JP6899754B2 - Circular accelerator and particle beam therapy system - Google Patents

Description

The present invention relates to an accelerator that accelerates heavy ions such as protons or carbon ions.

As a background technique of the present invention, Patent Document 1 states, "Providing an accelerator capable of efficiently emitting ion beams having different energies." "The accelerator has a circular vacuum vessel including a circular return yoke. Incident electrode. Is arranged closer to the inlet side of the beam emission path in the return yoke than the central axis of the vacuum vessel. The magnetic poles are arranged radially around the incident electrode in the return yoke from the incident electrode. , Alternate with magnetic poles in the circumferential direction of the return yoke. In the vacuum vessel, in the orbital concentric region where there are multiple beam orbits centered on the incident electrode, and around this region, from the incident electrode An orbital eccentric region is formed in which a plurality of eccentric beam orbits exist. In the orbital eccentric region, the beam orbits become dense between the incident electrode and the entrance of the beam emission path, and the beam orbit is dense with the incident electrode as a base point. The invention that the distance between the beam orbits becomes wider on the 180 ° opposite side of the entrance of the beam emission path is disclosed.

International release 2016/09 2621

The variable energy accelerator described in Patent Document 1 is a type of accelerator that accelerates a beam orbiting in a main magnetic field with a high-frequency electric field. Although such accelerators are expected to be used in the field of radiology, they are desirable to be small in order to efficiently use the installation space in medical facilities. Therefore, an object of the present invention is to provide a variable energy accelerator that can be easily miniaturized.

One of the representative circular accelerators of the present invention in achieving the above object is an electromagnet including an orbital surface in which the ions are accelerated in a circular accelerator that accelerates ions to generate an ion beam, and the ions. It has a high-frequency cavity that forms an electric field for acceleration and an emission path for extracting the ions after acceleration, and the electromagnet has a magnetic flux between the center of gravity of the orbital surface and the base end of the emission path. To form a magnetic field distribution on the orbital surface in which the magnetic flux density gradually decreases toward the outer edge of the orbital surface from the point where the density becomes maximum and the magnetic flux density becomes maximum, and the high-frequency cavity forms the electric field. The first electrode and the frequency modulator for modulating the frequency of the electric field applied to the first electrode are provided.

According to the present invention, it is possible to provide a variable energy accelerator that can be easily miniaturized.

It is an overall outline of the accelerator 1 of the first embodiment. It is the internal equipment arrangement drawing of the accelerator 1. It is a graph which shows the energy dependence of the orbiting frequency of the accelerator 1. This is the design trajectory shape of the accelerator 1. It is a graph which shows the energy dependence of the main magnetic field of accelerator 1. It is a figure which shows the movement in the normalized phase space of the orbiting beam of accelerator 1. It is a figure which shows the particle beam therapy system using the accelerator 1.

The accelerator of Example 1, which is a preferred embodiment of the present invention, will be described below with reference to FIGS. 1 to 6.

The accelerator 1 of this embodiment is a frequency modulation type variable energy accelerator. This accelerator 1 is a circular accelerator that has a magnetic field constant with time as a main magnetic field and accelerates protons orbiting in the main magnetic field by a high-frequency electric field. Figure 1 shows its appearance. The accelerator 1 has electromagnets 11 that can be divided into upper and lower parts, and these electromagnets 11 excite a main magnetic field in a vacuum region (hereinafter, referred to as an orbital plane) through which an accelerating beam passes.

A rough appearance of the accelerator 1 will be described with reference to FIG. First, on the surface of the electromagnet 11, a plurality of raceway surface through ports for communicating the inside and the outside of the accelerator 1 are provided. These through holes are mainly provided to draw out the current supply line to the equipment installed inside the accelerator 1, the signal line connected to those equipment to the external space, and the accelerated beam. Some are provided for use. FIG. 1 shows an example thereof, that is, a through-hole 111 for taking out an accelerated beam, through-holes 112 and 113 for pulling out a conductor connected to a coil 13 of an electromagnet 11 to the outside, and a through-hole for high-frequency power input. A mouth 114 is provided.

The device connected to the through hole 114 in FIG. 1 is a high frequency power supply having a frequency modulation function. This high-frequency power supply is connected to the D electrode (first electrode) 221 installed in the inner space of the electromagnet 11 via the through port 114. The high-frequency power supply has a function of outputting electric power at an appropriate frequency for accelerating ions orbiting in the inner space of the electromagnet 11, and includes a rotary variable capacitor 212 in the example shown in FIG. .. The capacitance is exemplified by that controlled by the servomotor 214, but is not limited thereto. The supply line for supplying electric power to the rotary variable capacitor 212 is omitted in the drawings.

Further, the through hole 115 provided on the upper surface of the electromagnet 11 is an ion introduction path, and the ions to be accelerated through the through hole 115 are inside the inner space of the electromagnet 11, or more accurately, inside the electromagnet 11. It is supplied to a substantially disk-shaped space (orbital plane 20) defined by an acceleration orbit existing in the sky. The position of the through-hole 115 into which the ion is introduced is different from that of a general circular accelerator, and will be described in detail later.

Further, FIG. 1 shows only the minimum configuration for explaining the appearance of the accelerator 1. Actually, a vacuum pump for maintaining the inside of the accelerator 1 in a vacuum and a temperature control means for controlling the temperature condition are provided, but these are omitted.

