JP2007075470A - Superconducting magnet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inexpensive superconducting magnet capable of keeping high reliability for a long period of time by shielding the invasion of a disturbance and suppressing the temporal change of a magnetic field. <P>SOLUTION: The superconducting magnet including: a container for storing liquid helium; a coil stored inside the container to generate a main magnetic field; and a radiation shield for covering the outer periphery of the container is provided with a second superconducting coil which is electrically independent of the coil for generating the main magnetic field and is arranged at the outer side of the radiation shield. In this case, the winding axis of the coil for generating the main magnetic field is made to be in parallel with the winding axis of the second superconducting coil. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a superconducting magnet, and more particularly to a superconducting magnet for a nuclear magnetic resonance apparatus that needs to generate a uniform magnetic field with excellent time stability.

  Conventionally, a magnet capable of suppressing fluctuation of an external magnetic field and shielding a leakage magnetic field, and a nuclear magnetic resonance apparatus using such a magnet have been known.

  For example, Patent Literature 1 describes a magnet that suppresses disturbance of leakage magnetic field and magnetic field uniformity, and an NMR analyzer and a medical MRI apparatus that use this magnet. This magnet suppresses the leakage magnetic field from the magnet to the outside and the disturbance of the magnetic field uniformity in the sample space by making one of the double outer cylinders of the cryogenic container or its lid part a magnetic material.

  Patent Document 2 describes that an MRI apparatus using a horizontal magnetic field type magnet or the like is installed in a magnetic shield.

Japanese Patent Laying-Open No. 2005-093464 JP 2002-172102 A

  An MRI apparatus is usually installed in a hospital, but it is difficult to maintain a uniform magnetic field in a place with a large vibration, for example, a place where a disturbance such as a large automobile, a train, or an elevator passes. In parallel with this, it is necessary to reduce the leakage magnetic field from the magnet to the outside as much as possible.

  An object of the present invention is to provide an inexpensive superconducting magnet such as an MRI apparatus or an NMR apparatus that can maintain high reliability over a long period of time by shielding intrusion of disturbance and suppressing temporal fluctuation of a magnetic field. There is.

  The superconducting magnet of the present invention includes a container for storing liquid helium, a coil for generating a magnetic field (also referred to as “main magnetic field”) stored in the container, and a radiation shield that covers the outer periphery of the container. This can be achieved by providing a second superconducting coil that is electrically independent from the coil that generates the magnetic field, outside the radiation shield.

  Generally, a shield coil for suppressing disturbance is more efficient when the superconducting loop is made as large as possible. Since the 2nd superconducting coil in this invention is wound around the radiation shield installed in the outer side in a magnet apparatus, it becomes possible to enlarge a superconducting loop. Thereby, it can shield efficiently. Therefore, for example, there is a merit that the shield efficiency is increased by 15 to 20% compared to the case where the second superconducting coil is disposed inside the liquid helium container.

  It is necessary to make the winding axis of the coil for generating the main magnetic field parallel to the winding axis of the second superconducting coil. This is a technique for suppressing magnetic field fluctuations due to disturbance. Experiments have confirmed that disturbances cannot be shielded when both winding axes are vertical.

  At this time, the second superconducting coil is disposed above and below the radiation shield. In other words, in order to shield the incoming disturbance, it is important to arrange them on the upper and lower edges when viewed from the coil for generating the main magnetic field. Thus, if the winding axis is the same and the coil is installed above the upper end and lower than the lower end of the coil for generating the main magnetic field, the winding shape and the installation location may differ from the design by about 25%. Further, there is no particular problem even if the winding density and winding accuracy differ by about 25%.

  Further, it is preferable that the coil for generating the main magnetic field is cooled by conduction by the immersed liquid helium, and the second superconducting coil is cooled by a refrigerator.

  In general, the MRI apparatus is equipped with a refrigerator to promote cooling, which cools the radiation shield. For this reason, the enlargement of the apparatus does not occur.

  The operating temperature of the second superconducting coil is preferably 10K to 35K.

