Rotary capacitor, round accelerator, and particle therapy system

Abstract

To provide a rotary capacitor which can stabilize frequency modulation while increasing the life of a component.SOLUTION: A rotary capacitor 22 includes: an external conductor 15 covering a stator electrode 32 and a rotor electrode 33; and a bypass capacitor 23 for electrostatically combining the rotor electrode 33 and an external conductor 15 to each other. An electrode pair of the bypass capacitors 23 (an external conductor side electrode 24 and a rotor side electrode 25) are set to extend along the direction of the axis of rotation of the rotor electrode 33 in the outer periphery side of a metal shaft part 35a. Also, at least one of the electrode pair of the bypass capacitor 23 has a cylindrical shape.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present disclosure relates to rotating condensers, circular accelerators and particle therapy systems. [Background technology]

[0002] As a circular accelerator that accelerates charged particles circulating in a main magnetic field and outputs them as a beam, a synchrocyclotron and an eccentric orbit accelerator that accelerate the charged particles by temporally modulating the frequency of an accelerating high-frequency electric field acting on the charged particles are known. This type of circular accelerator uses a superconducting coil to generate the main magnetic field, which makes it relatively easy to increase the strength of the main magnetic field, making it possible to reduce the cost by making the accelerator smaller, and is particularly applied to particle beam therapy systems.

[0003] In the above circular accelerator, a rotating capacitor is often used as a modulation element that modulates the accelerating high-frequency electric field. A rotating capacitor generally has a fixed stator electrode, a rotor electrode arranged opposite the stator electrode, and a rotation mechanism that rotates the rotor electrode. The rotation mechanism includes a shaft that rotatably supports the rotor electrode, and a bearing that supports the shaft. In addition, a vacuum rotating capacitor in which the electrode parts (stator electrode and rotor electrode) are provided in a vacuum is suitable as the rotating capacitor, and the shaft may be provided with a vacuum seal to maintain a vacuum state around the electrode parts.

[0004] Components such as the bearings and vacuum seals of the rotating condenser used in the circular accelerator may carry high-frequency currents associated with the accelerating high-frequency electric field from the circular accelerator. As a result, these components deteriorate over time due to the high-frequency currents. In other words, these components are consumables and need to be replaced periodically, but it is desirable to extend their lifespan in order to reduce the frequency of replacement.

[0005] In response to this, Patent Document 1 discloses a circular accelerator having a rotating capacitor in which the rotor electrodes of the rotating capacitor are insulated from an electrically conductive housing in terms of direct current and are capacitively coupled to the housing. In this circular accelerator, it is possible to allow high-frequency current to escape from the rotor electrodes to the conductive housing, thereby reducing the high-frequency current flowing through the shaft and bearings.

[0006] Furthermore, Non-Patent Document 1 discloses an accelerator in which a bypass capacitor that releases high-frequency current flowing from the rotor electrode to the shaft to the housing is installed between the rotor electrode and the shaft, separately from the rotor electrode. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Special Publication No. 2014-533884 [Non-patent literature]

[0008] [Non-Patent Document 1] Seishi Kikuchi, A 160 cm synchro and variable energy ordinary cyclotron, journal of the physical society of japan, Vol.15, No.1, 1960 Summary of the Invention [Problem to be solved by the invention]

[0009] In the technology described in Patent Document 1, the only configuration for dissipating high-frequency current into the housing is the rotor electrode of the variable capacitor. Therefore, when the electrostatic capacitance between the stator electrode and rotor electrode is small, heat is generated in the shaft and bearing due to dielectric loss caused by high-frequency power, which weakens the mechanical strength of the shaft and bearing.

[0010] In addition, in the technology described in Non-Patent Document 1, a bypass capacitor that releases high-frequency current to the housing is installed between the rotor electrode and the shaft, which reduces heat generation in the shaft and the bearing. However, because a bypass capacitor is installed between the rotor electrode and the shaft, the distance between the rotor electrode and the bearing becomes longer, which causes problems such as shaft vibration being more likely to occur and unstable frequency modulation.

[0011] An object of the present invention is to provide a rotating condenser, a circular accelerator, and a particle beam therapy system that are capable of stabilizing frequency modulation while extending the life of components. [Means for solving the problem]

[0012] A rotating capacitor according to one embodiment of the present disclosure is a rotating capacitor having a fixed stator electrode, a rotor electrode opposing the stator electrode, and a shaft portion supporting the rotor electrode rotatably around a rotation axis, the rotating capacitor further having a housing covering the stator electrode and the rotor electrode, and a bypass capacitor electrostatically coupling the rotor electrode to the housing, wherein an electrode pair of the bypass capacitor is disposed on the outer diameter side of the shaft portion so as to extend in the direction of the rotation axis, and at least one electrode of the electrode pair is cylindrical in shape. Effect of the Invention

