WO2018051425A1 - Beam extraction method and circular accelerator using same - Google Patents

Abstract

Provided is an accelerator capable of continuous and energy-variable beam delivery with highly efficient beam usage. In prior art cyclotrons, it had not been possible to change the energy of an extracted beam, and continuous delivery of a beam had been difficult with synchrotrons. The invention applies a beam extraction method of accelerating ions circling in an isochronous magnetic field by a high-frequency electric field and extracting the result as a beam to the exterior, wherein the accelerator is such that the center positions of each of the orbits defined for each energy of the ions are disposed in a straight line in the orbit plane, and in the process in which the beam is kicked by a beam separation device to be placed on an orbit laying on an extraction route, the beam is caused to circle a plurality of times in the accelerator between the kicking and the placement on the extraction route.

Description

Beam extraction method and circular accelerator using the same

The present invention relates to an accelerator for accelerating heavy ions such as protons or carbon ions, and more particularly to a method for extracting a beam from the accelerator.

Accelerators are used to generate high-energy charged particles, especially nuclear beams, in particle beam therapy and physical experiments. Examples of the accelerator that obtains a beam having a kinetic energy per nucleon of around 200 MeV include the cyclotron described in Patent Document 1 and Patent Document 2, and the synchrotron described in Patent Document 3. A feature of the cyclotron is that a beam circulating in a static magnetic field is accelerated by a high-frequency electric field. As the beam is accelerated, the beam moves to an outer orbit and is extracted after reaching the maximum energy. Therefore, basically, the energy of the extracted beam is fixed. The synchrotron changes the frequency of the magnetic field of the electromagnet that deflects the beam and the frequency of the high-frequency electric field that accelerates the beam, so that the beam goes around a fixed orbit. Therefore, it is possible to extract the beam before reaching the design maximum energy, and the extraction energy can be controlled. Patent Document 4 also describes an accelerator capable of controlling the extraction energy.

JP 2014-160613 A JP 2014-020800 A JP 2014-186939 A JP2016-507949A

A particle beam irradiation apparatus using a synchrotron can generate a plurality of ion beams having different energies in the synchrotron, and can change the energy of the ion beam emitted from the synchrotron. However, since a plurality of deflecting electromagnets and a plurality of quadrupole electromagnets are required, the synchrotron cannot be downsized to some extent. Further, in the synchrotron, the extraction of the ion beam becomes like a pulse, and the extraction amount of the ion beam is reduced.

On the other hand, a particle beam irradiation apparatus using a cyclotron that can be made smaller than a synchrotron can continuously extract an ion beam and has a large amount of ion beam extraction. However, the energy of the ion beam generated in the cyclotron is constant, and it is impossible to extract an ion beam having a low maximum energy energy. For this reason, when a low-energy ion beam is required, such as when irradiating an ion beam to a certain layer of the affected area, a degrader provided in the beam transport system is used so that the ion beam reaches the layer. It is necessary to adjust the energy of the ion beam. When the degrader is used for adjusting the energy of the ion beam, problems such as an increase in the beam size of the ion beam due to the degrader and a decrease in the number of ions transmitted through the metal plate of the degrader occur.

For this reason, there is a demand for a proton therapy apparatus that can continuously extract ion beams having different energies and improve the extraction efficiency of the ion beams.

In the accelerator described in Patent Document 4, the beam detaching device detaches ion beams having different energies from the beam orbit at a plurality of positions in the radial direction of the annular coil, but the energy that circulates on the inner circumference side of the annular coil is low. In order to take out the ion beam and put it on the orbit, a large electric field or magnetic field is required locally in the beam detachment apparatus.

A feature of the present invention that achieves the above-described object is a beam extraction method in which ions are accelerated by a high-frequency electric field of a circular accelerator and extracted as a beam to the outside. The ions are determined for each ion energy in the circular accelerator. In a process in which the center positions of a plurality of orbits are circulated so that they are arranged at different positions in a certain direction in the orbital plane, the beam being circulated is kicked by a beam detaching device and placed on the orbit on the take-out path. The beam is circulated a plurality of times until it takes the extraction path.