Next, the internal structure of the accelerator 1 will be described with reference to FIG. FIG. 2 is a view in which the electromagnet 11 of the accelerator 1 shown in FIG. 1 is divided into upper and lower parts, and the electromagnet 11 on the lower side is viewed from above. Although there are some differences such as the presence or absence of the through hole 115, basically, the upper electromagnet 11 in FIG. 2 has the same structure as viewed from below. Further, in the following description, the distinction between the upper side and the lower side of the electromagnet 11 is omitted, and the electromagnet 11 is simply referred to as the electromagnet 11.

The electromagnet 11 has a return yoke portion 121, a top plate portion 122, a columnar magnetic pole portion 123, and a coil 13 as main configurations.

The return yoke portion 121 is a thick cylindrical member, and one end of each of the return yoke portions 121 serves as a contact portion when connecting the upper and lower electromagnets 11. Further, the end portion on the opposite side to the contact portion is connected to the top plate portion 122. The top plate portion 122 is a disk-shaped member having a diameter substantially matching the outer diameter of the return yoke portion 121. Therefore, the appearance of the structure in which the return yoke portion 121 and the top plate portion 122 are fastened is a cylindrical structure in which one end is closed.

In such a structure, the magnetic pole portion 123 is formed in the space corresponding to the inner cylinder. The magnetic pole portion 123 is a columnar member having the top plate portion 122 as a base and projecting into the space on the inner cylinder side of the return yoke portion 121. However, unlike the return yoke portion 121, when the electromagnets 11 arranged vertically facing each other are connected, the opposing magnetic pole portions 123 project from the top plate portion 122 to such an extent that they do not contact each other and form a gap.

The return yoke portion 121, the top plate portion 122, and the magnetic pole portion 123 are formed by machined from a pure iron ingot, but the members corresponding to the respective parts may be individually machined and then connected. It may be integrally machined according to the processing equipment and the size of the ingot that can be prepared. Also, of course, the large member may be formed by combining smaller parts.

A coil 13 is installed between the return yoke portion 121 and the magnetic pole portion 123, and as shown in FIG. 2, the inner diameter of the coil 13 roughly matches the outer diameter of the magnetic pole portion 123. The coil 13 and the return yoke portion 121 are insulated from each other, and the coil 13 and the magnetic pole portion 123 are insulated from each other. A conductor connected to the coil 13 is pulled out from the through ports 112 and 113, is connected to a power source provided outside the accelerator 1, and the magnetic pole portion 123 is magnetized by passing a current through the coil 13, and the raceway surface 20 is described later. The magnetic field is excited with a predetermined distribution.

Further, a trim coil 60 is provided as a coil for finely adjusting the magnetic field generated by the coil 13 and the magnetic poles 123 on the surfaces (magnetic poles 124) corresponding to the facing surfaces of the magnetic poles 123 arranged in pairs on the upper and lower sides. Be done. The aspect of the trim coil 60 in this embodiment is a group of coils having a plurality of different diameters as shown in FIG. 2, in which a coil having a smaller diameter is arranged on the inner diameter side of the coil having a larger diameter. Further, each coil is arranged so that the center is deviated in one direction as shown in FIG. The trim coil 60 and the magnetic pole surface 124 are insulated by an insulating member (not shown), and are fixed to the magnetic pole surface 124 by using a non-magnetic member or an adhesive. The magnitude of the current applied to each of the coils constituting the trim coil 60 can be individually adjusted, and is connected to an external power source via the through holes 112 and 113 in the same manner as the coil 13. By adjusting the exciting current individually for each system, the trim coil current is adjusted before operation so as to approach the main magnetic field distribution described later and realize stable betatron vibration.

The accelerator 1 having such a mechanical structure accelerates the ions supplied from the ion source 12 to generate an ion beam, and the orbital plane 20 is defined by the trajectory of the accelerating ions. The position of the raceway surface 20 is in the space formed when the return yoke portions 121 described above are connected vertically, and is equidistant to the upper and lower magnetic pole surfaces 124. Strictly speaking, the raceway surface 20 is not a flat surface but a disk-shaped space having a thickness, but for convenience of explanation, it is treated as a circular region having a diameter smaller than the inner diameter of the return yoke portion 121.

The orbital plane 20 is formed with the magnetic field distribution required for the desired acceleration. By providing the columnar magnetic pole portion 123, most of the magnetic field strength required on the raceway surface 20, that is, in the example shown in FIG. 5, a magnetic field of 4.9 T can be generated over the entire raceway surface 20. The magnetic field distribution to be generated on the raceway surface 20 is the outer edge of the raceway surface 20 (that is, the outer edge of the raceway surface 20) from that position with the maximum position (incident point) when the ions supplied from the ion source 12 ride on the raceway surface 20. It gradually decreases toward the inner diameter surface of the coil 13), and such a distribution of the magnetic field strength is realized by the trim coil 60 described later.

That is, the trim coil 60 is composed of a plurality of coils having different diameters, and the magnetic fields generated by the respective coils are superimposed to cause non-uniformity in the magnetic field on the raceway surface 20. When it is desired to maximize the magnetic field at the incident point as in this embodiment, the trim coil 60 is arranged at the position of the incident point by arranging the coil having the smallest diameter among the trim coils 60 so that the central axis passes through the incident point. The magnetic fields derived from all the coils constituting the above can be superposed and maximized. However, the present invention is not limited to this mode, and the shape of the magnetic pole portion 123 may be adjusted according to the required magnetic field distribution. That is, it may be processed so that the magnetic pole surface 124 has a conical or convex shape so as to project toward the incident point. In this case, the trim coil 60 can be omitted, or the number of coil arrangements can be reduced, so that the man-hours and the like can be reduced.

