UPGRADE TO THE 8-GEV ELECTRON LINAC FOR KEKB
A. Enomoto
National Laboratory for High Energy Physics (KEK)
1-1 Oho, Tsukuba-shi, 305, Japan
Abstractenergy upgrade is to be achieved by using 57 accelerator units
with an acceleration gain of 160 MeV each: the linac building
is being extended at the upstream end of the present linac inorder to increase the number of accelerator units from 40 to
57; for increasing the rf peak power, the klystron modulator
powers will be increased twice, the 30-MW klystrons replaced

The KEK/PF 2.5-GeV linac is under reconstruction for
KEKB (the B-Factory at KEK). The linac will be renewed in
the autumn of 1998 as an 8-GeV electron linac, which canprovide full-energy beams into the 8-GeV electron ring and
3.5-GeV positron ring of KEKB, while continuing theinjection of 2.5-GeV beams for the synchrotron-radiation (SR)facilities. The main goal of the injector linac is to achieve anenergy upgrade from 2.5 GeV to 8 GeV, as well as to increase
the positron intensity. This report covers recent construction
progress and the remarkable activities regarding the energy
upgrade and positron beam improvements.

LER

TSUKUBAIRWIGGLER
RF
NIKKO

HER
LER

RF

Introduction

0

KEKB includes an 8-GeV electron ring (HER: highenergy ring) and a 3.5-GeV positron ring (LER: low-energyring), which is under construction in the same tunnel for
TRISTAN (Fig.1). KEKB aims at a luminosity of 1x1034
cm-2s-1 by establishing a crossing angle (±11 mrad) collision
between 1.1-A electrons and 2.6-A positrons. In order to save
injection time, KEKB requires full-energy injection from thelinac for both electrons and positrons; furthermore, a positron
beam intensity ten times as much as the present linac produces
is required.
The present 2.5-GeV linac [1] was commissioned in early
1982 as an electron injector for the Photon Factory (PF)storage ring; a positron generator linac [2] was added during
1982-1985 for the TRISTAN project. Electron/positron beam
injection was started in the autumn of 1986 to the TRISTAN
accumulation ring (AR); the storage beam in the PF ring waschanged from electrons to positrons in 1988 autumn, resulting
in a stable long-life storage for SR experiments.
For KEKB, the linac will be reconstructed and expanded
as shown in Fig.2. The linac will be renewed so as to deliver
8-GeV electron / 3.5-GeV positron single-bunch beams aswell as 2.5-GeV multi-bunch beams for the SR rings. The
Sector P (positron generator)
1

2

3

4

5

Assembly
Hall

7

6

TRISTAN AR
e

4

3

2

1

4

5

6

7

4

1

WIGGLER

RF
FUJI
RF

12-GeV PS

e+

RF
OHO
RF

LER (3.5 GeV)

e+ / ePF 2.5-GeV
STORAGE RING

PF / KEKB
e+/ e - INJECTOR

Fig.1 Schematic plan-view of KEKB.

Move

2

3

5

4

6

7

8

2

1

3

Sector 1
New gunroom
Sector A

5

HER

-

3

2

4

5

6

7

8

1

2

3

4

Sector 2

Move

Sector B (new)8

KEKB

HER (8 GeV)

6

0-1

Gun

500 m

5

6

7

8

7

8

Sector 3

1

Extension
1

2

3

Sector C

8

1

2

3

4

5

Sector 1

6

7

8

Positron
Radiator

1

2

3

4

6

Sector 2

7

8

1

2

3

4

5

6

Sector 3

Fig. 2 Linac reconstruction from 2.5 GeV (upper) to 8 GeV (lower). The existing linac has 40 accelerator units which are divided into 5
sectors. The shadow areas are extension buildings to increase the number of accelerator units from 40 to 57.

by the 50-MW klystrons, and rf pulse compressors used.
These parameter changes are summarized in Table 1. The
basic design details have been reported elsewhere [3]. The
following section considers recent construction progress andthe research and developments regarding the KEKB injector
linac.

