JP7485593B2 - Accelerators and particle beam therapy equipment - Google Patents

Description

This disclosure relates to accelerators and particle beam therapy devices.

Particle beam therapy, a cancer treatment method, irradiates the affected area with a beam of charged particles such as protons or carbon ions. The particle beam therapy equipment used in particle beam therapy adjusts the energy and spatial spread of the charged particle beam to form a dose distribution that matches the shape of the affected area.

Particle beam therapy equipment includes an accelerator, a beam transport system, and an irradiation device. An accelerator is a device that accelerates a charged particle beam until it has the energy required for treatment. Examples of accelerators used in particle beam therapy include synchrotrons, cyclotrons, and synchrocyclotrons.

By miniaturizing the accelerator, the particle beam therapy device can be made smaller. By using a superconducting magnet as the electromagnet for deflecting the charged particle beam, the accelerator can be made smaller. One example of an accelerator that uses a superconducting electromagnet is the synchrocyclotron described in Non-Patent Document 1.

In a synchrocyclotron, a particle beam orbits in a static magnetic field formed by a superconducting coil, and is accelerated by a radio-frequency acceleration electric field synchronized with the orbit. In a synchrocyclotron, the orbital frequency of the beam decreases as the beam accelerates, so the frequency of the radio-frequency acceleration electric field is modulated in accordance with the decrease in the orbital frequency.

The beam's horizontal orbit in the synchrocyclotron is a concentric circle for each energy, and the beam that has reached the maximum designed energy is extracted from the extraction channel. In order to use the extracted beam for treatment, the beam must be decelerated by a scatterer so that the energy corresponds to the depth of the affected area.

In contrast to the synchrocyclotron, there is a circular accelerator described in Patent Document 1, which is an accelerator that can vary the energy of the extracted beam. In this circular accelerator, the main magnetic field distribution is formed so that the orbits of beams with different energies are offset from the center of the circular accelerator to one side in the radial direction. The circular accelerator in Patent Document 1 is characterized by extracting a beam of a specific energy by generating a disturbance in the main magnetic field in this way. This configuration makes it possible to change the energy of the extracted beam without using a decelerator such as a scatterer.

The scanning irradiation method described in Patent Document 1 is a method for forming an emitted beam into a dose distribution that matches the shape of the affected area. In the scanning irradiation method, the desired dose distribution is formed by scanning the beam using a scanning magnet installed upstream of the affected area.

JP 2019-133745 A

W. Kleeven, “The IBA Superconducting Synchrocyclotron Project S2C2”, Proceedings of Cyclotrons 2013

In an accelerator in which a beam is accelerated by circulating in a static magnetic field, such as the synchrocyclotron mentioned above, in order to extract the beam from the accelerator, it is necessary to pass the beam from the magnetic field region (called the main magnetic field) formed by the magnetic poles to accelerate and circulate the beam to the outside of the accelerator in the radial direction and reach the inner wall of the accelerator.

In general, the magnetic field between the main magnetic field and the inner wall of the accelerator decays radially outward, so there is a magnetic field gradient in the radial direction. This magnetic field gradient significantly varies the tune, which is the frequency of betatron oscillations per revolution of the beam.

When the horizontal and vertical tunes of the beam match integer and half-integer multiples of the other, resonances are excited and the amplitude of the betatron oscillations increases. The increase in the amplitude of the betatron oscillations leads to an expansion of the area through which the beam passes, which in turn causes beam losses due to collisions with equipment inside the accelerator.

One objective of this disclosure is to provide a technology that reduces beam loss due to betatron oscillations between the magnetic field and the inner wall of the accelerator.

The accelerator according to one aspect of the present disclosure includes a main electromagnet that forms a cylindrical region inside a yoke and forms a static magnetic field in the cylindrical region, a beam extraction path that transports a charged particle beam from inside the main electromagnet to the outside, an acceleration electric field applicator that applies a frequency-modulated acceleration electric field to accelerate the charged particle beam circulating in the static magnetic field, and a kicker that increases the amplitude of betatron oscillation of the charged particle beam accelerated by the acceleration electric field, and the main electromagnet creates a first magnetic field for causing the charged particle beam, the amplitude of which has been increased by the kicker, to enter the beam extraction path, and a second magnetic field for suppressing divergence of the charged particle beam between the region of the static magnetic field inside the yoke and the inner wall of the yoke.

A particle beam therapy device according to one aspect of the present disclosure is equipped with the above accelerator.

According to one aspect of the present disclosure, it is possible to reduce beam loss due to betatron oscillations between the main magnetic field and the inner wall of the accelerator.

FIG. 2 is an external view of the circular accelerator 30 of the first embodiment. 1 is a cross-sectional view of the circular accelerator 30 of the first embodiment taken along a central plane. 1 is a vertical cross-sectional view of a circular accelerator 30 according to a first embodiment. 3 is a graph showing the distribution of the main magnetic field on the r-axis shown in FIG. 2; 13 is a graph showing the distribution of magnetic field strength Bz in the r direction in a region including a resonance suppression magnetic field 46. FIG. 11 is an external view of a circular accelerator 30 according to a second embodiment. FIG. 11 is a cross-sectional view of the eccentric orbit accelerator of the second embodiment taken along a central plane. FIG. 11 is a vertical cross-sectional view of the eccentric orbit accelerator of the second embodiment. A conceptual diagram of the design trajectory for each energy level is shown. 1 is a graph showing the relationship between the amplitude _βr and phase φ of horizontal betatron oscillation for each energy. 1 is a graph showing the relationship between the amplitude βz and phase φ of vertical betatron oscillation for each energy. FIG. 11 is an overall configuration diagram of a particle beam therapy device according to a third embodiment.