On the other hand, the combination of the columnar magnetic pole portion 123 and the trim coil 60 provided on the surface thereof as in the present embodiment includes the facing surface of the columnar magnetic pole portion 123 and the facing surface of the columnar magnetic pole portion 123 when the return yoke portion 121 is vertically connected. The space defined from the inner diameter surface of the return yoke portion 121 can be provided wider than in the case where the shape of the magnetic pole portion 123 is conical. Therefore, there is an advantage that the installation work of the D electrode and the kicker magnetic field generating coil 311 described later becomes easy, and the fine adjustment of the arrangement becomes easy. In this embodiment, the magnetic field distribution is formed by the combination of the magnetic pole portion 123 and the trim coil 60, and the surface of the magnetic pole portion 123 with respect to the raceway surface 20 is parallel to the raceway surface 20.

Subsequently, the device for accelerating the ions in this embodiment will be described.

In generating the beam, first, ions are incident on the accelerator 1 from the incident portion 130 in a low energy state. An ion source 12 is installed in the incident portion 130, and electric power required for generating ions is supplied from the outside through the through port 115. The ions generated by the ion source 12 are drawn out to the raceway surface 20 by the voltage applied to the extraction electrode. The extracted ions are accelerated each time they pass through the acceleration gap 223 by a high-frequency electric field excited by the high-frequency cavity 21.

The high-frequency cavity 21 excites an electric field in the acceleration gap 223 by a λ / 4 type resonance mode, and the electric field accelerates ions. Regarding the high-frequency cavity 21, the portion between the magnetic pole portions 123 that overlaps the raceway surface 20 when viewed from the vertical direction in FIG. 1 and is installed so as to cover a part of the raceway surface is defined as a dee electrode. To do.

Further, assuming that the incident point side is the front end and the opposite side is the rear end, the high frequency cavity 21 is connected to the dee electrode 221 on the rear end side and is connected to the high frequency power supply through the through port 114 provided in the return yoke portion 121. Has a connection part. The connection portion corresponds to a part of the high frequency cavity 21, and may be regarded as a part of the D electrode 221. However, it is defined as described above for the sake of simplicity.

When the shape of the D electrode 221 is projected onto the raceway surface 20, the D electrode 221 is formed so as to have a substantially fan shape having a sharp central angle centered on the vicinity of the incident point and extending to the opposite side to the base end portion of the exit path. .. As will be described later, the D electrode 221 has a structure that is almost symmetrical with respect to the axis of symmetry.

The beam (a collection of ions in an accelerated state) is accelerated by an electric field excited by an acceleration gap 223 sandwiched by a ground electrode 222 facing the dee electrode 221. This electric field is generated from a high-frequency power source connected to the D electrode, and the frequency of the electric field needs to be an integral multiple of the orbital frequency of the beam in order to synchronize with the above-mentioned orbital frequency of the beam. .. In this accelerator 1, the frequency of the electric field is set to 1 times the frequency around the beam. The frequency of this electric field is several tens of MHz, and will be referred to as a high-frequency electric field hereafter.

The ground electrode 222 is provided parallel to the end face of the D electrode 221 with a certain gap (acceleration gap 223) between the D electrode 221. In other words, when the D electrode 221 is fan-shaped, it becomes a line corresponding to a radius. On the other hand, they are almost parallel.

A high-frequency electric field is formed by the difference between the voltage applied to the D electrode 221 and the potential of the ground electrode 222, and the beam passes through the acceleration gap 223 twice in the orbital plane 20 around the incident point. Although acceleration is received in each of the two passages, it is necessary to reverse the potential difference between the acceleration from the D electrode 221 to the ground electrode 222 and the acceleration from the ground electrode 222 to the D electrode 221. There are various modes for reversing the potential difference, but in this embodiment, the ground electrode 222 maintains 0 V, and the D electrode 221 is assumed to change the potential to both positive and negative to the same extent by the high frequency power supply. The ground electrode 222 is configured such that the potential is kept constant during the acceleration period (the time required to obtain the desired energy), and the voltage applied to the D electrode 221 is changed to the same level above and below with reference to this potential. You may.

Further, the arrangement of the D electrode 221 and the ground electrode 222 may be exchanged. That is, the ground electrode 222 may be configured such that the end face of the ground electrode 222 is the end face of the dee electrode 221 and the ground electrode 222 is arranged on the end face of the dee electrode 221.

The beam accelerated by the above configuration is accelerated to have a predetermined energy and then taken out from the accelerator 1. When the beam is taken out of the accelerator 1, the kicker magnetic field generating coil 311 is excited. The kicker magnetic field generating coil 311 is installed on the magnetic pole surface 124, and the coil is formed by passing a current through this coil. A kicker magnetic field, which will be described later, is superposed and excited with respect to the main magnetic field formed by the 13 and the magnetic pole portion 123.

Further, a septum electromagnet 312 for taking out is provided at one place of the magnetic pole surface 124, and in addition to this septum electromagnet 312, one suitable for taking out may be adopted. Assuming that the path for extracting the beam from the internal space of the electromagnet 11 to the outside is the exit path, the end portion of the septum electromagnet 312 protruding toward the internal space side of the electromagnet 11 is the base end portion of the exit path. The base end portion of this emission path is provided so as to be aligned in the order of the center of gravity of the orbital plane 20 (the geometric center when the orbital plane 20 is a two-dimensional finite region), the incident point of ions, and the base end portion. Has been done. Further, the base end portion of the emission path is provided along the outer edge of the magnetic pole portion 123 in order to avoid interference with the beam orbiting the raceway surface 20.

