Accelerators and particle beam therapy equipment

Abstract

To provide an accelerator capable of improving ion beam extraction efficiency.SOLUTION: A disturbance magnetic field region provided at the outer periphery of a main magnetic field region 30 of an accelerator includes a peeler region 31 in which the strength of the magnetic field decreases outward, a regenerator region 32 in which the magnetic field strength increases outwardly, and a substantially flat region 33 whose magnetic field strength is greater than the magnetic field strength of the peeler region 31 and smaller than the magnetic field strength of the regenerator region 32.SELECTED DRAWING: Figure 7

Description

[Technical field]

[0001] The present disclosure relates to an accelerator and a particle beam therapy device. [Background technology]

[0002] Particle beam therapy is a type of radiation therapy, and is a treatment method in which a tumor is irradiated with an ion beam such as a proton beam or a carbon beam to destroy cells in the tumor. A particle beam therapy system for performing particle beam therapy includes an ion source that generates ions, an accelerator that accelerates the ions generated by the ion source to form an ion beam, a beam transport system that transports the ion beam formed by the accelerator from the accelerator to a treatment room, a rotating gantry that changes the irradiation direction of the ion beam transported by the beam transport system toward the tumor, an irradiation system that irradiates the tumor with the ion beam from the rotating gantry, and a control system that controls these components.

[0003] In the particle beam therapy system, a circular accelerator such as a synchrotron, a cyclotron, or a synchrocyclotron is used as an accelerator. In recent years, in order to miniaturize the particle beam therapy system, the circular accelerator has been miniaturized. An effective means of miniaturizing the circular accelerator is to increase the strength of the main magnetic field that orbits the ion beam in the circular accelerator, and an effective means of increasing the strength of the main magnetic field is to apply a superconducting magnet to the main electromagnet that generates the main magnetic field. From the viewpoint of application of a superconducting magnet, a cyclotron and a synchrocyclotron that use a static main magnetic field are more effective than a synchrotron that dynamically adjusts the magnitude of the main magnetic field. Non-Patent Document 1 discloses a synchrocyclotron that uses a superconducting magnet as the main electromagnet.

[0004] In circular accelerators with a static main magnetic field, such as cyclotrons and synchrocyclotrons, the energy of the ion beam extracted to the outside is generally fixed, and the energy of the ion beam irradiated to the tumor is adjusted by attenuating the ion beam using a scatterer called a degrader outside the circular accelerator.

[0005] In response to this, Patent Document 1 discloses a circular accelerator that uses a static main magnetic field while varying the energy of the ion beam extracted to the outside, thereby eliminating the need to attenuate the ion beam externally.

[0006] The circular accelerator described in Patent Document 1 accelerates an ion beam circulating within the circular accelerator to a desired energy, and then applies a radio-frequency electric field to the ion beam in a direction (hereinafter referred to as the horizontal direction) that is approximately perpendicular to the ion beam's traveling direction and the magnetic pole gap direction of the main magnetic field (hereinafter referred to as the vertical direction). The ion beam to which the radio-frequency electric field is applied gradually increases in horizontal amplitude of betatron oscillation, which is an oscillation centered on a central orbit, and passes through a magnetic field region called a peeler magnetic field and a regenerator magnetic field for generating resonance of betatron oscillation formed around the central orbit. The ion beam passing through the peeler magnetic field and the regenerator magnetic field rapidly increases in horizontal amplitude of betatron oscillation, and enters a septum magnetic field for extraction, and is extracted to the outside of the accelerator. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2019-133745 A [Non-patent literature]

[0008] [Non-Patent Document 1] XiaoYu Wu, “Conceptual Design and Orbit Dynamics in a 250 MeV Superconducting Synchrocyclotron”, Ph. D. Thesis, submitted to Michigan State University Summary of the Invention [Problem to be solved by the invention]

[0009] The ion beam passing through the peeler magnetic field and the regenerator magnetic field rapidly increases in horizontal amplitude and vertical amplitude due to resonance. When the vertical amplitude of the ion beam increases, the extraction efficiency of the ion beam decreases. Therefore, the technology described in Patent Document 1 has room for improvement in the extraction efficiency of the ion beam.

