The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

R&D Status of the C-band Linear Accelerator Technology for e+e· Linear Collider
Hiroshi MATSUMOTO, Tsumoru SHINTAKE, Mitsuhiro YOSHIDA*,
Yuko TAKASU*, and Jong Seok OH**High Energy Accelerator Research Organisation (KEK)
1-1 Oho, Tsukuba, 305-0801, Japan*University of Tokyo**Pohang Accelerator Laboratory (PALIPOSTECH)
San-31, Pohang, 790-794, South Korea
Abstract
Hardware R&D on the C-band (5712-MHz) RF-systemfor the e+e- linear co llider started in 1996 at KEK. We developed three conventional and a periodic permanent magnet (PPM) type 50-MW class C-band klystrons (TOSHIBA
E3746 series and E3748 series), a smart modulator, a travelling-wave resonator (TWR), a high power model of an rfpulse compressor cavity is now under fabricating and thefirst HOM-free accelerator structure (Choke-mode type,full-scale high power model) [1], [2].
The c-band linac rf-system will use for the SASE-FEL(SCSS) project in the SPring-S [3], which is actually makestrings test of the main Iinac rf system for the linear collider.
Life test was carried out on the conventional klystrons
over 6000 hours in total, where they showed highly reliable
performance as high as conventional S-band klystrons. As aupgrading program to 1 TeV c.m. energy scale, we developed a new PPM klystron in 1999. The first PPM klystronwas generated 37 MW with 2.5-J.Lsec pulse width and 50 pps
repetition rate [4].
The first high power model of an rf compressor cavity
was used a low thermal expansion material (super Invar) to
make stable operation with 200k ofQ-value [5], and its highpower test will on a early 2002 in KEK.
The electrical performance on the HOM-free accelerating
structure was tested with beam at SLAC-ASSET facility inDecember 1998 [6]. The measured wakefield showed excellent performance on the HOM damping as exactly expectedfrom the theory.

C-bandFig. I: C-hand main linac tunnels. It has diameter of 4.5-m for theklystron gallery and 3.0-m for the linac.

1 Introduction

The e+e- linear collider (500-GeV c.m.) will use morethan 8000 accelerating structures, 4000 klystrons and theirpulsed-modulator power supply. We have to maintain thebeam operation stably for a long period in the physics run.
Since 2000 units of the rf-system will be used in the main
linac, each component has to be very reliable. And thepower efficiency has to be high enough to lower the operation cost. Additionally, the construction cost has to be quite
low. The main linac will be installed in two parallel tunnels
with circular cross-section with diameters of 3-m and 4.5-m
for the accelerator and klystron gallery,
COL:b:IlOO!:i!itLC
respectively
shown in Figure 1. The tunnels
LINAC RF-SYSTEM
LINEAR
will be constructed in a very stable stratum
GeV FOR THO LlNA<:S SUCh as granite.
The key to succeed in developing such rfsystem, which meets all of those demands, isthe choice of the optimum drive rf frequency.
The choice has to be made by compromising
the following contradictions.
For the point of view of energy efficiency
from rf to the beam in the accelerating
structure, a higher frequency is desirable, since
the shunt impedan~92becomes higher at higher
frequency ( r oc f).
On the other hand, at higher frequencies, the+•'u;,technique required to fabricate acceleratingstructure becomes higher, and mass production
becomes sever. Additionally, the peak output
power from the klystron generally goes down,, , '"'"""· "'"'' as the frequency becomes higher.
We chose C-hand 5712 MHz as theFie:. 2: One unit of the C-hand main linac.
optimum frequency. It is twice higher than the;"'/~,;<,';. 6 ~-H~~-l~~-•)t~-~'~lS~. >"v~~

Wll:o>/o;, :l~v~-~),

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S-band 2856 MHz commonly used in conventional electronlinear accelerators. The advantage in choosing the C-band
frequency is that we can obtain high acceleratin& gradientrequired for 500 GeV LC using existing technology.
This paper will describe results of the hardware R&D on
the C-band RF system and current status.
l System Description

Each unit in the main linac rf-system is composed of two
50-MW klystrons, their pulsed modulators, one rf-pulse
compressor, four 1.8-m-long Choke-mode accelerating
structures and associated waveguide-system as shown in
Figure 2. The accelerating gradient is 36-MV/m under thebeam loading. The total number of the RF-system toachieve 500-GeV c.m. energy is 2000 units.
3 Results of Hardware R&D
In April 1996, we started hardware R&D, and up to now(April2001) we have developed all of the hardware components and tested those performances, except the rf pulse
compressor high-power model. They are summarized inTable-I.

