Circular accelerator and particle therapy system

Abstract

In the present invention, a high-frequency acceleration electric field is used to accelerate beam particles, and the trajectory radius of the beam particles orbiting in a magnetic field changes depending on energy. The present invention comprises a magnetic pole 15 in which layered magnetic bodies 15aa, 15ba, 15ca, 15da, 15ab, 15bb, 15cb, 15db are stacked, and auxiliary coils 16 embedded in one or more of the layered magnetic bodies 15aa, 15ba, 15ca, 15da, 15ab, 15bb, 15cb, 15db constituting the magnetic pole 15. The auxiliary coils 16 are disposed such that magnetic field regions generated on the beam orbit by the auxiliary coils 16 overlap.

Description

Circular accelerator and particle therapy system

[0001] The present invention relates to a circular accelerator for accelerating heavy ions such as protons or carbon ions, and a particle beam therapy system.

[0002] Patent Document 1 describes a charged particle beam deflection device that deflects a charged particle beam along a central orbit having a predetermined radius of curvature, comprising first and second magnetic poles arranged opposite each other across a space through which the charged particle beam passes, a main coil wound around each of the first and second magnetic poles and forming a magnetic field in the space from the first magnetic pole to the second magnetic pole, a first auxiliary coil arranged on the inner side of the central orbit and forming a magnetic field in the space from the second magnetic pole to the first magnetic pole, and a second auxiliary coil arranged on the outer side of the central orbit and forming a magnetic field in the space from the first magnetic pole to between the second magnetic poles, and that forms a magnetic field region inside the central orbit that is weaker than the magnetic field at the central orbit, and forms a magnetic field region outside the central orbit that is stronger than the magnetic field at the central orbit.

[0003] Non-Patent Document 1 describes that the magnetic field distribution of the main magnetic field is corrected so as to satisfy the isochronous condition by exciting a trim coil installed between the magnetic poles.

[0004] Patent No. 5622598

[0005] TEION KOGAKU (J. Cryo. Soc. Jpn.) Vol. 43 No. 11 (2008)

[0006] High-energy nuclear beams used in particle beam therapy and physics experiments are generated using accelerators.

[0007] One type of accelerator is a circular accelerator, which deflects beam particles using a magnetic field generated by an electromagnet to make them orbit in a circular orbit, and accelerates the beam particles with each orbit using a high-frequency electric field generated in an accelerating cavity installed in the orbit.

[0008] Circular accelerators include static magnetic field accelerators, such as cyclotrons, synchrocyclotrons, and variable energy accelerators, in which the radius of curvature of the beam particle's orbit changes depending on the energy, and dynamic magnetic field accelerators, such as synchrotrons, in which the radius of curvature of the beam particle's orbit does not depend on the energy.

[0009] In a dynamic magnetic field accelerator, the beam can only be injected at a timing when the magnetic field strength corresponds to the energy of the injected beam, but in a static magnetic field accelerator, there are no restrictions on the injection timing due to changes in magnetic field strength, so quasi-continuous beam injection is possible and a high dose rate can be achieved.

[0010] However, in a synchrocyclotron, which is a type of static magnetic field accelerator, the magnetic field distribution is symmetrical with respect to the axis of the beam injection point, and the radius of curvature of the beam particle's trajectory increases with acceleration. Therefore, the beam cannot be extracted unless it is accelerated until it reaches the extraction equipment installation position, and the extraction energy is limited to the maximum value and cannot be changed.

[0011] Therefore, a degrader is installed in the beam transport system to reduce the energy of the beam extracted from the accelerator in order to irradiate a beam of the desired energy. However, beam loss occurs when the beam passes through the degrader, resulting in a decrease in the dose rate in the low-energy region.

[0012] In a static magnetic field type variable energy accelerator, which is an example of a static magnetic field accelerator that solves this problem, the magnetic field distribution is asymmetric with respect to the axis of the beam injection point. As a result, in this type of accelerator, the orbit center positions for each energy are different and eccentric in one direction, and there is a point where the orbits for each energy converge. By installing extraction equipment at the orbit convergence point, it is possible to extract a beam of the desired energy, and since a degrader is not required, there is no decrease in dose rate due to the degrader. Due to its ability to rapidly repeat acceleration and its energy tunability, variable energy accelerators are expected to achieve high dose rates across the entire range of extraction energies.

[0013] In general, the distribution of the main magnetic field in an accelerator electromagnet depends primarily on the magnetic pole shape, and the strength of the main magnetic field depends primarily on the magnetomotive force of the main coil. Therefore, in electromagnet design, the magnetic pole shape is determined using three-dimensional magnetic field simulations to create a magnetic field distribution that allows beam particles to circulate stably.

[0014] However, due to factors such as processing errors and installation errors, the manufactured electromagnet may not achieve the desired magnetic field distribution, making it necessary to correct the magnetic field distribution. One method for correcting the magnetic field distribution is to change the magnetic pole shape by shimming. Magnetic field correction by shimming requires disassembling the electromagnet to expose the magnetic pole surface, and requires repeated work of placing shims on the magnetic pole surface to adjust the magnetic field correction amount and measuring the magnetic field, which takes a lot of time for iteration.

