JP4228110B2 - Open magnet system - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an open-type magnet device, and more particularly to an open-type magnet device suitable for reducing magnet weight.
[0002]
[Prior art]
The open-type magnet device includes a pair of magnet assemblies that are arranged facing each other in the vertical direction and that cover a subject's test portion therebetween, and each magnet assembly serves as a normal conduction or superconductivity that serves as a magnetic field generation source. The main coil is composed of a coil and a ferromagnetic magnetic shield (ferromagnetic plate) disposed so as to surround the periphery of the main coil for reducing the leakage magnetic field from the main coil.
[0003]
[Problems to be solved by the invention]
However, as the magnetic field strength generated in the uniform magnetic field space region increases, the required amount of magnetic shielding tends to increase and the magnet weight tends to increase. When the weight increases, the load on the floor of the room to be installed becomes large, so that the floor needs to be strengthened, and the work for carrying in and installing the magnet becomes difficult and time is required.
[0004]
FIG. 7 is a graph schematically showing a change in the amount of magnetic flux with respect to the radial position at a certain height position in the ferromagnetic plate. From this figure, the amount of magnetic flux is small in the central portion and proceeds to the outer diameter side. It turns out that it increases rapidly. For this reason, in the case of a conventional ferromagnetic plate having a constant thickness, the magnetic flux density (magnetic flux amount / cross-sectional area) inside the ferromagnetic plate also changes substantially proportionally as shown in FIG. On the other hand, in order to suppress the leakage magnetic field, the ferromagnetic material needs to have a thickness that does not cause magnetic saturation. Therefore, it is necessary to set a sufficient thickness so as not to cause magnetic saturation near the outermost diameter portion where the magnetic flux density is the highest. For this reason, on the contrary, an excessive ferromagnetic thickness is used in the inner diameter portion, which causes an increase in the weight of the magnet.
[0005]
Further, for example, in the case of a magnet device using a magnetic shunting member such as a conventional pole piece disclosed in Japanese Patent Laid-Open No. 5-234746, the outside of the ferromagnetic plate is cut to reduce the weight. . This is because the inner side cannot be cut for convenience of disposing the main coil or the magnetic shunt member serving as the magnetic field generation source close to the ferromagnetic plate.
[0006]
In view of the above, an object of the present invention is to provide an open magnet device that effectively reduces the weight of a magnetic shield and suppresses a leakage magnetic field.
Thereby, since the weight of the whole magnet apparatus is reduced, the relaxation of the request about an installation environment, the carrying-in of an apparatus, and the efficiency of installation work are implement | achieved.
[0007]
[Means for Solving the Problems]
In the present invention, the weight of the entire magnet is reduced by thinning the portion having a low magnetic flux density, that is, the central portion in the radial direction of the ferromagnetic plate. Since this portion is far away from the main coil serving as a magnetic field generation source, the amount of magnetic flux is originally reduced. Therefore, even if the ferromagnetic plate is thinned, the influence on the suppression of the leakage magnetic field is small.
[0008]
Further, in the present invention, the magnetic flux density is made substantially constant in the radial direction as shown in FIG. 8 by reducing the thickness at the inner diameter side portion of the ferromagnetic plate. Thereby, the magnet weight can be reduced.
[0009]
Furthermore, the magnet to which the present invention is applied has a structure in which the main coil acting as a magnetic field generation source is separated from the ferromagnetic plate, so that the inside of the ferromagnetic plate, which is a uniform magnetic field space, can be cut freely. It is. This can increase the distance from the main coil serving as the magnetic field generation source to the ferromagnetic plate, compared with the case of cutting the outside, and thus it is possible to cut more ferromagnetic plates. Therefore, the effect of weight reduction is further increased.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
A first embodiment of the present invention will be described with reference to FIGS.