The second superconducting coil can be achieved by including magnesium and boron. A typical example is an MgB ₂ superconductor.

  Further, in a superconducting magnet including a container for storing liquid helium, a coil for generating a magnetic field accommodated in the container, and a radiation shield covering the outer periphery of the container, the coil for generating a magnetic field is connected in series. This can be achieved by providing a leakage magnetic field shield superconducting coil outside the radiation shield.

  This can be achieved by generating a uniform magnetic field using a superconducting magnet and measuring the magnetic resonance phenomenon of the specimen placed in this space.

  The MRI apparatus and NMR analysis apparatus of the present invention can be provided as an apparatus that achieves the object of the present invention by using the superconducting magnet of the present invention or in combination.

  According to the present invention, there are provided superconducting magnets such as an inexpensive MRI apparatus and NMR apparatus that can maintain high reliability over a long period of time by shielding the intrusion of disturbance and suppressing temporal fluctuation of the magnetic field. be able to.

  In the present invention, by wrapping the second superconducting coil around the radiation shield, two types of effects can be expected.

  The first is a function as an external magnetic field fluctuation shield coil. The external magnetic field fluctuation shield coil according to the present invention is applied to a superconducting magnet that needs to ensure a precise magnetic field distribution and uniformity, and is intended to suppress a change in the uniform magnetic field. Mainly used for MRI and NMR analyzers. In particular, the present invention is characterized in that the second superconducting coil, which is electrically independent from the coil that generates the main magnetic field housed in the liquid helium container, is provided outside the radiation shield. In contrast to the superconducting coil that generates the main magnetic field, the second superconducting coil has a very small current capacity of 1/10 to 1/100, and therefore has an electrically independent structure.

  The second is a function as a leakage magnetic field shield coil. The leakage magnetic field shield coil in the present invention is intended to reduce magnetic field leakage from the coil that generates the main magnetic field. In general, a method is employed in which a shield coil that generates a magnetic field in the opposite direction to the main magnetic field is disposed inside the magnet to cancel the leakage magnetic field to the outside. This also applies mainly to MRI apparatuses and NMR analyzers.

  The difference from the external magnetic field fluctuation shield coil is that a coil for generating a main magnetic field stored in a liquid helium container and a superconducting coil are connected in series. This is because the current capacity of the coil that generates the main magnetic field and the second superconducting coil needs to be the same. In this case, the wire used for the coil that generates the main magnetic field and the leakage magnetic field shield coil It is necessary to connect the used wire with low resistance. Therefore, a superconducting wire that does not realize superconducting connection cannot be used.

The second superconducting coil is installed outside the radiation shield. However, if NbTi wire is used, if some disturbance energy enters, the superconducting wire rises to Tc or more, and in the worst case, quenching occurs. For this reason, a material having a high Tc is required. Among several superconducting materials, it is most effective to use a MgB ₂ superconductor in consideration of the convenience in producing a wire or coil, ease of low resistance connection, and the like.

Since the Tc of MgB ₂ is 39K, the inventors have confirmed through experiments that the performance necessary for the second superconducting coil can be maintained even if the temperature rises to about 35K.

  As a similar coil of the second superconducting coil in the present invention, there is a shim coil. The shim coil is a magnetic field correction coil installed for the purpose of improving the magnetic field uniformity. Generally, it is static because it corrects a distorted magnetic field. On the other hand, the second superconducting coil of the present invention can be said to be dynamic because temporal magnetic field fluctuations are suppressed in minutes or seconds. From the above, the second superconducting coil is different from a so-called shim coil.

  Thus, according to the magnet of the present embodiment, the design and structure of the magnet are facilitated. It is also considered that there is an effect that a uniform magnetic field is maintained for a long time.

  Furthermore, it is thought that an effect can be acquired at low cost. In addition, in the sample space at the center of the magnet, the magnetic field uniformity is not disturbed, and the magnetic field fluctuation due to the disturbance can be solved.

  Hereinafter, the present invention will be described based on examples. However, the present invention is not limited to these.

