Patent Application
20140031235
The present invention relates to a superconducting coil, a superconducting magnet, and a method for manufacturing a superconducting coil.
Japanese Patent Laying-Open No. 2008-153372 discloses a superconducting coil formed by winding a bismuth-based superconducting wire having a band shape. The superconducting wire is wound to form a racetrack shape having a straight portion and an arc portion.
If excessive stress is applied to a superconducting wire during manufacturing or use of a superconducting coil, the superconducting wire is damaged and reliability of the superconducting coil may lower. For example, in winding a superconducting wire around a core during manufacturing of a superconducting coil, a portion of start of winding, that is, an inner circumferential portion, is prone to damage because it is smaller in radius of curvature than a portion of end of winding. In order to avoid such damage, strength of the superconducting wire should only be increased by increasing a thickness thereof. Normally, however, a superconducting coil should have a prescribed number of turns, and in that case, a greater thickness of a superconducting wire leads to increase in size of a superconducting coil. Thus, in a superconducting coil having a prescribed number of turns, reliability of a superconducting coil and reduction in size thereof has had trade-off relation.
Then, an object of the present invention is to provide a superconducting coil, a superconducting magnet, and a method for manufacturing a superconducting coil, which is capable of achieving reduction in size while ensuring high reliability in a superconducting coil having a prescribed number of turns.
A superconducting coil according to the present invention has an oxide superconductor, and has an inner circumferential portion, an outer circumferential portion, and a welding portion. The inner circumferential portion is formed by winding one of first and second superconducting wires each having a band shape. The outer circumferential portion is formed by winding the other of the first and second superconducting wires around the inner circumferential portion. The welding portion joins the first and second superconducting wires to each other by welding between the inner circumferential portion and the outer circumferential portion. The first superconducting wire is higher in strength than the second superconducting wire. The second superconducting wire is smaller in thickness than the first superconducting wire.
According to the superconducting coil of the present invention, of the inner circumferential portion and the outer circumferential portion, one requiring higher strength can be formed from the first superconducting wire, while one requiring lower strength can be formed from the second superconducting wire. Namely, a portion requiring higher strength can be formed from a superconducting wire higher in strength, while a portion requiring lower strength can be formed from a superconducting wire smaller in thickness. Therefore, a superconducting coil having a prescribed number of turns can achieve reduction in size while ensuring high reliability.
The inner circumferential portion may be formed by winding the first superconducting wire. In addition, the outer circumferential portion may be formed by winding the second superconducting wire.
Thus, the inner circumferential portion wound with a diameter of curvature smaller than that of the outer circumferential portion is formed from a superconducting wire higher in strength. Therefore, damage of a superconducting wire caused by a small diameter of curvature can be suppressed.
The first and second superconducting wires joined to each other by the welding portion may be wound to form a racetrack shape having a straight portion and a curved portion. In addition, at least a part of the welding portion may be located at the curved portion.
Thus, at least a part of the welding portion is located at the curved portion during manufacturing of the superconducting coil, so that winding with less loosening is achieved. Therefore, since a position of the welding portion is stabilized, the welding portion is less likely to displace during winding. Thus, damage of the second superconducting wire, that is, a superconducting wire smaller in thickness, at an end portion of the welding portion due to displacement of the welding portion can be prevented.
The welding portion may be located only at the curved portion.
If the welding portion is located across the straight portion and the curved portion, a portion of the welding portion located at the curved portion is less likely to displace as described above, whereas a portion located at the straight portion is likely to displace. Consequently, the welding portion is likely to deteriorate at a boundary between the straight portion and the curved portion. Such deterioration can be prevented by the welding portion located only at the curved portion.
In the superconducting coil above, the welding portion may have a length not shorter than 2 cm.
Thus, the welding portion can have electrical resistance of a value sufficiently low in terms of practical use.
In the superconducting coil above, there may be a height difference between the inner circumferential portion and the outer circumferential portion because a width of the band shape of the first superconducting wire is greater than a width of the band shape of the second superconducting wire. In this case, the superconducting coil may have a spacer portion burying the height difference.
Thus, a gap attributed to the height difference between the inner circumferential portion and the outer circumferential portion can be buried. Therefore, lowering in heat conduction due to this gap can be suppressed.
