Circular accelerator, particle beam therapy system, and method for controlling a 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 control method for a circular accelerator.

Background Art

[0002] Patent Document 1 describes an accelerator having a pair of magnets that form a magnetic field therebetween, an ion source that injects ions between the magnets, an acceleration electrode that accelerates the ions, and a beam emission path that extracts the ions to the outside, and in which a plurality of circular beam orbits in which ions of different energies respectively orbit, formed by the pair of magnets, converge on one hand, and the frequency of the high-frequency electric field applied by the acceleration electrode to the ions is modulated by the beam orbit.

[0003] Patent Document 2 describes a circular accelerator including an acceleration cavity for accelerating particles, a capacitive coupling type input coupler for supplying high-frequency power to the acceleration cavity, and a rotary capacitor for modulating the resonance frequency of the acceleration cavity, wherein the rotary side electrode of the rotary capacitor is connected to a floating potential electrode via an insulating support, and the distance between the tip of the input coupler and the floating potential electrode changes as the rotary side electrode rotates.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] High-energy nuclear beams used in particle beam therapy, physical experiments, etc. are generated using accelerators.

[0006] One type of accelerator uses a magnetic field generated by an electromagnet to deflect a beam of particles, causing them to orbit in a circular path. A high-frequency electric field generated in an accelerating cavity placed in the orbit is then used to accelerate the beam of particles with each orbit. Examples of such accelerators include synchrotrons, cyclotrons, and synchrocyclotrons such as static magnetic field variable energy accelerators (see Patent Document 1).

[0007] In a synchrotron, as the beam particles are accelerated, the magnetic field of bending electromagnets placed in the orbit is strengthened, causing the beam particles to orbit in a constant path. As the beam's orbit period decreases with acceleration, the period of the accelerating electric field also decreases in sync with the decrease in the beam's orbit period. However, because the orbit is constant regardless of energy in a synchrotron, the beam can be extracted at the desired energy using an output device placed in the orbit. However, due to heat generation caused by eddy currents in the dynamic magnetic field and disturbances in the magnetic field, the cycle speed of repeated acceleration is limited, resulting in intermittent beam irradiation.

[0008] In contrast, because a cyclotron is a static magnetic field type, the radius of curvature of the beam particle's trajectory increases as the beam particle, incident from the ion source located at the center of the accelerator, is accelerated. Since the trajectory length increases in proportion to the increase in beam particle velocity, the beam's orbit period is constant regardless of energy, and the period of the radio frequency accelerating electric field is constant. Furthermore, because a cyclotron is a static magnetic field type, problems caused by eddy currents due to a dynamic magnetic field do not occur, making quasi-continuous beam irradiation possible by emitting the beam in a fast, repetitive acceleration cycle.

[0009] However, since the radius of curvature of the beam particle trajectory increases with acceleration, the beam cannot be emitted until it reaches the installation position of the emission equipment, and the emission energy can only be changed to its maximum value. Therefore, in order to irradiate with a beam of the desired energy, a degrader is installed in the beam transport system to reduce the energy of the beam emitted from the accelerator. As the beam passes through the degrader, beam loss occurs, so the dose rate decreases in the low-energy region.

[0010] In a static magnetic field type variable energy accelerator, such as the one described in Patent Document 1 above, the radius of curvature of the beam particle's trajectory increases as the beam particle, incident from an ion source located at the center of the accelerator, is accelerated, similar to a cyclotron. Since the increase in trajectory length is not proportional to the increase in beam particle velocity, the period of the radio frequency accelerating electric field changes with the acceleration of the beam particle. Because a static magnetic field type variable energy accelerator uses a static magnetic field, it has the advantage of enabling quasi-continuous irradiation by emitting beams with a fast repetition cycle.

