JP2010050019A - Circular accelerator - Google Patents

Abstract

When a charged particle beam is emitted from a circular accelerator to the outside, a circular accelerator that makes correction of beam emission easy by statically correcting a change in tune caused by a change in beam trajectory and changing the tune linearly. provide.
An end pack is provided at a magnetic pole edge portion 32 where a charged particle beam of a deflecting electromagnet 3 enters and exits, and a first protrusion is formed on an edge portion 32a radially outward from a balance trajectory 33a of a central energy beam of the end pack 34. The portion 34a is provided with a second protrusion 34b on the edge 32b radially inward from the equilibrium trajectory 33a of the central energy beam, and between the flat portion end faces 34c, 34d of the first and second protrusions 34a, 34b. By providing the connecting surface 34e connected by an arcuate curve or an elliptical arc curve, the betatron frequency of beams having different acceleration energies is constant or linear with respect to energy.
[Selection] Figure 3

Description

  The present invention relates to a circular accelerator that receives a low energy beam and emits an accelerated high energy beam.

Conventionally, circular accelerators such as synchrotrons orbitally accelerate a charged particle beam, transport the beam extracted from its equilibrium trajectory through a beam transport system, and irradiate a desired object with cancer treatment for particle beam medical use. It is used for diagnosis of the affected area.
In such a circular accelerator, resonance of the betatron oscillation of the beam has been used in order to continuously emit accelerated charged particles. The resonance of betatron oscillation is the following phenomenon. The charged particles circulate around the equilibrium orbit of the circular accelerator while vibrating left and right (horizontal direction) or up and down (vertical direction). This is called betatron vibration, and the amplitude of vibration is called betatron vibration amplitude. The frequency of betatron vibration per round orbit is generally called tune (betatron frequency). The tune can be controlled by a deflection electromagnet or a quadrupole electromagnet provided on the orbit. When the minority part of the tune is brought close to a / b (a and b are integers) and at the same time, a resonance-generating multipole magnet (for example, a hexapole electromagnet) is excited, a large number of charges are circulated. Among the particles, the amplitude of the betatron oscillation of a charged particle having a betatron oscillation amplitude of a certain level or more increases rapidly. This phenomenon is called resonance of betatron oscillation, and the boundary portion where the beam amplitude becomes unstable due to a sudden increase in vibration amplitude, that is, the boundary portion between the stable region and the unstable region is called a stability limit (separatory). Normally, this separatrix is generally considered on the phase plane. The phase plane is the amount of deviation from the beam's equilibrium trajectory on the horizontal axis, and the inclination from the beam's equilibrium trajectory on the vertical axis. The orbiting beam stably circulates in an ellipse that is a closed space on the phase plane, and when the vibration amplitude increases, that is, in the case of a beam orbiting a position far from the equilibrium orbit, the shape in the phase space Becomes a triangle from an ellipse. In either case, the beam circulates stably, but if the amplitude becomes larger and exceeds a certain boundary, the beam cannot circulate stably. At this time, on the phase plane, the beam that can no longer circulate stably goes out of the triangle. This triangular boundary becomes a boundary line that separates a stable region and an unstable region for a charged particle beam, and this boundary is called a separatrix. The magnitude of the betatron oscillation amplitude near the resonance stability limit depends on the deviation from the decimal part of the tune, and becomes smaller as the deviation is smaller. The beam outside the separatrix becomes unstable and is gradually extracted out of the circular accelerator. If acceleration is continued with the magnetic field strength of the deflecting electromagnet kept constant, the beam's equilibrium trajectory will shift radially outward, but this will change the tune, and if the separatrix does not change, The circulating beam moves out of the separatrix and is gradually extracted from the radially outer side beam from the separatrix to become an outgoing beam. By controlling the beam tune in this way, by taking a part of the beam out of the separatrix and emitting it out of the circular orbit, so-called resonant emission requires fine tuning of the tune, and adjustment of the emission parameters Takes a lot of time.

