The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

THE C-BAND (5712-MHZ) RF SYSTEM FOR e+e- LINEAR COLLIDER
H. Matsumoto, S. S. Win, Shigeru Takeda, KEK, Tsukuba Japan
Y. Takasu, M. Yoshida, University of Tokyo, Tokyo Japan
H. Baba, T. Shintake, SCSS Group, RIKEN, Harima JapanJ. S. Oh, PAL/POSTECH, Pohang, Republic of Korea
Abstract
Hardware R&D on the C-band (5712-MHz) RF-system
for an e+e- linear collider started in 1996 at KEK. We
have already developed three conventional and one periodic permanent magnet (PPM) type 50-MW class klystrons, a smart modulator, and the first HOM-free accelerator structure (Choke-mode type, full-scale high powermodel) [1], [2], [3], [4]. A very stable ceramic high voltage monitor was successfully tested up to 367-kV with a4.5-µsec pulse. A very good agreement with the expecteddivision ratio and signal waveforms was obtained fromhigh power tests. A new C-band SiC type high power rfload, advancing the power handling capability up to 50MW is now being designed. It should have excellent massproduction characteristics as it uses circularly symmetric
TM011 chained cavities [5], [6]. The first high power test
should be completed at KEK by the end of 2003. The firsthigh power prototype of an rf compressor cavity made ofa low thermal expansion material (super Invar) was designed to provide stable operation even with a very highQ of 200-k [7], it is now under high power testing at KEK.
The C-band linac rf-system will be used for the SASEFEL (SCSS) production project at SPring-8 [8], but SCSS
will also serve to verify the design and components,which can eventually be deployed for the main linac rf
system in a future linear collider.

INTRODUCTION
To be interesting, an e+e- linear collider must be a verylarge-scale machine. For example, the e+e- collider underconceptual design now at KEK would have a main linac
for each of two beams, and in total more than 8000 accel-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.

erating structures, 4000 klystrons and modulators would
be used. Therefore, the hardware we develop has to meetvery strict demands for: (1) High reliability, (2) Simplicity, (3) Reduced construction cost, (4) Reasonable power
efficiency, and (5) Operational ease. This list provides aguideline and the boundary conditions for our design
work. Of all the free system parameters that we may vary,
the choice of the drive rf frequency plays the determinative role in setting the system performance as well as
most other hardware details.
For reasons previously argued in [2], we propose that
the C-band frequency is the best choice to meet all of thedemands.

SYSTEM DISCRIPTION
Each unit in the main linac rf-system is composed of
two 50-MW klystrons, their pulse modulators, one
rf-pulse compressor, four 1.8-m-long choke-mode
accelerating structures and associated wave-guidesystem as shown in Fig. 2. The accelerating gradientis 35-MV/m under full beam loading. This accelerating gradient was chosen as practicable after studying
results from S-band frequency high gradient tests
done between 1987 and 1994 at KEK. The total RFsystem to achieve 500-GeV C.M. energy would beon the order of 2000 units.

RESULTS OF HARDWARE R&D

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

E-mail: hiroshi.matsumoto@kek.jp

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

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

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. In Phase-II R&D, we have to refine the design
details to reduce costs before starting mass-production. As
a part of the R&D program aiming toward a 1-TeV C.M.
energy scale linear collider, we developed the first PPMklystron (TOSHIBA E3748 series) in 1999 [4]. We chosea NdFeB magnet material (Model N40A, Shin-Etsu Co. inJapan). The Hot Isostatic Pressing (HIP) technique has
been applied for the first time to fabricate the magnetic
circuit for the PPM klystron. An output power of 37-MW
was generated with a 2.5-µsec pulse at a 50-pps repetition
rate.

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, whosefeatures are: (1) Direct HV charging from an inverter
power supply, (2) No de’Q-ing circuit, (3) Much smaller
in size than the usual modulator, (4) Use existing low-risk
reliable circuit components.
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 linear collider. The timing
variation (jitter and drift) of the pulse output is around 2nsec (at 3σ) over a 4 hour run at 50-pps [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.
Testing was begun in March 2003 at SPring-8. We obtained the results expected for the pulse wave shape, voltage and flatness, as well as verification of the operational

Figure 3: A fist inverter type H. V. power supply (TOSHIBA).

repeatability and also markedly reduced EMI and noise
generation.
A new prototype H.V. power supply was developedby 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 test prototype as shown in Fig. 3.

