Ion source and neutron capture therapy device

Abstract

The invention provides an ion source and a neutron capture therapy device capable of reducing the influence of discharge between electrodes. The plasma electrode (90) has at least a first electrode member (91) and a second electrode member (92), and the first electrode member (91) is a magnetic material. In a radial direction orthogonal to the axial direction, a distance (R3 ') from an end portion (91b) on the outer peripheral side of the first electrode member (91) to the lead-out opening (66) is equal to or greater than a distance (R1 ') from an end portion (44b) on the outer peripheral side of the first lead-out electrode (44) to the lead-out opening. This makes it possible to suppress the generation of a leakage magnetic field from the outer peripheral side of the first electrode member. Since the generation of a leakage magnetic field can be suppressed, the generation of discharge between the plasma electrode and the first extraction electrode (44) can be suppressed. The second electrode member is a member having a melting point and / or thermal conductivity higher than that of the magnetic body. Therefore, even if discharge occurs, the first electrode member, which is a magnetic body, can be protected by the second electrode member.

Description

[0001] This application claims priority based on Japanese Patent Application No. 2023-216639 filed on December 22, 2023. The entire content of the Japanese application is incorporated herein by reference. Technical Field

[0002] The present invention relates to an ion source and a neutron capture therapy device. Background Art

[0003] There is known an ion source configured to generate plasma in a plasma chamber and extract ions from the plasma chamber using extraction electrodes (for example, refer to Patent Document 1). Ions generated in the plasma chamber are extracted to the outside of the plasma chamber according to the potential difference between the plasma electrode and the extraction electrode in the plasma chamber.

[0004] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2021-176120

[0005] For example, in the case of generating plasma by inputting microwaves, if a magnetic field coaxial with the direction of extracting ions is formed by a coil or the like, the microwave absorption efficiency is improved, and the ion generation efficiency is improved. However, a strong electric field is applied between the extraction electrode and the plasma electrode. If the magnetic field leaks to this part, the following problems occur. That is, electrons move in a manner of winding around the magnetic field. Such electrons cause discharge between the electrodes. In the case of generating discharge, there may be a problem that the plasma electrode is damaged. Summary of the Invention

[0006] Accordingly, an object of the present invention is to provide an ion source and a neutron capture therapy device capable of reducing the influence of discharge between electrodes.

[0007] The ion source according to the present invention includes: a plasma electrode provided in a plasma chamber for generating ions by plasma; and an extraction electrode opposed to the plasma electrode and extracting ions from the plasma chamber. The plasma electrode has at least a first electrode member and a second electrode member, and an extraction opening for extracting ions is formed. The first electrode member is a magnetic body, and the second electrode member is a member having at least one of a melting point and thermal conductivity higher than that of the magnetic body. In a second direction orthogonal to a first direction in which the plasma electrode and the extraction electrode are opposed, the distance from the end portion on the outer peripheral side of the first electrode member to the extraction opening is greater than or equal to the distance from the end portion on the outer peripheral side of the extraction electrode to the extraction opening.

[0008] In the ion source according to the present invention, the plasma electrode has at least a first electrode member and a second electrode member, and the first electrode member is a magnetic body. Thus, the magnetic field is guided to the outer peripheral side by the magnetic body (i.e., the first electrode member), so that the leakage magnetic field toward the space between the plasma electrode and the extraction electrode can be suppressed. In particular, in the second direction orthogonal to the first direction in which the plasma electrode and the extraction electrode face each other, the distance from the end portion on the outer peripheral side of the first electrode member to the extraction opening is equal to or greater than the distance from the end portion on the outer peripheral side of the extraction electrode to the extraction opening. Thus, the leakage magnetic field generated from the outer peripheral side of the first electrode member can be suppressed. In this way, since the generation of the leakage magnetic field can be suppressed, the discharge between the plasma electrode and the extraction electrode can be suppressed. Also, the second electrode member is a member having at least one of a higher melting point and higher thermal conductivity than the magnetic body. Therefore, even if a discharge occurs, the magnetic body (i.e., the first electrode member) can be protected by the second electrode member. Thus, the influence of the discharge between the electrodes can be reduced. Also, since the damage of the first electrode member can be suppressed, the frequency of failures and the frequency of maintenance can be reduced.

[0009] The material of the second electrode member may be selected from tungsten, tantalum, and molybdenum. In this case, the second electrode member can have a high melting point.

[0010] The material of the second electrode member may be selected from copper, silver, and aluminum. In this case, the second electrode member can have high thermal conductivity.

[0011] The ion source may further include a magnetic field generating unit that is provided on the outer peripheral side of the plasma chamber and generates a magnetic field in the first direction in the plasma chamber, and the end portion on the outer peripheral side of the first electrode member is disposed at a position separated from the magnetic field generating unit toward the inner peripheral side. In this case, the leakage magnetic field generated from the end portion on the outer peripheral side of the first electrode member can also be suppressed.

[0012] The ion source may further include a magnetic field generating unit that is provided on the outer peripheral side of the plasma chamber and generates a magnetic field in the first direction in the plasma chamber, and the first electrode member extends to the magnetic field generating unit. In this case, the leakage magnetic field generated from the end portion on the outer peripheral side of the first electrode member can be further suppressed.

[0013] The magnetic field generating unit may include a coil and a magnetic yoke, and the first electrode member has a connecting portion connected to the magnetic yoke of the magnetic field generating unit. In this case, the first electrode member can be connected to the magnetic circuit of the coil through the connecting portion, and the generation of the leakage magnetic field can be further suppressed.

[0014] The neutron capture therapy device according to the present invention includes: an accelerator having an ion source that generates ions, and accelerating and emitting a particle beam; and an irradiation unit that generates neutron rays by the particle beam and irradiates the neutron rays to an object. In the neutron capture therapy device, the ion source includes: a plasma electrode provided in a plasma chamber that generates ions by plasma; and an extraction electrode opposed to the plasma electrode and extracting ions from the plasma chamber. The plasma electrode has at least a first electrode member and a second electrode member, and an extraction opening for extracting ions is formed. The first electrode member is a magnetic body, and the second electrode member is a member having at least one of a melting point and thermal conductivity higher than that of the magnetic body. In a second direction orthogonal to a first direction in which the plasma electrode and the extraction electrode are opposed, a distance from an outer peripheral side end portion of the first electrode member to the extraction opening is greater than or equal to a distance from an outer peripheral side end portion of the extraction electrode to the extraction opening.

