Proceedings of the 20th Linear Accelerator Meeting in Japan (September 6—8,1995tOsakatJapan)

ATF 1.54 GeV Linac for Linear C ollider
H. Matsumoto, M. Akemoto, T. Asaka*, H. Hayano, T. Korhonen, T. Naito andJS. Takeda
KEK, National Laboratory for High Energy Physics
1-1 Oho, Tsukuba-Shi, Ibaraki-Ken} 305, Japan* Tohoku-Gakuin university
1-31-1 Chuo, Tagajo-Shi, Miyagi-Ken, 985, JapanA bstract
The 1.54 GeV S-band linac for the ATF must be able to inject a multi-bunch beam of electrons into the damping
ring. To meet the energy goal of 1.54 GeV given site constraints, an accelerating gradient of 33 MV/m has to beachieved under beam loading conditions. The ATF Linac is currently operating only the 80 MeV injector as a recognizedbeam accelerator. A 200 kV thermionic electron gun produces a 2.8 ns spaced 20jnulti-bunched beam which is used forbeam diagnostic equipment development, perfonnance evaluations, and beam emittance and bunch length
measurements. With 2xlO10 electrons per bunch the measured normalized emittance and bunch length obtained were<8x10-5 m.rad (lo ) and 11-14 ps (FWHM) respectively at a beam energy of 75 MeV. These data were obtained using a
new method of bunch-by4>unch monitor system and optical transition radiation emitted from a 1 mm thick of stainlessplate. The first beam commissioning of the linac will start at the end ofSeptember 1995.

Introduction
The Linear Collider-1 (LC-1) project [1], which is
proposed for the next generation af high energy physics
research, requires collision energies in the 300 to 500
GeV region (from experimental results of the SLC andLEP), In order to actually realize such a large scaleaccelerator in a realistic length, a high gradient type linac
is necessary to provide high beam energy gains in shorter
distances. A further very important design consideration
is improving the overall reliability of such a large scaleaccelerator system by minimizing the number of activeelements, such as klystrons and their pulse modulators,etc.
The ATF has been started at KEK to efficiently
conduct research on high gradient linac related matters.
It consists of a 1.54 GeV S-band linac (including apositron test stand) and a 1.54 GeV damping ring,followed by an extraction line with a bunch-compressor.
ATF researchers are working together with manycollaborators at universities and industry groups inJapan as well as foreign institutes such as GERN,DESY, PAL, SEFT, and SLAC at this time.
As a first step, an 80 MeV injector part wasestablished and has been operating since August 1993.
The other main linac components are installed within
90% at this time. The first beam commissioning of the
1.54 GeV S-band linac will start at end of September
1995. The status of the experiments and the results
achieved so for will be presented.
The 1.54 GeV injector linac
The main parameters of the linac are listed in Table
1 .The 1.54 GeV injector linac comprises an 80 MeV pre­injector linac, eight units of regular accelerator sections,
two units of energy-compensation accelerating
structures, beam diagnostics and a positron target test
stand [2]. The linac has to accelerate the multi-bunch
beam with an energy spread of less than 1% (full width of90%) at the end of linac. To achieve this, a new idea of anenergy-compensation system (ECS) will be used. The
system consists of two 3 m-long accelerator structures
which, are driven by two different frequencies at2856+4.32727 MHz and 2856-4.32727 MHz. With theECS both the bunches within the train obtain a different
energy gain caused by the phase shifts during passage
through the two structures. The ECS should reduce theenergy spread from about 5% to 0.2% peak-to-peak with
2x1010 electrons per bunch.

Table 1 Main parameters of 1.54 GeV injector linac
BeaoL&acamelm
1.54
Beam energy
Energy spread< 1%
Normalized emittance< 3xl0*420
Number of bunches
Bunch population
2 x l0 10
Bunch separation
2.8<10Bunch length
Machine88
Total length
80 MeV pre-injector
18Linac70
Regular accelerating sectiws
Operation frequency
2856
2n/3
Phase shift/cell
Structure length
Number of structures
Accelerating field gradient31647.5(33*)
85
4.58GeVfull width of 90% beam
m-rad(la)electronsns
ps (FWHM)
mmm (active length=48 m)
MHzconstant gradient,
traveling wavem
MeV/m,* with beam loading
MW (maximum)ixs

Klystron peak power
Klystron pulse width
Number of klystrons
RF pulse compression
Dual-iris SLED type
Power gain
at peak5
Number of SLED cavities 8
Number of modulators
8200 MW of peak power
Energy compensation svstem
f0 + 4.32727 MHzAccelerating structures
f0 - 4.32727 MHz
f0=2,856 MHzKlystron peak power
50MW (SLAC5045)
RF pulse width
1.0Us
2
Number of klystrons