(1) INJECTION BEAM
energyelectron(GeV)
positron(GeV)(ns)
pulse length(ps)bunch width (1V)
particle(charge)/pulse
electron(nC)
positron(nC)(pps)
pulse repetition
emittance (1V)
electron(m)positron(m)energy width (1V)
electronpositron(2) MAIN LINAC(MHz)
frequency(Ps)
filling time
accelerator structure
accelerator unit length (m)accelerator unit number
total
before positron radiator
standby, energy tuning
energy gain per unit
with SLED(MeV)
without SLED
(MeV)
(MW)
input rf power / unit
energy multiplication(3) PRE-INJECTOR
Guntype (cathode)normalized emittance(m)
sub-harmonic buncher
SHB-1 frequency(MHz)
SHB-2 frequency(MHz)
Prebuncher / Buncher
frequency(MHz)phase velocity (Prebun.)phase velocity (Bun.)
Output beamenergy(MeV)energy spread (1V)
normalized emittance(m)Construction progress
The KEKB project was approved in FY 1994 as a fiveyear program; half of this time has already passed. All of the
TRISTAN experiments were completed by the end of 1995,
and the old TRISTAN accelerator and detectors were removed

Table 1 Change in the major parameters to the KEKB injector.
PRESENTKEKBPRESENTKEKB(4) POSITRON PRODUCTION
Radiator
2.5
8.0
material
tantalum(Ta) tungsten(W)
2.5
3.5
thickness
(mm)
8
14
<2
single bunch
diameter
(mm)
8
4
~5
~5
Primary electronenergy
(GeV)
0.25
3.7
particle / pulse
2 x 1091 x 1011
8 x 109
6 x 1010(0.32)(1.28)(charge / pulse)
(nC)
(16)
(10)
positron production rate
2 x 108
4 x 109(0.032)(0.64)
after the DC solenoid
6.5%
>6.5%
(e+/e-GeV)
25
50
final
1.8%
>1.8%
(e+/e-GeV)
Focusing system
type
quarter-wave transformer
4 x 10-8
6.4 x 10-8
normalized acceptance(m)
8 x 10-7
6 x 10-3
8.8 x 10-7
0.2%
0.22%

0.125%
0.125%

2856
0.5
T.W., 2p/3-mode, semi-C.G.
9.6
40
3
~ 3+1

57
26
4+2

62.5
20

160
90
40
1.8

triode (EIMAC Y796)
7 x 10-6
119.00114.24571.202856
0.7 c
0.7 ~ 1 c
40

60
1.2%
~ 6 x 10-5(5) RF SOURCE
Modulator
pfn charging voltage (kV)45
45
total capacitance (PF)
0.29
0.60
stored energy
(J)
295
610
impedance
( :)
6.0
4.7
output width (FWHM) (Ps)
3.5
5.6
voltage
(kV)
22.5
22.5
power
(MW)
80
108
Pulse transformer
step-up ratio
1:12
1:13.6
core bias
nouse
Klystronbeam voltage
(kV)
270
305
current(A)
295
354
output power max.
(MW)
33
46.5
power ave
(MW)
~ 27
~ 41
width
(Ps)
1.8
3.8
efficiency
44%46%(6) SYNCHRONIZATION BETWEEN LINAC AND RING
synchronization(ns)
96.2886
W0:
f0:
(MHz)
10.3854
Linac
fLinac (2856MHz)
5x5x11f0
SHB-1
(MHz)
11f0(114)
SHB-2
(MHz)
5x11f0(571)
KEKB
fRing
(MHz)
7x7f0(509)
frevolution(kHz)
7x7f0/n(99)harmonic number n
5120 (29x10)

