JPH0935899A - Take-out method for charged particle beam - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a beam take-out method which fixedly arrange a take-out beam characteristic and outgoing efficiency, regardless of outgoing energy. SOLUTION: After accelerating beam is ended, a high frequency electric field is applied to increase emittance. By adjusting intensity of the high frequency electric field, emittance when started beam outgoing is fixedly properly arranged, regardless of outgoing energy. By applying a high frequency electric field simultaneously with acceleration, decreasing emittance is suppressed, and arrange emittance constant at the end of acceleration, regardless of outgoing energy.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a beam extracting method for a circular accelerator which orbits a charged particle beam.

[0002]

2. Description of the Related Art In a circular accelerator, a charged particle beam such as electrons and ions is accelerated to orbit, and the charged particle beam taken out from the orbit is transported by a transport system and used for physical experiments and medical treatment. In conventional extraction of charged particle beams, AIP Conference Proceedings No. 127 (1983) (AIP Conference
The resonance of the betatron oscillation of the beam has been used as discussed in Proceedings, pages 53-61.

[0003] The resonance of betatron oscillation is the following phenomenon. The charged particles orbit while oscillating in the horizontal and vertical directions of the orbital plane that orbits, and this oscillation is called betatron oscillation. The frequency around the orbit of betatron oscillation is called a tune, and the tune can be controlled by a bending electromagnet or a quadrupole electromagnet provided on the orbit. The betatron oscillation amplitude differs depending on the particles that make up the beam, and particles with large amplitude to particles with small amplitude are mixed. Therefore, the beam diameter is determined by the maximum value of the betatron oscillation amplitude.

In the conventional example, when the tune is approximated to an integer ± 1/3 and the sextupole electromagnet for resonance generation provided on the orbit is excited at the same time, the amplitude of the charged particles in the orbit is above a certain boundary. The betatron oscillation amplitude of the charged particles that it possesses increases rapidly. This phenomenon is called betatron oscillation resonance, and the boundary is called the stability limit. The size of the betatron oscillation amplitude at the stability limit of resonance depends on the deviation from the tune integer ± 1/3, and this tune is gradually changed to the integer ± 1/3.
That is, the size of the stability limit is gradually decreased, and among the charged particles in the orbit, the charged particles with large betatron oscillation amplitude first generate resonance, and then the charged particles with small oscillation amplitude sequentially generate resonance. And gradually extracted the charged particle beam.

Further, Japanese Laid-Open Patent Publication No. 5-198397 discloses a circular accelerator for extracting a beam by increasing the betatron oscillation amplitude and generating resonance while keeping the tune and stability limit during extraction constant. . In this method, the stability limit is set to about the size of the charged particle beam, and the betatron oscillation amplitude is increased with the stability limit kept constant.

[0006]

The prior art has the following problems.

The spread of the betatron oscillation amplitude is called emittance, and a graph in which the horizontal axis represents the position of the particle and the vertical axis represents the deviation (orbital gradient) of the motion direction of the particle from the traveling direction (FIG. 2, phase space and Called) corresponding to the area occupied by the beam. The charged particles gain momentum in the traveling direction by acceleration, but the momentum in the direction perpendicular to the traveling direction does not change. Therefore, the direction of particle motion is aligned with the direction of beam travel as it accelerates. Therefore, the emittance decreases as the emitted energy increases. Conventionally, the size of the stability limit formed at the start of extraction has been set to about the emittance, so that the size depends on the emitted energy. Therefore, there has been a problem that various parameters of the outgoing beam such as the orbital gradient of the extracted charged particles and the outgoing efficiency change with each outgoing energy.

An object of the present invention is to provide a beam extraction method and apparatus for making various parameters of an extraction beam such as extraction beam characteristics and extraction efficiency constant regardless of the extraction energy.

[0009]

The first means for achieving the above object is a means for aligning the emittance in the horizontal or vertical direction immediately before extraction regardless of the emitted energy.

The second means is a means for keeping the emittance substantially constant at the start of acceleration or during the course of acceleration to the end of acceleration.