[0010] An object of the present disclosure is to provide an accelerator and a particle beam therapy system capable of improving ion beam extraction efficiency. [Means for solving the problem]

[0011] An accelerator according to one embodiment of the present disclosure is an accelerator that accelerates an ion beam while causing it to circulate using a main magnetic field and an accelerating radio frequency electric field, and includes a main magnetic field generating device having a plurality of magnetic poles arranged opposite each other and exciting the main magnetic field in the space between the magnetic poles, an extraction channel for extracting the ion beam, a displacement unit for displaceing the ion beam circulating in the main magnetic field region where the main magnetic field is excited to the outside of the main magnetic field region, and a disturbance magnetic field region provided on the outer periphery of the main magnetic field region for exciting a magnetic field that disturbs the ion beam displaced to the outside and guides it to the extraction channel, and the disturbance magnetic field region has a first region in which the magnetic field strength decreases as it moves outward, a second region in which the magnetic field strength increases as it moves outward, and a third region in which the magnetic field strength is greater than the magnetic field strength of the first region and less than the magnetic field strength of the second region. Effect of the Invention

[0012] According to the present invention, it is possible to improve the ion beam extraction efficiency. [Brief description of the drawings]

[0013] [Figure 1] 1 is a configuration diagram of a particle beam therapy system according to an embodiment of the present disclosure. [Diagram 2] FIG. 2 is a perspective view of a main field magnet that generates the main magnetic field. [Diagram 3]FIG. 2 is a longitudinal cross-sectional view taken along a vertical plane of the main field magnet. [Figure 4] FIG. 2 is a cross-sectional view taken along the mid-plane of the main field magnet. [Diagram 5] FIG. 2 is a diagram showing the magnetic field distribution on the center line of the main magnetic field. [Figure 6] FIG. 2 is a diagram for explaining the circular orbit of an ion beam. [Figure 7] FIG. 2 is a schematic diagram showing the magnetic field distribution on the mid-plane of the main magnetic field. [Figure 8] FIG. 13 shows the radial distribution of the magnetic field on the mid-plane at the magnetic pole periphery. [Figure 9] FIG. 13 is a diagram for explaining a comparative example. [Figure 10] FIG. 2 is a cross-sectional view of the main field magnet taken along another vertical plane. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. EXAMPLES

[0015] FIG. 1 is a diagram showing the overall configuration of a particle therapy system according to an embodiment of the present disclosure. The particle therapy system 1001 shown in FIG. 1 is a system for irradiating an ion beam formed by accelerating ions in an accelerator 1004 described later to a subject. In this embodiment, the accelerator 1004 accelerates an ion beam using hydrogen ions, i.e., protons, as an ion beam to any energy within a predetermined range and emits the ion beam. In this embodiment, the predetermined range is a range from 70 MeV to 235 MeV. However, the ion beam may be a heavy particle ion beam using helium and carbon, and the extraction energy, which is the energy of the ion beam to be emitted, is not limited to a range from 70 MeV to 235 MeV.

[0016] 1 is installed on the floor of a building (not shown). The particle therapy system 1001 also includes an ion beam generator 1002, a beam transport system 1005, a rotating gantry 1006, an irradiation device 1007, a treatment planning device 1008, and a control system 1009. The ion beam generator 1002 also includes an ion source 1003 and an accelerator 1004.

[0017] The ion source 1003 is an ion introduction device that supplies ions to the accelerator 1004. The accelerator 1004 accelerates the ions supplied from the ion source 1003 to form an ion beam, and emits the ion beam. A high frequency power supply 1036, which is a power supply for driving the accelerator 1004, a coil excitation power supply 1057, and an extraction channel power supply 1082 are connected to the accelerator 1004. In addition, an ion beam current measuring device 1098 that measures the current of the ion beam is connected to the accelerator 1004. The ion beam current measuring device 1098 includes a moving device 1017 and a position detector 1039. The accelerator 1004 will be described in more detail later.

[0018] The beam transport system 1005 is a transport system that transports the ion beam extracted from the accelerator 1004 to the irradiation device 1007, and has an ion beam path 1048 through which the ion beam passes. The ion beam path 1048 is connected to an extraction channel 1019 for extracting the ion beam in the accelerator 1004 and the irradiation device 1007. In addition, in the ion beam path 1048, electromagnets for transporting the ion beam from the accelerator 1004 to the irradiation device 1007 are arranged in this order: a plurality of quadrupole electromagnets 1046, a bending electromagnet 1041, a plurality of quadrupole electromagnets 1047, a bending electromagnet 1042, a quadrupole electromagnet 1049, a quadrupole electromagnet 1050, a bending electromagnet 1043, and a bending electromagnet 1044.

[0019] The rotating gantry 1006 is configured to be rotatable about a rotation axis 1045, and is a rotating device that revolves the irradiation device 1007 around the rotation axis 1045. A part of an ion beam path 1048 is installed in the rotating gantry 1006. Among the electromagnets for transporting the ion beam, a bending electromagnet 1042, quadrupole electromagnets 1049 and 1050, and bending electromagnets 1043 and 1044 are installed in the rotating gantry 1006.

[0020] The irradiation device 1007 is attached to the rotating gantry 1006 and is connected to the ion beam path 1048 downstream of the bending electromagnet 1044 .