A superconducting magnet according to the present invention has the superconducting coil described above, a heat insulating container, and a power supply.
The heat insulating container accommodates the superconducting coil. The power supply is connected to the superconducting coil.
According to the superconducting magnet of the present invention, of the inner circumferential portion and the outer circumferential portion of the superconducting coil, one requiring higher strength can be formed from the first superconducting wire, while one requiring lower strength can be formed from the second superconducting wire. Namely, a portion requiring higher strength can be formed from a superconducting wire higher in strength, while a portion requiring lower strength can be formed from a superconducting wire smaller in thickness. Therefore, in a superconducting magnet having a superconducting coil having a prescribed number of turns, while strength required of the superconducting coil can be ensured, reduction in size of the superconducting coil can be achieved by using a superconducting wire smaller in thickness. Therefore, a superconducting magnet can be reduced in size while reliability of the superconducting magnet is ensured.
A method for manufacturing a superconducting coil according to the present invention is a method for manufacturing a superconducting coil having an oxide superconductor, and has the following steps.
An inner circumferential portion is formed by winding one of first and second superconducting wires each having a band shape. After the inner circumferential portion is formed, the first and second superconducting wires are joined to each other by welding. After the first and second superconducting wires are joined to each other, an outer circumferential portion is formed by winding the other of the first and second superconducting wires around the inner circumferential portion. The first superconducting wire is higher in strength than the second superconducting wire. The second superconducting wire is smaller in thickness than the first superconducting wire.
According to the method for manufacturing a superconducting coil of the present invention, the welding portion is formed after the inner circumferential portion is formed. Therefore, damage of a superconducting wire due to the welding portion is not caused during formation of the inner circumferential portion.
As described above, according to the present invention, in a superconducting coil having a prescribed number of turns, reduction in size of a superconducting coil can be achieved while high reliability is ensured.
An embodiment of the present invention will be described hereinafter with reference to the drawings. It is noted that the same or corresponding elements in the drawings below have the same reference characters allotted and description thereof will not be repeated.
Referring mainly to
Superconducting wire 10 is formed by joint of first and second superconducting wires 11, 12 each having a band shape to each other with a welding portion 74. It is noted that “welding” herein is a concept encompassing “soldering”. Therefore, the “welding portion” may be a “soldering portion”.
Preferably, at least a part of welding portion 74 is located at curved portion CR. More preferably, welding portion 74 is located only at curved portion CR.
Welding portion 74 joins first and second superconducting wires 11, 12 to each other over a joint length SL (
Superconducting coil 80 has an inner circumferential portion 73 and an outer circumferential portion 75 in a two-dimensional layout as shown in
Referring mainly to
In addition, first superconducting wire 11 is higher in strength than second superconducting wire 12. It is noted that “strength” herein refers to tensile strength and bending strength. Therefore, superconducting wire 11 is higher in tensile strength and bending strength than second superconducting wire 12. Tensile strength is measured, for example, as a value of tensile stress at which a critical current through a superconducting wire lowers to 95%, and a greater value thereof indicates higher strength. Bending strength is measured, for example, as a diameter of curvature at which a critical current through a superconducting wire lowers to 95%, and a smaller value thereof indicates higher strength. For example, first superconducting wire 11 has tensile strength of 270 MPa, second superconducting wire 12 has tensile strength of 130 MPa, first superconducting wire 11 has bending strength of 60 mm, and second superconducting wire 12 has bending strength of 70 mm.
First and second superconducting wires 11, 12 have widths W1 and W2, respectively. Each of widths W1 and W2 is close approximately to a dimension W (an approximate dimension of superconducting coil 80 in a direction of axis of winding in
Specifically, in the present embodiment, first superconducting wire 11 is formed by sandwiching a wire similar to second superconducting wire 12 between a pair of lamination portions 11a in a direction of thickness. With this structure, thickness T1 is greater than thickness T2, and first superconducting wire 11 is higher in strength than second superconducting wire 12. Lamination portion 11a is made, for example, of stainless steel. The pair of lamination portions 11a is joined with a pair of soldering portions 11b being interposed therebetween. The pair of soldering portions 11b sandwiches a wire similar to first superconducting wire 12 in a direction of width. With this structure, width W1 is greater than width W2.
