Proceedings of LINAC 2004, Lübeck, Germany

DEVELOPMENT OF A C-BAND ACCELERATING MODULE FOR
SUPERKEKB
S. Ohsawa, T. Kamitani, T. Sugimura, K. Kakihara, M. Ikeda, T. Oogoe,S. Yamaguchi, and K. Yokoyama
High Energy Accelerator Research Organization, KEK, 1-1, Oho, Tsukuba, Ibaraki, Japan
which influence depends on the positron energy: higher
the energy, smaller the effect. Therefore 8GeV positrons
would be useful and desirable also for the KEKB insteadof the present energy 3.5GeV. The requirement andimportance of the beam seem to be growing, especially sounder the present status in which beam intensities are still
kept rising in both KEKB rings to increase luminosity.
Then exchange of energies would be more urgent issue
that should be realized quickly, if possible, before theSuperKEKB project starts. It would have an importantrole to escape the influence of the electron cloud effect on
positrons. Our strategy to get 8GeV positrons is basicallysimple [2].

Abstract
A C-band accelerating module has been constructed inKEKB/PF linac, and beam acceleration tests have beenperformed during 10-month operation. The purpose is toinvestigate C-band feasibility and stability of accelerationin the region beyond 40MV/m. The C-band accelerating
module is expected to be promising for accelerating
positrons up to 8GeV instead of 3.5GeV for both of thepresent KEK B-factory and SuperKEKB project in order
to upgrade the luminosity. Last summer a 1m-long Cband accelerating section installed in the KEKB/PF linac.
Accelerating field corresponding to 41 MV/m was
successfully achieved in October in a beam test. Presentstatus of C-band accelerator development is reported.

INTRODUCTION
The KEK-B factory is making highest luminosities(>1.3×1034 cm-1s-1) in the world, where 3.5-GeV electrons
and 8-GeV positrons are colliding. Toward higher
luminosities a future project SuperKEKB is underconsideration, of which target luminosities arise in theorder of 1-5 ×1035 cm-1s-1 [1]. In order to put into practicesuch a high goal, requirements to the injector linac should
inevitably become severe for all values such as beam
intensities, energies and emittances etc., as are listed intable 1. Some schemes of linac upgrade have been
considered what should be improved to meet therequirements as well as possibility and feasibility, andconsideration is still going on.
Table 1: Upgrade requirements
KEKB
SuperKEKB
Beam energy e+
3.5 GeV
8.0 GeV
e8.0 GeV
3.5 GeVStored current e+
2.6 A
4.1 A
e1.1 A
9.4 A
Linac beam e+
0.6 nC x 2
1.2 nC x 2
e1.0 nC x 1
2.5 nC x 2
Smaller emittance to fit IR&C-band structure aperture
Faster e+/e- mode switching for continuous injection

Figure 1: Layout of an upgrade scheme of theKEKB/PF linac
Presently positrons are produced at a production target
which is installed halfway of the KEKB/PF linac, as isshown in Fig.1, and then accelerated up to 3.5 GeV in thefollowing half of the linac. The most simple and feasible
way to get 8-GeV positrons would be increasing theaccelerating fields in the second half of the linac. This
scheme is direct method and does not request any newbuildings; however, the accelerating field strengths should
be increased double from the present value of 21MV/m to42 MV/m. How can we realise such a high accelerating
field, that is the question to be solved. It is obvious thatsuch a high field could not be obtained without increasing

Among the requirements, there has been a subject
useful not only SuperKEKB but also for the KEKB. It is aplan to exchange energies of electrons and positrons:
positrons become 8GeV instead of 3.5GeV, and electrons
become 3.5GeV from 8GeV. The purpose is to avoid
positron instability in the ring due to electron cloud, ofFigure 2: Layouts are showing how to replace
accelerating modules from S-band to C-band.

___________________________________________

satoshi.ohsawa@kek.jp

1Proceedings of LINAC 2004, Lübeck, Germany

values achieved in high power test operations. Most of thevalues exceeded successfully specifications.

the accelerating frequency of S-band. Otherwise RF
power of four times higher than the present value 41MW
would be necessary. That would be impossible due to lackof powers of electricity and cooling water, and lack ofspace for the extra klystrons and modulators in theklystron gallery.
The only way we can take seems to be introducing ahigher accelerating frequency instead of S-band. From thepractical points of view, the frequency was automaticallydecided on C-band (5712MHz), of which accelerating
structures could be expected to give apertures large
enough for accelerating positrons with large emittances, atleast, at high energy region. Furthermore a C-band
klystron has been fortunately already developed, and iscommercially available.
In order to achieve the average accelerating field of42MV/m, it is not enough merely to replace acceleratingstructures to C-band ones, but also necessary to double
the RF powers for each accelerating section This meansthat one accelerating module with a S-band klystronshould be replaced, for instance as is shown in Fig. 2,
with two C-band modules, each of which has a C-band
klystron.
Outline of our energy upgrade is shown in Table 2: inthe second half of the linac after the positron production
target, most of the S-band Accelerating modules should
be replaced by C-band ones, except for the beginning partfollowing the target, where the positron beam emittance isstill so large that the beam size is not small enough foraccelerating in the C-band structures.