[0015] In the neutron capture therapy device according to the present invention, the plasma electrode has at least a first electrode member and a second electrode member, and the first electrode member is a magnetic body. Thus, the magnetic field is guided to the outer peripheral side by the magnetic body (i.e., the first electrode member), so that the leakage magnetic field toward between the plasma electrode and the extraction electrode can be suppressed. In particular, in a second direction orthogonal to a first direction in which the plasma electrode and the extraction electrode are opposed, a distance from an outer peripheral side end portion of the first electrode member to the extraction opening is greater than or equal to a distance from an outer peripheral side end portion of the extraction electrode to the extraction opening. Thus, the leakage magnetic field generated from the outer peripheral side of the first electrode member can be suppressed. In this way, since the generation of the leakage magnetic field can be suppressed, the discharge between the plasma electrode and the extraction electrode can be suppressed. Also, the second electrode member is a member having at least one of a melting point and thermal conductivity higher than that of the magnetic body. Therefore, even if a discharge occurs, the magnetic body (i.e., the first electrode member) can be protected by the second electrode member. Thus, the influence of the discharge between the electrodes can be reduced. Also, since the damage of the first electrode member can be suppressed, the frequency of failures that cause the inability to irradiate neutron rays can be reduced, and thus the frequency of maintenance can be reduced.

[0016] According to the present invention, there is provided an ion source and a neutron capture therapy device capable of reducing the influence of discharge between electrodes. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a schematic diagram showing a neutron capture therapy device 100 including the ion source 10 according to an embodiment of the present invention.

[0018] Figure 2 is a schematic structural diagram of the ion source 10.

[0019] Figure 3 is a schematic structural diagram showing an enlarged view of the vicinity of the plasma chamber of the ion source 10.

[0020] Figure 4 is a cross-sectional view showing in more detail Figure 3 the structure of the plasma electrode 90 shown.

[0021] Figure 5 shows a state in which the end portion 91b on the outer peripheral side of the first electrode member 91 is arranged at the same position in the radial direction as the end portion 44b on the outer peripheral side of the first extraction electrode 44 (on the reference line SL1).

[0022] Figure 6 shows a state in which the end portion 91b on the outer peripheral side of the first electrode member 91 is arranged at the same position in the radial direction as the end portion 84a on the inner peripheral side of the coil 84 (on the reference line SL2).

[0023] Figure 7 is a diagram showing the results of a simulation test on a two-dimensional magnetic field calculation system by making a model of the ion source 10.

[0024] Figure 8 is a diagram showing the models of Examples 1 to 5.

[0025] Figure 9 is a diagram showing the results of a simulation test of the magnetic field strength of Examples 1 to 5.

[0026] Figure 10 is a diagram showing the models of Example 5 and Example 6.

[0027] Figure 11 is a diagram showing the results of a simulation test of the magnetic field strength of Example 5 and Example 6.

[0028] Figure 12 is a schematic cross-sectional view showing the second electrode member of the ion source according to the modified example.

[0029] In the figure: 10 - ion source, 16 - magnetic field generator (magnetic field generating section), 44 - first extraction electrode, 84 - coil, 86 - yoke, 90 - plasma electrode, 91 - first electrode member, 92 - second electrode member, 100 - neutron capture therapy device, 102 - treatment section (irradiation section), 112 - accelerator. Detailed Embodiments

[0030] Hereinafter, with reference to the accompanying drawings, preferred embodiments of the present invention will be described in detail.

[0031] Figure 1FIG. 0 is a schematic diagram of a neutron capture therapy apparatus 100 including the ion source 10 according to an embodiment of the present invention. The neutron capture therapy apparatus 100 is an apparatus for treating cancer using boron neutron capture therapy (BNCT: Boron Neutron Capture Therapy). The neutron capture therapy apparatus 100 includes a treatment unit 102, a treatment table 160, and a moving mechanism 110.

[0032] The treatment unit 102 (irradiation unit) has an irradiation port 106 for irradiating a patient 150 with neutron rays N. The treatment unit 102 is composed of a structure or the like that arranges the irradiation port 106 or the moving mechanism 110. The treatment unit 102 is provided in a treatment room 101. The irradiation port 106 is provided on a vertical wall portion of the treatment room 101. Neutron rays N are emitted horizontally from the irradiation port 106. The irradiation port 106 includes a collimator 120 and a peripheral wall 115 of the irradiation unit, which will be described later. The moving mechanism 110 is a mechanism that can move the treatment table 160 on which the patient 150 is placed in the treatment unit 102. The moving mechanism 110 is provided at a position in front of the irradiation port 106 in the treatment room 101.

[0033] In the treatment unit 102, for example, neutron rays N are irradiated onto a tumor of a patient 150 who has been injected with boron ( 10 B). The irradiation port 106 irradiates the patient 150 placed on the treatment table 160 with neutron rays N (particle beam).

[0034] The neutron capture therapy apparatus 100 includes an accelerator 112. The accelerator 112 has an ion source 10, which will be described later. The accelerator 112 accelerates ions generated by the ion source and emits a particle beam R. For example, as the accelerator 112, a cyclotron, a linear accelerator, or the like can be used.

[0035] The particle beam R emitted from the accelerator 112 is transmitted to a target arrangement unit 130 after passing through a transmission path 109 called a beam duct that maintains a vacuum inside and through which the beam can pass. The target arrangement unit 130 is a part where a target 111 is arranged and has a mechanism for holding the target 111 in a posture during irradiation. The target arrangement unit 130 arranges the target 111 at a position facing the end (beam outlet) of the transmission path 109. The particle beam R emitted from the accelerator 112 travels through the transmission path 109 toward the target 111 arranged at the end of the transmission path 109. A plurality of electromagnets 104 (quadrupole electromagnets, etc.) and a scanning electromagnet 116 are provided along the transmission path 109. The plurality of electromagnets 104, for example, use electromagnets to adjust the beam axis of the particle beam R.

[0036] The scanning electromagnet 116 scans the particle beam R and controls the irradiation of the particle beam R onto the target 111. The scanning electromagnet 116 controls the irradiation position of the particle beam R onto the target 111.

[0037] The neutron capture therapy apparatus 100 generates neutron beams N by irradiating a target 111 with a particle beam R, and emits the neutron beams N toward a patient 150 . The neutron capture therapy apparatus 100 includes a target 111 , a shield 108 , a deceleration member 139 , and a collimator 120 .

[0038] The target 111 is irradiated with the particle beam R to generate a neutron beam N. The target 111 is a solid-shaped component formed of a material that generates a neutron beam N by irradiating the particle beam R. Specifically, the target 111 is formed of, for example, beryllium (Be), lithium (Li), tantalum (Ta), and tungsten (W), and has, for example, a disk-shaped solid shape with a diameter of 160 mm. In addition, the target 111 is not limited to a disk shape, and may be other shapes.

