Acceleration cavity of circular accelerator, circular accelerator, and particle beam treatment device

Abstract

Provided are an acceleration cavity of a circular accelerator, a circular accelerator, and a particle beam treatment device that make it possible to improve reliability by a simpler method. The acceleration cavity 10 used for a circular accelerator 39 comprises: a rotating capacitor 22 that makes the capacitance variable by changing facing areas between a pair of electrodes 32, 33 facing each other; and an outer conductor 15 that forms a path through which a high-frequency current flows. The outer conductor is provided so as to surround the rotating capacitor, and includes a magnetic shield member 150 that blocks a magnetic field from the outside, and a conductive member 151 that forms the path through which the high-frequency current flows.

Description

Accelerating cavity of circular accelerator, circular accelerator and particle beam therapy device

[0001] The present invention relates to an accelerating cavity of a circular accelerator, a circular accelerator, and a particle beam therapy device.

[0002] Synchrocyclotrons and eccentric orbit accelerators are known as circular accelerators that maintain a constant main magnetic field strength and modulate the frequency of accelerating radio frequency waves. These circular accelerators can be made smaller and less expensive because they can generate a high magnetic field with a superconducting coil. Therefore, these circular accelerators are particularly suitable for particle beam therapy systems.

[0003] In synchrocyclotrons and eccentric orbit accelerators, a rotating condenser is used as a mechanism for modulating the frequency of the radio frequency waves that accelerate a charged particle beam (see Patent Documents 1 and 2). A rotating condenser generally includes a stator electrode, a rotor electrode disposed opposite the stator electrode, a rotating shaft that rotates the rotor electrode, and a bearing that supports the rotating shaft.

[0004] JP 2019-157556 A U.S. Pat. No. 10,722,735

[0005] When the magnetic field leaking from the circular accelerator acts on the rotating capacitor, eddy currents are generated in the electrodes of the rotating capacitor, which causes the rotating capacitor to heat up. Therefore, in Patent Document 1, multiple notches are formed on the side of the rotating blades to make it difficult for eddy currents to flow and suppress the occurrence of eddy current loss. In Patent Document 2, a magnetic shield separate from the magnet yoke is provided outside the superconducting magnet to suppress the magnetic field leaking from the circular accelerator.

[0006] In Patent Document 1, multiple notches are formed on the side of the rotating blades of the rotating capacitor, which makes manufacturing the rotating capacitor time-consuming. The rotating blades need to be manufactured with high precision, but adding multiple notches is time-consuming and increases manufacturing costs. In Patent Document 2, a magnetic shield separate from the magnetic yoke is provided on the outside of the superconducting magnet, which increases the number of parts and manufacturing costs.

[0007] An object of the present invention is to provide an accelerating cavity for a circular accelerator, a circular accelerator, and a particle beam therapy system that can improve reliability using a simpler method.

[0008] An accelerating cavity according to one aspect of the present disclosure is an accelerating cavity for use in a circular accelerator, and includes a rotating capacitor whose capacitance is variable by changing the facing area of ​​a pair of opposing electrodes, and an outer conductor that forms a path for passing a high-frequency current, the outer conductor being arranged to surround the rotating capacitor and including a magnetic shielding member that blocks an external magnetic field and a conductive member that forms the path for passing the high-frequency current.

[0009] The accelerating cavity of the present disclosure can supply high frequency current to a circular accelerator while reducing the influence of external magnetic fields on the rotating capacitor.

[0010] 9 is an explanatory diagram showing an overview of a circular accelerator according to the present embodiment. FIG. 1 is a perspective view showing the appearance of a circular accelerator. FIG. 2 is a cross-sectional view of the circular accelerator. FIG. 3 is a cross-sectional view taken along line A-A in FIG. 3. FIG. 4 is a diagram showing beam activation by energy. FIG. 5 is a diagram showing the motion pattern of the circular accelerator. FIG. 6 is a cross-sectional view showing an accelerating cavity and a rotating condenser. FIG. 7 is a cross-sectional view taken along line B-B in FIG. 7. FIG. 8 is a cross-sectional view showing a rotating condenser according to Modification 1. FIG. 9 is a cross-sectional view taken along line CC in FIG. 9. FIG. 10 is a diagram showing the configuration of a particle beam therapy system. FIG. 11 is a cross-sectional view showing a rotating condenser according to Modification 2. FIG. 12 is an explanatory diagram showing a rotating condenser according to Modification 3. FIG. 13 is an explanatory diagram showing a part of a rotating condenser according to Modification 4. FIG. 14 is an explanatory diagram showing a part of a rotating condenser according to Modification 5.