According to the present invention, each ion beam having different energy can be efficiently emitted from the accelerator.

It is the whole accelerator shape of a present Example. It is an internal structure of the accelerator of a present Example. It is magnetic field distribution which the massless septum of the accelerator of a present Example excites. It is magnetic field distribution along the beam orbit of the accelerator of a present Example. It is a design orbit of the accelerator of the present embodiment. It is magnetic field distribution in the track surface of the accelerator of a present Example. It is a figure which shows the dependence of the beam energy of the accelerator of the present Example tune. It is a figure which shows the magnetic field intensity which the massless septum of the accelerator of a present Example should excite.

The inventors have made various studies in order to realize an accelerator that can continuously extract an ion beam like a cyclotron and can extract an ion beam with different energy like a synchrotron.

The inventors first focused on widening the mutual spacing of the beam orbits of the ion beam that circulates in the vacuum vessel of the cyclotron (the spacing between the beam orbits in the radial direction of the vacuum vessel). Increasing the interval between the beam orbits, that is, increasing the interval between the beam orbits (turn separation) increases the diameter of the vacuum vessel and enlarges the cyclotron. This goes against the downsizing of the accelerator. In addition, in conventional cyclotrons, concentric beam orbits are drawn in a vacuum vessel, and it is difficult to ensure high-energy turn separation, so it is difficult to efficiently emit ion beams with different energies. there were.

In the cyclotron, there is an approximately circular beam passage region, and an ion source is installed so that ions are incident near the center of this region. The inventors move the incident point existing at the center of the beam passing region in the cyclotron to the beam extraction port side formed in the beam passing region, so that the ions from the ion source are not the center of the beam passing region but the beam. It was considered to enter the vacuum vessel at a position shifted to the take-out port side. As a result, the distance between the beam orbits formed in the vacuum vessel is close between the ion incident position where the ions are incident from the ion source and the beam extraction port, which is 180 ° opposite to the beam extraction port in the vacuum vessel. Between the position and the ion incident position, the distance between the beam orbits formed in the vacuum vessel can be widened, contrary to the position between the ion incident position and the beam extraction port.

The inventors have created a new accelerator capable of efficiently emitting each ion beam having different energy by applying such a concept of the beam orbit.

Each embodiment of the present invention to which an accelerator newly created by the inventors described above is applied will be described below with reference to the drawings.

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 this embodiment is a variable energy continuous wave accelerator capable of continuously outputting a beam with variable energy. The accelerator 1 is a circular accelerator that accelerates a charged particle beam that circulates in a constant magnetic field with a constant frequency (isochronous) by a high-frequency electric field. The appearance is shown in FIG. The accelerator 1 forms an outer shell by a magnet 11 that can be divided into upper and lower portions, and the inside is evacuated. The magnet 11 has a plurality of through-holes. Among them, the extraction beam through-hole 111 for extracting the accelerated beam, the extraction openings 112 and 113 for extracting the internal coil to the outside, and the high-frequency power input through-hole 114 are the upper and lower magnetic poles. It is provided on the connection surface. An ion source 12 is installed above the magnet 11, and a beam enters the accelerator 1 through the beam entrance through hole 115.