Example 1
FIG. 1 is a conceptual diagram of a superconducting magnet in the present invention. This figure is a simplified version of the image of a nuclear magnetic resonance apparatus, but in reality, many correction coils and the like are installed in order to obtain the required performance.

  There is a uniform magnetic field space 1 at the center of the superconducting magnet.

  In the case of an MRI apparatus, since it is necessary to take an image while a human is lying on his / her back, a space is provided through which a person's shoulder width and abdomen can pass.

  On the other hand, in the case of an NMR analyzer, since the sample is a solution or the like in a test tube, the uniform magnetic field space is usually several mm to several cm.

  A superconducting coil 3 that generates a main magnetic field is housed in a liquid helium container 2.

  The superconducting coil 3 that generates the main magnetic field is cooled to a temperature of 4.2 K while being immersed in the liquid helium 4. A radiation shield 5 is placed on the outside thereof. These are all stored in the vacuum heat insulating container 6.

The superconducting coil of the present invention (hereinafter referred to as “external magnetic field fluctuation shield coil” to be distinguished from the superconducting coil 3 that generates the main magnetic field) 7 is wound around the radiation shield 5. The temperature at the position where the coil was wound was 25K in a steady state. In this embodiment, the MgB ₂ superconducting wire 8 is used for the external magnetic field fluctuation shield coil 7. After heat-treating the wire at 600 ° C in advance, the surface of the wire is insulated with enamel, and the MgB ₂ superconducting wire 8 with a diameter of 0.8 mm is brought into contact with the outer surface of the radiation shield 5 and wound in a ring shape for 10 turns to shield the external magnetic field fluctuation shield Coil 7 was obtained. At this time, in order to shield the invading disturbance, the superconducting coil 3 generating the main magnetic field was installed above the upper end and below the lower end.

  The external magnetic field fluctuation shield coil 7 was directly wound at a position 0.5 m away from the coil center of the superconducting coil 3 that generates the main magnetic field. Thereafter, the external magnetic field variation shield coil 7 was reinforced with an epoxy resin. The winding directions of both coils were parallel. This is because if the winding direction is different, the disturbance, which is a feature of the present invention, cannot be suppressed. Example 2 describes the study results.

  FIG. 2 is a schematic circuit diagram of a connection portion of the external magnetic field fluctuation shield coil 7. The tip 9 of the wire and the rear end 10 of the wire were connected to form a superconducting loop. In this embodiment, the connecting portion 11 is configured using Pb-50Bi solder.

The connection portion 11 is desirably as low resistance as possible, but the required resistance value is (1) the magnitude of the inductance of the coil in the entire circuit, (2) the resistance value including the connection portion of the entire circuit, and (3) It depends on how long it will run in steady state. In this example, considering the above three points, both ends were connected so as to have a resistance of 10 ⁻¹¹ Ω or less. A heater wire 12 is wound around the connecting portion 11.

  Two external magnetic field fluctuation shield coils 7 having a superconducting loop were arranged in the height direction as shown in FIG. The energization current of the coil varies according to the design specifications at that time. However, the accuracy of the winding state and the installation location is not so important, and if the deviation is about 25%, no problem occurs.

In this embodiment, MgB ₂ wire is used for the external magnetic field fluctuation shield coil 7. The diameter of this wire was 0.8 mm. FIG. 3 shows an example of a schematic cross-sectional view of the MgB ₂ superconducting wire produced.
In the cross section of the MgB ₂ superconducting wire 8, a superconducting filament 14 is filled or included in a metal sheath material 13. The shape of the cross section is not limited to a round shape, and various wire shapes from a fan shape to a wide ultra-thin tape can be used as long as they satisfy the specifications required for a superconducting magnet. But it doesn't matter.

  The superconducting coil 3 for generating the main magnetic field was produced using NbTi wire. The diameter of the wire is 1 mm. This wire was wound into a coil shape to obtain a superconducting coil having an outer diameter of 1.0 m and a height of 2.0 m. The current value (operation current) for obtaining the necessary main magnetic field is 400A.

