Circular accelerator, particle therapy system, and control method of circular accelerator

Abstract

To provide a circular accelerator capable of extending the charge storage time compared to conventional methods, a particle therapy system, and a control method of the accelerator.SOLUTION: An accelerator 1 is an accelerator that accelerates beam particles using a frequency-modulated high-frequency accelerating electric field to change the orbital radius of the beam particles orbiting in the magnetic field which changes with energy. At least one charge storage coil 301 is provided vertically above or below the beam trajectory of the beam particles, which controls the dynamic magnetic field generated within the installation range so that the circular frequency of the beam particles changes.SELECTED DRAWING: Figure 3

Description

[Technical field]

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

[0002] Patent Document 1 describes an accelerator having a pair of magnets that form a magnetic field between them, an ion source that injects ions between the magnets, an acceleration electrode that accelerates the ions, and a beam extraction path that extracts the ions to the outside, in which a number of circular beam orbits formed by the pair of magnets, each of which has ions of different energy, are converged on one side, and the frequency of the high-frequency electric field applied to the ions by the acceleration electrode is modulated according to the beam orbit.

[0003] Patent document 2 describes a circular accelerator that includes an accelerating cavity for accelerating particles, a capacitively coupled input coupler for supplying high frequency power to the accelerating cavity, and a rotating capacitor for modulating the resonant frequency of the accelerating cavity, in which a floating potential electrode is connected to the rotating electrode of the rotating capacitor via an insulating support, and the distance between the tip of the input coupler and the floating potential electrode changes as the rotating electrode rotates. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent registration No. 6714146 [Patent Document 2] JP 2019-91595 A Summary of the Invention [Problem to be solved by the invention]

[0005] High-energy nuclear beams used in particle beam therapy and physics experiments are generated using accelerators.

[0006] One type of accelerator is one that deflects beam particles with a magnetic field generated by an electromagnet to make them go around a circular orbit, and accelerates the beam particles for each revolution using a high-frequency electric field generated in an accelerating cavity installed on the orbit. Examples of such accelerators include synchrotrons, cyclotrons, and synchrocyclotrons such as static magnetic field type variable energy accelerators (see Patent Document 1).

[0007] In a synchrotron, as the beam particles accelerate, the magnetic field of the bending electromagnets installed on the orbit is strengthened, causing the beam particles to orbit in a constant orbit. Since the orbital period of the beam decreases as the beam accelerates, the period of the accelerating electric field also decreases in sync with the decrease in the orbital period of the beam. However, since the orbit is constant in a synchrotron regardless of energy, the beam can be extracted at the desired energy by the extraction equipment installed on the orbit. However, the cycle speed of repeated acceleration is limited due to heat generation caused by eddy currents due to dynamic magnetic fields and disturbances in the magnetic field, resulting in intermittent beam irradiation.

[0008] In contrast, since a cyclotron is a static magnetic field type, the radius of curvature of the beam particle's orbit increases as the beam particles injected from the ion source installed in the center of the accelerator accelerate. Since the orbit length increases in proportion to the increase in the beam particle's speed, the beam's rotation period is constant regardless of the energy, and the period of the high-frequency acceleration electric field is constant. In addition, since a cyclotron is a static magnetic field type, problems caused by eddy currents due to dynamic magnetic fields do not occur, making it possible to irradiate the beam quasi-continuously by extracting the beam with a fast repeated acceleration cycle.

[0009] However, because the radius of curvature of the beam particle trajectory increases with acceleration, the beam cannot be extracted unless it accelerates until it reaches the extraction device installation position, and the extraction energy cannot be changed to its maximum value. Therefore, in order to irradiate a beam with the desired energy, a degrader that reduces the energy of the beam extracted from the accelerator is installed in the beam transport system. Since beam loss occurs when the beam passes through the degrader, the dose rate decreases in the low energy region.

[0010] In the static magnetic field type variable energy accelerator described in the above-mentioned Patent Document 1, as in the cyclotron, the radius of curvature of the beam particle trajectory increases as the beam particles injected from the ion source installed at the center of the accelerator accelerate. Since the increase in the trajectory length is not proportional to the increase in the beam particle speed, the period of the high-frequency accelerating electric field changes with the acceleration of the beam particles. Since the static magnetic field type variable energy accelerator uses a static magnetic field, it has the advantage that quasi-continuous irradiation is possible by beam extraction with a fast repeating cycle.

[0011] In a cyclotron, the magnetic field distribution is symmetrical about the ion source, whereas in a static magnetic field type variable energy accelerator, the magnetic field distribution is asymmetrical about the ion source. Therefore, in a static magnetic field type variable energy 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.