[0011] The following describes the disclosed accelerating cavity, circular accelerator, and particle beam therapy system according to embodiments of the present invention. The following embodiments are merely examples, and the present invention is not limited to the specific aspects described below. The present invention itself can be modified into various forms other than the following embodiments. Furthermore, the accelerating cavity according to the present invention is suitable for a circular accelerator, but is not limited to that application. The circular accelerator according to the present invention is suitable for a particle beam therapy system, but is not limited to that application.

[0012] The accelerating cavity of this embodiment includes an iron member as a magnetic shielding member. The accelerating cavity of this embodiment can accelerate a particle beam by supplying a high-frequency current to the accelerating gap, and can also suppress heat generation caused by the action of an external magnetic field on the rotating capacitor. In the accelerating cavity of this embodiment, the outer conductor, which is part of the accelerating cavity, has a magnetic shielding function, so there is no need to include a magnetic shielding member separate from the outer conductor.

[0013] The configuration of the circular accelerator 39 will be described below with reference to Figures 1 to 5. Figure 1 is an explanatory diagram showing an overview of the circular accelerator 39. Details of each component of the circular accelerator 39 will be described in Figure 2 and subsequent figures.

[0014] The upper and lower main electromagnets 40 are arranged facing each other, and an acceleration gap 11, which is part of the acceleration cavity 11, is formed between the main electromagnets 40. As will be described later, the acceleration cavity 11 is a device that accelerates charged particles by applying an electric field to them in a constant magnetic field between the main electromagnets 40. A coil 42 is wound around the main electromagnet 40.

[0015] The accelerating cavity 10 includes, for example, a dee electrode 12, a dummy dee electrode 13, an inner conductor 14, an outer conductor 15, and a rotating capacitor 22. The dee electrode 12 is a hollow electrode through which the beam passes and is connected to the inner conductor 14. The dummy dee electrode 13 is an electrode at earth potential and is connected to the outer conductor 15 that surrounds and encases the inner conductor 14. An accelerating gap 11 is formed between the dee electrode 12 and the dummy dee electrode 13. A radio-frequency electric field is formed in the accelerating gap 11.

[0016] The rotating capacitor 22 is a device for modulating the resonant frequency of the accelerating cavity 10. 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 (not shown), a stator electrode 32, a rotor electrode 33, and a shaft 35 connecting the rotor electrode 33 to the motor. A detailed structural example of the rotating capacitor 22 will be described later with reference to FIG. 7.

[0017] The capacitance of the rotating capacitor 22 can be varied over time by changing the opposing area between the stator electrode 32 and the rotor electrode 33. This changes the resonant frequency of the accelerating cavity 10, thereby forming a frequency modulation pattern. An acceleration voltage frequency-modulated by the rotating capacitor 22 is generated in the acceleration gap 11 between the dee electrode 12 and the dummy dee electrode 13.

[0018] The outer conductor 15 is provided so as to surround the rotating capacitor 22, and is formed by including an iron member 150 as an example of a "magnetic shielding member" that blocks the magnetic field from the electromagnet 40, and a copper member 151 as an example of a "conductive member" that forms a path for passing a high-frequency current. The outer conductor 15 includes the iron member 150 formed from iron such as SS400 so as to have the shape of the outer conductor 15, an inner copper thin film 151 formed on the inner surface, which is one surface of the iron member 150, and an outer copper thin film 152 formed on the outer surface, which is the other surface of the iron member 150.

[0019] The iron member 150 is not limited to iron, and any material that can shield magnetic fields and has the strength to form the shape of the outer conductor 15 may be used. The copper thin films 151 and 152 are not limited to copper, and may be formed from metal materials such as gold, gold alloys, silver, and silver alloys, or may be formed from non-metallic materials that are conductive. Therefore, the copper member may also be called a "conductive member," "conductive layer," "conductive thin film," etc.

[0020] For example, by immersing the iron member 150 in a plating solution of a predetermined composition, copper thin films (copper plating) 151, 152 of a predetermined thickness can be formed almost simultaneously on both surfaces (or the front and back surfaces) of the iron member 150.

[0021] The thickness of the iron member 150 is set so that the leakage magnetic field applied to the rotating capacitor 22 is equal to or less than a desired value. The thickness of the copper thin films 151 and 152 is set so that the high-frequency power required to accelerate the charged particle beam circulating within the acceleration gap 11 can be transmitted.

[0022] Here, it is not necessary to form the copper thin films 151, 152 on both surfaces (or the surface and back surface) of the iron member 150, but they need only be formed on the surface 151, which includes the high-frequency transmission surface where a high-frequency current path is formed. In other words, only the copper thin film 151 may be formed on the iron member 150.

[0023] Fig. 2 is a perspective view showing the appearance of the circular accelerator 39. Fig. 3 is a cross-sectional view showing the transverse section (center plane) of the circular accelerator 39. Fig. 4 is a cross-sectional view taken along line A-A in Fig. 3, showing the longitudinal section of the circular accelerator 39.

