KEK C-BAND RF-SYSTEM R&D STATUS FOR LINEAR COLLIDER
H. Matsumoto, Su Su 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 C-bandklystrons (TOSHIBA E3746 series and E3748 series), asmart modulator, and the first HOM-free accelerator
structure (Choke-mode type, full-scale high power model)[1], [2], [3], [4].
A very stable ceramic high voltage monitor was
successfully tested up to 367-kV with a 4.5-µsec pulse. A
very good agreement with the expected division ratio andsignal waveforms was obtained from high power tests.
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 avery high Q of 200-k [5], it is now under high power testing at KEK.
A new C-band SiC type high power rf-load, advancing
the power handling capability up to 50-MW is now being
designed. It should have excellent mass production characteristics as it uses circularly symmetric TM011 chainedcavities. The first high power test should be completed at
KEK by the end of 2003.
The C-band linac rf-system will be used for the SASEFEL (SCSS) production project at SPring-8 [6], 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.

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

1 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 7000 accelerating structures, 3500 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 forour design work. Of all the free system
C-bandLCC-band LINAC RF-SYSTEM LINEAR COLLIDER
parameters that we may vary, the choice of
AC POWER LINE
5712 MHz, S = 500 GeV FOR TWO LINACS
the drive rf frequency plays the determinaSMART MODULATOR
RF-SYSTEM: 1789 UNITSMODULATORS: 3560tive role in setting the system performance
KLYSTRONS: 3560as well as most other hardware details. For
ACC. STRUCTURES : 7120
ACTIVE LENGTH: 12.8 KM
reasons previously argued in [2], we proWALL-PLUG POWER :130 MW
pose that the C-band frequency is the best
HV INVERTER POWER SUPPLY
choice to meet all of the demands.
50 MW KLYSTRON
PM
47kV, 37kJ/s

PFN
MODULEPFN
MODULEPFN
MODULE

23.5 kV
H.V. pulse

BEAM LOADING COMPENSATION USING PHASE -TO-AMPLITUDE MODULATION

50 MW
2.5 µs

RF-PULSE
COMPRESSORWAVEGUIDEø1ø2400 MW, 0.5 µsx 4.0,η = 80%

Klystron
350 kV317 A
100 ppsµ P = 1. 53
η = 45%AM

V2TO STRUCTURES

95 MW, 0.5 µs
Q-MAGNET

Ea=36 (45) MV/m
ACCELERATOR
BPM

GIRDER
ACTIVE MOVER8m

3 π 4, a λ = 0.13~0.17, vg=0.012~0.035c, r=60 MOhm/m, Tf=286 ns,
τ = 0.53

ø2

COMPRESS PULSE COMPRESSOR INPUTOUTPUT RF-POWER

1.8 m-LONG

ø1

V1
1:15

WR-187 Rectangular
LOSS ~5%

ACCELERATOR
Beam

100 MW
2.5 µsPHASE SHIFT

1:15

100 MW, 2.5 µs
0°

TIME
180°
400 MW,0.5 µs

ACTIVE MOVER

ALIGNMENT TORELANCE : σ ~ 30 µm / STRUCTURE

TIME

Linear Collider
By T. Shintake (KEK)
March, 1998, H. MATSUMOTO & T. SHINTAKE, KEK

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

E-mail: hiroshi.matsumoto@kek.jp

2 SYSTEM DISCRIPTION
Each unit in the main linac rf-system iscomposed of two 50-MW klystrons, theirpulse modulators, one rf-pulse compressor,
four 1.8-m-long choke-mode accelerating
structures and associated wave-guidesystem as shown in Figure 2. The accelerating gradient is 36-MV/m under full beamloading. This accelerating gradient waschosen as practicable after studying results

from S-band frequency high gradient tests done between
1987 and 1994 at KEK. The total RF-system to achieve
500-GeV c.m. energy would be on the order of 2000 units.

3 RESULTS OF HARDWARE R&D
In April 1996, we started hardware R&D, and as ofJune 2002 we had developed most of the hardware components and tested their performances, with the exceptionof the high-power rf pulse compressor.