[0012] By installing the extraction equipment at the orbit convergence point, it is possible to extract a beam of the desired energy, and there is no decrease in the dose rate due to the degrader. Because of its ability to rapidly accelerate repeatedly and its energy variability, variable energy accelerators are expected to provide a high dose rate over the entire range of extraction energies. Furthermore, if the amount of accumulated charge in the accelerator can be increased, further improvements in the dose rate can be expected.

[0013] In a static magnetic field type variable energy accelerator, beam particles are supplied from the ion source sequentially for each period of the accelerating electric field, but not all of them can be accelerated. Whether the particles supplied from the ion source (supplied particles) are appropriately accelerated or not is determined by the period of the accelerating electric field and the rotation period of the beam particles.

[0014] The rotation period of the beam particles is determined by the energy of the beam and the magnetic field in the rotation region. Since the energy and magnetic field are constant for the particles (supplied particles) supplied from the ion source, the initial rotation period (initial period) of the supplied particles is constant.

[0015] In a static magnetic field type variable energy accelerator, the frequency of the accelerating radio frequency electric field and the rotation frequency of the beam are synchronized at an integer multiple ratio, so particles accelerated at a certain phase of the accelerating electric field are accelerated at the same phase in the next turn. Therefore, the supplied particles (reference particles) supplied at a timing when the initial period and the period of the accelerating radio frequency electric field match are accelerated most efficiently.

[0016] On the other hand, a particle accelerated in a phase earlier than the acceleration phase is accelerated more than a particle accelerated in the acceleration phase, so it is accelerated in a delayed phase in the next turn. Conversely, a particle accelerated in a phase later than the acceleration phase is accelerated less than a particle accelerated in the acceleration phase, so it is accelerated in an advanced phase in the next turn.

[0017] In this way, particles with timing shifted from the specified acceleration phase move in the direction returning to the acceleration phase, and this action allows them to oscillate stably even within the phase plane (propagation direction) consisting of momentum and phase. This oscillation is called synchrotron oscillation.

[0018] In accelerators, the stable region of synchrotron oscillation, defined by the energy difference ΔE and phase difference Δφ between the reference particle and the orbiting particle, is called a bucket. Stably orbiting beam particles change position on a fixed trajectory within the bucket unless subjected to an external force.

[0019] The beam particle injected earlier than the reference particle passes through the acceleration cavity more times than the reference particle, and is therefore positioned in the +ΔE direction. On the other hand, the beam particle injected later passes through the acceleration cavity less times than the reference particle, and is therefore positioned in the -ΔE direction. The period of the accelerating radio frequency electric field is designed based on the reference particle, and the period of the accelerating radio frequency electric field increases as the energy of the reference particle increases.

[0020] Therefore, ΔE is the energy threshold of the bucket (ΔE max ), the difference between the initial period and the period of the high frequency electric field increases, and the supplied particles cannot be accelerated. maxThe period during which the supplied particles can be appropriately accelerated is called the charge accumulation time, which is determined by the frequency modulation (FM) speed of the accelerating electric field and the magnetic field in the orbit that affects the rotation period of the beam particles. The longer the charge accumulation time, the more the amount of charge that can be accumulated in the accelerator can be increased, and therefore the dose rate of the irradiation beam can be increased.

[0021] Patent Document 2 shows an accelerating electric field generator using a cavity resonator that uses a rotating capacitor to perform frequency modulation of the accelerating electric field. The rotating capacitor is composed of a stationary plate called the stator and a rotating plate called the rotor. The overlapping area between the rotor and the stator changes depending on the rotation angle of the rotor, which changes the capacitance of the capacitor and the resonant frequency of the cavity resonator.

[0022] Therefore, in a static magnetic field accelerator that uses a rotating capacitor to frequency-modulate the accelerating electric field, once the rotor and stator of the rotating capacitor are manufactured and their shapes are determined, it is difficult to adjust the FM speed. Because the charge accumulation time, which affects the dose rate, is determined by the magnetic field and FM speed, it is difficult to extend the charge accumulation time without remaking the rotor and stator.

[0023] The present invention provides a circular accelerator, a particle beam therapy system, and an accelerator control method that can extend the charge accumulation time compared to the conventional methods. [Means for solving the problem]

[0024] The present invention includes multiple means for solving the above problems. One example is a circular accelerator that accelerates beam particles using a frequency-modulated high-frequency acceleration electric field, and in which the orbital radius of the beam particles circulating in a magnetic field changes depending on the energy, and includes at least one coil installed vertically above or below the beam orbit of the beam particles, which controls a dynamic magnetic field generated within the installation range so as to change the orbital frequency of the beam particles. Effect of the Invention

[0025] According to the present invention, the charge accumulation time can be extended as compared with the conventional art. Problems, configurations and effects other than those described above will become apparent from the following description of the embodiments. [Brief description of the drawings]