The following describes an embodiment of the present invention with reference to the drawings.

Figure 1 is an external view of the circular accelerator 30 according to the first embodiment. Figure 2 is a cross-sectional view of the circular accelerator 30 cut at the center plane. Figure 3 is a vertical cross-sectional view of the circular accelerator 30 (view taken along the A-A' arrow in Figure 2).

The circular accelerator 30 in this embodiment is an accelerator that accelerates charged particles and can be used in a particle beam therapy device.

The outer shell of the circular accelerator 30 is formed by the main electromagnet 40, which can be separated in the vertical direction. The region inside the main electromagnet 40 where the beam is accelerated (hereinafter referred to as the "acceleration region") is kept in a vacuum. An ion source 50 is installed above the main electromagnet 40 to generate a beam of ions to be injected into the main electromagnet 40.

The beam generated by the ion source 50 passes through a low-energy beam transport system 51 and reaches an ion injection section 52 provided near the center of the main magnetic pole 38. The beam that reaches the ion injection section 52 is injected into the acceleration region inside the main electromagnet 40 by the ion injection section 52. An ECR ion source, a laser ion source, or the like can be used as the ion source 50. When the beam is injected from outside, for example, the beam is injected into the acceleration region through an electrostatic inflector 53. The ion source 50 may be placed inside the evacuated acceleration region inside the main electromagnet 40. In that case, a PIG-type ion source or the like is preferable.

The main electromagnet 40 consists of a main magnetic pole 35, a yoke 37, and a main coil 38. The yoke 37 forms the exterior of the main electromagnet 40 and defines an approximately cylindrical region inside. The main coil 38 is an annular coil and is installed along the inner wall of the yoke 37. The main coil 38 is a superconducting coil. A cryostat 36 is installed around the main coil 38. The cryostat 36 cools the main coil 38. The main magnetic poles 35 are installed above and below the inner periphery of the main coil 38, facing each other. The magnetic field formed by the energized main coil 38 and main magnetic pole 35 is called the main magnetic field. The acceleration region is a region for accelerating the beam in the main magnetic field.

The yoke 37 has multiple through holes. Of these, the beam through hole 44, the coil through hole 47, the vacuum through hole 48, and the radio frequency system through hole 49 are provided on the connection surface of the yoke 37. The beam through hole 44 is a through hole for emitting an accelerated beam. The coil through hole 47 is a through hole for drawing out various coil conductors inside the yoke 37 to the outside. The vacuum through hole 48 is a through hole for drawing a vacuum inside the yoke 37. The radio frequency system through hole 49 is a through hole for providing the radio frequency acceleration cavity 10.

The radio frequency acceleration cavity 10 is a λ/2 resonant cavity, and has a dee electrode 12, a dummy dee electrode 13, an inner conductor 14, an outer conductor 15, and a rotating capacitor 31. The dee electrode 12 is a D-shaped hollow electrode, and is connected to the inner conductor 14. The dummy dee electrode 13 is an electrode connected to the outer conductor 15 that surrounds the inner conductor 14, and is at ground potential. The dummy dee electrode 13 forms an acceleration gap 11 between the dee electrode 12 and the dee electrode 13. An acceleration voltage that is frequency modulated by the rotating capacitor 31 is generated in the acceleration gap 11 between the dee electrode 12 and the dummy dee electrode 13. The acceleration gap 11 illustrated in FIG. 2 is for the case where the number of harmonics is 1, that is, the orbital frequency and the acceleration frequency are the same. The shape of the acceleration gap 11 is designed according to the orbital shape of the beam.

The input coupler 20 is a device for supplying radio frequency power to the radio frequency acceleration cavity 10, and is connected to the inner conductor 14 by electrostatic or magnetic coupling. Power is supplied to the input coupler 20 from the acceleration radio frequency power supply 21, and radio frequency power is supplied to the inner conductor 14 through the input coupler 20. This generates a radio frequency acceleration voltage for accelerating the beam in the acceleration gap 11 between the dee electrode 12 and the dummy dee electrode 13, and this radio frequency acceleration voltage generates a radio frequency electric field.

The rotating capacitor 31 is a device for modulating the resonant frequency of the radio frequency acceleration cavity 10, and includes a motor 32, a fixed electrode 33, and a rotating electrode 34 facing the fixed electrode 33. The fixed electrode 33 is formed on the inner conductor 14. The rotating electrode 34 is adjacent to the outer conductor 15, and is not physically connected to the outer conductor 15, but is electrically connected to the outer conductor 15 via electrostatic capacitance. Note that this configuration is one example, and as another configuration, the fixed electrode 33 may be formed on the outer conductor 15, and the rotating electrode 34 may be electrostatically coupled to the inner conductor 14.