from the tunnel. From late 1996, the first magnet for LER is
to be installed.
As for the linac, before the KEKB project was formallyapproved, the linac group had discussed ways to upgrade theenergy, and conducted feasibility studies using the existing
linac [4,5]; consequently, the way mentioned above wasadopted as a reasonable one, because the linac must continue
injection for the SR experiments and the upgrade should be
performed only during annual shutdowns.
Since the project began, the upgrading of the existing 2.5GeV linac has gradually been performed. By the end of
FY1995, 32 high-power klystron pulse-modulators of 40
existing units had been upgraded; of 5 the sub-boosters, eachof which will drive 8 klystrons, 2 were replaced by new ones
for the SLED system [6]; 12 SLED©s and 50-MW klystrons
were installed (Fig.3), and the rf conditioning was finished in
10 units. These units were tested in order to prove theacceleration gain and stability of the beam energy [7]. During
the rf conditioning, electric discharge, which causes a strong

higher beam powers; also, the layout was changed so as toreplace two 4-m accelerator structures by two 1-m ones andtwo 2-m ones. A positron-production study was recently
begun; a preliminary result concerning the electron-topositron conversion rate is as follows : 5.4% e+/e-GeV afterFig.4 Positron generator replacement at a high-energy point of
the upgraded linac. The accelerator structures behind thepositron production target were improved in order toreduce the coupler field.

the DC solenoid with a 500 MeV and 3.2 nC single bunch.
The beam-transport system of the existing linac is being
replaced so as to accommodate higher-energy beams, adding
beam-position monitors accompanying the quadrupole
magnets.
Expansion buildings are being added to two areas: one is
the most upstream part; the other is the 180 degree-bend part(¦arc§) of the new linac layout, which forms a J-shape (seeFig.2). These are under construction (Fig.5) and will becompleted by October and December, 1996, respectively.

Fig.3 RF-source upgrade of the existing linac for KEKB: themodulator powers were doubled; the PFN cabinets becametall-boy in order to make a capacitance doubling; and the
klystron outputs were upgraded from 30 MW to 50 MW
while maintaining the height.

vacuum degradation, was frequently observed in one unit.
The cause is now under investigation in connection with the rf
rise time and phase-switching time. The upgrading of theexisting linac will be almost completed by the end of FY1996.
In order to conduct a study regarding the production andacceleration of a high-current, single-bunch primary electron
beam for positron production, two sub-harmonic bunchers(SHBs) were inserted between the gun and the prebuncher.
The results obtained by a streak-camera system using opticaltransition radiation indicated that this system can produce abunch of about 12 ps FWHM at 10 nC [8].
The positron production target and the positron focusing
system have already been moved to a higher energy point(Fig.4). The target was newly fabricated in order to be used at

Fig.5 Extension buildings being constructed in order to increase
the number of the accelerator units from 40 to 57.

Research and Developments
Compact high-power klystron
For KEKB, the 30-MW klystrons used for the PF 2.5-GeV
linac will be replaced by the 50-MW klystrons, which shouldbe operated at an average output of 41 MW, 4 Ps, 50 pps. For
this purpose, the 30-MW klystron has been improved at KEK.
The design concept is to increase the output power while
keeping its original size, so that such equipment as thefocusing magnet and pulse-transformer tank can be utilized,
and that the height of output port is not changed.
In 50-MW klystrons, which were successfully developed
at KEK [9], the overall size of the klystron assembly was notchanged. However, the cathode insulator and the dimensions
around the anode were improved so as to decrease the fieldstrength at this part; the cathode diameter increased from 80 to
85 mm in order to decrease the current density; the distance
between the anode and the input cavity was increased; thefocusing field distribution was optimized so as to improve theefficiency on the basis of computer simulations.
The applied voltage has been increased from 270 to 305 kV.
At this voltage, it can output more than 50 MW pulses with anefficiency of more than 46%.
RF compression system

newly fabricated. These are 2S/3-mode traveling-wave diskloaded structures operated at 2856 MHz. In order to distributethe HEM-mode frequency, the structures have five sets ofdifferent disk-hole apertures, which have been decreased by
75 Pm per cell from the input to the output, making anapproximately flat field against wave attenuation through thestructure. The input/output couplers are those of the cavity
type, whose field asymmetry due to the coupling-iris iscorrected by a dip on the opposite side of this iris.
The fabrication method used for the PF injector linac isunique compared to those widely used in other accelerators.
The disk-loaded structure is made by an ¦electroplating
method§: disks and spacers, which were processed to a final
dimension and inspected by measuring resonance frequency,
are made one body by electroplating to a thickness of 5 mm.
The motivation for developing this method was rather tofacilitate mass production by eliminating any tuning after
machining and welding. In a modern view of accelerator
physics, it should be noticed that this method is only a ¦cool
method§ carried out at room temperature, thus eliminating any
unexpected or uncontrollable HOM resulting from dimplingof the spacer surface.

