Rotating capacitor, circular accelerator, and particle beam treatment system

Abstract

Provided is a rotating capacitor which makes it possible to stabilize frequency modulation while lengthening the lives of components. A rotating capacitor 22 comprises: an outer conductor 15 that covers a stator electrode 32 and a rotor electrode 33; and a bypass capacitor 23 that electrostatically couples the rotor electrode 33 and the outer conductor 15. An electrode pair (an outer conductor-side electrode 24 and a rotor-side electrode 25) of the bypass capacitor 23 is installed on the outer diameter side of a metal shaft part 35a so as to extend along the direction of the rotation axis of the rotor electrode 33. At least one electrode of the electrode pair of the bypass capacitor 23 has a cylindrical shape.

Description

Rotating condenser, circular accelerator and particle therapy system

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

[0002] As circular accelerators that accelerate charged particles circulating in a main magnetic field and output them as a beam, synchrocyclotrons and eccentric orbit accelerators, which accelerate charged particles by temporally modulating the frequency of an accelerating radio-frequency electric field acting on the charged particles, are known. This type of circular accelerator uses superconducting coils to generate the main magnetic field, which makes it relatively easy to increase the main magnetic field strength, thereby enabling cost reduction through compactness of the accelerator, and is therefore particularly applied to particle beam therapy systems.

[0003] In the above-mentioned circular accelerator, a rotating capacitor is often used as a modulation element for modulating the accelerating radio frequency electric field. A rotating capacitor generally has a fixed stator electrode, a rotor electrode arranged opposite the stator electrode, and a rotation mechanism for rotating the rotor electrode. The rotation mechanism includes a shaft for rotatably supporting the rotor electrode and bearings for supporting the shaft. Furthermore, a vacuum rotating capacitor, in which the electrode parts (stator electrode and rotor electrode) are placed in a vacuum, is suitable as a rotating capacitor, and the shaft may be provided with a vacuum seal to maintain a vacuum around the electrode parts.

[0004] Components used in circular accelerators, such as bearings and vacuum seals, may carry high-frequency currents associated with the accelerating high-frequency electric field from the circular accelerator. These components therefore deteriorate over time due to the high-frequency currents. These components are consumables and must be replaced periodically, but extending their lifespan is desirable 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, high-frequency current can be released 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, separate from the rotor electrode.

[0007] Special table 2014-533884 publication

[0008] Seishi Kikuchi, A 160 cm synchro and variable energy ordinary cyclotron, journal of the physical society of japan, Vol.15, No.1, 1960

[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] Furthermore, in the technology described in Non-Patent Document 1, a bypass capacitor is installed between the rotor electrode and the shaft to allow high-frequency current to escape to the housing, thereby reducing heat generation in the shaft and bearing. However, because a bypass capacitor is installed between the rotor electrode and the shaft, the distance between the rotor electrode and the bearing increases, which creates the problem of shaft vibration being more likely to occur and frequency modulation being less stable.

[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.

[0012] A rotating capacitor according to one aspect of the present disclosure is a rotating capacitor having a fixed stator electrode, a rotor electrode facing the stator electrode, and a shaft portion that supports the rotor electrode rotatably around a rotation axis, and also has a housing that covers the stator electrode and the rotor electrode, and a bypass capacitor that electrostatically couples the rotor electrode to the housing, wherein a pair of electrodes of the bypass capacitor are 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 pair of electrodes is cylindrical.

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

[0014] 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 of a circular accelerator according to an embodiment of the present disclosure; FIG. 3 is a cross-sectional view showing a longitudinal section of a circular accelerator according to an embodiment of the present disclosure; FIG. 4 is a diagram showing an example of a rotating condenser according to an embodiment of the present disclosure; FIG. 5 is a cross-sectional view showing a transverse section of a rotating condenser; FIG. 6 is a cross-sectional view showing a transverse section of a bypass capacitor; FIG. 7 is a diagram showing an equivalent circuit of the capacitance of a rotating condenser; FIG. 8 is a diagram showing a modified example of a rotating condenser according to an embodiment of the present disclosure; FIG. 9 is a diagram showing an equivalent circuit of the capacitance of a modified rotating condenser; FIG. 10 is a diagram showing another modified example of a rotating condenser according to an embodiment of the present disclosure; FIG. 11 is a diagram showing another modified example of a rotating condenser according to an embodiment of the present disclosure; FIG. 12 is a diagram showing a particle beam therapy system according to an embodiment of the present disclosure;

[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. Note that identical or similar reference numerals are used to designate identical or corresponding components in the drawings, and repeated description of these components may be omitted.