[0039] The deceleration member 139 decelerates (reduces the energy of) the neutron beam N generated by the target 111. The deceleration member 139 may have a laminated structure formed of a layer 139A that mainly decelerates fast neutrons included in the neutron beam N and a layer 139B that mainly decelerates epithermal neutrons included in the neutron beam N.

[0040] The shield 108 shields the generated neutron beam N and the gamma rays generated with the generation of the neutron beam N from being released to the outside. The shield 108 is provided to surround the deceleration member 139. The upper and lower parts of the shield 108 extend further upstream of the particle beam R than the deceleration member 139.

[0041] The collimator 120 shapes the irradiation area of ​​the neutron beam N and has an irradiation port 120a for the neutron beam N to pass through. The collimator 120 is, for example, a block-shaped component having the irradiation port 120a in the center. The collimator 120 is mounted on the wall of the part where the neutron beam N is irradiated into the treatment room 101 (i.e., the irradiation part peripheral wall 115).

[0042] The moving mechanism 110 is a so-called six-axis mechanism that moves the treatment bed 160 on which the patient 150 is placed in six-axis directions. The moving mechanism 110 enables horizontal movement and rotational movement of the treatment bed 160. In this embodiment, the moving mechanism 110 supports the patient 150 placed on the treatment bed 160 and moves the patient 150 together with the treatment bed 160.

[0043] Next, refer to Figure 2 , the detailed structure of the ion source 10 is described. Figure 2 1 is a schematic diagram of the structure of the ion source 10. Figure 2As shown, the ion source 10 is configured as follows: microwave power is input along the magnetic field line direction into a plasma chamber 12 to which a magnetic field satisfying the electron cyclotron resonance (ECR) condition or a magnetic field higher than this is applied, to generate a high-density plasma and extract ions. The ion source 10 is configured to generate a plasma of a source gas through the interaction of a magnetic field and microwaves, and extract ions from this plasma to the outside of the plasma chamber 12.

[0044] The intensity of the magnetic field satisfying the ECR condition is uniquely determined for the frequency of the microwave used. In the case where the microwave frequency is, for example, 2.45 GHz, a magnetic field of 87.5 mT (875 gauss) is required. Hereinafter, for the sake of convenience of explanation, the magnetic field satisfying the ECR condition is sometimes referred to as a resonance magnetic field.

[0045] The ion source 10 includes an ion source body 14. The ion source body 14 includes a plasma chamber 12, a magnetic field generator 16 (magnetic field generating section), and a vacuum container 18.

[0046] The plasma chamber 12 has a cylindrical shape with both ends. Hereinafter, for the sake of convenience of explanation, the direction from one end of the plasma chamber 12 to the other end is sometimes referred to as the axial direction. Also, the direction orthogonal to the axial direction is sometimes referred to as the radial direction, and the direction around the axial direction is sometimes referred to as the circumferential direction. However, this does not necessarily mean that the plasma chamber 12 has a rotationally symmetric shape. The axial direction corresponds to the "first direction" in the claims. The radial direction corresponds to the "second direction" in the claims. In the illustrated example, the plasma chamber 12 has a cylindrical shape, but the plasma chamber 12 may have any shape as long as it can appropriately accommodate the plasma. Also, the axial length of the plasma chamber 12 may be longer or shorter than the radial length of the end of the plasma chamber 12.

[0047] The magnetic field generator 16 is provided to apply a magnetic field to the plasma chamber 12. The magnetic field generator 16 is disposed around the plasma chamber 12. The magnetic field generator 16 is configured to generate a magnetic field along the central axis of the plasma chamber 12. The direction of its magnetic field lines is indicated by an arrow M in Figure 2 . The magnetic field generator 16 is configured to generate a resonance magnetic field or a magnetic field with a higher intensity on at least a part of the axis of the plasma chamber 12. The magnetic field generator 16 can also generate a magnetic field lower than the resonance magnetic field on at least a part of the axis of the plasma chamber 12.

[0048] The vacuum container 18 is a housing for accommodating the plasma chamber 12 in a vacuum environment. The vacuum container 18 is also a structure for holding the magnetic field generator 16. The plasma chamber 12 has a vacuum window 24 for receiving microwaves into the interior. The plasma chamber 12, the magnetic field generator 16, and the vacuum container 18 will be described in more detail later.

[0049] The ion source 10 is provided with a microwave supply system 26. The microwave supply system 26 is configured to input microwave power into the plasma chamber 12 through the vacuum window 24. The microwave supply system 26 includes a microwave source 28, a waveguide 30, and a matching unit 32. The microwave source 28 is, for example, a magnetron. The microwave source 28 outputs microwaves at a frequency of 2.45 GHz, for example. The waveguide 30 is a microwave transmission circuit for transmitting the microwaves output from the microwave source 28 to the plasma chamber 12. One end of the waveguide 30 is connected to the microwave source 28, and the other end is connected to the vacuum window 24 through the matching unit 32. The matching unit 32 is provided for microwave matching.

[0050] Thus, the microwaves are introduced from the microwave supply system 26 into the plasma chamber 12 through the vacuum window 24. The introduced microwaves propagate inside the plasma chamber 12 toward the end of the plasma chamber 12 that faces the vacuum window 24. The Figure 2 propagation direction of the microwaves is indicated by an arrow P in the figure. The propagation direction P of the microwaves is the same as the direction M of the magnetic field lines based on the magnetic field generator 16. Therefore, the propagation direction P of the microwaves is aligned with the axis of the plasma chamber 12.

[0051] In addition, the microwave supply system 26 is provided with a microwave detector 33 disposed on the waveguide 30. The microwave detector 33 includes, for example, a directional coupler for monitoring the incident power incident on the plasma chamber 12 and the reflected power reflected from the plasma chamber 12. The microwave detector 33 is configured to output the measurement result to the control device C.

[0052] The ion source 10 is provided with a gas supply system 34. The gas supply system 34 is configured to supply the raw material gas for the plasma to the plasma chamber 12. The gas supply system 34 includes a gas cylinder 36 as a gas source and a gas flow controller 38. The end of the gas pipe 40 of the gas supply system 34 is connected to the plasma chamber 12 after passing through the vacuum container 18. The gas pipe 40 is connected to the side wall 64 of the plasma chamber 12, for example. The gas flow controller 38 includes an on-off valve for connecting or disconnecting the gas cylinder 36 from the plasma chamber 12 and a flow control valve for adjusting the gas flow rate flowing from the gas cylinder 36 to the plasma chamber 12. In this way, the raw material gas is supplied from the gas cylinder 36 to the plasma chamber 12 at a controlled flow rate.