[0021] The irradiation device 1007 has scanning magnets 1051 and 1052, a beam position monitor 1053, and a dose monitor 1054. The scanning magnets 1051 and 1052, the beam position monitor 1053, and the dose monitor 1054 are arranged in a casing (not shown) of the irradiation device 1007. Furthermore, the scanning magnets 1051 and 1052, the beam position monitor 1053, and the dose monitor 1054 are arranged along the central axis of the irradiation device 1007, that is, along the beam axis of the ion beam.

[0022] The scanning magnets 1051 and 1052 each deflect the ion beam and configure a scanning system that scans the ion beam in directions approximately perpendicular to each other in a plane approximately perpendicular to the central axis of the irradiation device 1007. The beam position monitor 1053 and the dose monitor 1054 are disposed downstream of the scanning magnets 1051 and 1052. The beam position monitor 1053 measures the passing position of the ion beam. The dose monitor 1054 measures the dose of the ion beam.

[0023] A treatment table 1055 on which a patient 2001 to be examined lies is disposed downstream of the irradiation device 1007 so as to face the irradiation device 1007 .

[0024] The treatment planning device 1008 generates details of ion beam irradiation for the patient 2001 as a treatment plan, and notifies the control system 1009. The irradiation details include, for example, the ion beam irradiation area, irradiation energy, irradiation angle, and number of irradiations.

[0025] The control system 1009 is a control unit that controls the ion beam generator 1002, the beam transport system 1005, the rotating gantry 1006, and the irradiation device 1007 in accordance with a treatment plan notified from the treatment planning device 1008, and irradiates the patient 2001 with the ion beam.

[0026] The control system 1009 includes a central controller 1066 , an accelerator / transport controller 1069 , a scan controller 1070 , a rotation controller 1071 , and a database 1072 .

[0027] The central control device 1066 controls the ion beam generator 1002, the beam transport system 1005, the rotating gantry 1006, and the irradiation device 1007 via the accelerator / transport system control device 1069, the scan control device 1070, and the rotation control device 1071 in accordance with the treatment plan notified from the treatment planning device 1008, and irradiates the patient 2001 with the ion beam.

[0028] The accelerator / transport system control device 1069 controls the ion beam generator 1002 and the beam transport system 1005. The scan control device 1070 controls the irradiation device 1007. Specifically, the scan control device 1070 controls the scanning magnets 1051 and 1052 based on the measurement results of the beam position monitor 1053 and the dose monitor 1054 to scan the ion beam. The rotation control device 1071 controls the rotating gantry 1006. The database 1072 stores the treatment plan notified from the treatment planning device 1008. The database 1072 may also store various information used by the central control device 1066.

[0029] The central control device 1066 also has a CPU (Central Processing Unit) 1067, which is a central processing device, and a memory 1068 connected to the CPU 1067. The database 1072, the accelerator / transport system control device 1069, the scan control device 1070, and the rotation control device 1071 are connected to the CPU 1067 in the central control device 1066.

[0030] The CPU 1067 reads a program for controlling each device constituting the particle beam therapy system 1001 according to the treatment plan stored in the database 1072, and executes the read program to execute a control process for controlling each device in the particle beam therapy system 1001. Specifically, the CPU 1067 outputs commands to each device via the accelerator / transportation control device 1069, the scan control device 1070, and the rotation control device 1071 to control each device and irradiate the patient 2001 with an ion beam according to the treatment plan. The memory 1068 is used as a work area for the program, and stores various data used and generated in the processing of the CPU 1067.

[0031] The program executed by the CPU 1067 may be one program or may be divided into multiple programs. A part or all of the processing by the program may be realized by dedicated hardware. The program may be installed in the central control device 1066 from the database 1072, or may be installed in the central control device 1066 from a program distribution server or an external storage medium (not shown). Each device in the control system 1009 may be configured in a form in which two or more devices are connected by wire or wirelessly.

[0032] <Accelerator 1004> Next, the accelerator 1004 of the ion beam generator 1002 will be described in more detail with reference to Figures 1 to 4. Figure 2 is a perspective view of the accelerator 1004, Figure 3 is a longitudinal sectional view of the accelerator 1004 taken along a vertical plane 3, and Figure 4 is a transverse sectional view of the accelerator 1004 taken along an intermediate plane 2.

[0033] (Main field magnet 1) 2 to 4, the accelerator 1004 has a main magnetic field magnet 1. The main magnetic field magnet 1 is a main magnetic field generating device that generates a main magnetic field for circulating an ion beam, and as shown in Fig. 2, the main magnetic field magnet 1 has an upper return yoke 4 and a lower return yoke 5 each having a substantially disk-like shape when viewed in the vertical direction.