Next, the internal structure of the accelerator will be described with reference to FIG. Inside the magnet 11 is a cylindrical space 20 formed by a cylindrical inner wall, and an annular coil 13 is installed along the inner wall. By passing a current through the coil 13, the magnet 11 is magnetized, and a magnetic field is excited in the magnet 11 with a predetermined distribution. Magnetic poles 121 to 124 are installed inside the coil 13, and a cylindrical return yoke 14 is provided outside the coil 13. The beam circulates in the internal space 20 and accelerates. The energy of the extraction beam is a minimum of 70 MeV to a maximum of 235 MeV, and the circular frequency of the beam is 19.82 MHz. The magnetic poles 121 to 124 form four sets of irregularities along the beam trajectory, and the magnetic field acting on the beam is a low magnetic field in the concave portion and a high magnetic field in the convex portion. In this way, the strength of the magnetic field along the beam trajectory is added, and the average value of the magnetic field along the trajectory is made proportional to the relativistic γ factor of the beam, while the orbiting time of the orbiting beam is made constant regardless of energy. The betatron oscillation stably occurs in the beam orbital plane and in the direction perpendicular to the orbital plane. The magnetic pole recess is provided with high-frequency cavities 31 and 32 for exciting a high-frequency electric field, an extraction septum electromagnet 40, and a coil 50 for generating a kicker magnetic field. The kicker magnetic field adopts a massless septum method that applies a magnetic field only to a specific position as will be described later, and a current is passed through a coil installed symmetrically in a direction perpendicular to the trajectory plane with respect to the beam. Excites the magnetic field. The beam is incident on the accelerator 1 in the form of low energy ions from the incident point 120. The incident beam is accelerated every time it passes through the electric field gap by the high frequency electric field excited by the high frequency cavity. The accelerator 1 determines the beam trajectory so that the beam trajectory center moves in one direction on the same plane according to the acceleration of the beam. Accordingly, the magnetic pole shape and the coil arrangement are mirror-symmetric with respect to the center plane so that the in-plane component of the magnetic field at the center plane is zero. Further, as a result of the magnetic field distribution being symmetrical with respect to the axis AA 'in the center plane, the magnetic poles 121 and 124 and 122 and 123 have symmetrical shapes. Each magnetic pole is provided with a trim coil for fine adjustment of the magnetic field, and the trim coil current is adjusted before operation so as to ensure isochronism and stability of betatron oscillation.

The accelerator 1 has a control device 117, and the control device 117 accelerates the ion source 12, the coil 13, the high-frequency cavities 31 and 32, the extraction septum electromagnet 40, the kicker magnetic field generation coil 50, etc. , Control to exit from the accelerator.

Next, the trajectory configuration, which is a feature of this accelerator, will be described. The trajectory of each energy is shown in FIG. As for the orbit, the trajectory of 50 energy types is shown by a solid line from the maximum energy of 235 MeV every magnetic rigidity of 0.04 Tm. The dotted line is a line connecting the same circular phase of each orbit and is called an isochronous line. Isochronous lines are plotted for each orbital phase π / 18 from the aggregation region. The acceleration gap is set along the isochronous line. In the low energy region below 50 MeV, the trajectory is centered around the incident point of the ion like the crafted tron. In the recess in which the massless septum 50 is installed, the tracks are in a positional relationship apart from each other. The points where the orbits are gathered densely will be referred to as an aggregation area, and the discrete areas will be referred to as an orbital discrete area.

The center position or the center of gravity position of the substantially circular orbit determined for each ion energy is in a different position along a straight line toward one direction in the orbital plane every time the energy changes. In other words, the substantially circular orbit for each energy is formed so as to be eccentric in a certain direction as the energy increases.

In the orbital discrete region, the beam spreads over a wide region in the center plane according to the energy. By appropriately determining the excitation position of the magnetic field by the massless septum, the beam of energy corresponding to the excitation position receives a kick. . The beam shifted from the predetermined equilibrium trajectory by this kick is incident on a septum electromagnet installed at an aggregation point downstream of the half circumference. The septum magnet provides the beam with the deflection necessary to place the extracted beam on a defined design trajectory on the extraction path 140. Specifically, a magnetic field in a direction that cancels the main electromagnet magnetic field is excited, and the beam is guided to the extraction path 140.