[0024] The circular accelerator 39 is a device that accelerates a beam by a frequency-modulated radio-frequency electric field in a main magnetic field of constant intensity over time. As an example, a circular accelerator that accelerates a proton beam up to 235 MeV will be described, but the circular accelerator 39 may also be a device that accelerates a heavy particle beam such as helium or carbon.

[0025] The circular accelerator 39 is an eccentric orbit accelerator that forms a main magnetic field so as to eccentrically shift the beam orbit toward the entrance 82 of the beam extraction path, and can extract the beam with an energy that can be arbitrarily changed between 70 MeV and 235 MeV.

[0026] As shown in Figures 2 and 4, the outer shell of the circular accelerator 39 is formed by a main electromagnet 40 that can be separated in the vertical direction. An acceleration region is formed on the central plane within the main electromagnet 40, and this acceleration region is evacuated. Hereinafter, the orbit that a beam passes through from the time the beam starts accelerating in the acceleration region until its energy reaches its maximum energy of 235 MeV is referred to as a circular orbit. Of the circular orbits, the orbit through which a beam with a maximum energy of 235 MeV passes is referred to as a maximum energy orbit 80 (see Figure 3). The orbit through which a beam with an energy of 70 MeV passes is referred to as a minimum extraction energy orbit 81. The plane on which the circular orbit draws a spiral is referred to as an orbital plane or orbital plane. A two-dimensional polar coordinate system of the orbital plane is defined with the center of the acceleration region as the origin, and the axis extending radially outward from the center is referred to as the r-axis.

[0027] As shown in FIG. 4 , the main electromagnet 40 includes a main magnetic pole 38, a yoke 41, and a main coil 42. The yoke 41 forms the exterior of the main electromagnet 40. An approximately cylindrical region is formed inside the yoke 41. The main coil 42 is an annular superconducting coil and is installed along the inner wall of the yoke 41. A cryostat 60 is installed around the main coil 42, and the main coil 42 is cooled by the cryostat 60. The main magnetic poles 38 are installed above and below the inner periphery of the main coil 42, facing each other. The vertical magnetic field excited by passing a current through the main coil 42 and formed by the main magnetic pole 38 is called the main magnetic field. The main magnetic field is used to form an eccentric orbit. The acceleration region is a region for accelerating the beam in the main magnetic field.

[0028] As shown in FIG. 3 , a plurality of through-holes are formed in the yoke 41. The yoke 41 is formed with, for example, a beam through-hole 46, a coil through-hole 48, a vacuum through-hole 49, and a radio-frequency system through-hole 50. The beam through-hole 46 is a through-hole for emitting an accelerated beam. The coil through-hole 48 is a through-hole for drawing out various coil conductors installed in the yoke 41 to the outside. The vacuum through-hole 49 is a through-hole for evacuating the acceleration region. The radio-frequency system through-hole 50 is a through-hole for the acceleration cavity 10, and is provided at the connection surface of the upper and lower magnetic poles.

[0029] As shown in Fig. 2, an ion source 53 is installed above the main electromagnet 40. The ion source 53 generates a beam of ions to be incident on the main electromagnet 40. The beam generated by the ion source 53 passes through a low-energy beam transport system 54 and then through an ion injection section 52 to be injected into the acceleration region inside the main electromagnet 40. An ECR ion source or the like can be used as the ion source 53. The ion source 53 may also be placed inside the evacuated acceleration region inside the main electromagnet 40, in which case a PIG-type ion source or the like is suitable.

[0030] As shown in FIG. 3 , the ion injection unit 52 is positioned on the centerline closer to the beam extraction path entrance 82 than the mechanical center of the acceleration region. A beam of charged particles generated by the ion source 53 passes through the low-energy beam transport system 54, passes through the ion injection unit 52, and is injected into the acceleration region inside the main electromagnet 40 by an inflector electrode (not shown) or the like. The injected beam is accelerated in a radio-frequency electric field and circulates in the main magnetic field while increasing in energy. As the beam is accelerated, the radius of curvature of its trajectory increases, and the beam traces a spiral trajectory from the center of the acceleration region outward. The radio-frequency wave that accelerates the beam can be referred to as a first radio-frequency wave.

[0031] The accelerating cavity 10 is a λ / 2 resonant cavity and, as described above, includes, for example, a dee electrode 12, a dummy dee electrode 13, an inner conductor 14, an outer conductor 15, and a rotating capacitor 22. The dee electrode 12 is a hollow electrode through which the beam passes and is connected to the inner conductor 14. The dummy dee electrode 13 is an electrode at earth potential and is connected to the outer conductor 15 that surrounds the inner conductor 14. An accelerating gap 11 is formed between the dee electrode 12 and the dummy dee electrode 13. A radio-frequency electric field is generated in the accelerating gap 11.

