The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Present Status of R&D for Linear Colliders
Koji TAKATA
KEK, High Energy Accelerator Research Organization
I-I Oho, Tsukuba-shi, Ibaraki-ken, 305 Japan

Abstract
This review is concentrated mainly on the present status of
linear collider R&D work done at KEK together with
university teams. But it has been carried on in almost allareas under a solid frame of international collaboration. The
KEK team prepared a design study note of an X-band TeV
linear collider. We hope in a couple of years a more realistic
engineering design will appear, which would lead to actual
construction of such a collider early in 2000's.

1 Introduction
An electron~positron linear collider in an energy region
on the order of I TeV in the center of mass energy (EcM) is
one of the energy frontier machines, its construction in early
2000's being eagerly awaited by high energy physicists.
Linear collider designs for which R&D work is under way
may fall into three categories which, respectively, are based.upon normal conducting linac technologies, superconducting
Iinac technologies and two-beam accelerator principles.
R&D works along each lines have intensively been carriedout at several representative laboratories all over the world
for over IO years. The first type has been considered mainly
at KEK (Japan) under the name of JLC (Japan Linear
Collider), SLAC (USA) under the name of NLC (NextLinear Collider), at DESY (Germany) under the name of
SBLC (S-band Linear Collider) and BINP (Protvino, Russia)
under the name of VLEPP. The second one at
DESY(Germany) under the name of TESLA. The third one
at CERN (Switzerland) under the name of CLIC and partly
at LLL (Livermore, USA) under the name of TBNLC (Twobeam Next Linear Collider).
·My talk will be mainly concentrated on the first type
and particularly on the results ever obtained at KEK throughcollaboration with SLAC, BINP and other institutes both
inside and outside Japan. For the R&D works of the othertypes, the reader may be referred to a report I and informationon the Web2 site. But at first I will make a brief review ofresults of all the three works.

2 Review of General Design Parameters
The most important parameter besides the energy E'.cM
is the luminosity given by(1)

Eq. (I), N is the number of particles per bunch, frep thelinac repetition rate, Hb the number of bunches per linac
pulse and O'x (O'y) the horizontal (vertical) size of the bunch
cross section at the collision point.
Parameters have been surveyed by primarily considering
two important sociological boundary conditions: the total
AC power s a few MW and the length s a few 10 km.
Those are now not so much scattered among different design
options. As a typical example, let us cite those studied at
KEK 3 with the name JLC. Assuming an X-band (fRF =II.424 GHz) main linac operating at a gradient of 56
MV/m, we have the following numbers: N = 7xl09, frep =150Hz, Db= 85, as= 260 nm and d\il = 3.1 nm. In mostdesigns, two beams collide at an finite crossing angle ofabout several mrad, the bunch length O'z is chosen aroundI 00 J.Lm in order to avoid a luminosity decrease due to an
incomplete overlapping of two colliding bunches.
The very flat beam at the interaction point is a very
conspicuous feature for any design option. It is to reduce theself magnetic field of a bunch and accordingly the paircreation rate by beam strahlung when a particle runs throughan opposing bunch.
Figure I shows a schematic picture of the layout for the
JLC. Basic ingredients are an injector, main linac and beam
delivery line. The main component of the injector is a 2
GeV damping ring where beam emittances are damped
enough so that the above small beam sized can be achievedat the interaction point. Beams for the damping ring areprovided by an S-band linac of the same energy. For
positron beams, another electron linac operating at around10 GeV is necessary for positron production. Between thedamping ring and the main linac, a bunch compressor
section is necessary to reduce the bunch length by almost
one fiftieth. The main Iinac is a simple sequence of a unit
comprising four 1.3 m long X-band traveling wave
structures and focusing quads. The beam delivery line
comprises a scraper section where unwanted backgrounds are
cut off and a matching section where bunch shapes at the
linac exit are transformed into those required at theinteraction point.
One of the most important requirements is severealignment tolerances on the order of O.I J.lm to several J.Lm
for qtmds, position monitors and accelerating structures fromthe output of the damping ring through the main linac to
the interaction point. Misalignments in those components
would deflect particles in various ways, which results inemittance blow up at the interaction point.

3 Low Emittance Beams
which is required to be on the order of at least several times
1033 cm-2s-I at EcM = 0.5 TeV and should be
proportionately larger at higher energies, if searches for the
Higgs boson a:nd physics beyond the standard model should
bring out meaningful results within several to ten years. In

In a linear collider complex, the damping ring is the
only accelerator where beam emittances can be squeezed. In
the main linac and beam delivery line, the normalized
emittances (y E, where y is the Lorentz factor and E theactual emittance) are conserved as long as each components~4-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

are completely aligned, otherwise an emittance blow up
occurs to a degree depending on the magnitude of
misalignments. The normalized emittances to achieve theI·o.s km·I

11 km

above mentioned sizes of the beam cross section are y Ex =3x10-6 rad·m and YEy = 3xlo-8 rad·m.