[0011]

According to the first means, the state of the beam at the time of extraction is uniform regardless of the extraction energy, and various beam parameters such as the orbital gradient of the extraction beam and the extraction efficiency can be made constant. Since the beam state at the time of extraction is the same regardless of the extraction energy, the setting conditions of the electromagnet at the time of extraction can be unified, and the accelerator control can be simplified and stabilized. Moreover, the change of the emitted energy can be simplified.

According to the second means, the state of the beam at the end of acceleration is aligned regardless of the extraction energy, so that various beam parameters such as the orbit gradient of the extraction beam and the extraction efficiency can be made constant. Thereby, simplification and stabilization of the accelerator control and simplification of change of the emitted energy can be realized. In addition, the second means does not require time for adjusting emittance, so that the operation efficiency is improved.

[0013]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a flow chart showing the outline of the method of operating the circular accelerator according to the present invention. A feature of the present invention is that the emittance is adjusted during the acceleration and extraction of the beam.

First, a first embodiment of the present invention, that is, a method of operating a circular accelerator for adjusting emittance after finishing acceleration will be described with reference to FIG.

FIG. 9 is a diagram showing the outline of the present invention, which is a circular accelerator for emitting an accelerated beam by utilizing resonance called third-order resonance. The circular accelerator comprises a deflection electromagnet 1, a quadrupole electromagnet 2 having a converging force or a diverging force, a high frequency accelerating cavity 4, a multipole electromagnet for resonance excitation 5, a deflector 8 for emission, and the like. In the coordinate system, the beam circulation direction is s, the horizontal direction is x (the outside is positive), and the vertical direction is y. The beam revolves around the central orbit 14, which is a revolving orbit, with betatron oscillation.

FIG. 5 is a flow chart showing the operating procedure of the circular accelerator of this embodiment. A description will be given below according to this flow.

The charged particle beam from the pre-stage accelerator is incident on the circular accelerator through the injector. The trajectory of the incident beam is kept circular by a deflection electromagnet, passes through the high-frequency accelerating cavity 4 every revolution, and is accelerated each time. FIG.
23a shows the distribution of particles when the kinetic energy of the beam is accelerated to E0, and 23b shows the distribution of particles when the kinetic energy of the beam is accelerated to E1 (E1> E0) which is the emission energy. In this figure, the horizontal axis represents the horizontal position x of each particle in the beam in the horizontal direction, and the vertical axis represents the horizontal trajectory gradient x ′ = dx / ds, which is called a phase space. The area in the phase space in which the particles are distributed is called the emittance and has the same value in any part of the circular accelerator. The emittance decreases in inverse proportion to the momentum of the beam with acceleration. Therefore, the emittance of the beam accelerated to the extraction energy becomes smaller as the extraction energy becomes higher. The emittance when the kinetic energy of the beam is E0 is ε0, and the emittance when it is E1 is ε1.
(Ε1 <ε0).

After the acceleration to the emission energy E1, the quadrupole electromagnets 2 and 3 are adjusted to adjust the horizontal tune to 5/3.
Close to and set to 1.67. Next, using the high-frequency electric field applying electrode 6, a high-frequency electric field containing a frequency component synchronized with the period of betatron oscillation of particles is applied in the horizontal direction of the beam. Then, the particles gradually increase in betatron oscillation amplitude and emittance increases. The intensity of the high frequency electric field is adjusted to the intensity that increases the emittance to ε0 during the time T until the start of emission. FIG. 6 shows an example of temporal changes in the strength of the high-frequency electric field applying electrode during operation of the accelerator according to the present embodiment, along with temporal changes in the magnetic field strength of the deflection electromagnet.

When the multipole electromagnet 5 for resonance excitation (a hexapole electromagnet in this embodiment) is excited, the third resonance is excited in particles having a large betatron oscillation amplitude. FIG. 3 shows trajectories of particles in the beam on the phase space for each orbit at a position where the deflector 8 for emission is installed. A shaded triangular region 23a in FIG. 3 represents the range of charged particles in the phase space when the emittance is ε0, and a region 23b represents the range when the emittance is ε1. The triangular frame line indicates the stability limit 21 in the phase space. As shown in the figure, the magnetic field strength of the sextupole electromagnet is selected so that the size of the stability limit substantially matches the range of the charged particles in the phase space when the output energy is E0. Since the emittance is ε0 due to the application of the high frequency electric field, the range of the beam coincides with the region 23a of FIG.