[0026] [Figure 1] 1 is a schematic diagram of an overall configuration of a particle beam therapy system according to an embodiment. [Diagram 2] FIG. 1 is a diagram showing an overall outline of a circular accelerator according to an embodiment. [Diagram 3] FIG. 2 is a diagram showing the internal equipment layout of the circular accelerator of the embodiment. [Figure 4] FIG. 2 is a timing diagram for explaining the accumulation process of the circular accelerator of the embodiment. [Diagram 5] FIG. 2 is a schematic diagram for explaining the movement of buckets in the circular accelerator of the embodiment. [Figure 6] FIG. 2 is a schematic diagram for explaining the movement of buckets in the circular accelerator of the embodiment. [Figure 7] FIG. 2 is a schematic diagram for explaining the movement of buckets in the circular accelerator of the embodiment. [Figure 8] FIG. 2 is a schematic diagram for explaining the movement of buckets in the circular accelerator of the embodiment. [Figure 9] FIG. 2 is a schematic diagram for explaining the movement of buckets in the circular accelerator of the embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0027] The circular accelerator, particle beam therapy system, and circular accelerator control method of the present invention will be described with reference to Fig. 1 to Fig. 9. In the drawings used in this specification, the same or corresponding components are denoted by the same or similar reference numerals, and repeated description of these components may be omitted.

[0028] 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 apparatus according to an embodiment.

[0029] The particle beam therapy system 10 of this embodiment shown in FIG. 1 comprises an accelerator 1, a rotating gantry 2, an irradiation device 3 including a scanning magnet, a treatment table 4, a control device 7 that controls the operation of each of these devices, and an accelerator control device 8.

[0030] In a particle beam therapy system 10, a beam extracted from an accelerator 1 is transported to an irradiation device 3 by a rotating gantry 2. The transported ion beam is shaped to match the affected area by adjusting the beam energy in the irradiation device 3 and / or the accelerator 1, and a predetermined amount of the beam is irradiated to the affected area of ​​a patient 5 lying on a treatment table 4.

[0031] 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 it as input data for the accelerator control device 8. The accelerator control device 8 controls the injection, acceleration, and extraction of the charged particle beam in the accelerator 1, and supplies a beam of the required dose and energy.

[0032] Next, an accelerator according to a first embodiment, which is a preferred embodiment of the present invention, will be described below with reference to FIGS.

[0033] First, an overview of the accelerator will be described with reference to Fig. 2. Fig. 2 shows an outline of the overall configuration of the accelerator 1.

[0034] The accelerator 1 shown in FIG. 2 is a circular accelerator that uses a static magnetic field to deflect and circulate beam particles, and accelerates the beam particles for each revolution using a frequency-modulated high-frequency accelerating electric field.

[0035] The accelerator 1 excites a main magnetic field in a region through which the beam passes (hereinafter referred to as a beam passing region) by means of a magnet 11 that can be separated into upper and lower portions, and the inside of the beam passing region is evacuated to a vacuum.

[0036] Magnet 11 has a number of through-holes, including beam extraction through-hole 111 for extracting the accelerated beam, outlets 112 and 113 for extracting the internal coil to the outside, and radio frequency power input through-hole 114, which are provided on the connecting surface between the upper and lower magnetic poles. Radio frequency cavity 21 is installed through radio frequency power input through-hole 114. In radio frequency cavity 21, a dee electrode for acceleration and a rotary capacitor 212 are installed, as described later.

[0037] An ion source 12 that generates beam particles is installed at a position shifted from the center of the upper portion of the magnet 11, at a different position in the radial direction, and the beam is injected into the accelerator 1 through a beam injection through hole 115.

[0038] Next, the internal structure of the accelerator 1 will be described with reference to Fig. 3. Fig. 3 is a diagram showing the layout of the internal equipment of the circular accelerator of this embodiment.

[0039] 3, the inside of magnet 11 has a cylindrical beam passing region 20 formed by a cylindrical inner wall, and an annular main coil 13 is installed along the inner wall. Magnet 11 is magnetized by passing a current through main coil 13, and a magnetic field with a predetermined distribution is excited in beam passing region 20.

[0040] Magnetic poles 15 are disposed inside the main coil 13 so as to face each other vertically and form the upper and lower boundaries of a beam passing region 20 , and a cylindrical return yoke 14 is provided on the outside of the main coil 13 .