6km

11 km

IP

Main Liriac

Main Linac

BC2

FFS, Collimator

Collimator, FFS

8GeV Pre-linac BC1

1.98GeV Linac e± Target•I

BC2
8GeV Pre-linac

1.98 GeV Damping Ring

1.98 GeV Damping Ring
Pre-damping Rin..,-<--c:::=F==::i:i10=G=e=V=L=in:>.aac

Spin Rotator BC1

0.5km~~~=:::~CJE-Gun
Spin Rotator 1.98G~V Llnac

E-Gun

30km

1.0 TeV Configuration

111111$111 @~ 1 / 1085 bunches x 150Hz

Positron108llllll.!illllll
85 bunches x 150Hz

1.4 nsec

Electron

Figure I JLC general layout: 500 GeV JLC-1 --7 1.5 TeV JLCTherefore the damping ring is a very crucial element in a
linear collider. But there was no ring which had achieved
such small emittances. Hence one of the major R&D
programs at KEK has been to construct a test damping ringand verify that those normalized emittances are actuallyachievable. Although an optimized energy is nearly 2.0
GeV, we started to construct a 1.54 GeV test ring because ofa limited space, together with its S-band injection linac of
the same energy at the KEK ATF(accelerator test facility) in1990.
Since its commissioning started November 1995, the Shand linac was under operation until the end of June 1996.
Then our efforts were directed toward commission of thedamping ring, which started January 22, 1997. The
operation was on a single-bunch mode at an energy of :::: 1.0
GeV. On February 26, beam made 2 turns and on February
7, a stored current of I rnA was attained with a life time of::::1 sec. Now the ring is operated at an energy of 1.3 GeV
with a single bunch of 7 x 109 electrons being stored.
In the following typical design features of the injection
linac and test ring are summarized. For details the reader
may be referred to a status report in this conference4.
1.54 Ge V Linac

The linac, being 90 m long, accelerates 20 bunches per
pulse to 1.54 GeV with a bunch population of 2.0 X 1010
electrons and bunch spacing of 2.8 ns. In total 19 2856
MHz structures of 3 m in length are employed, among
which one is a buncher and two are energy compensating
structures (ECS), with the others regular structures. Every
two regular structures are fed RF power by an 80 MW
klystron, whose output pulses 4.5 J.LS in length arecompressed into 1 J.LS pulses by a SLED system. The

unloaded gradient of around 45 MV/m has been achieved
without problems. The ECS is operated either at (28564.33) MHz or (2856+4.33) MHz, thus enabling tocompensate an energy droop from the 1st to 20th bunch due
to beam loading and also a droop inside individual bunches.
Structures are aligned with an accuracy of 50 J.Lm in order to
get a normalized emittance of 3 x 10-4 rad·m or less.
At present the linac is being operated at a beam energyof 1.3 GeV, with an average accelerating gradient of 26
MV /m. The repetition rate is only 0.4 Hz contrary to a
design value of 20 Hz due to radiation problems. The highradiation background may be arisen from beam energy tails
which are expected to be cured by reinforcing the present
357 MHz subharmonic buncher system.
2 Damping Ring

A main function of a damping ring should provide
beams of smallest emittances to the main linac at such arepetition rate as 150 Hz. Since the natural emittance is
proportional to y3, a low energy ring is preferred. But thedamping rate decreases similarly. A ring of too low anenergy suffers also a short Touschek life. Therefore acompromise is made at an energy around 2 GeV, withadding damping wigglers to make the damping time shorter.
A FOBO lattice is employed at the ATF ring, where thebending magnet is of a combined function type with a
defocusing component. For a lattice of this type the betatrondispersion function 11 is made very small at a bending
magnet where particles emit synchrotron radiation and thus
small emittances are achieved. By considering possibilitiesof handling polarized particles, the energy are chosen as 1.54
GeV, 1.98 GeV, 2.42 GeV···· , at which the spin
depolarization is most invulnerable, while particularly for
the ATF ring the energy is 1.54 GeV.