The beam orbiting the orbital plane 20 is displaced from the design orbit in the presence of the kicker magnetic field, moves to enter the septum electromagnet 312, and then follows the beam along the take-out channel 322 formed by the magnetic field of the septum electromagnet 312. Is taken out of the accelerator 1. The arrangement of the kicker magnetic field generating coil 311 may be different from that in FIG. 2 as long as it realizes the above-mentioned movement with respect to the orbiting beam. In the arrangement of FIG. 2, even if the kicker magnetic field is small, a sufficient change in position can be obtained by securing a distance to reach the septum electromagnet 312.

Further, in the orbital surface 20, the magnetic pole portion 123, the coil 13, the trim coil 60, and the kicker magnetic field generation coil so that the in-plane deviation (magnetic field component parallel to the orbital surface 20) of the main magnetic field with respect to the design orbital becomes almost 0. 311, The shape and arrangement of the take-out septum electromagnet 312 are plane-symmetric with respect to the raceway surface 20. The shapes of the magnetic pole portion 123, the dee electrode 221, the coil 13, and the trim coil 60 are line-symmetrical with respect to the straight line passing through the incident point and the center of gravity of the raceway surface 20, and the main magnetic field is also symmetrical with respect to the straight line. Has sex. In the following, unless otherwise specified, the main magnetic field is a magnetic field orthogonal to the orbital plane 20.

Next, the trajectory of the beam orbiting the inside of the accelerator 1 will be described. The beam is accelerated while orbiting in the orbital plane 20. The kinetic energy of the beam that can be taken out (hereinafter, simply referred to as energy) will be described below as a minimum of 70 MeV to a maximum of 235 MeV, but the present invention is not limited to this, and the energy range may be set as necessary. In accelerating the beam, the greater the energy, the smaller the orbital frequency of the beam. The relationship between these energies and the orbital frequency is shown in FIG. With the energy immediately after the incident, the beam reaching 76 MHz and 235 MeV orbits at 59 MHz.

Since the beam needs to orbit stably, in order to achieve this stability, the magnetic field portion 123 makes the main magnetic field uniform along the orbit of the beam, and the magnetic field decreases as the energy increases. It becomes a distribution. Under such a magnetic field, the beam oscillates stably in each of the radial direction of the orbital plane 20 and the direction perpendicular to the orbital plane 20.

The orbit of each energy is shown in FIG. The orbit has a circular orbit with a radius of 0.497 m corresponding to the orbit with the maximum energy of 252 MeV on the outermost side, and 51 circular orbits are divided into 51 by the magnetic rigidity (ratio of momentum and charge) from there to 0 MeV. Is illustrated. The dotted line is a line connecting the same orbital phases of each orbit, and is called an equal orbital phase line. In this accelerator 1, as the beam accelerates, the center of the design trajectory of the beam corresponding to each energy moves in one direction in the raceway surface 20, particularly toward the base end side of the emission path from the center of gravity of the raceway surface 20.

As a result of the movement of the center of each design trajectory, there are areas where orbits of different kinetic energies are close to each other and areas far from each other. When the latest points of the orbits are connected, a line segment orthogonal to each orbit is formed, and when the farthest points of the orbits are connected, a line segment orthogonal to the orbit is formed, and these two line segments exist on the same straight line. If this straight line is defined as the axis of symmetry, the shape of the orbit and the distribution of the main magnetic field are axisymmetric with respect to the plane that passes through the axis of symmetry and is perpendicular to the orbital plane. The equi-circular phase lines are plotted every orbital phase π / 20 from the aggregation region. The acceleration gap 223 formed between the dee electrode 221 and the ground electrode 222 facing the dee electrode 221 is installed along the equicircling phase line that orbits ± 90 degrees with respect to the line segment connecting the latest points of the orbits. ..

In order to generate the above-mentioned orbital configuration and stable betatron vibration around the orbit, in the accelerator 1 of this embodiment, the main magnetic field distribution in which the value of the magnetic field decreases toward the outside in the radial direction of deflection of the design orbit is used. Is forming. In addition, the magnetic field is constant along the design trajectory. Therefore, the design trajectory becomes circular, and the radius and orbit time of the beam increase as the energy of the beam increases. In such a system, particles that deviate slightly in the radial direction from the design orbit receive a restoring force that returns them to the design orbit, and at the same time, particles that deviate in the direction perpendicular to the orbital plane also receive a main magnetic field in the direction of returning to the orbital plane. Receives resilience from.

The design orbit refers to the orbit drawn by the beam according to the energy, that is, corresponds to the energy of any of the aggregates of orbits (points in the case of 0MeV) that can be continuously defined from 0 MeV to 252 MeV. It means an orbit and does not indicate only an orbit corresponding to a specific energy. Therefore, "deviation from the design trajectory in the radial direction (radial direction)" means that the energy deviates from the original trajectory corresponding to a certain energy, although the energy hardly changes. Further, the above-mentioned "restoring force" means a force that the beam deviated from the original orbit in this way receives in the direction of returning to the design orbit according to the energy.

If the magnetic field is appropriately reduced as the beam energy increases, the above-mentioned restoring force will always cause particles that deviate from the design orbit to return to the design orbit and vibrate in the vicinity of the design orbit. Will be done. This makes it possible to stably orbit and accelerate the beam. The vibration centered on this design trajectory is called betatron vibration. The value of the magnetic field in the beam of each energy is shown in FIG. The magnetic field reaches a maximum of 5T at the incident point and drops to 4.91T at the outermost circumference.

For accelerating the ions, high frequency power is introduced from the high frequency power supply 210 through the input coupler 211, and a high frequency electric field is excited in the acceleration gap 223 between the D electrode 221 and the ground electrode 222. In order to excite the high frequency electric field in synchronization with the orbit of the beam, the frequency of the electric field is modulated according to the energy of the orbiting beam.

