WO2019093110A1

WO2019093110A1 - Circular accelerator and particle beam treatment system

Info

Publication number: WO2019093110A1
Application number: PCT/JP2018/039216
Authority: WO
Inventors: 孝道青木; 隆光羽江; 風太郎えび名; 裕人中島
Original assignee: 株式会社日立製作所
Priority date: 2017-11-13
Filing date: 2018-10-22
Publication date: 2019-05-16
Also published as: JP2019091553A; JP6899754B2

Abstract

This circular accelerator for generating an ion beam by accelerating ions comprises: an electromagnet which includes an orbit plane for accelerating the ions; a high-frequency cavity for forming an electric field for accelerating the ions; and an exit route for extracting the ions after acceleration. The circular accelerator is characterized in that: the electromagnet forms on the orbit plane a magnetic field distribution such that the magnetic flux density is maximized between the center of gravity of the orbit plane and a proximal end portion of the exit route, and such that the magnetic flux density gradually decreases from the point at which the magnetic flux density is maximized to an outer edge of the orbit plane; and the high-frequency cavity is provided with a first electrode for forming the electric field and a frequency modulator for modulating the frequency of electric power applied to the first electrode.

Description

Circular accelerator and particle therapy system

The present invention relates to an accelerator for accelerating heavy ions such as protons or carbon ions.

As background art of the present invention, as "providing an accelerator capable of efficiently emitting ion beams different in energy" in Patent Document 1, "the accelerator has a circular vacuum vessel including a circular return yoke. Electrode for incidence" Are disposed on the entrance side of the beam exit path in the return yoke with respect to the central axis of the vacuum vessel, and the magnetic poles are disposed radially around the entrance electrode in the return yoke. In the vacuum vessel, an orbit concentric area in which a plurality of beam orbits centering on the entrance electrode exist, and around the area from the entrance electrode in the vacuum vessel, alternately arranged with the magnetic poles. An orbital eccentric region is formed in which a plurality of eccentric beam orbitals exist. In the orbital eccentric region, the beam orbital between the entrance electrode and the entrance of the beam outgoing path Becomes dense, the invention that the spacing between the beam orbit each other becomes wider "in 180 ° opposite the entrance to the beam emission path to base the incidence electrode is disclosed.

International Publication 2016/092621

The variable energy accelerator described in Patent Document 1 is a type of accelerator that accelerates a beam circulating in a main magnetic field by a high frequency electric field. Such an accelerator is expected to be used in the radiation medical field, but in order to efficiently use the installation space in a medical facility, it is desirable to be small. Then, this invention makes it a subject to provide the variable energy accelerator which is easy to achieve size reduction.

One of the representative circular accelerators of the present invention for achieving the above-mentioned object is an electromagnet containing an orbital plane in which the ions are accelerated, a circular accelerator for accelerating the ions to generate an ion beam, and the ions The electromagnet has a high frequency cavity for forming 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 plane and the proximal end of the emission path. A magnetic field distribution is formed on the orbital plane in which the magnetic flux density gradually decreases from the point where the density is maximum and the magnetic flux density is maximum toward the outer edge of the orbital plane, and the high frequency cavity forms the electric field. And a frequency modulator that modulates the frequency of the power applied to the first electrode and the first electrode.

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

1 is a general outline of an accelerator 1 of Example 1; FIG. 2 is a layout view of internal devices of an accelerator 1; It is a graph which shows the energy dependence of the circulation frequency of the accelerator 1. It is a design trajectory shape of the accelerator 1. It is a graph which shows the energy dependence of the main magnetic field of the accelerator 1. It is a figure which shows the movement on the normalization phase space of the circular beam of the accelerator 1. FIG. FIG. 1 shows a particle beam therapy system using an accelerator 1;

The accelerator according to the first embodiment which is a preferred embodiment of the present invention will be described below with reference to FIGS.

The accelerator 1 of the present embodiment is a frequency modulation type variable energy accelerator. The accelerator 1 is a circular accelerator having a temporally constant magnetic field as a main magnetic field and accelerating protons circulating in the main magnetic field by a high frequency electric field. FIG. 1 shows its appearance. The accelerator 1 has electromagnets 11 which can be divided up and down. The electromagnets 11 excite a main magnetic field in a vacuum region (hereinafter referred to as an orbital plane) through which a beam being accelerated passes.

The rough appearance of the accelerator 1 will be described based on FIG. First, on the surface of the electromagnet 11, a plurality of track surface through holes for communicating the inside and the outside of the accelerator 1 are provided. Those through-holes are mainly provided for extracting current supply lines for equipment installed inside the accelerator 1 and signal lines connected to the equipment to the external space, and the accelerated beam is taken out Some are provided for use. FIG. 1 shows an example of the method, in which an accelerated beam is taken out, a through hole 111 for taking out a beam, through

holes

112 and 113 for drawing out a conductor connected to the coil 13 of the electromagnet 11, and a 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. The high frequency power source is connected to the dee electrode (first electrode) 221 installed in the inner space of the electromagnet 11 through the through hole 114. The high frequency power supply has a function of outputting electric power at an appropriate frequency to accelerate ions circulating in the inner space of the electromagnet 11. In the example shown in FIG. 1, the rotary variable capacitance capacitor 212 is provided. . The capacity is exemplified by one controlled by the servomotor 214, but is not limited thereto. A supply line or the like for supplying power to the rotary variable capacitance capacitor 212 is omitted in the drawing.