[0016] Furthermore, the following embodiments are merely examples, and the present disclosure is not limited to the specific aspects described below. The present disclosure itself can be modified into various forms other than the following embodiments. For example, the rotating condenser according to the present disclosure can be suitably used in a circular accelerator, but is not limited to that application. Furthermore, the circular accelerator according to the present disclosure can be suitably used in a particle beam therapy system, but is not limited to that 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 AA' 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 (a direct current main magnetic field) of constant strength over time using a frequency-modulated accelerating radio-frequency electric field. In this embodiment, the circular accelerator 100 is described as a device that accelerates a proton beam to approximately 200 MeV. More specifically, the circular accelerator 100 is an eccentric orbit accelerator that forms a main magnetic field so that the beam's orbit is eccentric toward the beam extraction path entrance 82 for extracting the beam. The circular accelerator 100 can extract the beam with energy that can be arbitrarily changed between approximately 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 accelerator (e.g., a synchrocyclotron) in which the charged particle beam's orbit 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 travels through from the time the beam starts accelerating in the 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 extraction is possible, passes.

[0021] 1, an ion source 51 that generates charged particles that are 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. Note that the ion source 51 may also be installed within the acceleration region 101, in which case a PIG (Phillips Ionization Gauge) ion source or the like is suitable.

[0022] The ion source 51 is connected to an ion injection unit 52 provided in the acceleration region 101 inside 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, which is 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 in energy. As the beam is accelerated, the radius of curvature of its orbit increases, and the beam traces 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 installed on both sides of a central plane D in the upper and lower directions. Each of the main coils 42 is installed 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 below, is installed around the main coil 42. The main magnetic poles 43 are installed on both sides of the central plane D on the inner periphery of the main coil 42 in the upper and lower directions.

[0025] When current is supplied to the main coil 42, the main magnetic pole 43 excites a time-constant magnetic field called the main magnetic field in the acceleration region 101. The beam orbits on the orbital plane of the acceleration region 101 due to the influence of the main magnetic field. Furthermore, a peeler magnetic field region 44 and a regenerator magnetic field region 45, which are disturbance magnetic fields consisting of dipole and multipole magnetic fields, are formed inside the main electromagnet 40. A radio frequency kicker 70, the peeler magnetic field region 44, the 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 orbiting beam passing through its interior. The septum coil 83 is a coil for deflecting the beam horizontally outward. Note that the septum coil 83 may be any coil as long as it can deflect the beam passing through it toward the high-energy beam transport system 47. A passive configuration using a magnetic material or permanent magnet may be used instead of 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 extracting the accelerated beam, and is provided with a high-energy beam transport system 47 for extracting the beam from the inside of the yoke 41 to the outside. A septum coil 83 for extracting the beam to the outside is installed 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-holes 48 are through-holes for drawing out various coils (such as the main coil 42) installed inside the yoke 41. The vacuum through-holes 49 are through-holes for evacuating the acceleration region 101. The high-frequency system through-holes 50 are through-holes for inserting the acceleration cavity 10.

[0029] The accelerating 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 accelerating 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 generally fan-shaped shape with a predetermined divergence angle and an apex located near the ion injection section 52. The inner conductor 14 is connected to the dee electrode 12 and extends from the dee electrode 12 to the outside of the main electromagnet 40 via 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 opposite 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 accelerating radio-frequency voltage by modulating the resonant frequency of the accelerating cavity 10. The capacitance of the rotating capacitor 22 fluctuates over time, changing the resonant frequency of the accelerating cavity 10 and forming a frequency modulation pattern at the resonant frequency. The accelerating radio-frequency voltage, the frequency of which is modulated by the rotating capacitor 22, and the radio-frequency electric field caused by the accelerating radio-frequency voltage are generated in the accelerating gap 11. The accelerating gap 11 is formed according to the shape of the beam trajectory. In this embodiment, the number of harmonics is 1, meaning that the orbital frequency at which the beam orbits is approximately equal to the acceleration frequency, which is the frequency of the accelerating radio-frequency voltage (more specifically, the frequency of the accelerating electric field caused by the accelerating radio-frequency voltage).