[0034] The upper return yoke 4 and the lower return yoke 5 have shapes that are approximately symmetrical with respect to the mid-plane 2. The mid-plane 2 passes roughly through the center of the main magnetic field magnet 1 in the vertical direction and approximately coincides with the orbital plane of the ion beam accelerated in the accelerator 1004.

[0035] Moreover, the upper return yoke 4 and the lower return yoke 5 are approximately perpendicular to the mid-plane 2, and have a shape that is approximately symmetrical with respect to the vertical plane 3, which is a plane that passes through roughly the center of the main magnetic field magnet 1 on the mid-plane 2. Note that in Fig. 2, the intersection of the mid-plane 2 with the main magnetic field magnet 1 is indicated by a dashed line, and the intersection of the vertical plane 3 with the main magnetic field magnet 1 is indicated by a dashed line.

[0036] As shown in FIG. 3, in the space surrounded by the upper return yoke 4 and the lower return yoke 5, two coils 6 are arranged substantially symmetrically with respect to the intermediate plane 2. The coils 6 are superconducting coils, and are made of superconducting wires using a superconductor such as niobium titanium. The coils 6 are installed inside a cryostat (not shown), which is a cooling mechanism for cooling the coils 6, and are cooled by the cryostat to a certain temperature (the temperature at which the coils 6 exhibit complete diamagnetism) or lower. The coils 6 are also drawn out to the outside of the main field magnet 1 by the coil drawing wiring 1022 shown in FIG. 1 and connected to a coil excitation power supply 1057. The coil excitation power supply 1057 is a power supply that supplies power to the coils 6, and is controlled by an accelerator / transport system control device 1069.

[0037] Further, a vacuum vessel 7 is provided inside the coil 6 in the space surrounded by the upper return yoke 4 and the lower return yoke 5. The vacuum vessel 7 is a vessel for maintaining the interior in a vacuum state, and is made of, for example, stainless steel. Inside the vacuum vessel 7, an upper magnetic pole 8 and a lower magnetic pole 9 are arranged symmetrically across the mid-plane 2, and are respectively coupled to the upper return yoke 4 and the lower return yoke 5. The upper return yoke 4, the lower return yoke 5, the upper magnetic pole 8 and the lower magnetic pole 9 are made of, for example, pure iron or low-carbon steel with reduced impurity concentration.

[0038] The main magnetic field magnet 1 having the above configuration forms a main magnetic field that applies a magnetic field in the vertical direction to the internal acceleration space 20 centered on the intermediate plane 2. The main magnetic field is roughly uniform within the intermediate plane 2, but has a slightly non-uniform intensity distribution.

[0039] The strength of the main magnetic field is designed so that ions supplied from the ion source 1003 circulate stably as an ion beam within the acceleration space 20 according to the principle of weak focusing. The principle of weak focusing indicates that ions circulate stably as an ion beam when the main magnetic field monotonically decreases as it approaches the outer periphery and the gradient is included between a predetermined upper limit value and a predetermined lower limit value.

[0040] 5 is a diagram showing the intensity distribution on the center line of the main magnetic field. The center line is the intersection line between the intermediate plane 2 and the vertical plane 3. In this embodiment, the direction along the intersection line is the Y-axis direction, and the direction perpendicular to the Y-axis direction on the intermediate plane 2 is the X-axis direction.

[0041] 5, the strength of the main magnetic field is greatest at a given position O1 that is shifted in the Y-axis direction from the magnetic pole center O2, which is the center of the upper magnetic pole 8 and the lower magnetic pole 9 in the direction of the mid-plane 2, and gradually decreases as it approaches the outer periphery of the upper magnetic pole 8 and the lower magnetic pole 9. Note that, hereinafter, position O1 may also be referred to as the center of the main magnetic field distribution.

[0042] (Ion source 1003) In the example of FIG. 2, the ion source 1003 is installed above the main magnetic field magnet 1. The upper return yoke 4 and the upper magnetic pole 8 are provided with a through hole 17 for guiding ions from the ion source 1003 to a position O1 in the acceleration space 20. A central axis (ion injection axis) 12 of the through hole 24 is approximately perpendicular to the mid-plane 2 and passes through to the position O1. The ion source 1003 is disposed above the through hole 17, and introduces ions to the position O1 in the acceleration space 20 through the through hole 17. The ion source 1003 may be installed inside the main magnetic field magnet 1. In this case, the through hole 17 is not necessary.