Second superconducting wire 12 may be, for example, a bismuth (Bi)-based superconducting wire. Specifically, second superconducting wire 12 has a plurality of superconductors 12a extending in a longitudinal direction and a sheath portion 12b covering the entire perimeter of the plurality of superconductors 12a. Sheath portion 12b is in contact with superconductor 12a. Each of the plurality of superconductors 12a is preferably a bismuth-based superconductor having, for example, Bi—Pb—Sr—Ca—Cu—O-based composition, and in particular, a material containing such a Bi 2223 phase that an atomic ratio among bismuth and lead:strontium:calcium:copper is represented in an approximated manner by substantially a ratio of 2:2:2:3 is optimal. A material for sheath portion 12b is made, for example, of silver or a silver alloy. It is noted that a single superconductor 12a may be provided.
A method for manufacturing superconducting coil 80 will now be described.
Referring to
Referring to
Referring to
It is noted that, in order to avoid displacement of the end portion of the first superconducting wire where welding portion 74 has been formed during this joint, this end portion is preferably fixed to inner circumferential portion 73 in advance. This fixation can be achieved, for example, by using a polyimide tape.
By winding second superconducting wire 12 around inner circumferential portion 73 after first and second superconducting wires 11, 12 are joined as above, outer circumferential portion 75 is formed. In winding second superconducting wire 12, tensile force is applied to second superconducting wire 12 in a longitudinal direction thereof. In a case where welding portion 74 is located at curved portion CR, this tensile force applies inward force to welding portion 74. Therefore, superconducting wire 10 in the vicinity of welding portion 74 is wound with less loosening.
As above, superconducting coil 80 (
According to superconducting coil 80 in the present embodiment, of inner circumferential portion 73 and outer circumferential portion 75, one requiring higher strength can be formed from first superconducting wire 11, while one requiring lower strength can be formed from second superconducting wire 12. Namely, a portion requiring higher strength can be formed from a superconducting wire higher in strength, while a portion requiring lower strength can be formed from a superconducting wire smaller in thickness. Consequently, an average value of dimension T (
More specifically, inner circumferential portion 73 is formed by winding first superconducting wire 11, and outer circumferential portion 75 is formed by winding second superconducting wire 12. Thus, inner circumferential portion 73 wound at a diameter of curvature smaller than that of outer circumferential portion 75 is formed from a superconducting wire higher in strength. Therefore, damage of a superconducting wire due to a small diameter of curvature can be suppressed.
In a case where at least a part of welding portion 74 is located at curved portion CR, winding with less loosening is achieved during manufacturing of superconducting coil 80 because at least a part of welding portion 74 is located at curved portion CR. Therefore, since a position of welding portion 74 is stabilized, welding portion 74 is less likely to displace during manufacturing of superconducting coil 80. Thus, second superconducting wire 12, that is, a superconducting wire smaller in thickness, can be prevented from being damaged (such as rupture RP in
In a case where welding portion 74 is located only at curved portion CR, welding portion 74 is not provided at straight portion ST where loosening is likely during manufacturing of superconducting coil 80. Therefore, since a position of welding portion 74 is further stabilized, welding portion 74 is further less likely to displace during manufacturing of superconducting coil 80. Thus, second superconducting wire 12, that is, a superconducting wire smaller in thickness, can further be prevented from being damaged at an end portion of welding portion 74 due to displacement of welding portion 74. Alternatively, if welding portion 74 is located across straight portion ST and curved portion CR, during manufacturing of superconducting coil 80, a portion of welding portion 74 located at curved portion CR is less likely to displace as described above, while a portion located at straight portion ST is likely to displace. Consequently, the welding portion tends to deteriorate at a boundary between straight portion ST and curved portion CR. Such deterioration can be prevented by welding portion 74 located only at curved portion CR.
In a case where welding portion 74 has a length not shorter than 2 cm in superconducting coil 80 above, welding portion 74 can have electrical resistance of a value sufficiently small in terms of practical use.
