WO2025094510A1 - Circular accelerator and particle therapy system - Google Patents

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

The present invention relates to a circular accelerator and a particle beam therapy system that accelerates heavy ions such as protons or carbon ions.

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 a magnetic field region that is weaker than the magnetic field at the central orbit is formed inside the central orbit, and a magnetic field region that is stronger than the magnetic field at the central orbit is formed outside the central orbit.

Non-Patent Document 1 describes how the magnetic field distribution of the main magnetic field is corrected to satisfy the isochronous condition by exciting a trim coil installed between the magnetic poles.

Patent No. 5622598

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

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

One type of accelerator is the circular accelerator, which deflects beam particles with the magnetic field generated by electromagnets 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.

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.

In a dynamic magnetic field accelerator, the beam can only be injected 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.

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 entrance point, and the radius of curvature of the beam particle's trajectory increases with acceleration. This means that the beam cannot be extracted unless it accelerates until it reaches the extraction equipment installation position, and the extraction energy is limited to the maximum value and cannot be changed.

Therefore, in order to irradiate a beam of the desired energy, a degrader is installed in the beam transport system to reduce the energy of the beam emitted from the accelerator. However, beam loss occurs when the beam passes through the degrader, and the dose rate drops in the low energy region.

In a static magnetic field type variable energy accelerator, which is an example of a static magnetic field type 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 center positions of the orbits 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 reduction in dose rate due to the degrader. Due to its ability to perform rapid repeated acceleration and its energy variability, variable energy accelerators are expected to provide a high dose rate across the entire range of extraction energies.

In general, in accelerator electromagnets, the distribution of the main magnetic field depends mainly on the magnetic pole shape, and the strength of the main magnetic field depends mainly on the magnetomotive force of the main coil. Therefore, in electromagnet design, the magnetic pole shape is determined using 3D magnetic field simulations so that a magnetic field distribution is formed that allows beam particles to circulate stably.

However, due to factors such as processing errors and installation errors, the manufactured electromagnet may not provide the desired magnetic field distribution, making it necessary to correct the magnetic field distribution. One method of 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 iterations of placing shims on the magnetic pole surface to adjust the amount of magnetic field correction and measuring the magnetic field.

In Non-Patent Document 1, there is no need to disassemble the electromagnet, and 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. On the other hand, in order to install the coil so that it does not interfere with the beam, it is necessary to widen the distance between the magnetic poles. 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, so the magnet must be made larger in order to ensure the beam circulation area.

In order to place magnetic field distribution correction coils in cyclotrons, synchrocyclotrons, and static magnetic field type variable energy accelerators without increasing the distance between the magnetic poles, we are considering a method of embedding the coils in the electromagnets as described in Patent Document 1.

Patent Document 1 shows an example of an electromagnet used in synchrotrons and the like that deflects a charged particle beam along a central orbit with a specified radius of curvature, in which an auxiliary coil capable of correcting the magnetic field distribution is embedded inside the yoke, thereby positioning the magnet without increasing the distance between the magnetic poles.

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

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 type variable energy accelerator, it is necessary to form a complex magnetic field distribution that changes nonlinearly toward the outer periphery of the accelerator so that the beam circulates stably throughout the beam orbital region from injection to extraction. To achieve this, the magnetic field generating regions of the 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.

The present invention provides a circular accelerator and particle beam therapy system that can form and correct the magnetic field distribution with high precision while keeping the accelerator's electromagnet size small.

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

The present invention makes it 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 clear from the explanation of the following examples.

1 is a schematic diagram of an overall configuration of a particle beam therapy system according to an embodiment. 1 is a schematic diagram of an overall configuration of an accelerator according to an embodiment of the present invention. FIG. 2 is a diagram showing the internal equipment layout of the accelerator of the embodiment. FIG. 2 is a diagram showing the layout of embedded coils as viewed from above the accelerator of the embodiment. FIG. 2 is a diagram showing the layout of embedded coils as viewed from a cross section of the accelerator of the embodiment. FIG. 2 is a schematic diagram of the magnetic field distribution of the accelerator of the embodiment. 1 is a diagram for explaining a method for determining the position of an embedded coil in an accelerator according to an embodiment. FIG. FIG. 2 is a diagram showing an outline of the connection between the coils and the power supply of the accelerator of the embodiment. FIG. 11 is an explanatory diagram of a quadrant coil which is a second modified example of the auxiliary coil of the accelerator of the embodiment. 10 is a diagram showing an outline of a connection between the split coils and a power supply in the first modified example of FIG. 9 . FIG. 2 is an explanatory diagram of a five-split coil which is a modified example of the auxiliary coil of the accelerator of the embodiment. 12 is a diagram showing an outline of a connection between the split coil and a power supply in the second modified example of FIG. 11 .