[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 electrostatic coupling or magnetic coupling. 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 self-excitation or external excitation.

[0033] When there is a connection surface perpendicular to the direction of the high-frequency current flow, 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 and fluctuations in the gap between the stator electrode 32 and the rotor electrode 33, which will be described later using Figure 4 and other figures. A bellows or the like may be used instead of the RF contact.

[0034] Furthermore, 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 an inductive load (coil) for adjusting the resonant frequency of the accelerating cavity 10. A polyimide film or the like is provided between the inner conductor 14 and the outer conductor 15 in the stub 16, providing insulation against direct current but conductivity against high-frequency current. The resonant frequency of the accelerating 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 attachment position of the stub 16. The stub 16 also serves as an entrance for passing water-cooling piping, signal lines, power supply lines, etc. from the inner conductor 14 to the Dee electrode 12. Although only one stub 16 is provided in the example shown in FIG. 2, two or more stubs 16 may be provided.

[0035] The rotating capacitor 22 will now be described in more detail.

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

[0037] 4, the rotating capacitor 22 is installed at the end of the accelerating cavity 10 opposite 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 interior of the outer conductor 15, where 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, making them equal in potential. This gap also allows the rotor electrode 33 to rotate without physical contact with the outer conductor 15. Alternatively, 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] Figure 5 is a cross-sectional view taken along line B-B' in Figure 4, showing a transverse cross section of the rotating capacitor. As shown in Figure 5, the stator electrode 32 and the rotor electrode 33 have a periodic symmetric structure with notches in the circumferential direction according to a desired modulation pattern so as to achieve 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 fluctuates over time.

[0040] 5, the above-described cyclic symmetry structure has eight-fold symmetry, and therefore 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 or greater than eight. The greater the number of cyclic symmetries, the lower the rotation speed of the motor 31 can be, which suppresses axial vibration and thereby extends the life of the bearing 30 and the vacuum seal 29, and further allows for precise modulation of the high-frequency frequency.

[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 the shaft portion that serves as the 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 (rotational axis direction) of the metal shaft portion 35a. The rotor electrode 33 is provided on the rotor electrode support portion 35b. Furthermore, a 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 rotational axis of the motor 31 is approximately perpendicular to the direction in which the dee electrodes 12 extend. More specifically, in the example of FIG. 4 , the rotational axis of the motor 31 is attached facing upward, perpendicular to the direction in which the dee electrodes 12 extend. The input coupler 20 and the high-frequency power supply 21 are provided on the opposite side of the rotational axis of the motor 31 from the dee electrodes 12. In this case, a double-supported state is formed with loads on both sides of the bearing 30, which stabilizes the rotation of the rotor electrode 33 and reduces axial vibration. The rotational axis of the motor 31 does not need to be attached facing upward, perpendicular to the direction in which the dee electrodes 12 extend. It may be attached facing downward, perpendicular to the direction in which the dee electrodes 12 extend, or may be attached parallel to the direction in which the dee electrodes 12 extend. FIG. 2 shows an example in which the rotational axis of the motor 31 is attached parallel to the direction in which the dee electrodes 12 extend. Furthermore, when the rotating shaft of the motor 31 is attached so that it faces 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, and therefore a vertically upward force is applied to the rotor electrode 33 due to external air pressure. This reduces the load on the bearing 30 of the shaft 35. Furthermore, the bearing 30 and 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, which maintain the interior in a vacuum state, making maintenance easier.

[0043] 4, the motor 31 has a structure in which its rotation 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 component 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 may be, for example, a lip seal, a double O-ring, a Wilson seal, or a bellows seal. If the rotation speed of the motor 31 is 2000 rpm or less, a magnetic fluid seal may be used as the vacuum seal 29. In this case, the sliding properties are improved, thereby enabling the life of the vacuum seal 29 to be extended.

[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 facilitates maintenance work such as replacing the bearing 30. Furthermore, since there is no need to release the vacuum for such maintenance work, downtime of the circular accelerator 100 can be reduced. Furthermore, even if the grease used in the bearing 30 generates dust, the generated dust is released into the atmosphere, thereby preventing deterioration of the vacuum in the space maintained in a vacuum state by the vacuum seal 29, and making it possible to prevent discharge, beam loss, and the like. Note that although only one bearing 30 is provided in this embodiment, two or more bearings 30 may be provided.