Further, a through hole 115 provided on the upper surface of the electromagnet 11 is an ion introduction path, and ions to be accelerated through the through hole 115 are inside the electromagnet 11, more precisely, inside the electromagnet 11. It is supplied to a generally disc-like space (track surface 20) defined by the trajectory of acceleration existing in the hollow part. The position of the through hole 115 for introducing ions 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. In practice, a vacuum pump for maintaining the inside of the accelerator 1 in vacuum and temperature control means for controlling temperature conditions 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 of the electromagnet 11 of the accelerator 1 shown in FIG. 1 divided up and down and the electromagnet 11 on the lower side viewed from above. Basically, the electromagnet 11 shown in FIG. 2 has the same structure as viewed from below, although there are some differences, such as the presence or absence of the through hole 115. 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.

The electromagnet 11 mainly includes a return yoke portion 121, a top plate portion 122, a cylindrical magnetic pole portion 123, and a coil 13.

The return yoke portion 121 is a thick cylindrical member, and when the upper and lower electromagnets 11 are connected, one end thereof is a contact portion. The end opposite to the contact portion is connected to the top plate portion 122. The top plate portion 122 is a disk-like member having a diameter that substantially matches the outer diameter of the return yoke portion 121. Therefore, the external appearance of the structure in which the return yoke portion 121 and the top plate portion 122 are fastened becomes a cylindrical structure whose 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 portion and protruding into the space on the inner cylinder side of the return yoke portion 121. However, unlike the return yoke portion 121, when connecting the electromagnets 11 which are disposed to face each other in the vertical direction, the opposing magnetic pole portions 123 project from the top plate portion 122 so as not to contact each other to form a gap.

The return yoke portion 121, the top plate portion 122, and the magnetic pole portion 123 are formed by cutting out an ingot of pure iron, but members corresponding to the respective portions may be separately cut out and then connected. Depending on the size of the processing apparatus and the size of the prepared ingot, it may be integrally cut out. Also, as a matter of course, large members may be formed by combining smaller parts.

The coil 13 is disposed between the return yoke portion 121 and the magnetic pole portion 123, and the inner diameter of the coil 13 substantially matches the outer diameter of the magnetic pole portion 123 as shown in FIG. The coil 13 and the return yoke portion 121 and the coil 13 and the magnetic pole portion 123 are respectively insulated. A conductor connected to the coil 13 is pulled out from the through

holes

112 and 113 and connected to a power supply provided outside the accelerator 1 and a current flows through the coil 13 to magnetize the magnetic pole portion 123 and to be described later Excite the magnetic field with a predetermined distribution.

A trim coil 60 is provided on the surface (magnetic pole surface 124) corresponding to the opposing surfaces of the magnetic pole portions 123 arranged in a pair at the top and bottom as a coil for finely adjusting the magnetic field generated by the coil 13 and the magnetic pole portion 123. Be An aspect of the trim coil 60 in the present embodiment is a coil group having a plurality of different diameters as shown in FIG. 2, in which a coil with a smaller diameter is disposed on the inner diameter side of the larger diameter coil. Also, each coil is arranged such that the center is biased in one direction as shown in FIG. The trim coil 60 and the pole face 124 are insulated by an insulating member (not shown) and fixed to the pole face 124 using a nonmagnetic member or an adhesive. The magnitude of the current supplied to each of the coils constituting the trim coil 60 can be adjusted individually, and is connected to an external power supply through the through

holes

112 and 113 in the same manner as the coil 13. By adjusting the excitation current individually for each system, the trim coil current is adjusted before operation so as to approximate a main magnetic field distribution described later and achieve stable betatron oscillation.

The accelerator 1 having such a mechanical structure accelerates the ions supplied from the ion source 12 to generate an ion beam, and the orbit plane 20 is defined by the orbit of the ions during acceleration. The position of the raceway surface 20 is in the space formed when the above-described return yoke portion 121 is vertically connected, and is equidistant to the upper and lower magnetic pole surfaces 124. The raceway surface 20 is a disc-like space that is not a flat surface but a thickness in a strict sense, but is treated as a circular area having a diameter smaller than the inner diameter of the return yoke portion 121 for convenience of explanation.

The orbital plane 20 has a magnetic field distribution required for the desired acceleration. By providing the cylindrical magnetic pole portion 123, it is possible to generate a magnetic field of 4.9 T over most of the required magnetic field strength in the orbital plane 20, that is, in the example shown in FIG.