Conventionally, the following four methods are generally known as methods for performing such resonance emission.
[Method 1] Gradually reduce the size of the separatrix from the initial large state, and first generate resonance in charged particles having a large betatron oscillation amplitude among the circulating charged particles, and then charge particles having a small oscillation amplitude. Resonance is generated sequentially, and the charged particle beam is gradually emitted to the irradiation chamber.
[Method 2] The stability limit is made substantially constant by keeping the tune substantially constant, and the amplitude of the betatron oscillation of the beam is increased by high frequency to generate resonance.
[Method 3] The stability limit is made substantially constant by keeping the tune substantially constant, the amplitude of the betatron oscillation of the beam is increased by high frequency to increase the beam to the boundary of the stability limit, and then the quadrupole electromagnet is excited. The separatrix is slightly reduced and the charged particle beam is gradually extracted.
[Method 4] The stability limit is made substantially constant by keeping the tune substantially constant, the beam is gradually accelerated by the high frequency acceleration electric field, and the beam outside the separatrix is gradually taken out.

  In any of the above methods, the charged particles circulate around the central orbit while vibrating. In the conventional example, the hexapole electromagnet or the like is temporally controlled to correct the change in the tune due to various factors and emit the beam. As a specific example, Patent Document 1 discloses the following example. Yes. That is, the betatron frequency (tune) due to the deviation of the equilibrium trajectory due to the change in the excitation current of the deflection electromagnet, the quadrupole electromagnet, the function coupling type electromagnet, etc. is prevented, and the charged particle beam is stably emitted. Therefore, in addition to the 6-pole electromagnet for resonance emission, a 6-pole electromagnet that cancels the change in the tune caused by the excitation current of the deflection electromagnet and the 4-pole electromagnet is provided, and the tune by the excitation current of the deflection electromagnet and the 4-pole electromagnet is provided. An excitation current is applied to the six-pole electromagnet so as to give a diverging force and a converging force to cancel the change in the beam.

Japanese Patent Laid-Open No. 11-074100

However, in the revolving accelerator shown in Patent Document 1 above,
(1) In order to prevent changes in the tune due to the shift of the equilibrium trajectory caused by changes in the excitation current of the deflecting electromagnet and other electromagnets, it is necessary to perform complex control of the hexapole electromagnet etc. It takes time.
(2) Even when the emission energy is the same, in the case of resonance emission, the charged particle beam travels on different beam trajectories in the process of reducing the separatrix, so that the tune change due to the trajectory change is prevented. Complicated control is required, and a large amount of beam adjustment time is required.

  The present invention has been made in order to solve the above-described problems, and the tune changes substantially linearly even when the equilibrium trajectory is displaced. To provide a low-cost circular accelerator capable of controlling the control substantially linearly, that is, capable of stably emitting a beam with simple control, and thereby shortening the beam adjustment time. It is in.

  A circular accelerator in which a charged particle beam according to the present invention orbits an equilibrium orbit includes a deflection electromagnet that generates a deflection magnetic field, a hexapole electromagnet that generates a magnetic field that corrects a difference in betatron oscillation due to a difference in energy of the charged particle beam, The deflection electromagnet is formed on the circumferential end surface of the magnetic pole in the same plane as the magnetic pole surface and extending in the circumferential direction. End packs of magnetic material, and the end packs have projections protruding in the circumferential direction separated from each other radially outside and inside, and a projection shape on the magnetic pole surface is formed between the projections. They are connected by a connecting surface that is a part of a cylindrical surface that becomes an arc of a predetermined curvature, or a part of an elliptical cylindrical surface that becomes an elliptical arc having a predetermined short axis and long axis, and the circumferential end faces of the protrusions are parallel to each other. Napeira Those having a flat end face is, the circumferential length of the end pack point shortest, in which are arranged on a line extending in the radial direction center position of the circumferential end face of the pole in the circumferential direction.

  In the circular accelerator provided with the deflection electromagnet described above, the time dependence of the magnetic field strength of the hexapole electromagnet at the time of resonant emission becomes substantially linear, and the adjustment of the emission parameter when the energy of the charged particles changes is facilitated. It is possible to greatly shorten the initial beam adjustment period at the time of construction, after long-term operation suspension or after partial modification of the apparatus. In addition, such an effect has an effect of improving driving reliability and realizing a low-cost circular accelerator. In addition, since the protrusions of the end pack are connected by the connecting surface as described above, the change in the shape of the connecting surface and the change between the protrusion and the connecting surface are gradual, making it insensitive to the effects of processing and installation errors. The necessity for forming the end pack with high accuracy is reduced, and the cost can be reduced accordingly.