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The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

A beam blow-up in a bunch train has to be also
considered. Regarding this effect, long range wake fields
should be made smallest in the time length of one train(about 120 ns). Since a dominant component of the longrange wake is the TM 11 mode, structures underconsideration are made to have a cell-to-cell dispersion of theTM 11 resonance frequency so that contributions of eachcells are canceled out as a whole. Such a structure, called OS(detuned structure), is similar to a constant gradient (CG)structure where the aperture radius is made gradually smaller
for a cell farther from the structure entrance. Such a
modification of cell dimensions is optimized to get eversmaller TM 11 wake fields for a OS, while it is to get a
uniform acceleration field for a CG structure. A similar typeof structure, called DDS (damped detuned structure) is also
under development. It has not only the cell-to-cell TM 11
frequency dispersion but also a small damping port at eachcell to lower external Q's of transverse modes.
The a/A range for the JLC design of a 1.3 m long
structure is from 0.20 at the structure entrance to 0.14 at theexit with an average of 0.166. With such a rather large
aperture and a high accelerating gradient as described above,
the peak power of input pulses to each structures has to beas high as 130 MW. A single klystron is practically unable
to cope with such a high rating. It is rather essential to add
some pulse compression system between a klystron andstructures to enhance the peak power by reducing the pulse
length. The SLED system is a typical one. Pulse shapes ofa SLED is too spiky and not fitted with linear collider
linacs. Several improved versions have been invented to getflat pulses: BPC (binary pulse compression), SLED II,
VLEPP open resonator and DLDS (delay line distribution
system). The last one is now being seriously considered for
JLC as well as NLC, whose technical features are discussed
below.
Taking those factors into consideration, we have adesign scheme for a unit of the JLC main linac as shown in
Figure 2.

The presently achieved beam parameters are as follows
where the numbers in parentheses are the target values:
beam energy= 1.3 GeV (1.54 GeV) , bunch population = 7
X w9 (2 X wlO), injection rate = 1.65 Hz (25 Hz), RF
voltage = 200 kV (800 kV), horizontal/vertical COD's = 6
mm (1 mm), tunes at Vx = 14.2, Vy = 8.4, Vz = 0.005,
beam life time "" 120 sec (80 sec), damping times withwigglers on 'tx = 18 ms (6.8 ms), 'ty = 41 ms (9.1 ms), 'tz= 18 ms (5.5 ms), bunch length = 7 mm (5.8 mm),
emittances ex "" 1.7 X 10-8 rad·m (1.4 X IQ-9 rad·m) and cy"" 2.0 x 10-9 rad·m (1.0 x w-11 rad·m).
3 Polarized Sources
The availability of polarized beams would be of vital
importance for high energy machines. I would like to stressthat a great contribution was made by a Japanese teamS in
developing polarized electron sources using strained GaAs
layers or superlattices. Guns based on that technique is now
incorporated into SLC and routinely operated with a highand stable polarization.
For polarized positron generation, an idea based on paircreation through compton scattering with circular-polarizedlaser lights6 has been pursued at the ATF.

3 Main Linac
JLC as well as NLC assumes a main linac operating at
11.424 GHz, 4 times the S-band frequency 2856 MHz. The
most typical feature of accelerating structures for a linear
collider is a relatively large ratio of the aperture radius towave length, a/A, compared with conventional linacs. It isbecause short range transverse wake fields, which are excitedby off-centered particles, are proportional to a-4 and as large
an aperture radius a as possible is required to make anemittance blow-up of individual bunches as small as
possible.
Reference line

2- XB-72K klystrons
67MW, BOOns

79.4m TE01, 118.1 mm $ WG.

~==v=~=c==0=·=96=3=·=1=.5=2=~=o=po=w=e=r=lo=s=s==========~X
39.7m
128MW, 250ns I structure

129MW, 250ns I structure

Beam line
130MW, 250ns I structure

Figure 2 Unit of the JLC main linac with a 4/3 - DLDS

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The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Accelerating Structure
The 1.3 m long JLC DS is of a conventional 2n/3 TW
structure with a pitch of 8.75 mm and the aperture a varying
from 5.23 min to 3.67 mm. The average shunt impedanceis about 75 MQ/m and filling time around 106 ns. The
frequency distribution of the TM11 mode over 150 cells issuch that a gaussian cr of about 2% is achieved around 15.5
GHz. Each cell is fabricated from OFHC copper with anaccuracy better than 1 flm by fine machining processes
developed at KEK. Most cells for the DDS's installed at the
SLAC NLC Test Accelerator Facility (NLCTA) were also
machined in the same way in Japan. In order to stack cells
together into a 1.3 m structure, a diffusion bonding
technique is used which is absolutely free from any brazing
materials. The technique, developed at KEK, has
successfully been applied for several JLC DS's. The
diffusion bonding was also applied at SLAC to stack DDS
cells machined in Japan.
Our most concern is the long range transverse wakefield. With one JLC structure, the wake was measured at the
ASSET facility in the SLC of SLAC last summer. At theexperiment a leading bunch passes through the structure at a
off-axis position. Transverse wake fields generated thus aremeasured by a trailing monitor bunch injected into thestructure at various timings. Field patterns in time domain
were in a good agreement with simulation results obtainedby codes in independently developed at SLAC and KEK 7.
High gradient test have been carried out for shorter(about 30 em) structures and there are no serious problems
encountered in order to achieve the above mentioned design
values.

