Particle accelerator

Abstract

To improve the beam transmission efficiency of particles even when the magnetic field gradient of a focusing element built into a drift tube with an internal focusing element is small due to the use of a permanent magnet.SOLUTION: In a drift tube linear accelerator (10) with built-in focusing elements, which serves as a particle accelerator that accelerates particles within an accelerating cavity (11) by electromagnetic fields formed in the cavity by a plurality of drift tubes (12) each having a focusing element built therein on a beam axis (P) along which particles pass, each focusing element built into the drift tube converges the particle beam in a specific direction. The drift tube linear accelerator (10) is configured such that drift tubes with built-in focusing elements (12), including a first type (12A) and a second type (12B), in which the direction of the focusing force acting on the particle beam is the same, are arranged adjacent to each other along the beam axis.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] SUMMARY OF THE INVENTION Embodiments of the present invention relate to a particle accelerator with a drift tube incorporating a focusing element. [Background technology]

[0002] Drift tube accelerators (DTLs) are widely used as particle accelerators for accelerating protons and heavy particles. Among these DTLs, IH-type DTLs, which excite the H-mode electromagnetic field, have high shunt impedance in the low-velocity region and can achieve particle acceleration with high acceleration efficiency.

[0003] In drift tube accelerators, acceleration is performed by an electric field with a frequency equivalent to microwaves. However, when accelerating a beam of protons or deuterons, which are particles with a current value of several tens of milliamperes or more, the particles repel each other due to the space charge effect, causing the particle beam to diverge significantly. This divergent beam then collides with the walls of the accelerating cavity, reducing the beam transmittance. To solve this problem, particle accelerators using focusing magnets have been proposed. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 5602855 [Non-patent literature]

[0005] [Non-Patent Document 1] KURENNOY, Sergey S., et al. H-mode accelerating structures with PMQ focusing for low-beta ion beams. Proc. IPAC10, 2010, 828. [Non-patent document 2] KURENNOY, Sergey S., et al. H-mode accelerating structures with permanent-magnet quadrupole beam focusing. Physical Review Special Topics-Accelerators and Beams, 2012, 15.9: 090101. Summary of the Invention [Problem to be solved by the invention]

[0006] Patent Document 1 discloses a proposal to improve the beam transmittance by introducing a focusing magnet between a pre-accelerator, such as a radio frequency quadrupole accelerator (RFQ), and an IH-type DTL, thereby improving the beam mismatch of particles entering the acceleration cavity.

[0007] Furthermore, Non-Patent Documents 1 and 2 state that by incorporating a magnet that focuses a particle beam in a drift tube accelerator and focusing the beam in two orthogonal directions, it becomes possible to accelerate particles while suppressing the expansion of the beam diameter, and that even in the case of an IH-type DTL, by incorporating a focusing magnet, it becomes possible to accelerate particles while suppressing the beam diameter, even for particles with a large current value.

[0008] Fig. 9 is a schematic longitudinal sectional view showing a conventional drift tube accelerator with built-in focusing elements. Fig. 10 shows the arrangement of drift tubes in the drift tube accelerator with built-in focusing elements of Fig. 9. As shown in Fig. 10, in a conventional drift tube accelerator with built-in focusing elements 100, within an accelerating cavity 101, a drift tube with built-in focusing elements 102 that focuses a particle beam in the horizontal direction, a drift tube without built-in focusing elements 103 that does not have a built-in focusing element, a drift tube with built-in focusing elements 104 that focuses a particle beam in the vertical direction, and a drift tube without built-in focusing elements 103 are sequentially arranged (arranged) along a beam axis O to form one period, and these drift tubes 102, 103, 104, and 103 are arranged periodically.

[0009] Incidentally, when accelerating monovalent He ions or the like with a small mass:charge ratio, if focusing element-integrated drift tubes 102, 104, whose focusing elements (focusing magnets) have beam focusing forces in different directions, are arranged alternately along the beam axis O, the magnetic field gradient that the focusing elements must have to reduce the beam diameter becomes very large in each of the focusing elements of the focusing element-integrated drift tubes 102, 104. In particular, when permanent magnets are used as focusing elements, it may be difficult to achieve the large magnetic field gradient described above because the residual magnetic flux density of the permanent magnet material is fixed.