3.1 Klystron R&D
We have successfully developed a 50-MW class solenoid focus type klystron, which meets 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 programaiming toward a 1-TeV c.m. energy scale linear collider,we developed the first PPM klystron in 1999 [4]. We
chose a NdFeB magnet material (Model N40A, Shin-Etsu
Co. in Japan). The Hot Isostatic Pressing (HIP) technique
has been applied for the first time to fabricate themagnetic circuit for the PPM klystron. An output powerof 37-MW was generated with a 2.5-µsec pulse at a 50pps repetition rate.

3.2 Modulator Power Supply
We focussed our modulator R&D work on reducing thefabrication cost and improving the reliability. In 1993, theconcept of a “Smart Modulator” was proposed by Prof. M.
H. Cho and the author [7]. As a first step, we developed aprototype modulator, whose features 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: the Electric Measurement Inc.
Model EMI-303L. The smart modulator was built in a
metal cabinet of compact size 160 (w) x 200 (H) x 120(D) cm. It is now in daily use driving a 50-MW klystron.
The total accumulated run time has exceeded 20,000hours. The fluctuation in the measured output voltage wasmeasured to be lower than ±0.17% (at 3σ), which meetsthe energy stability requirement for the linear collider.
The timing variation (jitter and drift) of the pulse output isaround 2-nsec (at 3σ) over a 4 hour run at 50 pps [8].
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 (Width) x 100 (Height) x 100 (Deep)cm, see Figure 3 [8]. The first prototype was developed
by NICHIKON Co. 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 verificationof the operational repeatability and also markedly reduced
EMI and noise generation.

Fig 3: Closed type compact modulator.

3.3 Pulsed High-Voltage Monitor
We have developed a very stable and accurate highvoltage monitor, to be used for monitoring the klystronpulse voltage, see Figure 4 [9]. Since it uses a ceramicmaterial for the capacitive type voltage divider (CVD),
the monitoring capacitance division ratio can be kept
quite stable even under temperature changes, or changes
in the set-up configuration, or mechanical stresses applied
to the monitor port through the input lead. We successfully operated the monitor up to 367-kV, and 5-µsec pulse
widths at a 50-pps repetition rate. The maximum voltage
in the CVD test was limited by the modulator output.

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

3.4 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 [10], 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. There is no unusual vacuum outgassing found even at the high power operation. The test
set up is shown in Figure 5.

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

4 FUTURE R&D
Fig 5: High power test stand for the rf pulse compressor cavity.

Fig. 6: Typical pulse compressor cavity rf power waveforms.

3.5 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 Figure 7. One particular advantage is that since
the main parts are completely axially symmetric, they canbe machined on a turning lathe; thus this type of cavity
has a big advantage in mass production because of its
easier machining. The load is now at the detailed designstage, and we expect that high power tests at KEK will becompleted by the end of 2003.

Fig. 7: First model of 50-MW SiC type C-band rf load.

3.6 C-band Accelerating Structure
A C-band Choke-Mode type damped structure wasdeveloped in 1998; its performance was tested with

The 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 inan 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 [11]. 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 an initial
beam energy of 500-MeV.

5 REFERENCE[1] T. Shintake et al., “C-band Main Linac RF System for LinearCollider ”, 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] M. Yoshida et al., "RF Pulse Compressor using Low Thermal Expansion Invar Material", 26th Linear Accelerator
Meeting in Japan, Aug.1~3, 2001,[6] T. Shintake et al., "The SPring-8 Compact SASE Source(SCSS)", SPIE2001, 29 July – 3 August 2001, San Diego,California, USA[7] J-S. Oh et al, “Efficiency Analysis of the First 111-MW Cband Klystron–Modulator System for Linear Collider”, KEKPreprint 98-32, March 1998.
[8] 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.
[9] Y. Takasu et al., “Pulsed High Voltage Monitor Using Ceramic”, 27th Linac meeting August 7-9, 2002, Kyoto.
[10] M. Yoshida et al., “RF Pulse Compressor System”, 27thLinac meeting August 7-9, 2002, Kyoto.
[11] 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.