The high-frequency cavity 21 using the resonance mode needs to sweep high-frequency frequencies in a range wider than the width of resonance. Therefore, it is necessary to change the resonance frequency of the cavity. The control is performed by changing the capacitance of the rotary variable capacitor 212 connected to the D electrode 221. The rotary variable capacitor 212 controls the capacitance generated between the conductor plate directly connected to the rotary shaft and the outer conductor by the rotation angle of the rotary shaft 213. That is, the rotation angle of the rotation shaft 213 is changed as the beam accelerates.

The behavior of the beam from the beam incident to the extraction of the accelerator 1 will be described. First, a low-energy beam is output from the ion source 12, and the beam is guided to the orbital plane 20. The beam incident on the orbital plane 20 is accelerated by a high-frequency electric field, its energy increases, and the radius of gyration of the orbit increases. The beam is then accelerated by a high frequency electric field while ensuring directional stability. That is, instead of passing through the acceleration gap 223 at the time when the high-frequency electric field is maximized, the acceleration gap 223 is passed when the high-frequency electric field is decreasing in time.

Then, since the frequency of the high-frequency electric field and the orbital frequency of the beam are synchronized at a ratio of exactly an integral multiple, the particles accelerated in the phase of the predetermined accelerating electric field are accelerated in the same phase in the next turn. On the other hand, a particle accelerated in a phase earlier than the acceleration phase has a larger acceleration amount than a particle accelerated in the acceleration phase, and therefore receives acceleration in a delayed phase in the next turn. On the contrary, a particle accelerated in a phase slower than the acceleration phase at that time has a smaller acceleration amount than a particle accelerated in the acceleration phase, so that the particle is accelerated in the advanced phase in the next turn. In this way, the particles at the timing deviated from the predetermined acceleration phase move in the direction of returning to the acceleration phase, and by this action, the particles can oscillate stably even in the phase plane (traveling direction) composed of the momentum and the phase.

This vibration is called synchrotron vibration. That is, the accelerating particles are gradually accelerated while synchrotron oscillating, and reach a predetermined energy to be taken out. In order to extract a predetermined extraction beam with a target energy, one or a plurality of coils of the kicker magnetic field generating coil 311 are selected based on the target energy and a predetermined exciting current is passed.

The beam of the target energy orbits along the design trajectory when no current is passed through the kicker magnetic field generating coil 311. However, when the current is passed through the kicker magnetic field generating coil 311, the target energy is reached. The beam deviates from the orbit due to the kick magnetic field caused by the kicker magnetic field generating coil 311. That is, the betatron vibration in the orbital plane is excited by the kicker magnetic field generating coil 311. When the position of the kick by the kicker magnetic field generation coil 311 and the position of the aggregation point are in an appropriate positional relationship, it is possible to displace the beam radially outward at the aggregation point by the kick by the kicker magnetic field generation coil 311.

In a series of operations in which the beam accelerates while orbiting in the main magnetic field, as described above, betatron vibration, which is vibration in the plane perpendicular to the orbit, momentum deviation, and synchrotron vibration, which is vibration in the phase of accelerated high frequency, are generated. To do. By these two kinds of vibrations, the beam can be stably accelerated to a predetermined energy without losing the beam. However, the betatron vibration generated by the above-mentioned principle needs to secure a force in the direction of returning the particles that have deviated from the orbit, that is, a converging force.

In a system in which the convergence force increases in proportion to the orbital displacement, it is possible to stably oscillate betatron even with a large displacement, but the magnetic field distribution of general accelerators including this accelerator has a displacement of 6 poles or more. There is a component that changes non-linearly with respect to. The non-linear distribution reduces the convergence force and raises an upper limit on the amplitude of the betatron oscillation. Since particles displaced beyond the upper limit are lost, it is necessary to reduce the non-linear distribution of the magnetic field and reduce the disturbance given to the orbiting beam in order to secure the beam amount. As a countermeasure against this disturbance, the main magnetic field is formed so as to have the distribution described above.

In addition, one of the disturbances given to the beam is derived from acceleration by a high-frequency electric field. The beam does not change its spatial position when it is accelerated, whereas the acceleration increases the kinetic energy. Then, the ions that were on the design orbit before the acceleration also moved outward due to the change in kinetic energy, and were disturbed by moving inward as a displacement seen from the design orbit. In other words, the transition from the pre-acceleration design trajectory to the post-acceleration design trajectory causes the betatron vibration, which was stable with respect to the pre-acceleration design trajectory, to be designed based on the post-acceleration design trajectory. It is generated inside the orbit. This is defined as the disturbance that accompanies acceleration. The dee electrode 221 of the accelerator 1 of this embodiment is devised so that the amplitude of the betatron vibration generated by the disturbance caused by the acceleration does not increase.

The beam is accelerated in the gap between the D electrode 221 and the ground electrode 222, and is accelerated twice when the beam enters the region covered by the D electrode 221 and when it exits the D electrode 221 per round. The displacement of the beam caused by these two accelerations can be reduced by optimizing the shape of the dee electrode 221 so as to cancel each other out. The principle will be described with reference to FIG. Introduces the concept of normalized topological space to describe betatron oscillations.