[0032] RF power to the accelerating cavity 10 is supplied by an RF power supply 21 via an input coupler 20. The input coupler 20 is coupled to the accelerating cavity 10 by either electrostatic coupling or magnetic coupling. As a result, an RF acceleration voltage for accelerating the beam and an RF electric field due to the RF acceleration voltage are generated in the accelerating gap 11.

[0033] As described above, the rotating capacitor 22 modulates the resonant frequency of the accelerating cavity 10. By varying the capacitance of the rotating capacitor 22 over time, the resonant frequency of the accelerating cavity 10 is changed, thereby forming a frequency modulation pattern. The acceleration voltage frequency-modulated by the rotating capacitor 22 is generated in the acceleration gap 11 between the dee electrode 12 and the dummy dee electrode 13. The acceleration gap 11 shown in Fig. 3 is an acceleration gap with a harmonic number of 1, i.e., an acceleration gap in which the orbital frequency and the acceleration frequency are the same, and is formed according to the trajectory shape of the beam.

[0034] 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 or externally excited system.

[0035] The main magnetic field that realizes the eccentric orbit will be described below. The main magnetic field may be a magnetic field whose strength is constant in the circumferential direction, or an AVF (Azimuthal Varying Field) type magnetic field. In either type of magnetic field, the main magnetic field distribution is an anisochronous magnetic field.

[0036] Under certain beam stabilization conditions, a beam that deviates slightly from the designed orbit in the radial direction is subjected to a restoring force that returns it to the designed orbit, and a beam that deviates in a direction perpendicular to the orbital plane is subjected to a restoring force from the main magnetic field in the direction that returns it to the orbital plane. In other words, the beam oscillates in the betatron direction near the designed orbit, and is accelerated while circulating stably. In addition, for a full-energy beam, the betatron frequency (horizontal tune) ν ｒ is set to a value close to 1. The main magnetic field distribution described above is formed by the main pole 38 and the trim coils and pole pieces (neither of which are shown) installed on the surface of the main pole 38. These components are arranged symmetrically above and below the orbital plane, so the main magnetic field, on the orbital plane, only has a magnetic field component in a direction perpendicular to the orbital plane.

[0037] Figure 5 shows the orbits for each energy. Fifty energy orbits are shown by solid lines, starting from the maximum energy of 235 MeV and in increments of 0.04 Tm in magnetic rigidity. The dotted lines connect the orbits with the same orbital phase, and are called iso-orbital phase lines. The iso-orbital phase lines are plotted every π / 20 orbital phase from the concentration region. The acceleration gap 11 formed between the dee electrode 12 and the dummy dee electrode 13 is located along the iso-orbital phase line. More specifically, the dee electrode 12 has a hollow, fan-like shape with its tip near the center of the concentric orbit and its radius aligned with the iso-orbital phase line.

[0038] The trajectory of the beam in the low energy region is close to a concentric trajectory centered near the ion injection section 52, as in conventional cyclotrons. The trajectories of higher energy are densely concentrated near the entrance 82 of the beam extraction path. Conversely, near the inner conductor 14, the trajectories of each energy are spaced apart from one another. The point where these trajectories are densely concentrated is called the concentrated region, and the area where they are dispersed is called the dispersed region. By forming such an trajectory arrangement and extracting the beam from near the concentrated region, the required beam kick amount can be reduced, making it easier to extract a beam with variable energy.

[0039] 6A to 6C, the process from when the beam is injected into the circular accelerator 39 to when it is extracted from the circular accelerator 39 will be described. ｃａｖ and frequency f, which is the frequency of the radio frequency electric field applied to the beam by the radio frequency kicker 70. ｅｘｔ 6B shows a graph representing the relationship between the acceleration voltage V generated in the acceleration gap 11 and the time T. ａｃｃ and the high frequency voltage V applied to the high frequency kicker 70. ｅｘｔ 6C shows a graph representing the relationship between the current of the incident beam and the current of the outgoing beam and the time T. The high frequency wave applied to the high frequency kicker 70 corresponds to an example of the second high frequency wave.

[0040] One acceleration period is the acceleration voltage V ａｃｃ It starts with the rise of the accelerating voltage V ａｃｃ When the beam is sufficiently increased, the beam is injected from the ion source 53 into the circular accelerator 39 (time T2). １ After this time, the RF capture of the beam is completed. The captured beam, i.e., the beam that is ready for acceleration among the injected beams, is then accelerated by the accelerating voltage V ａｃｃ (time T3). When the beam energy reaches the energy to be extracted, the accelerating RF power is turned off (time T4). ２ After this time, the acceleration voltage V ａｃｃis turned OFF (time T5), and the beam moves along a certain orbit. Note that the individual charged particles forming the beam oscillate in a direction perpendicular to the beam's orbit as it moves around. This oscillation is called betatron oscillation, and its frequency is called the betatron frequency. The frequency per revolution is called the tune, and the beam's displacement on the r-axis outward from the orbital plane per revolution is called the turn separation. For the orbiting beam, the betatron oscillation in the orbital plane and perpendicular to the beam's orbit is called the horizontal betatron oscillation, and the tune is called the horizontal tune. This betatron oscillation has the property that, when an appropriate high-frequency voltage is applied, resonance occurs and the amplitude increases rapidly.