[0043] (Output Channel 1019) 2 to 4, the accelerator 1004 has an extraction channel 1019 that extracts an ion beam and extracts it to a beam transport system 1005. In this embodiment, the extraction channel 1019 has an electromagnet (not shown) and is arranged outside the acceleration space 20, more specifically, on the outer periphery closer to the center O1 of the main magnetic field distribution on the Y axis of the upper magnetic pole 8 and the lower magnetic pole 9. The extraction channel 1019 has an opening 1019a near the Y axis, and takes in an ion beam of desired energy from the opening 1019a and extracts it to the outside of the accelerator 1004 through a through hole 18 provided in the upper return yoke 4 and the lower return yoke 5. The through hole 18 is provided at the tip of the beam transport system 1005, and the extracted ion beam is guided to the irradiation device 1007 through the beam transport system 1005.

[0044] The power supply lines that supply power to the electromagnets of the extraction channel 1019 are drawn out from the accelerator 1004 through through holes 15 provided in the upper return yoke 4 and the lower return yoke 5, and are connected to the extraction channel power supply 1082 shown in FIG. 1. The extraction channel power supply 1082 is a power supply that can supply power to the output channel, and is controlled by the accelerator / transport system control device 1069. The extraction channel 1019 may be composed of only a magnetic material, without including an electromagnet. In this case, no power supply for the extraction channel 1019 is required.

[0045] (RF Acceleration Cavity 1037) The accelerator 1004 also has a radio frequency acceleration cavity 1037, which is a component for accelerating ions injected into the acceleration space 20 to form an ion beam. The radio frequency acceleration cavity 1037 includes a pair of dee electrodes 1037a arranged on either side of the mid-plane 2. The dee electrode 1037a has a sector shape when viewed from the vertical direction. The dee electrode 1037a is arranged so that the apex (center) of the sector is located near the center O1 of the main magnetic field distribution and covers a part of the orbit of the ion beam including the magnetic pole center O2.

[0046] A ground electrode (not shown) is arranged opposite the radial end face of the Dee electrode 1037a, and an accelerating electric field, which is an accelerating high-frequency electric field for accelerating the ion beam, is formed between the radial end face of the Dee electrode 1037a and the ground electrode.

[0047] Since the Dee electrode 1037a is formed in a sector shape with the position O1 as the apex, the accelerating electric field can be applied so that the direction of travel of the circulating ion beam is parallel to the accelerating electric field, that is, at a position where an axis parallel to the X-axis that passes through the center of each orbit about which the ion beam revolves intersects with each orbit.

[0048] The radio frequency acceleration cavity 1037 is drawn out to the outside of the main magnetic field magnet 1 through a through hole 16 provided along the Y-axis direction between the upper return yoke 4 and the lower return yoke 5, and is connected to a waveguide 1010 at its outside. A radio frequency power supply 1036 is connected to the waveguide 1010. The radio frequency power supply 1036 is a power supply that supplies power to the radio frequency acceleration cavity 1037 through the waveguide 1010, and is controlled by an accelerator / transportation control device 1069. A radio frequency electric field is excited between the dee electrode 1037a and the ground electrode as an accelerating electric field by the power supplied from the radio frequency power supply 1036.

[0049] The orbit radius of the orbit of the ion beam revolving in the acceleration space 20 gradually increases with the acceleration of the ion beam, as described later. In order to appropriately accelerate the ion beam, the acceleration electric field needs to be synchronized with the ion beam, and for this purpose, the resonance frequency of the radio frequency acceleration cavity 1037 needs to be modulated according to the energy of the ion beam. The modulation of the resonance frequency is performed, for example, by adjusting the inductance or capacitance of the radio frequency acceleration cavity 1037. As a method for adjusting the inductance or capacitance of the radio frequency acceleration cavity 1037, a known method can be used. For example, when adjusting the capacitance, the capacitance of a variable capacitance capacitor connected to the radio frequency acceleration cavity 1037 is controlled to modulate the resonance frequency.

[0050] (Orbital density) FIG. 6 is a diagram for explaining the orbits of the ion beams circulating in the acceleration space 20, and shows each orbit 125 of the ion beams with different energies.

[0051] Ions introduced from the ion source 1003 into the acceleration space 20 are formed into an ion beam by a high-frequency electric field, which is an acceleration electric field, and circulate in the acceleration space 20. As shown in FIG. 5, the main magnetic field in the acceleration space 20 is maximum at a position O1 shifted from the magnetic pole center O2, and gradually decreases as it approaches the outer periphery of the upper magnetic pole 8 and the lower magnetic pole 9. In this case, the ion beam with low energy circulates along an orbit centered on the position O1. As the ion beam is accelerated by the high-frequency electric field, the orbit radius becomes larger and the center of the orbit gradually approaches the position O2 of the central axis 13 of the upper magnetic pole 8 and the lower magnetic pole 9. The orbit 127 of the ion beam with maximum energy shown in FIG. 4 has a shape that is approximately along the outer periphery of the upper magnetic pole 8 and the lower magnetic pole 9, and its center roughly coincides with the position O2.

