Circular accelerator and particle therapy system

Abstract

To provide a circular accelerator and a particle beam therapy system that are capable of improving beam extraction efficiency.SOLUTION: A circular accelerator 1 for accelerating and extracting a beam of charged particles orbiting in a magnetic field comprises: a first magnetic field region 26 in which the beam's closed orbits for respective energy are eccentric, the first magnetic field region having a magnetic field gradient of a magnetic field weakening toward the outer peripheral side; and a second magnetic field region 27 having a magnetic field gradient of a magnetic field strengthening toward the outer peripheral side. A boundary 28 between the first magnetic field region and the second magnetic field region is positioned downstream in the beam's travel direction of a predetermined region 16 in which intervals of the beam's closed orbits for respective energy are minimized.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a circular accelerator and a particle beam therapy system. [Background technology]

[0002] Particle beam therapy is performed by accelerating a charged particle beam (hereinafter simply referred to as a beam) using an accelerator such as a synchrotron or cyclotron, and irradiating a lesion such as cancer with the accelerated beam (Patent Document 1). Patent Document 1 describes a circular accelerator that accelerates and extracts a beam circulating in a static magnetic field, and a particle beam therapy system using the same. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2019-133745 A Summary of the Invention [Problem to be solved by the invention]

[0004] The circular accelerator described in Patent Document 1 arranges a circular closed orbit (hereinafter referred to as the central orbit) formed by beam particles with different kinetic energies (hereinafter simply referred to as energy) in a static magnetic field so that it is eccentric toward the beam extraction port from the accelerator, and extracts beams of different energies outside the accelerator from the same beam extraction port.

[0005] Furthermore, the circular accelerator described in Patent Document 1 accelerates a beam (hereinafter referred to as a circulating beam) circulating in the accelerator to a desired energy, and then applies a radio-frequency voltage to the circulating beam in a direction (hereinafter referred to as a horizontal direction) perpendicular to the beam travel direction and the magnetic pole gap direction (hereinafter referred to as a vertical direction). The horizontal amplitude of the oscillation (hereinafter referred to as a betatron oscillation) of the beam particles centered on the central orbit to which the radio-frequency voltage is applied gradually increases, and the beam particles come into contact with a magnetic field distribution called a peeler magnetic field and a regenerator magnetic field formed around the central orbit, which generates resonance of the betatron oscillation. The horizontal amplitude of the betatron oscillation of the beam particles that come into contact with the peeler magnetic field and the regenerator magnetic field increases rapidly, and the beam particles enter a septum magnetic field for extraction, and are extracted to the outside of the accelerator.

[0006] Therefore, the circular accelerator described in Patent Document 1 is an accelerator that accelerates a beam in a static magnetic field, but is capable of switching the energy of the beam extracted from the accelerator within a predetermined range (for example, from 70 MeV to 230 MeV).

[0007] On the other hand, in the circular accelerator described in Patent Document 1, the vibration of the beam particles may diverge in the vertical direction during beam extraction, and the beam particles may be lost in the accelerator, resulting in a decrease in the beam extraction efficiency from the accelerator. The decrease in beam extraction efficiency leads to a decrease in the beam current irradiated from the accelerator and an increase in the treatment time of a particle beam therapy system using the accelerator.

[0008] The present invention has been made in view of the above problems, and an object of the present invention is to provide a circular accelerator and a particle beam therapy system that can improve the beam extraction efficiency. [Means for solving the problem]

[0009] In order to solve the above problems, the circular accelerator according to the present invention is a circular accelerator that accelerates and extracts a beam of charged particles circulating in a magnetic field, in which the closed orbit for each energy of the beam is eccentric, and the accelerator is provided with a first magnetic field region having a magnetic field gradient in which the magnetic field weakens toward the outer periphery, and a second magnetic field region having a magnetic field gradient in which the magnetic field strengthens toward the outer periphery, and the boundary between the first magnetic field region and the second magnetic field region is located downstream in the direction of beam propagation of a specified region where the spacing between the closed orbits for each energy of the beam is narrowest. Effect of the Invention

[0010] According to the present invention, when the amplitude of the horizontal betatron oscillation increases, the gradient of the first magnetic field and the gradient of the second magnetic field increase simultaneously, so that the balance between the vertical converging and diverging forces acting on the beam can be maintained, and beam loss can be suppressed. [Brief description of the drawings]