An embodiment of the circular accelerator and particle beam therapy system of the present invention will be described with reference to Figs. 1 to 12. Note that in the drawings used in this specification, identical or corresponding components are given the same or similar reference numerals, and repeated explanations of these components may be omitted.

First, the overall configuration of a particle beam 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 beam therapy system according to an embodiment.

The particle beam therapy system 10 of this embodiment shown in FIG. 1 is composed of 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.

In the particle beam therapy system 10, the beam emitted from the accelerator 1 is transported to the irradiation device 3 by the 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 is irradiated to the affected area of the patient 5 lying on the treatment table 4.

The irradiation device 3 includes a dose monitor and 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 entrance, acceleration, and exit of the charged particle beam in the accelerator 1, and supplies a beam of the required dose and energy.

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

The accelerator 1 shown in Figure 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.

As shown in Figure 2, the accelerator 1 uses a magnet 11 that can be separated into upper and lower parts to excite a main magnetic field in the area through which the beam passes (hereafter referred to as the beam passing area), and the inside of the beam passing area is evacuated.

The magnet 11 has multiple through-holes, including an extraction 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 connection surface between the upper and lower magnetic poles. A high-frequency cavity 23 is installed through the high-frequency power input through-hole 114.

The high-frequency cavity 23 is equipped with a dee electrode for acceleration and a rotary variable capacitor 212, as described below.

In addition, an ion source 12 is installed above the magnet 11 at a position offset from the center and at a different radial position, and a beam is injected into the accelerator 1 through a beam injection through-hole 115.

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 Figure 3. The coil arrangement of the accelerator 1 of this embodiment will be explained using a projection onto the XY plane between the layered magnetic material 15aa and layered magnetic material 15ab of the magnetic pole 15 shown in Figure 4, and a YZ cross-sectional view of the magnet 11 shown in Figure 5.

As shown in Figures 3 and 4, there is a cylindrical return yoke 14 at the outermost side of the magnet 11, which reduces leakage magnetic flux and concentrates the magnetic flux in the cylindrical beam passing area 22 inside. Inside the return yoke 14, an annular main coil 13 is installed along the inner wall. Inside the main coil 13, magnetic poles 15 are installed so as to face each other vertically to form the upper and lower boundaries of the beam passing area 22.

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

As a cooling structure for the auxiliary coil 16, the auxiliary coil 16 may be made of a hollow conductor capable of internal cooling, and a cooling liquid may be flowed through a cooling flow path formed in the inner space of the auxiliary coil 16. In the case of such a cooling structure, power and cooling liquid can be supplied to the auxiliary coil 16 by using a power supply line 24 having a cooling supply pipe 25 built therein that supplies cooling liquid to the inner space of the auxiliary coil 16. By making the auxiliary coil 16 of a hollow conductor capable of internal cooling in this way, the surface area where the cooling liquid and the auxiliary coil 16 come into contact can be increased, and the cooling efficiency can be improved. Therefore, the cooling structure occupies a smaller portion of the magnetic pole, and a structure suitable for miniaturization can be achieved. Note that the cooling structure for the auxiliary coil 16 is not limited to an internal cooling structure, and another structure may be a structure in which a cooling pipe for flowing a cooling liquid for cooling the auxiliary coil 16 is buried inside the layered magnetic bodies 15aa, 15ba, 15ca, 15da, 15ab, 15bb, 15cb, and 15db.

Note that although the auxiliary coil 16 is embedded inside all of the layered magnetic material, it may be embedded only in a portion of it. Also, although the thickness of the layered magnetic material and the thickness of the auxiliary coil 16 are shown to be the same, this is not required and is not a limitation. Furthermore, although the thickness of each layer of the layered magnetic material is shown to be approximately the same, this is not required and they can be different thicknesses.

