JP2014160613A - Cyclotron - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cyclotron capable of improving beam lead-out efficiency.SOLUTION: A cyclotron 1 comprises: a pair of magnetic poles provided while being paired with a circulation track of a beam interposed therebetween, each including a plurality of protrusions and a plurality of recesses arrayed alternately in a circumferential direction and forming hill regions 25h each held between the protrusions and valley regions 25v each held between the recesses along the circulation; dee electrodes 5a, 5b provided in the valley region 25v; and a high frequency generation part 30 disposed at an outer circumferential side in a radial direction of the circulation track of the beam in at least one valley region 25v other than the valley region 25v in which the dee electrodes 5a, 5b are provided, and generating a high-frequency electric field for accelerating the beam.

Description

  The present invention relates to a cyclotron.

  Conventionally, as a technique in such a field, a cyclotron described in Patent Document 1 below is known. In this cyclotron, charged particles sent from an ion source are accelerated by an electric field created by an RF acceleration cavity, and after being sufficiently accelerated, a charged particle beam is extracted outside the cyclotron by an extraction device such as an electrostatic deflector. . As the beam is accelerated, the beam trajectory gradually shifts to the outside. When the beam is extracted, the electrostatic deflector septum is made thinner than the amount of beam trajectory shift per turn (turn separation). , Strip off the beam.

JP 2010-186631 A

The beam extraction efficiency in the cyclotron of the above system is
η = 1−t / δ
It is expressed by the approximate expression of Where t is the thickness of the septum and δ is the turn separation.
Also, turn separation δ due to beam acceleration is
δ = {R / γ (γ + 1)} · {ΔK / K}
It is expressed by the approximate expression of Where R is the radius of the orbit at the time of extraction, γ is the Lorentz factor, K is the kinetic energy of the beam, and ΔK is the amount of energy increase per turn.

  Miniaturization is desired for this type of cyclotron. However, if the cyclotron is downsized, the orbit radius R of the beam becomes smaller, thereby reducing the turn separation and lowering the beam extraction efficiency. When the extraction efficiency is lowered, the ion source is burdened to obtain the same beam current, and activation inside the cyclotron due to beam loss becomes a problem.

  In view of the above problems, an object of the present invention is to provide a cyclotron capable of increasing the beam extraction efficiency.

  The cyclotron according to the present invention is provided in a pair with a circular orbit of the beam interposed therebetween, each having a plurality of convex portions and a plurality of concave portions arranged alternately in the circumferential direction, and a hill region sandwiched between the convex portions. And a valley region sandwiched between recesses along a circular orbit, a de-electrode provided in the valley region, and a beam in at least one valley region other than the valley region in which the de-electrode is provided And a high-frequency generator that generates a high-frequency electric field for accelerating the beam.

  According to this cyclotron, since a high frequency generator for generating a high frequency electric field for accelerating the beam is provided separately from the de-electrode, the amount of energy increase (ΔK) per turn of the beam is increased. The turn separation (δ) increases, and as a result, the beam extraction efficiency (η) improves.

  As a specific configuration, the high-frequency generator includes a metal high-frequency electrode provided on the median plane and a metal frame that covers the periphery of the high-frequency electrode when viewed in a cross section orthogonal to the beam traveling direction. The frame may be provided with a slit extending on the median plane on the inner peripheral side of the high-frequency electrode.

  According to the present invention, it is possible to provide a cyclotron capable of increasing the beam extraction efficiency.

It is a top view inside a cyclotron according to the present invention. It is a schematic diagram of a pair of magnetic pole with which the cyclotron of FIG. 1 is provided. FIG. 3 is a cross-sectional view taken along the line III-III of FIG. 1, showing a vicinity of an acceleration cavity included in the cyclotron of FIG. 1.

  Hereinafter, embodiments of a cyclotron according to the present invention will be described in detail with reference to the drawings. In the cyclotron 1 of the present embodiment, it is assumed that the spiral orbit B of the charged particle beam is on the horizontal plane. Note that the cyclotron of the present invention may be arranged so that the orbit B is on the vertical plane.

  As shown in FIG. 1, the cyclotron 1 includes a vacuum vessel 3, de-electrodes 5 a and 5 b, a deflector 7, and a magnetic channel 9. The vacuum container 3 is a container for maintaining the acceleration space of charged particles in a high vacuum state. In the vacuum vessel 3, a pair of magnetic poles 21 and 23 are provided for forming a magnetic field necessary for particle acceleration. The magnetic poles 21 and 23 have a circular shape in plan view, and are symmetrical with respect to a median plane 27 (FIG. 3) described later. Further, the magnetic poles 21 and 23 are arranged facing each other in the vertical direction (a direction perpendicular to the paper surface of FIG. 1) with the orbit B of the charged particle beam interposed therebetween. A coil is arranged around each of the magnetic poles 21 and 23, and a magnetic field is generated between the magnetic pole 21 and the magnetic pole 23.