[0010] Furthermore, if the magnet outer radius is increased in order for the focusing elements (focusing magnets) to increase the magnetic field gradient, the outer diameter of the focusing-element-integrated drift tubes 102 and 104 will also increase, reducing the Q value, which represents the cavity performance of the accelerating cavity 101 in the focusing-element-integrated drift tube accelerator 100. This increases the amount of heat lost in the walls of the accelerating cavity 101, the focusing-element-integrated drift tubes 102 and 104, and the focusing-element-free drift tube 103, and reduces the power efficiency of the focusing-element-integrated drift tube accelerator 100.

[0011] The embodiments of the present invention have been made in consideration of the above circumstances, and have an object to provide a particle accelerator that can improve the beam transmittance of particles even when the magnetic field gradient of the focusing element built into the focusing element-integrated drift tube is small compared to the magnetic field gradient caused by the permanent magnet. [Means for solving the problem]

[0012] A particle accelerator in an embodiment of the present invention is characterized in that a plurality of drift tubes incorporating focusing elements, each of which incorporates focusing elements that focus the particle beam in a specific direction, are arranged on a beam axis through which the particles pass within a cavity, and the particles are accelerated within the cavity by electromagnetic fields generated by these drift tubes incorporating focusing elements, and the drift tubes incorporating focusing elements that apply the focusing force to the particle beam in the same direction are arranged adjacent to each other on the beam axis. [Effects of the Invention]

[0013] According to the embodiment of the present invention, even if the magnetic field gradient of the focusing element incorporated in the focusing element-integrated drift tube is small compared to the magnetic field gradient caused by a permanent magnet, it is possible to improve the beam transmittance of particles. [Brief explanation of the drawings]

[0014] [Figure 1] 1 is a schematic vertical cross-sectional view showing a drift tube accelerator with built-in focusing elements as a particle accelerator according to a first embodiment. [Figure 2] FIG. 2 is a schematic longitudinal cross-sectional view of a portion of FIG. 1 showing the arrangement of drift tubes. [Figure 3] FIG. 3 is a schematic cross-sectional view of the drift tube with a first-type focusing element built in shown in FIGS. 1 and 2, viewed from the particle beam traveling direction. [Figure 4] FIG. 3 is a schematic cross-sectional view of the drift tube with a second-type focusing element built in shown in FIGS. 1 and 2, viewed from the particle beam traveling direction. [Figure 5] 10 is a graph showing the relationship between the magnetic field gradient and the outer radius of a focusing magnet as a focusing element. [Figure 6] Graph showing the relationship between the maximum beam diameter of a drift tube accelerator with built-in focusing element and the magnetic field gradient of the focusing element (focusing magnet). [Figure 7] FIG. 10 is a schematic vertical cross-sectional view showing a drift tube accelerator with built-in focusing elements as a particle accelerator according to a second embodiment. [Figure 8] Graph showing the relationship between the Q value of a drift tube accelerator with built-in focusing elements and the outer radius of the drift tube. [Figure 9] FIG. 1 is a schematic longitudinal cross-sectional view showing a conventional drift tube accelerator with a built-in focusing element. [Figure 10] FIG. 10 is a schematic longitudinal cross-sectional view of a portion of FIG. 9 showing the arrangement of drift tubes. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. [A] First embodiment (Figs. 1 to 6) Fig. 1 is a schematic longitudinal sectional view showing a drift tube accelerator with built-in focusing elements as a particle accelerator according to the first embodiment. In this drift tube accelerator with built-in focusing elements 10 as a particle accelerator shown in Fig. 1, a drift tube with built-in focusing elements 12 and a drift tube without built-in focusing elements 13 are arranged on a beam axis P along which particles pass in an accelerating cavity 11, and particles are accelerated in the accelerating cavity 11 by electromagnetic fields generated by these drift tubes 12 and 13.