[0011] [Figure 1] Schematic diagram showing a cross section of a circular accelerator along a plane parallel to the beam orbit. [Diagram 2] FIG. 2 is an explanatory diagram showing the relationship between a cross section taken along a plane perpendicular to the beam orbit of a circular accelerator and the strength of the main magnetic field. [Diagram 3] FIG. 2 is an explanatory diagram showing the shape of the central orbit of a circular accelerator. [Figure 4] 11 is an explanatory diagram showing the change in distance from the central orbit of a beam particle that comes into contact with a peeler magnetic field and a regenerator magnetic field in a comparative example in which the peeler magnetic field and the regenerator magnetic field are arranged on either side of an orbit concentration region; FIG. [Diagram 5] 4 is an explanatory diagram showing the change in distance from the central orbit of beam particles in contact with the peeler magnetic field and the regenerator magnetic field in the magnetic field arrangement of the first embodiment. FIG. [Figure 6] FIG. 11 is an explanatory diagram showing the change in distance from the central orbit of beam particles that come into contact with the peeler magnetic field and the regenerator magnetic field when the regenerator magnetic field is placed radially outward of the peeler magnetic field in the second embodiment. [Figure 7]FIG. 11 is a schematic diagram showing a cross section along a plane parallel to the beam orbit of a circular accelerator according to the third embodiment. [Figure 8] FIG. 2 is an explanatory diagram showing the shape of the central orbit of a circular accelerator. [Figure 9] FIG. 13 is a schematic diagram of a particle beam therapy system using a circular accelerator according to a fourth embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. In this embodiment, as described later, the boundary between the peeler magnetic field 26 and the regenerator magnetic field 27 is located downstream of the orbit concentration region 16 in the beam traveling direction, so that when the amplitude of the horizontal betatron oscillation increases with the orbit concentration region 16 as a node, the gradient of the peeler magnetic field 26 and the gradient of the regenerator magnetic field 27 increase simultaneously, and the balance between the converging force and the diverging force in the vertical direction can be maintained, thereby suppressing the loss of the beam.

[0013] In this embodiment, a circular accelerator 1 that accelerates and extracts a charged particle beam circulating in a magnetic field is disclosed, in which closed orbits 30, 31, 32, and 33 for each energy of the charged particle beam are eccentric, and the circular accelerator 1 is equipped with a first magnetic field region 26 having a magnetic field gradient in which the magnetic field becomes stronger toward the outer periphery of the circular accelerator 1, and a second magnetic field region 27 having a magnetic field gradient in which the magnetic field becomes weaker toward the outer periphery of the circular accelerator 1, and the boundary between the first magnetic field region 26 and the second magnetic field region 27 is located downstream in the beam progression direction of the region 16 where the interval between the closed orbits for each energy becomes narrower.

[0014] According to this embodiment, it is possible to realize a circular accelerator capable of irradiating a large current regardless of the beam energy, and a particle beam therapy system capable of shortening the time required for beam irradiation. EXAMPLES

[0015] The first embodiment will be described with reference to Figures 1 to 5. Figure 1 is a schematic diagram of a cross section parallel to the beam orbit plane of a circular accelerator 1. Figure 2 is a schematic diagram of a cross section taken along arrows B1-B2 in Figure 1, and is a schematic diagram of a cross section perpendicular to the beam orbit plane of the circular accelerator 1.

[0016] The outer shell of the circular accelerator 1 is formed by a main electromagnet 10 that can be separated in the vertical direction in Fig. 2. Inside the main electromagnet 10, a space through which the beam passes (hereinafter referred to as the beam circulation region) is provided, and the beam circulation region is maintained in a vacuum.

[0017] An ion source 11 that generates a beam to be injected into the circular accelerator 1 is installed above the main electromagnet 10. The beam generated by the ion source 11 is injected into a beam circulation region inside the main electromagnet 10 via a low-energy beam transport system 12. As the ion source 11, for example, an ECR (Electron Cyclotron Resonance) ion source can be used.

[0018] The ion source 11 may be disposed in the beam circulation region. In that case, a PIG (Penning Ionization Gauge) type ion source or the like can be used. If a PIG type ion source is used, the circular accelerator 1 can be made smaller in the height direction.