FIG. 2 is a perspective view schematically showing only the magnetic poles 21 and 23. As shown in the figure, the magnetic poles 21 and 23 are cylindrical. The terms “radial direction” and “circumferential direction” used below are:
It shall mean the radial direction and the circumferential direction of a circle which is the contour shape of the magnetic poles 21 and 23 viewed from the direction of FIG. On the upper surface of the magnetic pole 21, four convex portions 21a and four concave portions 21b that are spirally curved are alternately arranged in the circumferential direction. Also, on the lower surface of the magnetic pole 23, four convex portions 23a and four concave portions 23b that are spirally curved are alternately arranged in the circumferential direction. The convex portion 21a and the convex portion 23a, and the concave portion 21b and the concave portion 23b are arranged with a gap therebetween so as to be plane-symmetric with respect to the median plane 27 (FIG. 3). Here, the convex portions 21a and 23a of the magnetic poles 21 and 23 are portions protruding toward the median plane 27 (see FIG. 3), and the concave portions 21b and 23b are recessed so as to be separated from the median plane 27. It is the part that is. The median plane 27 is a plane on which a circular orbit B where the charged particle beam accelerates and travels is located. Strictly speaking, the charged particle beam travels while oscillating in the direction in which the magnetic poles 21 and 23 are opposed to each other (vertical direction in FIG. 3). A plane having a median value is the median plane 27. In addition, the shape of convex part 21a, 23a and recessed part 21b, 23b is not restricted to the shape curved in the above spirals, A fan shape may be sufficient.

  Between the magnetic pole 21 and the magnetic pole 23, a narrow gap hill region 25h sandwiched between the convex portion 21a and the convex portion 23a and a wide gap valley region 25v sandwiched between the concave portion 21b and the concave portion 23b are formed. Has been. A spiral orbit B of the charged particle beam is formed on the plane of symmetry between the magnetic poles 21 and 23 (median plane 27; see FIG. 3).

  The de-electrodes 5 a and 5 b are electrodes that generate an electric field for accelerating charged particles inside the vacuum vessel 3. The de-electrodes 5a and 5b are both arranged in the valley region 25v and arranged so as to face each other in the radial direction. The de-electrodes 5a and 5b are formed in a shape along the shape of the valley region 25v in plan view. An inflector 11 that deflects charged particles sent from an ion source (not shown) provided outside or inside the cyclotron 1 and sends it on the median plane 27 is disposed at the center of the magnetic pole 21. .

  The deflector 7 is a device for deflecting a charged particle beam that circulates in the orbit B in a magnetic field and pulling it out to the extraction orbit. The deflector 7 has an introduction gap capable of variably applying an electric field to the charged particles, and guides and deflects the charged particle beam into the introduction gap. The magnetic channel 9 is a device for converging the charged particle beam in the horizontal direction. The magnetic channel 9 has an introduction gap that can give a magnetic field gradient in a fixed manner, and guides a charged particle beam to the introduction gap to converge the beam in the horizontal direction. In the cyclotron 1, three magnetic channels 9 a, 9 b and 9 c are installed as the magnetic channel 9.

  In the cyclotron 1, a magnetic field is generated between the magnetic pole 21 and the magnetic pole 23, and a high frequency voltage is applied to the deelectrodes 5a and 5b, whereby the charged particle beam is accelerated and spiraled on the median plane 27. The orbit B of the vehicle advances. The charged particle beam that has reached the outer peripheral side in the radial direction passes through the introduction gap of the magnetic channel 9a, then is deflected by the introduction gap of the deflector 7, further passes through the introduction gap of the magnetic channels 9b and 9c, and passes through the beam extraction duct. Pulled out.

  The cyclotron 1 further includes an acceleration cavity 31 provided in at least one valley region 25v other than the valley region 25v provided with the de-electrodes 5a and 5b. The acceleration cavity 31 is disposed on the outer peripheral side in the radial direction of the circular orbit B of the charged particle beam, and has a function of accelerating the charged particle beam.

  As shown in FIG. 3, the accelerating cavity 31 is seen in a metal high-frequency electrode 35 extending in the traveling direction of the charged particle beam and in a cross section (cross section shown in FIG. 3) orthogonal to the traveling direction of the charged particle beam. And a metal frame 37 that covers the periphery of the high-frequency electrode 35. The high-frequency electrode 35 is supported inside the frame body 37 via a support member 39. Further, the cyclotron 1 includes a high frequency generator 30 for forming an acceleration cavity 31. The high frequency generator 30 includes a high frequency electrode 35, a frame 37, a support member 39, a waveguide 41 described later, and a tuner 43 described later.

  The high-frequency electrode 35 is disposed at a position close to the outer peripheral side (end portion on the outer peripheral side) in the valley region 25v, and is positioned on the median plane 27. The high-frequency electrode 35 has a U-shaped cross section that is open on the radial inner side (the center side of the orbit of the charged particle beam), and the charged particle beam passes through the inner portion of the U-shaped cross section. It is supposed to be. The frame 37 has a C-shaped cross section and has a slit 37 a extending on the median plane 27. The slit 37a is formed to extend horizontally at a position on the center side (inner circumference side) of the orbit of the charged particle beam. Due to the presence of the slit 37 a, the frame 37 does not become an obstacle to the charged particle beam traveling on the median plane 27, and the charged particle beam can enter the inside of the frame 37.

