Halbach type magnetic circuit

Abstract

To provide a cylindrical Halbach type magnetic circuit capable of suppressing a change in the intensity of a magnetic field generated in an internal space even if the temperature of a magnet changes.SOLUTION: A Halbach type magnetic circuit 1 includes a permanent magnet assembly 2 in which a plurality of permanent magnet pieces 21-28 whose residual magnetic flux density decreases with temperature rise are arranged in a cylindrical Halbach array, and a magnetic shunt alloy plate 3 arranged so as to face at least one axial end face of the permanent magnet assembly. The permanent magnet piece may be a rare earth metal-based permanent magnet piece.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a Halbach-type magnetic circuit. [Background technology]

[0002] A cylindrical Halbach-type magnetic circuit can generate a magnetic field with two or four poles in its internal space by combining the magnetization directions of the multiple permanent magnet pieces that make up the magnetic circuit. Patent Document 1 shows a magnetic circuit that generates a two-pole magnetic field, and Patent Document 2 shows a magnetic circuit that generates a four-pole magnetic field.

[0003] The two-pole magnetic field formed by a Halbach magnetic circuit has uniform strength and a unidirectional magnetic field. Such Halbach magnetic circuits are primarily used as a means of applying a unidirectional magnetic field to substrates in the semiconductor manufacturing process, or as magnetic field generators for basic research.

[0004] In ECR ion sources, a sextupole magnetic field is often used to satisfy the ECR conditions. A Halbach-type magnetic circuit is used to generate the sextupole magnetic field.

[0005] In accelerator facilities, magnetic fields with different numbers of poles, such as dipole, tetrapole, hexapole, and octopole, are used appropriately to control the movement of charged particles moving at high speed. Electromagnets are primarily used to generate magnetic fields, but it is expected that magnetic circuits using permanent magnets will increasingly be used to save power and reduce size. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-56903 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-128681 Summary of the Invention [Problem to be solved by the invention]

[0007] Halbach-type magnetic circuits are used when a strong magnetic field is required. Rare-earth permanent magnets with high residual magnetic flux density are often used as permanent magnets that make up Halbach-type magnetic circuits. The main rare-earth permanent magnets are neodymium magnets and samarium magnets. The residual magnetic flux density of many permanent magnets changes with temperature, so the magnetic field strength in the internal space of a Halbach-type magnetic circuit also changes with temperature. Depending on the application, it may be necessary to minimize changes in magnetic field strength, but to do so, another means of counteracting temperature-dependent changes is required.

[0008] The present invention has been made in consideration of the above circumstances, and aims to provide a cylindrical Halbach-type magnetic circuit in which changes in the strength of the magnetic field generated in the internal space can be suppressed even when the temperature of the magnet changes. [Means for solving the problem]

[0009] In order to achieve the above object, a Halbach-type magnetic circuit according to one embodiment of the present invention comprises a permanent magnet assembly in which a plurality of permanent magnet pieces, whose residual magnetic flux density decreases with increasing temperature, are arranged in a cylindrical Halbach array, and a magnetic shunt alloy plate arranged to face at least one axial end face of the permanent magnet assembly. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a cylindrical Halbach-type magnetic circuit in which the change in the strength of the magnetic field generated in the internal space is suppressed even when the temperature of the magnet changes. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a perspective view of a Halbach-type magnetic circuit. [Figure 2] FIG. 2 is a front view of the permanent magnet assembly. [Figure 3] FIG. 2 is a partial enlarged view of a permanent magnet assembly. [Figure 4] 2 is a graph showing magnetic properties of a magnetic shunt alloy. [Figure 5] 4 is a graph showing a magnetic field distribution of the Halbach-type magnetic circuit according to the first embodiment. [Figure 6] 10 is a graph showing a magnetic field distribution of a Halbach-type magnetic circuit according to Comparative Example 1. [Figure 7] 10 is a graph showing a magnetic field distribution of a Halbach-type magnetic circuit according to Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings, but the present invention is not limited to the embodiments described below.

[0013] As a result of intensive research, the inventors of the present invention have found that by arranging a magnetic shunt alloy plate adjacent to the axial end faces of a cylindrical Halbach-type magnetic circuit, i.e., the end faces excluding the inner and outer peripheral faces, it is possible to suppress changes in the magnetic field strength in the internal space with respect to temperature.