Pulsed High-Voltage Monitor
We have developed a very stable and accurate highvoltage monitor, to be used for monitoring the klystronpulse voltage, see Fig. 4 [11]. Since it uses a ceramic material for the capacitive type voltage divider (CVD), themonitoring capacitance division ratio can be kept quite
stable even under temperature changes, or changes in theset-up configuration, or mechanical stresses applied to themonitor port through the input lead. We successfully operated the monitor up to 367-kV, and 4.5-µsec pulse
widths at a 50-pps repetition rate. The maximum voltage
in the CVD test was limited by the modulator output.

Figure 4: Ceramic high-voltage monitor. Left: A cup type monitor. Right: High voltage waveform at 367-kV and 4.5-µsec.

A New C-band 50-MW SiC Type RF Load
There are no available 50-MW class rf loads for Cband (5712-MHz) frequencies in the world. Therefore wehave constructed a new type rf load using SiC ceramicwith an indirect water-cooling structure, this upgraded thepower handling density of the SiC material from 100-W
per cc to 300-W per cc, allowing a much more compact
size. Also, using circular mode TM011 chained cavities asshown in Fig. 5. One particular advantage is that since themain parts are completely axially symmetric, they can bemachined on a turning lathe; thus this type of cavity has a
big advantage in mass production because of its easiermachining. For impedance matching between the waveguide and the cavity-chain of TM011 mode cavities, wechose an inductive iris-type coupler, which is structurally
simpler than the doorknob-type coupler. The load is now
at the detailed design stage, and we expect that highpower tests at KEK will be completed by the end of 2003.

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

cavity dimensions by a small amount. The new structure
is fabricated by brazing. High power tests of the structure
will begin at Spring-8 in Decmber 2003.

FUTURE R&DFig. 5: First model of 50-MW SiC type C-band rf load.

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 [12], 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 90-MW pulse,
0.5-µsec wide at a 50-pps repetition rate with a total multiplication factor of 3.0. The rf compressor provide thevery good thermal stabilities, which is better than conventional copper cavities. The test set up is shown in Fig. 6.
There is no unusual vacuum out-gassing found even at thehigh power operation as shown in Fig. 7.

Figure 6: High power test stand for rf pulse compressor cavity.

Figure 7: Typical pulse compressor cavity rf power waveforms.

C-band Accelerating Structure
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 thecavity. This problem can be resolved by changing theThe first stage of the C-band R&D between 1996 and1998 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 [13]. SCSS is comprisedof a buncher system and 1-GeV injector followed by a 20m long in-vacuum undulator. It should be able to produce
X-rays in the water-window range of 1- to10-nm wavelength. The first operation of the c-band linac at initial
beam energy of 500-MeV.

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 Linear Accelerator Meeting in Japan, Aug.01~03,2001, Tsukuba, Japan[5] S. S. Win et. al., “A new C-Band 50 MW Class SiC RFload”, Proc. of 28th LINAC Meeting, July 30-August 1,
2003, Tokai, J-PARK.
[6] S. S. Win et. al., “A new C-Band 50 MW Class SiC RFload. II”, Proc. of 14th Symposium on Accelerator Scienceand Technology, November 11-13, 2003, Tsukuba Japan.
[7] M. Yoshida et al., "RF Pulse Compressor using Low Thermal Expansion Invar Material", 26th Linear Accelerator
Meeting in Japan, Aug.1~3, 2001,[8] T. Shintake et al., "The SPring-8 Compact SASE Source(SCSS)", SPIE2001, 29 July – 3 August 2001, San Diego,California, USA[9] J-S. Oh et al, “Efficiency Analysis of the First 111-MW Cband Klystron–Modulator System for Linear Collider”,
KEK Preprint 98-32, March 1998.
[10] H. Matsumoto et al., “Closed type Compact Modulator for
50 MW C-band Klystron”, Power Modulator Conference &High Voltage Workshop, June 30 – July 3, 2002, Hollywood, CA.
[11] Y. Takasu et al., “Pulsed High Voltage Monitor Using Ceramic”, 27th Linac meeting August 7-9, 2002, Kyoto.
[12] M. Yoshida et al., “RF Pulse Compressor System”, 27thLinac meeting August 7-9, 2002, Kyoto.
[13] T. Shintake et al., “Progress on the C-band Accelerator for
the Spring-8 Cpmpact SASE Source”, proc. of PAC2003,May 2003, Portland, Oregon, USA.