[0015] In Non-Patent Document 1, the magnetic field can be corrected in a short time by changing the current value input to the coil based on the results of magnetic field measurements and beam position measurements using a beam monitor, without the need to disassemble the electromagnet. On the other hand, in order to install the coil so as not to interfere with the beam, the distance between the magnetic poles must be widened. As a result, the radius of curvature of the beam orbit increases due to a decrease in magnetic field strength, and the beam circulation area decreases due to an increase in the leakage magnetic field area. Therefore, in order to ensure the beam circulation area, the magnet must be made larger.

[0016] In order to arrange magnetic field distribution correction coils without increasing the distance between magnetic poles in a cyclotron, synchrocyclotron, or static magnetic field variable energy accelerator, a method of embedding the coils in an electromagnet as in Patent Document 1 is being considered.

[0017] Patent Document 1 shows an example of an electromagnet used in a synchrotron or the like that deflects a charged particle beam along a central orbit having a predetermined radius of curvature, in which an auxiliary coil capable of correcting the magnetic field distribution is embedded inside the yoke, thereby arranging the electromagnet without increasing the distance between the magnetic poles.

[0018] In Patent Document 1, the two auxiliary coils located inside and outside the central beam orbit have independent magnetic field generating regions and are used to create a magnetic field strength difference in the radial direction.

[0019] On the other hand, when applying a configuration in which auxiliary coils are embedded in electromagnets as in Patent Document 1 to adjust the magnetic field of a cyclotron, synchrocyclotron, or static magnetic field variable energy accelerator, it is necessary to form a complex magnetic field distribution that changes nonlinearly toward the accelerator's periphery so that the beam circulates stably throughout the beam orbital region from injection to extraction. To achieve this, the magnetic field generating regions of multiple coils must overlap and be arranged so that the coils do not interfere with each other, which is difficult to achieve with an auxiliary coil arrangement in which each magnetic field generating region is independent as in Patent Document 1.

[0020] The present invention provides a circular accelerator and a particle beam therapy system that are capable of forming and correcting a magnetic field distribution with high precision while keeping the size of the accelerator's electromagnets small.

[0021] The present invention includes multiple means for solving the above-mentioned problems, and one example thereof is a circular accelerator that accelerates beam particles using a radio-frequency acceleration electric field, and the orbital radius of the beam particles circulating in a magnetic field changes depending on the energy, the circular accelerator comprising a magnetic pole section made of stacked magnetic materials, and coils embedded inside one or more layered magnetic materials that make up the magnetic pole section, and the coils are arranged so that the magnetic field regions generated on the beam circulation orbit by each of the coils overlap.

[0022] According to the present invention, it is possible to form and correct the magnetic field distribution with high precision while keeping the electromagnet size of the accelerator small. Problems, configurations, and effects other than those described above will become apparent from the following description of the embodiments.

[0023] 11 is a schematic diagram of the overall configuration of a particle beam therapy system of an embodiment; FIG. 12 is an overall outline of an accelerator of an embodiment; FIG. 13 is a diagram of the internal equipment layout of an accelerator of an embodiment; FIG. 14 is a diagram of the layout of embedded coils as seen from the top of the accelerator of an embodiment; FIG. 15 is a diagram of the layout of embedded coils as seen from a cross section of the accelerator of an embodiment; FIG. 16 is a schematic diagram of the magnetic field distribution of the accelerator of an embodiment; FIG. 17 is a diagram explaining a method for determining the position of embedded coils in the accelerator of an embodiment; FIG. 18 is a diagram showing an outline of the connection between the coils and a power supply of the accelerator of an embodiment; FIG. 19 is an explanatory diagram of a four-split coil which is a modified example 2 of the auxiliary coil of the accelerator of an embodiment; FIG. 19 is an explanatory diagram of a five-split coil which is a modified example of the auxiliary coil of the accelerator of an embodiment; FIG. 11 is a diagram showing an outline of the connection between the split coils and a power supply of the modified example 2 of FIG.

[0024] Embodiments of the circular accelerator and particle beam therapy system of the present invention will be described with reference to Figures 1 to 12. In the drawings used in this specification, identical or corresponding components are designated by identical or similar reference numerals, and repeated explanations of these components may be omitted.

[0025] First, the overall configuration of a particle therapy system equipped with an accelerator will be described with reference to Fig. 1. Fig. 1 is a diagram showing an outline of the overall configuration of a particle therapy system according to an embodiment.

[0026] The particle beam therapy system 10 of this embodiment shown in FIG. 1 comprises an accelerator 1, a rotating gantry 2, an irradiation device 3 including a scanning magnet, a treatment table 4, a control device 7 that controls the operation of each of these devices, and an accelerator control device 8.

[0027] In a particle beam therapy system 10, a beam extracted from an accelerator 1 is transported to an irradiation device 3 by a rotating gantry 2. The transported ion beam is shaped to match the affected area by adjusting the beam energy in the irradiation device 3 and / or the accelerator 1, and a predetermined amount of ion beam is irradiated onto the affected area of ​​a patient 5 lying on a treatment couch 4.

[0028] The irradiation device 3 includes a dose monitor, which monitors the dose irradiated to each irradiation spot on the patient 5. Based on this dose data, the control device 7 calculates the required dose for each irradiation spot and uses this as input data for the accelerator control device 8. The accelerator control device 8 controls the injection, acceleration, and extraction of the charged particle beam in the accelerator 1, and supplies a beam of the required dose and energy.