80 MeV pre-injector linac
The pre-injector rf system consists of a 200 kV
thermionic electron gun, a beam chopper, two 357 MHz
standing wave Sub-Harmonic Bunchers (SHB), a 2856
MHz 7-cell traveling wave buncher and a 3 m longregular accelerator structure. The beam transport iscomposed of the 25 Helmholtz coils, a matching sectionof beam lattice, an energy analyzer magnet with a 45°
bend angle and beam diagnostics.
Thermionic electron gun [2]The gun uses a triode with a model EIMAC Y646-E
or Y796 grid-cathode assembly. The energy of the beam

is 200 kV in typical operation. The 20 bunches jfrom thegun are extracted by applying an rf voltage to the grid,
which is generated by a 2 kW rf amplifier with a 316 Vpeak-to-peak rf voltage at 50 Q load impedance. It is
synchronized with the 357 MHz of the SHBs. The main
parameters of the gun are listed in table 2, andexperimental results with multi-bunch generation are
shown in Figure 1.
Table 2 Main param eters of the gun
Beam energy
Number of pulses
Bunch width
Bunch separation
Bunch population
Population tolerance

20020
12.8> 3 x 1010
<±1.0

kV
ns(FWHM)ns%'夂。
.
ニ:てギニ
Further, to measure the bunch by bunchcharacteristics, a new monitor system with an 1 GHz
bandwidth has been developed and has been used at the80 MeV pre-injector since 1993. These new beamdiagnostics confirmed that they are very useful to makeprecise machine tuning.
OTR monitor [2, 3, 4]
The OTR is generated by 1 mm thick of stainlessplate mounted an sl beam axis at.a 45° angle. A gated ，camera with a 3 ns gate width is used for beam profile
measurements. A streak-camera which has 0.6 ps oftime resolution is used for bunch length measurements.
The measured bunch length has to be corrected fordispersion effects. Figure 3 shows the measured bunch
length (a) and energy spread (b) of accelerated multi­bunch beam at the end of the 80 MeV pre-injector.
o7coeoS6乏£ cal Msdoun【】 HMd0610eoo)>

fa(a) Bunch length measurement using streak-camera with 2.8 ibunch spacing
Figure 1 Multi-bunch beam from gun measured by acore current monitor

81JH

''丨
「卞丨
丨，'""II■ ' ' I"7-—- 0.9

iam energy

The 375 MHz SHB cavities [2]The beam bunching cavities use two 357 MHz SHBsand a 2856 MHz traveling wave cavity. The 20 bunchesfrom the gun have a 1 ns (FWHM) bunch length. Since
each bunch contains more than 2x1010 electrons acumulative loading voltage in the bunching cavities isproduced. To reduce the phase shift of the cavity voltage
due to beam loading, low R/Q SHBs have been developed
as shown in Figure 2. Two low R/Q SHBs which have R/Q= 4 5 Q were introduced to increase the beam tuning
range. Two 5 kW, 30 jus pulse width, solid-state amplifiers
are use to drive the SHBs.
Frequency tuner:.... ... i........... i......... i........測0.8'：

KT

0.7a>
运80ぶ'

•〜 ー卜:

0.60.5\ Energy spread i—79J50.4— ；0*3.. •… L一 し •丄 …

:ー 1丄..しュ105

10

15

2025

0.2
30Bunch number(b) Beam energy and energy spread with the 5xl09 electrons perbunch, and 2.8 ns bunch spacing
Figure 3 Measurement made with the OTR monitor system

Figure 3-a shows measured bunch lengths of 11-14 ps at
the center of the bunch train. Each black circle and its
error bar in Figure 3-a show the bunch length averaged
from the five data measured at the same bunch, and themaximum fluctuation of data. From Figure 3-b, the beamenergies reduce according to beam loading of the multi­bunch trains.
Multi-bunch emittance monitor [2, 3]
Figure 2 Low R/Q type SHB

Beam diagnostics
The 1.54 GeV injector linac will use conventional
diagnostic monitors including core current monitors (CT),screen profile monitors, stripline type beam position
monitors, a bunch length measurement system which
detects Optical Transition Radiation (OTR) using astreak-camera, wire scanners and beam loss monitors.

To measure the bunch-by-bunch beam emittance, a
new measurement system has already been developed. AMCP-PMT with a gate function is used for detection of
measured gamma rays produced by a scanning wire. The
beam size of any single bunch can be by adjusting thetiming of the 3 ns pulse. It takes about 5 minutes for anemittance measurement with 20 bunches as shown in
Figure 4. The measured data show that the normalized
emittance of each bunch is less than the 8xl0-5 m.radwhich is smaller than a specification of a 1.54 GeV linac

(3xl(H m rad). The emittance of each bunch does notstrongly depend on the bunch order. Variations between
bunches may be due to beam jitters during
measurement.

output. Figure 5 shows the first rf-processing of theregular accelerator unit. From this experiment, it wasalso found that once rf-processing had been completed,even after bringing the system up to atmosphericconditions, reestablishing vacuum, operation at 200 MW
input was possible within a few hours.