[0019] The main electromagnet 10 includes, for example, magnetic poles 15, a yoke 13, and a coil 14. The yoke 13 forms the exterior of the main electromagnet, and defines a substantially cylindrical area inside the yoke 13. The coil 14 is an annular coil, and is installed along the inner wall of the yoke 13. The magnetic poles 15 are formed on the inner peripheral side of the coil 14, facing each other in the vertical direction.

[0020] The main electromagnet 10 is excited by passing a current through the coil 14. When the main electromagnet 10 is excited, a magnetic field distribution (hereinafter referred to as the main magnetic field) required for the beam to circulate is formed between the opposing magnetic poles 15.

[0021] The strength of the main magnetic field along the arrows B1-B2 is shown at the bottom of Figure 2. The main magnetic field forms a non-axially symmetric distribution with respect to the central axis O15 of the magnetic pole 15. The absolute value of the magnetic field gradient on the B1 side of the edge of the magnetic pole 15 is greater than the absolute value of the magnetic field gradient on the B2 side. The region at the edge of the magnetic pole 15 where the absolute value of the magnetic field gradient is greater than the surrounding area is called the orbit concentration region 16.

[0022] The yoke 13 is provided with a plurality of through holes 17. The through holes 17 are used, for example, to extract the beam from the circular accelerator 1, to extract the coil 14 to the outside (not shown), and to install a radio frequency acceleration cavity 18.

[0023] The RF acceleration cavity 18 includes, for example, a dee electrode 19, a dummy dee electrode 20, and a resonator 21. The RF acceleration cavity 18 is connected to a RF power supply 40. When RF power is supplied from the RF power supply 40 to the RF acceleration cavity 18, a RF voltage required for accelerating the beam is induced in an acceleration gap 22 between the dee electrode 19 and the dummy dee electrode 20.

[0024] The circular accelerator 1 has, as devices for extracting a beam, a radio frequency kicker 23, a septum coil 24, and an extraction channel 25. On the outer periphery of the magnetic pole 15, a magnetic field distribution called a peeler magnetic field 26 and a regenerator magnetic field 27 is formed.

[0025] The procedure for accelerating and extracting a beam using the circular accelerator 1 will be described. The beam generated in the ion source 11 is injected into the main electromagnet 10 via the low-energy beam transport system 12. The beam injected into the main electromagnet 10 is deflected by the main magnetic field generated by the main electromagnet 10 and moves in an orbit inside the main electromagnet 10. The circular closed orbit formed by the beam orbiting inside the main electromagnet 10 is called the central orbit. The oscillation of the beam particles around the central orbit is called betatron oscillation. The plane on which the central orbit is formed is the orbital plane of the circular accelerator 1. The number of oscillations per revolution of the central orbit is called the tune.

[0026] Here, since the strength of the main magnetic field is constant during the operation of the circular accelerator 1, the beam forms a central orbit that differs for each energy. FIG. 3 is a schematic diagram of the central orbit for each energy in the circular accelerator 1.

[0027] The central trajectory 30 from the energy at the time of beam injection (hereinafter referred to as the injection energy) to a predetermined energy (hereinafter referred to as the minimum extraction energy) is a concentric circle whose radius increases as the energy increases.

[0028] A beam with energy higher than the minimum extraction energy forms an eccentric central orbit 32, with the orbit center approaching the center of the magnetic pole 15 as the energy increases. Central orbit 31 is the central orbit at the minimum extraction energy. Central orbit 33 is the central orbit at the maximum extraction energy. The energy at which the center O15 of the magnetic pole 15 and the center O33 of the central orbit coincide is called the maximum extraction energy. In the energy region above the minimum extraction energy, the central orbits are concentrated in the orbit concentration region 16.

[0029] The acceleration gap 22 accelerates the beam to a target energy by applying a radio frequency voltage (hereinafter referred to as acceleration voltage) parallel to the beam's direction of travel. The beam is accelerated while describing a spiral trajectory in a region where the energy is lower than the minimum extraction energy, and is accelerated while describing an eccentric spiral trajectory in a region where the energy is higher than the minimum extraction energy.