[0053] The ion source body 14 is provided with an extraction electrode system 42. The extraction electrode system 42 is configured to extract ions from the plasma through the extraction opening 66 of the plasma chamber 12. The extraction electrode system 42 includes a first extraction electrode 44 and a second extraction electrode 46. The first extraction electrode 44 is disposed between the plasma chamber 12 and the second extraction electrode 46. The terminal portion 62 having the extraction opening 66 is arranged with the first extraction electrode 44 with a gap therebetween, and the first extraction electrode 44 and the second extraction electrode 46 are arranged with a gap therebetween. The first extraction electrode 44 and the second extraction electrode 46 are each formed in a ring shape, for example, and have an opening portion at the center for allowing the ions extracted from the plasma chamber 12 to pass through.

[0054] The purpose of setting the first extraction electrode 44 is to extract positive ions from the plasma and prevent electrons from returning to the plasma chamber 12 from the beam line 52. Therefore, a negative high voltage is applied to the first extraction electrode 44. In order to apply a negative high voltage to the first extraction electrode 44, a first extraction power supply 48 is provided. The second extraction electrode 46 is grounded. Also, a positive high voltage is applied to the vacuum vessel 18. In order to apply a positive high voltage to the vacuum vessel 18, a second extraction power supply 50 is provided. The absolute value of the positive high voltage applied to the vacuum vessel 18 is greater than the absolute value of the negative high voltage applied to the first extraction electrode 44. Thereby, an ion beam 20 of positive ions is extracted from the plasma chamber 12. The direction of extracting the ion beam 20 from the plasma chamber 12 is the same as the propagation direction P of the microwave.

[0055] The ion source 10 is provided with a beamline 52 for transporting the ion beam 20 extracted through the extraction electrode system 42. The beamline 52 is connected to the ion source body 14 on the side opposite to the microwave supply system 26. The beamline 52 is a vacuum vessel communicating with the vacuum vessel 18. The beamline 52 is installed on the vacuum vessel 18 in a manner insulated from the vacuum vessel 18 of the ion source body 14. Therefore, a bushing 54 is provided between the beamline 52 and the vacuum vessel 18.

[0056] The bushing 54 maintains the vacuum in the beamline 52 and the vacuum vessel 18 while maintaining the withstand voltage between the vacuum vessel 18 and the grounded side. The bushing 54 is made of an insulating material. The bushing 54 has an annular shape and surrounds the extraction electrode system 42. The bushing 54 is installed to be sandwiched between the mounting flanges of the vacuum vessels of the beamline 52 and the ion source body 14 respectively.

[0057] A vacuum exhaust system 56 is provided for providing a vacuum environment to the vacuum chamber 18 and the plasma chamber 12. In the illustrated example, the vacuum exhaust system 56 is provided on the beam line 52. Since the beam line 52 communicates with the vacuum chamber 18 and the plasma chamber 12, the vacuum exhaust system 56 can evacuate the vacuum chamber 18 and the plasma chamber 12. The vacuum exhaust system 56 includes, for example, a high vacuum pump such as a cryopump or a turbomolecular pump.

[0058] The ion source 10 may include a control device C for controlling the output of the ion beam 20. The control device C controls each component of the ion source 10 to control the plasma generated in the plasma chamber 12, thereby controlling the output of the ion beam 20. The control device C is configured, for example, to control the operations of the microwave supply system 26, the gas supply system 34, and the coil power supply 76. The control device C can, for example, control the output of the ion beam 20 by adjusting at least one of the flow rate of the source gas, the microwave power, and the magnetic field strength.

[0059] The plasma chamber 12 is configured to generate and maintain a plasma in its internal space. Hereinafter, the internal space of the plasma chamber 12 may sometimes be referred to as the plasma generation space 58.

[0060] The plasma chamber 12 includes a starting end portion 60, a terminal end portion 62, and a side wall 64. The starting end portion 60 and the terminal end portion 62 face each other with the plasma generation space 58 therebetween. The side wall 64 surrounds the plasma generation space 58 and connects the starting end portion 60 and the terminal end portion 62. Thus, the plasma generation space 58 is defined inside the vacuum chamber 18 by the starting end portion 60, the terminal end portion 62, and the side wall 64. When the plasma chamber 12 has a cylindrical shape, the starting end portion 60 and the terminal end portion 62 are disk-shaped, the side wall 64 is cylindrical, and the end of the side wall 64 is fixed to the outer peripheral portions of the starting end portion 60 and the terminal end portion 62.

[0061] The starting end portion 60 has a vacuum window 24. The vacuum window 24 may occupy the entire starting end portion 60 or may be formed on a part (for example, the central portion) of the starting end portion 60. One side of the vacuum window 24 faces the plasma generation space 58, and the other side of the vacuum window 24 faces the microwave supply system 26. The vacuum window 24 seals the inside of the plasma chamber 12 as a vacuum. The propagation direction P of the microwave is perpendicular to the vacuum window 24. The vacuum window 24 is formed of a dielectric with low dielectric loss (for example, alumina or boron nitride, etc.). In addition, parts of the plasma chamber 12 other than the vacuum window 24 are formed of a non-magnetic metal material such as stainless steel or aluminum.

[0062] At least one extraction opening 66 is formed in the terminal end portion 62. The extraction opening 66 is formed at a position facing the vacuum window 24 with the plasma generation space 58 therebetween. That is, the vacuum window 24, the plasma generation space 58, and the extraction opening 66 are arranged along the axial direction of the plasma chamber 12.

[0063] The vacuum vessel 18 has a double-cylinder structure formed integrally with the plasma chamber 12. That is, the plasma chamber 12 is the inner cylinder of the vacuum vessel 18, and an outer cylinder 68 that houses the plasma chamber 12 is provided on its outer side. The outer cylinder 68 can be in the shape of a cylinder coaxial with the plasma chamber 12. There is a gap between the outer cylinder 68 and the side wall 64 of the plasma chamber 12, and the end portion of the gas pipe 40 of the gas supply system 34 enters this gap and is installed on the side wall 64. The vacuum vessel 18 is formed of, for example, a non-magnetic metal material.

[0064] The vacuum vessel 18 may not be formed integrally with the plasma chamber 12. The vacuum vessel 18 and the plasma chamber 12 can be a separable structure so that they can be separated. Also, the plasma chamber 12 can be constituted by the vacuum vessel 18 itself. Thus, in the case where the vacuum vessel 18 also serves as the plasma chamber 12, it is only necessary to install an end plate having an extraction opening 66 on the side of the beam line 52 of the outer cylinder 68.