Embodiment 1 FIG.
Embodiment 1 of the present invention will be described with reference to the drawings.
FIG. 1 is a diagram showing an equipment arrangement of a circular accelerator 100 according to the first embodiment. As is well known, the circular accelerator 100 accelerates charged particles incident from the former stage accelerator 9 via the beam transport system 1 while circulating around the equilibrium orbit 4 that is the orbit, and then passes through the emission device 7. Charged particles are supplied to an irradiation chamber (not shown) via the outgoing beam transport system 8.
As shown in FIG. 1, a circular accelerator 100 includes an incident device 2 that receives a charged particle, for example, a proton beam, transported from a previous accelerator 9, a high-frequency acceleration cavity 5 that gives energy to the charged particle, and a beam trajectory. A deflection electromagnet 3 that bends and a hexapole electromagnet that excites resonance at the time of emission of the accelerated charged particle beam, that is, generates a magnetic field for dividing the betatron oscillation of the charged particle beam into a stable region and a resonance region; An emission device 7 is provided for emitting a proton beam having an increased betatron oscillation amplitude to the beam transport system 8 for emission.

2 is a side view of the deflecting electromagnet 3, and FIG. 3 is an end pack 34 at the magnetic pole edge portion 32 of the magnetic pole 31 of the deflecting electromagnet 3 into and out of which the charged particle beam is seen from the AA arrow direction in FIG. FIG. 3 is an enlarged view for explaining a balanced trajectory 4 of a charged particle beam.
In FIG. 2, the deflecting electromagnet 3 is provided with a magnetic pole 31 having a magnetic pole surface 31 a opposed via a magnetic pole gap G, and a coil 39. The coil 39 is wound around a magnetic pole combined with an end pack described later. The balanced trajectory 4 shown in FIG. 1 includes a balanced trajectory 33a of a beam having a central energy corresponding to the beam central trajectory shown in FIG. 3, a balanced trajectory 33b of a beam (high energy beam) higher than the central energy, and an energy higher than the central energy. Is a collective term for the balanced trajectory 33c of a low beam (low energy beam). An end pack 34 is attached to the magnetic pole edge portion 32 at the entrance and exit of the charged particle beam of the magnetic pole 31.

  As shown in FIG. 3, the end pack 34 of magnetic material is attached to the circumferential end surface 31b of the magnetic pole 31 of the deflection electromagnet 3, and is provided extending in the circumferential direction so as to form the same plane as the magnetic pole surface 31a. Yes. The end pack 34 is provided with a first protrusion 34a on the outer edge portion 32a on the radially outer side and a second protrusion 34b on the inner edge portion 32b on the radially inner side. . Between the first and second protrusions 34a and 34b, a part of a cylindrical surface whose projected shape onto the magnetic pole surface 31a is an arc having a predetermined curvature, or an elliptical arc having a predetermined short axis and long axis The end packs 34a and 34b are connected by a connecting surface 34e which is a part of the elliptic cylindrical surface, and the end faces in the circumferential direction have flat portion end surfaces 34c and 34d which are parallel planes. The shortest point P of the circumferential length is arranged on a line extending in the circumferential direction from the radial center position Q of the circumferential end face of the magnetic pole 31. Then, the circular equilibrium orbit of the charged particle beam having the center energy of the emission energy passes on the figure obtained by projecting the connecting surface 34e of the end pack 34 on the circular equilibrium orbit plane, and the position where the circular particle orbit passes has passed through the end pack 34. It has the structure which comes to radial inside rather than the point P where the circumferential direction length becomes the shortest.

  In general, in a deflection electromagnet having a finite magnetic pole width (radial length), the magnetic field strength (deflection magnetic field) at the magnetic pole edge is highest at the center position of the magnetic pole width, and the magnetic field strength becomes weaker as it deviates from the center position. Therefore, the beam convergence force felt by charged particle beams having different energies is different, and the tune (betatron frequency) determined from the beam convergence force is also different. Further, when the center position of the magnetic pole width and the equilibrium orbit position of the center energy are shifted as in the first embodiment, the fluctuation of the tune due to the energy becomes nonlinear. It is the main object of the present invention including the first embodiment to make the energy dependence of the tune from nonlinear to linear.