[0013] According to the present disclosure, it is possible to stabilize frequency modulation while extending the life of components. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a perspective view showing an external appearance of a circular accelerator according to an embodiment of the present disclosure. [Diagram 2] FIG. 2 is a cross-sectional view showing a cross section of a circular accelerator according to an embodiment of the present disclosure. [Diagram 3] FIG. 1 is a cross-sectional view showing a longitudinal section of a circular accelerator according to an embodiment of the present disclosure. [Figure 4]FIG. 1 illustrates an example of a rotating capacitor according to an embodiment of the present disclosure. [Diagram 5] FIG. 2 is a cross-sectional view showing a cross section of a rotating capacitor. [Figure 6] FIG. 2 is a cross-sectional view showing a cross section of a bypass capacitor. [Figure 7] FIG. 2 is a diagram showing an equivalent circuit of the capacitance of a rotating capacitor. [Figure 8] FIG. 13 illustrates a modified example of a rotating capacitor according to an embodiment of the present disclosure. [Figure 9] FIG. 13 is a diagram showing an equivalent circuit of the capacitance of a rotating capacitor according to a modified example. [Figure 10] FIG. 13 illustrates another modified example of a rotating capacitor according to an embodiment of the present disclosure. [Figure 11] FIG. 13 illustrates another modified example of a rotating capacitor according to an embodiment of the present disclosure. [Figure 12] FIG. 1 illustrates a particle therapy system according to an embodiment of the present disclosure. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Hereinafter, embodiments of a rotating condenser, a circular accelerator, and a particle beam therapy system according to the present disclosure will be described with reference to the drawings. In each drawing, identical or corresponding components are designated by identical or similar reference numerals, and repeated description of these components may be omitted.

[0016] In addition, the following embodiment is merely an example, and the present disclosure is not limited to the following specific aspects. The present disclosure itself can be modified into various forms other than the following embodiment. For example, the rotating condenser according to the present disclosure can be suitably used in a circular accelerator, but is not limited to this application. In addition, the circular accelerator according to the present disclosure can be suitably used in a particle beam therapy system, but is not limited to this application.

[0017] Fig. 1 is a perspective view showing the appearance of a circular accelerator according to an embodiment of the present disclosure. Fig. 2 is a cross-sectional view showing a transverse section (center plane) of the circular accelerator. Fig. 3 is a cross-sectional view taken along line A-A' in Fig. 2, showing a longitudinal section of the circular accelerator.

[0018] The circular accelerator 100 shown in Figs. 1 to 3 is a device that accelerates and extracts a beam of charged particles circulating in a main magnetic field (DC main magnetic field) of constant intensity in time, using a frequency-modulated accelerating high-frequency electric field for acceleration. In this embodiment, a device that accelerates a proton beam to about 200 MeV will be described as the circular accelerator 100. More specifically, the circular accelerator 100 is an eccentric orbit type accelerator that forms a main magnetic field so that the orbit of the beam is eccentric to the side of the beam extraction path entrance 82 for extracting the beam, and can extract the beam by arbitrarily changing the energy of the beam between about 70 MeV and 200 MeV. However, the circular accelerator 100 is not limited to this example, and may be, for example, a device that accelerates a heavy particle beam such as helium or carbon, or an isocentric circular accelerator (for example, a synchrocyclotron) in which the orbit of the charged particle beam is not eccentric.

[0019] 1 and 3, the outer shell of the circular accelerator 100 is formed by a main electromagnet 40 that can be divided in the vertical direction across a central plane D. An acceleration region 101, which is a substantially cylindrical space for accelerating a beam, is formed inside the main electromagnet 40. The acceleration region 101 is formed substantially symmetrically across the central plane D and is evacuated.

[0020] The spiral orbit that the beam follows from when acceleration begins in acceleration region 101 until the beam energy reaches the maximum energy of about 200 MeV is called a circular orbit. Figure 2 shows, among the circular orbits, a maximum energy orbit 80 through which a beam with a maximum energy of about 200 MeV passes, and a minimum extraction energy orbit 81 through which a beam with an energy of 70 MeV, the lowest value at which the beam can be extracted, passes.

[0021] 1, an ion source 51 that generates charged particles to be incident on the main electromagnet 40 is installed above the main electromagnet 40. The ion source 51 is, for example, an ECR (Electron Cyclotron Resonance) ion source. The ion source 51 may be installed in the acceleration region 101, in which case a PIG (Phillips Ionization Gauge) ion source is preferable.

[0022] The ion source 51 is connected to an ion injection unit 52 provided in an acceleration region 101 in the main electromagnet 40 via a low-energy beam transport system 53, and charged particles generated by the ion source 51 pass through the low-energy beam transport system 53 and are injected into the acceleration region 101 via the ion injection unit 52. The ion injection unit 52 is located on the side of the beam extraction path entrance 82 for extracting the beam to the outside, relative to the physical center of the acceleration region 101, on a center line C passing through the center of the circular accelerator 100.