[0041] V ａｃｃ At the same time as the high frequency voltage V ｅｘｔ The application of the high frequency voltage V to the high frequency kicker 70 is started. ｅｘｔ The application of the acceleration voltage V ａｃｃ It is not necessary to turn off the high frequency voltage V ｅｘｔ The application of the acceleration voltage V may start immediately before, simultaneously with, or immediately after the start of shutting off the acceleration high frequency (time T4). ａｃｃ The energy to be extracted can be determined by the acceleration voltage V ａｃｃ The voltage can be controlled by the application time.

[0042] The high frequency voltage of the high frequency kicker 70 rises quickly with a response of several microseconds if the high frequency kicker 70 is not a resonator structure and is designed so that the capacitance is an appropriate value. Betatron oscillation has the property that the amplitude increases resonantly when the product of either the horizontal tune or the decimal part of the horizontal tune and the orbital frequency of the beam is approximately the same as the frequency of the applied high frequency voltage. Therefore, the frequency f of the high frequency voltage ｅｘｔ is the horizontal tune of the maximum energy beam, ν ｒ The decimal part of Δν ｒ and the rotation frequency f of the beam of energy to be extracted. ｒｅｖ The product Δν ｒ ×f ｒｅｖ Alternatively, the product Δνｒ ×f ｒｅｖ As a result, the amplitude of the horizontal betatron oscillation continues to increase resonantly, and the beam eventually reaches the peeler magnetic field region 44 and the regenerator magnetic field region 45 located on the outer periphery of the maximum energy orbit 80 (time T6).

[0043] The beam that reaches the peeler magnetic field region 44 is kicked toward the outer periphery of the orbital plane. The beam that reaches the regenerator magnetic field region 45 is kicked toward the inner periphery of the orbital plane. Kicking refers to deflecting the beam by applying an electric or magnetic field. The quadrupole magnetic field component of the peeler magnetic field region 44 kicks the beam further toward the outer periphery, increasing the turn separation. At the same time, the magnetic field of the regenerator magnetic field region 45 suppresses abrupt fluctuations in the horizontal tune of the beam, preventing the beam from being lost due to betatron oscillation diverging in the vertical direction, which is 90 degrees perpendicular to the horizontal direction, before the beam is extracted. When the magnetic field strengths of the peeler magnetic field region 44 and the regenerator magnetic field region 45 are appropriately adjusted, 2ν ｒ A resonance condition for betatron oscillation of ≈2 can occur, increasing the turn separation.

[0044] 3, a septum coil 43 is installed at the entrance 82 of the beam extraction path. When a turn separation that greatly exceeds the thickness of a coil conductor (not shown) installed on the inner periphery of the septum coil 43 is obtained, the beam is guided into the septum coil 43, is sufficiently deflected, and is then guided to the high-energy beam transport line 47 and extracted.

[0045] Immediately after the start of application of the radio frequency voltage to the radio frequency kicker 70 (time T5), the time until the beam is extracted can be shortened by applying as large a radio frequency voltage as possible and quickly increasing the amplitude of the beam. The radio frequency voltage is reduced immediately before the beam reaches the peeler magnetic field region 44 or the regenerator magnetic field region 45 (time T6), and the amount of the beam traveling to the peeler magnetic field region 44 and the regenerator magnetic field region 45 can be adjusted, thereby enabling precise control of the beam extraction current. The radio frequency voltage V ｅｘｔ Instead of lowering the frequency, the beam extraction current can be changed by sweeping the frequency of the radio frequency applied to the radio frequency kicker 70 or by changing the phase of the radio frequency of this gear. This utilizes the property that the betatron frequencies of the charged particles contained in the beam vary over a certain distribution (tune spread). By changing the frequency of the radio frequency and changing the band of the distribution of the frequencies of the charged particles that cause resonance, the beam extraction current can be changed.

[0046] From the start of beam extraction (time T6) to time t ４ After that, the high frequency voltage V ｅｘｔ By stopping the application of the voltage t, the beam emission is stopped (time T7). ４ By adjusting the value, the beam emission time can be controlled.

[0047] By controlling the high frequency voltage applied to the high frequency kicker 70, the beam extraction current can be adjusted, and by stopping the application of the high frequency voltage, the beam extraction can be stopped. Therefore, the spot dose required for scanning irradiation can be irradiated with a single extracted pulse beam without excess or deficiency, and the dose rate is improved. For example, as shown in FIG. 6, from the start of beam extraction (time T6) to the time t ４ The high frequency voltage V ｅｘｔ If the application of voltage T7 is continued, the beam can be emitted up to time T7'.