[0065] One end of the vacuum vessel 18 is closed by an end plate 70, and the other end is open toward the beam line 52. The starting end portion 60 of the plasma chamber 12 is formed at the center portion of the end plate 70. The outer peripheral portion of the end plate 70 extends radially to the outside of the outer cylinder 68. An installation flange 72 for installing the bushing 54 is provided at the end portion of the vacuum vessel 18 on the beam line 52 side. The installation flange 72 extends radially outward from the outer cylinder 68. The axial lengths of the vacuum vessel 18 and the plasma chamber 12 are equal, and the installation flange 72 and the terminal end portion 62 of the plasma chamber 12 are in the same axial position. The axial lengths of the vacuum vessel 18 and the plasma chamber 12 may also be different.

[0066] The vacuum vessel 18 is formed with a magnet holding portion 74 for holding the magnetic field generator 16. The magnet holding portion 74 is formed, for example, on the outer surface of the outer cylinder 68 of the vacuum vessel 18. In the present embodiment, the magnetic field generator 16 is provided outside the vacuum vessel 18 (i.e., in the atmosphere). The magnetic field generator 16 is arranged so as to surround the vacuum vessel 18. However, in another example, the vacuum vessel 18 may also have a magnet holding portion 74 for holding the magnetic field generator 16 inside the vacuum vessel 18 (i.e., in a vacuum). At this time, the same effect as in this example can also be obtained. Thus, the magnetic field generator 16 is arranged to surround the plasma generation space 58.

[0067] The magnetic field generator 16 includes a coil that generates a magnetic field in the axial direction toward the plasma chamber 12. In this example, the plasma chamber 12 and the vacuum container 18 are cylindrical, the coil is formed in a ring shape, and the wire is wound around the circumference of the plasma chamber 12. The magnetic field generator 16 includes a coil power supply 76 that causes an electric current to flow through the coil. In addition, the number of coils included in the magnetic field generator 16 is not particularly limited, and it may include one coil or may include a plurality of coils arranged in the axial direction of the plasma chamber 12.

[0068] Figure 3 is a schematic structural diagram of the vicinity of the plasma chamber of the ion source 10 on an enlarged scale. As Figure 3 shown, the ion source 10 includes: the above-described plasma chamber 12 that generates ions through plasma; and extraction electrodes 44 and 46 that extract ions from the extraction opening 66 of the plasma chamber 12 to the outside. The plasma chamber 12 includes a terminal portion 62 that determines the terminal of the plasma generation space 58 in the axial direction. The terminal portion 62 faces the first extraction electrode 44 across an extraction gap 78 in the axial direction. In Figure 3 the central axis CL1 of the plasma chamber 12 is shown. The direction in which the central axis CL1 extends is the axial direction. Also, sometimes the side where ions are output in the axial direction is referred to as the "downstream" side, and the opposite side is referred to as the "upstream" side. Also, sometimes the side far from the central axis CL1 is referred to as the "outer circumference" side, and the side close to the central axis CL1 is referred to as the "inner circumference" side.

[0069] On the other hand, a vacuum window 24 is provided at the start end portion 60 of the plasma chamber 12. The vacuum window 24 has a double-layer structure including a window main body 80 and a window protection member 82. The window protection member 82 is the inner layer of the vacuum window 24 facing the plasma generation space 58, and the window main body 80 is the outer layer of the vacuum window 24 adjacent to the window protection member 82 on the waveguide 30 side. The window protection member 82 covers the window main body 80 to protect the window main body 80 from electrons flowing back from the outside of the plasma chamber 12 through the extraction opening 66. The window main body 80 is, for example, a plate of alumina, and the window protection member 82 is, for example, a plate of boron nitride. In addition, in order to protect the side wall 64 of the plasma chamber 12 from the plasma, a liner (for example, made of boron nitride) that covers the inner surface of the side wall 64 may also be provided.

[0070] A magnetic field generator 16 that generates a magnetic field in the axial direction is provided around the side wall 64 of the plasma chamber 12. The magnetic field generator 16 includes: an annular coil 84 that surrounds the plasma chamber 12 with the central axis CL1 as the center; and a magnetic yoke 86 that is attached to the coil 84. The magnetic yoke 86 is provided adjacent to the outer circumference of the coil 84 and both ends in the axial direction. As the material of the magnetic yoke 86, a magnetic material such as iron can be used.

[0071] The terminal portion 62 includes an end wall 69 and a plasma electrode 90. The end wall 69 is a wall that protrudes from the end portion on the downstream side of the side wall 64 toward the inner peripheral side. The plasma electrode 90 is an electrode disposed in the plasma chamber 12 and having an extraction opening 66 that opens axially from the plasma chamber 12. The plasma electrode 90 expands radially outward perpendicular to the axial direction about the central axis CL1. The plasma electrode 90 has a shape that is symmetric with respect to the central axis CL1. The above-mentioned extraction opening 66 that penetrates axially is formed at the center of the plasma electrode 90. Through the above-mentioned second extraction power supply 50 (reference Figure 2 ), a high voltage is applied to the plasma electrode 90 (the second electrode member 92).

[0072] The plasma electrode 90 includes a first electrode member 91, a second electrode member 92, and a third electrode member 93. The second electrode member 92 is disposed on the downstream side in the axial direction of the first electrode member 91. The third electrode member 93 is disposed on the upstream side in the axial direction of the first electrode member 91.

[0073] The first electrode member 91 is a member for suppressing the leakage magnetic field from the plasma chamber 12. The first electrode member 91 is a magnetic body. Specifically, as the magnetic body, a soft magnetic body such as iron can be used. In addition, as the magnetic body, cobalt, nickel, etc. can also be used. The second electrode member 92 is a member for protecting the first electrode member 91 from the discharge when a discharge occurs between the first extraction electrode 44 and the plasma electrode 90. The second electrode member 92 is a member having at least one of a melting point and a thermal conductivity higher than that of the magnetic body of the first electrode member 91. As the second electrode member 92, a non-magnetic body can be used. As the material of the second electrode member 92, a material selected from tungsten, tantalum, and molybdenum, which are high melting point materials, can be used. As the material of the second electrode member 92, a material with high thermal conductivity can be selected from copper, silver, and aluminum. The third electrode member 93 is a member for protecting the first electrode member 91 from the plasma. As the third electrode member 93, an insulator having plasma resistance, such as boron nitride or alumina, can be used.

[0074] The first extraction electrode 44 is located on the downstream side of the plasma electrode 90 and at a position separated from the plasma electrode 90, and is opposed to the plasma electrode 90 in the axial direction. The second extraction electrode 46 is located on the downstream side of the first extraction electrode 44 and at a position separated from the first extraction electrode 44, and is opposed to the first extraction electrode 44 in the axial direction. The extraction electrodes 44 and 46 have openings 44a and 46a through which the extracted ions pass. In the present embodiment, the extraction electrodes 44 and 46 have a conical shape that slopes toward the upstream side in the axial direction as it goes from the outer peripheral side toward the inner peripheral side. Therefore, the first extraction electrode 44 is closest to the plasma electrode 90 at the position near the opening 44a.