[0030] When the beam is accelerated to the target energy, the accelerating voltage applied from the accelerating gap 22 is stopped, and the beam orbits in the main magnetic field with the energy constant. The target energy at which the beam acceleration is stopped is selected within the range from the minimum extraction energy to the maximum extraction energy depending on the application of the circular accelerator 1.

[0031] After the beam acceleration stops, the RF kicker 23 applies a horizontal RF voltage (hereinafter referred to as extraction voltage) to the beam to increase the amplitude of the horizontal betatron oscillation of the beam particles. Since the RF kicker 23 is installed in the orbit concentration region 16, a single RF kicker 23 can apply RF voltage to all the energies within the extraction range.

[0032] Since the rotation frequency of the beam depends on the energy, the frequency of the extraction voltage is controlled to a value suitable for increasing the amplitude of the horizontal betatron oscillation. B , the horizontal tune of the beam N x , where n is a natural number, the frequency of the high-frequency voltage is F R is set to the vicinity of Eq. 1 or Eq. 2.

[0033] F R =nF B N x ...(Formula 1)

[0034] F R =nF B (1-N x )...(Formula 2)

[0035] The beam particles, whose amplitude of horizontal betatron oscillation has increased, come into contact with the peeler magnetic field 26 and the regenerator magnetic field 27, which are located on the outer periphery of the central orbit at the maximum extraction energy.

[0036] The peeler magnetic field 26 is a magnetic field that weakens the strength of the main magnetic field with increasing distance from the center of the magnetic pole 15. The regenerator magnetic field 27 is a magnetic field that strengthens the strength of the main magnetic field with increasing distance from the center of the magnetic pole 15. In order to form the peeler magnetic field 26 and the regenerator magnetic field 27, a shim structure (not shown) that changes the magnetic field distribution is formed in the magnetic pole 15.

[0037] The peeler magnetic field and the regenerator magnetic field may be formed using magnetic field correction coils arranged in the main electromagnet 10. When the peeler magnetic field 26 and the regenerator magnetic field 27 are formed using coils, the strength of the peeler magnetic field and the regenerator magnetic field can be adjusted according to the energy of the extracted beam. The peeler magnetic field 26 and the regenerator magnetic field 27 may be formed using both a shim structure formed in the magnetic pole 15 and magnetic field correction coils. In this case, the strength of the peeler magnetic field 26 and the regenerator magnetic field 27 can be adjusted while suppressing the current required for the magnetic field correction coils.

[0038] The amplitude of the horizontal betatron oscillation of the beam particles that come into contact with the peeler magnetic field 26 and the regenerator magnetic field 27 increases rapidly due to the resonance of the betatron oscillation, and the beam enters the septum coil 24 for beam extraction. The septum coil 24 deflects the beam in a direction away from the center of the magnetic pole 15 or in a direction toward the center of the magnetic pole 15. As a result, the beam is extracted from the accelerator 1 through the through hole 17.

[0039] The direction and amount by which the septum coil 24 deflects the beam are adjusted according to the energy of the extracted beam. Multiple septum coils 24 may be used to extract the beam. In this case, the horizontal position and gradient (the rate of change of the horizontal position along the beam progression direction) of the beam extracted from the circular accelerator 1 can be adjusted independently.

[0040] Instead of the septum coil 24, a magnetic field correction structure (hereinafter referred to as a magnetic channel) made of a magnetic material such as iron may be used to extract the beam. The magnetic channel does not require a power source for excitation, but the magnetic field strength cannot be adjusted according to the energy of the beam. However, by using the magnetic channel in combination with the septum coil 24, it is possible to extract beams of different energies from the same position while suppressing the excitation current of the septum coil 24.

[0041] A method for improving the efficiency of extracting a beam from the circular accelerator 1 will be described. The peeler magnetic field 26 has the effect of converging the beam in the vertical direction. The regenerator magnetic field 27 has the effect of diverging the beam in the vertical direction. Therefore, beam particles that come into contact with the peeler magnetic field 26 and the regenerator magnetic field 27 orbit in the main magnetic field while being subjected to both the converging force from the peeler magnetic field 26 and the diverging force from the regenerator magnetic field 27 in the vertical direction.