[0011] In a cyclotron, the magnetic field distribution is axially symmetric with respect to the ion source, whereas in a static-field variable energy accelerator, the magnetic field distribution is asymmetric with respect to the ion source. Therefore, in a static-field variable energy accelerator, the orbital center positions differ for each energy level, resulting in eccentricity in one direction, and there are points where the orbits for each energy level converge.

[0012] By installing the emission equipment at the orbital convergence point, it is possible to emit a beam of the desired energy without dose rate reduction due to degraders. Due to its ability to perform rapid repetition acceleration and its energy variability, variable energy accelerators are expected to provide high dose rates across the entire emission energy range. Furthermore, if the amount of stored charge in the accelerator can be increased, even greater dose rate improvements can be expected.

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

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

[0015] In a static magnetic field variable energy accelerator, the frequency of the accelerating high-frequency electric field and the beam's rotation frequency are synchronized in an integer multiple ratio. Therefore, particles accelerated at a predetermined phase of the accelerating electric field will be accelerated at the same phase in the next turn. Consequently, the supply particles (reference particles) supplied at a timing where the initial period and the period of the accelerating high-frequency electric field coincide are accelerated most efficiently.

[0016] On the other hand, particles accelerated at a phase earlier than the acceleration phase receive a greater acceleration than particles accelerated at the acceleration phase, and therefore receive acceleration at a later phase in the next turn. Conversely, particles accelerated at a phase later than the acceleration phase at a given time receive a smaller acceleration than particles accelerated at the acceleration phase, and therefore receive acceleration at an advanced phase in the next turn.

[0017] Thus, particles whose timing is off from a predetermined acceleration phase move in the direction of returning to the acceleration phase, and this action allows them to oscillate stably within the phase plane (direction of propagation) 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. Unless subjected to external forces, the stably orbiting beam particle changes position along a fixed trajectory within the bucket.

[0019] Beam particles incident at an earlier timing than the reference particle pass through the accelerating cavity more times and are therefore positioned in the +ΔE direction. Conversely, beam particles incident at a later timing pass through the accelerating cavity less times and are therefore positioned in the -ΔE direction. The period of the accelerating high-frequency electric field is designed based on the reference particle, and the period of the accelerating high-frequency electric field increases as the energy of the reference particle increases.

[0020] Therefore, ΔE is the energy threshold of the bucket (ΔE max If ΔE exceeds ΔE, the difference between the initial period and the period of the high-frequency electric field widens, and the supplied particles can no longer be accelerated. maxAs described below, the period during which supply particles can be appropriately accelerated is called the charge accumulation time, and is determined from the magnetic field on the orbital path that affects the frequency modulation (FM) speed of the accelerating electric field and the orbital period of the beam particles. The longer the charge accumulation time, the larger the amount of charge that can be accumulated in the accelerator, and thus it becomes possible to increase the dose rate of the irradiation beam.

[0021] In Patent Document 2, an accelerating electric field generation device using a cavity resonator that performs frequency modulation of an accelerating electric field using a rotary capacitor is shown. The rotary capacitor is composed of a stationary electrode plate called a stator and a rotating electrode plate called a rotor. The capacitance of the capacitor changes as the area where the rotor and the stator overlap changes according to the rotation angle of the rotor, and the resonance frequency of the cavity resonator changes.

[0022] Therefore, in a static magnetic field type accelerator that performs frequency modulation of an accelerating electric field using a rotary capacitor, it has been difficult to adjust the FM speed once the shapes of the rotor and stator of the rotary capacitor are determined. Since the charge accumulation time that affects the dose rate is determined from the magnetic field and the FM speed, it has been difficult to extend the charge accumulation time without remanufacturing the rotor and stator.

[0023] The present invention provides a circular accelerator, a particle beam therapy system, and an accelerator control method capable of extending the charge accumulation time as compared with the prior art.