[0014] A cylindrical Halbach-type magnetic circuit is equipped with multiple permanent magnet pieces. The number of permanent magnet pieces that make up the cylinder is generally 8 to 24. Each permanent magnet piece is obtained by dividing the cylinder along the circumferential direction equally according to the number of permanent magnet pieces used. The magnetization direction of each permanent magnet piece is arranged so that it rotates once or multiple times when it makes one full turn along the circumferential direction of the cylinder. The more times the magnetization direction rotates, the greater the number of poles of the magnetic field generated in the internal space.

[0015] The residual magnetic flux density of many permanent magnets, including rare earth permanent magnets, decreases with increasing temperature, resulting in a decrease in the strength of the magnetic field they generate. Similarly, the magnetic field decreases with increasing temperature in Halbach-type magnetic circuits made up of permanent magnets.

[0016] A magnetic shunt alloy is an FeNi-based alloy whose magnetic flux density decreases as the temperature rises. When a magnetic shunt alloy plate is attached to a permanent magnet, some of the magnetic flux generated by the magnet short-circuits to the magnetic shunt alloy plate. When the magnet temperature rises, the residual magnetic flux density of the magnet decreases, while the magnetic flux density of the magnetic shunt alloy plate decreases, reducing the amount of short-circuiting. As a result, it is possible to suppress changes in the magnetic field that occur with temperature changes.

[0017] A Halbach-type magnetic circuit is composed of multiple permanent magnet pieces with different magnetization directions, but if there is a large difference in the short circuit amount between each magnet, the magnetic field distribution state may differ depending on whether or not a magnetic shunt alloy plate is present, so in order to make the short circuit amount the same for each magnet, it is desirable to attach a magnetic shunt alloy plate to the magnet surface parallel to the magnetization direction of each magnet.Therefore, in order to make the short circuit amount the same for each magnet of a Halbach-type magnetic circuit, it is desirable to attach a magnetic shunt alloy plate to the flat surface that corresponds to the axial end face of the cylindrical Halbach-type magnetic circuit, which is the magnet surface parallel to the magnetization direction of each magnet.

[0018] According to the present invention, it is possible to suppress changes in the magnetic field caused by temperature changes in the internal space of a cylindrical Halbach-type magnetic circuit. The cylindrical Halbach-type magnetic circuit according to the present invention is suitable for applications where changes in the magnetic field are problematic, and is particularly suitable for accelerator applications where even small changes in the magnetic field can adversely affect controllability.

[0019] Hereinafter, a Halbach-type magnetic circuit according to an embodiment of the present invention will be described. As shown in Figures 1 and 2, the Halbach-type magnetic circuit 1 includes a permanent magnet assembly 2 formed by arranging eight permanent magnet pieces 21 to 28 in a cylindrical Halbach array, and an annular magnetic shunt alloy plate 3 disposed so as to face both axial end faces of the permanent magnet assembly 2. Arrows YX indicate the axial direction of the Halbach-type magnetic circuit 1.

[0020] The eight permanent magnet pieces 21 to 28 are all equal in size and are arranged in order in the circumferential direction, and as a whole form a cylindrical permanent magnet assembly 2. The magnetization directions of all of the permanent magnet pieces 21 to 28 are parallel to a plane perpendicular to the axial direction YX.

[0021] Specifically, the magnetization direction of permanent magnet piece 21 is the direction indicated by arrow Y1, i.e., radially outward. The magnetization direction of permanent magnet piece 22 is the direction indicated by arrow Y2, i.e., the clockwise direction when the permanent magnet assembly 2 shown in Figure 2 is likened to an analog watch. The magnetization direction of permanent magnet piece 23 is the direction indicated by arrow Y3, i.e., radially inward. The magnetization direction of permanent magnet piece 24 is the direction indicated by arrow Y4, i.e., counterclockwise. The magnetization direction of permanent magnet piece 25 is the direction indicated by arrow Y5, i.e., radially outward. The magnetization direction of permanent magnet piece 26 is the direction indicated by arrow Y6, i.e., clockwise. The magnetization direction of permanent magnet piece 27 is the direction indicated by arrow Y7, i.e., radially inward. The magnetization direction of permanent magnet piece 28 is the direction indicated by arrow Y8, i.e., counterclockwise.

[0022] A quadrupole magnetic field is generated in the internal space (hollow portion) 29 of the permanent magnet assembly 2. That is, a north pole is generated near the inner circumferential surfaces of the permanent magnet pieces 23 and 27, and a south pole is generated near the inner circumferential surfaces of the permanent magnet pieces 21 and 25.