[0041] The magnetic poles 15 are magnetized by passing a predetermined excitation current through the main coil 13, and a magnetic field is generated between the magnetic poles 15. In order to excite a predetermined magnetic field distribution, the magnetic poles 15 have a shape in which the distance (gap) between the upper and lower magnetic poles that constitute the magnetic poles 15 changes spatially depending on the horizontal position. The magnetic poles 15 in this embodiment are shaped such that the distance (gap) between the upper and lower magnetic poles is smallest at the incident point and increases toward the periphery, and an example will be described in which the magnetic field is largest at the ion incident point and decreases toward the periphery, but the distance between the magnetic poles 15 may be large at the incident point and decreases toward the periphery. The shape of such magnetic poles 15 is not limited to these magnetic pole shapes as long as it can realize a magnetic field distribution that stabilizes the orbital motion of the beam in the beam passing region 20.

[0042] Furthermore, the magnetic pole shape and coil arrangement are symmetrical with respect to a plane (orbital plane) that passes through the gap center, and on the orbital plane, there is only a magnetic field component perpendicular to the orbital plane.

[0043] The magnetic pole shape is symmetrical about the axis AA' in the central plane, which results in a symmetrical distribution of the magnetic field.

[0044] If we define a right-handed coordinate system with the injection point 120 as the origin and the y-axis pointing from the origin to the center of the accelerator, the magnetic field distribution of the cyclotron is symmetrical with respect to the origin, but the magnetic field distribution formed by magnetic pole 15 is not symmetrical with respect to the origin, and the gradient of the magnetic field distribution in the positive y-axis direction becomes gentler away from the beam injection point. As a result, in the low energy region below 80 MeV, the orbit is centered near injection point 120 like the cyclotron, but as the energy of the beam particles increases, the center of the beam orbit moves in one direction on the same plane, resulting in the formation of an orbit confluence point where the orbits are densely concentrated.

[0045] In addition, Dee electrodes 31 and 32 for exciting a high-frequency electric field, an extraction septum electromagnet 40, a massless septum coil 50 for generating a kicker magnetic field, a magnetic field distribution adjustment coil 60, and a charge accumulation coil 301 are provided.

[0046] The kicker magnetic field is a massless septum type that applies a magnetic field only to a specific position in the radial direction of the accelerator. The kicker magnetic field is excited by passing electric current through a pair of coils that are installed symmetrically in a direction perpendicular to the beam orbit plane.

[0047] The charge storage coil 301 controls a local dynamic magnetic field generated within an installation range including a beam injection region so as to change the rotation frequency of the beam particles. The charge storage coil 301 of this embodiment will be described taking as an example a pair of coils installed vertically above and below the beam trajectory of the beam particles so as to be vertically symmetrical.

[0048] In this embodiment, the charge storage coil 301 is configured so that a pair of coils are arranged vertically symmetrically above and below the beam orbit, thereby making it possible to maintain higher stability of the beam in the vertical direction (z-axis direction) relative to the orbital plane of the beam particles (xy plane).

[0049] The charge storage coil 301 consists of two coils, one above and one below, which are smaller than the magnetic field distribution adjustment coil 60. In order to generate a dynamic magnetic field near the incidence point 120, the charge storage coil 301 is arranged, one above and one below the beam orbital plane, and parallel to the beam orbital plane so that the horizontal position of the accelerator 1 includes the incidence point 120 where the beam particles generated by the ion source 12 are incident.

[0050] In this embodiment, a pair of charge storage coils 301, one each installed above and below the beam orbit plane, has been described as an example, but any configuration in which a charge storage coil is placed either vertically above or below the beam orbit may be used, and a charge storage coil composed of multiple coils may also be used. Furthermore, when there is a pair vertically above and below, their radial positions do not need to be completely aligned, and can be changed as appropriate according to the design.

[0051] In addition, by increasing the size of the charge storage coil 301, when a beam particle is supplied at a later timing, the beam particle supplied at an earlier timing is accelerated and the radius of curvature of the trajectory is expanded, thereby reducing the possibility that the particle supplied at an earlier timing will go outside the charge storage coil 301, which is desirable in terms of increasing the number of stored particles. In other words, since the inside of the charge storage coil 301 is the region where the bucket position can be controlled, it is desirable to make the charge storage coil 301 sufficiently large, and the size of the charge storage coil 301 is desirable to cover at least the entire orbit of the beam particle at the timing of the end of beam injection.

[0052] In the accelerator 1 of this embodiment, the incidence point is located at a position shifted from the center of the electromagnet toward the orbit convergence point, and therefore, an example has been described in which the charge accumulation coil 301 is also disposed at a position shifted toward the orbit convergence point. However, when the present invention is applied to an accelerator such as a synchrotron cyclotron in which the beam incidence point is located at the center of the electromagnet, the charge accumulation coil 301 is disposed at the center of the electromagnet.