Means for Solving the Problems

[0024] The present invention includes a plurality of means for solving the above problems. For example, it is a circular accelerator that accelerates beam particles using a high-frequency accelerating electric field whose frequency is modulated, and the orbital radius of the beam particles orbiting in a magnetic field changes according to the energy. It is installed above or below the vertical direction of the beam orbit of the beam particles so as to control the traveling magnetic field generated within the installation range so that the orbital frequency of the beam particles changes. The time from the start of excitation to the end of excitation is shorter compared to the beam acceleration time of the circular accelerator. It includes at least one coil. <0​

Advantages of the Invention

[0025] According to the present invention, the charge accumulation time can be extended compared with the prior art. Problems, configurations, and effects other than those described above will be clarified by the description of the following embodiments.

Brief Description of the Drawings

[0026] [Figure 1] It is a schematic diagram of the overall configuration of the particle beam therapy apparatus of the embodiment. [Figure 2] It is a diagram showing the overall schematic of the circular accelerator of the embodiment. [Figure 3] It is an internal equipment layout diagram of the circular accelerator of the embodiment. [Figure 4] It is a timing diagram for explaining the accumulation process of the circular accelerator of the embodiment. [Figure 5] It is a schematic diagram for explaining the movement of the bucket in the circular accelerator of the embodiment. [Figure 6] It is a schematic diagram for explaining the movement of the bucket in the circular accelerator of the embodiment. [Figure 7] It is a schematic diagram for explaining the movement of the bucket in the circular accelerator of the embodiment. [Figure 8] It is a schematic diagram for explaining the movement of the bucket in the circular accelerator of the embodiment. [Figure 9] It is a schematic diagram for explaining the movement of the bucket in the circular accelerator of the embodiment. max 25>

Modes for Carrying Out the Invention

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

[0028] First, the overall configuration of the particle beam therapy system equipped with an accelerator will be explained using Figure 1. Figure 1 is a schematic diagram of the overall configuration of the particle beam therapy apparatus in the example.

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

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

[0031] The irradiation device 3 incorporates a dose monitor and monitors the dose irradiated to each irradiation spot on 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 ejection of the charged particle beam in accelerator 1 and supplies a beam with the required dose and energy.

[0032] Next, an accelerator of Example 1, which is a preferred embodiment of the present invention, will be described below with reference to Figures 2 to 9.

[0033] First, we will explain the overview of the accelerator using Figure 2. Figure 2 shows a schematic of the overall configuration of accelerator 1.

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

[0035] Accelerator 1 uses a magnet 11 that can be divided into upper and lower sections to excite a main magnetic field within the region through which the beam passes (hereinafter referred to as the beam passage region), and the inside of the beam passage region is evacuated.

[0036] The magnet 11 has multiple through-holes, of which an extraction beam through-hole 111 for extracting the accelerated beam, extraction holes 112 and 113 for drawing out the internal coil to the outside, and a high-frequency power input through-hole 114 are provided on the connection surface of the upper and lower magnetic poles. A high-frequency cavity 21 is installed through the high-frequency power input through-hole 114. As will be described later, an acceleration D-electrode section and a rotary capacitor 212 are installed in the high-frequency cavity 21.

[0037] The ion source 12 that generates the beam particles is installed at a position offset from the center of the top of the magnet 11, at different radial positions, and the beam is injected into the accelerator 1 through the beam injection port 115.

[0038] Next, the internal structure of accelerator 1 will be explained using Figure 3. Figure 3 is a diagram showing the internal equipment layout of the circular accelerator in this embodiment.

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

[0040] Inside the main coil 13, magnetic poles 15 are positioned opposite each other vertically, forming the upper and lower boundaries of the beam passage region 20, and the outside of the main coil 13 is equipped with a cylindrical return yoke 14.

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

[0042] Furthermore, the magnetic pole shape and coil arrangement are symmetrical with respect to the plane (orbital plane) passing through the center of the gap, and on the orbital plane, they only have magnetic field components perpendicular to the orbital plane.