The 50-MW klystron with an rf compression system is to
be used for the rf source of the KEKB injector linac. Threetypes of SLED systems were considered: the original SLED
fabricated at SLAC [6], a modified SLED developed by theJapan Linear Collider group [10], and a resonant-ring typecompression system (RRCS) developed by the injector linac
group [11]. The former two use double TM015-cylindricalcavities with a 3-dB power divider; on the contrary, the RRCS
has a simple structure comprising a single resonant ring.
However, we decided to adopt the JLC-type SLED, which
is improved in a low-gradient electric field around the cavitywaveguide coupling irises by using a two-hole couplingsystem. The RRCS was not adopted for two reasons: (1) the
energy multiplication factor is 8% lower (this corresponds 4 or
5 accelerator units against the total 57 units); (2) the radiation
is higher (40 PSv/h at a 40-MW input), while not being
detectable in JLC-type at more than 50 MW. These defects
are due to the choice of the TE20-like mode in a rectangular
waveguide for the transverse cross section of resonant ring.
The detail structure of the JLC-type SLED was further
modified in order to facilitate fabrication and handling in theexisting linac: the processing precision and the welding
structure/method were optimized so that they are sufficient for
obtaining a Q-value of about 100,000 (theoretical value
107,000); the tuner function was improved so as to facilitatesmooth adjustments with the necessary resolution (2 kHz in
resonant frequency); the drive mechanism of the detuner
needle was replaced by a solenoid type; the position of theneedle was magnetically sensed while producing an electricalsignal; and easily observable indicators were attached.

These basic design and fabrication method are also
followed in the new structures. However, in the old positron
generator, where two 4-m structures were used in a solenoidal
magnetic field, the acceleration field in the accelerator
structure installed immediately after the positron production
target was not sufficient due to frequent electric breaking inthe structure. Most marks due to arcing were found aroundthe input coupler and the first disk. From this experience, in
KEKB 1-m long structure was decided to be used after the
positron production target; the coupler structure is being
renewed in order to decrease the field strength when a higher
input power is used. The coupler dimensions (2b:innerdiameter, W:iris aperture), were determined by a computer
simulation [12]. These structures will be tested under higher
input power this autumn.

Accelerator structure

Positron beam increase

For the linac expansion, the number of regular accelerator
section was increased from 160 to 228. The deficits are being

One of the major target is to increase the positron
intensity. The required intensity of 4 x 109 positrons (0.64nC) per bunch was determined so that the injection time fromPHASE (DEGREE)

3
2
1
0

-1
-2
-3

0

10

20
30
NUMBER OF CELL

40

50

Fig. 6 Typical results of a nodal shift measurement: the standard
deviation of phase error is about 0.9 degrees; the 2-m
accelerator structures were fabricated by a ¦electroplating
method§ without any dimpling after machining.