[0029] Next, an accelerator according to a preferred embodiment of the present invention will be described below with reference to Figures 2 to 5. Figure 2 shows an outline of the overall configuration of the accelerator 1.

[0030] The accelerator 1 shown in FIG. 2 is a circular accelerator that accelerates beam particles using a frequency-modulated high-frequency accelerating electric field, and the orbital radius of the beam particles circulating in the magnetic field changes depending on the energy.

[0031] As shown in FIG. 2, the accelerator 1 excites a main magnetic field in a region through which the beam passes (hereinafter referred to as the beam passing region) by means of a magnet 11 that can be separated into upper and lower parts, and the inside of the beam passing region is evacuated.

[0032] The magnet 11 has a plurality of through-holes, including an extracted beam through-hole 111 for extracting the accelerated beam, coil connection through-holes 112 and 113 for extracting the internal coil to the outside, and a high-frequency power input through-hole 114, which are provided on the connecting surfaces of the upper and lower magnetic poles. A high-frequency cavity 23 is installed through the high-frequency power input through-hole 114.

[0033] As will be described later, the high frequency cavity 23 is provided with a dee electrode portion for acceleration and a rotary variable capacitor 212 .

[0034] An ion source 12 is installed above the magnet 11 at a position offset from the center and at a different position in the radial direction, and a beam is injected into the accelerator 1 through a beam injection through-hole 115 .

[0035] Next, the structure of the accelerator 1 will be explained using a cross-sectional view of the XY plane at the vertical center of the magnet 11 shown in Fig. 3. Also, the coil arrangement of the accelerator 1 of this embodiment will be explained using a projection view onto the XY plane between the layered magnetic bodies 15aa and 15ab of the magnetic pole 15 shown in Fig. 4, and a YZ cross-sectional view of the magnet 11 shown in Fig. 5.

[0036] 3 and 4, a cylindrical return yoke 14 is located at the outermost side of the magnet 11, reducing leakage magnetic flux and concentrating magnetic flux in an internal cylindrical beam passing region 22. An annular main coil 13 is installed inside the return yoke 14 along the inner wall. Magnetic poles 15 are installed inside the main coil 13 so as to face each other vertically and form the upper and lower boundaries of the beam passing region 22.

[0037] As shown in Figure 5, the magnetic pole 15 has a structure in which layered magnetic materials 15aa, 15ba, 15ca, and 15da are stacked on the upper side of the orbital beam on the vertically upper side, and a structure in which layered magnetic materials 15ab, 15bb, 15cb, and 15db are stacked on the lower side of the orbital beam, and auxiliary coils 16aa, 16ba, 16ca, 16da, 16ab, 16bb, 16cb, and 16db are embedded inside all of the layered magnetic materials 15aa, 15ba, 15ca, 15da, 15ab, 15bb, 15cb, and 15db, respectively.

[0038] The cooling structure of the auxiliary coil 16 may be configured such that the auxiliary coil 16 is made of a hollow conductor capable of internal cooling, and a coolant flows through a cooling channel formed in the hollow portion of the auxiliary coil 16. In this cooling structure, power and coolant can be supplied to the auxiliary coil 16 by using a power supply line 24 incorporating a cooling supply pipe 25 that supplies coolant to the hollow portion of the auxiliary coil 16. By configuring the auxiliary coil 16 as a hollow conductor capable of internal cooling, the surface area in contact with the coolant and the auxiliary coil 16 can be increased, thereby improving cooling efficiency. Therefore, the cooling structure occupies a smaller area within the magnetic pole, making it more suitable for miniaturization. Note that the cooling structure of the auxiliary coil 16 is not limited to an internal cooling structure. Alternatively, a cooling pipe for flowing coolant to cool the auxiliary coil 16 may be embedded inside the layered magnetic bodies 15aa, 15ba, 15ca, 15da, 15ab, 15bb, 15cb, and 15db.

[0039] Although the auxiliary coil 16 is embedded in all of the layered magnetic material, it may be embedded in only a portion of the layered magnetic material. Also, although the thickness of the layered magnetic material and the thickness of the auxiliary coil 16 are shown to be the same, they do not have to be the same and are not limited thereto. Furthermore, although the thickness of each layer of the layered magnetic material is shown to be approximately the same, they do not have to be the same and may be different thicknesses.

[0040] The magnetic pole 15, which is composed of stacked layered magnetic materials 15aa, 15ba, 15ca, 15da, 15ab, 15bb, 15cb, and 15db, is positioned vertically at the same position as the main coil 13 that generates the base magnetic field, as shown in Figure 5, and is positioned horizontally within the area surrounded by the main coil 13 that generates the base magnetic field, as shown in Figures 4 and 5.

[0041] In particular, as shown in FIG. 4, when the auxiliary coil 16 is projected onto the XY plane, the auxiliary coil 16 is arranged in an eccentric circular shape, and as shown in FIG. 5, when viewed in the YZ cross section, the auxiliary coil is arranged shifted in the vertical direction.

[0042] Furthermore, among the auxiliary coils 16aa, 16ba, 16ca, 16da, 16ab, 16bb, 16cb, and 16db, the radius of the auxiliary coils 16aa and 16ab, which are closest in the vertical direction to the beam circulation orbit, is smallest, and the radii increase in the order of auxiliary coils 16ba and 16bb, auxiliary coils 16ca and 16cb, and auxiliary coils 16da and 16db, i.e., as they move away from the beam.