[0048] Furthermore, if the beam remains circulating in the accelerator after extraction, the high frequency voltage V ｅｘｔBy applying the acceleration voltage V again, the beam extraction can be resumed (time T8), and the beam can be used for the next spot irradiation without having to be injected, captured, and accelerated again. In other words, since the beam can be extracted multiple times within one acceleration period, the charge injected from the ion source 53 can be used without waste, and the dose rate can be further improved. ａｃｃ When the voltage Vcc starts to rise, a new acceleration cycle begins (time T10).

[0049] 7, the rotating capacitor 22 will be described in detail. As described above, the rotating capacitor 22 is installed at the end of the acceleration cavity 10 opposite the dee electrode 12. The rotating capacitor 22 includes, for example, a motor 31, a stator electrode 32, a rotor electrode 33, a shaft 35 connecting the rotor electrode and the motor 31, a rotary joint 34, a vacuum seal 29, a bearing 30 for the shaft 35, and a holder 28.

[0050] The stator electrode 32 is formed on the inner conductor 14. The rotor electrode 33 is adjacent to the outer conductor 15 and is not physically connected to the outer conductor 15, but is electrically connected to the outer conductor 15 via capacitance. Conversely to this configuration, the stator electrode 32 may be formed on (physically connected to) the outer conductor 15, and the rotor electrode 33 may be electrostatically coupled to the inner conductor 14.

[0051] Figure 8 is a cross-sectional view taken along line B-B' in Figure 7. The stator electrode 32 and rotor electrode 33 have a cyclically symmetric structure with cutouts of any shape in the circumferential direction in order to realize the frequency modulation pattern shown in Figure 6(a). By changing the opposing area between the stator electrode 32 and rotor electrode 33, the electrostatic capacitance formed between the stator electrode 32 and rotor electrode 33 varies over time.

[0052] 8, this cyclic symmetry structure is eight-fold symmetric, so the frequency modulation pattern is repeated eight periods for each rotation of the motor 31. If the number of cyclic symmetries is further increased, the motor rotation speed can be reduced, and the life of the vacuum seal and bearing parts can also be improved.

[0053] The shaft 35 is installed so as to pass through the center of the motor 31. A rotary joint 34 is installed at the end of the shaft 35, and cooling water is supplied into the shaft 35. The cooling water is used to cool the rotor electrode 33. The motor 31 shown in FIG. 7 is merely an example, and the motor used in the rotating capacitor 22 may have a structure other than a structure in which the shaft and the rotating axis are shared. For example, a motor may be installed beside the shaft, and the shaft may be driven via a gear, pulley, or the like.

[0054] The holder 28 is water-cooled and holds and cools the vacuum seal 29 and bearing 30. The vacuum seal 29 is installed on the dee electrode 12 side and vacuum seals around the shaft 35. The bearing 30 that supports the shaft 35 is installed on the opposite side of the dee electrode 12. In other words, the bearing 30 is installed on the atmosphere side. Because the bearing 30, which is a consumable item, is installed on the atmosphere side, maintenance work such as replacing the bearing 30 is easy. Furthermore, since there is no need to release the vacuum for such maintenance work, downtime of the circular accelerator 39 can be reduced. Even if dust is generated from the grease used in the bearing 30, it is located in the atmosphere and does not cause a deterioration in the vacuum level, so problems such as discharge or beam loss do not occur.

[0055] A lip seal, double O-ring, Wilson seal, bellows seal, etc. may be used as the vacuum seal 29. If the rotation speed of the motor 31 is 2000 rpm or less, a magnetic fluid seal may be used, which improves sliding properties and is expected to extend the seal life.

[0056] The stator electrode 32, rotor electrode 33, inner conductor 14, outer conductor 15, and shaft 35, which can serve as paths through which high-frequency current flows, are all made of conductive materials. As described above, the outer conductor 15 of this embodiment has a copper thin film 151 provided on the surface of the iron member 150 (at least the surface that can serve as a path through which high-frequency current flows). The iron member 150, which forms the main shape of the outer conductor 15, reduces the leakage magnetic field from the main electromagnet 40 to a desired value or less.

[0057] Fig. 9 shows a rotary capacitor according to Modification 1. In order to reduce high-frequency current flowing near the vacuum seal 29 and bearing 30, a bypass capacitor 23 may be installed on the vacuum side of the vacuum seal 29, as shown in Fig. 9. The bypass capacitor 23 includes a holder-side electrode 24 and a shaft-side electrode 25 that face each other. The holder-side electrode 24 is an electrode fixed to a conductive holder 28' connected to the outer conductor 15, and the shaft-side electrode 25 is an electrode fixed on the shaft 35.

