JP2000046999A - Uniform magnetic field generator - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a uniform magnetic field generator where the structure of a coil is simple and which can simplify a cryostat and a supporting structure. SOLUTION: A coil group A is formed by laying out like a plane a main coil 1, a trim coil 2 and a shield coil 3 which have approximately similar two-dimensional forms. A coil group B that is constituted like the coil group A is laid out in parallel at intervals in a direction perpendicular to the plane of the coil group A. The current whose direction is opposite to that through the main coil 1 is passed through the trim coil 2 and the shield coil 3. A magnetic field generated by the trim coil 2 works so as to correct a magnetic field of the main coil 1 and improve the magnetic field uniformity in a uniform magnetic field area in the center. A magnetic field generated by the shield coil 3 works so as to reduce an outside leak magnetic field below a prescribed value. A uniform magnetic field is generated in the center of the area sandwiched between two pairs of coil groups A and B.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a uniform magnetic field generator capable of generating a spatially uniform magnetic field and having a small leakage magnetic field and useful for medical equipment, accelerators, and the like.

[0002]

2. Description of the Related Art A uniform magnetic field generator capable of generating a spatially uniform magnetic field having a small magnetic field gradient is used for an MRI, an accelerator and the like. For example, in a ring cyclotron apparatus for accelerating a charged particle beam, a sector electromagnet is used as a magnetic field generator for bending the trajectory of the charged particles in a ring shape, and a beam injection system for injecting the charged particle beam into the ring cyclotron apparatus Alternatively, a uniform magnetic field generator for bending a beam path is arranged in a beam extraction system for extracting a charged particle beam accelerated by a ring cyclotron apparatus to the outside.

A uniform magnetic field generator arranged in a beam incidence system or a beam extraction system of a ring cyclotron device is required to generate a uniform magnetic field on a beam path and to have a small leakage magnetic field to the outside. This is because, if there is a stray magnetic field, the charged particle beam passing through the orbit near the magnetic field generator is affected, and the performance of the ring cyclotron apparatus is reduced.

FIG. 11 is a schematic perspective view of a uniform magnetic field generator disposed in a beam incidence system and a beam extraction system of a conventional ring cyclotron apparatus, and FIG. 12 is a sectional view taken along the line AA 'of FIG. The uniform magnetic field generator includes main coils 100, 101, and 10 for generating a uniform magnetic field in the y direction in an area 106 along a curved beam path in the xz plane of the drawing.
3, 104, and the leakage magnetic field to the outside of the device, in particular, x in FIG.
Shield coil 10 for canceling the on-axis leakage magnetic field
2,105. Each coil 100-1 is generated so that a spatially uniform magnetic field can be generated on the beam path.
05 is curved along the beam path.

To generate a uniform magnetic field in the region 106,
It is preferable that the line connecting the center of the region 106 and the center of the cross section of the magnetic field generating coil is arranged at ± 30 ° with respect to the x-axis. Therefore, as shown in the cross-sectional view of FIG. 12, the cross-sectional centers of the main coils 100 and 101 and the cross-sectional center of the shield coil 102 are located on a line at 30 ° with respect to the x-axis. The center and the cross-sectional center of the shield coil 105 are -30 with respect to the x-axis.
° are located on the line. Also,
The main coils 101 and 103 have their ends 101 so as not to block the beam passing through the region 106.
It has a three-dimensional shape rising in the y-direction at a, 101b, 103a, and 103b (see FIG. 11).

[0006]

In order to generate a uniform magnetic field, the main coil and the shield coil are usually arranged such that the cross section of the coil is located on a line of about 30 °.
However, if this arrangement is adopted, the coil shape including the end shape of the main coil becomes three-dimensional and complicated as described above, and the production is not easy. In addition, since the direction in which the uniform magnetic field region can be accessed is limited to both ends, it is disadvantageous when this uniform magnetic field generator is used for other purposes. Furthermore, NiT operating this uniform magnetic field generator at cryogenic temperature
In the case of using a superconducting coil such as b, Nb ₃ Sn or the like, there is a problem that the shape of a cryostat that cools the coil with a coolant such as liquid helium becomes complicated or the coil supporting structure becomes complicated. SUMMARY OF THE INVENTION The present invention has been made in view of the above problems of the related art, and has as its object to provide a uniform magnetic field generator in which a coil structure is simple and a cryostat and a support structure can be simplified.