The open-type magnet device 10 has a pair of upper and lower magnet assemblies 12 and 14 facing each other, and each of the pair of upper and lower magnet assemblies 12 and 14 has a main coil 18 composed of a ferromagnetic plate 16 and a superconducting coil. A uniform magnetic field space region 20 is generated between the pair of upper and lower magnet assemblies 12 and 14 by exciting these main coils 18. The pair of upper and lower magnet assemblies 12 and 14 are supported by two ferromagnetic columns 22 and magnetically couple the ferromagnetic plates 16 in the upper and lower magnet assemblies 12 and 14. Each main coil 18 made of a superconducting coil is housed in a cooling container 24, and the upper and lower cooling containers 24 are connected by connecting pipes 26.
[0011]
In this embodiment, as is apparent from FIGS. 2 and 3, the vertical cross-sectional shape of the ferromagnetic plate 16 holds the main surface on the side far from the uniform magnetic field space region substantially flat, while maintaining the uniform magnetic field space region. The main surface close to is processed so that its thickness gradually decreases from the outer diameter side toward the center.
More specifically, the vicinity of the surface coupled with the ferromagnetic column 22 on the main surface close to the uniform magnetic field space region is parallel to the distant main surface, and a substantially conical recess is provided near the center. ing.
[0012]
Although only one set of coils is shown in FIG. 3, in order to obtain high magnetic field uniformity, about 3 to 6 sets of coils are usually used in the vertical direction. However, since a coil having a large magnetomotive force is generally disposed at an outer position having a large diameter, the change in the amount of magnetic flux in the radial direction described with reference to FIG. 7 is basically the same. Therefore, with respect to the magnetic field generating source composed of a plurality of sets of main coils, the inner diameter side portion of the ferromagnetic plate can be made thin as described above, and the same effect can be obtained.
[0013]
FIG. 4 shows a second embodiment of the present invention.
In the present embodiment, in order to facilitate the processing of the ferromagnetic plate 16, a structure in which four unit ferromagnetic plates 160, 162, 164, 166 having a shape with a hole having a different diameter at the center are stacked. It was. Then, the unit ferromagnetic plate 160 having a hole with a smaller diameter than the far side of the uniform magnetic field space region is arranged, and the unit ferromagnetic plates with the holes having a larger diameter are stacked in order, so that the entire ferromagnetic plate is positioned more than the outer diameter side. The center is formed to be thinner. By doing in this way, processing of each ferromagnetic material plate becomes easy and workability of processing can be improved.
Further, in the present embodiment, a hole penetrating through the central portion is formed. Therefore, this hole can be used for introducing illumination light into the gantry, blowing air into the gantry, cooling the gradient coil, wiring of various signal cables, and the like.
[0014]
The unit ferromagnetic plates 160, 162, 164, and 166 are all formed with holes, but the unit ferromagnetic plate 160 on the far side of the uniform magnetic field space region is a flat plate. Also good.
Further, the thickness of each unit ferromagnetic plate may be different. Also, the hole portion on the inner diameter side of each unit ferromagnetic plate may be tapered so as to approximate the shape of the ferromagnetic plate of the first embodiment of FIG. Good.
[0015]
FIG. 5 shows a third embodiment of the present invention.
In the present embodiment, the ferromagnetic plate is disposed on the side closer to the uniform magnetic field space region, the first ferromagnetic plate 28 formed in a flat plate shape, and the second ferromagnetic material disposed on the far side and formed in a donut shape. It is composed of two types of ferromagnetic plates composed of plates 160 and 162. The first ferromagnetic plate 28 serves as a structural material that can withstand electromagnetic force acting between the first ferromagnetic plate 28 and the main coil 18. Since electromagnetic force acts between the main coil 18 and the ferromagnetic shield, it is the simplest and easiest to obtain this with the innermost ferromagnetic plate 28 to obtain structural strength. It is. Since the yield strength is proportional to the square of the thickness, it is advantageous to increase the thickness of one sheet rather than stacking plates of the same thickness. Therefore, the first ferromagnetic plate 28 is made thicker than the constituent members of the second ferromagnetic plates 160 and 162 so that the electromagnetic force and the weight of the ferromagnetic plates 160 and 162 can be supported. .
[0016]
Conversely, the second ferromagnetic plate on the side far from the uniform magnetic field space can be thickened to assume the role as a structural material, and the first ferromagnetic plate can be fixed with a bolt or the like. In this case, the ferromagnetic plate closer to the uniform magnetic field space can increase the diameter of the hole at the center, and thus has a greater weight reduction effect.