[0053] The charge accumulation coil 301 is connected to a dynamic magnetic field control device 302 that controls a dynamic magnetic field generated near the incidence point 120. The dynamic magnetic field control device 302 includes a function generator that generates a current waveform to excite the charge accumulation coil 301 and a high-speed bipolar power supply, and constitutes a part of the accelerator control device 8 described above.

[0054] The beam is injected into the accelerator 1 in the form of low-energy ions from an injection point 120. The injected beam is accelerated by the radio frequency electric field excited by the radio frequency cavity 21 every time it passes through an electric field gap.

[0055] The magnetic pole 15 and the dee electrodes 31 and 32 are shaped symmetrically with respect to the yz plane.

[0056] In the magnetic field inside the accelerator 1 configured as above, particles that deviate from the designed orbit in the horizontal and vertical directions are subjected to a restoring force that returns them to the designed orbit, causing them to oscillate around the designed orbit. This oscillation is called betatron oscillation, and it allows the beam particles to stably orbit and accelerate around the beam.

[0057] Further, the magnetic pole 15 is provided with a magnetic field distribution adjusting coil 60 for fine adjustment of the magnetic field, and the current of the magnetic field distribution adjusting coil 60 is adjusted before operation to ensure stability of the betatron oscillation.

[0058] The high-frequency cavity 21 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 an input coupler 211. The high-frequency cavity 21 is connected to dee electrodes 31 and 32 inserted in the gap, and a high-frequency electric field is excited between the dee electrodes 31 and 32 and the ground electrode 35.

[0059] In the accelerator of the present invention, 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 as used in the present invention, it is necessary to sweep the high-frequency frequency in a range wider than the width of the resonance. For this reason, it is necessary to change the resonant frequency of the cavity.

[0060] This control is performed by changing the capacitance of a variable-capacitance rotary capacitor 212 installed at the end of the cavity. The rotary capacitor 212 controls the capacitance generated between the conductor plates (rotor and stator) directly connected to the rotating shaft and the external conductor by the rotation angle of the rotating shaft 213. In other words, the rotation angle of the rotating shaft 213 is changed in accordance with the acceleration of the beam.

[0061] In order to extract a desired beam at a desired energy, one or more coils of the massless septum coil 50 are selected based on the desired energy and a desired excitation current is passed through them.

[0062] When no current is passed through the massless septum coil 50, the beam of the target energy revolves along its designed trajectory, but when current is passed through the massless septum coil 50, the beam that has reached the target energy is deviated from the trajectory by a kick magnetic field caused by the massless septum coil 50.

[0063] As described above, the deviated beam oscillates stably around the designed orbit. That is, betatron oscillations are excited in the orbital plane by the massless septum coil 50.

[0064] 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, the kick by the massless septum coil 50 can displace the beam at the convergence point radially outward.

[0065] Next, we will describe the behavior of the beam from injection to extraction in the accelerator 1 of this embodiment. The operation of the accelerator 1 of this embodiment consists of three steps: injection, acceleration, and extraction.

[0066] In the injection step, a beam of low-energy ions is output from the ion source 12, which is arranged at a radially different position from the center of gravity of the annular main coil 13, and the beam is guided to the beam passing region through the injection point 120, which is the inner region of the innermost circular orbit.

[0067] After passing through the accumulation process described later, the beam injected into the beam passing region 20 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, but since the period of the accelerating radio frequency electric field is controlled so as to be synchronized with the orbital period of the beam particles, the period of the accelerating radio frequency electric field also increases.

[0068] In the subsequent acceleration step, the beam is accelerated while ensuring stability in the direction of travel due to the high-frequency electric field. In other words, the beam does not pass through the acceleration gap when the high-frequency electric field is at its maximum, but when the high-frequency electric field is decreasing over time. 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 certain phase of the accelerating electric field are accelerated at the same phase in the next turn.

[0069] On the other hand, particles accelerated in a phase earlier than the acceleration phase are accelerated in a delayed phase in the next turn because the amount of acceleration is greater than that of particles accelerated in the acceleration phase. Conversely, particles accelerated in a phase later than the acceleration phase are accelerated in a delayed phase in the next turn because the amount of acceleration is smaller than that of particles accelerated in the acceleration phase.

[0070] In this way, the particles undergo synchrotron oscillation while being accelerated, and gradually accelerate until they reach the specified energy level at which they are extracted.

[0071] 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 50, and 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 (extraction step).

[0072] So far, the behavior of the beam from beam injection to extraction in the accelerator 1 of the present invention has been described. However, the greatest feature of the accelerator 1 of the present invention is the charge accumulation process using the charge accumulation coil 301 .