The magnetic pole 15, which is composed of stacked layered magnetic materials 15aa, 15ba, 15ca, 15da, 15ab, 15bb, 15cb, and 15db, is arranged so that its vertical position is the same as that of the main coil 13 that generates the base magnetic field, as shown in FIG. 5, and its horizontal position is within the area surrounded by the main coil 13 that generates the base magnetic field, as shown in FIGS. 4 and 5.

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.

Furthermore, the radius of each of the auxiliary coils 16aa, 16ba, 16ca, 16da, 16ab, 16bb, 16cb, and 16db is the smallest for the auxiliary coils 16aa and 16ab, which are located vertically closest to the beam orbit, and the radius increases in the order of auxiliary coils 16ba, 16bb, auxiliary coils 16ca, 16cb, and auxiliary coils 16da and 16db, that is, the further away from the beam they are.

The incident point 120 of the beam particles is contained within the magnetic field region generated by the layered magnetic bodies 15aa, 15ba, 15ca, 15da, 15ab, 15bb, 15cb, and 15db of each layer by the auxiliary coil 16.

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 to each other individually or collectively, and the integrated magnetic poles are fastened to the yoke.

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 area 22 between the magnetic poles 15. The main coil 13 excites a magnetic field that serves as the base for the 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 can also be excited for the purpose of forming the main magnetic field distribution.

In the accelerator 1 of this embodiment, so long as it is possible to generate a desired magnetic field distribution in the beam circulation region from beam injection to beam 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) that passes through the center of the gap, and on the orbital plane, there is only a magnetic field component in a direction perpendicular to the orbital plane.

The shape of magnetic pole 15 is symmetrical with respect to axis AA' in the central plane, which results in a symmetrical distribution of the magnetic field. If a right-handed coordinate system is defined with the incidence point 120 as the origin and the Y-axis pointing in the direction toward the center of accelerator 1 from the origin, 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 direction of the Y-axis becomes gentler away from incidence point 120.

As a result, in the low energy region below the extraction energy, the orbit is centered near the entrance point 120, just like a cyclotron, but as the energy of the beam particles increases, the center of the beam's orbit moves in one direction on the same plane, resulting in the formation of an orbital convergence point where the orbits are densely packed. Particles that deviate 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, allowing them to orbit stably.

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

The high-frequency cavity 23 excites an electric field in the acceleration gap by a λ/4 type resonance mode. High-frequency power is introduced from an external high-frequency power source through a coupler 211. The high-frequency cavity 23 is connected to the 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 the ground electrode 35.

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 response to the energy of the circulating beam.

In a cavity using a resonant mode such as that used in this embodiment, it is necessary to sweep the high frequency range over a wider range than the width of the resonance. To do this, it is also necessary to change the resonant frequency of the high frequency cavity 23. This is controlled 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 plate (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.

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 method 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 orbit plane.

In order to extract a specified extracted beam with a target energy, one or more coils of the massless septum coil 50 are selected based on the target energy and a specified excitation current is passed through them. When no current is passed through the massless septum coil 50, the beam with the target energy revolves along its designed orbit, but when current is passed through the massless septum coil 50, the beam that has reached the target energy is deviated from the orbit by the kick magnetic field caused by the massless septum coil 50. As described above, the beam that has deviated from the orbit oscillates stably around the designed orbit. In other words, the massless septum coil 50 excites betatron oscillations in the orbital plane. When the position of the kick by the massless septum coil 50 and the position of the convergence point are in an appropriate positional relationship, it is possible to displace the beam radially outward at the convergence point by the kick by the massless septum coil 50.

This section describes the behavior of the beam from injection to extraction from the accelerator 1 of this embodiment.

The operation of Accelerator 1 consists of three steps: injection, acceleration, and extraction.

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 passage region 22 through the injection point 120.

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 since the period of the accelerating radio frequency electric field is controlled to synchronize with the orbital period of the beam particles, the period of the accelerating radio frequency electric field increases.