The second ferromagnetic plates 160 and 162 have holes with different diameters, and the second ferromagnetic plates 160 and 162 are located farther from the uniform magnetic field space region so that the thickness of the second ferromagnetic plates decreases toward the center. A second ferromagnetic plate 160 having a small-diameter hole is disposed, and a second ferromagnetic plate 162 having a large-diameter hole is disposed on the near side.
[0017]
In each of the embodiments described above, the main coil is simply described, but either a superconducting magnet or a normal conducting magnet can be used as the main coil.
[0018]
【The invention's effect】
ADVANTAGE OF THE INVENTION According to this invention, while reducing the weight of an open type magnet, the leakage magnetic field of the magnetic shield provided in each of a pair of magnet assembly arrange | positioned facing up and down can fully be reduced.
[Brief description of the drawings]
FIG. 1 is an external view of an open type magnet apparatus to which the present invention is applied.
FIG. 2 is an external view in which a part of an upper magnet assembly main part for explaining a first embodiment of the open type magnet apparatus of the present invention is cut.
FIG. 3 is a schematic cross-sectional view of the left half of the main part of the upper magnet assembly for explaining the first embodiment of the open type magnet apparatus of the present invention.
FIG. 4 is a schematic cross-sectional view of the left half of the main part of the upper magnet assembly for explaining a second embodiment of the open type magnet apparatus of the present invention.
FIG. 5 is a schematic cross-sectional view of the left half of the main part of the upper magnet assembly for explaining a third embodiment of the open magnet apparatus of the present invention.
FIG. 6 is a diagram showing a strong position at a certain height position in a ferromagnetic shield for suppressing a leakage magnetic field provided in a pair of upper and lower magnet assemblies constituting an open magnet device for explaining the principle of the present invention; The graph which showed typically the change of the amount of relative magnetic flux with respect to the relative distance to the outer diameter direction from the magnetic body shield center.
FIG. 7 is a graph schematically showing a difference in relative magnetic flux density with respect to a relative radial distance inside a ferromagnetic shield provided in a magnet assembly of the present invention and a conventional apparatus.
8 is a graph showing the relative radial distance in three ferromagnetic plates constituting a ferromagnetic shield provided in a magnet assembly corresponding to the second embodiment of the present invention shown in FIG. A graph schematically showing the difference in relative magnetic flux density.
9 is relative to the relative radial distance in the three ferromagnetic plates constituting the ferromagnetic shield provided in the magnet assembly in the fourth embodiment of the present invention shown in FIG. 6; The graph which shows the difference in magnetic flux density typically.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Open type magnet apparatus 12 Upper magnet assembly 14 Lower magnet assembly 16 Ferromagnetic plates 160, 162, 164, 166 Unit ferromagnetic plate 18 Main coil 20 Uniform magnetic field space region 22 Ferromagnetic column 24 Cooling vessel 26 Connecting pipe 28 Reinforcing and supporting ferromagnetic plate 30 Gap

Claims (2)

A pair of magnet assemblies that generate a uniform magnetic field space region that is disposed facing each other in the vertical direction and that covers a test portion of the subject, and the pair of magnet assemblies generate a uniform magnetic field. And a magnetic shield for reducing leakage magnetic flux from the magnetic field generation source, and the main surface on the side far from the uniform magnetic field space region is maintained substantially flat in the profile of the vertical cross section of the magnetic shield, and The open magnet device is characterized in that the thickness is configured so as to be thinner toward the center in the radial direction ,
The magnetic shield is composed of a plurality of unit magnetic shields, and each unit magnetic shield is provided with a concentric hole, and the diameter of each hole increases in order from the side closer to the uniform magnetic field space region. The open-type magnet device according to claim 1, wherein the open-type magnet device is selected. Each of the magnet assemblies further includes a second magnetic shield for reinforcing and supporting the magnetic shield, and the second magnetic shield is attached to the magnetic shield on the side close to or far from the uniform magnetic field space region. The open-type magnet device according to claim 1 , wherein the open-type magnet device is provided adjacent to the open magnet device.

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