2 RF Source
Klystron
For JLC, a klystron with a peak output power of 67
MW and pulse duration of 750 ns is under consideration.
The output is distributed to 4 accelerating structures after afour-fold compression through the DLDS. The tube is
expected to operate at a beam voltage of 430 kV and acurrent of 338 A (micro-perveance = 1.2) for the above
output rating. The current is produced by an Iridium coatedmatrix cathode 72 mm in diameter. In order to save thepower loss at focusing solenoids, a PPM (periodic
permanent magnet) focusing scheme is conceived, though
all test tubes fabricated up to now have used conventionalsolenoids.
There are three key technical issues for completing a
klystron of the above ratings: de beam generation, output
cavity, and output window. With respect to the first issue,
de beam powers of 330 MW with 2 flS duration at a beam
voltage of 600 kV, generated by the 72 mm diametercathode, have been successfully transmitted through the drift
tube 10 mm in diameter and 40 em in length. As for output
windows, we are using two output windows of a TE11
mode type with tapered section to reduce electric field
intensities on the ceramic surfaceS. It has been shown that asingle window of this type can be operated at ratings of 84
MW and 700 ns, which is well above the design values.

The most serious problem as yet not fixed is that of theoutput cavity. For a single cell cavity, we achieved only 30
MW x 100 ns rating levels, where surface breakdowns limit
a further increase of power. In order to reduce the surface
fields, we are now testing a traveling wave structure
comprising 4 to 5 cells with larger apertures~ With this new
structure, it seems promising to fix breakdowns. But the depower to RF power conversion efficiency is yet so low· asaround 25%. It seems to critically depend on beam-cavity
interaction behaviors · at the output cavity. Only one
dimensional codes are available to simulate the interaction
in multi-cell structures. But a 3 dimensional treatment is
indispensable for such high density beams in these
extremely high power klystrons and preparations are being
made to install a 3D code MAGIC.
A short review of klystron R&D results at SLAC may
be made herewith. At the same X-band frequency, several
tubes with a solenoid focusing (type XL4) have been
fabricated which stably produces 75 MW, 1.1 flS output
pulses at a repetition rate over 60 Hz and an efficiency of
48%. SLAC has developed also a PPM tube which produces
56 MW, 1.1 flS output pulses at an efficiency as high as
60%. These good results seem solely due to full
exploitation of a 3D code.
Modulator
Efficiency improvement is also an essential issue formodulators. Energies contained at the rising and falling partsof a pulse are not negligible for conventional modulators
where the ramping time is typically over 500 ns. Since for
the JLC design the flat top length is only about 1 flS, it
would considerably improve an overall efficiency if shorter
ramping times are achieved.
For this purpose, modulators with a Blumlein PFN havebeen developed9. With the step-up ratio of the pulse
transformer being halved in this scheme, the rise time canalso be approximately halved. A modulator of this type has
actually been routinely used with a rise time less than 300
ns to test the X-band klystrons.
Power Distribution
The idea of the DLDS for RF pulse compression wasconceived at KEK 10 and some preliminary R&D work isunder way. A unit comprises 2n klystrons whose output
powers are combined with 3-dB couplers as shown in Figure
2. The output pulse is segmented into m sections in thetime domain where m s 2n. Thus we have m pulses with apulse width of lim of the original one with a peak power
multiplied by the factor 2n. A segment with an early timing
is sent upstream of the linac with an oversized roundwaveguide. This system uses no resonant element as SLEDII or others do, thus of only concern being power losses inwaveguides and 3-dB couplers which would be made verysmall.
The JLC design chooses the case with n=2 and m=3,
dubbed 4/3 DLDS. As shown in Figure 2, a waveguide is aslong as 80 m. This arises an issue of how to achieve a
sufficient phase stability in such a long transmission line. It
is being considered to use waveguide materials like superinvar which has a very small thermal expansion coefficient-7-