[0016] Here, the particles are charged particles such as protons, deuterons, and monovalent He ions. Accelerating cavity 11 excites (resonates) an electromagnetic field having a frequency equivalent to a microwave. Focusing element-integrated drift tube accelerator 10 may not have focusing element-free drift tube 13, and may be configured solely with focusing element-integrated drift tube 12 as the drift tube.

[0017] There are two types of drift tubes 12 with built-in focusing elements: a first-type drift tube 12A with built-in focusing elements 14 that focus the particle beam in a specific horizontal direction (X direction in FIG. 3), and a second-type drift tube 12B with built-in focusing elements 14 that focus the particle beam in another specific vertical direction (Y direction in FIG. 4). A drift tube 13 without built-in focusing elements is a drift tube that does not incorporate the above-mentioned focusing elements 14. Here, the focusing elements 14 are focusing magnets such as electromagnets, permanent magnets, or superconducting magnets.

[0018] 1, the focusing element-equipped drift tube 12 and the focusing element-free drift tube 13 are supported in the accelerating cavity 11 by using stems 15 as support members. The stems 15 are set so that the support direction of the adjacent focusing element-equipped drift tubes 12 and 13 is alternately changed, for example, by 180°. Furthermore, because the focusing element-free drift tube 13 does not incorporate a focusing element 14 and is lighter than the focusing element-equipped drift tube 12, the stems 15 supporting the focusing element-free drift tube 13 are formed thinner (with a smaller diameter) than the stems 15 supporting the focusing element-equipped drift tube 12.

[0019] As shown in Figures 3 and 4, the first-type focusing element-equipped drift tube 12A and the second-type focusing element-equipped drift tube 12B of the focusing element-equipped drift tube 12 are configured by arranging a plurality of blocked focusing elements 14 in a ring shape inside a tube main body 16. Each of the focusing magnets, which are the plurality of focusing elements 14, has a magnetic field direction I. Depending on the arrangement of these focusing elements 14, the magnetic field direction J acting on the particle beam on the beam axis P converges the particle beam in the horizontal direction (X direction) and diverges it in the vertical direction (Y direction) in the first-type focusing element-equipped drift tube 12A, while the second-type focusing element-equipped drift tube 12B converges the particle beam in the vertical direction (Y direction) and diverges it in the horizontal direction (X direction). As a result, the first-type focusing element-equipped drift tube 12A and the second-type focusing element-equipped drift tube 12B each function as a quadrupole magnet.

[0020] Furthermore, in the focusing magnet that is the focusing element 14, the magnetic field gradient depends on the magnet outer radius when the magnet inner radius is constant, as shown in Figure 5. However, the dependency of the magnetic field gradient in this focusing element 14 (focus magnet) on the magnet outer radius is not proportional, but the slope gradually decreases, and there is a limit to the magnetic field gradient even if the magnet outer radius is increased.

[0021] 1, first-type focusing element-equipped drift tubes 12A and second-type focusing element-equipped drift tubes 12B, which have the same direction of focusing force on the particle beam, are arranged adjacent to each other on the particle beam axis P. That is, as shown in FIG. 2, in focusing element-equipped drift tube accelerator 10, two first-type focusing element-equipped drift tubes 12A, two focusing element-free drift tubes 13, two focusing element-equipped drift tubes 12B, and two focusing element-free drift tubes 13 are arranged adjacent to each other on the particle beam axis P in accelerating cavity 11, with this arrangement forming one period, and focusing element-equipped drift tubes 12 and focusing element-free drift tubes 13 being arranged periodically.

[0022] In FIG. 1, the drift tubes 12 with built-in focusing elements (drift tubes 12A with built-in first type focusing elements and drift tubes 12B with built-in second type focusing elements) and the drift tubes 13 without built-in focusing elements are arranged in the accelerating cavity 11 in a number exceeding one period as described above, but they may also be arranged in the accelerating cavity 11 in a number equal to an integer multiple of one period (two periods, three periods, etc.), or in a number exceeding an integer multiple of one period.

