Proceedings o f the 21st Linear A ccelerator Meeting in Japan (September 3 0 -October 2,19%f Tokyo, Japan)〔30 p - 3 〕

C-BAND RF SYSTEM FOR e+e_ LINEAR COLLIDER
AT 500 GeV TO 1 TeV C.M. ENERGY
T. Shintake, N. Akasaka, K. Kubo, H. Matsumoto, S. Matsumoto, Shigeru Takeda,
K. Oide and K. Yokoya, P. Pearce*、H.S. Lee*^ and M.H.Cho*^
K. Watanabe*3, H. Baba*4
KEK Tsukuba Japan ，*l:CERN, Geneva, Switzerland, *2: PAL, Pohang Korea,
ネ3..Tohoku University, *4: NIHON KOSHUHA Co.
A b stract
A hardware R&D for the C-band (5712 MHz) rf system for a linear collider started in 1996 at KEK. An accelerating
gradient of 32 MV/m ( including beam loading ) will be generated by 50 MW C-band klystrons in combination with an
rf-compression system. The klystron and its power supply can be fabricated by conventional technology. The
straightness tolerance for the accelerating structures is 30 |im，which is also achievable with conventional fabrication
processes. No critical new technology is required in a C-band system. Therefore, a reliable system can be constructed atlow cost with a minimum of R&D studies. The first high-power test is scheduled for 1997.
In tro d u ctio n
The e+e' linear collider is a large-scale machine. In themain linacs for two beams, we use more than 8000
accelerating structures, 4000 klystrons and pulse
modulators. Therefore, the system must meet thefollowings demands:( 1 ) High reliability
To provide beams with reasonable availability, thesystem must be highly reliable; that is, the fault rate must
be negligibly small and the lifetime of the key devices
must be sufficiently long.
(2) Simple
For the same reason as (1)，the system must be simple.
This will also help to lower the construction cost and to
make hardware maintenance easier.
(3) Lower construction cost
We should try to reduce the construction cost while notsacrincing the system performance and reliability.
(4) Reasonable power efficiency
We assume that the maximum limit on the whole wallplug power in a site is 200 MW. To meet thisrequirement, the rf system must efficiently accelerate thebeam.
(5) Easy to operate
The machine operation should be easy. In the actualmachine operation, the system must have flexibly toaccelerate various patterns of the beam current.
Among the system parameters, the choice of the drive
rf frequency plays the most important role concerning thesystem pertormance as well as the hardware details. We
proposed the C-band frequency as being the best choice tomeet all of the demands listed above[l-4].
OverallParam eters

The overall parameters are listed in Table-I for 500-GeV
and 1-TeV C.M. energy. In the 500-GeV case, an
accelerating gradient of 31.7 MV/m is generated by a 50MW klystron, thus an active length of 7.3 + 7.3 km issufficient to reach 500 GeV C.M. energy. A luminosityof 6 .6 x 1033 /cm2/sec can be obtained using a 150 MW
wall-plug power. The details are described in ref. [3].

Table-I
CM Energy
TeV
0.5
1 .0
Number of electrons per bunch
1.4
1.1xlO 10
Number of bunches per pulse
72
Bunch separation
nsec
2 .8
Repetition frequency
Hz10050
Bunch length
mm0 .2------ RF-parameters --RF frequency
GHz5.712
Peak input power at cavity
MW 83.0165
Nominal accelerating gradientMV/m 40.0 56.0
Effective accelerating gradient
MV/m 31.7 46.4
Wall-plug power for RF (2 linacs) MW
150133------ Accelerating Structure
Number of structures per beam
4080 5748
Total length of cavities per beamkm
7.3 10.3
Structure TypeCG with choke-mode
Unit length of structure
m1.80
Iris radius/wavelength
0.13 - 0.17
Shunt-impedance
MO/m 59.2 -■47.0------ Pulse-compressor -----Compression Scheme
multi-cell coupled cavity
Pulse compression ratio
5
Pulse compression efficiency%
70—----Klystron ——-Klystron peak power
MW50.3 98.6Efficiency%
4570
Number of kly strons per beam
2040 2874
RF pulse lengthjiisec
2.5------ Modulator------Number of modulators per beam
2040 2874
Power efriciency from AC to pulse%75------ Beam Dynamics------Injection energy
GeV
20
Phase delay of rf-crestdeg 14.5 1 0 .0
Structure straightness tolerance[im30------ Final focus ™ Spot size at IP (horizontal)nm
318
318(vertical)
nm
4.3
3.0
Crossing angle (crab crossing)
mrad8 .08 .0
xlO ^lLuminosity
6 .67.0