[0075] Next, Figure 4 is a cross-sectional view showing in more detail Figure 3 the structure of the plasma electrode 90 shown. Refer to Figure 4 for an explanation of the positional relationship of the plasma electrode 90. In addition, for the end portion 44b on the outer peripheral side of the first extraction electrode 44, a reference line SL1 extending in the axial direction (first direction) is set. The distance from the center axis CL1 to the radius (outer diameter) of the end portion 44b on the outer peripheral side of the first extraction electrode 44 (i.e., the end portion 44b in the radial direction (second direction)) is set as the distance R1. For the end portion 84a on the inner peripheral side of the coil 84, a reference line SL2 extending in the axial direction is set. The distance from the center axis CL1 to the radius (inner diameter) of the end portion 84a on the inner peripheral side of the coil 84 (i.e., the end portion 84a in the radial direction) is set as the distance R2. The radial distances of the reference lines SL1 and SL2 from the center axis CL1 are constant at each position in the axial direction.

[0076] The extraction opening 66 is formed by a through-hole that axially penetrates the end wall 69, the first electrode member 91, the second electrode member 92, and the third electrode member 93. In the present embodiment, the extraction opening 66 is constituted by a circular through-hole centered on the center axis CL1. However, as long as the extraction opening 66 is a shape capable of extracting ions from the plasma chamber 12, its shape, size, etc. are not particularly limited. In addition, as Figure 4 shown, the inner diameters of the extraction openings 66 formed in the end wall 69, the first electrode member 91, the second electrode member 92, and the third electrode member 93 can be different from each other and can have different inner diameters. The extraction opening 66 has an inner peripheral edge 66a. The inner peripheral edge 66a is the portion with the smallest inner diameter in the extraction opening 66. In the present embodiment, the end portion of the inner peripheral edge of the second electrode member 92 becomes the inner peripheral edge 66a. However, the inner peripheral edges of other members can also become the inner peripheral edge 66a. For the inner peripheral edge 66a of the extraction opening 66, a reference line SL3 extending in the axial direction (first direction) is set. The distance from the radius (outer diameter) of the end portion 44b on the outer peripheral side of the first extraction electrode 44 (i.e., the end portion 44b in the radial direction (second direction)) to the extraction opening 66 (here, the reference line SL3) is set as the distance R1'. The distance from the radius (inner diameter) of the end portion 84a on the inner peripheral side of the coil 84 (i.e., the end portion 84a in the radial direction) to the extraction opening 66 (here, the reference line SL3) is set as the distance R2'. The radial distances of the reference lines SL1 and SL2 from the reference line SL3 are constant at each position in the axial direction.

[0077] The first electrode member 91 of the plasma electrode 90 has a disk-like shape that extends radially around the central axis CL1. The first electrode member 91 has an inner peripheral end 91a and an outer peripheral end 91b. The positional relationship of the inner peripheral end 91a of the first electrode member 91 can be appropriately changed within a range that does not affect the extraction of ions. However, in Figure 4 the example shown, it is disposed at a position more radially inward than the inner peripheral end 69a of the end wall 69, and is disposed at a position more radially outward than the inner peripheral openings 44a, 46a of the extraction electrodes 44, 46.

[0078] In the radial direction, the distance R3 from the outer peripheral end 91b of the first electrode member 91 to the central axis CL1 is equal to or greater than the distance R1 from the outer peripheral end 44b of the first extraction electrode 44 to the central axis CL1. In the radial direction, the distance R3' from the outer peripheral end 91b of the first electrode member 91 to the extraction opening 66 (here, the reference line SL3) is equal to or greater than the distance R1' from the outer peripheral end 44b of the first extraction electrode 44 to the extraction opening 66 (here, the reference line SL3). That is, the outer peripheral end 91b of the first electrode member 91 is disposed in the radial direction at the same position as or more radially outward than the outer peripheral end 44b of the first extraction electrode 44. Additionally, in Figure 4 the example shown, the outer peripheral end 91b of the first electrode member 91 is disposed at a position more radially outward than the outer peripheral end 69b of the end wall 69.

[0079] In the radial direction, the distance R3 from the outer peripheral end 91b of the first electrode member 91 to the central axis CL1 is equal to or less than the distance R2 from the inner peripheral end 84a of the coil 84 to the central axis CL1. In the radial direction, the distance R3' from the outer peripheral end 91b of the first electrode member 91 to the extraction opening 66 (here, the reference line SL3) is equal to or less than the distance R2' from the inner peripheral end 84a of the coil 84 to the extraction opening 66 (here, the reference line SL3). That is, the outer peripheral end 91b of the first electrode member 91 is disposed in the radial direction at the same position as or more radially inward than the inner peripheral end 84a of the coil 84.

[0080] Therefore, the outer peripheral end 91b of the first electrode member 91 only needs to be disposed in the radial direction on the reference line SL1, on the reference line SL2, or in the region between the reference line SL1 and the reference line SL2. Figure 5 This shows a state where the outer peripheral end 91b of the first electrode member 91 is disposed in the radial direction at the same position as the outer peripheral end 44b of the first extraction electrode 44 (on the reference line SL1). Figure 6It shows a state where the end portion 91b on the outer peripheral side of the first electrode member 91 is arranged at the same position (on the reference line SL2) in the radial direction as the end portion 84a on the inner peripheral side of the coil 84. In this state, the first electrode member 91 extends to the magnetic field generator 16.

[0081] The second electrode member 92 of the plasma electrode 90 has a substantially disk-shaped configuration that expands radially around the central axis CL1. The second electrode member 92 has a portion that protrudes toward the downstream side in the axial direction near the end portion 92b on the outer peripheral side, but its shape is not particularly limited. The second electrode member 92 has an end portion 92a on the inner peripheral side and an end portion 92b on the outer peripheral side. The positional relationship of the end portion 92a on the inner peripheral side of the second electrode member 92 can be appropriately changed within a range that does not affect the extraction of ions, but it is arranged in such a way as to ensure the radius required for the extraction opening 66. Also, in order to protect against the discharge from the first extraction electrode 44, the end portion 92a on the inner peripheral side of the second electrode member 92 can be arranged at a position more on the inner peripheral side than the end portion 91a on the inner peripheral side of the first electrode member 91. The end portion 92b on the outer peripheral side of the second electrode member 92 can be arranged to cover the first electrode member 91 within a range that allows discharge to be generated from the first extraction electrode 44. In Figure 4 the example shown, the end portion 92b on the outer peripheral side of the second electrode member 92 is arranged at a position more on the inner peripheral side than the end portion on the outer peripheral side of the first extraction electrode 44.