[0007]

In the present invention, the above object is achieved by arranging a main coil which mainly generates a uniform magnetic field, a shield coil which cancels out a leakage magnetic field, and the like in a plane. Usually, for example, an active shield type M
A uniform magnetic field can be generated by disposing a shield coil outside the main coil like an RI magnet. However, if the main coil and the shield coil are arranged in a plane to make the uniform magnetic field generator easy to use, it is necessary to increase the number of coils. According to the study of the present inventors, it has been found that, at this time, if the trim coil for correcting the magnetic field is arranged inside the main coil, the magnetic field uniformity of the uniform magnetic field region can be improved with a small number of coils.

FIG. 1 is a conceptual diagram of a uniform magnetic field generator according to the present invention. As shown, the uniform magnetic field generating apparatus according to the present invention includes a main coil 1, a trim coil 2 disposed inside the main coil 1, and a main coil 1.
Consisting of a shield coil 3 arranged outside the
Each coil has a substantially similar two-dimensional shape. A current in a direction opposite to that of the main coil 1 flows through the trim coil 2 and the shield coil 3. The magnetic field generated by the trim coil 2 acts to correct the magnetic field of the main coil 1 to increase the magnetic field uniformity in the central uniform magnetic field region, and the magnetic field generated by the shield coil 3 reduces the external leakage magnetic field to a predetermined value. In order to reduce the value below.

These are arranged in a plane as one coil group A. Preferably, the two sets of coil groups A and B are arranged in parallel at a distance in a direction perpendicular to the plane of the coil group so as to form a uniform magnetic field. Configure the generator. The uniform magnetic field is generated at a central portion of a region sandwiched between the two coil groups A and B. The uniform magnetic field region has a shape similar to the two-dimensional planar shape of the main coil, the trim coil, and the shield coil. Therefore, by selecting these coil shapes, a circular region, an elliptical region, a rectangular region, an elongated linear region, Any shape, such as a curved curved area, can be used.

The main coils are preferably arranged so as to be located on a line of ± 30 ° with respect to the median plane, similarly to the arrangement of the main coils shown in FIG. Due to the positional relationship between the trim coil and the shield coil, the magnetic field generated from the shield coil hardly affects the magnetic field uniformity in the uniform magnetic field region, and the magnetic field generated from the trim coil hardly affects the leakage magnetic field.

According to the uniform magnetic field generator of the present invention, the main coil does not have a rising portion as in the prior art,
The coil group can be formed in a three-dimensional shape, and a coil group including a main coil, a trim coil, and a shield coil can be arranged in a plane. Therefore, when a superconducting coil is used, the cryostat used for cooling the coil can have a simple plate-like structure, and the structure for supporting the coil in the cryostat can be simplified.

Further, since the shape of the coil is planar, it is easy to manufacture the magnet coil, and it is also possible to access the uniform magnetic field region from the side of the uniform magnetic field generator, which has been impossible in the past. . The main coil, the trim coil, and the shield coil can also be configured by stacking thin coils in multiple layers. In this case, the degree of freedom in coil arrangement can be increased, and a uniform magnetic field can be generated in a uniform magnetic field region by adjusting the current flowing through the coils of each layer.

[0013]

Embodiments of the present invention will be described below with reference to the drawings. Here, a uniform magnetic field generator used for a charged particle beam incidence system and a take-out system of a ring cyclotron device will be described as an example. However, the present invention is also applicable to medical diagnostic devices such as MRI and analysis using other uniform magnetic fields. Of course, it can be applied to various uses such as equipment.

FIG. 2 is a plan view conceptually showing a ring cyclotron apparatus. This ring cyclotron device is a uniform magnetic field generator 11 constituting a charged particle injection system.
a, 11b, 11c, sector electromagnets 12a, 12b,
12c, 12d, high-frequency accelerating cavities 13a, 13b, and uniform magnetic field generator 1 constituting a charged particle beam extraction system
4a and 14b. Uniform magnetic field generators 11a, 11
b, 11c, sector electromagnets 12a, 12b, 12c,
12d and the uniform magnetic field generators 14a and 14b both generate a DC magnetic field in a direction perpendicular to the plane of the paper.