[0043] The magnetic pole shapes are symmetrical with respect to axis AA' in the central plane, resulting 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 direction of the accelerator center as the y-axis relative to the origin, the magnetic field distribution of the cyclotron will be symmetrical with respect to the origin. However, the magnetic field distribution formed by the magnetic pole 15 will not be symmetrical with respect to the origin, and the gradient of the magnetic field distribution in the positive y-axis direction will be gentler farther away from the beam injection point. As a result, in the low-energy region below 80 MeV, the orbit will be centered near the injection point 120, similar to a cyclotron. On the other hand, as the energy of the beam particles increases, the center of the beam's orbit moves in one direction within the same plane, resulting in the formation of orbital convergence points where the orbits are densely packed together.

[0045] In addition, it is equipped with D-electrodes 31 and 32 for exciting high-frequency electric fields, a septum electromagnet 40 for extraction, a massless septum coil 50 for generating a kicker magnetic field, a coil 60 for adjusting the magnetic field distribution, and a coil 301 for charge storage.

[0046] The kicker magnetic field employs a massless septum system, applying the magnetic field only to specific locations along the accelerator's radial direction. The kicker magnetic field is excited by passing an electric current through a pair of coils that are symmetrically positioned perpendicular to the beam trajectory plane.

[0047] The charge storage coil 301 controls the local dynamic magnetic field generated within the installation range, including the beam incidence region, so that the orbital frequency of the beam particles changes. In this embodiment, the charge storage coil 301 is described as a pair of coils installed symmetrically vertically above and below the beam trajectory of the beam particles.

[0048] As in this embodiment, by arranging the charge storage coil 301 in such a way that a pair of coils are vertically symmetrical above and below the beam trajectory, the stability of the beam in the vertical direction (z-axis direction) with respect to the orbital plane (xy-plane) of the beam particles can be maintained at a higher level.

[0049] The charge storage coil 301 consists of two smaller coils, one above and one below, than the magnetic field distribution adjustment coil 60. To generate a dynamic magnetic field near the incident point 120, one coil is placed above and one below the beam trajectory plane, and the accelerator 1 is positioned parallel to the beam trajectory plane so that its horizontal position encloses the incident point 120 into which 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 trajectory, were described as an example. However, any configuration in which the charge storage coils are placed either vertically above or vertically below the beam trajectory is acceptable, and the charge storage coils may consist of multiple coils. Furthermore, when there is a pair of coils vertically above and vertically below, their radial positions do not need to be perfectly coincidental and can be changed as appropriate according to the design.

[0051] Furthermore, increasing the size of the charge storage coil 301 is desirable for increasing the number of stored particles, as it reduces the possibility of particles supplied at an earlier timing escalating and widening the radius of curvature of their orbits when beam particles supplied at a later timing are supplied, thereby increasing the number of stored particles. In other words, since the area inside the charge storage coil 301 is the region where the position of the buckets can be controlled, it is desirable to make the charge storage coil 301 sufficiently large, and it is desirable that the size of the charge storage coil 301 covers at least the entire orbit of the beam particles at the end of beam injection timing.

[0052] In the accelerator 1 of this embodiment, the injection point is located at a position shifted from the center of the electromagnet toward the orbital convergence point, so an example was described in which the charge storage coil 301 is also positioned at a position shifted toward the orbital convergence point. However, when applying the present invention to accelerators such as synchrotrons and cyclotrons where the beam injection point is at the center of the electromagnet, the charge storage coil 301 is located at the center of the electromagnet.

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

[0054] The beam is injected into accelerator 1 from the injection point 120 in a low-energy ion state. The injected beam is accelerated each time it passes through the electric field gap by the high-frequency electric field excited by the high-frequency cavity 21.

[0055] The shapes of the magnetic pole 15 and the D-electrodes 31 and 32 are symmetrical with respect to the yz plane.