Here, we will explain the behavior of the beam from when it enters the circular accelerator 30 until it is extracted. The beam injected from the ion source 50 into the circular accelerator 30 is accelerated by the high-frequency electric field, and orbits in the main magnetic field while increasing in energy. The radius of curvature of the beam's orbit increases as the beam accelerates, and it becomes spiral-shaped.

Here, within the acceleration region, the orbit that the beam follows from when it starts accelerating until it reaches maximum energy is called the circular orbit. The orbit through which the beam with maximum energy passes among the circular orbits is called the maximum energy beam orbit 80. The plane along which the circular orbit describes a spiral is called the orbital plane.

If we define a two-dimensional polar coordinate system for the orbital plane with the center of the acceleration region as the origin and the axis radially outward from the center as the r-axis, the main magnetic field satisfies the beam stabilization condition in which the n value expressed by equation (1) is greater than 0 and less than 1.

In equation (1), ρ is the deflection radius of the ideal orbit (hereafter referred to as the "design orbit"), Bz is the magnetic field strength, and ∂Bz/∂r is the magnetic field gradient in the r direction.

A beam that is slightly displaced radially from the design orbit experiences a restoring force in a direction that returns it to the design orbit. Also, a beam that is displaced perpendicular to the orbital plane experiences a restoring force from the main magnetic field in a direction that returns it to the orbital plane. The beam oscillation that results from these factors is called betatron oscillation, and the frequency of the betatron oscillation is called the betatron frequency.

The ∂Bz/∂r of the main magnetic field is designed so that the beam can be stably orbited and accelerated while undergoing betatron oscillations near the designed orbit.

The number of oscillations per revolution is called the tune, and the displacement of the beam on the r-axis outside the orbital plane per revolution is called the turn separation. Betatron oscillation in the orbital plane and perpendicular to the beam orbit is called the horizontal betatron oscillation, and the tune of the horizontal betatron oscillation is called the horizontal tune. When an appropriate radio frequency voltage is applied to the beam, the amplitude of the betatron oscillation increases due to resonance. For the entire range of beam energies, the horizontal tune ν _r is set to a value close to 1.

The magnetic field distribution of the main magnetic field described above (hereinafter referred to as the "main magnetic field distribution") is formed by the main pole 38 and the trim coils and/or pole pieces installed on the surface of the main pole 38. These components that form the main magnetic field distribution are arranged symmetrically with respect to the orbital plane. Therefore, on the orbital plane, the main magnetic field has only a magnetic field component in a direction perpendicular to the orbital plane.

The circular accelerator 30 in this embodiment has a radio frequency kicker 81, a septum coil 41, and a high energy beam transport system 45 as equipment for emitting a beam.

The beam accelerated by the circular accelerator 30 is emitted from the beam extraction path entrance 82 to the outside of the acceleration region. The septum coil 41 is disposed at this beam extraction path entrance 82. Two or more septum coils 41 may be disposed along the beam propagation direction.

The high-energy beam transport system 45 is a transport system for transporting the extracted beam from inside the main electromagnet 40 to the outside. This high-energy beam transport system 45 is located behind the septum coil 41, passing through the beam through-hole 44 and extending to the outside of the main electromagnet 40.

The radio-frequency kicker 81 is a device that applies a radio-frequency voltage to the beam passing through it. The septum coil 41 is a coil for deflecting the beam horizontally toward the outer periphery, and has two coil conductors (not shown).

The septum coil 41 is located on the outer periphery of the acceleration region, with one coil conductor in contact with the acceleration region. By passing a current through the two coil conductors, a magnetic field perpendicular to the beam's orbit is generated inside the septum coil 41. The beam that enters the septum coil 41 is deflected by the magnetic field and transported to the high-energy beam transport system 45.

In addition, inside the main electromagnet 40, a pole magnetic field region 42 and a regenerator magnetic field region 43 are formed, which are disturbance magnetic fields consisting of a bipolar magnetic field or a multipolar magnetic field.

In this embodiment, the peeler magnetic field region 42 is located on the upstream side of the orbit in which the beam travels, sandwiching the beam emission path entrance 82 and the high-frequency kicker 81, and the regenerator magnetic field region 43 is located on the downstream side.

To emit the beam, in addition to the above-mentioned radio frequency kicker 81, septum coil 41, and high energy beam transport system 45, the peeler magnetic field region 42 and regenerator magnetic field region 43 are used.

When the beam is accelerated to maximum energy in the acceleration gap 11 to which the radio frequency acceleration voltage is applied, the application of the radio frequency acceleration voltage to the acceleration gap 11 is stopped. As a result, the beam orbits on the maximum energy beam orbit 80.

A radio frequency kicker 81 is installed on the maximum energy beam orbit 80. When the beam enters the radio frequency kicker 81, the amplitude of the betatron oscillation of the beam increases due to the radio frequency voltage from the radio frequency kicker 81. The beam with the increased betatron oscillation amplitude eventually reaches the peeler magnetic field region 42 and the regenerator magnetic field region 43, which are installed on the outer periphery of the maximum energy beam orbit 80 at a predetermined distance from the maximum energy beam orbit 80.