Bunch number

Figure 4 Beam emittance measurements using the gated wire
scanner measurement system

The OTR system also can measure the emittancenot only bunch-by-bunch but can also measure a single­
shot profile. The normalized emittance is evaluated to be
2.5-2.8xl0-5 m rad which is smaller than that of wire
scanners, because it does not excludes contributions frombeam jitter.
H igh gradient 3-m long structure
A new 3m-long structure was fabricated in such a 、
way as to eliminate or at least improve all the negative
influences and factors shown in previous high gradientexperiments. This principally involved the construction of
the structure input and output couplers. To avoid theimperfections caused to the structure by rf tuning
adjustments, and the accompanying contamination,
direct precision macmning of the coupler cavity was used.
This was made possible by the use of a three-dimensional
electromagnetic analysis code (MAFIA) for a very careful
dimensional detail design [5]. Then Numerical Control(NC) machines were employed for the precision
machining. Further, in order to reduce the field emission
currents, the material for the aisk section was all HIP(Hot Isostatic Press) [6] processed, and care was taken
to ensure that no machine oil remained in the material as
it was fabricated. Table 2 shows the main parameters ofthe structure.
Table 2 Main acceleration structure parameters.
Operation frequency
Phase shift/cell
Electric field distribution
Structure length
Number of cells
Quality factor
Shunt impedance (= V2/2P)
Attenuation param eter
Group velocity / c
Filling time
Ep / Ea2856 MHz2n/S
Constant gradient3 m
86
13,00060 MQ/m
0.57
0.0204-0.0065
0.83 JiS
1.9-2.1100,200300

400

500

600700RF PROCESSING TIME [HOUR]

Figure 5. The first rf-processing of 汪regular accelerator unit.

SummaryThe 80 MeV injector has been operating since August
1993 to efficiently conduct R&D on the multi-bunch andhigh current linac. The new idea of bunch-by-bunch beam
monitors for size, emittance, energy spread and bunch
length has been developed using the OTR system. It wasconfirmed that the new monitors are very useful for
precise machine tuning. A new 3-m long structure
confirmed that 33 MV/m beam acceleration would be noproblem. The first beam commissioning of the 1.54 GeVinjector linac will start at the end of September 1995.
Acknowledgm ent
The authors wish to acknowledge Professors Y.
Kimura, K. Takata and the ATF group for theircontinuous encouragement. The experiment could nothave been performed without the help of Mrs. S.
Kanazawa, K. Nishitani, T. Matsui and S. Morita of ATC
corporation. The authors wish to thank these people for
their help. A special mention has to be made that thepresent work could not have been carried out without
many discussions with and information fromcollaborators at universities, industry and foreigninstitutes.
R eferences

Ep : Peak surface electric field.
Ea : Axial electric field.

The 3m-long structure obtained a maximum
accelerating gradient of 52 MV/m at an input peak rfpulse power of 200 MW without any problem. At thatpoint the average field emission current from thestructure was 0.34 nA per rf-pulse. A microscopic fieldenhancement factor B of 70 was obtained from FowlerNordheim plots [7]. Up to 200 hours of initial rfprocessing time was required to condition an accelerator
unit to operation with a 400 MW, l|ns SLED peak rf[ 1 ] The second Subcommittee on Future Project of High Energy
P h ysics in Japan (Chaired by S. K om am iya), "The Interim
Report of the Subcommittee on Future Project of HighEnergy Physics in Japan, July 12,1995[2] Edited by F. Hinode, S. Kawabata, H. Matsumoto, H. Oide,k. Takata, Seishi Takeda and J. Urakawa, KEK Internal
95-4, June 1995.
[3] H. Hayano, T. Asaka, H. Matsumoto, T. Naito, M. Schmitz
and S. Takeda, Proc. of 17th In tern a tio n a l L inear
Accelerator Conference, Tsukuba, Japan, 1994, pp. 236238.
[4] T. Asaka, H. Hayano and T. Naito, Proc. of the 20th LinearAccelerator Meeting in Japan, September 6-8, 1995
[5] S. Yamaguchi, Proceedings of III Workshop on JLC, KEK,February 18-29, 1992.
[6] H. Matsumoto, M. Akemoto, H. Hayano, T. Naito and S.
Takeda, Proc. of 17th International Linear Accelerator
Conference, Tsukuba, Japan, 1994, pp. 62-64.
[7] H. Matsumoto, M. Akemoto, H. Hayano, T. Naito and S.
Takeda, Proc. of Particle Accelerator Conference, May 1991,IEEE, New York, pp. 1008-1010.

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