In the subsequent acceleration step, the beam is accelerated while ensuring directional stability by the high-frequency electric field. In other words, the beam does not pass through the acceleration gap at the time when the high-frequency electric field is at its maximum, but when the high-frequency electric field is decreasing. Then, since the frequency of the high-frequency electric field and the orbital frequency of the beam are synchronized at an integer multiple ratio, particles accelerated at a specified phase of the acceleration electric field are accelerated at the same phase at the next turn. On the other hand, particles accelerated at a phase earlier than the acceleration phase are accelerated at a delayed phase at the next turn because their acceleration amount is greater than that of particles accelerated at the acceleration phase. Conversely, particles accelerated at a phase later than the acceleration phase are accelerated at a delayed phase at the next turn because their acceleration amount is smaller than that of particles accelerated at the acceleration phase. In this way, the particles being accelerated are gradually accelerated while oscillating (synchrotron oscillation) around a certain design phase, and reach the specified energy to be extracted.

In the extraction step, the beam that has reached a predetermined energy level is subjected to the effect of a kick magnetic field generated by the massless septum coil, travels from the focusing point to the extraction channel, which is the beam extraction path 140 formed by the extraction septum magnet 40, and is extracted outside the accelerator 1.

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

As an example, consider a case where the magnetic field distribution of the accelerator 1 is asymmetric with respect to the magnet center _Yc and the position _Ym where the magnetic field strength is maximum, with the magnetic field gradient being steep on the negative side of _Ym and gradual on the positive side (see Fig. 6). In this embodiment, the magnetic pole 15 has 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.

The final buried position, number of buried auxiliary coils, and magnetomotive force of the auxiliary coils 16 are determined by fine-tuning these parameters and repeatedly performing 3D magnetic field simulations to search for suitable 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 two-pole magnetic field is generated inside.

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

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, a base magnetic field |B _z1 | is formed inside the main coil 13, as shown in the following equation (1).

Therefore, the auxiliary coils 16 are arranged so that the magnetic field regions generated on the beam circulation orbit by each auxiliary coil 16 overlap. For example, the auxiliary coil 16a is embedded so that a magnetic field of Δ|B _z | is generated at positions Y ₂ and Y ₉ where the difference between the base magnetic field and the target magnetic field is a certain value Δ|B _z |. In this state, the auxiliary coil 16b is further embedded so that a magnetic field of Δ|B _{z |} is generated at positions Y ₃ and Y ₈ where the difference between the base magnetic field and the target magnetic field is a certain value Δ|B z |. By repeating the above procedure, a multi-stage magnetic field distribution is formed as shown in the following formula (2).

In this case, depending on the design magnetic field distribution, it is possible that one or more of the auxiliary coils 16 may be excited in the opposite phase to the other auxiliary coils 16.

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.

The arrangement of the auxiliary coils 16 determined in this way is used as the initial condition for the iteration of the 3D magnetic field simulation, and in places 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 final arrangement and magnetomotive force of the auxiliary coils 16 are determined by gradually bringing the generated magnetic field distribution closer to the target magnetic field distribution.

Ultimately, the magnetomotive force of each coil is based on the value determined by 3D magnetic field simulation, and is fine-tuned by magnetic field measurements to form the designed magnetic field with greater precision.

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 explained. FIG. 8 shows an overview of the connection between the coil and the power supply of the accelerator of this embodiment.

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

In Figure 8, four auxiliary coil power supplies 18, 19, 20, and 21 are shown, corresponding to 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 are required. That is, as mentioned above, the auxiliary coils 16 are provided to correct the difference between the target magnetic field distribution and the generated magnetic field, and since the magnetic field to be excited in each auxiliary coil 16 is often different, it is desirable to connect an independent power supply to each.

Furthermore, just as in the case where the main coil power supply is turned ON to excite the main coil during the start-up phase of a normal accelerator, 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.

Furthermore, in FIG. 4 and other figures, a case is described in which a circular coil is used as each of the auxiliary coils 16. However, the shape of the auxiliary coil 16 embedded inside the magnetic pole 15 is not limited to a circular coil. Below, modified examples are explained using FIG. 9 to FIG. 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 in FIG. 9 and the 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 in FIG. 11 and the power supply.

