DEVELOPMENT OF S-BAND ACCELERATING STRUCTURE
Sadao MiuraA), Heung-Soo Lee B), Hiroshi Matsumoto C)
Hirokazu MaesakaC), Takahiro InagakiC), Tsumoru ShintakeC)
A)
MITSUBISHI HEAVY INDUSTRIES, LTD.
1-1, Itozaki Minami 1-chome, Mihara, Hiroshima, 729-0393, JAPANB)
POHANG ACCELERATOR LABORATORY
San31, Hyoja-dong, Nam-gu, Pohang, Gyeongbuk, 790-784, Republic of Korea
C)
HIGH ENERGY ACCELERATOR RESEARCH ORGANIZATION
1-1 Oho, Tsukuba, Ibaragi, 305-0801, JAPANAbstract
In Pohang Accelerator Laboratory (PAL) in Korea
construction of XFEL (X-ray Free electron Lazar)
institution is under construction aiming at the completionin 2014[1]. Energy 10GeV of the linac part of thisinstitution and main frequency are planned in S-band(2856 MHz), and about 178 S-band 3m accelerating
structures are due to be used for this linac.
The oscillation of an X-ray laser requires very low
emittance electron beam. On the other hand, since theaccelerating structure which accelerates an electron beam
has a feed port of microwave (iris), the electromagnetic
field asymmetry of the microwave feeding device calledcoupler worsens the emittance of an electron beam.
MHI manufactured two kinds of S-band acceleratingstructures with which the electromagnetic fieldasymmetry of coupler cavity was compensated for
PALXFEL linac. We report these accelerating structures.

TWO TYPE PROTOTYPE S-BAND
ACCELERATING STRUCTURE FORPALXFEL
Fig.1 shows J-type S-band accelerating structure which
equipped J-type double feed coupler. Fig.2 shows Quasisymmetrical type S-band accelerating structure which
equipped quasi-symmetrical type single feed coupler.
These are 3m long, constant gradient, S-band
2856MHz accelerating structures. Cell number is 84
regular cells and 2 coupler cells. Material of thesestructures is oxygen free high conductivity copper. Allcell are machined by super-precision lathe, and surface
roughness of these cells are suppressed to 0.1μm or less.
And all cells are assembled by vacuum brazing. Table 1
shows the specifications of these S-band accelerating
structures.
Table.1：Specifications of S-band acceleratingstructures
Resonance Frequency 2,856 MHz
Phase Shift
2π/3
Accelerator TypeC.G.
Number of Cells
84+2 coupler cell
Quality Factor
13000Group Velocity
0.012c (average)
Shunt Impedance
49.3～60.0 MΩ/mAttenuation Constant
0.56
Filling Time
0.84 μsJ-type
Coupler type
Quasi-summetry

INTRODUCTION
MHI manufactured two kinds of every two S-band
accelerating structures as prototype machine for
PALXFEL linac, and supplied them to PAL in June andAugust 2012 respectively. 1st kind of accelerating
structures which MHI manufactured are S-band 3mstructure which equipped J-type double feed coupler,and one more kind equips quasi-symmetrical type single
feed coupler. Both accelerating structures, J-type andQuasi-symmetrical type, are the structures with which
the electromagnetic field asymmetry of coupler cavity
was compensated.

To Undulator
Soft X-ray

S-bandKlystronX-bandKlystronS-bandKlystronS-bandKlystron

BC1

BC2．．．

RFGUN

S-bandAccelerating
Structures

X-bandAccelerating
Structures

BC3．．．

．．．
S-band Accelerating Structures~3GeV

S-band Accelerating Structures

Fig.1: Linac layout of PAL XFEL Project- 1162 -．．．

To Undulator
Hard X-ray
10GeVQuasi- symmetrical type coupler have wave guide of
1/4-wave length in iris opposite side in order tocompensate asymmetry of electromagnetic field in thecoupler cavity.
The single feed type S-band accelerating structures
which MHI manufactured is operated by a high axial
gradient up to 30 MV/m in ATF/KEK and PLSⅡ/PAL.
The axial gradient of PALXFEL linac is 20 ～ 25 MV/m.
Thus an accelerating structure with this type of coupler isapplicable for PALXFEL linac.

Microwave
Fig.2: J-type S-band accelerating structure

Iris

Wave-guide

Beam hole

Electronbeam

Another Iris

λg/4

Fig.6: Quasi- symmetrical type coupler

ANALYSIS AND MEASUREMENT
Fig.3: Quasi-symmetrical type S-band accelerating
structure

J-TYPE COUPLER AND QUASISYMMETRICAL COUPLER
J-type double feed coupler that Professor Matsumoto in
KEK devised[2] has two irises although that has only one
connection with wave-guide. Therefore, the surface field
intensity at the iris is low, and low beam emittance ismaintainable since electromagnetic field in the coupler
cavity is symmetrical.

A decision of the coupler optimal shape was made by
the analysis by a computer and RF measurement of a testmodel. The computer code used the three-dimensional
electromagnetic field analysis code HFSS. The test
model was manufactured based on calculated data. The
coupler optimal shape was determined by correction
processing of the test model while performing RFmeasurement. RF measurement was performed by thenodal shifting method using the network analyzer.

Iris
Electron
λg/4
Microwave
beam

Fig.7: Analysis model (J-type)

2λg
Fig.4: J-type coupler- 1163 -S-Band Accelerating structure Phase error
PQ01

5f=2856.000MHz(30℃,vac)Σθi≦±2.5deg

4Phase error (deg)3210-1-2-3-4-50102030405060708090

cell no.

Fig.10: Example of Phase error (PQ01)
Fig.8: RF measurement (J-type)

SUMMARY
LOW LEVEL RF RESULTS
MHI manufactured two J-type S-band accelerating
structures and two quasi-symmetrical type S-band
accelerating structures. Table2 shows Final RF result.
Input and output VSWR are less than 1.05 (tolerance) at
operation frequency. Phase error is lass than 2.5 degree(tolerance).
Table.2：Low level RF results of S-band accelerating
structures
Accelerating structure type
ItemJ-type 01 J-type 02 Q-type 01 Q-type 02
Input VSWR
1.032
1.021
1.014
1.007
Output
1.023
1.037
1.044
1.04
Phase error±0.17deg ±0.18deg ±0.14deg ±0.11deg
Quality Factor 13668
13265
13621
13667
Group
0.012
0.012
0.012
0.012
Attenuation
0.551
0.567
0.551
0.551
Constant (τ)
Filling Time
0.84μs0.84μs0.84μs0.84μs・ MHI manufactured two prototype J-type S-band
accelerating structures and two prototype quasisymmetrical type S-band accelerating structures forPALXFEL.
・ These accelerating structures are due to have highpower test carried out in PAL.
・ MHI receive the order of 40 accelerating structuresfrom PAL, and are already preparing massproduction.

REFERENCES[1] J.-H. Han et al., “STATUS OF THE PAL-XFEL
PROJECT”, Proceedings of IPAC2012, New Orleans,Louisiana, USA.
[2] H. Matsumoto et al. “Fabrication of the C-band (5712MHz)
Choke-Mode Type Damping Accelerating Structure”,thProceedings of the 24 Linear Accelerator Meeting inJapan

Fig.9: Example of Input VSWR (PQ01)- 1164 -