The magnetic field distribution to be generated in the orbital plane 20 has a maximum position (incident point) when ions supplied from the ion source 12 get on the orbital plane 20 from the outer edge of the orbital plane 20 (ie, from that position). The distribution 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 constituted by a plurality of coils with different diameters, and the magnetic field generated by each coil is superimposed to cause non-uniformity in the magnetic field in the orbital plane 20. If it is desired to maximize the magnetic field at the incident point as in this embodiment, the trim coil 60 is disposed at the position of the incident point by arranging the coil of 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 that make up the signal can be superimposed and maximized. However, the present invention is not limited to this mode, and the shape of the magnetic pole portion 123 may be adapted to the required magnetic field distribution. That is, the magnetic pole surface 124 may be processed to have a conical shape or a convex shape projecting toward the incident point. In this case, since the trim coil 60 can be omitted or the number of arranged coils can be reduced, the number of steps can be reduced.

On the other hand, in the combination of the cylindrical magnetic pole portion 123 and the trim coil 60 provided on the surface as in the present embodiment, the opposing surface of the cylindrical magnetic pole portion 123 and the opposing surface of the cylindrical magnetic pole portion 123 when the return yoke portion 121 is joined vertically A space defined from the inner diameter surface of the return yoke portion 121 can be widely provided as compared with the case where the magnetic pole portion 123 has a conical shape. Therefore, there is an advantage that the installation work of the die electrode and the coiler magnetic field generating coil 311 to be described later becomes easy and the fine adjustment of the arrangement becomes easy. In the present 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 orbital plane 20 is parallel to the orbital plane 20.

Subsequently, an apparatus for accelerating ions in the present embodiment will be described.

In order to generate a beam, ions are first incident on the accelerator 1 from the entrance section 130 in a low energy state. The incident part 130 is provided with the ion source 12 and is supplied with power necessary for ion generation from the outside through the through hole 115. The ions generated by the ion source 12 are extracted to the orbital plane 20 by the voltage applied to the extraction electrode. The extracted ions are accelerated each time they pass through the acceleration gap 223 by the 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 the λ / 4 resonant mode, and accelerates the ions by this electric field. With regard to the high frequency cavity 21, in particular, a portion disposed between the magnetic pole portions 123 and overlapping the track surface 20 when viewed from the vertical direction in FIG. 1 and defining a portion of the track surface as a dee electrode Do.

Further, assuming that the high frequency cavity 21 has the incident point side as the front end and the opposite side as the rear end, the high frequency cavity 21 is connected to the dee electrode 221 on the rear end side and connected to the high frequency power source through the through hole 114 provided in the return yoke portion 121 Connection. The connection portion corresponds to a part of the high frequency cavity 21 and may be regarded as a part of the dee electrode 221, but is defined as described above for the simplicity of description.

The Dee electrode 221 is formed to have a substantially fan shape whose central angle is an acute angle centered on the vicinity of the incident point and extends to the opposite side with respect to the proximal end of the emission path when the shape is projected onto the orbital plane 20 . As described later, the dee electrode 221 has a substantially symmetrical structure with respect to the axis of symmetry.

A beam (a set of ions in an accelerated state) is accelerated by an electric field excited in an acceleration gap 223 sandwiched by the ground electrode 222 opposite to the dee electrode 221. This electric field is generated from a high frequency power source connected to the dee electrode, and the frequency of the electric field needs to be an integral multiple of the loop frequency of the beam in order to synchronize with the loop frequency of the aforementioned beam . In this accelerator 1, the frequency of the electric field is one time of the frequency of the beam. The frequency of this electric field is several tens of megahertz, and is hereinafter referred to as a high frequency electric field.

The ground electrode 222 is provided in parallel with the end face of the dee electrode 221 with a certain gap (acceleration gap 223) between the dee electrode 221, and in other words, when the dee electrode 221 has a fan shape, it corresponds to a line corresponding to a radius. It is almost parallel to the other.

A high frequency electric field is formed by the difference between the voltage applied to the dee electrode 221 and the potential of the ground electrode 222, and the beam passes through the acceleration gap 223 twice when it makes a circuit around the orbital plane 20 around the incident point. Although acceleration is received in each of two passes, it is necessary to reverse the potential difference between the acceleration from the dee electrode 221 toward the ground electrode 222 and the acceleration from the ground electrode 222 toward the dee electrode 221. There are various modes for reversing the potential difference, but in the present embodiment, the ground electrode 222 is maintained at 0 V, and the potential of the dee electrode 221 is changed equally to plus and minus 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 dee electrode 221 is changed to the same upper and lower level based on this potential. May be

Also, the arrangement of the dee electrode 221 and the ground electrode 222 may be interchanged. That is, the ground electrode 222 may be arranged such that the end surface of the ground electrode 222 is disposed on the end surface of the dee electrode 221 and the end surface of the dee electrode 221.

