JPH0613200A - High frequency accelerating cavity for charged particle accelerator - Google Patents

Abstract

PURPOSE:To improve the instability of a beam in a higher-order resonance mode by setting the sum total of cylindrical part length in an equator position on the inside surface of an accelerating cavity and a radius of curvature of an adjacent spherical surface constant, and making a ratio of the length to the radius of curvature differ from each other, and changing a cavity shape. CONSTITUTION:In a high frequency accelerating cavity 1, an inside surface shape is formed in an approximately spherical shape so as to be axially symmetrical with the charged particle beam axis 2 as its center. An input coupler installation hole 3 to supply acceleration energy to an equator position and a tuner installation hole 4 to adjust resonance frequency of the cavity are provided. When cavity length is 2XL1, straight line length in the equator position is L2, a radius of the cavity is R and a beam pipe diameter is R3, and when R1 and R2+L2 among these parameters are fixed, the resonance frequency in a higher-order resonance mode can be changed without changing an impedance value largely in a main higher-order resonance mode.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high frequency accelerating cavity used in a charged particle accelerator for accelerating charged particles such as electrons.

[0002]

2. Description of the Related Art A high-frequency accelerating cavity is used as an accelerating means of a charged particle accelerator used for the purpose of accelerating a charged particle beam of electrons or the like to a high energy on an orbit to obtain synchrotron radiation. Used. In the high-frequency acceleration cavity, the energy of the charged particles that accelerate and accumulate is GeV.
In the case of a charged particle accelerator of a class, a plurality of high-frequency accelerating cavities are required in one accelerator due to the limitation of the acceleration voltage due to discharge. All of the high-frequency accelerating cavities in which the multiple units are installed are excited in the acceleration mode with the same resonance frequency, but the high-frequency accelerating cavities have a higher-order resonance mode together with the acceleration mode. The high-order resonance mode causes destabilization of charged particles to be accelerated, exerts a force perpendicular to the traveling direction, and causes beam instability, and is not suitable for an accelerator that accelerates and accumulates a high-energy beam. It becomes a serious problem. In particular, when using a plurality of accelerating cavities as described above, if accelerating cavities with the same specifications are arranged, the frequencies of the higher-order resonance modes match, so that the timing of the adversely affecting force is In synchronization with the lap,
There is a possibility that the influence on the charged particles will be maximized. As a conventional means for improving the beam instability due to the higher-order resonance mode, the length of the blind cover that closes the port opened in the high-frequency acceleration cavity is adjusted. For example, according to a report by the Institute for High Energy Physics in September 1988, the beam instability of the 2.5 GeV electron storage ring at the synchrotron radiation experimental facility at the same site was determined by the four high-frequency waves used at the facility. It is reported that it was significantly improved by attaching a blind cover to the acceleration cavity and adjusting the length. According to the same document, the blind cover 26, 27 is attached to the high-frequency acceleration cavity 20 as shown in FIG. This figure shows a cross section of the high-frequency acceleration cavity 20 as seen from the beam downstream side. The center circle of the high-frequency acceleration cavity 20 shows a beam hole 25, and the input section 2 for supplying acceleration power to the equator position.
1 and a tuner unit 22 that adjusts the resonance frequency of the high-frequency acceleration cavity by changing the shape of the acceleration cavity 20. Ports 23 and 24 are provided.
A blind cover 26, 27 is attached to the. Blind cover 2
6 and 27 form a part of the accelerating cavity 20, it is explained that by adjusting this length, the resonance frequency of the higher-order resonance mode is adjusted and the beam instability due to the accelerating cavity is improved. ing.

[0003]

However, the blind cover by the above-mentioned conventional means is a cylindrical sealing member that is inserted from a port opened at the equator position of the acceleration cavity to close the port opening. Since it forms a part in the circumferential direction, it has the following problems. (1) Since the rate of change of the resonance frequency is different for each higher-order resonance mode, the length of the blind cover becomes large, especially for modes with a small rate of change, which causes a reduction in the acceleration power efficiency of the accelerating cavity. . (2) Since the setting of the blind cover length is determined by estimating the resonance frequency of the higher-order resonance mode during steady operation,
Designing a blind cover is difficult because it is difficult to estimate the resonance frequency of the higher-order resonance mode when a large amount of power is input. The present invention has been made in view of the above problems, and provides a high-frequency accelerating cavity of a charged particle accelerator for improving beam instability due to a higher-order resonance mode due to a shape change of the accelerating cavity without using a blind cover. With the goal.

[0004]

In order to achieve the above-mentioned object, the means adopted by the present invention is that a plurality of high-frequency accelerating cavities excited at the same resonance frequency are arranged in a charged particle accelerating device, and are placed at the equator position. An input part for supplying high frequency energy is provided in a cavity formed in a substantially spherical surface including a cylindrical part,
In a high-frequency acceleration cavity of a charged particle accelerator for accelerating charged particles passing through the cavity by applying high-frequency energy, the length of the cylindrical portion at the equator position on the inner surfaces of the plurality of high-frequency acceleration cavities and the length of the cylindrical portion adjacent to the cylindrical portion. The ratio of the length of the cylindrical portion to the radius of curvature of the spherical surface is made different by making the total sum of the radius of curvature of the spherical surface constant and the radius of curvature of the spherical surface different from each other.