On the other hand, the external magnetic field fluctuation shield coil 7 was manufactured using MgB ₂ wire. The diameter of the wire is 0.8mm. The shield current required for suppressing the change of the uniform magnetic field due to the fluctuation of the external magnetic field is approximately 2A. As described above, since the energization currents are different, it is necessary that both have independent electric circuits.

The operation method is as follows.
(1) When a current is applied to the superconducting coil 3 that generates the main magnetic field and the magnetic field is raised or lowered, the external magnetic field fluctuation shield coil 7 is heated with a heater to be in a normal conducting state.
(2) When 400 A is supplied to the superconducting coil 3 that generates the main magnetic field and enters a steady state (static magnetic field), the heating of the external magnetic field fluctuation shield coil 7 is stopped to bring it into a superconducting state.

  Thus, since the external magnetic field fluctuation shield coil 7 needs to switch between the superconducting state and the normal conducting state, the heater wire 12 is wound around the connecting portion 11 and the heater is turned off in the superconducting state. Turn on the heater to switch to the normal conduction state.

As described in (1) and (2) above, 400 A was energized to the coil that generates the main magnetic field to reach a steady state, and then the external magnetic field fluctuation shield coil 7 was put into a superconducting state. Then, an external disturbance was applied by intentionally applying an acceleration of 0.8 to 1.3 gal (cm / s ² ) from the outside to cause a magnetic fluctuation. And the magnetic field uniformity at that time was measured. As a comparison, measurement was also performed when the external magnetic field fluctuation shield coil 7 was not in a superconducting state.

  The results are shown in Table 1. When the external magnetic field fluctuation shield coil 7 is in a superconducting state, the magnetic field uniformity is 0.01 ppm or less even when a disturbance is applied, but when the external magnetic field fluctuation shield coil 7 is not energized, the magnetic field uniformity is 50 ppm or more. It was. As described above, it has been clarified that by forming the permanent current loop with the external magnetic field fluctuation shield coil 7 in the superconducting state, the magnetic field uniformity is not affected even by disturbance.

  Moreover, although the present Example is the structure which winds the external magnetic field fluctuation | variation shield coil 7 to the radiation shield 5, the same effect was acquired even when it wound so that it might contact with the outer wall of the liquid helium container 2 thermally. . When the external magnetic field fluctuation shield coil 7 is wound around the outer periphery of the superconducting coil 3 that generates the main magnetic field, the workability is poor and the yield is about 50% because the gap with the inner wall of the liquid helium container 2 is very narrow. However, by winding around the radiation shield 5 and the outer wall of the liquid helium container 2, the workability was greatly improved and the yield was 100%.

(Example 2)
As in the first embodiment, in the superconducting magnet configured as shown in FIG. 1, the winding axis of the superconducting coil 3 that generates the main magnetic field and the winding axis of the external magnetic field variation shield coil 7 are (1) parallel. (2) Magnetic field homogeneity of the superconducting coil 3 that generates a main magnetic field by intentionally giving a disturbance so as to be 0.8 to 1.3 gal (cm / s ² ) from the outside in the case of being vertical. The change of was measured.

  Table 2 shows the magnetic field uniformity of the external magnetic field fluctuation shield coil.

  As described above, it is confirmed that the winding axis of the superconducting coil 3 that generates the main magnetic field needs to be parallel to the winding axis of the external magnetic field fluctuation shield coil 7 in order to suppress the fluctuation magnetic field generated by the disturbance. did.

(Example 3)
As in the first embodiment, in the superconducting magnet configured as shown in FIG. 1, the superconducting coil 3 that generates the main magnetic field is cooled with liquid helium, and the external magnetic field variation shield coil 7 is a commercially available GM refrigerator with a temperature of 8K. Cooled to ~ 35K. And the disturbance was intentionally given so that it might become 0.8-1.3 gal (cm / s < ² >) from the outside, and the change of the magnetic field uniformity was measured.

In this example, measurement was performed for the case where NbTi wire, Nb ₃ Sn wire, MgB ₂ wire, Y-123 wire, and Bi-2212 wire were used for the external magnetic field fluctuation shield coil 7. Table 3 shows the magnetic field uniformity when the temperature of the external magnetic field fluctuation shield coil 7 is changed by the refrigerator.