The beam accelerated by the above configuration is extracted from the accelerator 1 after being accelerated to have a predetermined energy. 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 disposed on the magnetic pole surface 124, and a current is supplied to the coil to make the coil A kicker magnetic field to be described later is superimposed and excited on the main magnetic field formed by the magnetic pole portion 13 and the magnetic pole portion 123.

In addition, although a septum electromagnet 312 for extraction is provided at one location of the pole face 124, in addition to the septum electromagnet 312, one suitable for extraction may be employed. Assuming that the path for extracting the beam from the internal space of the electromagnet 11 to the outside is an emission path, the end of the septum electromagnet 312 projecting to the internal space side of the electromagnet 11 is the proximal end of the emission path. The proximal end of this emission path is provided on a straight line in the order of the center of gravity of the orbital plane 20 (geometric center when the orbital plane 20 is a two-dimensional finite area), the ion incident point, and the proximal end. It is done. Further, the proximal end of the emission path is provided along the outer edge of the magnetic pole portion 123 in order to avoid interference with the beam traveling around the raceway surface 20.

In the presence of the kicker magnetic field, the beam orbiting the orbital plane 20 is displaced from the design orbit, moves to impinge on the septum electromagnet 312 and then travels along the extraction 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 the above-described movement is realized with respect to the circulating beam. With the arrangement of FIG. 2, even if the kicker magnetic field is small, a sufficient change in position can be obtained by securing the distance to the septum electromagnet 312.

In addition, in the orbital plane 20, the magnetic pole portion 123, the coil 13, the trim coil 60, and the kicker magnetic field generating coil so that the deviation in the plane (the magnetic field component parallel to the orbital plane 20) with respect to the design orbit becomes substantially zero. 311, the shape and arrangement of the septum electromagnet 312 for taking out 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 symmetrical with respect to a straight line passing the incident point and the center of gravity of the orbital plane 20, and the main magnetic field is also symmetrical with respect to the straight line. Have sex. Hereinafter, the main magnetic field is a magnetic field orthogonal to the orbital plane 20 unless otherwise specified.

Next, the trajectory of the beam circulating inside the accelerator 1 will be described. The beam is accelerated while orbiting in the orbital plane 20. The kinetic energy of the extractable beam (hereinafter simply referred to as energy) is described as a minimum of 70 MeV to a maximum of 235 MeV, but is not limited thereto and the energy range may be set as needed. In accelerating the beam, the higher the energy, the smaller the frequency of the beam. The relationship between these energies and the circulation frequency is shown in FIG. At the energy immediately after incidence, the beam reaching 76 MHz and 235 MeV travels at 59 MHz.

Since the beam needs to be stably circulated, the main magnetic field is uniform along the beam trajectory by the magnetic pole part 123 in order to realize this stability, and the magnetic field decreases as the energy increases. It becomes distribution. Under such a magnetic field, the beam betatron oscillates stably with respect to the radial direction of the orbital plane 20 and the direction perpendicular to the orbital plane 20, respectively.

The trajectory 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 from that there are 51 circular orbits divided into 51 by magnetic rigidity (ratio of momentum to charge) up to 0 MeV. Is illustrated. The dotted line is a line connecting the same circulation phase of each orbit, and is called an equal circulation phase line. The accelerator 1 moves the center of the design trajectory of the beam corresponding to each energy in one direction in the orbital plane 20, particularly to the proximal end side of the emission path rather than the center of gravity of the orbital plane 20 as the beam accelerates.

As a result of the movement of each designed track center, there are areas where tracks of different kinetic energy are close to each other and areas that are separated from each other. When the closest points of the trajectories are connected, they become line segments orthogonal to the trajectories, and when the farthest points of the trajectories are connected, they become line segments orthogonal to the trajectories, and these two line segments exist on the same straight line. If this straight line is defined as an axis of symmetry, the shape of the orbit and the distribution of the main magnetic field pass through the axis of symmetry and become plane symmetric with respect to a plane perpendicular to the orbital plane. The equi-rotational phase lines are plotted for every circulation phase π / 20 from the aggregation region. The acceleration gap 223 formed between the dee electrode 221 and the opposing ground electrode 222 is installed along an equi-rotation phase line that rotates by ± 90 degrees based on the line segment connecting the closest points of the orbits. .

In order to generate the above orbit configuration and stable betatron oscillation around the orbit, in the accelerator 1 of this embodiment, the main magnetic field distribution in which the value of the magnetic field decreases as going outward in the deflection radius direction of the designed orbit It is formed. Also, the magnetic field is constant along the design trajectory. Thus, the design trajectory is circular, and as the energy of the beam increases, the radius and orbit time of the trajectory increase. In such a system, particles slightly deviated from the design trajectory in the radial direction receive restoring forces that return them to the design trajectory, and at the same time, particles shifted in the direction perpendicular to the track surface also return the main magnetic field to the track surface. Receive resilience from

The design trajectory refers to the trajectory drawn by the beam according to the energy, that is, corresponds to any energy of a collection of trajectories (points in the case of 0 MeV) 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 does not substantially change from the original trajectory corresponding to a certain energy, but deviates from the trajectory. Further, the above-mentioned "restoring force" means the force that the beam deviated from the original trajectory in this way receives in the direction to return to the design trajectory according to the energy.