[0046] The bypass capacitor 23 is a component 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 opposing electrodes: 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 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 face each other at their cylindrical side surfaces. This configuration electrostatically couples the outer conductor side electrode 24 and the rotor side electrode 25, electrically connecting the outer conductor 15 and the rotor electrode support portion 35b to high-frequency current. This allows high-frequency current to flow through the outer conductor 15 via the bypass capacitor 23, thereby suppressing high-frequency current flowing through the vacuum seal 29 and bearing 30. The outer conductor 15 is grounded.

[0049] The high-frequency current flowing through the vacuum seal 29 and bearing 30 can be calculated from the equivalent circuit of the capacitance of the rotating capacitor 22. FIG. 7 shows 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 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, i.e., the diameter and length of the cylindrical electrode, can be determined according to the allowable current of the vacuum seal 29 and bearing 30. In FIG. 7, the variable capacitance formed between the stator electrode 32 and 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] The rotating capacitor 22 may also 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 includes 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 is 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 is cylindrical.

[0053] Therefore, since the bypass capacitor 23, which is provided separately from the stator electrode 32 and the rotor electrode 33, is located on the outer diameter side of the metal shaft portion 35a, the shaft 35 can be made shorter than in the case of the technology described in Non-Patent Document 1, in which the bypass capacitor is attached to the tip of the shaft portion, thereby suppressing axial vibration. This enables stabilization of frequency modulation while extending the life of the components. Furthermore, since at least one electrode of the electrode pair of the bypass capacitor 23 is cylindrical, it is easier to balance the rotor electrodes 33. Furthermore, since the capacitances of the bypass capacitors 23 can be matched, the high-frequency current flowing through the vacuum seal 29, bearing 30, etc. can be made constant, simplifying electrical design. Furthermore, since the high-frequency current flowing through the bypass capacitor 23 flows more predominantly through the outermost electrode, it is easier to cool the areas that are 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, which makes it possible to suppress the occurrence of shaft vibration.

[0055] In this embodiment, the bypass capacitor 23 is provided between the rotor electrode 33 and the bearing 30. This makes it possible to more appropriately suppress the high-frequency current flowing through the bearing 30.

[0056] Furthermore, according to this embodiment, the bypass capacitor 23 is provided between the rotor electrode 33 and the vacuum seal 29. This makes it possible to more appropriately suppress the high-frequency current flowing through the vacuum seal 29.

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

[0058] (Modification 1) Fig. 8 is a diagram showing a rotating capacitor 22 according to Modification 1. The rotating capacitor 22 shown in Fig. 8 differs from the rotating capacitor 22 shown in Fig. 4 in that the shaft 35 includes an insulated shaft portion 35c in addition to the metal shaft portion 35a and the rotor electrode support portion 35b. The insulated shaft portion 35c constitutes the 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 shown in Fig. 8, the insulated shaft portion 35c is provided in a vacuum region sealed by a vacuum seal 29.

[0059] The material for forming the insulating shaft portion 35c may be any insulating material that can achieve the mechanical strength required for rotation of the rotor electrode 33, such as FRP (Fiberglass Reinforced Plastics), alumina, or 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, making it difficult for high-frequency current to flow. This reduces the high-frequency current flowing from the rotor electrode 33 to the metal shaft portion 35a, thereby further reducing the high-frequency current flowing from the shaft 35 to the vacuum seal 29 and bearing 30. This reduces the deterioration of the vacuum seal 29 and bearing 30, thereby extending their lifespan. Furthermore, the provision of the insulated shaft portion 35c allows the bypass capacitor 23 alone to sufficiently prevent high-frequency current from flowing to the protected components, thereby reducing the capacitance of the outer conductor-side electrode 24 and the rotor-side electrode 25. This allows the diameter and length of the outer conductor-side electrode 24 and the rotor-side electrode 25 to be reduced, thereby reducing axial vibration and stabilizing frequency modulation. Furthermore, the shorter electrodes allow for improved heat dissipation through heat transfer.