[0005]

According to the present invention, the shape parameters for determining the inner surface shape of the high-frequency accelerating cavities provided in a plurality of charged particle accelerators are changed based on a predetermined change criterion, and the resonance frequency of the acceleration mode is kept the same, Since the resonance frequency of the higher-order resonance mode can be changed individually, the resonance frequency of the higher-order resonance mode can be different for each of the plurality of high-frequency acceleration cavities. When the resonance frequencies of the higher-order resonance modes of the multiple high-frequency acceleration cavities are the same, the timing of the force that the higher-order resonance modes adversely affect the charged particle beam is synchronized with the orbit of the charged particles and Although the problem that the influence is maximized occurs, in the case of the present invention, the influence is reduced because the resonance frequencies of the higher-order resonance modes are different. The change of the shape of the inner surface of the cavity is performed by making the sum of the length of the cylindrical portion at the equator position on the inner surface of the cavity and the radius of curvature of the spherical surface adjacent to the cylindrical portion constant, and the length of the cylindrical portion and the radius of curvature of the spherical surface. It is realized by making the ratio of each different. As described above, since the change in the volume with respect to the change in the shape parameter of the inner surface of the cavity is large, the change in the resonance frequency of the higher-order resonance mode is larger than that in the conventional technique, and the adjustment is easy. In addition, since the structure is axisymmetric, it is easy to design multiple accelerating cavities and the stability of the accelerating beam is excellent.

[0006]

Embodiments of the present invention will be described below with reference to the accompanying drawings for the understanding of the present invention. The following embodiments are examples of embodying the present invention and do not limit the technical scope of the present invention. 1 is a cross-sectional view of a high-frequency acceleration cavity according to an embodiment of the present invention, FIG. 2 is a pattern diagram of inner surface shape change showing 1/4 of the inner surface shape in the cross-sectional view shown in FIG. 1 together with shape parameters, and FIG. FIG. 4 is a graph showing the dependence of the impedance values of the acceleration mode and the main higher-order resonance modes on the cavity inner surface shape. Figure 1
In the high frequency acceleration cavity 1, the inner surface is formed in a substantially spherical shape in axial symmetry about the charged particle beam axis 2 as a central axis, and an input coupler for installing an input coupler for supplying acceleration energy to the equator position is used. An installation hole 3 and a tuner installation hole 4 for installing a tuner for adjusting the resonance frequency of the cavity are opened. Since the acceleration cavity having such a shape can reduce the maximum electric field value on the inner surface, it is widely adopted as a high-frequency acceleration cavity using a superconducting material. In addition, this accelerating cavity has a feature that the impedance value of the higher-order resonance mode can be reduced compared to the accelerating cavity having a reentrant nose cone. The purpose of realizing the high-frequency acceleration cavity 1 in the present embodiment is to distribute the coupling impedance, which is an adverse effect of the higher-order resonance mode peculiar to the high-frequency acceleration cavity 1 on charged particles, over a wide frequency range, and to realize the charged particle accelerator. When a plurality of units are installed, the resonance frequency of the higher-order resonance mode is changed by changing the parameter that determines the inner surface shape of each high-frequency acceleration cavity 1 according to a predetermined change criterion. By the way, since the resonance frequencies of the higher-order resonance modes of the respective high-frequency acceleration cavities 1 in which a plurality of units are installed are different, the timing of the force that adversely affects the charged particles due to the coincidence of the frequencies of the higher-order resonance modes is the charged particle. It is not synchronized with the orbit of the, and the adverse effects of higher-order resonance modes are reduced. At this time, since the shape of the inner surface of the high-frequency acceleration cavity 1 determines the magnitude of the impedance value of the resonance mode, it is necessary to change only the resonance frequency without increasing the impedance value of the higher-order resonance mode.