[0023] The beam injected into the acceleration region 101 is accelerated by the accelerating radio frequency electric field and orbits in the main magnetic field while increasing its energy. As the beam accelerates, the radius of curvature of its orbit increases, and the beam describes a spiral orbit from the center of the acceleration region to the outside.

[0024] As shown in FIG. 3, the main electromagnet 40 has a yoke 41, a main coil 42, and a main magnetic pole 43. The yoke 41 forms the outer shell of the main electromagnet 40, and defines an acceleration region 101 inside. The main coils 42 are annular superconducting coils, and are disposed above and below the central plane D. Each of the main coils 42 is disposed along the inner wall of the yoke 41. A cryostat 60, which is a cooling mechanism for cooling the main coil 42 to a certain temperature (the temperature at which the main coil 42 exhibits complete diamagnetism) or lower, is disposed around the main coil 42. The main magnetic poles 43 are disposed on the inner periphery side of the main coil 42, above and below the central plane D.

[0025] When a current is supplied to the main coil 42, a time-constant magnetic field called the main magnetic field is excited in the acceleration region 101 by the main magnet 43, and the beam revolves on the orbital plane of the acceleration region 101 due to the influence of the main magnetic field. Inside the main electromagnet 40, a peeler magnetic field region 44 and a regenerator magnetic field region 45, which are disturbance magnetic fields consisting of a bipole magnetic field and a multipole magnetic field, are formed. A radio frequency kicker 70, a peeler magnetic field region 44, a regenerator magnetic field region 45, a septum coil 83, and a high energy beam transport system 47 are used for beam extraction. The radio frequency kicker 70 is a device that applies a radio frequency voltage to the revolving beam passing through its interior. The septum coil 83 is a coil for deflecting the beam horizontally toward the outer periphery. Note that the septum coil 83 may be any coil that can deflect the beam passing through its interior toward the high energy beam transport system 47, and may be replaced with a passive configuration using a magnetic material or a permanent magnet without using a coil.

[0026] 1 and 2, the yoke 41 is provided with a plurality of through holes. For example, the yoke 41 is provided with a beam through hole 46, a coil through hole 48, a vacuum through hole 49, and a high-frequency system through hole 50 as through holes.

[0027] The beam through-hole 46 is a through-hole for emitting the accelerated beam, and is provided with a high-energy beam transport system 47 for extracting and emitting the beam from inside the yoke 41 to the outside. A septum coil 83 for extracting the beam to the outside is provided at a beam extraction path entrance 82 located at the end of the high-energy beam transport system 47 on the inside side of the circular accelerator 100.

[0028] The coil through-hole 48 is a through-hole for drawing out various coils (such as the main coil 42) installed inside the yoke 41 to the outside. The vacuum through-hole 49 is a through-hole for drawing a vacuum in the acceleration region 101. The high-frequency system through-hole 50 is a through-hole for inserting the acceleration cavity 10.

[0029] The acceleration cavity 10 is a member that excites an accelerating radio frequency electric field for accelerating the beam injected from the ion injection section 52 into the acceleration region 101. In this embodiment, the acceleration cavity 10 is a λ / 2 resonant cavity, and includes a dee electrode 12, a dummy dee electrode 13, an inner conductor 14, and an outer conductor 15, and is connected to a rotating capacitor 22.

[0030] The dee electrode 12 is a hollow electrode through which the beam passes, and is provided at one end of the acceleration cavity 10. It has a substantially fan-shaped shape with a predetermined spread angle, with a vertex at a point near the ion injection section 52. The inner conductor 14 is a conductor connected to the dee electrode 12 and extends from the dee electrode 12 to the outside of the main electromagnet 40 through the high-frequency system through-hole 50. The outer conductor 15 is a conductor that surrounds the dee electrode 12 and the inner conductor 14, and also functions as a housing for the rotating capacitor 22. The dummy dee electrode 13 is an electrode at earth potential, and is connected to the outer conductor 15. The dummy dee electrode 13 is provided to face the dee electrode 12, and an acceleration gap 11 in which an accelerating high-frequency electric field is excited is formed between the dee electrode 12 and the dummy dee electrode 13.

[0031] The rotating capacitor 22 is a device for modulating the acceleration radio frequency voltage by modulating the resonance frequency of the acceleration cavity 10. The capacitance of the rotating capacitor 22 varies over time, thereby changing the resonance frequency of the acceleration cavity 10, and a frequency modulation pattern can be formed at the resonance frequency. The acceleration radio frequency voltage whose frequency is modulated by the rotating capacitor 22 and the radio frequency electric field due to the acceleration radio frequency voltage are generated in the acceleration gap 11. The acceleration gap 11 is formed according to the orbital shape of the beam. In this embodiment, the number of harmonics is 1, that is, the orbital frequency at which the beam orbits and the acceleration frequency which is the frequency of the acceleration radio frequency voltage (more specifically, the frequency of the acceleration electric field due to the acceleration radio frequency voltage) are approximately the same.