Subsequently, a high frequency electric field is applied to gradually increase the amplitude of the charged particles. Particles that are outside the stability limit, that is, whose betatron oscillation amplitude is larger than the stability limit, have a sharp increase in the oscillation amplitude every revolution due to resonance. The points drawn outside the stability limit in FIG. 3 represent the trajectory drawn by the particles when the stability limit is exceeded, and the numbers beside the points indicate the number of turns. The rate at which the beam moves out of the stability limit depends on the intensity of the high-frequency electric field, so the intensity of the high-frequency electric field is adjusted so that the required amount of beam is extracted during the time Te from the start to the end of extraction. To do.

30 in FIG. 3 (30i on the inner side and 30o on the outer side)
Indicates an electrode of the deflector 8 for emission of FIG. 1, particles colliding with the electrode 30 are lost, and particles entering the region between the electrodes 30 are taken out of the circular accelerator.
By making the intensity of the sextupole magnet and the deflector for extraction to be proportional to the momentum of the charged particles,
In addition to the extraction efficiency, various beam parameters match the output beam when the output energy is E0.

Also when changing the type of charged particles to be taken out, the emittance of the outgoing beam is adjusted by adjusting the emittance to ε0 and adjusting the intensity of the outgoing deflector.
In addition to extraction efficiency, various beam parameters are constant regardless of emission energy and particle type.

This embodiment can be applied to beam extraction using resonance other than the third resonance in the same manner.

[0025] Further, there are two for adjusting emittance and for emitting.
By providing different types of high-frequency electric field applying electrodes, frequency selection and high-frequency electric field strength adjustment can be controlled independently, and control can be simplified.

Next, a second embodiment of the present invention for adjusting the emittance while accelerating the beam will be described with reference to FIG. FIG. 7 is a flow chart showing the procedure for operating the circular accelerator of this embodiment. A description will be given below according to this flow.

The charged particle beam from the pre-stage accelerator is incident on the circular accelerator through the injector. The trajectory of the incident beam is kept circular by the bending electromagnet. It passes through the high-frequency acceleration cavity 4 every lap and is accelerated each time.

When the beam energy reaches E0, the emittance is ε0. Here, the high-frequency electric field applying electrode 6 is used to start applying a high-frequency electric field including a frequency component synchronized with the period of betatron oscillation of particles in the horizontal direction of the beam. The high frequency electric field is continuously applied while continuing the acceleration until the energy reaches the emission energy E1. At this time,
As the energy of the beam increases and the orbital frequency increases, the frequency of the high frequency electric field increases. The intensity of the high-frequency electric field is adjusted to the rate at which the momentum of the beam increases with acceleration, so that the emittance of the beam is kept substantially constant from when the energy reaches E0 to the end of acceleration. FIG. 8 shows an example of the temporal change in the strength of the high-frequency electric field applying electrode according to this example, along with the temporal change in the magnetic field strength of the deflection electromagnet.

When the acceleration is completed up to the emission energy E1, the emittance of the beam remains ε0. Next, when the tune is set to 1.503 by adjusting the strength of the quadrupole electromagnets 2 and 3 and the multipole electromagnet 5 for resonance excitation (the octupole electromagnet in this embodiment) is excited, particles having a large betatron oscillation amplitude are formed. The secondary resonance is excited. FIG. 4 shows trajectories of particles in the beam on the phase space for each orbit. The hatched circular region 23a in FIG. 4 represents the range of charged particles in the phase space, and the frame line indicates the stability limit 21 in the phase space. The magnetic field strength of the octupole electromagnet is selected so that the size of the stability limit is about the emittance ε0 of the beam.

