Econd Secant elastic modulus of conductor tape or conductor wire Ecors Secant elastic modulus of corset tape or corset wire Efoil Secant elastic modulus of corset foil element Icond Electrical current carried by a superconducting element 13 Acond Cross-sectional area of a conductor tape or conductor wire Bz Axial component of the magnetic field r Radius of a circular conductor turn 12

CITED REFERENCES

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