[0056] In the magnetic field inside accelerator 1 with the above configuration, particles that deviate horizontally and vertically from their design trajectory are subjected to a restoring force that returns them to their design trajectory, causing them to oscillate around the design trajectory. This oscillation is called betatron oscillation, and it allows the beam particles to stably orbit and accelerate within the beam.

[0057] Furthermore, the magnetic pole 15 is equipped with a magnetic field distribution adjustment coil 60 for fine-tuning the magnetic field, and the current in the magnetic field distribution adjustment coil 60 is adjusted before operation to ensure the stability of the betatron oscillation.

[0058] The high-frequency cavity 21 excites an electric field in the acceleration gap by a λ / 4 type resonant mode. High-frequency power is introduced from an external high-frequency power supply through the 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, the frequency of the electric field is modulated in accordance with the energy of the circulating beam in order to excite a high-frequency electric field in synchronization with the reciprocating beam. In a cavity using a resonant mode as used in the present invention, it is necessary to sweep the high-frequency range over a wider range than the resonance width. For this reason, the resonant frequency of the cavity also needs to be changed.

[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 conductive 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 as the beam accelerates.

[0061] To extract a predetermined extraction beam at the target energy, one or more of the massless septum coils 50 are selected based on the target energy and a predetermined excitation current is applied.

[0062] If no current is flowing through the massless septum coil 50, the beam with the target energy will orbit along its designed trajectory. However, if current is flowing through the massless septum coil 50, the beam that has reached the target energy will deviate from its trajectory due to the kick magnetic field generated by the massless septum coil 50.

[0063] As mentioned above, a beam that deviates from its orbit oscillates stably around the design orbit. In other words, the massless septum coil 50 excites betatron oscillations within the orbital plane.

[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 makes it possible to displace the beam radially outward at the convergence point.

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

[0066] In the injection step, a beam is emitted in the form of low-energy ions from an ion source 12 positioned radially different from the center of gravity of the annular main coil 13, and the beam is guided into the beam passage region through the injection point 120, which is the innermost region of the innermost orbit.

[0067] After undergoing the accumulation process described later, the beam injected into the beam passage region 20 is accelerated by the high-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 high-frequency electric field is controlled to synchronize with the orbital period of the beam particles, the period of the accelerating high-frequency electric field also increases.

[0068] In the subsequent acceleration step, the beam is accelerated while ensuring directional stability due to the high-frequency electric field. That is, instead of passing through the acceleration gap at the time when the high-frequency electric field is at its maximum, the beam passes through the acceleration gap when the high-frequency electric field is decreasing over time. As a result, since the frequency of the high-frequency electric field and the reciprocating frequency of the beam are synchronized in an integer multiple ratio, particles accelerated at a predetermined phase of the accelerating electric field will be accelerated at the same phase in the next turn as well.

[0069] On the other hand, particles accelerated at a phase earlier than the acceleration phase receive a greater acceleration than particles accelerated at the acceleration phase, and therefore receive acceleration at a later phase in the next turn. Conversely, particles accelerated at a phase later than the acceleration phase receive a smaller acceleration than particles accelerated at the acceleration phase, and therefore receive acceleration at an advanced phase in the next turn.

[0070] In this way, the accelerating particle undergoes synchrotron oscillations, gradually accelerating until it reaches the predetermined energy at which it is extracted.

[0071] Once the beam reaches a predetermined energy, it is subjected to a kick magnetic field generated by the massless septum coil 50 and enters the exit channel, which is the beam extraction path 140 formed by the extraction septum electromagnet 40 from the convergence point, and is extracted outside the accelerator 1 (exit step).

[0072] Up to this point, we have described the behavior of the beam from beam injection to extraction in the accelerator 1 of the present invention, but the most distinctive feature of the accelerator 1 of the present invention lies in the charge accumulation process using the charge accumulation coil 301.

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

[0074] This makes it possible to extend the charge accumulation time in the injection step compared to conventional methods, and to achieve an improvement in the dose rate by increasing the amount of accumulated charge.

