Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

PRESENT STATUS OF THE C-BAND ACTIVITIES AT KEK
Matsumoto1, A), S. S. WinA), Shigeru TakedaA), J. S. OhB), H. BabaC), T. ShintakeC), SCSS GroupC),
Y. TakasuD), M. YoshidaD), F. FurutaE)
A)
KEK, Tsukuba JapanB)
PAL/POSTECH, Pohang, Republic of Korea
C)
RIKEN, Harima JapanD)
University of Tokyo, Tokyo JapanE)
University of Nagoya, JapanAbstract
We have already developed three conventional and oneperiodic permanent magnet (PPM) type 50-MW class
klystrons, a smart modulator, and the first HOM-free accelerator structure (Choke-mode type, full-scale highpower model) [1], [2], [3], [4]. A very stable ceramic highvoltage monitor was successfully tested up to 367-kV
with a 4.5-µsec pulse. A new C-band SiC type high power
rf-load, advancing the power handling capability up to 50MW is now being designed. It should have excellent mass
production characteristics as it uses circularly symmetric
TM011 chained cavities [5], [6]. The first high power prototype of an rf compressor cavity made of a low thermal
expansion material (super Invar) was designed to provide
stable operation even with a very high Q of 200-k [7], it
was successfully tested the output rf power up to 135MW, 0.5-µsec pulse width and 50-pps repetition rate. The
C-band linac rf-system will be used for the SASE-FEL(SCSS) production project at SPring-8 [8], but SCSS will
also serve to verify the design and components, which caneventually be deployed for the main linac rf system in afuture linear collider.

1. INTRODUCTION
The C-band main linac has been developed motivatedby the urgent and essential physics program at the linearcollider. The C-band technology needs a minimum R&D
towards early construction and reliable operation. It enables an early start to the physics program, so as to be as
concurrent as possible with LHC operation. Once a new

Figure 2: One unit of the C-band main linac.

Figure 1: C-band main linac tunnels. The klystron gallery is 4.5m in diameter and the linac tunnel is 3.0-m in diameter.

particle threshold opens, all angles of the new physics canbe thoroughly studied in the clean experimental environment of e+e- collisions.
The main linac system is the heart of linear collider. Itis a huge system, which is a repetition of thousands ofcommon RF-units. Therefore, in order to realize a good
physics program, these RF-units have to meet strict requirements for: (1) High reliability, (2) Simplicity, (3)Easy operation, (4) Reasonable power efficiency, and (5)Low cost.
This guiding principle provides boundary conditions
for our design work. The C-band group has been working
extensively along this line. Especially the first three items
are crucial for the large system to be operated, prior to thedetailed discussions on energy efficiency or upgradeability. The rf-frequency commonly used for various
electron linear accelerators is the S-band (2.8-GHz).
The S-band technology was established about 1/2
century ago, and it has had a long experience.
The C-band frequency is only two times higher
than the S-band, hence the size of the accelerator
structure is a half of the S-band. Therefore, the current technology for fabricating the components caneasily meet the accuracy required for the C-band
main linac. Consequently, this directly results inhigh reliability, simplicity, and hence low cost.
The total C-band main linac RF-system for 500GeV C.M. energy includes about 2000 rf-units asshown in Fig 2. In total about 8000 accelerating
structures and about 4000 klystrons with modulators
are needed for the two main linacs. The number ofE-mail: hiroshi.matsumoto@kek.jp
36Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

each component is not large enough to induce a drastic
cost reduction caused by the mass-production. Therefore,
from the beginning of the designing of each component,
the efforts toward cost reduction are absolutely necessary.
For this purpose the C-band group was inventing novel
ideas and executing special R&D’s for cost reduction.
The cost for the main linac and that for the civil engineering were estimated with a help of industry. The total
reduction factor of the current price over that for the mass
production is currently estimated to be around 2.2, averaged over all the main linac components. The total cost iskept reasonably low, in spite of the low reduction rate.
This is simply due to the efforts described above. In addition, the fabrication of the C-band components needs neither a number of advanced machines nor complicatedprocesses.
Although the number of accelerator physicists in the Cband group is small, the R&D budget is not enough, andwe are suffered from political oppressions in the past, thegroup has efficiently worked out the design lead by theguiding principles described above with a major collaboration with industry. The reason that there are so many
supporters of the C-band group in the industry and in universities is simply because the C-band technology is easier than other technologies and it naturally fits the current
technology level of the industry. Obviously, if an international team of accelerator experts extensively study the Cband main linac, they will certainly see the advantages ofthe technology and further improvements can be expected.
In this report, the rf-unit, klystron, modulator, pulse
compression system, accelerating structure, alignment andsupport system for the structures and possible energy upgrade scenario for the C-band technology are described ineach section.

