JP4869374B2 - H-mode drift tube linear accelerator - Google Patents

Description

  In the present invention, a TE mode (H mode) that generates a magnetic field in the resonator longitudinal direction is excited in the resonator, and a current induced by the generated magnetic field flows through the stem from the resonator wall surface, and a plurality of currents are arranged in the acceleration axis direction. The present invention relates to an H-mode drift tube linear accelerator that accelerates charged particles by an accelerating electric field generated between drift tube electrodes.

  An H-mode drift tube linear accelerator (DTL) is induced in a resonator by arranging a plurality of drift tube electrodes in a resonator (sometimes serving as a vacuum vessel) that generates an electromagnetic field. An accelerating electric field is generated between adjacent drift tube electrodes (hereinafter referred to as a gap) by a magnetic field to accelerate charged particles. The drift tube electrode has a hollow cylindrical shape inside, and the charged particles are given acceleration energy by the electric field generated at the cylindrical thick part on the opposite surface of the drift tube electrode. Pass through.

By synchronizing the timing at which the high-frequency accelerating electric field is generated in the gap and the timing at which charged particles having a certain energy appear in the gap, the accelerating electric field generated one after another is efficiently applied to the charged particle energy. The distance from the center of the drift tube electrode to the center of the next drift tube electrode (referred to as the cell length) increases from the low energy side to the high energy side according to the energy of the charged particles.
The operating frequency is the resonant frequency of the resonator, and in order to change all the accelerating electric fields completely in the same phase, the cell length L is generally a value proportional to a half-integer multiple of the wavelength corresponding to the resonant frequency. It is necessary to be. The relationship between the cell length L and the resonance frequency is shown in Equation (1). However, the change in the particle velocity at the gap is ignored, and the average value is βs. λ is the wavelength at the resonance frequency.

The kinetic energy ΔW obtained by the charged particles moving in parallel with the acceleration axis passing through one cell of the linear accelerator is expressed by Equation (2).
However, in Equation (2), L is the cell length, q is the valence, g is the gap length, E _g is the average electric field strength generated in the gap, φ _S is the synchronous phase, and T is a charged particle called transit time factor. This is a factor representing the effect of changing the phase of the high-frequency voltage while passing through the gap.

In designing the arrangement of the drift tube electrodes, first, E _g g in Equation (2), that is, the acceleration voltage V is set. In general, E _g is made the same in all cells to eliminate electric field concentration in a certain cell, and the generated electric field is efficiently used for acceleration. If the average electric field strength according to the gap position is allowed, the maximum electric field is limited from the viewpoint of preventing discharge, so that the average electric field strength at other locations is lowered, resulting in a decrease in acceleration energy as a whole, and acceleration efficiency is reduced. Getting worse. In order to prevent this, it is preferable to make the average electric field strength as uniform as possible.
Even if the average electric field strength is uniform, if the gap length changes, the voltage determined by the product of both changes, so if the gradient voltage distribution that increases the gap length according to the acceleration energy is adopted, the acceleration energy increase is increased even on the output side. It can be maintained and the particles can be accelerated efficiently. In this case, the cell length and the gap length have a proportional relationship.

Here, E _g is determined using the kill-patrick discharge limit as an index. Research on the discharge limit of the electrode in vacuum was made by Kilpatrick in 1957, and the relational expression called the Kilpatrick discharge limit was clarified. That is, as the discharge limit due to the high-frequency electric field, the high-frequency frequency f (MHz) and the discharge limit E (MV / m) are expressed by a relational expression such as Expression (3).

In fact, it has also been demonstrated that an electric field exceeding the limit of the kill-patrick discharge can be applied stably. This is because a formula relating to the limit of the kill-patrick discharge was issued in 1957, and the technology such as vacuum and electrode surface at that time was lower than that at present, and was clarified by recent technological progress. However, Kilpatrick discharge limit, has become one of the criteria weigh the extent of even the current E _g.