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

Furthermore, it is not limited to the configuration shown in FIG. 9, which is divided into four equal parts in the circumferential direction, but it is also possible to use a configuration such as split auxiliary coils 261, 262, 263, 264, and 265, which are divided into two parts in the radial direction and the outer diameter side is divided into four parts in the circumferential direction, for a total of five parts.

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

In this way, by using a circular coil divided into four as in Figure 9, or a coil divided into five as in Figure 11, it is possible to form the designed magnetic field distribution with greater precision by making it possible to excite each divided area with a different magnetomotive force. With a simple circular coil, only one auxiliary coil power supply was required for each magnetic layer that makes up the magnetic pole, but with a divided auxiliary coil, a power supply is provided for each divided area.

The magnetomotive force of each divided area 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 a three-dimensional magnetic field simulation, as in the case of the simple circular auxiliary coil 16.

The position of the split auxiliary coil in the initial condition is determined by assuming that the magnetic field generated in the area where the split auxiliary coil is installed is a uniform bipolar magnetic field, and is determined so that the design magnetic field is formed by superimposing the split auxiliary coil in two dimensions. The magnetomotive force of each divided area of the split auxiliary coil is applied as the initial condition with the same value as in the case of a simple circular auxiliary coil.

In addition, when two or more auxiliary coils are stacked above and below the beam orbit, it is possible to combine a circular auxiliary coil 16 as shown in Figure 4 with a split auxiliary coil as shown in Figure 9 or Figure 12 in each layer as appropriate, allowing for a free and appropriate design depending on the magnetic field to be formed.

In the examples 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.

In addition, this embodiment describes an example 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-axisymmetrical design magnetic field is formed by exciting the main coil 13 and auxiliary coil 16. However, it can also be applied to an electromagnet that mainly forms a non-axisymmetrical design magnetic field by exciting the magnetic poles and main coil with a non-axisymmetrical shape, and the magnetic field distribution can be corrected by exciting the auxiliary coil.

Furthermore, although this embodiment describes an example of application to an accelerator electromagnet with a non-axisymmetric magnetic field distribution, it can also be applied to accelerator electromagnets with an axisymmetric magnetic field distribution, such as a cyclotron or synchrocyclotron. When applied to a cyclotron or synchrocyclotron, the arrangement of the embedded auxiliary coils is also axisymmetric.

Next, we will explain the effects of this embodiment.

The accelerator 1 of the present embodiment described above accelerates beam particles using a high-frequency acceleration electric field, and the orbital radius of the beam particles circulating in the magnetic field changes depending on the energy. It is equipped with a magnetic pole 15 in which layered magnetic materials 15aa, 15ba, 15ca, 15da, 15ab, 15bb, 15cb, and 15db are stacked, 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.

Therefore, by embedding the auxiliary coil 16 in the magnetic pole 15, it is not necessary to widen the magnetic pole gap to avoid interference between the equipment and the beam, as compared to when the coil is placed in a narrow magnetic pole gap, so it is possible to improve the concentration of the magnetic flux lines between the magnetic poles and reduce the leakage magnetic field. In addition, since the magnetic field regions generated on the beam orbit by each auxiliary coil 16 are arranged to overlap, it is possible to maintain the function of forming the main magnetic field distribution and/or correcting the main magnetic field distribution. Furthermore, since the magnetic pole 15 is made of layered magnetic materials 15aa, 15ba, 15ca, 15da, 15ab, 15bb, 15cb, and 15db, it is very easy to embed the auxiliary coil 16. These effects make it possible to form and correct the magnetic field distribution with high precision while keeping the electromagnet size of the accelerator 1 small.

In addition, the beam particle incidence point 120 is contained within the magnetic field region generated by each of the layered magnetic bodies 15aa, 15ba, 15ca, 15da, 15ab, 15bb, 15cb, and 15db by the auxiliary coil 16, making it possible to provide a configuration that is highly suitable for use in eccentric orbit synchrocyclotron accelerators and general synchrocyclotrons.

In addition, the auxiliary coil 16 is composed of a combination of multiple split auxiliary coils 161, 162, 163, 164, 261, 262, 263, 264, and 265, which is extremely suitable for forming magnetic fields with different strengths in the circumferential direction.