2. RESULTS OF HARDWARE R&D
In April 1996, we started hardware R&D, and as ofJune 2003 we had developed most of the main hardware
components and tested their performances, with the exception of the high-power rf pulse compressor.

2.1 Klystron R&D
We have successfully developed a 50-MW class solenoid focus type klystron (TOSHIBA E3746 series), whichmeets the required specifications of the 500-GeV linear
collider as shown in Fig 3. As a part of the R&D programaiming toward a 1-TeV C.M. energy scale linear collider,we developed the first PPM klystron (TOSHIBA E3748
series) in 1999 [4]. It was obtained an output power of 37MW was generated with a 2.5-µsec pulse at a 50-pps
repetition rate.

2.2 Modulator Power Supply
We focussed our modulator R&D work on reducing
the fabrication cost and improving the reliability. As afirst step, we developed a prototype modulator, whoseFigure 3: Typical efficiency and RF output power characteristics at saturation as a function of beam voltage.
features are: (1) Direct HV charging from an inverter
power supply, (2) Much smaller in size than the usual
modulator, (3) Use existing low-risk reliable circuit components such as thyratron tube for the PFN switching.
To reduce the modulator size and allow removing thede’Q-ing circuit from PFN, we employed an inverter type
DC-HV power supply (EMI-303L in U.S.A). A firstmodel of the smart modulator was built in a metal cabinetof compact size 160 (W) x 200 (H) x 120 (D) cm. The
fluctuation in the measured output voltage was measured
to be lower than ±0.17% (at 3σ), which meets the energy
stability requirement for the GLC [9].
The next step in the smart modulator development wasto install everything except for the inverting H.V. supply
in an insulation oil filled metal tank or cabinet of very
compact size: 150 (W) x 100 (H) x 100 (D) cm [10]. The
first prototype was developed by NICHIKON Co in Japan,
and it was tested in March 2003 at SPring-8. We obtained
the results expected for the pulse wave shape, voltage andflatness, as well as verification of the operational repeatability and also markedly reduced EMI and noise generation.
A new prototype H.V. power supply was developed
by TOSHIBA Co. in Japan and it was tested along withthe rest of the modulator beginning in March 2003 atSPring-8. It is also very compact, being only 48 (W), 45(H) and 63 (D) cm. It generates a maximum output voltage of 50-kV and provides an average power of 30-kW(or a peak of 37.5-kJ/sec); this supply can drive a 50-MW
klystron at up to a 60-pps repetition rate giving a 350-kV
beam voltage. We obtained an output voltage regulationof within ±0.1% with a prototype model.

2.3 RF Pulse Compressor
At the present the initial test of a high power model
has begun at KEK. This prototype uses a copper plated
invar metal for the rf cavity [11], this permits the temperature control system for the rf compressor to be simplified and thus contributes to reducing the cost of the totalsystem. The current experimental result is a 135-MW
peak output power, 0.5-µsec pulse width at 50-pps repetition rate with a total multiplication factor of 3.0. The rfcompressor provide the very good thermal stabilities,
which is much better than conventional copper cavities.
There is no unusual vacuum out-gassing found even at the37Proceedings of the 1st Annual Meeting of Particle Accelerator Society of Japanand the 29th Linear Accelerator Meeting in Japan (August 4 - 6, 2004, Funabashi Japan)

high power operation as shown in Fig. 4.

Figure 4: Typical pulse compressor cavity rf power waveforms.

2.4 C-band Accelerating Structure

3. FUTURE R&D
1998 was successfully completed. Beginning in 1999, thenext priority is to develop a 50-MW class PPM type klystron with power efficiency of higher than 50%.
On a parallel track, in order to evaluate the system performance in a realistic situation, at least one unit of the Cband system has to be built, installed and beam tested in
an operational machine.
To this end, a c-band linac rf system is now under fabricating for the 1-GeV electron injector linac for the SCSS
project (SASE-FEL) at SPring-8. SCSS is comprised of abuncher system and 1-GeV injector followed by a 20-m
long in-vacuum undulator. It should be able to produce
X-rays in the water-window range of 1- to 10-nm wavelength. The first operation of the C-band linac at initial
beam energy will be started range of 0.5- and 1-GeV [14].