As described above, when the temperature is 15K or higher for the NbTi line and 20K or higher for the Nb ₃ Sn line, the magnetic field uniformity deteriorates rapidly. This depends on the Tc of each superconducting material. That, NbTi superconductor Tc is 9.8K, since Nb ₃ Sn superconductors Tc is 18K, the external temperature exceeds Tc, can not be controlled variation of the magnetic field from the disturbance.

In this embodiment, the above five types of superconductors are used for the external magnetic field fluctuation shield coil 7, but oxide superconductors typified by Y-based, Bi-based, Ti-based, and Hg-based materials, and other organic superconductors, etc. The same effect can be obtained even if various superconductors having a relatively high Tc are used. However, when the temperature is set to 35K or lower using a GM refrigerator, it is preferable to use a MgB ₂ superconductor in consideration of the manufacturing cost and bending strain characteristics of the wire.

Example 4
FIG. 4 is a conceptual diagram of the superconducting magnet in the present invention. Although the basic configuration is the same as that of FIG. 1, the present embodiment is a superconducting magnet for the purpose of reducing the leakage magnetic field from the superconducting coil 33 that generates the main magnetic field, rather than suppressing the fluctuation of the external magnetic field. This figure is also a simplified diagram of an MRI apparatus and an NMR analyzer, but in reality, a large number of correction coils and the like are installed to obtain the required performance, and the structure is very complicated.

  There is a uniform magnetic field space 31 at the center of the superconducting magnet. It is composed of a superconducting coil 33 that generates a main magnetic field housed in a liquid helium container 32. The superconducting coil 33 that generates the main magnetic field is cooled to a temperature of 4.2 K while being immersed in the liquid helium 34. A radiation shield 35 is placed on the outside thereof. These are all housed in a vacuum insulation container 36.

The leakage magnetic field shield superconducting coil 37 of the present invention is wound around the outer wall of the liquid helium container 32. The temperature at which the coil was wound was 6.5K in a steady state. In this embodiment, MgB ₂ superconducting wire 38 is used for the leakage magnetic field shield superconducting coil 37.

After heat-treating the wire in advance at 700 ° C., an MgB ₂ superconducting wire 38 having a diameter of 1.6 mm, whose surface was insulated with enamel, was brought into thermal contact with the outer wall of the liquid helium container 32 and wound in a ring shape for three turns.

  The leakage magnetic field shield superconducting coil 37 was directly wound around the outer wall of the liquid helium container 32 at a position 0.5 m away from the coil center of the superconducting coil 33 that generates the main magnetic field. Thereafter, the leakage magnetic field shield superconducting coil 37 was impregnated and reinforced with an epoxy resin.

FIG. 5 is a circuit diagram of the connection part of the leakage magnetic field shield coil magnetic field. The tip 40 of the wire from which the leakage magnetic field shielded superconducting coil 37 is manufactured, the tip 41 of the wire from which the superconducting coil 33 generating the main magnetic field is produced, and the rear end 42 of the wire from which the leakage magnetic field shielded superconducting coil 37 is manufactured and the main The rear end 43 of the wire rod from which the superconducting coil 33 that generates a magnetic field was produced was connected so as to have a low resistance, thereby forming a connection portion 39.

The superconducting coil 33 that generates the main magnetic field was produced using NbTi wire. The diameter of the wire is 1.1mm. This wire was wound into a coil shape to obtain a superconducting coil having an outer diameter of 1 m and a height of 2 m. The current value (operation current) for obtaining the necessary main magnetic field is 400A. On the other hand, the leakage magnetic field shield superconducting coil 37 was manufactured using MgB ₂ wire. The diameter of the wire is 1.6mm. Since the superconducting coil 33 that generates the main magnetic field and the leakage magnetic field shield superconducting coil 37 are connected in series, 400 A is energized as a shield current.