The charged particle beam 10 enters the magnetic field generated by the sector electromagnets 12a, 12b, 12c and 12d through the uniform magnetic field generators 11a, 11b and 11c constituting the charged particle incident system. In the case of the example in the figure, the charged particle beam 10 is bent by 90 ° by each sector electromagnet, and draws a substantially rectangular trajectory. When passing through the high-frequency accelerating cavities 13a and 13b, the charged particle beam 10 is accelerated, gradually moves to the outer orbit in a spiral shape, and finally passes through the uniform magnetic field generators 14a and 14b constituting the charged particle beam extraction system to the outside. Is taken out.

Next, the structure of a uniform magnetic field generator of a charged particle beam incident system and an extraction system will be described. Here, the structure will be described by taking the uniform magnetic field generator 11a of the charged particle beam incident system as an example, but other uniform magnetic field generators 11b, 11b,
11c, 14a, and 14b have the same structure. FIG. 3 is a schematic perspective view of the uniform magnetic field generator 11a, and FIG.
It is B 'sectional drawing.

This uniform magnetic field generator comprises a first coil group consisting of three types of coils 20, 21, 22 arranged in a circular arc shape in a plane with each other, and a three-dimensional coil similarly arranged in a planar shape in a circular arc shape. A uniform magnetic field region 26 is formed by two coil groups of a second coil group consisting of coils 23, 24, and 25 of different types, and is located at the center of a space sandwiched by two planar upper and lower coil groups. Generates a uniform magnetic field in the y direction.

The first coil group includes a main coil 20 and
Trim coil 21 arranged inside main coil 20
And a shield coil 22 arranged outside the main coil 20. The second coil group similarly includes a main coil 23, a trim coil 24 disposed inside the main coil 23, and a shield coil 25 disposed outside the main coil 23. Two main coils 2
Currents in the same direction flow through 0, 23, and the trim coil 2
Currents in the opposite directions to the main coils 20, 23 are passed through the shield coils 22, 25 and the shield coils 22, 25, respectively.

As shown in FIG. 4, the main coils 20 and 23 are viewed from a cross section perpendicular to the coil group plane so that the cross-sectional area of the uniform magnetic field region 26 is minimized. The centers of the sections 20 and 23 are arranged so as to be located on a line of ± 30 ° with respect to a center plane (median plane) passing through the center of the uniform magnetic field region 26 and being parallel to the coil group plane. With such an arrangement, it is possible to minimize the six-pole component that becomes the lowest-order irregular magnetic field.

Next, the characteristics of the magnetic field generated by the uniform magnetic field generator will be described. As shown in FIG. 4, the cross-sectional dimension of the main coils 20 and 23 is 44 mm × 2.
4 mm, the cross-sectional dimensions of the trim coils 21 and 24 are 5 mm x
24 mm, and the cross-sectional dimensions of the shield coils 22 and 25 are 20
mm × 24 mm. Trim coils 21 and 2 from y-axis
4, the distance in the x direction to the center of the main coils 20 and 23, and the distance in the x direction to the center of the shield coils 22 and 25 are 29.0 mm and 55.4, respectively.
mm and 103.9 mm. The distance from the x-axis to the center of the first coil group 20, 21, 22 in the y direction and the center of the second coil group 23, 24, 25 in the y direction was 42 mm. A current having a current density of 180 A / mm ² is supplied to the main coil through the uniform magnetic field generator, and currents of 180 A / mm ² and 180 A /
The magnetic field intensity distribution was calculated in a two-dimensional plane under the condition of flowing in mm ² .

FIG. 5 shows the magnetic field strength B in the y direction along the x axis.
6 shows the distribution of y. FIG. 5 shows a magnetic field individually generated by the main coil, the trim coil, and the shield coil, and a combined magnetic field thereof. FIG. 6 shows the distribution of the leakage magnetic field in the x-axis direction, and FIG. 7 shows the y-direction magnetic field intensity B _{y at} x = 0.
When the value of B _y0 is B _y0 , B _y / B _y0 in the uniform magnetic field region 26
Of the x-axis direction is shown. Also, the coefficients of the (two-dimensional) multipole component are as shown in [Table 1] below.