[0058] The configuration of the bypass capacitor 23 is shown in Figure 10. Figure 10 is a cross-sectional view taken along line CC' in Figure 9. The holder-side electrode 24 and the shaft-side electrode 25 are electrodes that do not have any notches in the circumferential direction. This configuration increases the capacitance and reduces the impedance to high frequencies. As a result, high-frequency current flows more easily through this bypass capacitor 23, and the high-frequency current flowing through the vacuum seal 29 and bearing 30 is reduced, thereby improving the lifespan of the vacuum seal 29 and bearing 30 and reducing the frequency of maintenance work.

[0059] The holder-side electrode 24 and the shaft-side electrode 25 may have circumferential notches, similar to the stator and rotor electrodes. Although the presence of notches reduces the capacitance, they can also produce capacitance changes that contribute to resonance frequency modulation, similar to the stator and rotor electrodes. The holder-side electrode 24 and the shaft-side electrode 25 can be formed to have a rotation radius similar to that of the stator and rotor electrodes, increasing the capacitance and enhancing the bypass effect of high-frequency current.

[0060] When a magnetic fluid seal is used as the vacuum seal 29, the shaft 35 must be made of a magnetic material to form a magnetic path. This reduces both the deterioration of the sealing performance of the magnetic fluid seal caused by the leakage magnetic field and the eddy current loss generated in the rotor electrode 33.

[0061] As described above, in this embodiment, the outer conductor 15 has a hybrid structure made of the iron member 150 and the copper thin film 151 (or the copper thin films 151, 152), so that the leakage magnetic field can be reduced to a desired value or less by the iron member 150. If the entire rotating capacitor 22 is tightly covered with a conductive housing connected to the outer conductor 15, high-frequency noise that may be radiated from the shaft 35 into the surrounding space can be suppressed.

[0062] The configuration of a particle beam therapy system 100 according to this embodiment will be described below with reference to FIG. 11 . The particle beam therapy system 100 includes, for example, a circular accelerator 39, a rotating gantry 190, an irradiation device 192 including a scanning coil, a treatment couch 201, and a control device 191 for controlling these components. The beam extracted from the circular accelerator 39 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 of ion beam is irradiated onto the affected area target of a 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 a calculation device.

[0063] According to the rotating condenser 22 of this embodiment, the bearing 30, which is a consumable item, is installed on the atmospheric side, so that when performing maintenance work on the bearing 30 (e.g., replacing the bearing 30), it is not necessary to open the circular accelerator 39 to the atmosphere. In other words, it is possible to perform maintenance on the bearing 30 without opening the circular accelerator 39 to the atmosphere. This improves the efficiency of maintenance work on the bearing 30. Furthermore, because it is not necessary to open the circular accelerator 39 to the atmosphere, downtime of the circular accelerator 39 can be shortened. As a result, the operating time of a particle beam therapy system using the circular accelerator 39 can be extended, and patient throughput can be improved.

[0064] A rotating capacitor 22a according to Modification 2 will now be described with reference to Fig. 12 . The rotating capacitor 22a further includes a bearing 30a and a holder 28a in addition to the configuration of the rotating capacitor 22 shown in Fig. 7 . In Modification 2, the bearing 30 corresponds to an example of a first individual bearing, and the bearing 30a corresponds to an example of a second individual bearing. A shaft 35 is supported by the bearings 30 and 30a. The holder 28a holds the bearing 30a. The bearing 30 and the bearing 30a are installed at positions spaced apart from each other on the shaft 35.

[0065] When the bearings are installed on the atmospheric side, the shaft 35 becomes longer than when the bearings are installed on the vacuum side, but by installing the bearings 30, 30a at positions separated from each other, the shaft 35 can be stably supported.

[0066] It should be noted that three or more bearings may be installed and the shaft 35 may be supported by the three or more bearings.

[0067] A rotating capacitor 22c according to Modification 3 will be described with reference to FIG. 13 . In Modification 3, a cooling pipe 26, an example of a "cooling section," is disposed on the outer surface of the outer conductor 15, through which cooling water is circulated. More precisely, the cooling pipe 26 is wound around a predetermined location outside the outer copper thin film 152 of the outer conductor 15. Cooling water from a cooling water circulator (not shown) is pumped into the cooling pipe 26 to remove heat generated primarily in the iron member 150 of the outer conductor 15. A fluid other than water may be used as a coolant. The cooling pipe 26 is rust-proof and corrosion-resistant, but can be replaced if it corrodes. Therefore, compared to forming a cooling water passage through which cooling water flows inside the iron member 150 of the outer conductor 15, the structure of the iron member 150 can be simplified, and the cooling structure can be replaced separately from the iron member 150, improving convenience.