As described above, if the magnetic field is reduced appropriately as the energy of the beam increases, particles that deviate from the design trajectory always work in the direction to return to the design trajectory and vibrate near the design trajectory. It will be done. This makes it possible to stably orbit and accelerate the beam. Vibration centered on this design trajectory is called betatron vibration. The value of the magnetic field in each energy beam is shown in FIG. The magnetic field reaches a maximum of 5 T at the incident point and drops to 4.91 T at the outermost periphery.

In order to accelerate 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 dee electrode 221 and the ground electrode 222. The frequency of the electric field is modulated correspondingly to the energy of the circulating beam in order to excite the high frequency electric field in synchronization with the circulation of the beam.

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

The behavior of the beam from beam incidence to 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 the high frequency electric field, and its energy increases and the turning radius of the orbit is increased. After that, the beam is accelerated while securing the traveling direction stability by the high frequency electric field. That is, instead of passing through the acceleration gap 223 at the time when the high frequency electric field is maximum, the acceleration gap 223 is allowed to pass when the high frequency electric field decreases temporally.

Then, since the frequency of the high frequency electric field and the circulating frequency of the beam are synchronized at an integral multiple ratio, the particles accelerated in the phase of the predetermined acceleration electric field are accelerated in the same phase in the next turn. On the other hand, since particles accelerated in a phase earlier than the acceleration phase have a larger amount of acceleration than particles accelerated in the acceleration phase, they are accelerated in a delayed phase in the next turn. On the other hand, particles that are accelerated at a phase later than the acceleration phase at that time are accelerated at the advanced phase in the next turn because the amount of acceleration is smaller than particles accelerated at the acceleration phase. As described above, the particles having a timing shifted from the predetermined acceleration phase move in the direction returning to the acceleration phase, and by this action, they can be stably oscillated also in the phase plane (traveling direction) consisting of momentum and phase.

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

The beam of the target energy circulates along the design trajectory when the current is not supplied to the kicker magnetic field generating coil 311, but the target energy is reached when the current is supplied to the kicker magnetic field generating coil 311 The beam is deviated from the orbit by the kicking magnetic field caused by the kicker magnetic field generating coil 311. That is, betatron oscillation 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 convergence point are in an appropriate positional relationship, it is possible to displace the beam radially outward at the convergence point by the kick by the kicker magnetic field generation coil 311.

In a series of operations in which the beam accelerates in the main magnetic field, as described above, the betatron vibration, which is the vibration in the plane perpendicular to the orbit, the synchrotron vibration, which is the vibration in the phase of momentum shift and acceleration high frequency. Do. These two types of vibration allow the beam to stably accelerate to a predetermined energy without losing the beam. However, the betatron oscillation generated by the above principle needs to secure a force in the direction back to the orbit for the particles which are out of orbit, that is, a convergence force.

In a system in which the convergence force increases in proportion to the orbital displacement, betatron oscillation can be stably performed even at large displacements, but the magnetic field distribution of general accelerators including this accelerator has displacements of six or more poles. There is a component that changes non-linearly with The non-linear distribution reduces the convergence power and creates an upper limit on the amplitude of betatron oscillations. Since particles displaced above the upper limit are lost, it is necessary to reduce the non-linear distribution of the magnetic field and to reduce the disturbance given to the circulating beam in order to secure the beam volume. As a countermeasure against this disturbance, the main magnetic field is formed to have the above-mentioned distribution.

Moreover, one of the disturbances given to the beam is derived from acceleration by a high frequency electric field. While the beam does not change its spatial position at the time of acceleration, acceleration increases kinetic energy. Then, ions in the design trajectory before acceleration are also out of the design trajectory due to changes in kinetic energy, and are subjected to a disturbance that moves inward as displacement seen from the design trajectory. In other words, due to the transition from the design trajectory before acceleration to the design trajectory after acceleration, the betatron oscillation that was stable with respect to the design trajectory before acceleration is based on the design trajectory after acceleration. It will be generated inside the orbit. We define this as a disturbance associated with acceleration. The dee electrode 221 of the accelerator 1 of the present embodiment is provided with a device for preventing the amplitude of the betatron oscillation generated by the disturbance accompanying the acceleration from increasing.

The beam is accelerated in the gap between the dee electrode 221 and the ground electrode 222, but is subjected to two accelerations when the beam enters and leaves the area covered by the dee electrode 221 per round. The displacement of the beam caused by the two accelerations can be made smaller by canceling out each other by optimizing the shape of the dee electrode 221. The principle will be described with reference to FIG. We introduce the concept of normalized phase space to describe betatron oscillations.