As shown above, the drift tube electrode array assumes the electric field distribution generated in the gap with a rectangular waveform of the average electric field strength E _g, which assumes a voltage multiplied by the gap length to be accelerated in accordance with (2) The energy of the charged particles is calculated, converted into a velocity, and the cell length of equation (1) is determined.
However, since the discharge at the drift tube electrode which is actually a problem depends not only on _Eg but also on the surface electric field strength at the drift tube electrode, the drift tube electrode shape and the generated voltage are set to 2 to prevent discharge. It is necessary to verify by dimensional electrostatic field analysis.

  As one of the measures against electric discharge, the drift tube electrode is subjected to R chamfering at all corners to prevent electric discharge. The R chamfer dimension needs to be set so that the surface electric field intensity causing the discharge does not exceed the discharge reference value.

Examples of conventional techniques related to the shape of the drift tube electrode include, for example, increasing the outer diameter of the drift tube at the incident portion, decreasing the inner diameter of the drift tube at the incident portion, and incident in an H-mode drift tube linear accelerator. A technology that increases the capacitance between the drift tubes of the incident part, either by reducing the radius of curvature of the drift tube of the part, or any combination thereof, thereby increasing the first few gap voltages of the incident part Is disclosed (for example, see Patent Document 1).
In addition, to reduce the electric field that does not contribute to the accelerating electric field generated between the drift tubes, the drift tube surface where the accelerating electric field is generated is reduced by reducing the thickness at the end of the drift tube and making the cross-sectional shape close to an isosceles trapezoid. Has disclosed a drift tube type linac (see, for example, Patent Document 2).

Japanese Patent Laying-Open No. 2006-351233 (page 6, FIG. 5) Japanese Patent Laid-Open No. 11-121199 (2nd page, FIG. 12)

The H-mode drift tube linear accelerator shown in Patent Document 1 is designed to increase the first few gap voltages of the incident part by making the shape of the drift tube as described above. The drift tube type linac shown in FIG. 2 adopts the above-described shape to reduce the electric field that does not contribute to the formation of the acceleration electric field, thereby improving the power efficiency. In either case, in order to prevent discharge at the electrode end, the electrode end is subjected to R chamfering, but no consideration is given to actively reducing the surface electric field strength.
In such an H-mode type drift tube linear accelerator, when the cell length and the gap length increase in accordance with the energy of the charged particles, the electric field distribution that is originally the maximum at the center of the gap becomes coarse and dense in the gap. As a result, the electric field distribution is maximized near the end of the drift tube electrode rather than the center of the gap, and as a result, the surface electric field strength at the end of the drift tube electrode increases even though the acceleration energy gain obtained in the gap is the same. There is a problem in that the discharge limit may be exceeded, leading to discharge.

  The present invention has been made to solve the above-described problems, and reduces the surface electric field strength of the drift tube electrode that causes discharge as much as possible while maintaining the high-frequency acceleration electric field strength generated between the drift tubes. And it aims at obtaining the H mode type drift tube linear accelerator which can be operated in the stable state.

  In the H-mode type drift tube linear accelerator according to the present invention, a plurality of cylindrical drift tube electrodes supported by a stem are arranged with a predetermined gap on an acceleration axis in a resonator for exciting a TE mode. In the H-mode drift tube linear accelerator that accelerates the charged particle beam by generating an electric field between the drift tube electrodes, the drift tube electrode is subjected to R chamfering on the outer diameter side and the inner diameter side of the end face, The R chamfer on the outer diameter side of the drift tube electrode end located at both ends of the gap portion having a double-crested electric field distribution having a peak on both sides of the center portion of the gap portion is more than the R chamfer on the inner diameter side. Has a large radius of curvature, and the drift tube thick part where the R chamfer end on the inner diameter side and the R chamfer end on the outer diameter side are smoothly connected It is those that have been made.