According to the method for manufacturing superconducting coil 80 in the present embodiment, welding portion 74 is formed after inner circumferential portion 73 is formed. Therefore, unlike a case where inner circumferential portion 73 is wound after first and second superconducting wires 11, 12 are joined to each other with welding portion 74, damage of a superconducting wire, in particular rupture RP (
Though first superconducting wire 11 is employed for inner circumferential portion 73 and second superconducting wire 12 is employed for outer circumferential portion 75 in the present embodiment, in a case where reliability of outer circumferential portion 75 is particularly demanded, first superconducting wire 11 may be employed for outer circumferential portion 75 and second superconducting wire 12 may be employed for inner circumferential portion 73. In addition, width W1 of first superconducting wire 11 does not necessarily have to be greater than width W2 of the second superconducting wire. Moreover, a superconducting coil does not necessarily have to be in a racetrack shape, and the shape may be circular or polygonal.
Referring to
Spacer portion 91 is a spacer burying at least a part of height difference D (
Spacer portion 91 is preferably formed from a sheet made of an insulator, and specifically, it is formed from a prepreg sheet or an FRP (Fiber Reinforced Plastic) sheet.
Cooling plates 93 are arranged to sandwich each superconducting coil 80. Cooling plate 93 serves to thermally connect superconducting coil 80 to a refrigerator head (not shown). Insulating plate 92 is inserted between cooling plate 93 and superconducting coil 80. The plurality of superconducting coils 80 are stacked in a direction of axis of winding with cooling plate 93 and insulating plate 92 being interposed therebetween.
According to the present embodiment, spacer portion 91 can bury a gap created by height difference D. Therefore, lowering in heat conduction caused by this gap (such as lowering in heat conduction between outer circumferential portion 75 and cooling plate 93) can be suppressed.
In addition, in a case where a material for spacer portion 91 is a prepreg sheet or FRP, a difference in coefficient of thermal expansion between spacer portion 91 and superconducting wire 10 can be decreased.
It is noted that, in a case where a superconducting coil is directly cooled by such a fluid as liquid nitrogen, it is not necessary to provide cooling plate 93.
Referring to
According to superconducting magnet 100 in the present embodiment, of inner circumferential portion 73 and outer circumferential portion 75 (
It is noted that, instead of providing refrigerator head 103, a low-temperature fluid such as liquid nitrogen may be used.
Referring to
Referring to
It is noted that, since features other than the above are substantially the same as those in the third embodiment described above, the same or corresponding elements have the same reference characters allotted and description thereof will not be repeated.
Hoop stress is applied to superconducting wire 10 of superconducting coil 290 by magnetic field H generated by superconducting coil 390. Hoop stress becomes greater in proportion to a distance r from the center of winding. Therefore, if a superconducting coil is formed simply by winding one type of superconducting wire, hoop stress applied to the outer circumferential portion is greater than hoop stress applied to the inner circumferential portion.
According to the present embodiment, the inner circumferential portion is formed from second superconducting wire 12 smaller in thickness. As such, while superconducting coil 290 is reduced in size, the outer circumferential portion to which great hoop stress is likely to be applied is formed from first superconducting wire 11 higher in strength. Thus, lowering in reliability attributed to hoop stress can be suppressed.
Hoop stress applied to superconducting wire 10 forming superconducting coil 290 (
Simulation conditions are as follows. A superconducting wire having width W1=4.5 mm, thickness T1=0.30 mm, tensile strength of 270 MPa, and bending strength of 60 mm was employed as first superconducting wire 11 (
As a result of calculation, hoop stress applied to second superconducting wire 12 forming the inner circumferential portion of superconducting coil 29 was 81 MPa at the innermost portion (r=50 mm) and 121 MPa at the outermost portion (r=75 mm). These stresses were within the range of tensile strength of 130 MPa of second superconducting wire 12.
In addition, hoop stress applied to first superconducting wire 11 forming the outer circumferential portion of superconducting coil 29 was 89 MPa at the innermost portion (r=75 mm) and 119 MPa at the outermost portion (r=100 mm). These stresses were within the range of tensile strength of 270 MPa of first superconducting wire 12.
It should be understood that the embodiments and the example disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the embodiments above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.
10 superconducting wire; 11 first superconducting wire; 12 second superconducting wire; 73 inner circumferential portion; 74 welding portion; 75 outer circumferential portion; 80, 90 superconducting coil; 91 spacer portion; 92 insulating plate; 93 cooling plate; 100 superconducting magnet; 101 heat insulating container; 102 power supply; 103 refrigerator head; CR curved portion; and D height difference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2011-120092 | May 2011 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2012/060916 | 4/24/2012 | WO | 00 | 10/2/2013 |