[0023] Here, the magnetic field gradient of the focusing elements 14 incorporated in each of the adjacent first-type focusing element-containing drift tubes 12A is set to, for example, the same value. Similarly, the magnetic field gradient of the focusing elements 14 incorporated in each of the adjacent second-type focusing element-containing drift tubes 12B is also set to, for example, the same value. Note that the drift tubes 13 not incorporating focusing elements that make up one period of the arrangement of the focusing element-containing drift tubes 12 and the focusing element-free drift tubes 13 do not necessarily have to exist as described above, and may be arranged between adjacent first-type focusing element-containing drift tubes 12A or second-type focusing element-containing drift tubes 12B.

[0024] Let the array pattern of one cycle of the drift tube 12 with a convergence element and the drift tube 13 without a convergence element in FIG. 2 be M, and the array pattern of the drift tubes 102, 104 with a convergence element and the drift tube 103 without a convergence element in FIG. 10 be N. The relationship between the maximum beam diameter of the drift tube accelerators 10, 100 with a convergence element and the magnetic field gradient of the convergence element (convergence magnet) is shown in FIG. 6. From this FIG. 6, it can be seen that even when the magnetic field gradient of the convergence element is the same value (for example, 250 T / m), the maximum beam diameters in the drift tube accelerators 10, 100 with a convergence element are significantly different. When the convergence element (convergence magnet) has a magnetic field gradient value below a certain value due to restrictions such as material and shape, for example, like a permanent magnet, the array pattern M in FIG. 2 can make the maximum beam diameter of the drift tube accelerator with a convergence element smaller.

[0025] Due to being configured as above, according to the first embodiment, the following effects (1) and (2) can be achieved. (1) As shown in FIGS. 1 and 2, since the beam convergence directions by the respective convergence elements 14 of the adjacent drift tubes 12 with a convergence element (the first - type drift tube 12A with a convergence element, the second - type drift tube 12B with a convergence element) are the same, the function of converging the beam in the same direction can be borne by a plurality of adjacent drift tubes 12 with a convergence element. Therefore, the magnetic field gradient required for the convergence element 14 of each drift tube 12 with a convergence element can be set to a small value within the range of the magnetic field gradient by a permanent magnet. As a result, even for a drift tube accelerator 10 with a convergence element having a convergence element 14 made of a permanent magnet and having a small magnetic field gradient, as shown in FIG. 6, the maximum beam diameter of the particles can be reduced, and the beam transmission rate of the drift tube accelerator 10 with a convergence element can be improved.

[0026] (2) Because the magnetic field gradient of the focusing elements 14 of the focusing element-integrated drift tubes 12 (first-type focusing element-integrated drift tubes 12A and second-type focusing element-integrated drift tubes 12B) can be set to a small value within the range of the magnetic field gradient produced by the permanent magnets, the outer diameter of the focusing elements 14 can be made small, as shown in Fig. 5. As a result, the outer diameter of the focusing element-integrated drift tubes 12 can be set small, so that the Q factor, which represents the cavity performance of the focusing element-integrated drift tube accelerator 10, can be maintained high, and a focusing element-integrated drift tube accelerator 10 with good power efficiency can be realized.

[0027] [B] Second embodiment (Figs. 7 and 8) 7 is a schematic longitudinal sectional view showing a drift tube accelerator with built-in focusing elements as a particle accelerator according to the second embodiment. In this second embodiment, parts similar to those in the first embodiment are designated by the same reference numerals as in the first embodiment, and descriptions thereof will be simplified or omitted.

[0028] The second embodiment of the particle accelerator, i.e., the drift tube accelerator 20 with built-in focusing elements, differs from the first embodiment in that the outer diameter of the drift tube 21 without built-in focusing elements 14 is smaller than the outer diameter of the drift tubes 12 with built-in focusing elements (drift tube 12A with built-in first type focusing elements and drift tube 12B with built-in second type focusing elements).