  As shown in FIG. 3, the deflection electromagnet 3 employed in the circular accelerator according to the first embodiment has a magnetic pole surface 31a of the magnetic pole 31 on the magnetic pole edge portion 32 (circumferential end surface) of the deflection electromagnet 3. It has the end pack 34 of the magnetic material formed by extending in the circumferential direction on the same plane. The end pack 34 has flat end surfaces 34c and 34d, which are flat surfaces at circumferential ends, and are separated from each other radially inside and outside, first and second protrusions 34a and 34b, A portion of the cylindrical surface in which the projection shape onto the magnetic pole surface 31a or the orbiting equilibrium orbital surface 4 is an arc having a predetermined curvature, or an ellipse that is an elliptical arc having a predetermined short axis and long axis, between the protrusions 34a and 34b. The connecting surface 34e is a part of the cylindrical surface. The flat portion end surfaces 34c and 34d of the first and second protrusions 34a and 34b are parallel to each other. When the end pack 34 is not provided, the magnetic field strength (deflection magnetic field) at the magnetic pole edge portion 32 described above is highest at the central position Q of the magnetic pole width W, and becomes weaker as the distance from the central position increases. By installing the pack 34, it is possible to reduce the influence of the magnetic field intensity nonuniformity. That is, the circumferential magnetic pole length of the deflection electromagnet 3 at the center Q of the magnetic pole width W where the magnetic field strength is high due to the endpack 34 is set, and the circumferential magnetic pole length at a position away from the center Q of the magnetic pole width W where the magnetic field strength is low. It is shorter than As already described, when this long and short shape is formed by a part of a cylindrical surface having a predetermined curvature, or a part of an elliptical cylindrical surface having a predetermined short axis and a long axis, the hexapole electromagnet 6 at the time of resonance emission The time dependence of the magnetic field intensity becomes substantially linear, and the beam adjustment period can be greatly shortened. By shortening the adjustment period, there are energy saving effects and cost reduction effects for the shortened period. Further, as described above, the connecting surface 34e that is smooth and has no singularity connects the protrusions 34a and 34b, so that the change due to the position of the shape is gradual, and it is caused by the shape error caused by the processing accuracy and the installation accuracy. Insensitive to placement errors. Therefore, the effect that high precision is not required for processing and installation is also derived.

  FIG. 4 shows a horizontal beam convergence characteristic necessary for stable beam emission in the circular accelerator 100 employing the conventional deflection electromagnet 3 not equipped with the end pack 34 according to the present invention. Calculations were made using a three-dimensional magnetic field and an orbital analysis code, and the energy dependence of the tune required at that time was shown. Since only the tune in the horizontal direction is controlled by resonance emission, only the dependency in the horizontal direction is shown. However, in order to make the conditions the same as when the end pack 34 according to the present invention to be described later is equipped, the magnetic pole 31 does not have the curved end face 34e at the end of the end pack 34 in FIG. 3, that is, the first and second The calculation was performed under the condition that the flat end faces 34c and 34d of the protrusions 34a and 34b are connected on the same surface. As shown in FIG. 3, the beam 33c having a lower energy than the center energy passes through the inner diameter side of the deflection electromagnet 3, and the beam 33b having a higher energy than the center energy passes through the outer diameter side of the deflection electromagnet. The magnetic field strength distribution in the magnetic pole portion is weaker in the radially inner portion 32b of the magnetic pole edge than in the radially outer portion 32a of the magnetic pole edge. (Dispersion is negative) is stronger than outside.

  On the other hand, FIG. 5 shows a circular accelerator 100 that employs the deflecting electromagnet 3 equipped with the end pack 34 shown in FIG. 3, and shows the horizontal beam convergence characteristic necessary for stable beam emission. The solid line B indicates the energy dependence of the tune required at that time, calculated using the dimensional magnetic field and the trajectory analysis code. For comparison, the result shown in FIG. Under the above condition, the energy dispersion dependency of the tune has linearity. Therefore, time control of the magnetic field strength of the hexapole electromagnet 6 or the like for satisfying the emission condition is very simple. The solid line B in FIG. 5 shows the result that there is no energy dependence, but it is not essential that there is no energy dependence of the tune, but it is essential that it has a substantially linearity. . The reason is as follows. At the time of emission, the 6-pole electromagnet 6 is excited to set the separatrix to a predetermined size. When the hexapole electromagnet 6 is excited, the horizontal dependency of the energy dependence of the tune is linear when the hexapole electromagnet 6 is not excited, but the inclination changes. It is. Therefore, it is sufficient if it is linear, and its inclination changes, so there is no reason to stick to this.