[0042] When the focusing force of the peeler magnetic field 26 is stronger than the divergence force of the regenerator magnetic field 27, the vertical tune of the beam particles increases. On the other hand, when the focusing force of the peeler magnetic field 26 is weaker than the divergence force of the regenerator magnetic field 27, the vertical tune of the beam particles decreases. The vertical tune of the circular accelerator 1 is adjusted to be in the range of 0 to 0.5 before the beam particles come into contact with the peeler magnetic field 26 and the regenerator magnetic field 27.

[0043] When the vertical tune increases and approaches 0.5, or decreases and approaches 0, resonance occurs in the vertical betatron oscillation, causing a sudden increase in the amplitude of the vertical betatron oscillation and resulting in beam loss. Therefore, the strengths of the peeler magnetic field 26 and the regenerator magnetic field 27 must be set so that the vertical tune of the beam particles does not change significantly.

[0044] In order to extract beams of different energies, an orbit concentration region 16 is formed in the circular accelerator 1. The main magnetic field in the orbit concentration region 16, like the peeler magnetic field 26, changes in a direction that weakens the main magnetic field as it moves away from the center of the magnetic pole 15.

[0045] The peeler magnetic field 26 and the regenerator magnetic field 27 form a magnetic field distribution in which the absolute value of the strength increases nonlinearly with the distance from the central orbit in order to obtain a magnetic field of the strength required for extraction while suppressing the influence on the accelerating beam. More specifically, the peeler magnetic field 26 and the regenerator magnetic field 27 are formed to have a sextupole magnetic field component whose strength is proportional to the square of the distance from the central orbit. Therefore, the absolute value of the magnetic field gradient formed by the peeler magnetic field 26 and the regenerator magnetic field 27 increases with distance from the central orbit. In contrast, the gradient of the main magnetic field in the orbit concentration region 16 is necessary to apply a focusing force to the accelerating beam, but is not necessary for beam extraction. Therefore, the magnetic field gradient of the main magnetic field in the orbit concentration region 16 is constant regardless of the distance from the central orbit.

[0046] The change in distance from the central orbit of a beam particle that comes into contact with the peeler magnetic field 26 and the regenerator magnetic field 27 will be explained using Figures 4 and 5. Figure 4 is a schematic diagram of a configuration compared to this embodiment. Figure 5 is a schematic diagram of the configuration of this embodiment.

[0047] In the comparative example shown in Fig. 4, the peeler magnetic field 26 and the regenerator magnetic field 27 are arranged approximately evenly on either side of the orbit-intensive region 16. In the present embodiment shown in Fig. 6, the boundary 28 between the peeler magnetic field 26 and the regenerator magnetic field 27 is set downstream of the orbit-intensive region 16. That is, in this embodiment, the pair of the peeler magnetic field 26 and the regenerator magnetic field 27 is shifted downstream in the beam traveling direction. The horizontal axis in Figs. 4 and 5 indicates the position in the beam traveling direction. The intersection 0 with the vertical axis corresponds to the orbit-intensive region 16.

[0048] When a peeler-shaped magnetic field (the peeler magnetic field 26 and the magnetic field of the orbit central region 16) and the regenerator magnetic field 27 are applied to the beam particles, the amplitude of the horizontal betatron oscillation increases with the peeler-shaped magnetic field as node n, as shown in Figures 4 and 5. In the vicinity of the central orbit, the magnetic field change of the orbit central region 16 is larger than the magnetic field change of the peeler magnetic field 26, so the amplitude of the horizontal betatron oscillation increases with the orbit central region 16 as node n.

[0049] In the comparative example of Fig. 4, where the peeler magnetic field 26 and the regenerator magnetic field 27 are arranged on either side of the orbit-focused region 16, the magnetic field gradient of the regenerator magnetic field 27 increases with an increase in the amplitude of the horizontal betatron oscillation, whereas the magnetic field gradient of the orbit-focused region 16 does not change from its initial state. Since the vertical focusing and diverging forces are proportional to the magnetic field gradient, in the comparative example of Fig. 4, the vertical diverging force becomes dominant with an increase in the amplitude of the horizontal betatron oscillation, and the vertical tune approaches 0, causing beam loss.

[0050] In contrast, in the present embodiment shown in FIG. 5, the boundary 28 between the peeler magnetic field 26 and the regenerator magnetic field 27 is located downstream of the orbit concentration region 16 in the beam propagation direction, so that when the amplitude of the horizontal betatron oscillation increases with the orbit concentration region 16 as node n, the strength of the peeler magnetic field received by the beam particles simultaneously increases nonlinearly.