[0032] High frequency power for generating an accelerating high frequency voltage is supplied from a high frequency power supply 21 to the accelerating cavity 10 via an input coupler 20. The input coupler 20 is coupled to the accelerating cavity 10 by either an electrostatic coupling method or a magnetic coupling method. The high frequency power supply 21 supplies high frequency power at a frequency that follows the change in the resonant frequency of the accelerating cavity 10 by either a self-excited method or a separately-excited method.

[0033] When there is a connection surface perpendicular to the direction in which the high frequency current flows, such as the connection between the dee electrode 12 and the inner conductor 14, electrical continuity is ensured by providing an RF (Radio Frequency) contact on the connection surface. This makes it possible to suppress dimensional changes in the accelerating cavity 10 due to thermal expansion, as well as fluctuations in the gap between the stator electrode 32 and the rotor electrode 33, which will be described later with reference to Fig. 4 etc. A bellows or the like may be used instead of the RF contact.

[0034] In addition, a cylindrical stub 16 extending coaxially is formed on each of the inner conductor 14 and the outer conductor 15. The stub 16 functions as a substitute for an inductive load (coil) for adjusting the resonance frequency of the acceleration cavity 10. A polyimide film or the like is provided between the inner conductor 14 and the outer conductor 15 in the stub 16, and the film is insulated from direct current electricity but is conductive to high frequency electricity. The resonance frequency of the acceleration cavity 10 can be adjusted by adjusting the length of the stub 16, the ratio of the diameter of the inner conductor 14 to the diameter of the outer conductor 15 in the stub 16, and the position where the stub 16 is attached. The stub 16 is also used as an entrance for passing water cooling pipes, signal lines, power supply lines, etc. from the inner conductor 14 to the Dee electrode 12 side. In the example of FIG. 2, only one stub 16 is provided, but two or more stubs may be provided.

[0035] The rotating capacitor 22 is described in more detail below.

[0036] FIG. 4 is a diagram showing an example of the rotating capacitor 22 of the present embodiment.

[0037] 4, the rotating capacitor 22 is installed at the end of the acceleration cavity 10 opposite to the dee electrode 12. The rotating capacitor 22 includes a motor 31, a stator electrode 32, a rotor electrode 33, a shaft 35, a rotary joint 34, a vacuum seal 29, a shaft bearing 30, and a bypass capacitor 23.

[0038] The stator electrode 32 is fixed on the inner conductor 14. The rotor electrode 33 is provided inside the outer conductor 15 (in the gap between the inner conductor 14 and the outer conductor 15) so as to face the stator electrode 32. The inside of the outer conductor 15 in which the rotor electrode 33 is provided is maintained in a vacuum state. A small gap is provided between the outer conductor 15 and the rotor electrode 33, which electrostatically couples the outer conductor 15 and the rotor electrode 33 to have the same potential. The gap allows the rotor electrode 33 to rotate without being in physical contact with the outer conductor 15. The stator electrode 32 may be provided on the outer conductor 15, and the rotor electrode 33 may be provided inside the outer conductor 15 (in the gap between the inner conductor 14 and the outer conductor 15). In this case, a small gap is provided between the inner conductor 14 and the rotor electrode 33.

[0039] Fig. 5 is a cross-sectional view taken along line B-B' in Fig. 4, showing a cross section of the rotating capacitor. As shown in Fig. 5, the stator electrode 32 and the rotor electrode 33 have a periodic symmetric structure having cutouts in the circumferential direction according to a desired modulation pattern so as to realize modulation of the resonant frequency of the accelerating cavity 10. As a result, as the rotor electrode 33 rotates, the area of ​​the opposing portion between the stator electrode 32 and the rotor electrode 33 changes over time, and the electrostatic capacitance formed between the stator electrode 32 and the rotor electrode 33 varies over time.

[0040] 5, since the above-mentioned cyclic symmetry structure has eight-fold symmetry, the frequency modulation pattern is repeated eight periods for each rotation of the motor 31. However, the cyclic symmetry structure is not limited to this example, and the number of cyclic symmetries may be less than eight or more than eight. The larger the number of cyclic symmetries, the lower the rotation speed of the motor 31 can be, and the more the axial vibration can be suppressed, thereby extending the life of the bearing 30 and the vacuum seal 29, and furthermore, the frequency of the high frequency can be modulated with high precision.