[0073] In the accelerator 1 of this embodiment, preferably, in order to synchronize the rotation period of the injected beam particles to the change over time of the period of the accelerating high-frequency electric field, the time from the start of excitation to the end of excitation is set to be sufficiently shorter than the time required from the injection step to the removal step (hereinafter, referred to as the operation cycle, synonymous with the beam acceleration time of the accelerator 1), and the injected beam particles are positioned at the center of the bucket by generating a dynamic magnetic field in the beam injection region by the charge storage coil 301, which changes so that the rotation frequency of the beam particles is synchronized with the frequency of the high-frequency acceleration electric field.

[0074] This makes it possible to extend the charge accumulation time in the incident step compared to the conventional method, and to achieve an improvement in the dose rate due to an increase in the amount of accumulated charge.

[0075] The excitation timing of this charge storage coil 301 is preferably set to coincide with the beam injection timing of the beam particles from the ion source 12, and it is preferable that the excitation be repeated at each beam injection timing from the ion source 12 during operation of the accelerator 1.

[0076] The behavior of the charge storage coil 301 and the dynamic magnetic field control device 302 in the storage process will be described below with reference to Fig. 4 to Fig. 9. Fig. 4 shows a timing chart for explaining the storage process, and Fig. 5 to Fig. 9 show explanatory diagrams for explaining the movement of the bucket by the charge storage coil 301.

[0077] The dynamic magnetic field control device 302 generates the desired current waveform shown in FIG. 4 and starts exciting the charge storage coil 301 so that the desired magnetic field expressed by the following equation (1) is generated in the charge storage coil 301 during the period during which particles are desired to be supplied from the ion source 12, from time t0 before the beam particles start to be supplied from the ion source 12. ΔB=(2πm / q)Δf(t) … (1)

[0078] Here, in equation (1), m is the mass of the beam particle, q is the charge of the beam particle, and Δf(t) is the frequency difference between the accelerating electric field at time t when a particle is supplied from the ion source 12 and the accelerating electric field at the supply time of a reference particle (hereinafter referred to as the reference time).

[0079] Therefore, Δf(t) becomes Δf(t)=0 at the reference time t2, and at the reference time t2, the magnitude relationship between the frequency of the high frequency electric field at time t and the frequency of the high frequency electric field at the reference time is reversed, and the sign of Δf(t) is reversed. Although Fig. 4 to Fig. 9 show examples of Δf(t) expressed as a linear function, this distribution does not depend on the functional form.

[0080] Also, ΔB represents the dynamic magnetic field generated by charge accumulation coil 301, and the sign of ΔB is determined so that the direction of change in the rotation frequency of the beam particles due to the dynamic magnetic field is the same as the direction of change in the frequency of the high frequency acceleration electric field.

[0081] From time t0, the charge accumulation coil 301 starts to be excited, and at time t1 (the beam particle accumulation start time) when the magnetic field of the charge accumulation coil 301 is at a minimum, the radio frequency electric field is excited and the supply of the beam particles starts.

[0082] Here, an example will be described in which the beam particle accumulation start time t1 coincides with the time when the radio frequency electric field is excited, but the radio frequency electric field may be excited from a time prior to t1.

[0083] At the beam accumulation start time t1, in the accelerator 1 in this embodiment in which the rotation period increases with the increase in beam particle energy, the direction of the magnetic field generated by the charge accumulation coil 301 is in a direction that strengthens the main magnetic field.

[0084] In the accelerator 1 of this embodiment, the period of the accelerating radio frequency electric field is designed based on the reference particle, and therefore the period of the accelerating radio frequency electric field increases as the energy of the reference particle increases.

[0085] By exciting the charge storage coil 301, the magnetic field inside the charge storage coil 301 becomes stronger, and the rotation period of the supply particles becomes shorter, so that the rotation period of the supply particles can be made closer to the period of the high frequency electric field at time t1, and the supply particles can be accelerated most efficiently. Therefore, the bucket is formed around the main supply particles, and the supply particles are arranged at the center of the bucket.

[0086] As shown in Fig. 6, which shows the state of the timing of t=t1 in Fig. 5, the supply particle 411 at time t1 is accelerated in the high-frequency electric field from a timing earlier than the reference particle, and therefore its energy is higher than that of the reference particle. If this energy difference is ΔE1, the supply particle 411 at time t1 is disposed in the +ΔE1 direction with respect to the reference particle, but the newly formed bucket 401 is formed by being shifted overall in the +ΔE1 direction with respect to the bucket when the charge storage coil 301 is not excited (substantially the same as the bucket 402 in Fig. 7), and therefore exists stably while tracing a closed trajectory within the bucket 401.

[0087] When the accumulation of beam particles starts, the magnetic field of the charge accumulation coil 301 is changed in the opposite phase direction in accordance with the formula (1) in conjunction with the frequency modulation of the accelerating electric field.