FIG. 6 shows the calculation result of the time dependence of the required magnetic field strength of the hexapole electromagnet 6 when performing resonant emission. The broken lines A and B in the figure correspond to the conditions when A and B in FIG. 5 are calculated, respectively. In other words, this corresponds to the case where A is a conventional type and B is a deflection electromagnet 3 according to the present invention. As shown in the figure, under the condition A, it is necessary to change the magnetic field strength of the hexapole electromagnet 6 every time, and a great amount of adjustment time is required for the initial beam adjustment. On the other hand, in the case of B, the time dependency of the strength of the hexapole electromagnet 6 is substantially linear, and therefore the adjustment of the hexapole electromagnet at the time of emission can be simplified as compared with the conventional case, and the beam adjustment period can be reduced. Significant shortening is possible.
FIG. 7 shows the calculation result of the time change of the beam current at the time of beam extraction in the case of B in FIG. It can be seen that a stable beam is emitted continuously.

Embodiment 2. FIG.
FIG. 8 is an enlarged view for explaining the end pack 34 and the balanced trajectory of the charged particle beam in the magnetic pole edge portion 32 according to the second embodiment. The difference between FIG. 8 and FIG. 3 of the first embodiment described above is that the charged particle beam having the center energy of the emission energy on the figure obtained by projecting the connecting surface 34e of the end pack 34 onto the orbital equilibrium orbital surface. The balanced trajectory 33a passes, and the passing position is located radially inward from the point Q at which the end pack 34 has the shortest circumferential length, and the rest is the same as FIG. Therefore, explanation of the reference numerals is omitted.
Further, FIG. 9 shows an AA cross section and a BB cross section of FIG. As can be seen from the figure, the end pack 34 is provided with an inclined surface K ₁ at a predetermined angle θ ₁ at a predetermined distance l ₁ from the circumferential end surface 31 b of the magnetic pole 31 with which the end pack 34 is closely attached. Further, the inclined surface K ₁ is provided with an inclined surface K ₂ at a predetermined angle θ ₂ at a position advanced by a predetermined distance l _{2 in the} same direction, and the gap G between the magnetic pole surfaces of the deflection electromagnet 3 is formed by the charged particle beam. It is larger with respect to the outer direction of the bending electromagnet 3 on the orbit.
In this way, the orbital equilibrium trajectory 33a of the charged particle beam having the central energy of the emission energy passes through the inner side in the radial direction from the point Q at which the circumferential length of the end pack 34 is the shortest. Since the deflecting electromagnet 3 is configured to include the end pack 34 provided with the inclined surfaces K ₁ and K ₂ on the 31, in addition to the same effect as the first embodiment, the magnetic field shape due to the magnetic field saturation at the magnetic pole edge portion 32. There is an effect to prevent the disturbance.
In the second embodiment, the example in which the _two inclined surfaces K ₁ and K ₂ are provided has been described, but the number of inclined surfaces is not limited to this. Further, when the curved shape is used instead of the straight line, the magnetic field accuracy is further improved. The shape of the inclined surface is determined by performing an optimization calculation by electromagnetic field calculation so that the end of the magnet is not easily saturated. This shape is the same as the end shape of a conventional bending electromagnet.

Embodiment 3 FIG.
FIG. 10 is an enlarged view for explaining the end pack 34 in the magnetic pole edge portion 32 and the balanced trajectory 4 of the charged particle beam according to the third embodiment.
The end pack 34 shown in FIG. 10 is the same as the end pack shown in FIG. 3 except that the flat surface ends 34c and 34d of the two protrusions are not located on the same plane. As can be seen from FIG. 10, the first protrusion 34a has a stretching length of L ₁ to the circumferential direction of the charged particle beam, the second protrusion 34b is having a stretching length of the L ₁ greater than L ₂ It is.
Thus, by making the extension length of the second protrusion 34b of the end pack 34 larger than the extension length of the first protrusion 34a, the action of the magnetic field on the charged particle beam on the inner diameter side compared to the magnetic pole outer diameter side. This is effective when the passing position of the charged particle beam 33a having the center energy is greatly deviated from the point Q at which the circumferential length of the end pack 34 is the shortest.
This is because the magnetic field intensity in the vicinity of the magnetic pole edge portion 32 is weaker than that of the magnetic pole center portion, and in order to correct it, it is necessary to increase the extension length of the protrusion. In addition, as is generally known, even in a configuration in which an edge angle is provided in the magnetic pole edge portion 32 of the deflection electromagnet 3 in order to give the charged particle beam 4 a converging action, the magnetic field strength of the radially inner portion 32b of the edge is also provided. Can be reduced compared to the radially outer portion 32a. As a result, the same effects as those of the first embodiment can be obtained.