[0043] Furthermore, an incident point 120 of the beam particles is contained within a magnetic field region generated by the layered magnetic materials 15aa, 15ba, 15ca, 15da, 15ab, 15bb, 15cb, and 15db by the auxiliary coil 16.

[0044] The method of fixing the auxiliary coil 16 to each of the layered magnetic bodies 15aa, 15ba, 15ca, 15da, 15ab, 15bb, 15cb, and 15db, and the method of fixing the layered magnetic bodies 15aa, 15ba, 15ca, 15da, 15ab, 15bb, 15cb, and 15db to the return yoke 14 are not particularly limited, but examples include a method in which the layered magnetic bodies are aligned using position adjustment pins, and then the layered magnetic bodies are fastened together individually or collectively, and the integrated magnetic poles are fastened to the yoke.

[0045] The magnet 11 is magnetized by passing a predetermined excitation current through the main coil 13 and auxiliary coil 16, and a magnetic field is generated in the beam passage region 22 between the magnetic poles 15. The main coil 13 excites a magnetic field that serves as the base of a desired magnetic field distribution, and the auxiliary coil 16 excites a magnetic field that corrects the base magnetic field distribution. Note that the auxiliary coil 16 is not limited to exciting a magnetic field that corrects the base magnetic field distribution, and it can also be excited for the purpose of forming the main magnetic field distribution.

[0046] In the accelerator 1 of this embodiment, as long as a desired magnetic field distribution can be generated in the beam circulation region from beam injection to extraction, it does not matter how the distance (gap) between the magnetic poles 15 changes depending on the position within the magnet. Furthermore, the shape of the magnetic poles 15 and the arrangement of the auxiliary coils 16 are symmetrical with respect to a plane (orbital plane) passing through the gap center, and on the orbital plane, only the magnetic field component is in a direction perpendicular to the orbital plane.

[0047] The shape of magnetic pole 15 is symmetrical with respect to axis AA' in the central plane, resulting in a symmetrical magnetic field distribution. If a right-handed coordinate system is defined with incidence point 120 as the origin and the Y-axis extending from the origin toward the center of accelerator 1, the magnetic field distribution of the cyclotron is symmetrical with respect to the origin, but the magnetic field distribution formed by magnetic pole 15 is not symmetrical with respect to the origin, and the gradient of the magnetic field distribution in the positive Y-axis direction becomes gentler at distances from incidence point 120.

[0048] As a result, in the low energy region below the extraction energy, the orbit is centered near the injection point 120, just like a cyclotron, but as the energy of the beam particles increases, the center of the beam orbit moves in one direction on the same plane, resulting in the formation of an orbit convergence point where the orbits are densely concentrated. Particles that have deviated horizontally and vertically from the design orbit in the magnetic field inside the accelerator 1 configured as described above receive a restoring force that returns them to the design orbit, causing them to oscillate (betatron oscillation) around the design orbit, thereby orbiting stably.

[0049] In the accelerator 1, the beam particles are accelerated every time they pass between the electrodes by a high frequency electric field excited between the dee electrodes 31 and 32. The dee electrodes 31 and 32 have shapes symmetrical with respect to the yz plane.

[0050] The high-frequency cavity 23 excites an electric field in the acceleration gap in a λ / 4 resonance mode. High-frequency power is introduced from an external high-frequency power supply through a coupler 211. The high-frequency cavity 23 is connected to dee electrodes 31 and 32 inserted in the gap, and a high-frequency electric field is excited between the dee electrodes 31 and 32 and a ground electrode 35.

[0051] In the accelerator 1 of this embodiment, in order to excite a high frequency electric field in synchronization with the rotation of the beam, the frequency of the electric field is modulated in accordance with the energy of the circulating beam.

[0052] In a cavity using a resonant mode such as that used in this embodiment, it is necessary to sweep the high-frequency frequency over a range wider than the width of the resonance. To do this, it is necessary to change the resonant frequency of the high-frequency cavity 23. This control is performed by changing the capacitance of the rotary variable capacitor 212 installed at the end of the high-frequency cavity 23. The rotary variable capacitor 212 controls the capacitance generated between the conductor plates (rotor and stator) directly connected to the rotating shaft and the external conductor by the rotation angle of the rotating shaft 213. In other words, the rotation angle of the rotating shaft 213 is changed in accordance with the acceleration of the beam.

[0053] The accelerator 1 is equipped with an extraction septum magnet 40 and a massless septum coil 50 for generating a kicker magnetic field for beam extraction. The kicker magnetic field uses a massless septum system that applies a magnetic field only to a specific position in the accelerator's radial direction. The kicker magnetic field is excited by passing a current through a pair of coils that are installed symmetrically in a direction perpendicular to the beam orbital plane.