[0041] Returning to the description of FIG. 4, the shaft 35 connects the motor 31 and the rotor electrode 33, and rotatably supports the rotor electrode 33. The shaft 35 includes a metal shaft portion 35a and a rotor electrode support portion 35b. The metal shaft portion 35a is a shaft portion that serves as a rotation axis of the rotor electrode 33, and in this embodiment, is installed so as to penetrate the center of the motor 31. The rotor electrode support portion 35b is provided at one end of the metal shaft portion 35a. The rotor electrode support portion 35b is a disk-shaped member that is provided along a radial direction that is approximately perpendicular to the extension direction (rotation axis direction) of the metal shaft portion 35a. The rotor electrode 33 is provided on the rotor electrode support portion 35b. In addition, the rotary joint 34 is provided at the other end of the metal shaft portion 35a. The rotary joint 34 supplies cooling water into the shaft 35. Pipes (not shown) for passing cooling water supplied from the rotary joint 34 are provided inside the shaft 35 and the rotor electrode 33, and the cooling water cools the shaft 35 and the rotor electrode 33. This makes it possible to easily suppress deformation of the shaft 35 and the rotor electrode 33 due to thermal expansion.

[0042] In the example of FIG. 4, the rotating capacitor 22 is attached so that the rotation shaft of the motor 31 is approximately perpendicular to the extension direction of the dee electrode 12. More specifically, in the example of FIG. 4, the rotation shaft of the motor 31 is attached vertically upward to the extension direction of the dee electrode 12. The input coupler 20 and the high-frequency power source 21 are provided on the opposite side of the rotation shaft of the motor 31 from the dee electrode 12. In this case, since a double-supported state is formed with a load on both sides of the bearing 30, the rotation of the rotor electrode 33 is stabilized and axial vibration can be reduced. The rotation shaft of the motor 31 does not need to be attached vertically upward to the extension direction of the dee electrode 12, but may be attached vertically downward to the extension direction of the dee electrode 12 or may be attached parallel to the extension direction of the dee electrode 12. FIG. 2 shows an example in which the rotation shaft of the motor 31 is attached parallel to the extension direction of the dee electrode 12. Furthermore, when the rotating shaft of the motor 31 is attached facing downward in a direction perpendicular to the extension direction of the dee electrode 12, the inside of the outer conductor 15 in which the rotor electrode 33 of the rotating capacitor 22 is provided is in a vacuum state, so that a vertically upward force is applied to the rotor electrode 33 due to the external air pressure. This reduces the load on the bearing 30 of the shaft 35. Furthermore, the bearing 30 and the motor 31 are located below the vacuum seal 29 and outside the area surrounded by the outer conductor 15, shaft 35, and vacuum seal 29 that maintain the inside in a vacuum state, making maintenance easier.

[0043] 4, the motor 31 has a structure in which the rotating shaft and the metal shaft portion 35a are shared, but this is not limited to this example. For example, the motor 31 may be installed near the metal shaft portion 35a, and the motor 31 and the shaft 35a may be mechanically connected via gears, pulleys, etc.

[0044] The vacuum seal 29 is a member provided between the shaft 35 and the outer conductor 15, and seals the space surrounded by the shaft 35 and the outer conductor 15 so as to maintain a vacuum state. The vacuum seal 29 is, for example, a lip seal, a double O-ring, a Wilson seal, or a bellows seal. When the rotation speed of the motor 31 is 2000 rpm or less, a magnetic fluid seal can be used as the vacuum seal 29. In this case, the sliding property is improved, and the life of the vacuum seal 29 can be improved.

[0045] The bearing 30 is a member that supports the shaft 35, and is installed between the vacuum seal 29 and the motor 31. Therefore, the bearing 30 is installed in the atmosphere. This makes it easy to perform maintenance work such as replacing the bearing 30. In addition, since it is not necessary to release the vacuum for the maintenance work, the downtime of the circular accelerator 100 can be reduced. Even if the grease used in the bearing 30 generates dust, the generated dust is released into the atmosphere, so that it is possible to suppress deterioration of the vacuum level in the space where the vacuum state is maintained by the vacuum seal 29, and it is possible to suppress discharge, beam loss, and the like. In this embodiment, only one bearing 30 is provided, but two or more bearings 30 may be provided.

[0046] The bypass capacitor 23 is a member for reducing high-frequency current flowing through the components to be protected, and is installed on the side sealed by the vacuum seal 29 (vacuum side). In this embodiment, the components to be protected are the vacuum seal 29 and the bearing 30.

[0047] The bypass capacitor 23 includes a pair of electrodes facing each other, that is, an outer conductor side electrode 24 and a rotor side electrode 25. The outer conductor side electrode 24 is an electrode fixed to the outer conductor 15, and the rotor side electrode 25 is an electrode fixed to the outer diameter side of the metal shaft portion 35a at the rotor electrode support portion 35b.