3.1 The klystron R&D
We have successfully developed the 50-MW class klystron, which meets specifications of the 500-GeV linearcollider. The R&D achievement level is 900AI, the rest of
10% is that the klystron was tested at 50-pps, while the design parameter assumes 100-pps operation. This is limitedby the maximum power of the inverter capacitor charger,
which we purchased at the beginning of the project. At 100pps, the heat load becomes twice higher at the beam collector in klystron. Since the present design has enough margins,
we expect no serious problems in 100-pps operation. In
Phase-11 R&D, we have to refine the design details to reduce cost before starting the mass-production. As a part ofthe R&D program toward 1 TeV c.m. energy scale linear
collider, we developed the first PPM klystron in 1999 asshown in Figure 3. We
chose the NdFeB magnet(Model N40A, Shin-Etsu
Co. in Japan), which has aresidual flux density (Br) of
1.22 Testa and a coercive
force of 10.5 k-Oersted. HotI
isostatic pressing (HIP)~Itechnique
has been fJrStly
applied to fabricate a magII .. ....,,_____ .,....c.a,
netic circuit in a PPM klystron. Simply stacking disksof the magnetic stainlesssteel (Mag-SUS) and oxygen-free-copper (OFC) al-tematively, and, they werebonded in one block with-··diffusion bonding. No brazFig. 3: Cut away view of the first ing-alloys were used in thisC-band (5712-MHz) 50 MW-class process. An output power
PPM type klystrons (TOSHIBA- of 37 MW was generated
with 2.5-~sec pulse widthE3747).
SJt2MIII,==::.
-··

and 50 pps repetition rate.

3.2 The RF pulse compressor (cold model)
A new scheme was proposed as a flat-pulse rf-pulse
compressor by T. Shintake in 1996, and the demonstrated
generation of a flat-pulse in 1996 as shown in Figure 4. We
have obtained the 3 .25 ofpower gain with 65% ofefficiency. The first highpower model is using aninvar metal as the rf cavity
with copper plating, thetemperature control system
to .the rf compressor will be
simplified and contributes toreduce cost of the total system. The high power modelFig. 4: A flat-pulse output wave- is now under fabricating,form from rfpulse compressor
and it will be tested end of. this year (200 1) in KEK.
3.3 The C-hand accelerating structure
The C-band Choke-Mode type damped structure was developed in 1998, its performance was tested with ASSET at
SLAC as shown in Figure 5. The powerful HOM damping
performance was proven. We found parasitic resonance atvery high frequency around 23 GHz, which is caused by
EM field trapping inside the cavity. It can be solved bychanging the cavity dimensions by small amount. For thebeam alignment, the cavity type RF-BPM was developedand its performance was tested with ASSET beam [7]. The
position acc~y of 1O-J.11D and the resolution of 1-JJ.m wereconfirmed. High power test of the structure will start in
middle of2002 in SPring-8.

Fig. 5: The C-band accelerating structure of 1.8-m long, 91 cells.

3..1 Modulator power supply
Accordingly we focussed our R&D work on reducing thefabrication cost and improving the reliability. In 1993, theconcept of the i Smart Modulatori was proposed by Prof. M.
H. Cho and the author. As a fJrSt step, we developed a prototype modulator, whose features are: (1) Direct HV charging from an inverter power supply, (2) No deQ-ing circuit,(3) Much smaller size than the usual modulator and (4) Use
existing reliable circuit components.
To reduce the modulator size and allow removing thedeQ-ing circuit from PFN, we employed an inverter type
DC-HV power supply: Model EMI-303L (Electric-53-The 13th Symposium on Accelerator Science ·and Technology, Suita, Osaka, Japan, October 2001

Measurement
Inc. The smart
modulator wasinstalled in a
metal cabinet ofcompactsize
160 (w) x 200(H) x 120 (D)em as shown inFigure 6. It is
now in daily usefor driving the50-MW
Fig. 6: First prototype Smart Modulator.
klystron.
Thetotal run time has exceeded 6000 hours. The fluctuation of
the measured output voltage was lower than ±0.17% (at 3o),which meets the energy stability requirement for the linear
collider. The timing variation (jitter and drift) of the pulse
output is around 2-nsec (at 3o) for 4 hour run at 50 pps [8],[9].
The second step of the smart modulator will install into
the insulation oil filled one box metal cabinet with very
compact size 150 (W) x 100 (H) x 100 (D) em as shown inFigure 6 [1 0]. It is now under deigning and will obtain middleof2002.
4FutureR&D
Hardware R&D items, should be completed before theLC construction as following updating. Targets of those
R&Dis are: (1) Reduce construction cost of machine. (2)Improve hardware reliability and (3) Reduce operation cost
(improve power efficiency). Here we briefly explain theR&D subjects and expected solutions.