[0023] 3 shows the permanent magnet piece 28 as seen from the radial outside. The magnetic flux flows as indicated by arrows Y81 and Y82. That is, part of the magnetic flux generated from the permanent magnet piece 28 is short-circuited to the magnetic shunt alloy plate 3. Although not shown, part of the magnetic flux generated from each of the other permanent magnet pieces 21 to 27 is also short-circuited to the magnetic shunt alloy plate 3. Therefore, the magnetic field generated in the internal space 29 is slightly weaker than when the magnetic shunt alloy plate 3 is not provided.

[0024] When the magnet temperature rises, the residual magnetic flux density of the magnet decreases, and the amount of short circuit to the magnetic shunt alloy plate also decreases. Therefore, even if the temperature changes, it is possible to suppress changes in the magnetic field in the internal space 29.

[0025] As mentioned above, the magnetization direction of the permanent magnet pieces 21 to 28 is parallel to a plane perpendicular to the axial direction YX. Therefore, a flow of magnetic flux that short-circuits to the magnetic shunt alloy plate occurs in each permanent magnet piece. Because there is no difference in the amount of short-circuiting to the magnetic shunt alloy plate depending on the permanent magnet piece, the difference in magnetic field distribution state due to the presence or absence of the magnetic shunt alloy plate can be suppressed.

[0026] However, attaching a magnetic shunt alloy plate to the inner circumferential surface of the permanent magnet assembly 2 is undesirable because it could cause differences in the amount of short-circuiting to the magnetic shunt alloy plate depending on the permanent magnet piece, thereby changing the magnetic field distribution in the internal space 29. Furthermore, attaching a magnetic shunt alloy plate to the inner circumferential surface would also result in a problem of reducing the available internal space 29. The required internal space size is determined by the intended use, and in the case of accelerator applications, it is determined by the size of the vacuum chamber that runs through the interior. Therefore, attaching a magnetic shunt alloy plate to the inner circumferential surface of the permanent magnet assembly would require expanding the inner diameter of the permanent magnet assembly by the thickness of the magnetic shunt alloy plate. However, expanding the inner diameter would significantly reduce the magnetic field that can be generated. One possible solution would be to increase the outer diameter of the permanent magnet assembly, but because the outer periphery is far from the internal space, a significant increase in the outer diameter would be necessary to compensate for the magnetic field reduction caused by increasing the inner diameter. This would result in a significant increase in weight, size, and cost. Therefore, attaching a magnetic shunt alloy plate to the inner circumferential surface is undesirable.

[0027] The inner and outer circumferential surfaces of the permanent magnet assembly 2 shown in Fig. 2 are both circular when viewed in the axial direction, but this is not limited to this. At least one of the inner and outer circumferential surfaces of each permanent magnet piece may be flat. In this case, at least one of the inner and outer circumferential surfaces of the permanent magnet assembly 2 is a regular octagon when viewed in the axial direction. The outer and inner circumferential shapes of the annular magnetic shunt alloy plate may be regular octagonal to match the shape of the end faces of the permanent magnet assembly, or may remain circular.

[0028] In the Halbach-type magnetic circuit 1 shown in FIG. 1, the inner and outer diameters of the magnetic shunt alloy plate 3 respectively match the inner and outer diameters of the permanent magnet assembly 2. As mentioned above, in many applications the inner diameter of the permanent magnet assembly 2 is designed to be as small as possible. Making the inner diameter of the magnetic shunt alloy plate 3 smaller than the inner diameter of the permanent magnet assembly 2 is undesirable, as this could hinder the use of the internal space 29. It is preferable that the inner and outer diameters of the magnetic shunt alloy plate 3 respectively match the inner and outer diameters of the permanent magnet assembly 2, as this allows for efficient short-circuiting of the magnetic flux.

[0029] Making the outer diameter of the magnetic shunt alloy plate 3 larger than the outer diameter of the permanent magnet assembly 2 does not have much effect in terms of temperature compensation, but making the outer diameter of the magnetic shunt alloy plate larger than the outer diameter of the permanent magnet assembly 2 in order to hold and fix the magnetic shunt alloy plate is significant from a structural standpoint, and does not reduce the effect of temperature compensation. Generally, a structure for holding the permanent magnet assembly 2 is present outside the outer periphery of the permanent magnet assembly 2, and it is structurally significant to make the outer diameter of the magnetic shunt alloy plate larger than the outer diameter of the permanent magnet assembly 2 so that the magnetic shunt alloy plate can be fixed to this structure.