[0054] To extract a desired beam with a desired energy, one or more coils of the massless septum coil 50 are selected based on the desired energy, and a desired excitation current is applied to them. When no current is applied to the massless septum coil 50, the beam with the desired energy revolves along its designed trajectory. However, when current is applied to the massless septum coil 50, the beam that reaches the desired energy deviates from the trajectory due to a kick magnetic field generated by the massless septum coil 50. As described above, the deviated beam oscillates stably around the designed trajectory. In other words, the massless septum coil 50 excites betatron oscillations within the trajectory plane. When the position of the kick generated by the massless septum coil 50 and the position of the focusing point are appropriately positioned, the kick generated by the massless septum coil 50 can displace the beam radially outward at the focusing point.

[0055] The behavior of the beam from injection into the accelerator 1 of this embodiment to extraction will be described below.

[0056] The operation of the accelerator 1 consists of three steps: injection, acceleration, and extraction.

[0057] In the injection step, a beam of low-energy ions is output from the ion source 12, which is located at a radially different position from the center of gravity of the annular main coil 13, and the beam is guided to the beam passing region 22 through the injection point 120.

[0058] In the acceleration step, the beam injected into the beam passing region 22 is accelerated by the radio frequency electric field, and its energy increases. As the energy increases, the radius of rotation of the orbit increases and the magnetic field on the orbit decreases. As a result, the orbital period of the beam particles increases, and the period of the accelerating radio frequency electric field is controlled to synchronize with the orbital period of the beam particles, so that the period of the accelerating radio frequency electric field also increases.

[0059] In the subsequent acceleration step, the beam is accelerated while ensuring directional stability due to the radio-frequency electric field. That is, rather than passing through the acceleration gap when the radio-frequency electric field is at its maximum, the beam passes through the acceleration gap when the radio-frequency electric field is decreasing. Since the frequency of the radio-frequency electric field and the orbital frequency of the beam are synchronized at an integer multiple ratio, particles accelerated at a given phase of the acceleration electric field are accelerated at the same phase in the next turn. On the other hand, particles accelerated at a phase earlier than the acceleration phase are accelerated at a greater rate than particles accelerated at the acceleration phase, so they are accelerated at a delayed phase in the next turn. Conversely, particles accelerated at a phase later than the acceleration phase are accelerated at a smaller rate than particles accelerated at the acceleration phase, so they are accelerated at an advanced phase in the next turn. In this way, the accelerating particles oscillate (synchrotron oscillation) around a certain designed phase, gradually accelerating until they reach the specified energy to be extracted.

[0060] In the extraction step, the beam that has reached a predetermined energy level is subjected to the action of a kick magnetic field caused by the massless septum coil, and travels from the convergence point to the extraction channel, which is the beam extraction path 140 formed by the extraction septum electromagnet 40, and is extracted out of the accelerator 1.

[0061] Next, we will explain the coil arrangement method, which is a feature of this embodiment, for forming a magnetic field that stably accelerates the beam in the beam circulation region from beam injection to extraction using the main coil 13 and auxiliary coil 16, particularly the method for determining the embedding position of the coil within the magnetic pole.

[0062] For example, the magnetic field distribution of the accelerator 1 is ｃ and the position Y where the magnetic field strength is at its maximum value ｍ is asymmetric with respect to Y ｍ Consider a case where the magnetic field gradient is steeper on the negative side and gentler on the positive side (see Figure 6). In this example, the magnetic pole 15 is assumed to have a flat shape, and a method for determining the conditions for forming such a complex magnetic field distribution will be described using the magnetic pole shape, coil arrangement, and coil current value as parameters.

[0063] The final embedded positions, number of embedded auxiliary coils 16, and magnetomotive force are determined by fine-tuning these parameters and repeatedly performing three-dimensional magnetic field simulations to search for appropriate parameters, but the initial values ​​of each parameter are determined using an approximate calculation method that assumes that no magnetic field is generated outside the coil and that a uniform dipole magnetic field is generated inside.

[0064] In this case, a stepped magnetic field distribution is formed by superimposing multiple dipole magnetic fields generated by the auxiliary coil 16. The position and magnetomotive force of the auxiliary coil 16 are determined as initial conditions for the iteration so that the peak of the stepped magnetic field is on the desired magnetic field distribution.

[0065] 7 shows an example in which the auxiliary coil 16 is arranged to form a target magnetic field distribution. When only the main coil 13 is present, the base magnetic field |B ｚ１ | is formed.

[0066]

[0067] Therefore, the auxiliary coils 16 are arranged so that the magnetic field regions generated on the beam orbit by the individual auxiliary coils 16 overlap. For example, when the difference between the base magnetic field and the target magnetic field is a certain value Δ|B ｚ Position Y where |２ , Y ９ Δ|B ｚ In this state, the difference between the base magnetic field and the target magnetic field is a certain value Δ|B ｚ Position Y where | ３ , Y ８ Δ|B ｚ The auxiliary coil 16b is further embedded so as to generate a magnetic field of |. By repeating the above procedure, a multi-stage magnetic field distribution is formed as shown in the following equation (2).

[0068]

[0069] At this time, depending on the designed magnetic field distribution, it is conceivable that one or more of the auxiliary coils 16 may be excited in an opposite phase to the other auxiliary coils 16 .

[0070] In reality, it is not possible to generate a step-like magnetic field distribution, and it is thought that the magnetic field distribution will be as shown by the solid line in the figure.