The beam that reaches the peeler magnetic field region 42 is kicked toward the outer periphery of the orbital plane. The beam that reaches the regenerator magnetic field region 43 is kicked toward the inner periphery of the orbital plane. The kick from the quadrupole magnetic field component of the peeler magnetic field region 42 further increases the amplitude of the betatron oscillation of the beam, increasing the turn separation of the beam. In addition, the magnetic field of the regenerator magnetic field region 43 suppresses sudden fluctuations in the horizontal tune of the beam, preventing the beam from being lost due to the betatron oscillation diverging in the vertical direction perpendicular to the horizontal direction before the beam is emitted from the circular accelerator 30.

When sufficient turn separation is obtained, the beam enters the septum coil 41, which kicks it outside the orbital plane. The beam kicked by the septum coil 41 passes through the high-energy beam transport system 45 and is emitted outside the circular accelerator 30.

The increase in turn separation described above is much greater due to the peeler magnetic field region 42 and the regenerator magnetic field region 43 than due to the radio frequency kicker 81. Therefore, by adjusting the radio frequency voltage applied by the radio frequency kicker 81, the amount of the beam that reaches the peeler magnetic field region 42 and the regenerator magnetic field region 43 among the beams orbiting on the maximum energy beam orbit 80 can be adjusted. As a result, if the application of the radio frequency voltage to the radio frequency kicker 81 is stopped during the beam extraction, the beam can be prevented from reaching the peeler magnetic field region 42 and the regenerator magnetic field region 43, and the extraction of the beam from the circular accelerator 30 can be interrupted. In addition, if the application of the radio frequency voltage to the radio frequency kicker 81 is resumed, the extraction of the beam can be resumed. Furthermore, the intensity of the beam extracted from the circular accelerator 30 can be controlled by controlling one or more of the intensity, amplitude, phase, and frequency of the radio frequency voltage applied to the radio frequency kicker 81.

The peeler magnetic field region 42 and the regenerator magnetic field region 43 are regions where a multipole magnetic field exists that acts on the beam passing through them. This multipole magnetic field includes at least a quadrupole magnetic field component. This multipole magnetic field may further include a quadrupole or more multipole magnetic field or a bipole magnetic field. The peeler magnetic field region 42 has a magnetic field gradient such that the main magnetic field weakens toward the radially outer periphery. Conversely, the regenerator magnetic field region 43 has a magnetic field gradient such that the main magnetic field strengthens toward the radially outer periphery. The peeler magnetic field region 42 can also be a region where the main magnetic field decreases at the pole end.

The peeler magnetic field region 42 and the regenerator magnetic field region 43 are each located on the outer periphery of the maximum energy beam orbit 80, in a region of a predetermined azimuth angle on either side of the beam extraction path entrance 82. In addition, it is desirable that the peeler magnetic field region 42 and the regenerator magnetic field region 43 are located at a position on the outer periphery side of the maximum energy beam orbit 80 by a width greater than the amplitude of the betatron oscillation before resonance, so that the beam before the amplitude of the betatron oscillation increases due to the high frequency kicker 81 does not enter the peeler magnetic field region 42 or the regenerator magnetic field region 43. It is also desirable that the peeler magnetic field region 42 is located upstream of the beam extraction path entrance 82 in the beam propagation direction, and the regenerator magnetic field region 43 is located downstream, but the reverse is also possible.

In the vicinity of the peeler magnetic field region 42 and the regenerator magnetic field region 43, multiple magnetic pole pieces and/or coils made of a magnetic material are fixed to the yoke 37 by a non-magnetic material to form the desired multipole magnetic field. For example, for each of the peeler magnetic field region 42 and the regenerator magnetic field region 43, multiple magnetic pole pieces form a multipole magnetic field, and coils form a bipole magnetic field. The multiple magnetic pole pieces and coils can be positioned close to each other, or can be positioned at spatially separated positions.

Figure 4 is a graph showing the distribution of the main magnetic field on the r-axis shown in Figure 2.

The magnetic field strength Bz of the main magnetic field shows a slightly decreasing magnetic field gradient ∂Bz/∂r on the r-axis from the center of the acceleration region to the maximum energy beam orbit 80. In this region, the n value of formula (1) satisfies the stabilization condition, and the beam orbits stably. However, in the range on the r-axis where the regenerator magnetic field region 43 exists on the orbit, the magnetic field gradient ∂Bz/∂r shows that the magnetic field strength Bz of the main magnetic field rises sharply. In this region, the beam is unstable and is kicked toward the inner circumference of the orbital plane. In addition, although not shown, the distribution of the main magnetic field in the r-direction in the range where the peeler magnetic field region 42 exists on the orbit shows a magnetic field gradient ∂Bz/∂r in which the magnetic field strength Bz drops sharply in the peeler magnetic field region 42, opposite to the regenerator magnetic field region 43. In this peeler magnetic field region 42, the beam is unstable as in the regenerator magnetic field region 43, and opposite to the regenerator magnetic field region 43, the beam is kicked toward the outer circumference of the orbital plane.