[0052] Therefore, as shown in FIG. 6, the ion beam's circular orbit 125 is dense between position O1 and position Y1 at the end of the acceleration space 20 in the Y-axis direction, and is sparse between position O1 and position Y2 at the end in the Y-axis direction on the opposite side, sandwiched between position O2, the center of the upper magnetic pole 8 and the lower magnetic pole 9.

[0053] 4, the center of the orbit 127 of the maximum energy beam, which corresponds to the maximum energy (235 Mev) of the extractable ion beams, among the orbits 125, roughly coincides with the magnetic pole center O2. Also, the center O3 of the orbit 126 of the minimum energy beam, which corresponds to the minimum energy of the extractable ion beams, is on the line segment connecting the magnetic pole center O2 and the center O1 of the main magnetic field distribution.

[0054] (Ion beam extraction) The accelerator 1004 has a high-frequency kicker 40, a peeler region 31, a regenerator region 32, and a substantially flat region 33 as a mechanism for guiding the ion beam circulating within the acceleration space 20 to the extraction channel 1019, and extracts an ion beam having a predetermined range of energy by utilizing the density of the circular orbit 125.

[0055] The radio frequency kicker 40 is a displacement unit that displaces outward the ion beam circulating in the main magnetic field region in which the main magnetic field in the acceleration space 20 is excited. The radio frequency kicker 40 increases the amplitude of the betatron oscillation of the ion beam, for example, by applying a horizontal radio frequency electric field to the ion beam. This displaces the ion beam so that it passes through the peeler region 31, the regenerator region 32, and the approximately flat region 33. The peeler region 31, the regenerator region 32, and the approximately flat region 33 constitute a disturbance magnetic field region that disturbs the ion beam displaced by the radio frequency kicker 40 and excites a magnetic field that guides the ion beam to the extraction channel 1019.

[0056] FIG. 7 is a diagram for explaining the arrangement of the peeler region 31, the regenerator region 32, and the approximately flat region 33, and shows the magnetic field distribution on the intermediate plane 2 around which the ion beam circulates.

[0057] The magnetic field distribution shown in Fig. 5 is formed in the main magnetic field region 30 shown in Fig. 7. A peeler region 31, a regenerator region 32, and a substantially flat region 33 are formed on the outer magnetic pole periphery of the main magnetic field region 30. The peeler region 31 and the regenerator region 32 are located outside the dense region where the ion beam orbit 125 in the main magnetic field region 30 is dense.

[0058] Fig. 8 is a diagram showing the radial distribution of the magnetic field in the peeler region 31, the regenerator region 32, and the substantially flat region 33. The magnetic field distribution in the peeler region 31 corresponds to the magnetic field distribution along line AA' in Fig. 7, the magnetic field distribution in the regenerator region 32 corresponds to the magnetic field distribution along line BB' in Fig. 7, and the magnetic field distribution in the substantially flat region 33 corresponds to the magnetic field distribution along line CC' in Fig. 7.

[0059] The magnetic fields at the innermost positions (positions A, B, C) of the peeler region 31, the regenerator region 32, and the substantially flat region 33 are almost the same. The peeler region 31 is a first region in which the strength of the magnetic field decreases relatively greatly as it moves outward (from A to A'). The regenerator region 32 is a second region in which the strength of the magnetic field increases relatively greatly as it moves outward (from B to B'). The substantially flat region 33 is a third region in which the magnetic field is substantially constant. In this embodiment, the magnetic field of the substantially flat region 33 decreases slightly and more gently than the magnetic field of the peeler region 31 as it moves outward (from C to C'). Therefore, at the outer periphery of each region, the magnetic field of the peeler region 31 is the smallest, the magnetic field of the regenerator region 32 is the largest, and the magnetic field of the flat region 33 is between the magnetic fields of the peeler region 31 and the regenerator region 32.

[0060] The operation of extracting an ion beam having a desired energy from the accelerator 1004 will now be described.

[0061] In response to commands from the central controller 1066, the accelerator and transport controller 1069 causes the ion source 1003 to generate ions and introduces the ions through the through-hole 17 to position O1 in the acceleration space 20 in the main magnetic field magnet 1. The accelerator and transport controller 1069 uses the radio frequency acceleration cavity 1037 to generate an accelerating electric field in the acceleration space 20, accelerating the ions to form an ion beam. The formed ion beam increases its energy as it orbits.