[0088] As a result, as shown in FIG. 7, which shows the timing of t=t2 in FIG. 5, the bucket 402 gradually shifts overall in the −ΔE direction, and each time a beam particle is supplied from the ion source 12, the supplied particle 412 is positioned in the center of the bucket 402.

[0089] When the bucket does not move as in the conventional method, the particles present in the bucket exist stably while tracing a closed trajectory within the bucket.

[0090] In contrast, when the bucket 402 moves due to the magnetic field generated by the charge storage coil 301 as in this embodiment, if the speed at which the bucket 402 moves is sufficiently slow compared to the speed at which the beam particles orbit the closed locus within the bucket 402, the beam particles move within the bucket 402 while following the movement of the bucket 402.

[0091] 8, which shows the timing of t=t3 in Fig. 5, the magnetic field generated by the charge accumulation coil 301 becomes 0 at time t2 when the reference particle is supplied, and the magnetic field of the charge accumulation coil 301 becomes the opposite phase to that at the start of beam accumulation at time t3 when the accumulation of the beam particles is stopped. The time t3 when the accumulation of the beam particles is stopped is set by the dynamic magnetic field control device 302.

[0092] Here, an example is described in which the accumulation stop time t3 is set so that the midpoint between the accumulation start time t1 and accumulation stop time t3 of the beam particles is the reference time t2. In this case, the supplied particle 413 at time t3 is positioned in the -ΔE1 direction with respect to the reference particle, but the bucket 403 formed at the accumulation stop time t3 of the beam particles is formed by shifting overall in the -ΔE1 direction with respect to the case in which the charge accumulation coil 301 is not excited, i.e., the conventional bucket, and thus exists stably while tracing a closed trajectory within the bucket.

[0093] After accumulation stop time t3, the excitation amount of the charge accumulation coil 301 is reduced and excitation is stopped at time t4.

[0094] As shown in FIG. 9, which shows the timing of t=t4 in FIG. 5, at time t4, the magnetic field generated by the charge storage coil 301 becomes 0, and the generated bucket 404 returns to the position where it was generated at the reference time t2. At this time, the energy difference ΔE4 between the supply particle 414 supplied from the ion source 12 and the reference particle is ΔE4>ΔE max As a result, the feed particles 414 are positioned outside the bucket 404 and therefore do not accumulate.

[0095] With this, the operation of the charge storage process of the charge storage coil 301 in one operation cycle is completed.

[0096] For example, the energy difference ΔE1 between the beam particle supplied at time t1 and the reference particle is the bucket energy threshold ΔE max If the charge storage coil 301 is not used, the supplied particles are disposed outside the bucket and cannot be accelerated.

[0097] Next, the effects of this embodiment will be described.

[0098] The accelerator 1 of the present embodiment described above is an accelerator that accelerates beam particles using a frequency-modulated high-frequency acceleration electric field, and in which the orbital radius of the beam particles circulating in the magnetic field changes depending on the energy, and is equipped with at least one charge storage coil 301 installed vertically above or below the beam orbit of the beam particles, which controls the dynamic magnetic field generated within the installation range so as to change the orbital frequency of the beam particles.

[0099] As described above, by using the charge storage coil 301, it is possible to move the generation positions of the buckets 401, 403 in the ΔE direction or the −ΔE direction, and it is possible to position the supply particles 411, 413 at the center of the buckets 401, 403 and accelerate the supply particles.

[0100] In this way, by using the charge storage coil 301, charges can be stored even at times t1 and t3, whereas charges could be stored only at t=time t2 in Fig. 4 in the past, and therefore it is possible to extend the charge storage time, which means the period during which the particles 411, 412, and 413 supplied from the ion source 12 can be appropriately accelerated. This makes it possible to increase the amount of stored charge and increase the dose rate.

[0101] In addition, the accelerator 1 further includes an ion source 12 that generates beam particles, and the charge storage coil 301 is installed so that the horizontal position of the accelerator 1 includes an incidence point 120 at which the beam particles generated by the ion source 12 are incident, so that a larger number of incident beam particles can be accumulated.

[0102] Furthermore, the charge storage coil 301 generates a dynamic magnetic field so that the orbital frequency of the beam particles is synchronized with the frequency of the high-frequency acceleration electric field, thereby positioning the injected beam particles at the center of the bucket, making it possible to extend the charge storage time in the injection step and further increase the amount of accumulated charge.

[0103] Furthermore, since the time from the start to the end of excitation of the charge storage coil 301 is short compared to the acceleration time of the beam in the accelerator 1, the dynamic magnetic field excited by the charge storage coil 301 can be prevented from affecting the beam particles being accelerated.

[0104] Furthermore, the device is further provided with an ion source 12 that generates beam particles, and the charge storage coil 301 is excited at the timing of beam injection of the beam particles from the ion source 12, thereby enabling the injected beam particles to be reliably accumulated.