Here, the increase in the amplitude of the betatron oscillation caused by the high frequency kicker 81 is smaller than the increase in amplitude caused by the peeler magnetic field region 42 and the regenerator magnetic field region 43, so a configuration is shown in which the peeler magnetic field region 42 and the regenerator magnetic field region 43 are used to emit a beam from the circular accelerator 30. However, this configuration is not limited to this. As another configuration, the peeler magnetic field region 42 and the regenerator magnetic field region 43 are not used, and the high frequency kicker 81 can be used to increase the amplitude of the betatron oscillation to emit a beam from the circular accelerator 30.

The position of the high-frequency kicker 81 may be on the maximum energy beam orbit 80 and is not particularly limited, but as an example, the high-frequency kicker 81 is placed near the entrance 82 of the beam emission path as shown in FIG. 2.

Here, the betatron oscillation has a property that the amplitude increases resonantly when the product of either the tune or the fractional part of the tune and the rotation frequency of the beam is approximately equal to the frequency of the applied high frequency voltage. Therefore, in this embodiment, the frequency f _ext of the high frequency voltage applied by the high frequency kicker 81 is set to be approximately equal to the product Δν _r ×f _rev of the fractional part Δν _r of the horizontal tune ν _r of the maximum energy beam and the rotation frequency f _rev of the maximum energy beam. As a result, the amplitude of the horizontal betatron oscillation continues to increase resonantly, and the beam eventually reaches the peeler magnetic field region 42 and the regenerator magnetic field region 43. Note that the frequency f _ext of the high frequency voltage applied by the high frequency kicker 81 may be set to be equal to the product ν _r ×f _rev of the horizontal tune ν _r of the maximum energy beam and the rotation frequency f _rev of the maximum energy beam.

The beam is kicked toward the outer periphery when it passes through the peeler magnetic field region 42, and is kicked toward the inner periphery when it passes through the regenerator magnetic field region 43. Since both the peeler magnetic field region 42 and the regenerator magnetic field region 43 have a magnetic field gradient in the radial direction, the amount of kicking gradually increases as the beam makes multiple revolutions, and the turn separation increases. In other words, the turn separation can be increased by utilizing the resonance condition of the betatron oscillation of 2ν _r =2.

As described above, the septum coil 41 is installed at the entrance 82 of the beam extraction path. When the beam reaches a turn separation that greatly exceeds the thickness of the coil conductor installed on the inner circumference side of the septum coil 41, the beam is guided into the septum coil 41, where it is sufficiently deflected, guided to the high-energy beam transport system 45, and extracted from the circular accelerator 30.

In this case, if the linear combination of the horizontal tune _νr and the vertical tune _νz at the time of extraction is equal to a half integer or an integer, a resonant phenomenon of betatron oscillation called resonance is excited, causing beam divergence. In particular, in a circular accelerator where the n value in equation (1) satisfies the stabilization condition, it is possible to increase the beam current by suppressing the beam divergence caused by the resonance called Walkinshaw resonance.

Walkinshaw resonance occurs when the horizontal tune v _r and the vertical tune v _z satisfy the condition expressed by equation (2).

In addition, there is a width in which resonance is excited, called the resonance width, and in reality, resonance occurs if the right-hand side of equation (2) is equal to or smaller than a certain resonance width δ. The resonance width δ is expressed by equation (3).

In formula (3), _Jr is the function of canonical transformation of the horizontal particle coordinates with the design trajectory as the origin, and G is a complex number expressed by formula (4) and is called the driving term.

Here, in equation (4), j is the imaginary unit. ξ is an arbitrary phase. s is the coordinate in the beam propagation direction. ψr and ψz are the phase advances of the betatron oscillation in the horizontal and vertical directions, respectively. θ is the angular coordinate in the propagation direction coordinate s. _k2 is the second derivative of _Bz in the r direction divided by Bρ, as shown in equation (5), and represents the magnitude of the sextupole magnetic field component.

By setting the magnitude of the driving term to 0, the resonance width δ becomes 0, and resonance occurs only when equation (2) is satisfied, thereby reducing the beam divergence due to resonance.

From formula (4), it can be said that the magnitude of the driving term can be set to 0 or close to 0 by applying the resonance suppression magnetic field 46 that corrects _k2 to any section in the s direction on the outer periphery of the maximum energy beam orbit 80. By disposing a magnetic field (resonance suppression magnetic field 46) for suppressing the beam from diverging, separate from the magnetic fields (peer magnetic field and regenerator magnetic field) for making the beam enter the beam extraction path, between the cylindrical region in which the static magnetic field is formed inside the yoke 37 and the inner wall, it is possible to reduce beam loss due to an increase in the amplitude of betatron oscillation between the cylindrical region and the inner wall in the yoke 37. In this embodiment, the resonance suppression magnetic field 46 is created outside the region through which the beam passes during acceleration when viewed in the radial direction. It is possible to effectively suppress an increase in the amplitude of betatron oscillation in order to make the accelerated beam reach the inner wall of the yoke 37. Moreover, the resonance suppression magnetic field 46 is disposed upstream of the peeler magnetic field region 42 and downstream of the acceleration gap 11 in the orbit in which the beam orbits. The multipole magnetic field component included in the resonance suppressing magnetic field 46 may be, for example, a four-pole or more multipole magnetic field component. By applying the multipole magnetic field component, the magnitude of the driving term can be made close to zero, and the resonance width can be made close to zero.