[0062] Furthermore, by combining the bypass capacitor 23 with the insulated shaft portion 35c, it is possible to shorten the insulated shaft portion 35c compared to when the insulated shaft portion 35c alone suppresses the high-frequency current flowing through the vacuum seal 29 and the bearing 30. 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] (Modification 2) Fig. 10 is a diagram showing the rotating capacitor 22 of the accelerating cavity 10 according to Modification 2. The rotating capacitor 22 shown in Fig. 10 differs from the rotating capacitor 22 shown in Fig. 4 in that it includes multiple (specifically, two) bypass capacitors 23, i.e., multiple pairs of outer conductor-side electrodes 24 and rotor-side electrodes 25 that constitute the bypass capacitor 23. In this case, the opposing area between the outer conductor-side electrode 24 and the rotor-side electrode 25 can be increased, allowing the diameter and length of the outer conductor-side electrode 24 and the rotor-side electrode 25 to be reduced. This reduces vibration and stabilizes frequency modulation. Furthermore, the shorter electrodes allow for improved heat dissipation through heat transfer.

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

[0065] (Modification 3) FIG. 11 is a cross-sectional view of the bypass capacitor 23 according to Modification 3, 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 25 a 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 each other with respect to high frequency currents. This allows high frequency currents to flow through the outer conductor 15 via the bypass capacitor 23, making it possible to suppress high frequency currents flowing through the vacuum seal 29 and the bearing 30.

[0068] Furthermore, the rotor-side electrode 25 is lightweight due to the cutouts 25a, which reduces axial vibration and stabilizes frequency modulation. Even in this case, the outer conductor-side electrode 24 is cylindrical, so the capacitance of the bypass capacitor 23 remains constant. This allows the high-frequency current flowing through the vacuum seal 29, bearing 30, etc. to be constant, simplifying electrical design.

[0069] As 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 adjusting the irradiation device 192 and beam energy, and a predetermined amount is irradiated onto 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 these embodiments. Those 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 of the described configurations.

[0074] 10: Acceleration cavity 11: Acceleration 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 portion 35b: Rotor electrode support portion 35c: Insulated shaft portion 36: Electromagnetic shield 40: Main electromagnet 41: Yoke 42: Main coil 43: Main magnetic pole 51: Ion source 52: Ion injection portion 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 rotating capacitor having a fixed stator electrode, a rotor electrode facing the stator electrode, and a shaft portion supporting the rotor electrode rotatably around a rotation axis, the rotating capacitor further comprising: a casing covering the stator electrode and the rotor electrode; and a bypass capacitor electrostatically coupling the rotor electrode to the casing, wherein a pair of electrodes of the bypass capacitor are 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 of the electrodes of the electrode pair is cylindrical in shape.

2. The rotating capacitor according to claim 1, further comprising a motor provided on said shaft portion for rotating said shaft portion, said bypass capacitor being provided on the motor side relative to said rotor electrode.

3. The rotating capacitor according to claim 1, further comprising a bearing provided on said shaft portion and supporting said shaft portion, said bypass capacitor being provided between said rotor electrode and said bearing.

4. The rotating capacitor according to claim 1, further comprising a vacuum seal provided on said shaft and capable of sealing said stator electrode and said rotor electrode while maintaining them in a vacuum state, said bypass capacitor being provided between said rotor electrode and said vacuum seal.

5. The rotating capacitor of claim 1, wherein said shaft portion has an insulating portion formed of an insulating material.

6. The rotating capacitor of claim 1, wherein the bypass capacitor is a plurality of capacitors.

7. The rotating capacitor of claim 1, wherein one electrode of the electrode pair is cylindrical, and the other electrode of the electrode pair is cylindrical with a notch formed therein.

8. A circular accelerator comprising: a rotating condenser according to claim 1; and an accelerating cavity for accelerating charged particles using an accelerating high frequency voltage modulated by said rotating condenser.

9. The circular accelerator according to claim 8, wherein the accelerating cavity has a Dee electrode which generates the accelerating radio frequency voltage, an inner conductor which supplies radio frequency power to the Dee electrode for generating the accelerating radio frequency voltage, and an outer conductor which surrounds the Dee electrode and the inner conductor, the rotating capacitor is attached so as to be electrostatically coupled with the inner conductor, and the housing is the outer conductor.

10. The circular accelerator according to claim 9, wherein said rotating capacitor is mounted so that said axis of rotation is substantially perpendicular to the direction in which said dee electrodes extend.

11. A particle beam therapy system comprising: the circular accelerator according to claim 8; and an irradiation device for irradiating a patient with the charged particles accelerated by the circular accelerator.

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