  As described above, 400 A was energized, and after reaching a steady state, the leakage magnetic field to the outside was measured. As a comparison, measurement was also performed when no leakage magnetic field shield superconducting coil 37 was installed. The measurement location is a position 50 cm from the lower end of the vacuum heat insulating container 36 of the superconducting magnet.

  The results are shown in Table 4. When the leakage magnetic field shield superconducting coil 37 was installed, the leakage magnetic field was 5 gauss or less, but when the leakage magnetic field shield superconducting coil 37 was not installed, the leakage magnetic field was 300 gauss or more.

  As described above, it has been clarified that the leakage magnetic field can be significantly reduced by connecting the superconducting coil 33 that generates the main magnetic field and the leakage magnetic field shield superconducting coil 37 in series as in the present embodiment.

In this embodiment, the MgB ₂ superconductor is applied to the leakage magnetic field shield superconducting coil 37. However, the Tc of oxide superconductors typified by Y-based, Bi-based, Tl-based, and Hg-based materials, and other organic superconductors. The same effect can be obtained even if various superconductors having a relatively high value are used. However,
If it can be used at a temperature of 35K or lower, considering the manufacturing cost and bending strain characteristics of the wire,
It is preferred to use a MgB ₂ superconductor.

  In addition, the inventors have confirmed through experiments that the same effect can be obtained even when the leakage magnetic field shield superconducting coil 37 is wound around the radiation shield 35.

The conceptual diagram of the superconducting magnet in this invention. The circuit schematic diagram of the connection part of an external magnetic field fluctuation shield coil. The cross-sectional schematic diagram of a superconducting wire. The conceptual diagram of the superconducting magnet in this invention. The circuit diagram of the connection part of a leakage magnetic field shield coil.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1,31 ... Uniform magnetic field space, 2,32 ... Liquid helium container, 3,33 ... Superconducting coil which produces | generates a main magnetic field, 4,34 ... Liquid helium, 5,35 ... Radiation shield, 6,36 ... Vacuum insulation container, 7 ... External magnetic field fluctuation shield coil, 8, 38 ... MgB ₂ superconducting wire, 9, 10 ... Wire tip end side of external magnetic field fluctuation shield coil, 11, 39 ... Connection part, 12 ... Heater wire, 13 ... Metal sheath material, 14 ... superconducting filament, 37 ... leakage magnetic field shield superconducting coil,
40 ... Lead end of wire rod of leakage magnetic field shield coil, 41 ... Front end of wire rod of superconducting coil generating main magnetic field, 42 ... Rear end portion of wire rod of leakage magnetic field shield coil, 43 ... Rear end of wire rod of superconducting coil generating main magnetic field Department.

Claims (8)

In a superconducting magnet including a container for storing liquid helium, a coil for generating a magnetic field housed in the container, and a radiation shield covering the outer periphery of the container,
A superconducting magnet, wherein a superconducting coil that is electrically independent from a coil that generates the magnetic field is provided outside the radiation shield.   The superconducting magnet according to claim 1, wherein a winding axis of the coil for generating the magnetic field and a winding axis of the superconducting coil are made parallel to each other.   The superconducting magnet according to claim 1, wherein the coil for generating the magnetic field is cooled by immersed liquid helium, and the superconducting coil is cooled by a refrigerator.   The superconducting magnet according to claim 1, wherein the superconducting coil has an operating temperature of 10K to 35K.   The superconducting magnet according to claim 1, wherein the superconducting coil is made of a superconductor containing magnesium and boron. In a superconducting magnet including a container for storing liquid helium, a coil for generating a magnetic field housed in the container, and a radiation shield covering the outer periphery of the container,
A superconducting magnet, wherein a leakage magnetic field shield superconducting coil connected in series with the coil for generating a magnetic field is provided outside the radiation shield.   A nuclear magnetic resonance apparatus system that generates a uniform magnetic field using the superconducting magnet according to claim 1 and measures a magnetic resonance phenomenon of an inspection object placed in the space. A nuclear magnetic resonance apparatus system that generates a uniform magnetic field using the superconducting magnet according to claim 6 and measures a magnetic resonance phenomenon of an inspection object placed in the space.

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