The main electromagnet 40 is equipped with a generator that creates the resonance suppression magnetic field 46, which may be, for example, a coil or a pole piece, or both, that is placed on the surface of the main magnetic pole 38. Forming the resonance suppression magnetic field 46 with a coil increases the degree of freedom for adjustment, making it possible to realize a robust device. Alternatively, the generator that creates the resonance suppression magnetic field 46 may be a piece of iron that is placed on the surface of the main magnetic pole 38. Because the resonance suppression magnetic field 46 is formed with a piece of iron, there is no need to excite the device each time it is used, making it possible to easily and quickly use the device.

Figure 5 is a graph showing the distribution of magnetic field strength Bz in the r direction in a region including the resonance suppression magnetic field 46. The magnetic field strength Bz shown in Figure 5 has been corrected by the resonance suppression magnetic field 46.

In addition, in this embodiment, as shown in FIG. 2, the resonance suppression magnetic field 46 is applied to only one location. However, this is not limited to this. As another example, the resonance suppression magnetic field 46 may be multiple in any region in the s direction.

In addition, in formula (4), as an example, the product of the first power of the horizontal betatron vibration amplitude βr and the square of the vertical betatron vibration amplitude _βz is used. In formula (4), the magnitude and driving term of the sextupole magnetic field component are large in the region where the product of the horizontal betatron vibration amplitude and the square of the vertical betatron vibration amplitude is large, so that the resonance width can be reduced by providing a second magnetic field that reduces the magnitude of the sextupole magnetic field component in the region where the product of the horizontal betatron vibration amplitude and the square of the vertical betatron vibration amplitude is large. However, this is not limited to this. As another example, the product of the real power of the horizontal betatron vibration amplitude βr and the real power of the vertical betatron vibration amplitude _βz can be used.

The above explanation is about reducing the resonance width of Walkinshaw resonance, but the resonance width of other types of resonance can also be reduced to zero by applying a multipole magnetic field component that sets the magnitude of the driving term to zero.

The circular accelerator of Example 2 will be described with reference to the drawings. Here, the description of the configuration of this example that is the same as that of Example 1 will be omitted, and only the configuration that differs from Example 1 will be described.

The circular accelerator of this embodiment is an eccentric orbit accelerator in which the main magnetic field is formed to eccentrically orbit the beam toward the entrance 82 of the beam extraction path so that the energy of the extracted beam can be freely changed between 70 MeV and 235 MeV.

Figure 6 is an external view of the circular accelerator 30 of the second embodiment. Figure 7 is a cross-sectional view of the eccentric orbit accelerator of the second embodiment cut at the center plane. Figure 8 is a vertical cross-sectional view of the eccentric orbit accelerator of the second embodiment (view taken along the A-A' arrow in Figure 7).

In the circular accelerator 30 of Example 2, the radius of the orbit around which the beam travels increases as the beam accelerates, and as the radius increases, the center of the orbit moves in a specified direction (to the right in FIG. 7). In the opposite direction to the direction in which the center of the orbit moves (to the left in FIG. 7), there is a region where the orbit becomes dense according to the speed, and the beam extraction path entrance 82 is located near this region. Because the orbit becomes dense near the beam extraction path entrance 82, a beam of the desired energy can be easily emitted.

The structural changes in the eccentric orbit accelerator shown in Figures 7 and 8 from the circular accelerator of Example 1 shown in Figures 2 and 3 are the shapes of the dee electrode 12 and dummy dee electrode 13, and the shape of the acceleration gap 11 formed between them.

Here, the line passing through the rotation axis of the rotating condenser 31 and the center of the circle of the acceleration region is defined as the center line. The ion injection section 52 is positioned on the center line closer to the entrance 82 of the beam extraction path than the center of the acceleration region. In addition, although not shown, the shapes of the upper and lower opposing surfaces of the main magnetic pole 38 for forming the main magnetic field described below are also significantly different from those in Example 1. The configuration of the radio frequency kicker 81 also differs in structure from that in Example 1 in order to apply a radio frequency electric field to multiple energy orbits.

Figure 9 shows a conceptual diagram of the design orbit for each energy. The dotted lines connect the same orbital phases of each orbit, and are called iso-orbital phase lines. The iso-orbital phase lines are plotted for every orbital phase π/4, including the orbital phase φ = 0 set in the concentration region. The acceleration gap 11 formed between the dee electrode 12 and the opposing dummy dee electrode 13 is installed along the iso-orbital phase line.

More specifically, the dee electrode 12 has a hollow sector-like shape surrounded by two curves that extend from the tip near the center of the minimum emission energy orbit 83 to the circumference along an iso-rotating phase line, and an arc connecting the points on the circumference reached by the two curves. The dummy dee electrode 13 has an outer shape that has two curves that respectively face the two curves of the dee electrode 12.