[0048] FIG. 6 is a cross-sectional view taken along the line CC' in FIG. 4, showing the cross section of the bypass capacitor 23. As shown in FIG. 6, the outer conductor side electrode 24 and the rotor side electrode 25 of the bypass capacitor 23 have a cylindrical shape with a circular cross section, and extend along the extension direction of the shaft 35. The outer conductor side electrode 24 and the rotor side electrode 25 have cylindrical side surfaces facing each other. With this configuration, the outer conductor side electrode 24 and the rotor side electrode 25 are electrostatically coupled, and the outer conductor 15 and the rotor electrode support portion 35b can be electrically connected to the high-frequency current. This makes it possible to pass the high-frequency current through the outer conductor 15 via the bypass capacitor 23, and to suppress the high-frequency current flowing through the vacuum seal 29 and the bearing 30. The outer conductor 15 is grounded.

[0049] The high-frequency current flowing through the vacuum seal 29 and the bearing 30 can be calculated from the equivalent circuit of the capacitance of the rotating capacitor 22. FIG. 7 is a diagram showing the equivalent circuit of the capacitance of the rotating capacitor 22. As shown in FIG. 7, the high-frequency current flowing from the stator electrode 32 and the rotor electrode 33 is distributed to a parallel circuit having the capacitance Cpass of the bypass capacitor 23, the capacitance Cv of the vacuum seal, and the capacitance Cbe of the bearing 30. The capacitance of the bypass capacitor 23, that is, the diameter and length of the cylindrical electrode, can be determined according to the allowable current of the vacuum seal 29 and the bearing 30. In FIG. 7, the variable capacitance formed between the stator electrode 32 and the rotor electrode 33 is shown as capacitance Crotco.

[0050] The stator electrode 32, rotor electrode 33, inner conductor 14, outer conductor 15, outer conductor side electrode 24, rotor side electrode 25 and metal shaft portion 35a, which can be paths through which the high-frequency current described above flows, are members made of conductive materials.

[0051] Furthermore, the rotating capacitor 22 may be provided with an electromagnetic shield 36 that covers the bypass capacitor 23, the vacuum seal 29, the shaft bearing 30, and the motor 31. The electromagnetic shield 36 can prevent high-frequency power that cannot be completely suppressed by the bypass capacitor 23 from leaking to the outside.

[0052] As described above, according to this embodiment, the rotating capacitor 22 has the outer conductor 15 that covers the stator electrode 32 and the rotor electrode 33, and the bypass capacitor 23 that electrostatically couples the rotor electrode 33 to the outer conductor 15. The electrode pair (the outer conductor side electrode 24 and the rotor side electrode 25) of the bypass capacitor 23 are disposed on the outer diameter side of the metal shaft portion 35a so as to extend in the direction of the rotation axis of the rotor electrode 33. Furthermore, at least one of the electrode pair of the bypass capacitor 23 has a cylindrical shape.

[0053] Therefore, since the bypass capacitor 23 provided separately from the stator electrode 32 and the rotor electrode 33 is installed on the outer diameter side of the metal shaft portion 35a, the shaft 35 can be made shorter than the bypass capacitor attached to the tip of the shaft portion as in the technology described in Non-Patent Document 1, and the occurrence of axial vibration can be suppressed. Therefore, it is possible to stabilize the frequency modulation while extending the life of the parts. In addition, since at least one electrode of the electrode pair of the bypass capacitor 23 is cylindrical, it is easy to balance the rotor electrodes 33. Furthermore, since the electrostatic capacitance of the bypass capacitor 23 can be made uniform, the high-frequency current flowing through the vacuum seal 29, the bearing 30, etc. can be made constant, and the electrical design can be made easier. In addition, since the high-frequency current flowing through the bypass capacitor 23 flows mostly through the outermost electrode, it is easy to cool the part that is most likely to generate heat.

[0054] In this embodiment, the bypass capacitor 23 is provided closer to the motor 31 than the rotor electrode 33. In this case, the length of the shaft 35 can be made shorter, making it possible to suppress the occurrence of axial vibration.

[0055] Furthermore, in this embodiment, the bypass capacitor 23 is provided between the rotor electrode 33 and the bearing 30. Therefore, the high-frequency current flowing through the bearing 30 can be more appropriately suppressed.

[0056] Furthermore, according to this embodiment, the bypass capacitor 23 is provided between the rotor electrode 33 and the vacuum seal 29. Therefore, the high frequency current flowing through the vacuum seal 29 can be more appropriately suppressed.

[0057] Next, a modified example of the rotating capacitor 22 will be described.