[0022]

[Table 1]

From these figures and Table 1, it can be seen that the uniform magnetic field generating apparatus according to the present invention generates a sufficiently uniform magnetic field in the uniform magnetic field region 26.
It can be seen that the leakage magnetic field generated inside and outside the device is very small, and that the coefficient of multi-pole components other than the dipole component is almost 1% or less. It turns out that it can be demonstrated. When the coils constituting the uniform magnetic field generator are superconducting coils, the superconducting coils need to be cooled. FIG. 8 is a schematic sectional view of a coil group of a uniform magnetic field generator according to the present invention using a cryostat as a cooling device.

Since the main coil 20 (23), the trim coil 21 (24), and the shield coil 23 (25) are arranged in a plane, the cross-sectional shapes of the coil containers 30, 33 for storing these coils are simple. A flat container having a rectangular shape can be employed. When current is applied to the coils to excite them, electromagnetic force acts on each coil, and the entire coil container tends to deform. Therefore, at the time of design, the coil container 3 is designed to withstand the expected electromagnetic force.
0, 33, or the coil containers 30, 3
The thickness and shape of the coil container are determined so that the deformation amount of No. 3 is equal to or less than the allowable value.

In order to reduce the amount of heat entering the coil containers 30 and 33, 8
0K shields or radiation shields 31 and 34 are provided. Furthermore, a cryostat can be formed by adopting a structure in which these coil containers 30, 33 and 80K shields 31, 34 are housed in vacuum containers 32, 35. The coil container 30 is fixed in the vacuum container 32 by support structures 36 and 37, and the coil container 33 is fixed in the vacuum container 35 by support structures 38 and 39. The upper and lower vacuum vessels 32 and 35 are connected and fixed by belt-like support frames 40 provided at several places. As can be seen from the above description, since the arrangement of the innermost coil in cross section is planar, there is an advantage that the cross sectional shape of the entire cryostat is very simple.

FIG. 9 is a schematic sectional view of a uniform magnetic field generator employing another coil cooling method. For simplicity, the illustration of the 80K shield outside the coil container, the vacuum container, the support structure for fixing the upper and lower coil groups, and the like are omitted in FIG.
In this example, the coil containers 45 and 46 are blocks made of a material having high thermal conductivity such as aluminum and copper. The superconducting coils 20 to 22 and 23 to 25 are coil containers 45 and 46, respectively.
Embedded in the inside of the high thermal conductivity block constituting the above. The coil containers 45 and 46, that is, the high thermal conductivity blocks are provided with refrigerator units 47 and 4 such as GM refrigerators.
Cool at 8. With such a structure, each coil can be brought into a superconducting state by conduction cooling without circulating a coolant such as liquid helium in the coil container.

FIG. 10 is a schematic sectional view showing another example of the uniform magnetic field generator according to the present invention. In the uniform magnetic field generating apparatus of this example, a combination of a main coil, a trim coil, and a shield coil, which are thinly formed, are stacked (three layers in the illustrated example) to form an entire coil group. That is, the upper coil group includes a main coil 50a, a trim coil 51a, and a shield coil 52a, which are made thin, a main coil 50b, and a trim coil 5a.
1b, the layer of the shield coil 52b and the main coil 50
c, the trim coil 51c and the shield coil 52c are layered. Similarly, the lower coil group includes a main coil 53a, a trim coil 54a, and a shield coil 55a, a thin layer, a main coil 53b, a trim coil 54b, a shield coil 55b, a main coil 53c, a trim coil 54c, Shield coil 55
It is configured by stacking layers of c.

Each thin coil can be easily wound using an NC machine because the coil is flat. By stacking thin coils in layers, the degree of freedom of coil arrangement increases,
Furthermore, it is possible to generate a precise uniform magnetic field by adjusting the value of the current flowing through each coil.

[0029]

According to the present invention, coils for generating a uniform magnetic field can be arranged in a plane, and the performance and convenience of the uniform magnetic field generator can be improved.

[Brief description of the drawings]

FIG. 1 is a conceptual diagram of a uniform magnetic field generator according to the present invention.

FIG. 2 is a plan view conceptually showing a ring cyclotron device.