[0062] When the ion beam reaches a desired energy, the accelerator / transport system control device 1069 turns off the power supplied to the radio frequency acceleration cavity 1037 and turns on the radio frequency kicker 40. This causes a radio frequency electric field to be applied to the ion beam superimposed on the main magnetic field. As a result, the orbit 125 of the ion beam is displaced in the radial direction (toward position Y1). For example, as shown in FIG. 7, when the ion beam is the lowest energy beam, the orbit 126 is displaced in the radial direction as shown in FIG. 7 as an orbit 126', and when the ion beam is the highest energy beam, the orbit 127 is displaced in the radial direction as shown in FIG. 7 as an orbit 127'.

[0063] As a result, the ion beam passes through the peeler region 31 and the regenerator region 32. This causes a resonance of horizontal betatron oscillation called 2 / 2 resonance, and the ion beam diverges in the radial direction and reaches the opening 1019a of the extraction channel 1019. The ion beam completely leaves the circular orbit by the extraction channel 1019, and is extracted to the outside of the accelerator 1004 through the through-hole 18.

[0064] FIG. 9 is a diagram for explaining a conventional method for extracting an ion beam, which is described in Non-Patent Document 1, as a comparative example.

[0065] In the conventional method, only the ion beam having the maximum energy is the target for extraction. Therefore, the orbit 128 of the ion beam orbiting the main magnetic field region 30A is formed concentrically, and the orbit expands as the ion beam is accelerated, so that the ion beam passes through the peeler region 31A and the regenerator region 32A. At this time, since only the ion beam having the maximum energy needs to be extracted, the peeler region 31A is formed over the entire circumference of the magnetic pole periphery except for the regenerator region 32A. In other words, the magnetic field gradient of the peeler region 31A and the magnetic field gradient of the regenerator region 32A were designed taking into consideration only the ion beam having the maximum energy.

[0066] In contrast to this, in this embodiment, the energy of the extracted ion beam is variable, so it is necessary to form the peeler region 31 and the regenerator region 32 in the region through which not only the maximum energy beam but also the minimum energy beam passes.

[0067] In order to appropriately induce resonance by betatron oscillation, the product of the magnitude of the magnetic field gradient and the length passing through the region having the magnetic field gradient is important. As shown in Fig. 7, the length passing through the peeler region 31 and the regenerator region 32 becomes shorter as the energy of the ion beam decreases. Therefore, in order to induce resonance in a low-energy ion beam, it is preferable to increase the magnetic field gradient so as to compensate for the short passing length passing through the peeler region 31 and the regenerator region 32.

[0068] An ion beam having energy greater than that of the lowest energy beam passes through the same peeler region 31 and regenerator region 32 as the lowest energy beam. Therefore, if a region in which a gradient magnetic field is distributed, such as the peeler region 31A of the comparative example, is formed instead of the approximately flat region 33, the product of the magnitude of the magnetic field gradient and the length passing through the region having the magnetic field gradient becomes too large, and the ion beam becomes unstable in the horizontal and vertical directions.

[0069] For this reason, in this embodiment, the peeler region 31 is limited to a narrow range, and the magnetic pole periphery excluding the peeler region 31 and the regenerator region 32 becomes an approximately flat region 33 having an approximately constant magnetic field as shown in Fig. 8. Note that, since it is natural for the magnetic field to decrease at the magnetic pole periphery, and the ion beam is stabilized due to the principle of weak convergence, in this embodiment, the magnetic field in the approximately flat region 33 has a slight decreasing gradient.

[0070] In this embodiment, in order to make the magnetic field gradient in the peeler region 31 larger than that in the comparative example, as shown in Fig. 3, the gap distance L between the upper magnetic pole 8 and the lower magnetic pole 9 at the position 41 sandwiching the peeler region 31 is much wider than the gap distances at the positions sandwiching the regenerator region 32 and the substantially flat region 33. Fig. 10 is a longitudinal cross-sectional view of the accelerator 1004 taken along a vertical plane passing through the regenerator region 32, and shows the gap distance M at the position 42 sandwiching the regenerator region 32 and the gap distance N at the position 43 sandwiching the substantially flat region 33. As shown in Fig. 10, the gap distance N is wider than the gap distance M.

[0071] In this embodiment, by utilizing the fact that the gap interval L at the position 41 sandwiching the peeler region 31 is large, the opening 1019a which is the entrance of the output channel 1019 is provided at the position of the peeler region 31. However, the opening 1019a does not have to be provided at the position of the peeler region 31.

[0072] As described above, according to this embodiment, the disturbance magnetic field region provided on the outer periphery of the main magnetic field region 30 of the accelerator 1004 has the peeler region 31 in which the magnetic field strength decreases toward the outside, the regenerator region 32 in which the magnetic field strength increases toward the outside, and the approximately flat region 33 in which the magnetic field strength is greater than that of the peeler region 31 and less than that of the regenerator region 32. This makes it possible to shorten the distance that the ion beam passes through the peeler region 31 and the regenerator region 32, which are regions in which resonance of the betatron oscillation of the ion beam occurs, and therefore makes it possible to suppress an increase in the amplitude of the ion beam in the vertical direction, and to improve the ion beam extraction efficiency.