[0058] (Variation 1) Fig. 8 is a diagram showing a rotating capacitor 22 according to a first modified example. The rotating capacitor 22 shown in Fig. 8 differs from the rotating capacitor 22 shown in Fig. 4 in that the shaft 35 has an insulating shaft portion 35c in addition to the metal shaft portion 35a and the rotor electrode support portion 35b. The insulating shaft portion 35c constitutes a shaft portion together with the metal shaft portion 35a, and is an insulating portion provided between the metal shaft portion 35a and the rotor electrode support portion 35b. In the example of Fig. 8, the insulating shaft portion 35c is provided in a vacuum region sealed by a vacuum seal 29.

[0059] The material for forming the insulated shaft portion 35c may be an insulator that can provide the mechanical strength required for rotation of the rotor electrode 33, and examples of such materials include FRP (Fiberglass Reinforced Plastics), alumina, and aluminum nitride.

[0060] Fig. 9 is a diagram showing an equivalent circuit of the capacitance of the rotating capacitor 22 of this modified example. The equivalent circuit shown in Fig. 9 differs from the equivalent circuit shown in Fig. 7 in that a circuit having the capacitance Ci of the insulating shaft portion 35c is added so as to be in parallel with the circuit having the capacitance Cpass of the bypass capacitor 23 and in series with the circuits having the capacitance Cv of the vacuum seal and the capacitance Cbe of the bearing.

[0061] In this modification, the insulated shaft portion 35c has a small capacitance Ci and is difficult for high-frequency current to flow through, so that the high-frequency current flowing from the rotor electrode 33 to the metal shaft portion 35a can be reduced, and the high-frequency current flowing from the shaft 35 to the vacuum seal 29 and the bearing 30 can be further reduced. Therefore, the vacuum seal 29 and the bearing 30 are less likely to deteriorate, and the life of the vacuum seal 29 and the bearing 30 can be extended. In addition, the provision of the insulated shaft portion 35c makes it possible to sufficiently suppress the high-frequency current from flowing to the components to be protected by the bypass capacitor 23 alone, so that the capacitance of the outer conductor side electrode 24 and the rotor side electrode 25 can be reduced. Therefore, the diameter and length of the outer conductor side electrode 24 and the rotor side electrode 25 can be reduced, and as a result, the shaft vibration can be reduced and the frequency modulation can be stabilized. In addition, the shorter electrodes allow for better heat dissipation by heat transfer.

[0062] In addition, by combining the bypass capacitor 23 with the insulated shaft portion 35c, it is possible to shorten the insulated shaft portion 35c compared to suppressing the high-frequency current flowing through the vacuum seal 29 and the bearing 30 using only the insulated shaft portion 35c. This allows stable frequency modulation while ensuring mechanical strength. Furthermore, if mechanical strength can be ensured, the entire shaft portion may be composed of the insulated shaft portion 35c.

[0063] (Variation 2) FIG. 10 is a diagram showing the rotating condenser 22 of the acceleration cavity 10 according to the second modification. The rotating condenser 22 shown in FIG. 10 is different from the rotating condenser 22 shown in FIG. 4 in that a plurality of bypass condensers 23 (specifically, two) are provided, that is, a plurality of pairs of the outer conductor side electrode 24 and the rotor side electrode 25 constituting the bypass condenser 23 are provided. In this case, it is possible to increase the facing area of ​​the outer conductor side electrode 24 and the rotor side electrode 25, and therefore it is possible to reduce the diameter and length of the outer conductor side electrode 24 and the rotor side electrode 25. This makes it possible to reduce vibration and stabilize frequency modulation. In addition, the shorter electrodes allow for better heat dissipation by heat transfer.

[0064] Although two sets of the outer conductor side electrodes 24 and the rotor side electrodes 25 are provided in FIG. 10, three or more sets may be provided.

[0065] (Variation 3) FIG. 11 is a cross-sectional view of the bypass capacitor 23 according to the third modification, and shows a cross section corresponding to the cross section taken along line CC' in FIG.

[0066] The bypass capacitor 23 shown in FIG. 11 differs from the bypass capacitor 23 shown in FIG. 6 in that a notch 25a is formed in the rotor-side electrode 25 in the cross-sectional shape.

[0067] 6, the outer conductor side electrode 24 and the rotor side electrode 25 are electrostatically coupled, and the outer conductor 15 and the rotor electrode support portion 35b can be electrically connected to the high-frequency current. This allows the high-frequency current to flow through the outer conductor 15 via the bypass capacitor 23, making it possible to suppress the high-frequency current flowing through the vacuum seal 29 and the bearing 30.

[0068] In addition, the rotor side electrode 25 is made lighter by the cutout 25a, which reduces axial vibration and stabilizes frequency modulation. Even in this case, the outer conductor side electrode 24 has a cylindrical shape, so the capacitance of the bypass capacitor 23 is constant. This makes it possible to keep constant the high-frequency current flowing through the vacuum seal 29, bearing 30, etc., making it easier to design the electrical system.