Table 3: C-band R&D main items and maximum values
achieved in test operations.
Klystron
E3746
Compact Modulator (1/3size)
Sub-booster
RF window
Accelerating section
Dummy load
3-dB hybrid power divider
Wave guide flange
Pulse compressor, SKIP

Accelerating Section
A first prototype accelerating section of C-band was
designed basically as a half size of the present 2-meterlong S-band section that is used in the KEKB/PF linac [4].
Therefore it is 1m long with a quasi-constant field
gradient. By comparing with S-band values, we could
determine with confidence precise sizes of each cavity
from a few test cavities
After RF processing of totally 300 hours, which was
performed at the test stand with 0.5 µs RF pulses, theacceleration field reached to the level of 41.8 MV/m.
Then the C-band accelerating section had been installedin the KEKB/PF linac in September 2003. RF power froma C-band klystron fed into a single 1m-long section
during the first 10-month operation without a pulse
compressor, which was still under investigation last year,
but installed recently August 2004 [5].
After the 10-month operation at 50 pps with 0.5 µs RF,
we opened and directly observed the couplers andaperture disks August 2004, and found that the inputcoupler iris was discoloured at peripherals and there weremany discharge spots especially at the aperture corner ofthe first disk. This observation is consistent with othermeasurements such as analyses using the reflection andtransmission waves or acoustic sensors. It is obvious thatthe iris is the main source of discharges. In the secondprototype accelerating section, the coupler structure will
be changed so that it will have a thicker and wider iris
without sharp edges in order to avoid discharge due to RF
heating.

Table 2: Energy upgrade scheme with C-band accelerating
sections

Acc. field
Presentupgrade

S-bandsection
21 MV/m
231 m
46 m

C-bandsection
42 MV/m
0185 m43MW, 2µs, 50pps
45kV, 2µs, 50pps
400W, 2µs, 50pps
300MW, 2µs, 50pps
41MV/m, 0.5µs, 50pps
100MW, 0.5µs, 50pps
200MW, 0.5µs, 50pps
200MW, 0.5µs, 50pps
200MW, 0.5µs, 50pps

e+ energy
gain
4.8 GeV
8.0 GeV

DEVELOPMENT OF C-BAND MODULE
We decided in 2002 to start investigation of C-band
accelerating structures how would be operational stability
and reliability at accelerating fields as high as 42MV/m
for both of KEKB and SuperKEKB. We started
developing from April 2002 a C-band accelerating
module to fine difficulties, if any, as quick as possible.
Although almost of all elements necessary for the Cband module should had been designed from thebeginning except for the klystron, until July 2003
minimum elements, such as those listed in Table 3, hadbeen prepared for making an acceleration test except for
an RF pulse compressor [3]. Then after confirming highpower performances in the test stand, we installed them in
the KEKB/PF linac at the #44 unit where was empty andready for C-band. The relevant main items that have beendeveloped for C-band are listed in Table 3 with maximum

Figure 3: The first prototype 1m-long C-band accelerating
structure.

2

Proceedings of LINAC 2004, Lübeck, Germanyin the C-band module in the linac for long time operation
with beam acceleration.
Although SLED-type RF pulse compressors are used inthe KEKB/PF linac, we adopted a different mode cavity
for the C-band pulse compressor [5]. That is TE038-mode
used in the LIPS [6], because a Q factor much higher
than100,000 is necessary to achieve the same fieldmultiplication factor as the S-band pulse compressors.
This requirement comes from a difference of RF pulse
lengths: C-band RF pulses have a half-length of the Sband. Although SKIP has many more nodes in the cavities
than SLED, the C-band wavelength is half of S-band one.
Therefore Mechanical sizes of SKIP become about thesame as the S-band SLED.

Dummy Load
A dummy load was developed for C-band based on theS-band one which is used in the KEKB/PF linac. The
dummy load that consists of 13 pairs of SiC buttons havebeen tested up to 100 MW peak and 2 kW average powers.

Figure 4: Photograph of a dummy load for C-band.
First Acceleration Test
During 10-month operation, the accelerating fieldstrengths of the C-band section have been measured byanalysing beam-energy gains as a function of theaccelerating phase. A field gradient of 41.2 MV/m wasachieved with the klystron output power of 43.8MW,
which was almost our goal.

Table 4: Comparison between KEKB-SLED andC-band SKIPKEKB-SLED C-band SKIP
Frequency
2856 MHz
5712 MHzRF pulse length
4.0 µs2.0 µsResonance mode
TE015
TE038
Length
33.59 cm
30.72 cm
Cavity diameter
20.51 cm
23.28 cm
Q value (Qo)90,000130,000
Coupling
6.4
6.6

Figure 5: The first prototype 1m-long accelerating section
installed in the beam line of the KEKB/PF linac.

Figure 7: Photograph of SKIP at test stand.

ACKNOWLEDGEMENT
The authors would like to give their sincere thanks toall staff members of KEKB/PF linac and Mitsubishi
Heavy Industries, LTD for their powerful cooperation.

REFERENCES[1] J. W. Flanagan, Y. Ohnishi, et al., Letter of Intent for
KEK Super B Factory, Part III: Accelerator Design,
KEK Report 04-4
[2] T. Kamitani, EPAC2002, Paris, p.1088.
[3] S. Michizono, et al., “KEKB Injector Linac andUpgrade for SuperKEKB”, this conference.
[4] T. Kamitani, et al., “Development of C-band
accelerating section for SuperKEKB”, this conference.
[5] T. Sugimura, et al., “SKIP- a Pulse Compressor for
SuperKEKB ”, this conference.
[6] A. Fiebig, et al., CERN/PS 87-45(RF) March, 1987.

Figure 6: A result of beam acceleration.
Pulse Compressor（ SKIP）
We have manufactured this year a C-band pulse
compressor “SKIP”, which stands for SuperKEKB
Injector Pulse Compressor. After 170-hour RF processing,
the peak output power attained 200 MW at a repetition
rate of 50 pps for 43 MW input. SKIP has been installed3