4.1 KlystronOne of the most costly parts in klystron is the processing
time of the high temperature baking. This process will bealso a bottleneck in the mass-production. To reduce baking
time, we will refine the material quality used in the klystron,and also surface treatment after machining.

4.2 Modulator
In the mass production, circuit components will be delivered at quite low cost. The assembling process will be thebottleneck and most costly. The modular design will solve
this problem. Dividing the modulator into blocks and fabricating them in different supplier, we can assemble them bysimply plug-in to the modulator cabinet. This design also
makes the machine maintenance faster, and the machine
downtime shorter.
We have to also set up an R&D program to improve reliability and life-time of the thyratron-tube. Introducing highquality material in thyratron parts, such as, insulation ceramics, anode electrode, grid, and cathode, we expect thatthe tube lifetime will be dramatically improved.

4.3 RF Pulse Compressor
We need higher efficiency and easier operation. Using
rising part of the modulator pulse, we can rescuer beamenergy in the klystron, as a result the system power efficiency will be improved. To do this, we need a digital phasemodulator, with intelligent feedback loop. It will also maintain a constant energy gain in the system.

4.4 Accelerating Structure
An optimizing the cavity dimensions, speed up the machining and refine the fabrication process. Automated machine for the cell machining will dramatically reduce thefabrication cost.

4.5 Waveguide and other components
The waveguide is a hidden item of the most costly partsin the rf-system. To reduce cost, minimize number of typeof waveguide components, and simplify the design details,
and introduce automated machining.
5Test linac
To integrate developed hardware components, a test linac(at least 1-GeV) has to be constructed, and the system reliability has to be investigated with a long term of beam operation.
The ftrst stage of the C-band R&D between 1996 and1998 was successfully completed. For 1999, the ftrst priority is to develop a 50-MW class PPM type klystron withpower efficiency of higher than 50%.
For the next step, in order to examine the system performance in a realistic situation, one unit of the C-band system has to be installed and tested with beam in an existingmachine.
The c-band linac rf system now under fabricating for 1GeV electron injector linac for the SCSS project (SASEFEL) in SPring-8. Its comprises of 1-GeV injector include
the buncher system and 20-m long vacuum type undulator
can be applied the water-window with range of 1- and 10nm of the light wavelength. The first operation of the cband linac at beam energy of 500-MeV will start on a early
2004, second step will accelerates the beam energy of 1GeV in a early 2007.
References[I] T. Shintake et al., IC-band Main Linac RF System for Linear Collider i, KEK preprint 96-122, September. 1996.
[2] H. Matsumoto et al, I Fabrication of the C-band (5712 MHz) ChokeMode Type Damped Accelerator Structurei, KEK preprint 98-143,
September 1998.
[3] T. Shintake et al., "The SPring-S Compact SASE Source (SCSS)",SPIE2001, 29 July ft 3 August 2001, San Diego, California, USA[4] H. Mlltsumoto et al., "DEVEWPMENT OF THE C-BAND (5712MHz) 50 MW CLASS PPM Klystron(II)", 26th Linear Accelerator
Meeting in Japan, Aug.Ol--()3,2001, Tsukuba, Japan[5] M. Yoshida et al., "RF Pulse Compressor using Low Thermal Expansion Invar Material", 26th Linear Accelerator Meeting in Japan,Aug.l:-3, 2001,[6] T. Shintake et al., I The first Wakefield Test on the C-band ChokeMode Accelerating Structurei, KEK preprint 99-11, May 1999.
[7] T. Shintake, "Development of Nanometer Resolution RF-BPMs",
HEAC99,[8] J-S Oh et al, I Efficiency Issue in C-band KlystronftModulator System
for LinearCollideri, KEK Preprint97-51, May 1997.
[9] J-S. Oh et al, I Efficiency Analysis of the First 111-MW C-band KlystronftModulator System for Linear Collideri, KEK Preprint 98-32,
March 1998.
[IOJH. Baba et al., "Closed-Type Modulator for 50 MW C-band Klystron", 2001 Linac Meeting in Japan [Japanese], 26th Linear Accelerator Meeting in Japan, Aug.Ol-03,200 I, Tsukuba, Japan-54-