[0082] The third electrode member 93 of the plasma electrode 90 has a disk-shaped configuration that expands radially around the central axis CL1. The third electrode member 93 has an end portion 93a on the inner peripheral side and an end portion 93b on the outer peripheral side. The positional relationship of the end portion 93a on the inner peripheral side of the third electrode member 93 can be appropriately changed within a range that does not affect the extraction of ions, but in order to protect the first electrode member 91 from the plasma, it is arranged at a position where the first electrode member 91 is not exposed. Also, in order to protect the first electrode member 91 from the plasma, the end portion 93b on the outer peripheral side of the third electrode member 93 is arranged at a position more on the outer peripheral side than the end portion 69a on the inner peripheral side of the end wall 69.

[0083] Next, the effects of the ion source 10 and the neutron capture therapy device 100 according to the present embodiment will be described.

[0084] In the ion source 10 according to the present embodiment, the plasma electrode 90 has at least a first electrode member 91 and a second electrode member 92, and the first electrode member 91 is a magnetic body. Thus, the magnetic field is guided to the outer peripheral side by the magnetic body (i.e., the first electrode member 91), so that the leakage magnetic field toward the space between the plasma electrode 90 and the first extraction electrode 44 can be suppressed. In particular, in the radial direction (second direction) orthogonal to the axial direction (first direction) in which the plasma electrode 90 and the first extraction electrode 44 face each other, the distance R3' from the end portion 91b on the outer peripheral side of the first electrode member 91 to the extraction opening 66 (here, the reference line SL3) is equal to or greater than the distance R1' from the end portion 44b on the outer peripheral side of the first extraction electrode 44 to the extraction opening 66 (here, the reference line SL3). Thus, the leakage magnetic field generated from the outer peripheral side of the first electrode member 91 can be suppressed. For example, when the outer diameter of the magnetic body of the first electrode member 91 is smaller than the outer diameter of the first extraction electrode 44, a leakage magnetic field may be generated from the end portion 91b on the outer peripheral side of the first electrode member 91 into the space between the electrodes. In the present embodiment, since the generation of the leakage magnetic field can be suppressed, the discharge between the plasma electrode 90 and the first extraction electrode 44 can be suppressed. Further, the second electrode member 92 is a member having at least one of a higher melting point and higher thermal conductivity than the magnetic body. During discharge, electrons flow from the extraction electrode 44 to the plasma electrode 90, so that the plasma electrode 90 is locally heated at the discharge site. When the heated portion is a magnetic body, an instantaneous temperature rise may cause the material to melt, resulting in damage such as cracking. In contrast, in the present embodiment, even if a discharge occurs, the magnetic body (i.e., the first electrode member 91) can be protected by the second electrode member 92. Even if the temperature locally rises due to the discharge, as long as it is a material with a high melting point, it can withstand the temperature rise. Further, as long as it is a material with high thermal conductivity, even if the temperature rises, the heat can be rapidly diffused. Thus, the influence of the discharge between the electrodes can be reduced. Further, since the damage of the first electrode member 91 can be suppressed, the frequency of failures and the frequency of maintenance can be reduced.

[0085] Here, with reference to Figures 7 - 9 , the relationship between the outer diameter of the first electrode member 91 and the leakage magnetic field suppression effect will be described. Figure 7 FIG. is a diagram showing the results of a simulation test of a two-dimensional magnetic field calculation system by fabricating a model of the ion source 10. In Figure 7 , the darker the color of the portion, the stronger the magnetic field. In this simulation test, "FEMM" was used as the calculation software. As shown in Figure 8As shown, models of Example 1 to Example 5 were prepared. Example 1 is a model when the outer diameter of the first electrode member 91 is substantially the same as the outer diameter of the first lead electrode 44. Examples 2 to 4 are models in which the outer diameter of the first electrode member 91 is gradually increased. Example 5 is a model when the outer diameter of the first electrode member 91 is the same as the inner diameter of the coil 84. Figure 9 is a graph showing the magnetic field intensity on the central axis CL1 in Examples 1 to 5. The region of "E1" in the graph represents Figure 7 the beam extraction region denoted by "E1" in Figure 7 The region of "E2" in the graph represents Figure 9 the region of the plasma chamber 12 denoted by "E2" in

[0086] The material of the second electrode member 92 can be selected from tungsten, tantalum, and molybdenum. At this time, the second electrode member 92 can be made to have a high melting point.

[0087] The material of the second electrode member 92 can be selected from copper, silver, and aluminum. At this time, the second electrode member 92 can have high thermal conductivity.

[0088] The ion source 10 may further include a magnetic field generator 16, which is provided on the outer peripheral side of the plasma chamber 12 and generates an axial magnetic field in the plasma chamber 12, and the end portion 91b on the outer peripheral side of the first electrode member 91 is disposed at a position separated from the magnetic field generator 16 toward the inner peripheral side. At this time, leakage of the magnetic field from the end portion 91b on the outer peripheral side of the first electrode member 91 can also be suppressed.

[0089] The ion source 10 may further include a magnetic field generator 16, which is provided on the outer peripheral side of the plasma chamber 12 and generates an axial magnetic field in the plasma chamber 12, and the first electrode member 91 extends to the magnetic field generator 16. At this time, leakage of the magnetic field from the end portion 91b on the outer peripheral side of the first electrode member 91 can be further suppressed.

[0090] The neutron capture therapy device 100 according to this embodiment includes: an accelerator 112 having an ion source 10 that generates ions, and accelerating and emitting a particle beam; and an irradiation unit that generates neutron beams based on the particle beam and irradiates the object with the neutron beams. In the neutron capture therapy device 100, the ion source 10 includes: a plasma electrode 90 provided in a plasma chamber 12 that generates ions through plasma; and a first extraction electrode 44 opposed to the plasma electrode 90 and extracting ions from the plasma chamber 12. The plasma electrode 90 has at least a first electrode member 91 and a second electrode member 92, and an extraction opening 66 for extracting ions is formed. The first electrode member 91 is a magnetic body, and the second electrode member 92 is a member having at least one of a melting point and thermal conductivity higher than that of the magnetic body. In the radial direction (second direction) orthogonal to the axial direction (first direction) opposed to the plasma electrode 90 and the first extraction electrode 44, the distance R3' from the outer peripheral end 91b of the first electrode member 91 to the extraction opening 66 (here, the reference line SL3) is equal to or greater than the distance R1' from the outer peripheral end 44b of the first extraction electrode 44 to the extraction opening 66 (here, the reference line SL3).