A C-band Choke-Mode type damped structure wasdeveloped in 1998; its performance was tested withASSET at SLAC. As expected, its powerful HOM damping performance was proven. We found parasitic resonance at very high frequencies around 23-GHz. The resonance was caused by EM fields trapped inside the cavity.
This problem can be resolved by changing the cavity dimensions by a small amount. The new structure is fabricated by brazing. High power tests of the structure will bestared in July this year at Spring-8 [12].

4. REFERENCE[1] T. Shintake et al., “C-band Main Linac RF System for Linear
Collider ”, KEK preprint 96-122, September. 1996.
[2] H. Matsumoto et al, “Fabrication of the C-band (5712 MHz)
Choke-Mode Type Damped Accelerator Structure”, KEK
preprint 98-143, September 1998.
[3] T. Shintake et al., “The first Wakefield Test on the C-band
Choke-Mode Accelerating Structure”, KEK preprint 99-11,May 1999.
[4] H. Matsumoto et al., "DEVELOPMENT OF THE C-BAND(5712 MHz) 50 MW CLASS PPM Klystron (II)", 26th Lin2.5 Pulsed High-Voltage Monitor
ear Accelerator Meeting in Japan, Aug.01~03,2001, Tsukuba,We have developed aJapan[5] S. S. Win et. al., “A new C-Band 50 MW Class SiC RF
very stable and accurateload”, Proc. of 28th LINAC Meeting, July 30-August 1,
high-voltage monitor, to be
2003, Tokai, J-PARK.
used for monitoring the kly[6]S. S. Win et. al., “A new C-Band 50 MW Class SiC RF load.
stron pulse voltage [13].
II”, Proc. of 14th Symposium on Accelerator Science andSince it uses a ceramic maTechnology, November 11-13, 2003, Tsukuba Japan.
terial for the capacitive type[7] M. Yoshida et al., "RF Pulse Compressor using Low Thermalvoltage divider (CVD), theExpansion Invar Material", 26th Linear Accelerator Meeting
monitoring capacitance diviin Japan, Aug.1~3, 2001,
sion ratio can be kept quite[8] T. Shintake et al., "The SPring-8 Compact SASE Source(SCSS)", SPIE2001, 29 July – 3 August 2001, San Diego,
stable even under tempera- Figure 5: Measured voltage
California, USAture changes, or changes in waveform at -367-kV.
the set-up configuration, or mechanical stresses applied to [9] J-S. Oh et al, “Efficiency Analysis of the First 111-MW Cband Klystron–Modulator System for Linear Collider”, KEK
the monitor port through the input lead. We successfully
Preprint 98-32, March 1998.
operated the monitor up to 367-kV, and 4.5-µsec pulse[10] H. Matsumoto et al., “Closed type Compact Modulator for 50
widths at a 50-pps repetition rate, see Fig. 5. For the moMW C-band Klystron”, Power Modulator Conference &ment, the maximum voltage in the CVD test was limitedHigh Voltage Workshop, June 30 – July 3, 2002, Hollywood,by the modulator output voltage.
CA.
[11] M. Yoshida et al., “RF Pulse Compressor System”, 27th Li2.6 A New C-band 50-MW SiC Type RF Loadnac meeting August 7-9, 2002, Kyoto.
[12] T. Inagaki et al., “High Gradient Test of The C-band ChokeThere are no available 50-MW class rf loads for CMode Type Accelerating Structure”, 29th Linac meeting,band frequencies in the world. Therefore we have conChiba, August 4-6 2004.
structed a new type rf load using SiC ceramic with an [13] Y. Takasu et al., “Pulsed High Voltage Monitor Using Ceindirect water-cooling structure. We chose a circularramic”, 27th Linac meeting August 7-9, 2002, Kyoto.
mode TM011 chained cavities. One particular advantage is [14] T. Shintake et al., “Progress on the C-band Accelerator for
the Spring-8 Compact SASE Source”, proc. of PAC2003,
that since the main parts are completely axially symmetric,
May 2003, Portland, Oregon, USA.
they can be machined on a turning lathe; thus this type ofcavity has a big advantage in mass production because of
its easier machining.

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