[0071] The arrangement of the auxiliary coils 16 determined in this manner is used as the initial condition for the iteration of the three-dimensional magnetic field simulation, and in areas where there is a large difference between the target magnetic field distribution and the generated magnetic field, auxiliary coils 16 are added, moved, the magnetomotive force is changed, the magnetic pole shape is changed, etc., and the generated magnetic field distribution is gradually brought closer to the target magnetic field distribution, thereby determining the final arrangement and magnetomotive force of the auxiliary coils 16.

[0072] Ultimately, the magnetomotive force of each coil is determined based on the value determined by the three-dimensional magnetic field simulation, and is finely adjusted by magnetic field measurement to form the designed magnetic field with higher precision.

[0073] Next, a specific example of the power supply system for the auxiliary coil 16 will be described with reference to Fig. 8. The operation of this embodiment will be described. Fig. 8 shows an outline of the connection between the coil and the power supply of the accelerator of this embodiment.

[0074] As shown in FIG. 8 , the upper coil 13a and the lower coil 13b of the main coil 13 are connected to a main coil power supply 17, the upper auxiliary coil 16aa and the lower auxiliary coil 16ab are connected to an auxiliary coil power supply 18, the upper auxiliary coil 16ba and the lower auxiliary coil 16bb are connected to an auxiliary coil power supply 19, the upper auxiliary coil 16ca and the lower auxiliary coil 16cb are connected to an auxiliary coil power supply 20, and the upper auxiliary coil 16da and the lower auxiliary coil db are connected to an auxiliary coil power supply 21, each via a dedicated power supply line 24.

[0075] 8 shows an example in which there are four auxiliary coil power supplies 18, 19, 20, and 21, matching the number of auxiliary coils 16, but in reality, the same number of auxiliary coil power supplies as the number of auxiliary coils 16 determined in the electromagnet design is required. That is, as described above, the auxiliary coils 16 are provided to correct the difference between the target magnetic field distribution and the generated magnetic field, and therefore the magnetic fields to be excited in each of the auxiliary coils 16 are often different, so it is desirable to connect an independent power supply to each.

[0076] Furthermore, just as in the start-up phase of a normal accelerator, when the main coil power supply is turned on to excite the main coil, in the accelerator 1 of this embodiment, all auxiliary coil power supplies are also turned on at the same time that the main coil power supply is turned on during the start-up phase.

[0077] 4 and other figures illustrate the case where circular coils are used as the individual auxiliary coils 16. However, the shape of the auxiliary coils 16 embedded inside the magnetic poles 15 is not limited to circular coils. Modifications will be described below with reference to FIGS. 9 to 12. FIG. 9 is an explanatory diagram of a four-split coil, which is a modified example of the auxiliary coil, and FIG. 10 is a diagram showing an outline of the connection between the split coils of FIG. 9 and a power supply. FIG. 11 is an explanatory diagram of a five-split coil, which is a modified example of the auxiliary coil, and FIG. 12 is a diagram showing an outline of the connection between the split coils of FIG. 11 and a power supply.

[0078] As shown in Fig. 9, it is also possible to form split auxiliary coils 161, 162, 163, and 164 that are split into four in the circumferential direction. In this case, as shown in Fig. 10, a power supply 171 is connected to the upper split auxiliary coil 161a and the lower split auxiliary coil 161b (collectively split auxiliary coil 161), a power supply 172 is connected to the upper split auxiliary coil 162a and the lower split auxiliary coil 162b (collectively split auxiliary coil 162), a power supply 173 is connected to the upper split auxiliary coil 163a and the lower split auxiliary coil 163b (collectively split auxiliary coil 163), and a power supply 174 is connected to the upper split auxiliary coil 164a and the lower split auxiliary coil 164b (collectively split auxiliary coil 164).

[0079] Furthermore, the present invention is not limited to the configuration shown in FIG. 9 in which the coil is divided into four equal parts in the circumferential direction, but as shown in FIG. 11 , it is also possible to adopt a configuration in which the coil is divided into two parts in the radial direction and the outer diameter side is divided into four parts in the circumferential direction, resulting in a total of five divided auxiliary coils 261, 262, 263, 264, and 265.

[0080] In this case as well, as shown in FIG. 12 , a power supply 271 is connected to the upper split auxiliary coil 261 a and the lower split auxiliary coil 261 b (collectively referred to as the split auxiliary coil 261), a power supply 272 is connected to the upper split auxiliary coil 262 a and the lower split auxiliary coil 262 b (collectively referred to as the split auxiliary coil 262), a power supply 273 is connected to the upper split auxiliary coil 263 a and the lower split auxiliary coil 263 b (collectively referred to as the split auxiliary coil 263), a power supply 274 is connected to the upper split auxiliary coil 264 a and the lower split auxiliary coil 264 b (collectively referred to as the split auxiliary coil 264), and a power supply 275 is connected to the upper split auxiliary coil 265 a and the lower split auxiliary coil 265 b (collectively referred to as the split auxiliary coil 265).

[0081] In this way, it is possible to form the designed magnetic field distribution with higher precision by using a coil that divides a circular coil into four parts as shown in Figure 9 or a coil that divides it into five parts as shown in Figure 11, and by making it possible to excite each divided part with a different magnetomotive force. In the case of a simple circular coil, only one auxiliary coil power supply was required for each magnetic layer that makes up the magnetic pole, but in the case of a divided auxiliary coil, a power supply is prepared for each divided part.