FIG. 3 is a schematic perspective view of an example of a uniform magnetic field generator according to the present invention.

FIG. 4 is a sectional view taken along the line BB ′ of FIG. 3;

5 is a diagram showing the distribution of magnetic field strength B _y in the y direction along the x-axis.

FIG. 6 is a diagram showing an x-axis direction distribution of a leakage magnetic field.

7 is a diagram showing the x-axis direction distribution of B _y in the vicinity of the center.

FIG. 8 is a schematic cross-sectional view of an example of a coil group of a uniform magnetic field generator according to the present invention using a cryostat as a cooling device.

FIG. 9 is a schematic cross-sectional view of a uniform magnetic field generator using another coil cooling method.

FIG. 10 is a schematic sectional view showing another example of the uniform magnetic field generator according to the present invention.

FIG. 11 is a schematic perspective view of a uniform magnetic field generator disposed in a beam incidence system and a beam emission system of a conventional ring cyclotron device.

FIG. 12 is a sectional view taken along the line AA ′ of FIG. 11;

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Main coil, 2 ... Trim coil, 3 ... Shield coil, 10 ... Charged particle beam, 11a-11c ... Uniform magnetic field generator which comprises a charged particle incident system, 12a-12d
... Sector electromagnets, 13a, 13b ... High frequency accelerating cavities,
14a, 14b: Uniform magnetic field generator constituting a charged particle beam extraction system, 20, 23: Main coil, 21, 22
4: Trim coil, 22, 25: Shield coil, 26
... Uniform magnetic field region, 30, 33 ... Coil container, 31, 34
... 80K shield, 32,35 ... Vacuum container, 36-39
... Support structure, 40 ... Support frame, 45, 46 ... Coil container, 47,48 ... Refrigerator unit, 50a-50c ... Main coil, 51a-51c ... Trim coil, 52a-
52c: shield coil, 53a to 53c: main coil, 54a to 54c: trim coil, 55a to 55c ...
Shield coil, 56: uniform magnetic field region, 100 to 104
... Main coil, 102,105 ... Shield coil

Continuation of the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (Reference) H05H 13/00 G01N 24/06 520J (72) Inventor Akira Goto 2-1, Hirosawa, Wako-shi, Saitama Pref. RIKEN ( 72) Inventor Yasushige Yano 2-1 Hirosawa, Wako-shi, Saitama F-term in RIKEN (Reference) 2G085 AA11 BC04 BC09 4C096 AB42 AD08 CA02 CA15 CA22 CA27 CA35 CA36 CA39 CA40 CA52

Claims (8)

[Claims] 1. A uniform magnetic field generating apparatus for generating a spatially uniform magnetic field by combining a plurality of coils, wherein a coil group including a plurality of coils arranged in a plane is used, and a region surrounded by the coil group is used. And generating a spatially uniform magnetic field and reducing a leakage magnetic field outside the coil group to a predetermined value or less. 2. A method according to claim 1, further comprising: arranging the two coil groups in parallel, generating a spatially substantially uniform magnetic field in a central portion of a region sandwiched between the two coil groups, and reducing an external leakage magnetic field to a predetermined value. The uniform magnetic field generator according to claim 1, wherein the value is reduced to a value or less. 3. The coil group includes a main coil, a trim coil mainly acting to correct a magnetic field generated by the main coil to increase the magnetic field uniformity of the central portion, and an external leakage magnetic field by a predetermined amount. The uniform magnetic field generator according to claim 1 or 2, comprising a shield coil that mainly acts so as to reduce the value to a value less than or equal to the value. 4. The uniform magnetic field generator according to claim 3, wherein the trim coil is disposed inside the main coil, and the shield coil is disposed outside the main coil. 5. The uniform magnetic field generator according to claim 3, wherein the main coil is disposed in a direction substantially at 30 ° with respect to a center plane. 6. The uniform magnetic field generator according to claim 3, wherein said main coil, trim coil and shield coil are superconducting coils. 7. The uniform magnetic field generator according to claim 6, wherein said main coil, trim coil and shield coil are arranged in a flat cryostat. 8. The uniform magnetic field generator according to claim 1, wherein said main coil, trim coil and shield coil are each formed by superimposing thin coils in a multilayer. .