In regions where the beam energy is low, the beam trajectory is close to a concentric trajectory centered near the ion injection section 52, similar to that of a cyclotron. However, in regions where the beam energy is higher, the beam trajectory is tightly concentrated near the entrance 82 of the beam exit path, and conversely, the trajectories of each energy are separated from each other near the inner conductor 14.

The area where these orbits are densely packed is called the concentrated area, and the area where they are dispersed is called the dispersed area. By configuring the beam to be extracted from near the concentrated area, the amount of kick required to extract the beam can be reduced, making it easier to select the desired energy and emit the beam.

To create the orbit configuration described above and generate stable vibrations around the orbit, the accelerator of this embodiment creates a magnetic field distribution in which the magnetic field strength of the main magnetic field decreases toward the radially outer periphery by using the shape of the main pole 38 and the trim coils and/or pole pieces installed on its surface. In addition, the main magnetic field is a constant value on a line along the design orbit. Therefore, the design orbit at each energy is circular.

Next, we will explain how the beam is emitted.

To emit a beam from the circular accelerator 30, a high-frequency kicker 81 is installed near the concentration region where the trajectories of all energy beams are concentrated, and a peeler magnetic field region 42 and a regenerator magnetic field region 43 are placed on either side of it, a septum coil 41, and a high-energy beam transport system 45 are used.

In this embodiment, among the above components used for extraction, the configuration of the radio frequency kicker 81 is different from that of the first embodiment. The procedure for extracting the beam is basically the same as that described in the first embodiment. However, if the timing for cutting off the radio frequency acceleration voltage applied to the acceleration gap 11 and the timing for starting the application of the radio frequency voltage to the radio frequency kicker 81 are shifted forward, a beam of any energy can be extracted. By starting the application of the radio frequency voltage, the amplitude of the betatron oscillation of the beam of the desired energy is increased by the radio frequency kicker 81. Eventually, the beam reaches the peeler magnetic field region 42 and the regenerator magnetic field region 43, and is extracted from the circular accelerator 30.

The circular accelerator 30 of this embodiment is also a circular accelerator in which the n value in formula (1) satisfies the stabilization condition, as in the first embodiment. It is possible to increase the beam current by suppressing the beam divergence due to the resonance called Walkinshaw resonance. As in the first embodiment, the second embodiment can also reduce the beam divergence due to the resonance by setting the magnitude of the driving term to 0. In addition, according to formula (4), it is possible to set the magnitude of the driving term to 0 or close to 0 by applying the resonance suppression magnetic field 46 for correcting _k2 to any section in the s direction on the outer periphery of the maximum energy beam orbit 80.

The relationship between the amplitude _βr and phase φ of the horizontal betatron oscillation for each energy is shown in Fig. 10. The relationship between the amplitude _βz and phase φ of the vertical betatron oscillation for each energy is shown in Fig. 11. The amplitudes _βr and _βz are symmetrical with respect to the phase φ = π.

The amplitude _βr shown in Fig. 10 is maximum when the phase φ=π/2, 3π/2, while the amplitude _βz shown in Fig. 11 is maximum when the phase φ=0 and is minimum when φ=π.

From equation (4), it can be said that the degree of contribution of _k2 to the driving term becomes large in the s-direction coordinate where the amplitude _βr and the amplitude _βz are large. Therefore, in the circular accelerator 30 of this embodiment, since the amplitude βz is relatively flat with respect to the phase φ, the resonance suppression magnetic field ₄₆ is applied near the iso-periodic phase line of phase φ=π/2 and/or near the iso-periodic phase line of 3π/2 where the amplitude βr, which varies greatly with the phase φ, is maximized. Since the horizontal betatron oscillation amplitude is maximized near the phases π/2 (90 degrees) and 3π/2 (270 degrees), the resonance width can be reduced by providing a second magnetic field in the vicinity thereof that reduces the magnitude of the sextupole magnetic field component.

In the circular accelerator 30 of this embodiment, similar to the circular accelerator 30 of the first embodiment, the resonance suppression magnetic field 46 may be present in multiple regions in any s-direction. In addition, although the Walkinshaw resonance has been described in this embodiment, the resonance width can be set to 0 for other types of resonance in the same manner by applying a multipole magnetic field component so that the magnitude of the driving term is set to 0.

As a third embodiment, we will explain a particle beam therapy device that uses a circular accelerator.

Figure 12 is a diagram showing the overall configuration of a particle beam therapy device according to the third embodiment. The particle beam therapy device includes the circular accelerator 30 shown in the first or second embodiment, a rotating gantry 90, an irradiation device 92 including a scanning magnet, a treatment table 101, and a control device 91 for controlling them.

The beam emitted from the circular accelerator 30 is transported to the irradiation device 92 by the rotating gantry 90. The ion beam transported to the irradiation device 92 is shaped to match the affected area by adjusting the beam energy in the irradiation device 92, and a predetermined amount is irradiated to the affected area of the patient 100 lying on the treatment table 101.

The irradiation device 92 includes a dose monitor and monitors the dose irradiated to each irradiation spot on the patient 100. Based on this dose data, the treatment control device 91 calculates the required dose for each irradiation spot and uses this as input data for the accelerator control device 93. The accelerator control device 93 controls the entrance, acceleration, and exit of the charged particle beam in the circular accelerator 30.