Note that the inclined surfaces K ₁ and K ₂ provided in the end pack 34 shown in FIG. 9 of the _second embodiment may be provided in the end pack 34 of the first and third embodiments.

  The present invention is applicable to a medical accelerator for performing cancer treatment using a charged particle beam, diagnosis of an affected part, etc., particle beam irradiation to various materials, and a physical experiment accelerator.

It is a figure which shows the equipment arrangement | positioning of the circular accelerator of Embodiment 1. FIG. 3 is a side view of the deflection electromagnet of Embodiment 1. FIG. FIG. 3 is an enlarged view showing a magnetic pole edge portion according to the first embodiment. It is a figure which shows the energy dependence of the tune of a horizontal direction when not providing a curve part end surface in an end pack. FIG. 5 is a diagram illustrating energy dependence of a horizontal tune according to the first embodiment. FIG. 6 is a diagram showing time dependence of the strength of a hexapole electromagnet at the time of resonant emission according to the first embodiment. FIG. 3 is a diagram showing an emitted beam current at the time of beam emission according to the first embodiment. FIG. 6 is an enlarged view showing a magnetic pole edge portion according to a second embodiment. FIG. 6 is a diagram illustrating a cross section of a magnetic pole edge portion according to a second embodiment. FIG. 6 is an enlarged view showing a magnetic pole edge portion according to a third embodiment.

Explanation of symbols

3 deflection electromagnet, 4 balanced orbit, 6 hexapole electromagnet, 31 magnetic pole, 31a magnetic pole surface,
31b magnetic pole end face, 32 magnetic pole edge part, 32a edge outer side part,
32b Edge inner part, 33a Equilibrium orbit of central energy beam,
33b Balanced orbit of high energy beam, 33c Balanced orbit of low energy beam,
34 end packs, 34a first protrusions, 34b second protrusions,
34c, 34d flat part end face, 34e connecting face, 100 circular accelerator,
L ₁ stretch length of the first protrusion, L ₂ stretch length of the second protrusion,
W Magnetic pole width (endpack length).

Claims (5)

In a circular accelerator in which a charged particle beam circulates in an equilibrium orbit, the circular accelerator includes a deflection electromagnet that generates a deflection magnetic field and a hexapole electromagnet that generates a magnetic field that corrects a difference in betatron oscillation caused by a difference in energy of the charged particle beam. And an extraction device that extracts the charged particle beam from the equilibrium trajectory to the outside of the circular accelerator,
The deflection electromagnet has an end pack of magnetic material formed on the circumferential end surface of the magnetic pole in the same plane as the magnetic pole surface and extending in the circumferential direction. There are separated projections projecting in the circumferential direction, and between the projections, a part of a cylindrical surface whose projected shape onto the magnetic pole surface is an arc of a predetermined curvature, or a predetermined short axis and long axis It is connected by a connecting surface that is a part of an elliptical cylindrical surface that has an elliptical arc, and the circumferential end surfaces of the protrusions have flat end surfaces that are parallel to each other, and the circumferential length of the end pack A circular accelerator characterized in that the shortest point is arranged on a line extending in the circumferential direction from the radial center position of the circumferential end face of the magnetic pole. On the figure obtained by projecting the connection surface of the end pack onto the orbital equilibrium orbit plane, the orbital equilibrium orbit of the charged particle beam having the center energy of the emission energy passes, and the passing position is the circumference of the end pack. The circular accelerator according to claim 1, wherein the circular accelerator is located radially inward from a point having the shortest length in the direction. 3. The circular accelerator according to claim 1, wherein a circumferential length of the radially inner protrusion is larger than a circumferential length of the radially outer protrusion. 4. The distance between the magnetic pole surface of the end pack and the balanced track surface increases as the end pack approaches the end surface on the side where the end pack protrusion and the connecting surface are located from a predetermined position in the circumferential direction. The circular accelerator according to any one of claims 1 to 3, wherein a predetermined inclined surface is provided. The circular accelerator according to claim 4, wherein the predetermined inclined surface is formed of two or more inclined surfaces.

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