[0030] The temperature compensation effect can be achieved even if the inner and outer diameters of the magnetic shunt alloy plate do not necessarily match perfectly with the inner and outer diameters of the permanent magnet assembly 2. It is desirable to cover most of the end faces of the permanent magnet assembly 2 with the magnetic shunt alloy plate, but it is preferable to cover at least 50% of the area of ​​each end face. As the proportion covered by the magnetic shunt alloy plate decreases, the temperature compensation effect also decreases, so to compensate for this decrease, it is advisable to increase the thickness of the magnetic shunt alloy plate. However, increasing the thickness increases the axial length of the Halbach-type magnetic circuit 1, including the magnetic shunt alloy plate, which has the disadvantage of increasing its size. If the area of ​​the magnetic shunt alloy plate is at least 50% of the area of ​​the end faces of the permanent magnet assembly 2, the temperature compensation effect can be achieved without excessively increasing the thickness of the magnetic shunt alloy plate.

[0031] 1, the permanent magnet assembly 2 and the magnetic shunt alloy plate 3 are in contact, but the temperature compensation effect can be obtained even if there is a gap of several millimeters between them, so they do not necessarily have to be in contact. A non-magnetic member such as an aluminum plate may be placed between the permanent magnet assembly 2 and the magnetic shunt alloy plate 3.

[0032] The thickness of the magnetic shunt alloy plate does not need to be greater than necessary, and should be 50% or less of the axial length of the permanent magnet assembly 2. If the thickness of the magnetic shunt alloy plate is too small, the effect of temperature compensation will decrease, so it is preferable that the thickness of the magnetic shunt alloy plate be at least 5% or more of the axial length of the permanent magnet assembly 2.

[0033] Example 1 As an example, we created a Halbach-type magnetic circuit that generates a quadrupole magnetic field as shown in Figure 1, and measured the magnetic field in the internal space. The dimensions of the permanent magnet assembly 2 were an inner diameter of 50 mm, an outer diameter of 120 mm, and an axial length of 50 mm, and the dimensions of the magnetic shunt alloy plate 3 were an inner diameter of 50 mm, an outer diameter of 120 mm, and a thickness of 5 mm. In other words, the ratio of the thickness of the magnetic shunt alloy plate 3 to the axial length of the permanent magnet assembly 2 was 5 mm / 50 mm = 10%.

[0034] The number of permanent magnet pieces constituting the permanent magnet assembly 2 was eight, and the magnetization direction was as shown in Figure 2. The permanent magnet used was a samarium magnet, with a residual magnetic flux density of 1.1 T and a temperature coefficient of residual magnetic flux density of -0.03% / K.

[0035] Figure 4 shows the magnetic characteristics of the magnetic shunt alloy used. Figure 5 shows the values ​​of the quadrupole magnetic field versus radial position in the internal space as a result of the magnetic field distribution when the temperature of the permanent magnet assembly 2 is 20°C and 30°C. Symbol G11 shows the results when the temperature is 20°C, and symbol G12 shows the results when the temperature is 30°C. The magnetic field values ​​at 20°C and 30°C are nearly identical. These results confirm the effect of temperature compensation achieved by providing a magnetic shunt alloy plate.

[0036] <Comparative Example 1> The magnetic field was measured for the Halbach-type magnetic circuit in Example 1 with the magnetic shunt alloy plate removed, and the results are shown in Figure 6. Symbol G21 shows the result at 20°C, and symbol G22 shows the result at 30°C. It was confirmed that the value of the quadrupole magnetic field decreased as the temperature increased.

[0037] <Comparative Example 2> A permanent magnet assembly was prepared with an inner diameter of 60 mm and other dimensions identical to those of the permanent magnet assembly of Example 1. A magnetic shunt alloy plate with a thickness of 5 mm was attached to the inner peripheral surface of this permanent magnet assembly. The inner diameter of this Halbach-type magnetic circuit was 50 mm, and therefore the size of the internal space was the same as in Example 1. The magnetic shunt alloy plate used was made of the same material as in Example 1. Figure 7 shows the magnetic field distribution of this Halbach-type magnetic circuit. Symbol G31 represents the result at 20°C, and symbol G32 represents the result at 30°C. As can be seen from the figure, the effect of temperature compensation was obtained, but the magnetic field strength was significantly reduced compared to Example 1 shown in Figure 5.