[0051] As a result, in this embodiment, when the amplitude of the horizontal betatron oscillation increases, the gradient of the peeler-shaped magnetic field and the gradient of the regenerator magnetic field 27 increase simultaneously, so that the balance between the converging and diverging forces in the vertical direction is maintained, thereby suppressing the occurrence of beam loss.

[0052] 1 and 5, the center of the peeler magnetic field 26 is aligned with the center of the orbit-intensive region 16, but this is not limiting. The center of the peeler magnetic field 26 may be set at another location as long as the boundary 28 between the peeler magnetic field 26 and the regenerator magnetic field 27 satisfies the condition that it is located downstream of the center of the orbit-intensive region 16. If the center of the peeler magnetic field 26 is set upstream of the center of the orbit-intensive region 16, the regenerator magnetic field 27 approaches the orbit-intensive region 16, making it easier to bring low-energy beam particles in particular into contact with the regenerator magnetic field 27. EXAMPLES

[0053] The second embodiment will be described with reference to Fig. 6. In the following embodiments including this embodiment, differences from the first embodiment will be mainly described. Fig. 6 is a schematic diagram showing the change in distance from the central orbit of a beam particle that comes into contact with the peeler magnetic field 26 and the regenerator magnetic field 27.

[0054] In this embodiment, the regenerator magnetic field 27 is disposed closer to the outer periphery of the magnetic pole than the peeler magnetic field 26. In this case, it is possible to prevent beam particles with increased amplitude of horizontal betatron oscillation from contacting the regenerator magnetic field 27 before the peeler magnetic field 26, so that it is possible to suppress the occurrence of vertical resonance more effectively than in the first embodiment. EXAMPLES

[0055] The third embodiment will be described with reference to Figures 7 and 8. The circular accelerator 1A of this embodiment has orbit correction magnetic fields 29A and 29B. The circular accelerator 1A of this embodiment has the same configuration as the circular accelerator 1 described in the first embodiment, but differs from the first embodiment in that the orbit correction magnetic fields 29A and 29B are formed on the magnetic pole 15. Figure 7 is a schematic diagram showing a cross section of the circular accelerator 1A taken along a plane parallel to the beam orbit.

[0056] The orbit correction magnetic field 29A is a magnetic field in a direction that weakens the main magnetic field generated by the magnetic pole 15. The orbit correction magnetic field 29B is a magnetic field in a direction that strengthens the main magnetic field generated by the magnetic pole 15. The orbit correction magnetic fields 29A and 29B are formed at symmetrical positions with respect to the orbit concentration region 16. The strengths of the orbit correction magnetic fields 29A and 29B have different values ​​according to the energy of the orbital beams that intersect with the orbit correction magnetic fields 29A and 29B.

[0057] The orbit correction magnetic fields 29A and 29B can be formed by providing a shim structure (not shown) for generating a magnetic field to the magnetic pole 15. The orbit correction magnetic fields 29A and 29B are formed by forming a shim structure that partially changes the shape of the magnetic pole 15 so as to increase or decrease the gap between the upper and lower magnetic poles in the vertical direction. The strength of the orbit correction magnetic fields 29A and 29B can be changed according to the energy by changing the height and width of the shim structure from place to place.

[0058] The orbit correction magnetic fields 29A and 29B can also be formed by a correction magnetic field coil (not shown) inserted between the magnetic poles 15. In this case, the strength of the orbit correction magnetic fields 29A and 29B is controlled by the current flowing through the correction magnetic field coil. In order to set the strength of the orbit correction magnetic fields 29A and 29B to a value corresponding to the energy of the beam extracted from the circular accelerator 1A, the current flowing through the correction magnetic field coil is adjusted to a value corresponding to the beam energy.

[0059] When the orbit correction magnetic fields 29A and 29B are formed by the corrective magnetic field coils, the strength of the orbit correction magnetic fields 29A and 29B can be adjusted after the accelerator 1A is manufactured. The orbit correction magnetic fields 29A and 29B may be formed by using a shim structure and a corrective magnetic field coil in combination. In this case, the magnetic field distribution before the current is passed through the corrective magnetic field coils is already close to the state in which the necessary orbit correction magnetic fields 29A and 29B are added, so that the strength of the orbit correction magnetic fields 29A and 29B can be finely adjusted after the accelerator 1A is manufactured while suppressing the strength of the current passed through the corrective magnetic field coils.