The phase space is a plane defined by the displacement seen from the design trajectory and the inclination with respect to the design trajectory, and the particles having stable betatron vibration orbit in the phase space. The phase space is further normalized by the normalized matrix represented by the Twiss parameter for each point of the orbit, which is called the normalized phase space. In the unaccelerated situation, the betatron vibration of the particle is the normalized phase space. It can be regarded as a clockwise circular motion above (A). As shown in FIG. 6, when the beam goes around the orbital plane 20, it makes a circular motion in the normalized topological space by an angle of 2πν. However, ν is a betatron frequency, and the betatron frequency in the horizontal direction is about 0.99, which is slightly smaller than 1. Also, when considered in real space, x = 0 corresponds to the position of the design trajectory.

Now, for particles that move circularly in a normalized topological space, the disturbance of acceleration can be seen as a movement in the spatial direction in a normalized topological space. Due to the synchronization condition with high frequency, the two acceleration gaps need to be separated from each other by an angle of π on the orbit. Then, the motion from one acceleration gap to the other acceleration gap becomes a circular motion with an angle of πν on the normalized topological space.

Considering the moment when the particle passes through a certain acceleration gap, the particle is displaced inward (x <0) in the normalized topological space due to the disturbance accompanying the acceleration received by this passage (B). Next, since ν is almost 1, this particle finishes a circular motion at an angle of πν in the normalized moving space by the time it reaches the other acceleration gap, and the displacement is almost outward (x> 0). After that, it becomes almost parallel to the design trajectory (x'≈0) (C). Then, when it passes through the other acceleration gap, it receives disturbance due to acceleration again. Then, in the acceleration gap arrangement in which the left and right sides are line-symmetric with respect to the axis of symmetry as in the arrangement of the present embodiment, the former disturbance and the latter disturbance have almost the same magnitude (D). As a result, the trajectories on the series of normalized phase spaces of B, C, and D can be almost closed, that is, the disturbance due to the acceleration received during one round can be reduced, and the increase in the amplitude of the betatron vibration can be suppressed.

The displacement Δx in the normalized phase space due to the disturbance caused by the acceleration described above is given by Eq. 1 using the variance function η in the orbit, the momentum Δp obtained by the acceleration, the momentum p of the particles, and the Twiss parameter β.
Represented by.

Since η and β in the orbit of momentum p, which are two points symmetrical with respect to the axis of symmetry on the orbit, are equal to each other and Δp determined by the acceleration gap shape is also equal, the disturbance due to acceleration can be reduced as described above. Further, in a place where the momentum p is small, there may be a difference between Δp / p due to acceleration when entering the D electrode and Δp / p when exiting.

In that case, while keeping the position of the acceleration gap 223 symmetrical, the width of the acceleration gap 223, that is, the distance between the dee electrode 221 and the ground electrode 222 is increased on the side entering the dee electrode 221 and the transit time factor is reduced to increase Δp. Can be made smaller. In this case, the disturbance due to acceleration can be further reduced by adopting an asymmetrical shape rather than the symmetrical Dee electrode shape, and as a result, the amount of beam that can be accelerated can be increased.

In addition, in order to adjust the magnitude of the disturbance caused by the acceleration when entering the electrode and the disturbance caused by the acceleration when exiting the electrode, the position of the acceleration gap can be slightly shifted to shift the high frequency synchronization phase, or the D electrode. Alternatively, it is conceivable to change the shape of the ground electrode appropriately and change the electric field distribution on the left and right. In any case, the beam is disturbed when it makes one round in the accelerator, but the suppression of the increase in betatron vibration amplitude is achieved by making the trajectory in the normalized topological space resulting from the disturbance a closed trajectory. As a result, it becomes possible to realize a small accelerator with a large amount of beam that can be taken out.

According to the accelerator 1 of the present embodiment described above, in the case of the conventional cyclotron, a degrader (attenuation member) is required for energy change, and a large proportion of the ion beam is lost when passing through the degrader (attenuation member). Was there. However, the accelerator 1 of this embodiment does not need to correspond to a wide energy region as in the conventional case even if the installation of the degrader is omitted or is installed for fine adjustment of energy, so that the ion beam shielding performance It is possible to install a degrader with a low energy level, and it is possible to improve the utilization efficiency of the ion beam. Further, by omitting the installation of the degrader, the generation of neutrons and the like generated when the ion beam passes through the degrader can be suppressed, and the amount of the shielding member for shielding these can be reduced. That is, the degree of freedom in arranging the accelerator is improved, which also contributes to the efficiency of the space required for installation.

In addition, conventional variable energy accelerators and cyclotrons make the orbital average magnetic field proportional to the relativistic γ factor of the beam to keep the orbital time constant regardless of energy (the magnetic field distribution with this property is isochronous). Called a sexual magnetic field). Under an isochronous magnetic field, the magnetic field is modulated along the orbit to ensure beam stability in the orbital plane and in the direction perpendicular to the orbital plane, but in order to achieve both isochronism and beam stability. Requires a maximum (Hill) and a minimum (Valley) of the magnetic field. A non-uniform magnetic field with this distribution can be formed by making the distance (gap) between the magnetic poles narrow in the Hill region and wide in the Valley region. However, the difference between the Hill magnetic field and the Valley magnetic field is practically limited to the saturation magnetic flux density of the magnetic pole material which is a ferromagnet. That is, the difference between the Hill magnetic field and the Valley magnetic field is limited to about 2 Tesla.

On the other hand, when miniaturizing the above-mentioned conventional accelerator, it is necessary to increase the main magnetic field and reduce the deflection radius of the beam orbit, but the difference between the main magnetic field and the above-mentioned Hill magnetic field and Valley magnetic field is in a proportional relationship. However, the above-mentioned limit due to the saturation magnetic flux density becomes a factor that hinders miniaturization, but the accelerator of this embodiment adopts a structure in which such a limitation is relaxed, so that miniaturization can be achieved. It will be possible.