[0069] Furthermore, so long as at least one of the outer conductor side electrode 24 and the rotor side electrode 25 is cylindrical, the capacitance of the bypass capacitor 23 can be kept constant even if the other has any shape, and therefore the high-frequency current flowing through the vacuum seal 29, bearing 30, etc. can be kept constant.

[0070] FIG. 12 is a diagram showing a particle beam therapy system according to this embodiment.

[0071] The particle beam therapy system 300 shown in FIG. 12 includes a circular accelerator 100, a rotating gantry 190, an irradiation device 192 that includes a scanning coil and irradiates a patient with a charged particle beam emitted from the circular accelerator 100, a treatment table 201, and a control device 191 that controls these.

[0072] The beam extracted from the circular accelerator 100 is transported to the irradiation device 192 by the rotating gantry 190. The transported ion beam is shaped to match the shape of the affected area by the irradiation device 192 and adjustment of the beam energy, and a predetermined amount of the beam is irradiated to the affected area target of the patient 200 lying on the treatment couch 201. The irradiation device 192 includes a dose monitor and monitors the dose irradiated to the patient 200 for each irradiation spot. The control device 191 calculates the required dose for each irradiation spot based on this dose data, and outputs the calculation result to the calculation device.

[0073] The above-described embodiments of the present disclosure are examples for explaining the present disclosure, and are not intended to limit the scope of the present disclosure to only those embodiments. A person skilled in the art can implement the present disclosure in various other forms without departing from the scope of the present disclosure. For example, the present disclosure is not limited to those having all the configurations described. [Explanation of symbols]

[0074] 10: accelerating cavity 11: accelerating gap 12: dee electrode 13: dummy dee electrode 14: inner conductor 15: outer conductor 16: stub 20: input coupler 21: high frequency power supply 22: rotating capacitor 23: bypass capacitor 24: outer conductor side electrode 25: rotor side electrode 25a: notch 29: vacuum seal 30: bearing 31: motor 32: stator electrode 33: rotor electrode 34: rotary joint 35: shaft 35a: metal shaft section 35b: rotor electrode support section 35c: insulated shaft section 36: electromagnetic shield 40: main electromagnet 41: yoke 42: main coil 43: main magnetic pole 51: ion source 52: ion injection section 53: low energy beam transport system 60: cryostat 82: beam extraction path entrance 83: septum coil 100: Circular accelerator 101: Acceleration region 190: Rotating gantry 191: Control device 192: Irradiation device 200: Patient 201: Treatment table 300: Particle beam therapy system

Claims

1. A rotary capacitor having a fixed stator electrode, a rotor electrode facing the stator electrode, and a shaft portion rotatably supporting the rotor electrode around a rotation axis, a housing covering the stator electrode and the rotor electrode, and a bypass capacitor electrostatically coupling the rotor electrode to the housing, wherein the electrode pair of the bypass capacitor is installed on the outer diameter side of the shaft portion so as to extend in the direction of the rotation axis, and at least one electrode of the electrode pair has a cylindrical shape, the rotary capacitor.

2. The rotary capacitor according to claim 1, further comprising a motor provided in the shaft portion for rotating the shaft portion, wherein the bypass capacitor is provided on the motor side with respect to the rotor electrode.

3. The rotary capacitor according to claim 1, further comprising a bearing provided in the shaft portion for supporting the shaft portion, wherein the bypass capacitor is provided between the rotor electrode and the bearing.

4. The rotary capacitor according to claim 1, further comprising a vacuum seal provided in the shaft for sealing the stator electrode and the rotor electrode in a vacuum state, wherein the bypass capacitor is provided between the rotor electrode and the vacuum seal.

5. The rotary capacitor according to claim 1, wherein the shaft portion has an insulating portion formed of an insulator.

6. The rotary capacitor according to claim 1, wherein there are a plurality of bypass capacitors.

7. One electrode of the electrode pair has a cylindrical shape, and the other electrode of the electrode pair has a shape in which a notch is formed in the cylinder, the rotary capacitor according to claim 1.

8. A circular accelerator having the rotary capacitor according to claim 1, and an accelerating cavity for accelerating charged particles using an accelerating high-frequency voltage modulated by the rotary capacitor.

9. The accelerating cavity has a Dee electrode for generating the accelerating high-frequency voltage, an inner conductor for supplying high-frequency power for generating the accelerating high-frequency voltage to the Dee electrode, and an outer conductor surrounding the Dee electrode and the inner conductor, the rotary capacitor is attached so as to be electrostatically coupled to the inner conductor, and the housing is the outer conductor, the circular accelerator according to claim 8.

10. The circular accelerator according to claim 9, wherein the rotary capacitor is attached such that the rotation axis is substantially orthogonal to the direction in which the Dee electrode extends.

11. A circular accelerator according to claim 8, and An irradiation device that irradiates a patient with charged particles accelerated by the circular accelerator, a particle beam therapy system comprising the same.

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