[0060] A method for improving the beam extraction speed from the circular accelerator 1A by using the trajectory correction magnetic fields 29A and 29B will be described with reference to Fig. 8. Fig. 8 is a schematic diagram of the central trajectories for each energy in the circular accelerator 1A.

[0061] In the circular accelerator 1A of this embodiment, as in the first embodiment, the central orbit is concentric in the energy region lower than the minimum extraction energy, and the central orbit is eccentric in the energy region higher than the minimum extraction energy. Furthermore, in the circular accelerator 1A, since the orbit correction magnetic fields 29A and 29B are applied to the circulating beam, the central orbit moves toward the regenerator magnetic field 27 as the beam energy decreases in the energy region from the minimum extraction energy to the maximum extraction energy. The amount of movement of the central orbit becomes smaller as the energy increases, and becomes zero at the maximum energy, so that the shape of the central orbit 33 at the maximum energy is the same as that of the first embodiment.

[0062] In the circular accelerator 1A of this embodiment, in order to prevent the vertical betatron oscillation from becoming unstable when the beam is extracted, the boundary 28 between the peeler magnetic field 26 and the regenerator magnetic field 27 is located downstream of the orbit concentration region 16.

[0063] A low-energy beam has a smaller radius of the central orbit than a high-energy beam. Therefore, the amplitude of the horizontal betatron oscillation required to contact the regenerator magnetic field 16 increases as the energy decreases. For this reason, in the circular accelerator 1 of the first embodiment, when extracting a low-energy beam, it is necessary to greatly increase the amplitude of the horizontal betatron oscillation using the high-frequency kicker 23. Therefore, in the circular accelerator 1 of the first embodiment, it takes a long time before the beam extraction starts.

[0064] In contrast, in the circular accelerator 1A of this embodiment, the central orbit of the beam is corrected so that it approaches the regenerator magnetic field 27 as the energy decreases. Therefore, in the circular accelerator 1A, the amplitude of the horizontal betatron oscillation required for the beam to contact the regenerator magnetic field 27 in the low energy region is smaller than that in the first embodiment. As a result, in the circular accelerator 1A of this embodiment, the time required to start extracting the beam can be shortened in the low energy region, making it possible to complete the beam irradiation in a short time.

[0065] In the circular accelerator 1A of this embodiment, the central orbit is not corrected by the orbit correction magnetic fields 29A and 29B in the energy region lower than the minimum extraction energy. Therefore, the orbit correction magnetic fields 29A and 29B are not formed in the energy region lower than the minimum energy, that is, in the region where the central orbit for each energy is concentric. However, a sudden spatial change in the magnetic field strength may impair the stability of the circulating beam. Therefore, in the energy region lower than the minimum extraction energy, the strength of the orbit correction magnetic fields 29A and 29B is gradually attenuated as the energy decreases.

[0066] In the circular accelerator 1A of this embodiment, the orbit correction magnetic fields 29A and 29B are formed symmetrically with respect to the orbit centralized region 16, but this is just one example. The orbit correction magnetic fields 29A and 29B may have any shape as long as they are distributed to bring the central orbit closer to the regenerator magnetic field as a whole. The orbit correction magnetic fields 29A and 29B do not have to be formed symmetrically with respect to the orbit centralized region 16.

[0067] On the other hand, forming the trajectory correction magnetic fields 29A and 29B symmetrically with respect to the trajectory concentration region 16 has the advantage that the amount of movement of the central trajectory can be easily calculated. The trajectory correction magnetic fields may be formed in one place, or in three or more places. If the trajectory correction magnetic fields are formed in multiple places, the strength of the trajectory correction magnetic field per place is reduced. This makes it easier to manufacture the shim structure and the magnetic field correction coils, but complicates the design of the magnetic field distribution.