In order to generate the above-described orbit configuration and stable betatron oscillation around the orbit, the accelerator 1 of the present embodiment has a magnetic field distribution in which the minimum and maximum magnetic fields appear four times per revolution along the beam orbit. Adopted. FIG. 4 shows the magnetic field distribution along the trajectory. FIG. 4 shows the magnetic field distribution along the trajectories of energy 235 MeV, 200 MeV, 150 MeV, 70 MeV, and 7.5 MeV, the horizontal axis is 0 in the trajectory aggregation region, and the trajectory direction distance with the trajectory discrete region 1 in the half circumference downstream, The vertical axis is the magnetic field. In this way, by appropriately determining the amplitude while raising the average magnetic field with respect to energy, it is possible to realize a magnetic field distribution that is isochronous and oscillates in betatron even in an eccentric orbital arrangement like the present accelerator. FIG. 5 shows the magnetic field distribution on the center plane as an isomagnetic field diagram. In FIG. 5, the maximum magnetic field of 2.2T and the minimum magnetic field of 0.86T are represented by isomagnetic lines in 32 steps. The circles shown by broken lines in FIGS. 3, 2, and 5 are circles having a radius of 1494 mm, and the trajectory of all energy is included in the circle.

Under the above conditions, the evaluation result of the betatron frequency (tune) around the orbit is shown in FIG. The tune was calculated based on the magnetic field gradient obtained from the magnetic field of the orbit and the energy of the front and back. At low energy, the tune in the orbital plane is almost 1, which increases with acceleration. The tune in the direction perpendicular to the orbital plane is almost zero at low energy, and exists in the range of 0 to less than 0.5 in the entire energy region.

As shown in FIG. 2, the massless septum 50 has a plurality of current paths formed of copper wires arranged in a direction substantially parallel to the beam trajectory. By supplying a current to a specific current path among the plurality of current paths, the magnetic field on the right side (outer in the radial direction) as shown in FIG. 7 can be increased or decreased. As a result, the magnetic field application region can be localized, a magnetic field can be applied to a beam having a specific energy, the beam can be shifted from a predetermined design trajectory, and the beam can be introduced into the extraction septum electromagnet.

Now, in the accelerator 1 of the present embodiment, the number of laps in which the beam circulates in the accelerator from when the beam is kicked by the massless septum 50 to the extraction septum electromagnet 40 differs depending on each energy. For example, when a 235 MeV beam is taken out, the beam goes around the accelerator half way from kicking the beam with a massless septum to reaching the septum electromagnet. On the other hand, the 70 MeV beam enters the septum electromagnet 40 after the beam has made the accelerator 1 10.5 rounds. Thus, by changing the number of turns required for beam extraction for each energy, the necessary kick amount in the massless septum, that is, the magnetic field generated by the massless septum can be lowered. Further, if the number of laps is increased as the energy is lower, the local magnetic field generation performance required for the massless septum as a whole can be kept low, and there are also advantages in downsizing and ease of manufacture.

Explain the principle that can reduce the magnetic field. Since the in-plane tune at low energy is close to 1, when a half-turn is made in the accelerator after a kick is received at a certain location, the phase of the betatron oscillation also makes a half turn. Therefore, there is almost no displacement of the position half a round downstream from the kick position. However, since the tune is slightly deviated from 1 (for example, 0.02), as the number of laps is increased, the position shift caused by the kick can be increased even at the position downstream of the half circle. In the accelerator according to the present embodiment, the orbital displacement necessary to enter the entrance of the extraction septum electromagnet 40 is caused by the kick of the massless septum 50. FIG. 8 shows the evaluation results of the magnetic field to be excited by the massless septum 50 in the case where the massless septum 50 circulates a plurality of times from the kick to the removal by the massless septum 50 according to the above principle. FIG. 8 plots the required amount of massless septum magnetic field when the massless septum 50 is taken out from the kick a plurality of times. For comparison, a dotted line indicates a necessary excitation amount in the case of taking out from the massless septum 50 to the taking-out septum electromagnet in a half circle. From this result, when the massless septum 50 used in the present embodiment is taken out from the kick a plurality of times, the required amount of excitation can be reduced to 0.05 T or less. On the other hand, in the case of taking out from the massless septum kick to the taking-out septum electromagnet in a half turn, it takes about 0.15 T at maximum, and it can be seen that it is more advantageous to take out after a plurality of turns in terms of power consumption.