[0082] The magnetomotive force of each divided region of the divided auxiliary coils 161, 162, 163, 164, 261, 262, 263, 264, and 265 is finally determined by alternating between changing the embedded position, number of embedded coils, and magnetomotive force of the divided auxiliary coils and performing three-dimensional magnetic field simulation, as in the case of the simple circular auxiliary coil 16.

[0083] The initial position of the split auxiliary coil is determined by assuming that the magnetic field generated within the installation range of the split auxiliary coil is a uniform bipolar magnetic field, and is determined so that the designed magnetic field is formed by superposing the split auxiliary coils in two dimensions. The same value as for the simple circular auxiliary coil is applied as the initial condition for the magnetomotive force of each divided region of the split auxiliary coil.

[0084] In addition, when two or more auxiliary coils are stacked above and below the beam circulation orbit, it is possible to combine a circular auxiliary coil 16 as shown in Figure 4 etc. with a divided auxiliary coil as shown in Figure 9 etc. or Figure 12 etc. in each layer as appropriate, and it is possible to freely design as appropriate according to the magnetic field to be formed.

[0085] In the examples described so far, magnets 11 with flat magnetic pole shapes have been used as examples. However, when the leakage magnetic field is large, it is also possible to apply an axially symmetrical shape in which the distance between the magnetic poles decreases from the center of the magnet toward the outer periphery of the magnet in order to flatten the base magnetic field.

[0086] Also, in this embodiment, an example has been described in which the present invention is applied to a magnet with a magnetic pole shape that is axially symmetrical with respect to the magnet center, and a non-axisymmetric design magnetic field is formed by exciting the main coil 13 and auxiliary coil 16. However, it is also conceivable to apply the present invention to an electromagnet that mainly forms a non-axisymmetric design magnetic field by exciting the magnetic poles and main coil that are non-axisymmetrical, and to operate it so that the magnetic field distribution is corrected by exciting the auxiliary coil.

[0087] Furthermore, although the present embodiment describes an example in which the present invention is applied to an accelerator electromagnet with a non-axisymmetric magnetic field distribution, the present invention can also be applied to accelerator electromagnets with an axisymmetric magnetic field distribution, such as cyclotrons and synchrocyclotrons. When applied to a cyclotron or synchrocyclotron, the arrangement of the embedded auxiliary coils is also axisymmetric.

[0088] Next, the effects of this embodiment will be described.

[0089] The accelerator 1 of the present embodiment described above accelerates beam particles using a radio-frequency acceleration electric field, and the orbital radius of the beam particles circulating in the magnetic field changes depending on the energy. The accelerator 1 includes a magnetic pole 15 formed by stacking layered magnetic materials 15aa, 15ba, 15ca, 15da, 15ab, 15bb, 15cb, and 15db, and an auxiliary coil 16 embedded inside one or more layered magnetic materials 15aa, 15ba, 15ca, 15da, 15ab, 15bb, 15cb, and 15db that constitute the magnetic pole 15, and the auxiliary coils 16 are arranged so that the magnetic field regions generated on the beam circulation orbit by each auxiliary coil 16 overlap.

[0090] Therefore, by embedding the auxiliary coils 16 in the magnetic poles 15, it is not necessary to widen the magnetic pole gap to avoid interference between the equipment and the beam, compared to when coils are placed in a narrow magnetic pole gap, which improves the concentration of magnetic flux lines between the magnetic poles and reduces leakage magnetic fields. Furthermore, because the magnetic field regions generated on the beam orbit by the individual auxiliary coils 16 are arranged to overlap, it is possible to maintain the function of forming and / or correcting the main magnetic field distribution. Furthermore, the magnetic poles 15 are formed by stacking the layered magnetic materials 15aa, 15ba, 15ca, 15da, 15ab, 15bb, 15cb, and 15db, making it very easy to embed the auxiliary coils 16. These effects enable the formation and correction of the magnetic field distribution with high precision while maintaining the compact size of the electromagnet of the accelerator 1.

[0091] Furthermore, since the beam particle incidence point 120 is contained within the magnetic field region generated by each of the layered magnetic materials 15aa, 15ba, 15ca, 15da, 15ab, 15bb, 15cb, and 15db by the auxiliary coil 16, the configuration can be very suitably applied to eccentric orbit synchrocyclotron accelerators and general synchrocyclotrons.

[0092] Furthermore, the auxiliary coil 16 is configured by combining a plurality of divided auxiliary coils 161, 162, 163, 164, 261, 262, 263, 264, and 265, which is a very suitable configuration when it is desired to generate a magnetic field with different strengths in the circumferential direction.

[0093] Furthermore, the magnetic pole 15 can be configured in a more space-saving manner because its vertical position is located at the same position as the main coil 13 that generates the base magnetic field, and its horizontal position is located within the area surrounded by the main coil 13 that generates the base magnetic field.

[0094] Furthermore, when two or more auxiliary coils 16 are arranged above and below the beam orbit, the radius of the auxiliary coil 16 whose vertical position is closest to the beam orbit is the smallest, and the radius increases as it moves away from the beam, making it easier to form a main magnetic field distribution and / or a magnetic field that corrects the main magnetic field distribution closer to the beam incidence point.

[0095] <Others> The present invention is not limited to the above-described embodiments, and various modifications and applications are possible. The above-described embodiments have been described in detail to clearly explain the present invention, and the present invention is not necessarily limited to those having all of the described configurations.