After exciting the octupole electromagnet, the strength of the quadrupole electromagnets 2 and 3 is adjusted to gradually bring the tune closer to 3/2. The size of the stability limit is the resonance point of the tune (3/2)
The smaller the deviation from, the smaller the deviation becomes, and the stability limit is exceeded in order from the charged particles having a large betatron oscillation amplitude among the charged particles in the orbit. A charged particle exceeding the stability limit drastically increases its amplitude while drawing a locus along two curves extending from the stability limit. The points drawn outside the stability limit in FIG. 4 represent the trajectories of particles when the stability limit is exceeded, and the numbers beside the points indicate the number of turns.
The rate at which the beam moves out of the stability limit depends on the speed at which the tune is changed. The rate of change of the magnetic field strength of the quadrupole electromagnet is adjusted so that a required amount of beam is extracted during the time Te from the start of extraction to the end of extraction. The temporal change of the orbit gradient of the outgoing particles due to the change of the size of the stability limit is compensated by the temporal control of the strength of the outgoing deflector and the electromagnet of the transport system.

By making the intensity change pattern of the octupole electromagnet and the intensity change of the quadrupole electromagnet and the deflector for extraction proportional to the momentum of the charged particles, in addition to the emittance of the emitted beam and the extraction efficiency, various beam parameters have the emission energy E0. It corresponds to the outgoing beam at.

Also when changing the type of charged particles to be extracted, the emittance of the outgoing beam can be adjusted by adjusting the emittance to ε0 and adjusting the intensity of the outgoing deflector.
In addition to extraction efficiency, various beam parameters are constant regardless of emission energy and particle type.

[0033]

According to the present invention, the orbital gradient and extraction efficiency of the extraction beam can be made constant regardless of the extraction energy. Since the beam state at the time of extraction is the same regardless of the extraction energy, the setting conditions of the electromagnet at the time of extraction can be unified, and the accelerator control can be simplified and stabilized. Moreover, the change of the emitted energy can be simplified.

Further, when the emittance at the end of acceleration is always constant, the time for adjusting the emittance is not required, so that the operating efficiency is improved.

[Brief description of drawings]

FIG. 1 is a flowchart showing a method for operating a circular accelerator according to a first embodiment of the present invention.

FIG. 2 is an explanatory diagram showing a range of a beam in a phase space after completion of acceleration.

FIG. 3 is an explanatory diagram showing a stability limit in a phase space and a position of charged particles in each orbit according to the first embodiment.

FIG. 4 is an explanatory diagram showing a stability limit in a phase space and a position of charged particles in each orbit in Example 2;

FIG. 5 is a flowchart showing a method for operating a circular accelerator according to the first embodiment.

FIG. 6 is an explanatory diagram showing a change over time in the strength of the high-frequency electric field applying electrode in Example 1.

FIG. 7 is a flowchart showing a method for operating a circular accelerator according to a second embodiment.

FIG. 8 is an explanatory diagram showing a change over time in the strength of the high-frequency electric field applying electrode in Example 2.

FIG. 9 is an explanatory diagram showing a configuration of a circular accelerator according to an embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Bending electromagnet, 2 ... Focusing quadrupole electromagnet, 3 ... Divergence quadrupole electromagnet, 4 ... High frequency acceleration cavity, 5 ... Resonance excitation multipolar electromagnet, 6 ... High frequency electric field applying electrode, 8 ... Ejecting deflector, 9 ... Injector, 10 ... Beam, 11 ... Beam transport system, 12 ... Pre-accelerator, 13 ... Charged particle beam, 14 ...
Central orbit, 21 ... Stable limit, 23 ... Beam range, 30
…electrode

Claims (3)

[Claims] 1. A beam extraction method for accelerating and extracting a charged particle beam with a circular accelerator, wherein the emittance is increased before starting extraction of the charged particle beam. 2. A beam extraction method for accelerating and extracting a charged particle beam with a circular accelerator, wherein the emittance in the horizontal direction or the vertical direction at the start of extraction of the charged particle beam is made substantially constant regardless of the energy of the beam. And a method for extracting a charged particle beam. 3. A beam extraction method for extracting a charged particle beam by accelerating it with a circular accelerator, wherein the emittance in the horizontal or vertical direction is kept substantially constant at the start of acceleration or during the middle of acceleration to the end of acceleration. The extraction method of the charged particle beam.