The phase space is a plane defined by the displacement seen from the design trajectory and the inclination to the design trajectory as an axis, and the particles having stable betatron oscillation orbit in the phase space. The phase space is further normalized by a normalization matrix represented by a Twiss parameter for each point of the orbit, which is called a normalized phase space, and in the non-accelerated situation, the betatron oscillation of the particle is a normalized phase space. It can be regarded as a clockwise circular motion above (A). As shown in FIG. 6, when the beam travels around the orbital plane 20, it circularly moves in the normalized phase 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, in real space, x = 0 corresponds to the position of the design trajectory.

Now, for particles moving circularly in the normalized phase space, the acceleration disturbance can be viewed as a spatial movement in the normalized phase space. From the synchronization condition with the high frequency, the two acceleration gaps need to be separated by an angle π on the orbit. Then, the motion from one acceleration gap to the other is a circular motion at an angle ππ on the normalized phase space.

Here, considering the moment when the particle passes through a certain acceleration gap, the particle is displaced inside (x <0) on the normalized phase space by the disturbance accompanying the acceleration received in this pass (B). Subsequently, since ν is approximately 1, this particle has finished circular motion at an angle π 上 on the normalized moving space by the time the other acceleration gap is reached, and the displacement is almost outward (x> 0). Then, it becomes almost parallel (x '≒ 0) to the design trajectory (C). Then, when passing through the other acceleration gap, it is subjected to the disturbance accompanying the acceleration again. Then, in the case of the acceleration gap arrangement in which the left and right lines are symmetrical with respect to the symmetry axis as in the arrangement of this embodiment, the former disturbance and the latter disturbance have substantially the same size (D). As a result, the trajectory on the series of normalized phase spaces of B · C · D can be substantially closed, that is, the disturbance accompanying acceleration during one round can be reduced, and an increase in the amplitude of the betatron oscillation can be suppressed.

The displacement amount Δx on the normalized phase space due to the disturbance due to the acceleration described above is expressed by Equation 1 using the dispersion function η in the orbit, the momentum Δp obtained by acceleration, the momentum p of the particle, and the Twiss parameter β.

Since η and β in the trajectory of a certain momentum p at two points symmetrical to each other on the orbital symmetry axis are equal to each other and Δp determined by the acceleration gap shape is also equal, the disturbance accompanying acceleration can be reduced as described above. Furthermore, it is also conceivable that a difference may occur between Δp / p due to acceleration when entering the Dee electrode and Δp / p when exiting when the momentum p is small.

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 taken large on the side entering the dee electrode 221, and the transit time factor is lowered. Can be made smaller. In this case, by adopting an asymmetrical shape, it is possible to further reduce the disturbance associated with acceleration, and as a result, it is possible to increase the amount of beams that can be accelerated, as compared with the case of using a symmetrical Dee electrode.

Also, in order to adjust the magnitude of the disturbance associated with acceleration upon entering the electrode and the disturbance associated with acceleration upon exiting the electrode, the position of the acceleration gap may be slightly offset to shift the high frequency synchronization phase, or Alternatively, the shape of the ground electrode may be appropriately changed to change the electric field distribution between the left and right. In any case, although the beam is disturbed as it travels around in the accelerator, suppression of the increase in the betatron oscillation amplitude is achieved by making the trajectory on the normalized phase space resulting from the disturbance a closed trajectory. As a result, it becomes possible to realize a small accelerator with a large amount of removable beam.

In contrast to the accelerator 1 of the present embodiment described above, the conventional cyclotron requires a degrader (attenuation member) for energy change, and a large proportion of the ion beam is lost when passing it. The However, the accelerator 1 of the present embodiment omits the installation of the degrader, or even if it is installed for the fine adjustment of the energy, since it is not necessary to cope with the wide energy region as in the prior art, the shielding performance of the ion beam It is possible to install a lower degrader and to improve the utilization efficiency of the ion beam. Further, by omitting the installation of the degrader or the like, 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 the arrangement of the accelerator is improved, which also contributes to the efficiency of the space required for the installation.

Also, conventional variable energy accelerators and cyclotrons keep the orbiting time constant regardless of energy by making the mean magnetic field in orbit proportional to the relativistic γ factor of the beam (magnetic field distribution with this property is isochronous) Called the sex magnetic field). Under isochronous magnetic field, the beam stability in the orbital plane and in the direction perpendicular to the orbital plane is ensured by modulating the magnetic field along the orbit, but in order to achieve both isochronism and beam stability Requires the maximum (Hill) and minimum (Valley) of the magnetic field. A nonuniform magnetic field with this distribution can be formed by narrowing the distance (gap) between the magnetic poles 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 extent of the saturation magnetic flux density of the magnetic pole material which is a ferromagnetic substance. That is, the difference between the Hill magnetic field and the Valley magnetic field is limited to about 2 Tesla.

On the other hand, in the case of miniaturizing the conventional accelerator described above, it is necessary to increase the main magnetic field to reduce the deflection radius of the beam trajectory, but the difference between the main magnetic field and the aforementioned Hill magnetic field and Valley magnetic field is proportional. Although the above-mentioned limit due to the saturation magnetic flux density will be a factor to hinder the miniaturization, in the case of the accelerator of this embodiment, the structure is adopted in which such limitation is relaxed, and therefore, the miniaturization can be achieved. It becomes possible.