  A waveguide 41 and a tuner 43 are connected to the frame body 37. If the space surrounded by the frame 37 is an acceleration space A, the waveguide 41 applies a high-frequency electromagnetic field to the acceleration space A. The tuner 43 adjusts the frequency of the high-frequency electromagnetic field in the acceleration space A by changing the electromagnetic characteristics of the acceleration space A, for example, by inserting and removing a metal bar in the acceleration space A. With these configurations, an electromagnetic field having an appropriate frequency can be generated in the acceleration space A, and the charged particle beam passing through the acceleration space A can be accelerated.

  As described above, the charged particle beam is further imparted with kinetic energy when passing through the acceleration cavity 31, and then deflected by the deflector 7 and output to the outside of the cyclotron 1 as described above.

  Then, the effect of the cyclotron 1 which has the above high frequency generation parts 30 and the acceleration cavity 31 is demonstrated.

  According to this cyclotron, apart from the de-electrodes 5a and 5b, the high-frequency generator 30 for generating a high-frequency electric field for accelerating the charged particle beam is further provided, so that the charged particle beam is further accelerated. Therefore, the amount of energy increase per turn of the charged particle beam increases, thereby increasing turn separation, and as a result, the extraction efficiency of the charged particle beam is improved.

That is, the extraction efficiency of the charged particle beam in this type of cyclotron 1 is
η = 1−t / δ (1)
It is expressed by the approximate expression of Where t is the thickness of the septum of the deflector 7 and δ is the deviation of the beam trajectory per turn (turn separation).
Also, turn separation δ due to beam acceleration is
δ = {R / γ (γ + 1)} · {ΔK / K} (2)
It is expressed by the approximate expression of Where R is the orbit radius at the time of withdrawal, γ is the Lorentz factor, K is the kinetic energy of the beam, and ΔK is the amount of energy increase per turn.

  As described above, in the cyclotron 1, in addition to the acceleration by the normal de-electrodes 5a and 5b, the charged particle beam is further accelerated in the acceleration cavity 31, so that the amount of energy increase ΔK per turn of the charged particle beam is increased. Will increase. According to the above formulas (1) and (2), the turn separation δ increases as ΔK increases, and as a result, the extraction efficiency η of the charged particle beam improves. In this case, even if the orbit radius R of the charged particle beam becomes small, the extraction efficiency η can be ensured, which contributes to the downsizing of the cyclotron 1.

Further, since the charged particle beam is locally accelerated, the size of the acceleration cavity 31 can be reduced, and the power consumption can be reduced. The harmonics of the RF cavity related to the de-electrodes 5a and 5b are 2 (f = 96 MHz), but the harmonics of the RF cavity related to the acceleration cavity 31 are 4 (f = 192 MHz). If the harmonics is 4, the same turn separation can be obtained (at the extraction position) even if the di voltage is low, so that the power consumption of the RF cavity can be reduced. The reason why the harmonics of the RF cavity related to the de-electrodes 5a and 5b is 2 is to facilitate the extraction of the charged particle beam from the internal ion source. Further, by adjusting the RF phase of the acceleration cavity 31, it is easy to increase the turn separation at the extraction position even when isochronism is lost. Also, when adopting another beam extraction method, precessional extraction, the amplitude of precession can be changed to some extent by adjusting the acceleration voltage around the horizontal tune passing through 1. Therefore, the acceleration voltage can be adjusted.

  Although one embodiment of the present invention has been described above, the present invention is not limited to the above-described embodiment, and may be modified without changing the gist described in each claim. For example, in the cyclotron 1 of the embodiment, the spiral orbit B of the charged particle beam has been described as being on the horizontal plane. However, the cyclotron of the present invention is arranged so that the orbit B is on the vertical plane. Also good.

  DESCRIPTION OF SYMBOLS 1 ... Cyclotron, 5a, 5b ... Di electrode, 21 ... Magnetic pole, 21a ... Convex part, 21b ... Concave part, 23 ... Magnetic pole, 23a ... Convex part, 23b ... Concave part, 25h ... Hill area, 25v ... Valley area, 27 ... Median Plane, 27a ... slit, 30 ... high frequency generator, 35 ... high frequency electrode, 37 ... frame, B ... circular orbit.

Claims (2)

A plurality of convex portions and a plurality of concave portions, which are provided in a pair across the circular orbit of the beam and are alternately arranged in the circumferential direction, each have a hill region sandwiched between the convex portions and the concave portions. A pair of magnetic poles that form a sandwiched valley region along the orbit,
A de-electrode provided in the valley region;
A high frequency generator for generating a high frequency electric field for accelerating the beam, arranged on the outer peripheral side in the radial direction of the circular orbit of the beam in at least one valley region other than the valley region provided with the de-electrode,
A cyclotron characterized by comprising: The high-frequency generator includes a metal high-frequency electrode provided on a median plane, and a metal frame that covers the periphery of the high-frequency electrode when viewed in a cross section orthogonal to the traveling direction of the beam. ,
In the frame,
The cyclotron according to claim 1, wherein a slit extending on the median plane is provided on an inner peripheral side with respect to the high-frequency electrode.

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