[0075] The excitation timing of the charge storage coil 301 is preferably set to coincide with the beam incidence timing of the beam particles from the ion source 12, and it is desirable that the excitation is repeated each time the beam from the ion source 12 is incident during the 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 explained below with reference to Figures 4 to 9. Figure 4 shows a timing diagram to explain the storage process, and Figures 5 to 9 show explanatory diagrams to explain 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 Figure 4, and starts exciting the charge storage coil 301 from time t0, before beam particles begin to be supplied from the ion source 12, for the period during which particles are to be supplied from the ion source 12, so that the desired magnetic field represented by the following equation (1) is generated inside the charge storage coil 301. 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) represents the difference in frequency 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 time the reference particle is supplied (hereinafter, the reference time).

[0079] Therefore, Δf(t) = 0 at the reference time t2, and the relative magnitudes of the high-frequency electric field frequencies at time t and at the reference time t2 are reversed, causing the sign of Δf(t) to also be reversed. Figures 4 to 9 show examples where Δf(t) is expressed as a linear function, but this distribution is independent of the functional form.

[0080] Furthermore, ΔB represents the dynamic magnetic field generated by the charge storage coil 301, and the sign of ΔB is determined such that the direction of the change in the orbital frequency of the beam particles due to the dynamic magnetic field is the same as the direction of the frequency change of the high-frequency accelerating electric field.

[0081] At time t0, the charge storage coil 301 starts to be energized, and at time t1 (the start time of beam particle storage), when the magnetic field of the charge storage coil 301 is at its minimum, the high-frequency electric field is excited and the supply of beam particles begins.

[0082] Here, we will explain using an example where the start time t1 of beam particle accumulation coincides with the time when the high-frequency electric field is excited, but the high-frequency electric field may be excited at a time before t1.

[0083] At the beam storage start time t1, the direction of the magnetic field generated by the charge storage coil 301 is such that, in the accelerator 1 of this embodiment where the orbit period increases with increasing energy of the beam particles, the direction strengthens the main magnetic field.

[0084] In accelerator 1 of this embodiment, the period of the accelerating high-frequency electric field is designed based on a reference particle, so the period of the accelerating high-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 is strengthened, shortening the orbital period of the supplied particles. This makes it possible to bring the orbital period of the supplied particles closer to the period of the high-frequency electric field at time t1, thereby accelerating the supplied particles most efficiently. Consequently, the bucket is formed around the supplied particles, and the supplied particles are positioned at the center of the bucket.

[0086] As shown in Figure 6, which illustrates the timing at t=t1 in Figure 5, the supplied particle 411 at time t1 is accelerated by the high-frequency electric field earlier than the reference particle, so its energy is higher than that of the reference particle. If this energy difference is ΔE1, the supplied particle 411 at time t1 is positioned in the +ΔE1 direction relative to the reference particle. However, the newly formed bucket 401 is formed with an overall shift in the +ΔE1 direction relative to the bucket when the charge storage coil 301 is not excited (approximately the same as bucket 402 in Figure 7). Therefore, it exists stably while tracing a closed trajectory within bucket 401.

[0087] As beam particle accumulation begins, the magnetic field of the charge accumulation coil 301 is changed in the opposite phase direction according to equation (1), in accordance with the frequency modulation of the accelerating electric field.

[0088] As a result, as shown in Figure 7, which illustrates the timing at t=t2 in Figure 5, the bucket 402 gradually shifts overall in the -ΔE direction, and each time beam particles are supplied from the ion source 12, the supplied particles 412 are positioned in the center of the bucket 402.

[0089] If the bucket does not move as in conventional methods, the particles within the bucket will remain stable, following a closed trajectory within the bucket.