In the second embodiment, the accelerated nuclide is a carbon ion in the first embodiment. This accelerator is a frequency-modulated variable energy accelerator capable of extracting carbon ions in the range of kinetic energy of 140 MeV to 430 MeV per nucleon. Since the operating principle, device configuration, and operating procedure are the same as those in the first embodiment, they are omitted. The differences are the magnitude of the orbital radius and the relationship between the magnetic field and energy, and the relationship between the orbital frequency and energy. From the accelerator shown in Example 1, the product of the orbital radius and the magnetic field is proportional to the ratio of the magnetic rigidity of the beam. It can be realized by. Therefore, the beam is disturbed when it makes one round in the accelerator by the same method as in Example 1, but the betatron vibration amplitude is set to a closed trajectory in the normalized topological space resulting from the disturbance. Suppression of the increase in the beam is achieved, and it becomes possible to realize a small accelerator with a large amount of beam that can be taken out.

As a third embodiment, the particle beam therapy system using the accelerator described in Example 1 or Example 2 will be described. FIG. 7 is a schematic view of this embodiment.

The particle beam therapy system 410 has an accelerator 1 that generates a particle beam (ion beam), and the ion beam emitted from the accelerator 1 travels inside the duct 400. The ion beam traveling inside the duct 400 is deflected in a desired direction by the function of the deflection electromagnet 401, and is guided to an arbitrary position by providing a plurality of the deflection electromagnets 401. Further, a plurality of quadrupole electromagnets 402 are installed in the path (transport path) for transporting the ion beam, and the state of the ion beam is adjusted by the convergence or divergence action of the quadrupole electromagnet 402. The output of the deflecting electromagnet 401 and the quadrupole electromagnet 402 is configured to be adjusted by the energy of the passing ion beam, and the adjustment is controlled by the control device 408. Further, some of the deflection electromagnets 401 are configured to be rotatable around the rotation shaft 407, and by this rotation, the irradiation field forming device described later can be carried to an arbitrary rotation angle centered on the patient 409. it can. Each deflection electromagnet 401 may be fixed to the floor surface or wall surface of the facility via a fixture to hold the irradiation field forming device in a fixed position.

The ion beam carried to the vicinity of the treatment area via such a transport device passes through the irradiation field forming device and irradiates the affected area. There are various types of irradiation field forming devices, and FIG. 7 shows an example in which two scanning electromagnets 403 are used. The two scanning electromagnets 403 generate magnetic fields that are orthogonal to each other, and by controlling the outputs of the two orthogonal magnetic fields, the ion beam can be deflected toward a desired position.

A position monitor 404 that detects the passing position of the ion beam and a dose monitor 405 that measures the dose of the irradiated ion beam are installed downstream of the scanning electromagnet 403 (on the patient side of the scanning electromagnet 403). For each monitor, for example, a multi-wire proportional chamber can be used as a position monitor, and an ionization chamber can be used as a dose monitor. Of course, different types of monitors with suitable functions may be used.

The irradiation field forming device may be of a type in which a beam forming member such as a scatterer, a collimator, a range shifter, or a ridge filter or an energy adjusting member is provided on the downstream side of the scanning electromagnet 403. Further, it may be an irradiation field forming device in which the scanning electromagnet 403 is not installed and only the scatterer is installed. Further, the installation position of the position monitor 404 and the dose monitor 405 is not limited to the position shown in FIG. 7, and may be installed in the middle of the beam transport path.

On the downstream side of the scanning electromagnet 403, a support 406 for holding the patient 409 in a position suitable for treatment is provided. Although omitted in FIG. 7, the support base 406 is configured to be movable under the control of the control device 408. Further, the support base 406 may be of any type such as a bed type in which the patient 409 can take a supine or prone position, or a chair-like type suitable for the patient 409 in a sitting position. ..

Further, the operation of the deflection electromagnet 401, the quadrupole electromagnet 402, the scanning electromagnet 403, the position monitor 404, the dose monitor 405 and the support base 406 mentioned above is controlled by the control device 408. The control device 408 has an input interface (not shown), and is capable of receiving operation instructions of a medical staff or an assistant thereof. Although the control device 408 has been described as one device in this embodiment, this is an example, and a control device corresponding to each control target may be prepared and combined to form the control device 408.

The particle beam therapy system 410 having the above-mentioned devices as a configuration irradiates the affected area to be irradiated with proton beams or carbon beams (differences) depending on the position (depth) of the affected area with respect to the body surface. The energy of (collectively referred to as particle beams below) is set to an appropriate value and the patient is irradiated. The energy of the particle beam to be irradiated is determined by the treatment plan, and the information of the energy of the particle beam and the irradiation amount determined by the treatment plan is input to the control device 408. Upon receiving this, the control device 408 controls the accelerator 1 so that the particle beam of the indicated energy and irradiation amount is emitted. More specifically, the ion source 12 is required to supply ions, and the high frequency power supply 210 of the accelerator 1 is charged from the initial frequency (E = 0) shown in the graph of FIG. 3 to the target frequency (E = target). It requires an output of a frequency that gradually decreases toward (energy).

When an ion beam that has reached the target energy is generated by these controls, the kicker magnetic field generation coil 311 is operated to kick the ion beam out of the design orbit, and the septum electromagnet 312 is operated to emit the ion beam from the accelerator 1. Let me. The emitted ion beam travels inside the duct 400 while being controlled in direction and state by the deflection electromagnet 401 and the quadrupole electromagnet 402, and is controlled by the scanning electromagnet 403 toward the planned position. The irradiation amount of the ion beam is measured by the dose monitor 405, and when the target irradiation amount is reached, the control device 408 stops the operation of the kicker magnetic field generating coil 311 and the ion source 12 of the accelerator 1. After that, the above control is repeated so as to correspond to the next energy and irradiation amount specified in the treatment plan.