  According to the H-mode drift tube linear accelerator of the present invention, the R chamfer on the outer diameter side of the end portion of the drift tube electrode located at both ends of the gap portion having the electric field distribution having a two-peak shape is more than the R chamfer on the inner diameter side. The drift tube thick part has a large radius of curvature and the end part of the R chamfer on the inner diameter side and the end part of the R chamfer on the outer diameter side are smoothly connected, so the electric field distribution in the gap H-mode drift tube that can suppress discharge at the electrode part and operate in a stable state even in a wide gap where the surface electric field strength of the drift tube electrode increases and becomes greater near the end of the drift tube electrode than the center. A linear accelerator can be obtained.

It is sectional drawing which shows the H mode type drift tube linear accelerator by Embodiment 1 of this invention. It is sectional drawing which shows two drift tube electrodes of the output side among the drift tube electrodes used for FIG. It is sectional drawing which shows drift tube electrodes other than the electrode shown in FIG. 2 among the drift tube electrodes used for FIG. It is an acceleration electric field distribution map of the H mode type drift tube linear accelerator of FIG. It is a figure which shows the surface electric field strength in the drift tube electrode of the H mode type drift tube linear accelerator of FIG. It is sectional drawing which shows the other example of the drift tube electrode of the H mode type drift tube linear accelerator by Embodiment 1 of this invention. It is sectional drawing which shows the H mode type drift tube linear accelerator by Embodiment 2 of this invention. It is an acceleration electric field distribution map of the H mode type drift tube linear accelerator of FIG.

Embodiment 1 FIG.
Hereinafter, a description will be given based on the drawings. First, the configuration of the H-mode drift tube linear accelerator according to the first embodiment will be described with reference to FIG. The figure is a sectional view showing an IH (Interdigital-H) type linear accelerator which is one of H-mode type drift tube linear accelerators.

Drift tube electrodes 2a to 2e for accelerating charged particles are placed in a resonator 1 for generating an electromagnetic field mode (sometimes also serving as a vacuum vessel) from an incident side 3 which is a low energy side to a high energy side. Are arranged in the axial direction of the acceleration shaft 5 toward the emission side 4, and are supported by stems 6 extending alternately from the resonator 1 for each cell, with a predetermined gap 7 on the acceleration shaft 5. Installation is fixed. Further, an incident side end drift tube 8 a is arranged on the incident side 3 of the resonator 1, and an emission side end drift tube 8 b is arranged and fixed on the acceleration shaft 5 in the same manner. Note that the number of electrodes is an example, and is not limited to this figure.
The gap 7 is gradually widened from the incident side 3 toward the emission side 4, and the electrode lengths of the drift tube electrodes 2a to 2e are also gradually increased. That is, the case where the gradient voltage distribution in which the cell length is sequentially increased toward the emission side 4 is employed is shown.

  The drift tube electrode of the present embodiment will be described in detail later, but in order to suppress discharge at the electrode portion, a drift tube electrode 2e that is adjacent to and faces the emission side end drift tube 8b and forms a gap; The drift tube electrode 2d that forms a pair with the drift tube electrode 2e is characterized in that the shape of the two bodies is different from the shapes of the other drift tube electrodes 2a to 2c. Hereinafter, the characteristics of these electrode shapes will be described with reference to the drawings.

FIG. 2 is a cross-sectional view showing two electrode shapes of the drift tube electrodes 2d and 2e. Since the purpose is to explain the shape of the end, the length direction is shown in a reduced scale.
The outer diameter D of the drift tube electrode is determined from the inner diameter d of the drift tube electrode, which is the aperture through which the beam passes, and the thickness T that determines the capacitance generated in the gap.
The corners on the outer diameter side and inner diameter side of the end face are rounded to prevent discharge. In the following description, the radius of curvature of the R chamfer on the inner diameter side of the drift tube electrode is referred to as the inner diameter side R chamfer dimension, and the radius of curvature of the R chamfer on the outer diameter side is referred to as the outer diameter side R chamfer dimension.
In addition, the “end face” is a name for the side surface of the drift tube, and the portions where the corners of the end face are chamfered are collectively referred to as the “end part”.

The inner diameter side R chamfer dimension r is as small as possible so as not to unnecessarily reduce the electric field strength in the acceleration axis direction, and is the same for all the drift tube electrodes.
On the other hand, the outer diameter side R chamfer dimension R1 is set to a value larger than the inner diameter side R chamfer dimension r, and has a shape that is continuously and smoothly connected from the inner diameter side R chamfering end to the outer diameter of the drift tube electrode. More specifically, the inner diameter side R chamfer is formed in a range of 90 degrees with a radius of curvature r continuously from the surface of the inner diameter d, and the outer diameter side R chamfer is continuously set to 90 with a radius of curvature R1. The “drift tube thick portion” is formed in a range of degrees, and its end portion is smoothly continuous with the surface of the outer diameter D. The center point of both radii of curvature is on an axis 9 parallel to the acceleration axis 5.
However, FIG. 2 shows the best mode, and the connecting portion between the inner diameter side R chamfer and the outer diameter side R chamfer has a straight line portion (plane portion) perpendicular to the acceleration shaft 5 in consideration of workability and the like. Although it may be provided, it is desirable to provide a smooth continuous surface with as few straight portions as possible.

FIG. 3 is a cross-sectional view showing electrode shapes of the drift tube electrodes 2a to 2c, 8a and 8b other than the two drift tube electrodes 2d and 2e on the emission side described in FIG.
The inner diameter d, the wall thickness T, and the outer diameter D are the same as in FIG. All corners are R chamfered to suppress discharge, and as described above, the R chamfer dimension r on the inner diameter side is the same as the drift tube electrodes 2d and 2e in FIG. It is made as small as possible.
On the other hand, the outer diameter side R chamfering dimension R2 is larger than the inner diameter side R chamfering dimension r, but the surface electric field strength at the R chamfered portion only needs to be equal to or less than the discharge limit standard. A straight thick portion 10 perpendicular to the acceleration shaft 5 exists between the inner diameter side R chamfering end portion and the outer diameter side R chamfering end portion, which is smaller than the R chamfer dimension R1. Specific dimensions will be described later.

  Here, the entire cavity of the accelerator 1 is equivalent to the LC resonance circuit, and if it is equivalent to the inductance L in the cavity of the accelerator 1 in the region where the magnetic field is generated, mainly equivalent to the capacitance C in the gap, The resonance frequency F can be expressed as in Equation (4).

The drift tube electrode has a hollow cylindrical shape inside, and the capacitance is determined by the thick portion. The gap capacity C can be expressed as C∝S / g using the gap length g and the drift tube electrode thickness effective sectional area S. Accordingly, when the dimension of the thick portion of the drift tube electrode is reduced, the capacitance C is reduced proportionally, and in order to obtain a constant resonance frequency, it is necessary to increase the region where the magnetic field is generated, that is, the diameter of the resonator. Further, when the thick portion is reduced, the surface electric field strength related to the discharge is remarkably increased.
Therefore, in order to prevent the surface electric field strength from exceeding the discharge reference value, the thickness of the drift tube electrode is set to be larger than at least twice the inner diameter side R chamfering dimension r, and the gap where the surface electric field strength is increased. In the portion, the shape of the electrode end is as shown in FIG.

Next, the operation will be described.
Here, in the configuration of FIG. 1, the dimensions of each part of the drift tube electrode of FIGS. 2 and 3 are as follows as an example.
The number of cells is 6, the inner diameter d of the drift tube electrode is 14 mm, the outer diameter D is 36 mm, the inner diameter side R chamfer dimension r is 2 mm, and the outer diameter side R chamfer dimension is R1 for the drift tube electrodes 2d and 2e shown in FIG. = 9 mm, R2 = 5 mm for the drift tube electrodes 2a to 2c, 8a and 8b shown in FIG.
The synchronous phase is constant at −30 degrees, the electric field strength is 1.6 times the kill-patrick discharge limit, each cell is constant, and the voltage is a gradient voltage distribution that increases according to the charged particle energy.

FIG. 4 is a view showing an acceleration electric field distribution generated in the H-mode drift tube linear accelerator under the above-described conditions. The acceleration electric field distribution according to the first embodiment is indicated by a solid line as “present invention”, and as a “comparative example”, the case where all the drift tube electrodes have the shape of FIG. 3 (r = 2 mm, R2 = 5 mm) is indicated by a broken line. ing.
Here, the accelerating electric field strength on the vertical axis represents Emod = √ obtained from each electric field component Ex, Ey, Ez, where the acceleration axis direction is the Z axis, the stem direction is the Y axis, and the direction orthogonal thereto is the X axis. It means (Ex ² + Ey ² + Ez ² ). The horizontal axis indicates the acceleration axis direction Z of the resonator. A chevron waveform appears in the portion corresponding to each gap.

As shown in FIG. 4, the energy of the charged particles increases as it approaches the emission side, so that the gap distance is also increased. As shown, the shape changes to a two-crested shape in which a valley is generated at the center of the gap. For this reason, even if the voltage obtained by integrating the electric field distribution in the gap section is the same in the case of the mountain shape and the case of the two mountain shapes (that is, the energy gain given to the charged particles is the same) Even so, the “surface” field strength generated at the surface of the drift tube electrode forming the gap will be different for the ridge shape and the two ridge shape.
The electric field intensity E1 generated in the first gap on the incident side 3 and the electric field intensity E6 generated in the last gap on the emission side are not limited to the direction of the acceleration axis 5 at the resonator end. Since it also occurs in the direction around the resonator 1, it is reduced compared to other cells due to this influence.

  As can be seen from FIG. 4, the electric field distribution has two peaks in the gap formed by the two drift tube electrodes 2d and 2e on the exit side excluding the exit end drift tube 8b. By adopting the electrode shape as shown in FIG. 2 for the drift tube electrodes 2d and 2e forming the gap in which the electric field distribution having the two mountain shape is generated, in this part, the solid line is shown in comparison with the comparative example shown by the broken line. However, the difference between the peak value of the electric field intensity near the end of the drift tube electrode and the valley part of the electric field intensity at the center of the gap is reduced, and the peak value is also low. However, as described above, the voltage, which is a value obtained by integrating the electric field distribution over the gap length by the distance, is constant, and there is no change in the energy to which the charged particles are applied.

  As in the comparative example, when the difference between the peaks and valleys is large and the peak value of the electric field strength is high, the surface electric field strength exceeds the reference value, and there is a problem that discharge occurs between the gaps. By adopting the electrode shape as in this embodiment, the peak value of the electric field strength in the vicinity of the electrode end can be kept below the discharge reference value.

FIG. 5 is a diagram in which the surface electric field strength of the drift tube electrode is compared with the discharge reference electric field value, and the “present invention” is compared with the “comparative example” in the same manner as in FIG. 4.
The outer diameter side R chamfer dimension R1 of the two drift tube electrodes 2d and 2e (electrode numbers 5 and 6) installed on the emission side 4 is larger than the other drift tube electrodes (2a to 2c, 8a and 8b). As a result, the effective cross-sectional area of the thick portion of the drift tube electrode is reduced, and the capacitance on the exit side 4, which is the high energy side, is reduced when viewed as a whole in the H-mode type drift tube linear accelerator. Therefore, as a result of equalizing the electric field strength generated in each gap by increasing the resonator diameter on the high energy side by an amount corresponding to the decrease of C according to the equation (4), the result is that other than the two bodies on the high energy side Although the surface electric field strength at the drift tube electrodes (electrode numbers 1 to 4) slightly increases, the surface electric field strength at the drift tube electrodes (electrode numbers 5 to 7) installed on the high energy side decreases. It is below the discharge reference value.

Here, the surface electric field strength of the drift tube electrodes (electrode numbers 1 to 4) installed on the low energy side has a short gap length, so that an electric field distribution in a mountain shape is generated instead of two peaks as seen in FIG. Therefore, as shown in FIG. 5, the influence of the two drift tube electrodes 2d and 2e (electrode numbers 5 and 6) originally lower than the discharge reference value and installed on the high energy side is small and causes no problem.

  In the above description, only two electrode shapes, the drift tube electrode 2e adjacent to the emission side end drift tube electrode 8b and the drift tube electrode 2d paired with the drift tube electrode 2e, are made different from those shown in FIG. However, the present invention is not limited to these two bodies, but for a plurality of drift tube electrodes in which the electric field distribution becomes two peaks and the surface electric field strength increases because the cell length becomes longer according to the energy of the charged particles. You may comprise so that the drift tube electrode shown to may be applied.

Further, the drift tube electrode end portions located at both ends of the gap portion having the electric field distribution having a two-crest shape are the same shape and have a thick drift tube as shown in FIG. 3, but the electric field distribution becomes two peaks. Even if it is a drift tube electrode which forms a gap, as long as only one electrode surface exceeds the discharge limit, the electrode shape as shown in FIG. 6 may be used. That is, the drift tube electrode is a composite type drift tube electrode as shown in FIGS.
In FIG. 6, the shape of the electrode end facing the side where the surface electric field strength is high (usually the high energy side) is the inner diameter side R chamfer end portion and the outer diameter side having a larger curvature radius than that in FIG. The shape of the electrode end facing the side where the surface electric field strength is low (usually the low energy side) is the same as in FIG. It is a thing. As a result, in addition to the effect of reducing the surface electric field strength, the capacitance generated between the drift tubes is not substantially reduced unnecessarily, and the change of the resonator diameter for adjusting the capacitance can be reduced. Is obtained.

  As described above, according to the H-mode type drift tube linear accelerator of the first embodiment, the drift tube electrode has R chamfering on the outer diameter side and the inner diameter side of the end face, and the electric field distribution is the gap portion. The R chamfer on the outer diameter side of the end portion of the drift tube electrode located at both ends of the gap portion having the electric field distribution of two peaks having peaks on both sides of the central portion of the center portion has a larger radius of curvature than the R chamfer on the inner diameter side In addition, the drift tube thick part is formed in which the end part of the R chamfer on the inner diameter side and the end part of the R chamfer on the outer diameter side are smoothly connected, so the electric field distribution in the gap is closer to the drift tube electrode than in the center of the gap. Even if the surface electric field strength at the drift tube electrode increases and the discharge at the electrode section can be suppressed, the H-mode drift can be operated in a stable state. It can provide over blanking linear accelerator.

  In addition, since the drift tube electrode end portions located at both ends of the gap portion having the two-peak electric field distribution having peaks on both sides of the central portion of the gap portion are the drift tube thick portions having the same shape, the electric field distribution Even if it is a drift tube electrode that forms a gap with two peaks, if the surface electric field strength at one end of the electrode does not exceed the discharge limit, the shape of the end on one side can be adjusted to that of the other electrode. The capacitance generated between the drift tubes is not substantially unnecessarily reduced, and the change in the resonator diameter for adjusting the capacitance can be reduced.

  In addition, the connecting surface between the R chamfering end on the inner diameter side and the R chamfering end on the outer diameter side is a curved surface that is continuously connected without having a flat portion, so that energy applied to charged particles is reduced. Thus, the surface electric field strength can be effectively reduced.

Furthermore, the drift tube electrode formed with the thick drift tube portion in which the end portion of the R chamfer on the inner diameter side and the end portion of the R chamfer on the outer diameter side are smoothly connected is adjacent to the end drift tube electrode on the output side. Since the drift tube electrode and the drift tube electrode paired with the drift tube electrode are used, the electric field distribution has a two-peak shape particularly in the case of an H-mode drift tube linear accelerator employing a gradient voltage distribution. The resulting increase in surface electric field strength can be effectively suppressed, and operation can be performed in a stable state.

Embodiment 2. FIG.
FIG. 7 is a sectional view showing an IH type linear accelerator according to the second embodiment. The same parts as those in FIG. 1 are denoted by the same reference numerals, and description thereof will be omitted.
The resonator according to the present embodiment is a resonator that employs the APF method as a beam axis component convergence method. In the first embodiment, the cell length is a gradient voltage distribution that is sequentially increased toward the emission side. However, in the APF method, the sign of the synchronization phase is oscillatingly changed in several cell cycles. Therefore, although the cell length increases in a period of several cells, the adjacent cell length does not necessarily increase according to the energy of the charged particles.

  Therefore, in the present embodiment, the pair of drift tube electrodes forming the longest gap length excluding the first gap as viewed in the axial direction of the acceleration shaft 5 is used as shown in FIG. 2 described in the first embodiment. A drift tube electrode is applied. In the case of FIG. 7, the drift tube electrodes 2 c and 2 d correspond to the electrodes, and the end shape of the drift tube electrodes 2 c and 2 d is continuous without the flat portion between the R-chamfer end on the inner diameter side and the R-chamfer end on the outer diameter side. The curved drift tube is thick and connected smoothly. The other end shapes of the drift tube electrodes (2a, 2b, 2e, 8a, 8b) are as shown in FIG. 3 of the first embodiment.

In the configuration of FIG. 7, the number of cells is 6, and the electrode dimensions and voltage conditions are as follows as an example.
The drift tube electrodes 2c and 2d to which the shape of FIG. 2 is applied have an inner diameter d = 14 mm, an outer diameter D = 36 mm, an inner diameter side R chamfer dimension r = 2 mm, and an outer diameter side R chamfer dimension R1 = 9 mm. The other drift tube electrodes to which are applied have the same inner diameter, outer diameter, and inner diameter side R chamfer dimension, but the outer diameter side R chamfer dimension R2 = 5 mm. In addition, a comparative example in which all the electrodes have the end shape (r = 2 mm, R2 = 5 mm) in FIG. 2 is used.
The synchronous phase is not constant because the APF method is applied, the electric field strength is 1.6 times the limit of the kill-patrick discharge, each cell is constant, and the voltage is a gradient voltage distribution that increases according to the charged particle energy.

FIG. 8 is a diagram showing an accelerating electric field distribution generated in the accelerator 1 under the above conditions. This embodiment is shown as a solid line as “present invention” and “comparative example” is shown as a broken line.
As can be seen from the figure, the portion where the electric field distribution is two peaks is not necessarily the high energy side, and the electric field distribution is two peaks at the longest gap length. Therefore, the shape of the electrode end portions of the pair of drift tube electrodes 2c, 2d forming the gap having the electric field distribution of the two peaks is made as shown in FIG. 2, and the surface electric field strength at the gap portion is compared with the broken line. Compared to the example, it can be seen that, as shown by the solid line, the difference between the valley at the center of the gap and the peak value on both sides is reduced and the peak value is lowered. By lowering the peak values on both sides, the electric field strength in the vicinity of the electrode end can be kept below the discharge reference value.

  In the above description, the electrode shape as shown in FIG. 2 is applied only to the pair of drift tube electrodes that form the longest gap length excluding the first gap. Without application, the present invention is also applied to a pair of drift tube electrodes that subsequently form a long gap length, that is, other electrodes in which the electric field distribution is doubled and the surface electric field strength is increased due to the long cell length. Such a configuration may be adopted. When the accelerator is increased in size and the number of drift tube electrodes is increased, the electric field distribution becomes two peaks at a plurality of gaps.

In addition, the connecting portion between the inner diameter side R chamfer and the outer diameter side R chamfer has been described as having a smooth curved surface without having a flat portion, but in consideration of workability and the like, a straight line perpendicular to the acceleration shaft 5 is used. A part (plane part) may be formed somewhat.
Further, in the case where the electric field distribution becomes two peaks and only one drift tube electrode surface exceeds the discharge limit, a composite type drift tube electrode as shown in FIG. May be. The end of the outer diameter side R chamfered dimension is arranged so as to face the higher surface electric field strength.

  As described above, according to the H-mode drift tube linear accelerator according to the second embodiment, the drift tube thick portion is formed in which the inner radius side R chamfer end and the outer radius side R chamfer end are smoothly connected. Since the drift tube electrodes formed as a pair of drift tube electrodes forming the longest gap length excluding the first gap when viewed in the axial direction of the acceleration axis, the energy of charged particles such as the APF method is not necessarily used. Accordingly, even when the cell length does not increase, it is possible to appropriately reduce the surface electric field intensity and suppress the discharge at the place where the cell length where the electric field distribution becomes two peaks is the longest.

  Further, the drift tube electrode having the drift tube thick portion as described above has a pair of drift tube electrodes that form the longest gap length excluding the first gap when viewed in the axial direction of the acceleration axis. Then, because it is a pair of drift tube electrodes that form a long gap length, even if the accelerator becomes larger, the number of drift tube electrodes increases, and the number of gaps where the electric field distribution becomes two peaks increases, Electric field intensity can be reduced to suppress discharge.

DESCRIPTION OF SYMBOLS 1 Resonator 2a-2e Drift tube electrode 3 Incident side (low energy side) 4 Outgoing side (high energy side)
5 Acceleration axis 6 Stem 7 Gap 8a Incident side end drift tube 8b Emission side end drift tube electrode 9 Axis parallel to acceleration axis 10 Straight wall thickness d Inner diameter D Outer diameter T Thickness r Inner diameter side R chamfer dimension R1, R2 Outer Radius side chamfer dimension E1 Field strength generated in the first gap E6 Field strength generated in the last gap.

Claims (6)

A plurality of cylindrical drift tube electrodes supported by a stem are arranged with a predetermined gap on an acceleration axis in a resonator for exciting the TE mode, and an electric field is generated between the drift tube electrodes. In an H-mode drift tube linear accelerator for accelerating a charged particle beam,
The drift tube electrode has a chamfered corner on the outer diameter side and the inner diameter side of the end face,
The R chamfering on the outer diameter side of the end portion of the drift tube electrode located at both ends of the gap portion having a two-peak electric field distribution having a peak on both sides of the central portion of the gap portion is the inner diameter side. A thicker drift tube having a radius of curvature larger than that of the R chamfer, and smoothly connecting the end portion of the R chamfer on the inner diameter side and the end portion of the R chamfer on the outer diameter side. An H-mode drift tube linear accelerator. The H-mode drift tube linear accelerator according to claim 1,
The drift tube electrode end portions positioned at both ends of the gap portion having a double-crested electric field distribution having peaks on both sides of the central portion of the gap portion are the drift tube thick portions having the same shape. An H-mode drift tube linear accelerator characterized by comprising: In the H mode type drift tube linear accelerator according to claim 1 or 2,
The connecting surface of the inner diameter side R chamfering end portion of the drift tube thick portion and the outer diameter side R chamfering end portion is a curved surface continuously connected without having a flat portion. H-mode drift tube linear accelerator. In the H mode type drift tube linear accelerator according to any one of claims 1 to 3,
The drift tube electrode formed with the thick portion of the drift tube is a pair of drift tube electrodes that form the longest gap length excluding the first gap when viewed in the axial direction of the acceleration axis. H-mode drift tube linear accelerator. The H-mode drift tube linear accelerator according to claim 4,
The drift tube electrode formed with the thick portion of the drift tube is a drift tube electrode adjacent to the end drift tube electrode on the emission side, and a drift tube electrode paired with the drift tube electrode. Modal drift tube linear accelerator. In the H mode type drift tube linear accelerator according to any one of claims 1 to 3,
The drift tube electrode formed with the thick portion of the drift tube has a pair of drift tube electrodes that form the longest gap length excluding the first gap when viewed in the axial direction of the acceleration axis, and subsequently An H-mode drift tube linear accelerator characterized by being a pair of drift tube electrodes that form a long gap length.

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