[0091] In the neutron capture therapy device 100 according to this embodiment, the plasma electrode 90 has at least a first electrode member 91 and a second electrode member 92, and the first electrode member 91 is a magnetic body. Thus, the magnetic field is guided to the outer peripheral side by the magnetic body (i.e., the first electrode member 91), so that the leakage magnetic field toward the space between the plasma electrode 90 and the first extraction electrode 44 can be suppressed. In particular, in the radial direction (second direction) orthogonal to the axial direction (first direction) opposed to the plasma electrode 90 and the first extraction electrode 44, the distance R3' from the outer peripheral end 91b of the first electrode member 91 to the extraction opening 66 (here, the reference line SL3) is equal to or greater than the distance R1' from the outer peripheral end 44b of the first extraction electrode 44 to the extraction opening 66 (here, the reference line SL3). Thus, the leakage magnetic field generated from the outer peripheral side of the first electrode member 91 can be suppressed. In this way, since the generation of the leakage magnetic field can be suppressed, the discharge between the plasma electrode 90 and the first extraction electrode 44 can be suppressed. And the second electrode member 92 is a member having at least one of a melting point and thermal conductivity higher than that of the magnetic body. Therefore, even if a discharge occurs, the magnetic body (i.e., the first electrode member 91) can be protected by the second electrode member 92. Thus, the influence of the discharge between the electrodes can be reduced. And since the damage of the first electrode member 91 can be suppressed, the frequency of failures that cause the neutron beam N not to be irradiated can be reduced, and thus the maintenance frequency can be reduced.

[0092] The present invention is not limited to the above embodiment.

[0093] For example, as Figure 10As shown in Embodiment 6 of (b), the first electrode member 91 may have a connecting portion 95 connected to the yoke 86 of the magnetic field generator 16. In Embodiment 6, the first electrode member 91 extends to the coil 84, and the end portion 91b is connected to the yoke 86 via the connecting portion 95. At this time, the first electrode member 91 can be connected to the magnetic circuit of the coil 84 through the connecting portion 95, and the generation of leakage magnetic fields can be further suppressed. Figure 10 (a) in is an example in which the first electrode member 91 extends to the coil 84 but does not have a connecting portion 95, which corresponds to Figure 8 Embodiment 5 in. As Figure 11 shown, compared with Embodiment 5 without the connecting portion 95, Embodiment 6 with the connecting portion 95 can significantly reduce the leakage magnetic field in the beam extraction region. Thus, even when the outer diameter of the magnetic body is the same, the reduction degree of the leakage magnetic field can be further improved according to the shape. Thus, the shape of the magnetic body is not limited to a disk, and all axisymmetric shapes capable of providing a portion protruding in the axial direction can also be adopted.

[0094] In the above embodiment, the extraction electrodes 44 and 46 are inclined, but they can also be in the shape of a disk extending straight in the radial direction. In addition, when the extraction electrodes 44 and 46 are in the shape of a disk, discharge is likely to occur on the outer peripheral side. Therefore, it is preferable that the second electrode member 92 covers the outer peripheral side position of the first electrode member 91 to the extraction electrode 44.

[0095] The second electrode member 92 is not limited to the above embodiment. For example, as Figure 12 shown, the second electrode member 92 can also be a multilayer structure. Here, the second electrode member 92 has a first layer 92A with high thermal conductivity on the upstream side and a second layer 92B with a high melting point on the downstream side. The temperature of the second layer 92B on the downstream side rises instantaneously during discharge. Therefore, by making the second layer 92B have a high melting point, a second electrode member 92 with excellent durability can be realized.

[0096] In the above embodiment, the ion source 10 is applied to the accelerator 112 of the neutron capture therapy device 100. However, the use of the ion source 10 is not limited. For example, it can also be used as an ion source for an ion implantation device or other particle beam therapy devices. The ion source 10 is used as a monovalent ion source, for example. And the ion source 10 can also be used as an ion source for a proton accelerator or an X-ray source.

[0097] The above ion source 10 extracts positive ions, but whether the generated ions are positive or negative is not particularly limited.

[0098] Moreover, the ion source 10 is not limited to a microwave ion source, and the present invention can also be applied to all ion sources using a magnetic field (for example, an ECR ion source).

Claims

1. An ion source, characterized in that: have: A plasma electrode disposed in a plasma chamber for generating ions through plasma; and an extraction electrode, which is opposite to the plasma electrode and extracts the ions from the plasma chamber, The plasma electrode has at least a first electrode member and a second electrode member, and is formed with an extraction opening for extracting the ions. The first electrode member is a magnetic body. The second electrode member is a member having at least one of a melting point and a thermal conductivity higher than that of the magnetic body. In a second direction orthogonal to a first direction in which the plasma electrode and the extraction electrode are opposed, a distance from an outer peripheral end of the first electrode member to the extraction opening is greater than a distance from an outer peripheral end of the extraction electrode to the extraction opening.

2. The ion source according to claim 1, characterized in that The material of the second electrode member is selected from tungsten, tantalum and molybdenum.

3. The ion source according to claim 1, characterized in that The material of the second electrode member is selected from copper, silver and aluminum.

4. The ion source according to claim 1, characterized in that further comprising a magnetic field generating unit, the magnetic field generating unit being provided on the outer peripheral side of the plasma chamber and generating a magnetic field in the first direction in the plasma chamber, An outer peripheral end portion of the first electrode member is disposed at a position spaced apart from the magnetic field generating portion toward the inner peripheral side.

5. The ion source according to claim 1, characterized in that further comprising a magnetic field generating unit, the magnetic field generating unit being provided on the outer peripheral side of the plasma chamber and generating a magnetic field in the first direction in the plasma chamber, The first electrode member extends to the magnetic field generating portion.

6. The ion source according to claim 5, characterized in that The magnetic field generating unit comprises a coil and a yoke. The first electrode member includes a connection portion connected to the yoke of the magnetic field generating unit.

7. A neutron capture therapy device comprising: an accelerator having an ion source for generating ions, and accelerating the ions and emitting a particle beam; and an irradiation unit that generates neutron beams using the particle beam and irradiates the neutron beams toward an object, The neutron capture therapy device is characterized in that: The ion source comprises: A plasma electrode disposed in a plasma chamber for generating ions through plasma; and an extraction electrode, which is opposite to the plasma electrode and extracts the ions from the plasma chamber, The plasma electrode has at least a first electrode member and a second electrode member, and is formed with an extraction opening for extracting the ions. The first electrode member is a magnetic body. The second electrode member is a member having at least one of a melting point and a thermal conductivity higher than that of the magnetic body. In a second direction orthogonal to a first direction in which the plasma electrode and the extraction electrode are opposed, a distance from an outer peripheral end of the first electrode member to the extraction opening is greater than a distance from an outer peripheral end of the extraction electrode to the extraction opening.

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