[0029] 5, the outer radius of the focusing element 14 must be set large in the focusing element-embedded drift tube 12 (first-type focusing element-embedded drift tube 12A, second-type focusing element-embedded drift tube 12B). That is, in the focusing element-embedded drift tube 12, there is a limit to how small the outer radius of the focusing element 14 (magnet outer radius) can be set to ensure the magnetic field gradient required for the focusing element 14. Therefore, it is difficult to reduce the outer radius of the focusing element-embedded drift tube 12.

[0030] 8, the Q value, which represents the cavity performance of accelerating cavity 11, depends on the outer radii of drift tube 12 with focusing elements and drift tube 21 without focusing elements, and decreases as the outer radii of drift tube 12 with focusing elements and drift tube 21 increase. Therefore, in drift tube accelerator 20 with built-in focusing elements, drift tube 21 without focusing elements has no restriction on the magnet outer radius imposed by focusing element 14 (focusing magnet), and therefore it is possible to set the outer radius of drift tube 21 without focusing elements to be small.

[0031] As configured as above, the second embodiment provides the following effect (3) in addition to the effects (1) and (2) of the second embodiment.

[0032] (3) Since the outer diameter of the drift tube 21 without built-in focusing elements is configured to be smaller than the outer diameter of the drift tubes 12 with built-in focusing elements (drift tube 12A with built-in first type focusing elements and drift tube 12B with built-in second type focusing elements), the Q value of the drift tube accelerator 20 with built-in focusing elements can be further improved compared to the first embodiment, based on the dependency of the Q value on the drift tube outer radius shown in Fig. 8. As a result, the power efficiency of the drift tube accelerator 20 with built-in focusing elements can be improved even more than in the first embodiment.

[0033] Although several embodiments of the present invention have been described above, these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, substitutions, changes, and combinations can be made without departing from the spirit of the invention. Furthermore, such substitutions, changes, and combinations are included in the scope and spirit of the invention, and are also included in the inventions and their equivalents as set forth in the claims. [Explanation of symbols]

[0034] 10...Drift tube accelerator with built-in focusing element (particle accelerator), 11...Accelerating cavity, 12...Drift tube with built-in focusing element, 12A...Drift tube with built-in first-type focusing element, 12B...Drift tube with built-in second-type focusing element, 13...Drift tube without built-in focusing element, 14...Focusing element, 15...Stem (support member), 20...Drift tube accelerator with built-in focusing element (particle accelerator), 21...Drift tube without built-in focusing element

Claims

1. A particle accelerator in which a plurality of drift tubes incorporating focusing elements, each of which incorporates a focusing element for focusing a particle beam in a specific direction, are arranged on a beam axis through which particles pass within a cavity, and the particles are accelerated within the cavity by an electromagnetic field generated by these drift tubes incorporating focusing elements, A particle accelerator characterized in that the drift tubes with built-in focusing elements, which have the same direction of focusing force on the particle beam, are arranged adjacent to each other on the beam axis.

2. 2. The particle accelerator according to claim 1, further comprising a drift tube without a built-in focusing element disposed within the cavity on the beam axis through which the particles pass.

3. 3. The particle accelerator according to claim 1, wherein the magnetic field gradients in the focusing elements of the adjacently arranged focusing element-embedded drift tubes are set to the same value.

4. 3. The particle accelerator according to claim 1, wherein the drift tubes with built-in focusing elements are provided in a plurality of types with different directions of focusing force on the particle beam, and at least these drift tubes with built-in focusing elements are periodically arranged on the beam axis within the cavity.

5. 3. The particle accelerator according to claim 2, wherein the drift tube without built-in focusing elements has a smaller diameter than the drift tube with built-in focusing elements.

6. 3. The particle accelerator according to claim 2, wherein support members for supporting the drift tube without built-in focusing elements in the cavity are formed thinner than support members for supporting the drift tube with built-in focusing elements in the cavity.

7. 3. The particle accelerator according to claim 1, wherein the focusing element is an electromagnet, a permanent magnet, or a superconducting magnet.

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