As described above, according to the accelerator 1 having the configuration of the present embodiment, the two iron cores that are installed to face each other and form a magnetic field therebetween, the electrode that accelerates the ion beam, and the beam emission that extracts the ion beam to the outside And a beam detachment device for detaching the ion beam from the beam orbit at a plurality of positions in the radial direction of the annular coil. Further, a plurality of annular beam circular orbits around which ion beams of different energies each circulate formed by a plurality of magnetic poles formed on each of the two iron cores are focused at the entrance of the beam extraction path. Since a plurality of beam orbits are focused at the entrance of the beam exit path, each ion beam having a different energy separated from each beam orbit is easily incident on the entrance of the beam exit path. An ion beam can be emitted efficiently.

The beam detachment device can detach ion beams having different energies from the beam orbit at a plurality of positions in the radial direction of the annular coil, and can efficiently emit each ion beam having different energies. This beam detachment device gives a kick to the beam by a magnetic field. At this time, the kick with respect to the beam by the beam detaching device is applied over a plurality of turns up to the beam emission path, so that the beam can be efficiently extracted.

With the above configuration, it is possible to provide an accelerator capable of extracting a beam with a wide range of energy with high efficiency. In this embodiment, the massless septum is used as the orbit separation means for taking out. However, if a kick amount sufficient for the orbit separation is obtained, the force source may be changed to an electric field, and the kick direction is also vertical. You may change the direction. In the present embodiment, excitation according to a predetermined magnetic field distribution is realized by the magnetic pole shape and the trim coil current, but magnetic field excitation may be realized by either one.

DESCRIPTION OF SYMBOLS 1 Accelerator 11 Magnet 12 Ion source 13 Coil 14 Return yoke 20 Internal space 31-32 High frequency cavity 40 Extraction septum electromagnet 50 Massless septum coil 111 Extraction beam through-hole 112-113 Coil connection through-hole 114 For high-frequency input Through-hole 115 Through-holes 121 to 124 for beam incidence Magnetic pole protrusion 130 Incident point 140 Beam extraction path

Claims (6)

1. A beam extraction method in which ions are accelerated by a high-frequency electric field of a circular accelerator and extracted outside as a beam,
   In the circular accelerator, the ions circulate so that the center positions of a plurality of orbits determined for each ion energy are arranged at different positions in a certain direction in the orbital plane,
   A beam extraction method in which the beam being circulated is kicked by a beam detaching device and placed on a trajectory on the extraction path, and the beam is circulated a plurality of times from the kick to the extraction path.
2. 2. The beam extraction method according to claim 1, wherein the number of beam circumferences from the kick to the extraction path varies depending on the energy of the extracted beam.
3. 2. The beam extraction method according to claim 1, wherein the beam detachment device detaches the beam from the orbit by applying a magnetic field, and at least one of the strength and direction of the applied magnetic field varies depending on the energy of the extracted beam. A beam extraction method characterized by the above.
4. An accelerator characterized by extracting at least two beams having different energies using the extracting method according to claim 1.
5. Two iron cores installed opposite each other and forming a magnetic field between them,
   An electrode for accelerating the ion beam;
   A beam extraction path for extracting the ion beam to the outside;
   A beam detachment device for detaching the ion beam from the beam orbit at a plurality of radial positions of the ion beam orbital surface;
   A control device for controlling the beam detachment device,
   The control device excites the ion beam at a predetermined position in a radial direction of the ion beam trajectory surface so that the ion beam circulates a plurality of times until the ion beam is separated from the circular trajectory and enters the beam extraction path. Accelerator.
6. The accelerator according to claim 5, wherein
   The control device may be configured to reduce the number of times the beam circulates after the beam is released after the orbit is released from the outer periphery than the number of times the beam circulates after the beam is released from the inner periphery. An accelerator characterized by controlling the device.

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