The II th Symposium on Accelerator Science and Technology, Harima Science Garden City, I997

and also to use a signal RF of another frequency as areference.
3 C-hand Alternative Technologies
As a backup technology to the X-band ones as describedabove, a test linac unit operating at a C-band frequency of
5.712 has also been under development. It comprises a pulse
compressor of coupled cavity type which makes a SLED II
pulse line much shorter, a heavily damped accelerating
structure of choke-mode type and klystrons generating 50
MW x 2.5 !lS pulsesll.

structures, we are tightly coupled with SLAC team. At highgradient tests of X-band structures, people from CERN and
IHEP (Beijing) contributed a lot. For wake fieldcalculations, we got contributions not only from SLAC but
from Tsinghua University (Beijing). For S-band and X-band
klystron development, a lot of technical information was
brought from SLAC and BINP. Particularly in developing
X-band klystrons, BINP has contributed at designs of output
cavities and windows together with developing many
waveguide components. A prototype DLDS will be
constructed at a I 20 m long tunnel at BINP by a joint teamof BINP, KEK and others.

4 Final Focus
Experimental studies have been carrying on by using
46.6 GeV electron beams at the FFfB (Final Focus Test
Beam) facility at SLC of SLAC. It is a test stand, designed,
constructed and commissioned by a world wide linear
collider community. The Japanese group contributed in the
optics design, construction and installation of the final quadfamily, a beam profile monitor and a precise beam position
monitor, and a series of beam studies in commissioning
including beam tunings.
In May 1994, a spot size was measured to be as small as
70 nm by a novel profile monitor based on compton
scattering from a standing wave of a laser light12. An RF
cavity type beam position monitor was also tested yieldinga resolution of 30 nm. Focused spot sizes were found larger
than predicted from linac emittances. Furthermore they can
not be extrapolated to 0 in the 0 emittance limit. This
problem is not yet fixed and is intensively surveyed by theJapanese team.

5 Civil Engineering
Ba'lic studies are launched to survey reliable andeconomic methods to construct a tunnel as long as 30 km or
more13. At present the following two methods seempromising: NATM (New Austrian Tunneling Method) andTBM (Tunneling Boring Method). In the first method, atunnel is made by blasting with explosive materials, thenspraying shotcrete and left for a while until walls becamemechanically stable through a redistribution of rock stresses.
In the second method, a full-aperture boring machine
continues to excavate the tunnel, with concrete linings
being followed afterwards. The tunnel cross section would
be an elliptic one with a flat floor in the case of NATM
while a circle in the case of TBM. In both cases, however, agood waterproofing is a serious issue not only for avoiding
underground water contamination but for guaranteeing asolid support for linac structures.

I International Linear Collider Technical Review
Committee Report 1995, ed. G. A. Loew, SLAC-R-95-471,
SLAC
2 http://www.slac.stanford.edu/xorg/ilc-trc!ilc-trchome.html
3 JLC Design Study, KEK Report 97-l, April 1997
4 H. Hayano: "Commissioning of ATF Damping Ring,"
Status Report in this conference.
5 T. Nakanishi eta!: "Large Enhancement of Spin
Polarization Observed by Photoelectrons from a Strained
GaAs Layers," Physics Letters A 158(1991) p. 345.
6 T. Hirose: "Important Role of Polarized Positron Beams
for Future Linear Colliders," Tokyo Metropolitan
University Report TMU-HEP/Exp 95-8.
7 M. Yamamoto: "Study of Long Range Wake Fields of
Linac Accelerating Structures," Thesis, the Graduate
University for Advanced Studies, 1994.
8 Y. Otake et al: "Design and High-power Test of a TEllmode X-band RF Window with Taper Transitions,"
Proceedings of 1995 PAC (Dallas), p. 1590.
9 H. Mizuno et al: "A Blumlein Type Modulator for 100
MW Class X-band Klystron," Proceedings of 1993 PAC(Washington), p. 1321.
10 H. Mizuno et al: "A New RF Power Distribution
System for X-band Linac Equivalent to an RF Pulse
Compression Scheme of Factor 211 ," Proc. of 17th Int.
Linac Conference (Tsukuba), 1994, p. 463.
11 T. Shintake: "C-band Linac RF System for e+e- Linear
Collider," Proceedings of 1995 PAC (Dallas), p. 1099.
12 T. Shintake: "Proposal of a Nanometer Beam Size
Monitor for e+e- Linear Colliders," N.I.M. A311(1992), p.
453.
13 JLC Design Study, Ch. 17, KEK Report 97-1, April
1997.

6 International Collaboration
R&D work has been carried out under intensive
international
collaboration.
In
thedesignandcommissioning of the ATF accelerator complex, many
researchers of oversea institutes have been and will beeagerly participating, among whom SLAC people areplaying a dominant role. For the construction of X-band-8-