10...RF acceleration cavity, 11...acceleration gap, 12...Dee electrode, 13...dummy Dee electrode, 14...inner conductor, 15...outer conductor, 20...input coupler, 21...acceleration RF power supply, 30...circular accelerator, 31...rotating capacitor, 32...motor, 33...fixed electrode, 34...rotating electrode, 35...main magnetic pole, 36...cryostat, 37...yoke, 38...main coil, 38...main magnetic pole, 40...main electromagnet, 41...septum coil, 42...peer magnetic field region, 43...regenerator magnetic field region, 44...beam through hole, 45... High energy beam transport system, 46...resonance suppression magnetic field, 47...coil through hole, 48...vacuum through hole, 49...high frequency system through hole, 50...ion source, 51...low energy beam transport system, 52...ion injection section, 53...electrostatic inflector, 80...maximum energy beam trajectory, 81...high frequency kicker, 82...beam extraction path entrance, 83...minimum extraction energy trajectory, 90...rotating gantry, 91...treatment control device, 91...control device, 92...irradiation device, 93...accelerator control device, 100...patient, 101...treatment table

Claims (10)

A main electromagnet that defines a cylindrical region inside the yoke and generates a static magnetic field in the cylindrical region;
a beam extraction path for transporting the charged particle beam from the inside to the outside of the main electromagnet;
an accelerating electric field applicator that applies a frequency-modulated accelerating electric field to accelerate the charged particle beam circulating in the static magnetic field;
a kicker for increasing the amplitude of betatron oscillation of the charged particle beam accelerated by the accelerating electric field;
having
the main electromagnet creates a first magnetic field for causing the charged particle beam, the amplitude of which has been increased by the kicker, to enter the beam extraction path, and a second magnetic field for suppressing divergence of the charged particle beam between a region of the static magnetic field in the yoke and an inner wall of the yoke,
The second magnetic field is
being produced outside a region through which the charged particle beam passes during acceleration;
Contains four or more multi-pole magnetic field components,
The charged particle beam is disposed in the vicinity of a region where the product of the real power of the amplitude of the horizontal betatron oscillation of the charged particle beam and the real power of the amplitude of the vertical betatron oscillation is maximized.
Accelerator. the second magnetic field is disposed in the vicinity of a region where the product of the amplitude of the horizontal betatron oscillation and the square of the amplitude of the vertical betatron oscillation of the charged particle beam is maximized;
The accelerator of claim 1 . The main electromagnet is
a peeler magnetic field region for kicking the beam toward the outer periphery of the orbit, formed on the upstream side of the orbit in which the charged particle beam circulates, between the kicker and an entrance of the beam extraction path, and a regenerator magnetic field region for kicking the charged particle beam toward the inner periphery of the orbit, formed on the downstream side;
forming the second magnetic field upstream of the peeler magnetic field region;
The accelerator of claim 1 . the accelerating electric field applicator has a first electrode and a second electrode, and applies the accelerating electric field to a gap between the first electrode and the second electrode;
The main electromagnet forms the second magnetic field upstream of the peeler magnetic field region and downstream of the gap.
The accelerator of claim 3 . the radius of the orbit of the charged particle beam increases in accordance with the velocity of the charged particle beam, and the center of the orbit moves in a predetermined direction as the radius increases;
An accelerator according to any one of claims 1 to 4 . a region in which the trajectory according to the speed becomes dense in a direction opposite to the predetermined direction, and an entrance of the beam extraction path is disposed in the vicinity of the region;
The accelerator of claim 5 . A main electromagnet that defines a cylindrical region inside the yoke and generates a static magnetic field in the cylindrical region;
a beam extraction path for transporting the charged particle beam from the inside to the outside of the main electromagnet;
an accelerating electric field applicator that applies a frequency-modulated accelerating electric field to accelerate the charged particle beam circulating in the static magnetic field;
a kicker for increasing the amplitude of betatron oscillation of the charged particle beam accelerated by the accelerating electric field;
having
the main electromagnet creates a first magnetic field for causing the charged particle beam, the amplitude of which has been increased by the kicker, to enter the beam extraction path, and a second magnetic field for suppressing divergence of the charged particle beam between a region of the static magnetic field in the yoke and an inner wall of the yoke,
a radius of the orbit around which the charged particle beam moves increases in accordance with a velocity of the charged particle beam, and a center of the orbit moves in a predetermined direction as the radius increases;
a region in which the trajectory becomes dense according to the speed is present in a direction opposite to the predetermined direction, and an entrance of the beam extraction path is disposed in the vicinity of the region;
The second magnetic field is disposed in the vicinity of a region in the orbit of maximum energy where the rotation phase, with the reverse direction being 0 degrees, is 90 degrees or 270 degrees .
Accelerator . The accelerator of claim 1, wherein the main electromagnet includes a coil installed in a main pole that forms the static magnetic field in the main electromagnet, and the second magnetic field is created by the coil. The accelerator of claim 1, wherein the main electromagnet includes an iron piece disposed in a main pole that forms the static magnetic field in the main electromagnet, and the second magnetic field is created by the iron piece. A particle beam therapy system comprising the accelerator according to any one of claims 1 to 9 .

Images