System Description
Figure 1 shows a schematic diagram of one unit in themain linac rf-system. Two 50 MW klystrons are driven
by two high-voltage pulse modulators independently,followed by a 3dB hybrid power combiner and pulse
compressor to generate 350 MW peak power, which drives
four accelerating structures. The pulse-compression action
is performed by rotating the phase of the input rf-signal in
opposite directions in each klystron. By comoming twopowers at 3-dB hybrid, the phase modulation (PM) isconverted to the amplitude modulation (AM) of the rampwaveform, which compensates the beam loading effect in
the accelerating structure. The energy-storage cavity
consists of three coupled cavities using a low loss TEOlnmode.
We use a standard rectangular waveguide: EIA187(47.55 mm x 22.15 mm, 3.95-5.85 GHz), whose
attenuation constant is 0.03 dB/m (5% loss/m).
Figure 2 shows the key points concerning the C-band,
which make the system simple and reliable.
Klystron Power Supply & Pulse Transform er
The filling time of the accelerating structure scales astF = ^ Q ro c 0 )- ^ \0)
At the C-band, it becomes 280 nsec. Including the pulselength of the beam and a compression factor of five in therf-compression system, the rf-pulse at the klystron
becomes 2.5 jisec. Including the rise- and fall-times, thepulse-length of the high-voltage applied to the electrongun of the klystron becomes 3 |isec or longer, which isquite suitable to the conventional power-supply consistingof a Pulse Forming Network (PFN) and a step-up pulsetransformer. This type of power supply has been used inmany linear accelerators, owing to its high reliability andgood efficiency.
To charge high voltage into the PFN capacitors, weuse an inverter power supply. Modern technology forpower-semiconductor devices (such as IGBT) has improved
the power efficiency by better than 90%. Using this, we
can simplify our modulator design, making it modular
according to the required functions: the inverter power
supply (DC block), the PFN module (pulse formingblock), and the pulse-transformer tank (matching block toa klystron). With this approach, it becomes easier toreduce the cost, improve the reliability and easemaintenance. In the case of a failure, we simply replace
any broken block with a new one and send the old one to afactory for repair.
C-band K lystron
Since the drift-tube diameter is proportional to the rf
wavelength, we can use an electron beam with a larger
diameter than higher frequency bands. It also makes it
easier to design an electron gun with a larger cathode toextract a higher beam current. Therefore, we can design thebeam voltage as low as 350 kV, which enables the PFN
voltage to be as low as 43 kV. At this voltage level, it iseasier to obtain sutable PFN capacitors from existing
components ranges of various manufacturers.

RF Pulse Com pressor
We use a three-cell coupled-cavity pulse-compressor
instead of a delay-line type pulse-compressor. The cavity
is compact, having a length of 1 m, and its diameter is
160 mm. Therefore, it will be easier to fabricate at lower
cost. A computer-simulation code was made to simulate
the time response of the coupled-cavity system, which has
shown a maximum efficiency as high as 70%. The details
are reported in ref. [5].
Accelerating S tructure
We use a choke-mode cavity structure[6 ], in which allof the higher-order modes are heavily damped. Therefore,
the multi-bunch wakefield and any associated instability
will not harm the beam emittance. The only concern isthe single-bunch emittance dilution due to the short-range
wake-field, which is a strong function of the iris aperture.
We use relatively large iris-aperture: average <2a>=16mm. As a result, the straightness tolerance for one
structure becomes 30 |im or larger. This is a controllable
level in conventional fabrication techniques of the diskloaded structure. To eliminate any stress and make thestructure straighter, a low temperature brazing techniquewill be adopted[7]To align the structure with beam, we use an RF-BPM
attached to the structure. This type of RF-BPM was tested
using the FFTB beam line at SLAC in December
1995 [9]. It demonstrated a very high resolution of 44 nm
for a single bunch. Three RF-BPM were assembled in one
block, and the misalignment between them was measured
with electron beam. It was only 3 |im. This is a quite
promising result for a structure-alignment procedure.
The dark-current problem due to field-emission under thehigh accelerating gradient has been studied using computer
simulations [10]. From which, no serious contributions tothe background in the detector at IP is expected at C-band
frequency.
Hardware R&D Program
In January, 1996, the hardware R&D has been started atKEK. In 1997-1998, we will construct one unit of rfsystem. Since we use one klystron, the input rf will bedirectly amplitude modulated to demonstrate the flat-top
output from the compressor. The first klystron tube will
be available in 1997.
References[1 ]JLC Group, ' JLC-I ', KEK Report 92 -1 6 , December 1992,
A/H/M[2] T. Shintake, e t a l., "C-Band Linac RF-System for e+e"
Linear Collider", Proc. 1995 Particle Accelerator Conf. andInt. Conf. on High-energy Accelerators, May, 1995,Dallas, Texas, U.S.A., KEK Preprint 95-49[3] K. Yokoya, e t a l , "C-band Linear Collider with C.M.
Energy 500 GeV to 1 TeV M, Proc. 5th European Particle
Accelerator Conference (EPAC96), Sitges, BarcelonaSpain, 10-14, June 1996, KEK Preprint 96-68, July 1996 A[4] T. Shintake, et al” "C-band RF Main Linac System for
e+e- Linear Colliders at 500 GeV to 1 TeV C.M. energy",
Proc. 5th European Particle A ccelerator Conference