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

[0091] As shown in Figure 8, which illustrates the timing at t=t3 in Figure 5, the magnetic field generated by the charge storage coil 301 becomes 0 at time t2 when the reference particles are supplied, and at time t3 when the beam particle storage stops, the magnetic field of the charge storage coil 301 is in the opposite phase to when the beam storage started. This beam particle storage stop time t3 is set by the dynamic magnetic field control device 302.

[0092] Here, we will explain using an example in which the accumulation stop time t3 is set so that the midpoint between the beam particle accumulation start time t1 and accumulation stop time t3 is the reference time t2. In this case, the supplied particles 413 at time t3 are positioned in the -ΔE1 direction relative to the reference particles, but the bucket 403 formed at the beam particle accumulation stop time t3 is formed when the charge accumulation coil 301 is not excited, i.e., it is formed shifted overall in the -ΔE1 direction relative to the conventional bucket, and therefore exists stably while drawing a closed trajectory within the bucket.

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

[0094] As shown in Figure 9, which illustrates the timing at t=t4 in Figure 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 reference time t2. At this time, the energy difference ΔE4 between the supply particles 414 supplied from the ion source 12 and the reference particles is ΔE4 > ΔE max Therefore, the supplied particles 414 are placed outside the bucket 404 and do not accumulate.

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

[0096] For example, the energy difference ΔE1 between the beam particles supplied at time t1 and the reference particles is the energy threshold ΔE of the bucket. max If the value is greater than this, and the charge storage coil 301 is not used, the supplied particles will be placed outside the bucket and will not be able to be accelerated.

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

[0098] The accelerator 1 of this embodiment described above is an accelerator that accelerates beam particles using a frequency-modulated high-frequency accelerating electric field, and the orbital radius of the beam particles orbiting in a magnetic field changes with energy, and includes at least one charge storage coil 301 installed vertically above or vertically below the beam trajectory of the beam particles to control 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 becomes possible to move the generation positions of buckets 401 and 403 in the ΔE direction or the -ΔE direction, thereby positioning the supply particles 411 and 413 at the center of buckets 401 and 403 and accelerating the supply particles.

[0100] Thus, by using the charge accumulation coil 301, while conventionally charge accumulation was only possible at time t = time t2 in Figure 4, charge accumulation becomes possible at time t1 and time t3 as well. This makes it possible to extend the charge accumulation time, which represents the period during which the supplied particles 411, 412, and 413 supplied from the ion source 12 can be appropriately accelerated. As a result, it becomes possible to increase the amount of accumulated charge and thus increase the dose rate.

[0101] Furthermore, the accelerator 1 is equipped with an ion source 12 for generating beam particles, and the charge storage coil 301 is positioned such that the horizontal position of the accelerator 1 encompasses the incident point 120 into which the beam particles generated by the ion source 12 are injected, thereby enabling the accumulation of more incoming beam particles.

[0102] Furthermore, the charge storage coil 301 generates a dynamic magnetic field such that the orbital frequency of the beam particles is synchronized with the frequency of the high-frequency accelerating electric field. This allows the incident beam particles to be positioned at the center of the bucket, extending the charge storage time during the injection step and further increasing the amount of stored charge.

[0103] Furthermore, because the time from the start to the end of excitation of the charge storage coil 301 is shorter than the acceleration time of the beam of accelerator 1, the dynamic magnetic field excited by the charge storage coil 301 can suppress its effect on the beam particles during acceleration.

[0104] Furthermore, the system is further equipped with an ion source 12 that generates beam particles, and the charge storage coil 301 is excited at the beam incidence timing of the beam particles from the ion source 12, thereby ensuring the accumulation of the incident beam particles.

[0105] Furthermore, since the charge storage coil 301 is composed of a pair of coils positioned vertically above and vertically below the beam trajectory, the stability of the beam in the vertical direction (z-axis direction) with respect to the orbital plane (xy-plane) of the beam particles can be maintained at a higher level.