[0068] 14, the main parts of the acceleration cavity according to Modification 4 will be described. Instead of forming a copper thin film on both sides of the iron member 150, a copper thin film 151 may be formed only on the outer surface where a path through which a high-frequency current flows is formed. This makes it possible to prevent the formation of an unnecessary copper thin film 152.

[0069] 15 will be used to explain the main parts of an acceleration cavity according to Modification 5. In Modification 5, the copper thin film 151e is not formed on the entire outer surface of the iron member 150, but is formed only partially in an area including an area 153 where a path through which a high-frequency current may flow may exist. The area of ​​the copper thin film 151e may be set larger than the area of ​​the area 153 where a path through which a high-frequency current may flow may exist. This allows the copper thin film 151e to be formed efficiently.

[0070] The present invention is not limited to the above-described embodiments, and includes various modifications. The above-described embodiments have been described in detail to clearly explain the present invention, and are not necessarily limited to those including all of the described configurations. Furthermore, part of the configuration of one embodiment can be replaced with the configuration of another embodiment. Furthermore, the configuration of another embodiment can be added to the configuration of one embodiment. Furthermore, part of the configuration of each embodiment can be added to, deleted from, or replaced with other configurations.

[0071] The above-described embodiments clearly disclose the following configurations.

[0072] (Representation 1) An accelerating cavity for use in a circular accelerator, comprising a rotating capacitor whose capacitance is variable by changing the facing area of ​​a pair of opposing electrodes, and an outer conductor which forms a path for passing high-frequency current, the outer conductor being arranged to surround the rotating capacitor, and comprising a magnetic shielding member which blocks external magnetic fields and a conductive member which forms a path for passing high-frequency current.

[0073] (Representation 2) An acceleration cavity according to Representation 1, wherein the magnetic shielding member is configured so that the leakage magnetic field at the rotating capacitor is equal to or less than a desired value, and the conductive member is provided on the surface of the magnetic shielding member.

[0074] (Representation 3) The acceleration cavity according to either of Representations 1 or 2, wherein the conductive member is a copper thin film formed on the surface of the magnetic shielding member.

[0075] (Representation 4) An accelerating cavity according to any one of Representations 1 to 3, wherein the thickness dimension of the copper thin film is set so as to be able to propagate the high frequency power required to accelerate the charged particle beam circulating within the circular accelerator.

[0076] (Representation 5) An accelerating cavity according to any one of Representations 1 to 4, wherein the surface of the magnetic shielding member includes at least a high-frequency transmission surface through which a path of a high-frequency current is formed.

[0077] (Representation 6) An acceleration cavity according to any one of Representations 1 to 5, which is provided with a cooling section on the outside of the outer conductor for cooling the magnetic shielding member.

[0078] (Representation 7) A circular accelerator comprising an accelerating cavity according to any one of Representations 1 to 6.

[0079] (Representation 8) A particle beam therapy system including the circular accelerator according to any one of Representations 1 to 7, and an irradiation device that irradiates a patient with a charged particle beam extracted from the circular accelerator.

[0080] 10: accelerating cavity, 11: accelerating gap, 12: dee electrode, 13: dummy dee electrode, 14: inner conductor, 15: outer conductor, 22: rotating capacitor, 32: stator electrode, 33: rotor electrode, 39: circular accelerator, 100: particle beam therapy device.

Claims

1. An accelerating cavity for use in a circular accelerator, comprising: a rotating capacitor whose capacitance is variable by changing the facing area of a pair of opposing electrodes; and an outer conductor which forms a path for passing a high-frequency current, the outer conductor being arranged to surround the rotating capacitor, and comprising a magnetic shielding member which blocks external magnetic fields and a conductive member which forms the path for passing the high-frequency current.

2. An acceleration cavity according to claim 1, wherein the magnetic shielding member is configured so that the leakage magnetic field at the rotating capacitor is equal to or less than a desired value, and the conductive member is provided on the surface of the magnetic shielding member.

3. An acceleration cavity according to claim 2, wherein the conductive member is a copper thin film formed on the surface of the magnetic shielding member.

4. An accelerating cavity according to claim 3, wherein the thickness of the copper thin film is set so as to be able to transmit the high frequency power required to accelerate the charged particle beam circulating within the circular accelerator.

5. An acceleration cavity according to claim 4, wherein the surface of said magnetic shielding member includes at least a high frequency transmission surface along which a path for said high frequency current is formed.

6. An acceleration cavity according to claim 5, comprising a cooling section for cooling said magnetic shielding member outside said outer conductor.

7. A circular accelerator comprising an accelerating cavity according to any one of claims 1 to 6.

8. A particle beam therapy system comprising: the circular accelerator according to claim 7; and an irradiation device that irradiates a patient with the charged particle beam extracted from the circular accelerator.

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