(EPAC96), Sitges, Barcelona Spain, 10-14, June 1996,
KEK Preprint 96-69, July 1996 A[5] T. Shintake and N. Akasaka, "A New RF PulseCompressor using M ulti-Cell Coupled-Cavity System",
Proc. 5th European Particle A ccelerator Conference(EPAC96), Sitges, Barcelona Spain, 10-14, June 1996,
KEK Preprint 96-71, July 1996 A[6] H. Matsumoto, et al., ’'C-band Ghoke-Mode Accelerating
Structure for the Linear Collider", Proc. 5th European
P article A ccelerator C onference (EPA C96), Sitges,
Barcelona Spain, 10-14, June 1996, KEK Preprint 96-70,July 1996 A
T. Shintake, et al.， ’’HOM Free Linear A ccelerating
Structure for e+e- Linear Collider at C-band", Proc. 1995
Particle Accelerator Conf. and Int. Conf. on High-energy
Accelerators, May, 1995, Dallas, Texas, U.S.A.,
KEK
Preprint 95-48

T. Shintake, "The Choke Mode Cavity", Jpn. J. Appl.
Phys. Vol.31, pp.L1567-L1570, Part2, N o .llA 1992
[7] H. M atsum oto, et al” "Low T em perature B razing
Technique for the A ccelerators", Proc. 5th EuropeanP article A ccelerator C onference (EPA C 96), Sitges,Barcelona Spain, 10-14, June 1996, KEK Preprint 96-67,
July 1996 A[8] K. Kubo, et al” "Alignment Issue for C-band LinearCollider", Proc. 5th European Particle A ccelerator
Conference (EPAC96), Sitges, Barcelona Spain, 10-14,
June 1996, KEK Preprint 96-66, July 1996 A[9] The experimental results on RF-BPM will be publishedsoon.
[10] N. Akasaka, "Dark Current Simulation in High GradientA ccelerating Structure", Proc. 5th European Particle
A ccelerator Conference (EPAC96), Sitges, Barcelona
Spain, 10-14, June 1996, KEK Preprint 96-65, July 1996 A

MAIN LINAC RF-SYSTEM

C-band L I N E A R C O L L I D E R

AC POWER LINE

5712 MHz, V s =500 GeV FOR TWO LINACSRF-SYSTEM:2040 UNITSMODULATORS:4080

KLYSTRONS

|1m

4080

ACC. STRUCTURES: 8160ACTIVE LENGTH:1 4 .7 KM

WALL-PLUG POWER :: 1 5 0 MW
BEAM LOADING COMPENSATION USING PHASE -TO-AMPLITUDE MODULATIONACTIVE MOVER

3^ /4, a //. » 0.13-0.17, vgK0.0l2-0.035c,c, rs53 M /n

ACTIVE MOVER

ALIGNMENT TOBELANCE : a - 30 pm/STRUCTURE：L in ea r C o llid er ：By T. Shintake (KEK)

F ig .1 One unit of the C-band RF system.
What makes the C-band system simple & reliable.
Pulso Modulator
High reliability，good efficiency, simple( Low PFN voltage : 43 kV ^-> long life forthyratron & capacitors.
Efficiency AC to Pulse
Compact modular design

C-band JLCParameters are for 500 GeV C.M. Energy case.
In 1 TeV upgrade case, we use 100 MW klystron andlonger linacs by 3.5 km for each beams.
50 MW C-band KlystronHigh Reliability

Large Beam Drift Tube Diameter :16 mm |Lower Gun Voltage : 350 kV

JNo de-Q'ing ( sim ple )Inverter HV supply
Pulse Transformer
HV-Pulse length of 3 gsec is best fit toconventional pulse-transformer and

ard Rectangular Waveguide
SimpleSmall Loss 0.03 dB/m|

Loss = 5 % for 8 meter

JLow emittance dilution
Iris aperature <2a>=16 mr
Alignment torelance
30 urn/structure

Fig. 2 What makes the C-band system simple and reliable. The key points in the C-band rf-system.