In the second embodiment, the accelerated nuclide is carbon ion in the first embodiment. This accelerator is a frequency modulation type variable energy accelerator that can extract carbon ions in the range of 140 MeV to 430 MeV kinetic energy per nucleon. The operation principle, device configuration, and operation procedure are the same as those of the first embodiment and thus will be omitted. What is different is the relationship between the size of orbital radius and the relationship between magnetic field and energy, and the relationship between orbital frequency and energy, but from the accelerator shown in Example 1, the product of orbital radius and magnetic field is made proportional to the ratio of magnetic rigidity of beam. It can be realized by Therefore, although the beam is subjected to disturbance as it travels around the accelerator in the same manner as in Example 1, the betatron vibration amplitude is obtained by setting the locus on the normalized phase space resulting from the disturbance as a closed locus. The suppression of the increase of is achieved, which in turn makes it possible to realize a small accelerator with a large amount of removable beam.

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

The particle beam treatment 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 a plurality of deflection electromagnets 401 are provided to be guided to an arbitrary position. Also, a plurality of quadrupole magnets 402 are installed in a path for transporting the ion beam (transport path), and the state of the ion beam is adjusted by the convergence or diverging action of the quadrupole electromagnets 402. The outputs of the deflection electromagnet 401 and the quadrupole electromagnet 402 are configured to be adjusted by the energy of the passing ion beam, and the adjustment is controlled by the controller 408. In addition, some of the deflection electromagnets 401 are configured to be rotatable about the rotation axis 407, and by this rotation, the irradiation field forming device described later can be carried to any rotation angle around the patient 409. it can. Each of the deflection electromagnets 401 may be fixed to the floor or wall of the facility via a fixing tool, and the irradiation field forming apparatus may be held at 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 is irradiated to the affected area. There are various types of irradiation field forming apparatuses, but FIG. 7 gives an example in which two scanning electromagnets 403 are used. The two scanning electromagnets 403 are such that the generated magnetic fields are orthogonal to each other, and the ion beam can be deflected to a desired position by controlling the outputs of the two orthogonal magnetic fields.

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

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 downstream of the scanning electromagnet 403. In addition, it may be an irradiation field forming apparatus in which only the scatterer is installed without the scanning electromagnet 403 being installed. Further, the installation positions of the position monitor 404 and the dose monitor 405 are not limited to the positions shown in FIG. 7 and may be installed in the middle of the transport path of the beam.

On the downstream side of the scanning electromagnet 403, a support 406 for holding the patient 409 at a position suitable for treatment is provided. Although not shown in FIG. 7, the support base 406 is configured to be movable under the control of the control device 408. Also, the support stand 406 may be any type such as a bed type in which the patient 409 can take a supine or prone posture, or a type having a chair-like shape suitable for the patient 409 to take a sitting posture. .

The control device 408 controls the operations of the deflection electromagnet 401, the quadrupole electromagnet 402, the scanning electromagnet 403, the position monitor 404, the dose monitor 405, and the support stand 406 which have been described up to now. The control device 408 has an input interface (not shown), and can receive operation instructions from the medical staff or his / her assistant. 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 these may be combined to form the control device 408.

When irradiating the affected area to be irradiated, the particle beam treatment system 410 having the above-described devices constitutes proton beam or carbon beam (irradiated) according to the position (depth) of the affected area with reference to the body surface ( In the following, the energy of the particle beam (collectively called particle beam) is set to an appropriate value and the patient is irradiated. Although the energy of the particle beam to be irradiated is determined by the treatment plan, the information of the energy and the dose of the particle beam defined by the treatment plan is input to the control device 408. The control device 408 having received this controls the accelerator 1 so that particle beams of the instructed energy and irradiation amount are emitted. More specifically, the ion supply operation is requested to the ion source 12, and for the high frequency power supply 210 of the accelerator 1, from the initial frequency (E = 0) shown in the graph of FIG. It requires an output of decreasing frequency towards energy).

When an ion beam reaching the target energy is generated by these controls, the kicker magnetic field generating coil 311 is operated to kick the ion beam from the designed trajectory, and the septum electromagnet 312 is operated to eject the ion beam from the accelerator 1 Let 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 to a planned position. The dose of the ion beam is measured by the dose monitor 405, and when the target dose is reached, the controller 408 stops the operation of the kicker magnetic field generating coil 311 of the accelerator 1 and the ion source 12. Thereafter, the above control is repeated so as to correspond to the next energy and irradiation dose defined in the treatment plan.

According to the particle beam treatment system of this embodiment, since the ion beam of a wide energy necessary for treatment is emitted from the accelerator 1, the utilization efficiency of the ion beam as compared with the particle beam treatment system adopting the conventional cyclotron Can be dramatically improved. Moreover, since the amount of beams that can be taken out is large, the irradiation time can be shortened, and the number of patients per unit of time that can be irradiated in the facility can also be increased. Further, as mentioned above, since the restriction of the magnetic field is small, it is easy to miniaturize, which contributes to the miniaturization of the whole particle beam therapy system.