[0038] According to the above embodiment, the Halbach-type magnetic circuit 1 includes the magnetic shunt alloy plate 3. When the temperature rises, the residual magnetic flux density of each permanent magnet piece decreases, and the amount of short circuit to the magnetic shunt alloy plate also decreases. Therefore, even if the temperature changes, it is possible to suppress changes in the magnetic field in the internal space 29 of the Halbach-type magnetic circuit 1.

[0039] In particular, in accelerator applications, even small changes in the magnetic field affect the control of charged particles. The magnetic field generator for accelerators equipped with the Halbach-type magnetic circuit according to the above embodiment can minimize changes in the magnetic field even when the temperature changes.

[0040] <Other> FIG. 2 shows a permanent magnet assembly having permanent magnet pieces obtained by dividing a cylindrical body into eight parts, but the number of divisions is not limited to eight. A magnetic shunt alloy plate may be placed on only one of the axial end faces of the permanent magnet assembly. By placing a magnetic shunt alloy plate on both end faces, it is possible to prevent unevenness in the effect on the magnetic field in the internal space. Embodiments of the present invention are applicable to all permanent magnets whose remanence decreases with increasing temperature.

[0041] The following notes are provided regarding the embodiments described above. <Appendix 1> a permanent magnet assembly including a plurality of permanent magnet pieces arranged in a cylindrical Halbach array, the residual magnetic flux density of which decreases with increasing temperature; a magnetic shunt alloy plate disposed so as to face at least one axial end face of the permanent magnet assembly; A Halbach-type magnetic circuit comprising: <Appendix 2> 2. The Halbach-type magnetic circuit according to claim 1, wherein the permanent magnet pieces are rare earth permanent magnet pieces. <Appendix 3> 2. The Halbach-type magnetic circuit according to claim 1, wherein the magnetic shunt alloy plate is annular, and the inner diameter of the magnetic shunt alloy plate is the same as the inner diameter of the permanent magnet assembly. <Appendix 4> 2. The Halbach-type magnetic circuit according to claim 1, wherein the magnetic shunt alloy plate is annular and the outer diameter of the magnetic shunt alloy plate is the same as the outer diameter of the permanent magnet assembly. <Appendix 5> 2. The Halbach-type magnetic circuit according to claim 1, wherein 50% or more of the area of ​​the axial end face of the permanent magnet assembly faces the magnetic shunt alloy plate in the axial direction. <Appendix 6> 2. The Halbach-type magnetic circuit according to claim 1, wherein the thickness of the magnetic shunt alloy plate is 5% to 50% of the axial length of the permanent magnet assembly. <Appendix 7> 7. A magnetic field generator for an accelerator, comprising the Halbach-type magnetic circuit according to any one of claims 1 to 6.

[0042] Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various modifications and changes can be made based on the technical concept of the present invention. [Explanation of symbols]

[0043] 1 Halbach-type magnetic circuit 2. Permanent Magnet Assembly 21~28 Permanent magnet piece 29 Internal space (hollow part) 3 Magnetic shunt alloy plate

Claims

1. a permanent magnet assembly including a plurality of permanent magnet pieces arranged in a cylindrical Halbach array, the residual magnetic flux density of which decreases with increasing temperature; a magnetic shunt alloy plate disposed so as to face at least one axial end face of the permanent magnet assembly; A Halbach-type magnetic circuit comprising:

2. 2. The Halbach-type magnetic circuit according to claim 1, wherein the permanent magnet pieces are rare earth permanent magnet pieces.

3. 2. The Halbach-type magnetic circuit according to claim 1, wherein the magnetic shunt alloy plate is annular, and the inner diameter of the magnetic shunt alloy plate is the same as the inner diameter of the permanent magnet assembly.

4. 2. The Halbach-type magnetic circuit according to claim 1, wherein the magnetic shunt alloy plate is annular, and the outer diameter of the magnetic shunt alloy plate is the same as the outer diameter of the permanent magnet assembly.

5. 2. The Halbach-type magnetic circuit according to claim 1, wherein 50% or more of the area of ​​the axial end face of said permanent magnet assembly faces said magnetic shunt alloy plate in the axial direction.

6. 2. The Halbach-type magnetic circuit according to claim 1, wherein the thickness of the magnetic shunt alloy plate is 5% to 50% of the axial length of the permanent magnet assembly.

7. A magnetic field generating device for an accelerator, comprising the Halbach-type magnetic circuit according to any one of claims 1 to 6.

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