[0096] The embodiment of the present invention may be in the following form.

[0097] (1) A circular accelerator that accelerates beam particles using a high-frequency acceleration electric field, and in which the orbital radius of the beam particles circulating in a magnetic field changes depending on the energy, the circular accelerator comprising a magnetic pole section made of stacked magnetic materials and coils embedded inside one or more layered magnetic materials that make up the magnetic pole section, and the coils are arranged so that the magnetic field regions generated on the beam circulation orbit by each of the coils overlap.

[0098] (2) In the circular accelerator described in (1), the incident point of the beam particles is contained within a magnetic field region generated from each layer of the magnetic material by the coil.

[0099] (3) In the circular accelerator described in (1) or (2), the coil has a cooling passage through which a coolant flows, and a cooling supply pipe for supplying the coolant to the cooling passage is built into the power supply line.

[0100] (4) In the circular accelerator according to any one of (1) to (3), the coil is configured by combining a plurality of split coils.

[0101] (5) In the circular accelerator described in any one of (1) to (4), the magnetic pole section is arranged in the same vertical position as the main coil that generates the base magnetic field.

[0102] (6) In the circular accelerator described in any one of (1) to (5), the magnetic pole section is arranged in a region surrounded by a main coil that generates a base magnetic field, with its horizontal position being in a region surrounded by a main coil that generates a base magnetic field.

[0103] (7) In the circular accelerator described in any one of (1) to (6), when two or more coils are arranged above and below the beam circular orbit, the radius of the coil whose vertical position is closest to the beam circular orbit is smallest, and the radius increases as the coil is positioned further away from the beam.

[0104] (8) In the circular accelerator described in any one of (1) to (7), the beam particles are accelerated using the frequency-modulated high-frequency acceleration electric field, and the orbits of the beam particles with different energies are converged to one side.

[0105] (9) A particle beam therapy system including the circular accelerator according to any one of (1) to (8).

[0106] DESCRIPTION OF SYMBOLS 1...Accelerator (circular accelerator) 2...Rotating gantry 3...Irradiation device 4...Treatment table 5...Patient 7...Controller 8...Accelerator controller 10...Particle beam therapy system 11...Magnet 12...Ion source 13...Main coil 13a...Upper coil 13b...Lower coil 14...Return yoke 15...Magnetic poles 15aa, 15ab, 15ba, 15bb, 15ca, 15cb, 15da, 15db...Layered magnetic material 16, 16a, 16aa, 16ab, 16b, 16ba, 16bb, 16ca, 16cb, 16da, 16db...Auxiliary coil 17...Main coil power supply 18, 19, 20, 21...Auxiliary coil power supply 22...Beam passing area 23...High frequency cavity 24...Power supply line 25...Cooling supply piping 31, 32...Dee electrodes 35...Ground electrode 40...Extraction septum electromagnet 50...Massless septum coil 111...Through hole for extracted beam 112, 113...Through hole for coil connection 114...Through hole for inputting high frequency power 115...Through hole for beam injection 120...Incidence point 140...Beam extraction path 161, 162, 163, 164, 261, 262, 263, 264, 265...Split auxiliary coils 161a, 162a, 163a, 164a, 261a, 262a, 263a, 264a, 265a...Upper split auxiliary coils 161b, 162b, 163b, 164b, 261b, 262b, 263b, 264b, 265b...Lower split auxiliary coils 171, 172, 173, 174, 271, 272, 273, 274, 275...power supply 211...coupler 212...rotary variable capacitance capacitor 213...rotating shaft

Claims

1. A circular accelerator that accelerates beam particles using a radio-frequency accelerating electric field, and in which the radius of the orbit of the beam particles orbiting in a magnetic field changes depending on the energy, said circular accelerator comprising: a magnetic pole section made of laminated magnetic material; and coils embedded inside one or more layered magnetic materials that make up said magnetic pole section, said coils being arranged so that the magnetic field regions generated by each of said coils on the beam orbit overlap.

2. A circular accelerator according to claim 1, wherein the point of incidence of the beam particles is contained within a magnetic field region generated from each layer of the magnetic material by the coil.

3. A circular accelerator according to claim 1, wherein the coil is provided with a cooling passage through which a coolant flows, and a cooling supply pipe for supplying the coolant to the cooling passage is built into the power supply line.

4. A circular accelerator according to claim 1, wherein the coil is constructed by combining a plurality of split coils.

5. A circular accelerator according to claim 1, wherein the magnetic pole section is arranged in the same vertical position as a main coil that generates a base magnetic field.

6. A circular accelerator according to claim 1, wherein the magnetic pole section is arranged in a horizontal position within an area surrounded by a main coil that generates a base magnetic field.

7. A circular accelerator as claimed in claim 1, wherein when two or more of the coils are arranged above and below the beam circulation orbit, the radius of the coil whose vertical position is closest to the beam circulation orbit is the smallest, and the radius increases as the coil moves away from the beam.

8. A circular accelerator according to claim 1, wherein the beam particles are accelerated using the frequency-modulated high-frequency accelerating electric field, and the orbits of the beam particles having different energies are converged to one side.

9. A particle beam therapy system comprising the circular accelerator according to any one of claims 1 to 8.

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