A specific embodiment for changing the shape of the inner surface of the high frequency acceleration cavity 1 will be described below. FIG. 2 shows an inner surface shape pattern of a quarter portion of the high frequency acceleration cavity 1 shown in FIG. 1 and is shown together with each parameter for determining the inner surface shape. In FIG. 2, the inner surface of the cavity is formed by an arc having an outer radius R1 and an inner radius R2, and a straight line connecting the arc radii R1 and R2. Further, the cavity length is 2 * L1, the straight line length at the cavity equator is L2, the cavity radius is R, and the beam pipe diameter is R3. By changing one of these six types of shape parameters, the resonance frequency peculiar to the cavity changes. It is already known that the influence of changes in each shape parameter on the cavity impedance of this shape tends to be as shown in FIG. That is, the dependence of the shape parameters R1 and (R2 + L2) on the typical resonance mode is high. Therefore, if the shape parameters R1 and (R2 + L2) are fixed, it is possible to change the resonance frequency of the higher-order resonance mode without significantly changing the impedance value of the main higher-order resonance mode. Incidentally, in FIG. 3, (a) in FIG. 3 is a TM ₀₁₀ mode which is an acceleration mode,
(B), (c), and (d) are high-order resonance modes TM ₀₁₁ ,
It shows the shape parameter dependence of the impedance value in each of TM ₁₁₀ and TM ₁₁₁ modes. Using the change data of each higher-order resonance mode with respect to the shape parameter of the inner surface of the cavity, the resonance frequencies of the respective acceleration modes of the high-frequency acceleration cavities 1 to be installed in the charged particle accelerator are set to be the same, and A calculation example of setting the above-mentioned shape parameter that makes the resonance frequencies of the higher-order resonance modes different is shown below. The following calculation is a setting example for the high frequency acceleration cavity 1 in the 500 MHz band. R1 = 3 cm, L1 in FIG.
= 15 cm and R3 = 5 cm were fixed. R which is the cavity diameter
_Has a resonance frequency of 5 in the TM ₀₁₀ mode, which is the acceleration mode.
It was adjusted to be 00 MHz. Then, the characteristics of the main higher-order resonance modes were investigated when the sum of R2 and L2 was fixed at 11 cm and R2 was changed to 8 cm and 10 cm. As shown in Table 1, the results show that the resonance frequency is 5 MHz for three high-order resonance modes with high impedance values, with almost no change in impedance value.
I was able to change it.

[Table 1] For example, when the high-frequency acceleration cavity 1 according to the present embodiment is used in a charged particle accelerator that requires two high-frequency acceleration cavities,
The resonance frequencies of the acceleration modes of the two high-frequency acceleration cavities 1 are the same, and one has an inner surface shape of R2 = 8 cm, L2 = 3 cm,
If the other one has an inner surface shape of R2 = 10 cm and L2 = 1 cm, the resonance frequencies of the higher-order resonance modes that make the accelerated charged particle beam unstable are different from each other. The adverse effect on the beam is reduced.

[0008]

As described above, according to the present invention, the shape parameter that determines the inner surface shape of the high-frequency accelerating cavities provided in the charged particle accelerating apparatus is changed based on a predetermined change criterion, and resonance of the acceleration mode is achieved. Since the resonance frequencies of the higher-order resonance modes can be individually changed while keeping the frequencies the same, the resonance frequencies of the higher-order resonance modes of the plurality of high-frequency acceleration cavities can be made different. When the resonance frequencies of the higher-order resonance modes of the multiple high-frequency acceleration cavities are the same, the timing of the force that the higher-order resonance modes adversely affect the charged particle beam is synchronized with the orbit of the charged particles and Although the problem that the influence is maximized occurs, in the case of the present invention, the influence is reduced because the resonance frequencies of the higher-order resonance modes are different. The shape of the inner surface of the cavity is changed by making the sum of the length of the cylindrical portion at the equator position on the inner surface of the cavity and the radius of curvature of the spherical surface adjacent to the cylindrical portion constant, and by changing the length of the cylindrical portion and the radius of curvature of the spherical surface. It is realized by making the ratio of each different. As described above, since the change in the volume with respect to the change in the shape parameter of the inner surface of the cavity is large, the change in the resonance frequency of the higher-order resonance mode is larger than that in the conventional technique, and the adjustment is easy.
In addition, since the structure is axisymmetric, it is easy to design multiple accelerating cavities and the stability of the accelerating beam is excellent. Particularly in a medium- or large-sized charged particle accelerator, the charged particle beam can be stably accelerated, and the emittance is low and a large current can be accumulated.

[Brief description of drawings]

FIG. 1 is a sectional view of a high-frequency acceleration cavity according to an embodiment of the present invention.

FIG. 2 is a pattern diagram of a cavity inner surface shape of a portion corresponding to ¼ of FIG.

FIG. 3 is a graph showing the dependence of the impedance values of the acceleration mode and the main higher-order resonance modes on the cavity inner surface shape.

FIG. 4 is a sectional view of a high-frequency acceleration cavity according to a conventional example.

[Explanation of symbols]

 1 ... High-frequency accelerating cavity 3 ... Hole for installing input coupler L2 ... 1/2 of cylindrical length R2 ... Radius of curvature of spherical surface

Claims (1)

[Claims] 1. A plurality of high-frequency accelerating cavities excited at the same resonance frequency are arranged in a charged particle accelerator, and high-frequency energy is supplied to the cavities formed in a substantially spherical surface including a cylindrical portion at an equator position. In a high frequency accelerating cavity of a charged particle accelerating device which is provided with an input part of ## EQU1 ## and accelerates charged particles passing through the cavity by applying high frequency energy, A high-frequency acceleration of a charged particle accelerator, characterized in that the ratio of the length of the cylindrical portion to the radius of curvature of the spherical surface is made different by keeping the sum of the radius of curvature of the spherical surface adjacent to the cylindrical portion constant. cavity.