[0105] In addition, since the charge storage coil 301 is composed of a pair of coils arranged vertically above and below the beam orbit, on either side of the beam orbit, it is possible to maintain higher stability of the beam in the vertical direction (z-axis direction) relative to the orbital plane of the beam particles (xy plane).

[0106] <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.

[0107] For example, although the embodiment has been described with reference to an example in which the present invention is applied to an eccentric orbit accelerator, the present invention can also be applied to a synchrocyclotron.

[0108] Furthermore, the embodiment of the present invention may be in the following form.

[0109] (1) A circular accelerator that accelerates beam particles using a frequency-modulated high-frequency acceleration electric field, and in which the orbital radius of the beam particles orbiting in a magnetic field changes depending on the energy, and has at least one coil installed vertically above or below the beam orbit of the beam particles, which controls a dynamic magnetic field generated within the installation range so as to change the orbital frequency of the beam particles.

[0110] (2) The circular accelerator described in (1) further includes an ion source that generates the beam particles, and the coil is installed so that the horizontal position of the circular accelerator includes an incidence point at which the beam particles generated by the ion source are incident.

[0111] (3) In the circular accelerator according to (1) or (2), the coil generates a dynamic magnetic field so that the rotation frequency of the beam particles is synchronized with the frequency of the high-frequency accelerating electric field.

[0112] (4) In the circular accelerator described in any one of (1) to (3), the time from the start of excitation to the end of excitation of the coil is short compared with the acceleration time of the beam of the circular accelerator.

[0113] (5) In the circular accelerator described in any one of (1) to (4), an ion source for generating the beam particles is further provided, and the pair of coils are excited at the beam injection timing of the beam particles from the ion source.

[0114] (6) In the circular accelerator described in any one of (1) to (5), the coil is composed of a pair of coils arranged vertically above and below the beam orbit, sandwiching the beam orbit. [Explanation of symbols]

[0115] 1...Accelerator 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 14…Return yoke 15...Magnetic pole 20...Beam passing area 21...High frequency cavity 31, 32…Dee electrodes 35...Ground electrode 40…Removal septum magnet 50…Massless septum coil 60...Magnetic field distribution adjustment coil 111…Extraction beam hole 112,113…drawer opening 114...High frequency power input through hole 115: Beam injection hole 120…Incidence point 140…Beam extraction path 211…Input coupler 212...Rotary capacitor 213...Rotation axis 301...Charge storage coil 302...Dynamic magnetic field control device 401...Bucket at accumulation start time t1 402...Bucket at reference time t2 403...Bucket at storage stop time t3 404…Bucket at excitation stop time t4 411: Particles supplied at accumulation start time t1 412: Particles supplied at reference time t2 413…Supply particles supplied at accumulation stop time t3 414…Supply particles supplied at excitation stop time t4

Claims

1. A circular accelerator that accelerates beam particles using a frequency-modulated high-frequency accelerating electric field, and the orbit radius of the beam particles circulating in a magnetic field changes depending on the energy, At least one coil is provided vertically above or below the beam trajectory of the beam particles, and controls a dynamic magnetic field generated within the range of the coil so that the rotation frequency of the beam particles is changed. Circular accelerator.

2. 2. The circular accelerator according to claim 1, an ion source for generating the beam particles; The coil is installed so that the horizontal position of the circular accelerator includes an incidence point where the beam particles generated by the ion source are incident. Circular accelerator.

3. 2. The circular accelerator according to claim 1, The coil generates a dynamic magnetic field such that the rotation frequency of the beam particles is synchronized with the frequency of the high frequency acceleration electric field. Circular accelerator.

4. 2. The circular accelerator according to claim 1, The time from the start of excitation to the end of excitation of the coil is short compared with the acceleration time of the beam of the circular accelerator. Circular accelerator.

5. 2. The circular accelerator according to claim 1, an ion source for generating the beam particles; The coil is excited at the timing of the beam injection of the beam particles from the ion source. Circular accelerator.

6. 2. The circular accelerator according to claim 1, The coil is composed of a pair of coils arranged vertically above and below the beam orbit, with the beam orbit in between. Circular accelerator.

7. A circular accelerator according to any one of claims 1 to 6 is provided. Particle beam therapy system.

8. A method of operating a circular accelerator that accelerates beam particles using a frequency-modulated high-frequency accelerating electric field, comprising the steps of: changing the orbit radius of the beam particles orbiting in the magnetic field according to the energy; At least one coil is installed vertically above or below the beam trajectory of the beam particles, and a dynamic magnetic field generated within the installation range is controlled so that the rotation frequency of the beam particles is changed. How to operate a circular accelerator.

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