The circular accelerator according to the present invention and the particle beam therapy system using the same have been described above by way of examples. The present invention is not limited to the embodiments described above, but includes various modifications. For example, the embodiments described above are described in detail in order to explain the present invention in an easy-to-understand manner, and are not necessarily limited to those having all the configurations described. Also, part of the configuration of one embodiment can be replaced with the configuration of another embodiment, and the configuration of another embodiment can be added to the configuration of one embodiment. In addition, with respect to a part of the configuration of each embodiment, it is possible to add, delete, and replace other configurations.

For example, although the electromagnet in the above-mentioned embodiment assumed what adopted the general coil of normal conduction, you may adopt the superconductivity magnet which used the superconductivity coil. In addition, as a frequency modulator, a rotary variable capacitance capacitor has been exemplified, but this may be changed to a variable capacitance diode. Also, ions to be accelerated are not limited to protons, and helium or carbon may be adopted, and the magnetic field distribution and the frequency of the high frequency applied to the high frequency cavity may be adjusted according to the ion species to be accelerated and the required energy. .

DESCRIPTION OF SYMBOLS 1 accelerator 11 electromagnet 111 extraction beam through hole 112 to 113 coil connection through hole 114 high frequency power input through hole 115 beam incident through hole 12 ion source 13 coil 121 return yoke portion 122 top plate portion 123 magnetic pole portion 124 magnetic pole surface 130 entrance portion 20 orbital plane (orbital plane)
21 high frequency cavity 221 dee electrode (first electrode)
222 Grounding electrode (second electrode)
223 Acceleration gap 210 High frequency power supply 211 Input coupler 212 Rotary variable capacitance capacitor (frequency modulator)
213 Rotary shaft 214 Servomotor 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 stand 407 Rotary shaft 408 Control device 409 Patient 410 Particle therapy system

Claims

In a circular accelerator that accelerates ions to produce an ion beam,
An electromagnet including a track surface in which the ions are accelerated,
A high frequency cavity forming an electric field for accelerating the ions,
An emission path for extracting the ions after acceleration;
The electromagnet is a magnetic field in which the magnetic flux density is maximized between the center of gravity of the orbital plane and the proximal end of the emission path, and the magnetic flux density gradually decreases toward the outer edge of the orbital plane from the point where the magnetic flux density is maximal. Forming a distribution on the orbital plane,
The above-mentioned high frequency cavity is provided with a frequency modulator which modulates the frequency of the 1st electrode for forming the above-mentioned electric field, and the electric power applied to the 1st electrode. A circular accelerator.
A circular accelerator according to claim 1, wherein
The electromagnet is
It has a magnetic pole part which protrudes to the above-mentioned track surface,
The magnetic pole portion is
A plurality of coils with different diameters are provided on the surface of the magnetic pole portion facing the orbital plane, so that a coil with a smaller diameter is placed on the inner diameter side of the large diameter coil, or opposite to the orbital plane of the magnetic pole portion A circular accelerator characterized in that a surface has a convex shape projecting toward the orbital plane.
A circular accelerator according to claim 2, wherein
The first electrode is formed in a substantially fan shape whose central angle is an acute angle and extends to the opposite side of the proximal end of the emission path when the first electrode is projected onto the orbital plane,
A connecting portion with the frequency modulator is formed in a portion corresponding to the fan-shaped arc.
A circular accelerator according to claim 3, wherein
And a second electrode provided so as to face a gap corresponding to each side corresponding to a radius in the substantially fan-shaped first electrode,
The circular accelerator according to claim 1, wherein the second electrode is maintained at substantially the same potential during the acceleration period of the ions.
A circular accelerator according to claim 4, wherein
The first electrode and the second electrode are disposed such that the arrangement of the gap is substantially line symmetrical with respect to a straight line connecting the proximal end of the emission path and the center of gravity of the orbital plane. Circular accelerator.
A circular accelerator according to claim 4, wherein
The first electrode and the second electrode are disposed such that the size of the gap is asymmetric with respect to a straight line connecting the proximal end of the emission path and the center of gravity of the orbital plane. Circular accelerator.
A circular accelerator according to claim 4, wherein
The first electrode and the second electrode are connected to a straight line connecting the proximal end of the emission path and the center of gravity of the orbital plane, with respect to the disturbance received when ions accelerated in the orbital plane pass through the gap. A circular accelerator characterized in that it is installed to have symmetry.
A circular accelerator according to claim 1, wherein
The circular accelerator, wherein the frequency modulator comprises a rotary variable capacitance capacitor.
A circular accelerator according to claim 1,
A transport path for transporting the ion beam generated by the circular accelerator,
An irradiation field forming device for deflecting the ion beam transported through the transportation path to form a desired irradiation field;
Particle therapy apparatus comprising: