JP5307689B2 - Drift tube linear accelerator - Google Patents

Description

  The present invention provides a drift tube linear accelerator that supplies high-frequency power to a vacuum cylindrical resonator and accelerates charged particles with an electric field generated between electrodes (drift tubes) supported by rods (stems) in the cylindrical resonator. It is about.

  The drift tube linear accelerator is configured by arranging a plurality of hollow cylindrical drift tubes in a cylindrical resonator along the acceleration direction. A high frequency power is supplied into the cylindrical resonator, and a high frequency electric field generated between the drift tubes accelerates charged particles (for example, protons) along the acceleration direction. The arrangement of the drift tubes is designed so that charged particles are in the drift tubes when the direction of the high-frequency electric field is opposite to the acceleration direction.

  There are two types of electromagnetic field modes generated in the cylindrical resonator: TM mode (electric field is generated in the longitudinal direction of the cylindrical resonator) and TE mode (magnetic field is generated in the longitudinal direction of the cylindrical resonator). The drift tube linear accelerator using the TM mode includes an Alvara type drift tube linear accelerator. In this Alvare type drift tube linear accelerator, since the electromagnetic field mode in the cylindrical resonator is used as it is for the acceleration electric field generated between the drift tubes, the drift tube is supported by the stem so as to be suspended from the cylindrical resonator. On the other hand, drift tube linear accelerators using the TE mode include IH (Interdigital-H) type drift tube linear accelerators. In the IH drift tube linear accelerator, the electromagnetic field mode in the cylindrical resonator cannot be used for the acceleration electric field as it is, so the stems supporting the drift tube are alternately arranged from the top and bottom (or left and right) of the cylindrical resonator to induce the induced current. Indirectly generates an accelerating electric field between the drift tubes.

  For the drift tube linear accelerator using the TE mode, the drift tube installation position accuracy is particularly important. This is because the acceleration electric field strength generated between the drift tubes depends on the installation position of the drift tubes. Furthermore, when an APF (Alternating Phase Focused) self-convergence method is applied, the acceleration electric field generated between the drift tubes is also used for the convergence of charged particles. Therefore, it is necessary to adjust the acceleration electric field intensity with higher accuracy. Therefore, it is not a good idea to insert a stem into a cylindrical resonator that has been proposed as a drift tube support method for a conventional drift tube linear accelerator and use an RF contact at the boundary.

  Therefore, a structure is used in which the drift tube is provided with a pedestal for installation and fixing in addition to the stem, and a base for fixing the pedestal is provided in the cylindrical resonator. The drift tube is installed and fixed in the cylindrical resonator by screws using a screw hole provided at the drift tube arrangement position on the base and a through hole provided in the base. As a result, the high-precision alignment of the drift tube arrangement position can be performed with the allowable amount in the radial direction of the screw hole and the through hole. (For example, Patent Document 1 and Patent Document 2)

Japanese Patent Laying-Open No. 2007-157400 (FIG. 8) JP 2006-351233 A (FIG. 5)

The conventional drift tube linear accelerator has a problem that when an RF contact is inserted between the pedestal and the base, a potential difference is generated between the pedestal and the base due to a high-frequency current path, causing discharge.
The present invention has been made to solve the above-described problems, and an object of the present invention is to obtain a drift tube linear accelerator capable of suppressing discharge between a pedestal and a base and enabling stable operation.

  A drift tube linear accelerator according to the present invention includes a cylindrical resonator, a base fixed to the cylindrical resonator, and a plurality of drift tubes arranged linearly in the cylinder axis direction of the cylindrical resonator, and includes a plurality of drift tubes. At least one drift tube of the tubes is supported by a stem having a pedestal, and the pedestal is fixed to the base so that the installation position of the drift tube supported by the stem can be adjusted. In the drift tube linear accelerator in which the RF contact is inserted between the pedestal and the base, there is a gap between the pedestal and the base and there is no electrical contact portion in the outer peripheral portion of the RF contact.

  According to the present invention, there is a gap between the pedestal and the base of the outer peripheral part than the RF contact, thereby reducing the electric field strength generated by the potential difference, suppressing the discharge, and enabling the stable operation of the drift tube linear Accelerator can be provided.

It is external force sectional drawing which shows the drift tube linear accelerator by Embodiment 1 of this invention. It is an expanded sectional view which shows the principal part of the drift tube linear accelerator by Embodiment 1 of this invention. It is a schematic diagram which shows the mode in the drift tube linear accelerator which TE mode excited. FIG. 5 is an induced current path diagram between a pedestal and a base of the drift tube linear accelerator according to the first embodiment of the present invention. It is an expanded sectional view which shows the principal part of the drift tube linear accelerator by Embodiment 2 of this invention. It is an expanded sectional view which shows the principal part of the drift tube linear accelerator by the modification of Embodiment 2 of this invention. It is an expanded sectional view which shows the principal part of the drift tube linear accelerator by Embodiment 3 of this invention. It is an expanded sectional view which shows the principal part of the drift tube linear accelerator by Embodiment 4 of this invention. It is an expanded sectional view which shows the principal part of the drift tube linear accelerator by the modification of Embodiment 4 of this invention. It is an expanded sectional view which shows the principal part of the drift tube linear accelerator by Embodiment 5 of this invention. It is an expanded sectional view which shows the principal part of the drift tube linear accelerator by the modification of Embodiment 5 of this invention.

Embodiment 1 FIG.
1 is a schematic cross-sectional view showing a main part of a drift tube linear accelerator according to Embodiment 1 of the present invention. 1 is a side cross-sectional view in which a part of the right side of the side cross-section is omitted, the left-side view is a front-side cross-sectional view, the cross-section at the AA position in the right-side view is the left-side view, and the left-side view. The cross section at the B-B position is a diagram on the right side. In FIG. 1, 1 is a cylindrical resonator which is a vacuum vessel extending in the acceleration direction, and this cylindrical resonator 1 constitutes a drift tube linear accelerator. A pair of upper and lower bases 2 extending in the acceleration direction are provided in the cylindrical resonator 1. The base 2 is for generating a magnetic field generated in the acceleration direction by the TE mode up to the end of the cylindrical resonator 1. In the first embodiment, a pair of upper and lower bases 2 having the same shape are installed in consideration of the symmetry in the cylindrical resonator 1.

  Since the cavity shape of the cylindrical resonator 1 and the height, width, length, and cross-sectional shape of the base 2 affect the electromagnetic field distribution and resonance frequency generated in the cylindrical resonator 1, the adjacent drift tubes 4. It is determined by a three-dimensional electromagnetic field analysis so that the acceleration electric field strength generated between them has a desired value. In the first embodiment, the cross-sectional shape of the base 2 is a trapezoid, and the cylindrical resonator 1 side is installed as a lower bottom to enhance the installation strength.

  On the base 2, a pair of hollow cylindrical drift tubes 4 supported by the stem 3 are arranged in the acceleration direction, that is, a plurality of them. End drift tubes 5 are directly attached to end plates of the cylindrical resonator 1 at both ends in the acceleration direction of the cylindrical resonator 1 (the right end is omitted in FIG. 1), and a stem for supporting the end drift tube 5 is necessary. And not. The stems 3 are alternately arranged from the top and bottom of the cylindrical resonator 1 in order in the acceleration direction. The stem 3 is provided with a pedestal 6 at the opposite end to which the drift tube 4 is attached.

  In the drift tube linear accelerator using the TE mode, the induced current flows through the inner wall of the cylindrical resonator 1, the base 2, the stem 3, and the drift tube 4, so that the contact resistance at each contact portion greatly affects the amount of power supplied. Therefore, as an example of the drift tube linear accelerator in the first embodiment, an IH type drift tube linear accelerator having a great effect of the present invention will be described.

FIG. 2 is an enlarged cross-sectional view of the installation fixing surface of the pedestal 6 and the base 2 of the drift tube linear accelerator according to the first embodiment of the present invention. Here, the stem 3 has a cylindrical bar shape and the pedestal 6 has a disk shape, and the pedestal 6 has a width equal to or less than the width of the upper surface of the base 2 in the cross-sectional direction of FIG. This is to avoid an increase in surface resistance due to unnecessary discontinuous surfaces and unnecessary discharge at the edge. In the first embodiment, an example in which the stem diameter D1 is φ30 mm, the pedestal diameter D5 is φ60 mm, and the surface on the side where the base faces the pedestal, that is, the base upper surface width W1 is 60 mm will be described. The base 6 is provided with a through hole 11 for fixing with the base 2 and the screw 10 and an RF contact 13 on the outer periphery side of the base 6 from the center of the through hole, that is, a through hole machining position (Pitch Circle Diameter; PCD) D2. A groove 14 is provided. The thickness of the peripheral portion of the pedestal 6 on the outer side of the groove 14 is made thin so as to have a gap 16 for providing a gap between the pedestal 6 and the base 2 continuously from the groove outer periphery D4.

  In an example in which the through hole machining position (PCD) D2 is φ40 mm, the groove inner periphery D3 is φ51.2 mm, the groove outer periphery D5 is φ53.6 mm, the groove height d2 is 1.2 mm, and the gap height d1 is 0.5 mm. explain. In the present invention, there is a gap 16 for providing a gap between the pedestal 6 and the base 2 continuously from the groove outer periphery D4, that is, on the outer peripheral side of the pedestal 6 with respect to the RF contact 13, and between the pedestal 6 and the base in between. 2 is not in electrical contact. Further, the gap height d1 is set to be equal to or higher than the surface roughness of the surface of the base 6 or the base 2, preferably higher than the machining accuracy, for example, 30 μm or higher.

In order not to cause unnecessary heat concentration and discharge, a cap screw is used as the screw 10 so that it does not protrude from the upper surface of the pedestal 6 and is further subjected to silver plating to improve conductivity. Moreover, in order to install and fix between the base 6 and the base 2, it is not necessary to be the screw 10, and a post-processed pin or the like may be used. On the other hand, no post-adjustment after fabrication is required between the cylindrical resonator 1 and the base 2, between the stem 3 and the drift tube 4, and between the stem 3 and the pedestal 6, thus preventing the occurrence of contact resistance on the respective installation fixing surfaces. In order to do so, it is cut from an integrated object or fixed by electron beam welding or the like.

  FIG. 3 is a schematic diagram of a mode showing a generated magnetic field direction 20 and an induced current direction 21 in the drift tube linear accelerator excited by the TE mode. A high frequency power supply is supplied to excite the TE mode from the drift tube linear accelerator. Then, a magnetic field is generated in the acceleration direction. The induced current induced by the magnetic field flows through the drift tube 4, the inner wall of the cylindrical resonator 1, the base 2, the base 6, and the stem 3, and an accelerating electric field is generated between the adjacent drift tubes 4.

  FIG. 4 is a detailed view of the path of the induced current between the base 6 and the base 2. Since the induced current is a high-frequency current, the direction in which it flows at a constant period is reversed. Here, the case where the induced current flows from the base 2 to the base 6 will be described. Since the induced current is a high-frequency current, there is a skin effect, and the current density j of the conductor decreases as shown in the following equation (1) with respect to the depth δ.

ここでdは表皮深さで、電流が表面電流の1/e（約0.37）になる深さであり数式（２）で表される。

Here, d is the skin depth, which is the depth at which the current becomes 1 / e (about 0.37) of the surface current, and is expressed by Equation (2).

ここで、ρは導体の電気抵抗、ωは電流の角周波数、μは導体の透磁率である。
一般的にＩＨ型ドリフトチューブ線形加速器で用いられる運転周波数200MHz、導体として銅を例にとると、数式（１）、数式（２）より、表面から30μｍの深さでは表面電流密度の１０００分の１程度になり、導体表面でしか流れない事がわかる。

Here, ρ is the electrical resistance of the conductor, ω is the angular frequency of the current, and μ is the permeability of the conductor.
In general, the operating frequency of 200 MHz used in the IH drift tube linear accelerator and copper as an example of conductor are 1000 minutes of the surface current density at a depth of 30 μm from the surface according to Equations (1) and (2). It becomes about 1 and it turns out that it flows only on the conductor surface.

  4 will be described based on this phenomenon. The induced current 21 flowing on the surface of the base 2 moves to the base 6 via the RF contact 13 and flows to the stem 3 and the drift tube 4. The induced current does not move from the base 2 to the base 6 via the inside of the conductor. As a result, the contact between the base 2 and the base 6 is ensured by the RF contact 13, the contact resistance is low, and no unnecessary heat is generated. In this way, since the pedestal 6 having the likelihood of the through hole 11 is fixed to the base 2 with the screws 10, the position of the drift tube 4 can be finely adjusted, and the power supply amount can be reduced in a structure capable of high-precision alignment. There is an effect that can be reduced.

  As described above, most of the induced current flows on the surface of the base 2, moves to the base 6 via the RF contact 13 having a low contact resistance provided on the base 6, and flows to the stem 3 and the drift tube 4. Here, in consideration of the fact that the induced current is a high-frequency current and flows only to the surface, a potential difference is generated between the end of the base 6 and the base 2 by reciprocating to the RF contact 13. The electric field strength induced by this potential difference can be expressed by the following formula (3).

ここで、Ｖは台座６と土台２間の電位差、Ｅは台座６と土台２間の電界強度、ｄは台座６と土台２間のギャップ長である。

Here, V is a potential difference between the base 6 and the base 2, E is an electric field strength between the base 6 and the base 2, and d is a gap length between the base 6 and the base 2.

  If the design does not include the gap 16, a minute gap due to the surface roughness is opened between the base 6 and the base 2. When the gap is only open to the extent of surface roughness (several μm), the generated electric field strength is high even with a small potential difference, and there is a problem that discharge occurs. That is, since the high-frequency current flows only on the conductor surface and passes through the RF contact 13, a potential difference is generated between the base 6 and the base 2, and the gap between the base 6 and the base 2 is extremely narrow to the surface roughness level. For this reason, it was found that the electric field strength increased and discharge occurred. Therefore, in the first embodiment, in order to shorten the reciprocating distance to the RF contact 13 and reduce the generated potential difference, the groove 14 for installing the RF contact 13 is formed on the base 6 from the through hole installation position (PCD) D2. By providing the gap 16 between the pedestal 6 and the base 2 while being provided outside, the electric field strength in the gap is weakened, so that the discharge can be suppressed.

  Further, when the reciprocating distance to the RF contact 13 is long, there is a problem that the path of the high-frequency current becomes a resonator operation different from that in the electromagnetic field design. Electromagnetic field analysis of the drift tube linear accelerator was performed using electromagnetic field analysis software using a mesh. In the analysis, it is assumed that there is no gap 16 between the pedestal 6 and the base 2 and there is no contact resistance (considering surface resistance), so the induced current does not enter between the pedestal 6 and the base 2 and the base 2 It moves from the surface to the surface of the base 6 as it is. Therefore, when the reciprocating distance to the RF contact 13 becomes a length that cannot be ignored compared to the original high-frequency current path length, the resonator operation differs from the design time, and the resonance frequency and acceleration electric field strength of the manufactured drift tube linear accelerator are reduced. There was a problem different from the design value. The relationship between the high-frequency current path and the resonance frequency of the drift tube linear accelerator will be described below.

ここで、ｆはドリフトチューブ線形加速器の共振周波数、Ｌはインダクタンス成分、Ｃはキャパシタンス成分である。Ｃは主にドリフトチューブ４間に発生するキャパシタンスである。Ｌは一般のコイルの考え方を適用すると理解しやすく、コイルに発生するインダクタンスは次の数式（５）で表現される。 The resonance frequency of the drift tube linear accelerator is expressed by the following equation (4).

Here, f is a resonance frequency of the drift tube linear accelerator, L is an inductance component, and C is a capacitance component. C is a capacitance mainly generated between the drift tubes 4. L can be easily understood by applying a general coil concept, and the inductance generated in the coil is expressed by the following equation (5).

ここで、μは空芯の場合は1.0であり、ｎは巻き数、Ｓはコイルの断面積、ａはコイル
で囲まれる断面積の半径である。
ドリフトチューブ線形加速器でも数式（５）の考え方で説明する事ができ、この場合の断面積は、高周波電流の経路により囲まれる断面積とみなす事ができる。したがって、高周波電流経路長（circle）と共振周波数の関係は次の数式（６）として考える事ができ、共振周波数と高周波電流経路は反比例の関係にある。

Here, μ is 1.0 in the case of an air core, n is the number of turns, S is a sectional area of the coil, and a is a radius of a sectional area surrounded by the coil.
The drift tube linear accelerator can also be explained by the concept of Expression (5), and the cross-sectional area in this case can be regarded as a cross-sectional area surrounded by a high-frequency current path. Therefore, the relationship between the high frequency current path length (circle) and the resonance frequency can be considered as the following formula (6), and the resonance frequency and the high frequency current path are in an inversely proportional relationship.

次に、高周波電流経路とドリフトチューブ４間に発生する電界強度の関係を下記に説明する。ドリフトチューブ４間に発生する電界強度は、次の数式（７）で表されるファラデーの法則に従う。

Next, the relationship between the electric field strength generated between the high-frequency current path and the drift tube 4 will be described below. The electric field strength generated between the drift tubes 4 follows Faraday's law expressed by the following equation (7).

ここで、lはドリフトチューブ４間長、Ｅ_ＤＴはドリフトチューブ４間に発生する電界
強度、Ｂはドリフトチューブ線形加速器内に発生する磁界強度、ドットは時間微分を示し、Ｓは高周波電流の経路により囲まれる断面積である。
したがって、高周波電流経路長（circle）と加速電圧（数式（７）左辺）の関係は数式（８）として考える事ができ、加速電界強度（加速電圧）と高周波電流経路は２乗に比例する関係がある。

Here, l is the length between the drift tubes 4, E _DT is the electric field strength generated between the drift tubes 4, B is the magnetic field strength generated in the drift tube linear accelerator, the dots indicate time differentiation, and S is the path of the high-frequency current Is a cross-sectional area surrounded by.
Therefore, the relationship between the high-frequency current path length (circle) and the acceleration voltage (left side of Formula (7)) can be considered as Formula (8), and the relationship between the acceleration electric field strength (acceleration voltage) and the high-frequency current path is proportional to the square. There is.

本実施の形態１において円筒共振器１内径を一般的な値であるφ３２０ｍｍとすると、高周波電流経路の長さは、図２の本発明のＲＦコンタクト１３の設置方法では３％の増加にとどめることができ、設計値からの誤差を小さくする効果がある。

When the inner diameter of the cylindrical resonator 1 in the first embodiment is φ320 mm, which is a general value, the length of the high-frequency current path is limited to 3% in the installation method of the RF contact 13 of the present invention shown in FIG. It is possible to reduce the error from the design value.

  If the gap 16 is not provided, the gas remains in the groove 14 and is exhausted little by little from the minute gap between the pedestal 6 and the base 2, so that the degree of vacuum of the cylindrical resonator 1 deteriorates. In the drift tube linear accelerator described in the first embodiment, since the gap 16 is provided, no gas remains in the groove 14, and the effect of preventing the deterioration of the degree of vacuum is obtained.

Embodiment 2. FIG.
In the first embodiment, the shape of the gap 16 is such that the surface of the base 6 for forming the gap 16 is flat and parallel to the surface of the base 2 so that the gap is constant. A chamfered configuration may be used. FIG. 5 is an enlarged cross-sectional view of the pedestal portion of the drift tube linear accelerator according to the second embodiment of the present invention. The portion of the pedestal 6 where the gap 16 is formed, that is, the shape of the peripheral portion is changed to the shape of the C chamfer 23. Yes. You may comprise not only C chamfering but R round chamfer 24 which has roundness as shown in FIG. As described above, by making the shape of C chamfering or R chamfering, that is, by increasing the size of the gap 16 toward the outer peripheral portion, the same effect as in the first embodiment can be obtained, and the pedestal end portion can be obtained. There is an effect that the possibility of discharging at 22 can be further reduced. That is, the potential difference generated between the pedestal 6 and the base 2 is not strictly constant, and the potential difference increases toward the end of the pedestal 6 where the current path becomes longer. Accordingly, by performing R chamfering or C chamfering, the size of the gap 16 at the pedestal end 22 can be further increased, and further, by reducing the electric field concentration, the generated electric field strength can be suppressed and discharge can be further prevented. .

Embodiment 3 FIG.
FIG. 7 is an enlarged cross-sectional view showing a pedestal portion of the drift tube linear accelerator according to the third embodiment of the present invention. In the third embodiment, as shown in FIG. 7, the groove shape is a dovetail groove 25, that is, a groove having a groove bottom dimension larger than the opening dimension of the groove. Even when the dovetail 25 is configured, the same effects as those of the first and second embodiments are obtained, and the RF contact 13 is prevented from dropping when the base 6 is installed and fixed on the base 2. Since the RF contact 13 is highly conductive, if a general adhesive is used to prevent the RF contact 13 from being dropped carelessly, the conductivity is lost, the contact resistance increases, and the amount of supplied power increases. It can be prevented.

Embodiment 4 FIG.
FIG. 8 is an enlarged cross-sectional view of the pedestal portion of the drift tube linear accelerator according to the fourth embodiment of the present invention. As shown in FIG. 8, the height dimension d1 of the gap 16 may be larger than the height dimension d2 of the notch 141 for sandwiching the RF contact 13. By doing in this way, the electric field strength in the gap 16 can be further reduced, and there is an effect that discharge is hardly generated.
FIG. 9 is an enlarged cross-sectional view of a pedestal portion of a drift tube linear accelerator according to a modification of the fourth embodiment in which a dielectric material 17 such as ceramic is sandwiched between gaps 16. As the height dimension d1 of the gap 16 is increased, the electric field strength is reduced and creeping discharge does not occur on the surface of the dielectric, so that there is an effect that the dielectric material is sandwiched in the gap 16 and the RF contact placement is improved.

Embodiment 5 FIG.
FIG. 10 is an enlarged cross-sectional view showing the pedestal portion of the drift tube linear accelerator according to the fifth embodiment of the present invention. In the embodiments so far, the groove 14 and the notch 141 are provided in the base 6 in order to install the RF contact. However, the groove 14 may be provided on the base 2 side as shown in FIG. . FIG. 11 is an enlarged cross-sectional view showing a pedestal portion of a drift tube linear accelerator according to a modification of the fifth embodiment of the present invention. In FIG. 11, the groove 14 is provided on the base 2 side, and the gap 16 is formed by cutting the periphery of the groove 14. Even if the groove 14 is provided on the base 2 side shown in FIG. 10 and FIG. 11 and the outer periphery of the base 6 or the base 2 is cut away from the RF contact, the gap 16 is formed. In addition, the RF contact 13 can be installed and fixed stably, and the base 6 can be easily installed and fixed on the base 2.

  As described above, the present invention provides an RF contact between the base and the base to ensure electrical contact between the base of the stem supporting the drift tube and the base, and is provided outside the RF contact. A gap is provided in this portion so that the pedestal and the base do not make electrical contact, and the gap is desirably 30 μm or more.

1: Cylindrical resonator 2: Base 3 Stem 4: Drift tube 6: Base 10: Screw 13: RF contact 14: Groove 16: Gap 23: C chamfer 24: R chamfer 25: Dovetail d1: Gap height

Claims (3)

A cylindrical resonator; a base fixed to the cylindrical resonator; and a plurality of drift tubes arranged linearly in a cylindrical axis direction of the cylindrical resonator, and at least one of the plurality of drift tubes. The drift tube is supported by a stem having a pedestal, and the pedestal is fixed to the base so that the installation position of the drift tube supported by the stem can be adjusted, and between the pedestal and the base. In the drift tube linear accelerator in which the RF contact is inserted, there is a gap between the pedestal and the base and there is no electrical contact portion in the entire outer portion of the RF contact, and the dimension of the gap is outside. Drift tube linear accelerator characterized by becoming larger toward A cylindrical resonator; a base fixed to the cylindrical resonator; and a plurality of drift tubes arranged linearly in a cylindrical axis direction of the cylindrical resonator, and at least one of the plurality of drift tubes. The drift tube is supported by a stem having a pedestal, and the pedestal is fixed to the base with screws so that the installation position of the drift tube supported by the stem can be adjusted, and between the pedestal and the base. In the drift tube linear accelerator having an RF contact inserted between the base and the base, there is a gap between the base and the entire outer portion of the RF contact , and the RF contact A drift tube linear accelerator provided outside the screw . A cylindrical resonator; a base fixed to the cylindrical resonator; and a plurality of drift tubes arranged linearly in a cylindrical axis direction of the cylindrical resonator, and at least one of the plurality of drift tubes. The drift tube is supported by a stem having a pedestal, and the pedestal is fixed to the base so that the installation position of the drift tube supported by the stem can be adjusted, and between the pedestal and the base. In a drift tube linear accelerator in which an RF contact is inserted into a groove provided on either the pedestal or the base, the entire portion outside the RF contact is electrically connected between the pedestal and the base. a gap no contact portion, the dimensions of this gap, the drift tube linac, wherein less than the depth of the groove

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