According to the particle beam therapy system of this embodiment, an ion beam having a wide range of energy required for treatment is emitted from the accelerator 1. Therefore, the utilization efficiency of the ion beam is compared with the particle beam therapy system using a conventional cyclotron. Can be dramatically improved. In addition, since the amount of beam that can be taken out is large, the irradiation time can be shortened, and the number of patients per hour that can be irradiated at the facility can be increased. Further, as mentioned above, since the limitation of the magnetic field is small, it is easy to miniaturize the particle beam therapy system as a whole, which contributes to the miniaturization of the entire particle beam therapy system.

In the above, the circular accelerator of the present invention and the particle beam therapy apparatus using the same have been described with reference to examples. The present invention is not limited to the above-described examples, and includes various modifications. For example, the above-described embodiment has been described in detail in order to explain the present invention in an easy-to-understand manner, and is not necessarily limited to the one including all the described configurations. Further, it is possible to replace a part of the configuration of one embodiment with the configuration of another embodiment, and it is also possible to add the configuration of another embodiment to the configuration of one embodiment. Further, it is possible to add / delete / replace a part of the configuration of each embodiment with another configuration.

For example, as the electromagnet in the above-described embodiment, it is assumed that a general coil of normal conduction is adopted, but a superconducting magnet using a superconducting coil may be adopted. Further, as the frequency modulator, a rotary variable capacitor is given as an example, but this may be changed to a variable capacitance diode. Further, the accelerating ions are not limited to protons, but helium or carbon may be adopted, and the magnetic field distribution and the high frequency frequency applied to the high frequency cavity may be adjusted according to the accelerating ion species and the required energy. ..

1 Accelerator 11 Electromagnet 111 Take-out beam through-hole 112 to 113 Coil connection through-hole 114 High-frequency input through-hole 115 Beam-injection through-hole 12 Ion source 13 Coil 121 Return yoke part 122 Top plate part 123 Magnetic pole part 124 Magnetic pole surface 130 Incident part 20 Orbital surface (orbital surface)
21 High-frequency cavity 221 Dee electrode (first electrode)
222 Ground electrode (second electrode)
223 Acceleration Gap 210 High Frequency Power Supply 211 Input Coupler 212 Rotating Variable Capacitor (Frequency Modulator)
213 Rotating shaft 214 Servo motor 311 Kicker Magnetic field generation coil 312 Septum electromagnet 322 Extraction channel 60 Trim coil 400 Duct 401 Deflection electromagnet 402 Quadrupole electromagnet 403 Scanning electromagnet 404 Position monitor 405 Dose monitor 406 Support base 407 Rotating shaft 408 Control device 409 Patient 410 Particle beam therapy system

Claims (7)

In a circular accelerator that accelerates ions to generate an ion beam,
An electromagnet containing a raceway surface on which the ions are accelerated and having a magnetic pole portion protruding with respect to the raceway surface.
A high frequency cavity, which forms an electric field to accelerate the ions.
It has an emission path, for taking out the ions after acceleration, and has.
The electromagnet has a maximum magnetic flux density between the center of gravity of the orbital surface and the base end of the exit path, and the magnetic flux density gradually decreases toward the outer edge of the orbital surface from the point where the magnetic flux density is maximum. A distribution is formed on the orbital surface,
The magnetic pole portion is provided so that a plurality of coils having different diameters are placed on the surface of the magnetic pole portion facing the raceway surface so that a coil having a smaller diameter is placed on the inner diameter side of the coil having a larger diameter, or the magnetic pole portion. The surface facing the raceway surface has a convex shape protruding toward the raceway surface, and has a convex shape.
The high-frequency cavity includes a first electrode for forming the electric field and a frequency modulator that modulates the frequency of power applied to the first electrode .
When the first electrode is projected onto the raceway surface, the first electrode is formed so as to have an acute angle and a substantially fan shape extending to the opposite side of the base end portion of the emission path.
A circular accelerator characterized in that a connection portion with the frequency modulator is formed in a portion corresponding to the fan-shaped arc. The circular accelerator according to claim 1.
The substantially fan-shaped first electrode has a second electrode provided so as to face each other with a gap between the sides corresponding to the radius.
The second electrode is characterized in that it is maintained at a substantially equipotential potential during the acceleration period of the ions. The circular accelerator according to claim 2.
The first electrode and the second electrode are characterized in that the arrangement of the gap is substantially line-symmetric with respect to a straight line connecting the base end portion of the emission path and the center of gravity of the raceway surface. Circular accelerator The circular accelerator according to claim 2.
The first electrode and the second electrode are installed so that the size of the gap is asymmetric with respect to a straight line connecting the base end portion of the emission path and the center of gravity of the raceway surface. Circular accelerator. The circular accelerator according to claim 2.
The first electrode and the second electrode refer to a straight line connecting the base end of the exit path and the center of gravity of the orbital plane with respect to the disturbance received when the ions accelerated on the orbital plane pass through the gap. A circular accelerator characterized in that it is installed so as to have symmetry. The circular accelerator according to any one of claims 1 to 5.
The frequency modulator is a circular accelerator characterized by having a rotary variable capacitance capacitor. The circular accelerator according to any one of claims 1 to 6.
A transportation path for transporting an ion beam generated by the circular accelerator,
An irradiation field forming device that deflects an ion beam transported through the transport path to form a desired irradiation field.
A particle beam therapy device comprising.

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