Furthermore, the magnetic pole 15 is arranged so that its vertical position is the same as that of the main coil 13 that generates the base magnetic field, and its horizontal position is within the area surrounded by the main coil 13 that generates the base magnetic field, allowing for a more space-saving configuration.

In addition, when two or more auxiliary coils 16 are arranged above and below the beam orbit, the radius of the auxiliary coil 16 closest to the beam orbit in the vertical direction is smallest, and the radius increases the further away from the beam, making it easier to form the main magnetic field distribution and/or the magnetic field that corrects the main magnetic field distribution closer to the beam incidence point.

＜Other＞
The present invention is not limited to the above-mentioned embodiment, and various modifications and applications are possible. The above-mentioned embodiment has been described in detail to explain the present invention in an easily understandable manner, and the present invention is not necessarily limited to having all of the described configurations.

The present invention may take the following forms:

(1) A circular accelerator that accelerates beam particles using a high-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, the circular accelerator comprises a magnetic pole section made of stacked magnetic materials, and coils embedded inside one or more layers of magnetic materials that make up the magnetic pole section, and the coils are arranged so that the magnetic field regions generated by each of the coils on the beam orbit overlap.

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

(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.

(4) In the circular accelerator described in any one of (1) to (3), the coil is composed of a combination of multiple split coils.

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

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

(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 orbit, the coil whose vertical position is closest to the beam orbit has the smallest radius, and the radius increases the further away from the beam.

(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.

(9) A particle beam therapy system equipped with a circular accelerator described in any one of (1) to (8).

1...Accelerator (circular accelerator)
Reference Signs List 2...Rotating gantry 3...Irradiation device 4...Treatment table 5...Patient 7...Control device 8...Accelerator control device 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 region 23...High frequency cavity 24...Power supply line 25...Cooling supply piping 31, 32...Dee electrode 35...Ground electrode 40...Extraction septum electromagnet 50...Massless Septum coil 111...extraction beam through-holes 112, 113...coil connection through-hole 114...high frequency power input through-hole 115...beam injection through-hole 120...incidence point 140...beam extraction path 161, 162, 163, 164, 261, 262, 263, 264, 265...split auxiliary coils 161a, 162a, 163a, 164a, 261 a, 262a, 263a, 264a, 265a...Upper auxiliary divided coils 161b, 162b, 163b, 164b, 261b, 262b, 263b, 264b, 265b...Lower auxiliary divided coils 171, 172, 173, 174, 271, 272, 273, 274, 275...Power supply 211...Coupler 212...Rotary variable capacitor 213...Rotary shaft

Claims (9)

A circular accelerator that accelerates beam particles using a high-frequency accelerating electric field and changes the orbit radius of the beam particles orbiting in a magnetic field depending on the energy,
The circular accelerator includes a magnetic pole portion having a laminated magnetic body,
a coil embedded inside one or more layered magnetic bodies constituting the magnetic pole portion;
The coils are arranged so that magnetic field regions generated on the beam circular orbit by each of the coils overlap. 2. The circular accelerator according to claim 1,
a circular accelerator, the incident point of the beam particles being contained within a magnetic field region generated by the coil from each of the layers of the magnetic material. 2. The circular accelerator according to claim 1,
The coil has a cooling passage therein through which a cooling liquid flows,
A circular accelerator, wherein a cooling supply pipe for supplying a cooling liquid to the cooling path is built into the power supply line. 2. The circular accelerator according to claim 1,
A circular accelerator, wherein the coil is composed of a combination of a plurality of split coils. 2. The circular accelerator according to claim 1,
A circular accelerator, wherein the magnetic pole section is arranged in the same vertical position as a main coil that generates a base magnetic field. 2. The circular accelerator according to claim 1,
A circular accelerator, 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. 2. The circular accelerator according to claim 1,
A circular accelerator, wherein when two or more of the coils are arranged above and below the beam circular orbit, the coil whose vertical position is closest to the beam circular orbit has the smallest radius, and the radius increases as the coil moves away from the beam. 2. The circular accelerator according to claim 1,
A circular accelerator in which the beam particles are accelerated using the frequency-modulated high-frequency accelerating electric field, and the orbits of the beam particles of different energies are concentrated in one direction. A particle beam therapy system comprising the circular accelerator according to any one of claims 1 to 8.

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