[0106] <Other> It should be noted that the present invention is not limited to the embodiments described above, and various modifications and applications are possible. The embodiments described above are explained in detail for the purpose of clearly illustrating the present invention, and are not necessarily limited to those having all the configurations described.

[0107] For example, while the embodiment described an example of application to an eccentric orbit type accelerator, it can also be applied to synchrocyclotrons.

[0108] Furthermore, embodiments of the present invention may also be as follows.

[0109] (1) A circular accelerator that accelerates beam particles using a frequency-modulated high-frequency accelerating electric field, wherein the orbital radius of the beam particles orbiting in a magnetic field changes with energy, and comprises at least one coil installed vertically above or vertically below the beam trajectory of the beam particles for controlling the 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 comprises an ion source for generating the beam particles, wherein the coil is positioned such that the horizontal position of the circular accelerator encompasses the incident point into which the beam particles generated by the ion source are incident.

[0111] (3) In the circular accelerator described in (1) or (2), the coil generates a dynamic magnetic field such that the orbital 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 of (1) to (3), the time from the start of excitation to the end of excitation of the coil is shorter than the acceleration time of the beam of the circular accelerator.

[0113] (5) The circular accelerator according to any one of (1) to (4) further comprises an ion source that generates the beam particles, and the pair of coils are excited at the beam incidence timing of the beam particles from the ion source.

[0114] (6) In the circular accelerator described in any of (1) to (5), the coil is composed of a pair of coils arranged vertically above and vertically below the beam trajectory, flanking the beam trajectory. [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…Magnets 12…Ion source 13…Main coil 14…Return York 15...Magnetic pole 20... Beam passage region 21...High frequency cavity 31, 32... D-electrode 35...Ground electrode 40... Septum electromagnet for extraction 50… Massless septum coil 60…Coil for adjusting magnetic field distribution 111... Through-hole for extraction beam 112, 113… drawer openings 114… Through-hole for high-frequency power input 115... Beam injection port 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 the start time t1 of storage 402... Bucket at reference time t2 403... Buckets at storage stop time t3 404... Buckets at excitation stop time t4 411... Supply particles supplied at the start time t1 of accumulation 412... Supply particles supplied at reference time t2 413... Supply particles supplied at the 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, wherein the orbital radius of the beam particles orbiting in the magnetic field changes with energy, The system includes at least one coil, installed vertically above or below the beam trajectory of the beam particles, which controls the dynamic magnetic field generated within the installation area so as to change the orbital frequency of the beam particles, and whose excitation start time to excitation end time is shorter than the acceleration time of the beam of the circular accelerator. A circular accelerator.

2. In the circular accelerator described in claim 1, The ion source that generates the aforementioned beam particles is further provided, The coil is positioned such that the horizontal position of the circular accelerator encompasses the incident point into which the beam particles generated by the ion source are injected. A circular accelerator.

3. In the circular accelerator described in claim 1, The coil generates a dynamic magnetic field such that the orbital frequency of the beam particles is synchronized with the frequency of the high-frequency accelerating electric field. A circular accelerator.

4. In the circular accelerator described in claim 1, The ion source that generates the aforementioned beam particles is further provided, The coil is excited in accordance with the beam incidence timing of the beam particles from the ion source. A circular accelerator.

5. In the circular accelerator described in claim 1, The coils consist of a pair of coils positioned vertically above and vertically below the beam trajectory, flanking the beam trajectory. A circular accelerator.

6. The circular accelerator is provided according to any one of claims 1 to 5. Particle beam therapy system.

7. A method for operating a circular accelerator that accelerates beam particles using a frequency-modulated high-frequency accelerating electric field, The orbital radius of the beam particle orbiting in the magnetic field is changed by energy, At least one coil, positioned vertically above or below the beam trajectory of the beam particles, and having an excitation start-to-excitation time shorter than the beam acceleration time of the circular accelerator, controls the dynamic magnetic field generated within its installation range so as to change the orbital frequency of the beam particles. How to operate a circular accelerator.

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