[0068] The circular accelerator 1A of this embodiment improves the beam extraction efficiency, similarly to the circular accelerator 1 of the first embodiment. Furthermore, in the circular accelerator 1A of this embodiment, the time until the beam extraction can be shortened in the extraction of a low energy beam, so that the average beam current can be improved. As a result, when the circular accelerator 1A of this embodiment is used in a particle beam therapy system, it becomes possible to complete the treatment in a shorter time than when the circular accelerator 1 described in the first embodiment is used in the particle beam therapy system. EXAMPLES

[0069] A particle therapy system 100 using a circular accelerator 1 will be described with reference to Fig. 9. The particle therapy system 100 shown in Fig. 9 includes, for example, the circular accelerator 1, a beam transport system 50, and a rotating gantry 51, and these main components 1, 50, and 51 are each connected to a control device 60.

[0070] The control device 60 operates the circular accelerator 1 and controls the beam transport system 50 and the rotating gantry 51 based on a treatment plan created in advance by a treatment planning device (not shown), and irradiates the beam to an affected area 53 of a patient 52.

[0071] The particle beam therapy system 100 can change the energy of the beam extracted from the circular accelerator 1 within a range used in particle beam therapy (for example, a range from 70 MeV to 230 MeV). Therefore, the particle beam therapy system 100 does not need to install a degrader in the middle of the beam transport system 50 to adjust the energy irradiated to the patient 52, as in a particle beam therapy system using a cyclotron as an accelerator.

[0072] The particle beam therapy system 100 of this embodiment can prevent the occurrence of beam loss in the degrader, and therefore can obtain a high beam current regardless of the energy of the beam irradiated to the patient 52. Furthermore, the particle beam therapy system 100 of this embodiment can suppress the beam loss when extracting the beam from the circular accelerator 1, and therefore can improve the current of the beam irradiated to the patient 52, and can complete the treatment in a shorter time.

[0073] The present invention is not limited to the above-described embodiment. A person skilled in the art can make various additions and modifications within the scope of the present invention. The above-described embodiment is not limited to the configuration example shown in the attached drawings. The configuration and processing method of the embodiment can be appropriately changed within the scope of achieving the object of the present invention.

[0074] In addition, each component of the present invention can be selected arbitrarily, and the invention having the selected configuration is also included in the present invention. Furthermore, the configurations described in the claims can be combined in combinations other than those explicitly stated in the claims. [Explanation of symbols]

[0075] 1: circular accelerator, 10: main electromagnet, 11: ion source, 12: low energy beam transport system, 13: yoke, 14: coil, 15: magnetic pole, 16: orbit concentration region, 17: through hole, 18: radio frequency acceleration cavity, 19: dee electrode, 20: dummy dee electrode, 21: resonator, 22: acceleration gap, 23: radio frequency kicker, 24: septum coil, 25: channel, 26: peeler magnetic field, 27: regenerator magnetic field, 28: boundary, 29A, 29B: orbit correction magnetic field, 30 to 33: central orbit, 40: radio frequency power supply, 50: beam transport system, 51: rotating gantry, 60: control device, 100: particle beam therapy system

Claims

1. A circular accelerator that accelerates and extracts a beam of charged particles orbiting in a magnetic field, the closed orbit for each energy of the beam is eccentric; A first magnetic field region having a magnetic field gradient in which the magnetic field weakens toward the outer periphery; a second magnetic field region having a magnetic field gradient in which the magnetic field becomes stronger toward the outer periphery; The boundary between the first magnetic field region and the second magnetic field region is located downstream in the traveling direction of the beam from a predetermined region where the interval between the closed orbits for each energy of the beam is narrowest. Circular accelerator.

2. 2. The circular accelerator of claim 1, The boundary between the first magnetic field region and the second magnetic field region is located downstream in the beam propagation direction of a predetermined region in which the interval between closed orbits for each energy of the beam is narrowest, and the center of the first magnetic field region in the beam propagation direction is located upstream in the beam propagation direction of the predetermined region. Circular accelerator.

3. 3. A circular accelerator according to claim 1, The boundary between the first magnetic field region and the second magnetic field region is in a range from a point located downstream of the predetermined region in the traveling direction of the beam to a point less than 90 degrees away. Circular accelerator.

4. 4. A circular accelerator according to claim 1, The distance from the central trajectory of the beam to the second magnetic field region is greater than the distance from the central trajectory of the beam to the first magnetic field region. Circular accelerator.

5. A particle beam therapy system using the circular accelerator according to any one of claims 1 to 4.

Images