[0073] In this embodiment, the magnetic field strength in the substantially flat region 33 decreases more gradually toward the outside than in the peeler region 31. In this case, the principle of weak convergence allows the ion beam to stably pass through the peeler region 31 and the regenerator region 32.

[0074] In this embodiment, the ion source 1003 introduces ions into a predetermined position O1 on the extraction channel side of the magnetic pole center O2 in the main magnetic field region 30. The peeler region 31 and the regenerator region 32 are provided on the extraction channel 1019 side of the position O1 where the ions are introduced by the ion source 1003. In this case, it is possible to form a dense orbit on the extraction channel 1019 side, so that it is possible to reduce the kick amount required to extract the beam, and it is possible to easily extract a beam having a desired energy.

[0075] In addition, in this embodiment, since the approximately flat region 33 is larger than the combined region of the peeler region 31 and the regenerator region 32, it is possible to shorten the distance that the ion beam passes through the peeler region 31 and the regenerator region 32.

[0076] In this embodiment, the opening 1019a, which is the entrance of the emission channel 1019, is provided in the peeler region 31. In this case, the number of revolutions of the extracted ion beam until it reaches the opening 1019a can be reduced by about one or two times, compared to the case where the opening 1019a of the emission channel 1019 is provided outside the peeler region 31. This makes it possible to suppress the spread of the ion beam in the vertical and horizontal directions, thereby improving the quality of the extracted ion beam.

[0077] In this embodiment, the main magnetic field magnet 1 generates a main magnetic field so that the orbit around which the ion beam travels accelerates while being eccentric. This makes it possible to reduce the amount of kick required to extract the beam, and to easily extract a beam with the desired energy.

[0078] The above-described embodiments of the present disclosure are illustrative examples of the present disclosure, and are not intended to limit the scope of the present disclosure to only these embodiments. A person skilled in the art may implement the present disclosure in various other forms without departing from the scope of the present disclosure. [Explanation of symbols]

[0079] 1: main magnetic field magnet 4: upper return yoke 5: lower return yoke 6: coil 7: vacuum vessel 8: upper magnetic pole 9: lower magnetic pole 20: acceleration space 30: main magnetic field region 31: peeler region 32: regenerator region 33: approximately flat region 40: radio frequency kicker 1001: particle beam therapy system 1002: ion beam generator 1003: ion source 1004: accelerator 1005: beam transport system 1006: rotating gantry 1007: irradiation device 1008: treatment planning device 1009: control system 1019: extraction channel 1019a: aperture 1037: radio frequency acceleration cavity

Claims

1. An accelerator that accelerates an ion beam while rotating it by a main magnetic field and an accelerating high frequency electric field, A main magnetic field generating device having a plurality of magnetic poles arranged opposite to each other, and exciting the main magnetic field in a space between the magnetic poles; an exit channel for extracting the ion beam; a displacement unit that displaces the ion beam circulating in a main magnetic field region where the main magnetic field is excited to the outside of the main magnetic field region; a disturbance magnetic field region provided on the outer periphery of the main magnetic field region and exciting a magnetic field that disturbs the outwardly displaced ion beam and guides it to the exit channel; The disturbed magnetic field region is a first region in which the magnetic field strength decreases outwardly; a second region in which the magnetic field strength increases outwardly; a third region having a magnetic field strength greater than that of the first region and less than that of the second region at a position equidistant from the center of the magnetic pole; In the third region, the strength of the magnetic field decreases more gradually toward the outside than in the first region; a distance between first portions of the magnetic pole sandwiching the first region is greater than a distance between second portions of the magnetic pole sandwiching the second region; a distance between third portions of the magnetic pole sandwiching the third region is narrower than a distance between the first portions and wider than a distance between the second portions; An accelerator, wherein an entrance of the exit channel through which the ion beam is incident is provided in the first region.

2. an ion introduction device that introduces ions that form the ion beam into a predetermined position on the exit channel side of a center of the magnetic pole in the main magnetic field region; The accelerator according to claim 1 , wherein the first region and the second region are provided on a side of the output channel relative to the predetermined position.

3. The accelerator of claim 1 , wherein the third region is greater than the combined areas of the first region and the second region.

4. 2. The accelerator according to claim 1, wherein the main magnetic field generating device generates the main magnetic field so that the ion beam accelerates while moving in an eccentric orbit around the ion beam.

5. The accelerator of claim 1; and an irradiation device that irradiates the ion beam extracted from the accelerator.

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