vacancy to 2.6 A would be ten-several minutes. In the case ofa uniform fill into the 5120 ring rf buckets, the stored charge
per bucket is 5 nC; about eight injections per bucket are
therefore needed. This linac positron intensity is obtainable
when a primary electron beam of 6 x 1010 electrons (10 nC)
per bunch can be accelerated to the target, and the positron
production rate still be kept at 1.8% e+/e- GeV, as in the oldgenerator [13].
We have already experienced some difficulty concerning
high-current beam acceleration at 2-ns, 16-nC beam up to 250
MeV.
The investigation is still continuing using a
combination of relevant fields: the first is how to produce anintense single-bunch beam by the pre-injector [14]; thesecond concerns theoretical studies regarding the wake-field[15]; the third involves beam monitoring [16]; the forth, an
accelerator alignment [17]; and the fifth, a beam transport.
Although beam studies regarding the pre-injector has been
progressing, as mentioned before, the other studies are either
under investigation on paper or are being qualitatively
discussed.
Because of restrictions coming from the linac-ring beamtransport line, the standard deviation of energy spread must be
less than 0.125%. For the primary electron beam, the 180degree bending ¦arc§ in the expansion building was carefully
designed so as to be achromatic and isochronous to thesecond-order optics [18]. The final design comprises 6 bends
with quadrupoles and sextupoles, and satisfies less bunch andemittance growth for an energy spread of up to 1.2% VE/E.
Further, a bunch-compression system (BCS) will be
introduced before the radiator in order to suppress any
debunching effect in the positron focusing system. For the
produced positron beam, an energy compression system(ECS) will be used at the end of the linac.
e+ Bunch after ECS

60

e+ Energy after ECS

Injection Acceptance

Injection Acceptance

80
40

60
40

20
20
0

-75

-50

-25

0
0
3480
Phase (deg)

3490

3500

3510
3520
Energy (MeV)

Fig. 7 Estimated bunch/energy width of the positrons after ECS.

For increasing the positron yield, the focusing systembehind the positron production target will be improved.
Recently, a feasibility study was initiated regarding the use of
a super-conducting magnet. According to a preliminary study,a tapered shape solenoid field (6T) would improve the yieldby more than two-times as much as the present system [19].
Summary and future
The construction of the KEKB injector linac is
successfully progressing. Reconstruction for the energy
upgrade in the existing linac will be finished by the end of
FY1996. The expansion buildings will be completed by theend of 1996, and the construction of the expanded part will
start at the beginning of 1997. The pre-injector, including subharmonic bunchers, will first be moved to the most upstreamof the new building (before the construction is completed,injection for the PF ring will be made using a temporary preinjection system). Then seventeen accelerator units will be
installed sequentially. From the autumn of 1997, some ofthese units and ¦arc§ will be tested by the local control systeminstalled at the new sub-control station of the expansion
building. Most of the construction will progress during
FY1997, finished and connected to the existing linac by thesummer 1998, and commissioned in the autumn of 1998.
References[1] J. Tanaka, et al., "Operation of the KEK 2.5 GeV Electron
Linac", Proc of the 1984 Linear Accelerator Conference, Darmstadt,
Germany, May 7-11, 1984.
[2] A. Enomoto, et al., "Focusing System of the KEK Positron
Generator", Nucl. Instr. Meth. (NIM), A281, 1989, pp.1-9.
[3] I. Sato, et al. edited, ¦Design Report of PF Injector Linac
Upgrade for KEKB§ (in Japanese), KEK Report 95-18.
[4] H. Honma, et al., ¦Upgrade of the PF 2.5-GeV Linac Klystron
Modulator for the KEKB§, Proc. of the 1994 Internal Linac
Conference Tsukuba, Japan, August 21-26, 1994, pp. 436-438.
[5] T. Oogoe, et al., "High-Gradient Acceleration Test Using
Resonant Ring", Proc. of the 1994 Internal Linac Conference
Tsukuba, Japan, August 21-26, 1994, pp.266-268.
[6] Z. D. Farkas, et al., "SLED: A Method of Doubling SLAC's
Energy", Proc. of the IX International Conference on High-Energy
Accelerators, SLAC, U.S.A., May 2-7, 1974, pp.1-9.
[7] H. Hanaki, et al., ¦Test Operation of the PF Linac RF System
Upgraded for the KEKB Injector§, Proc. of the 5th European Particle
Accelerator Conference, Sitges, Spain, June 10-14, 1996, to be
published.
[8] S. Ohsawa, et al., "Pre-injector of the KEK 2.5-GeV Linac andHigh-Current Single-Bunch Beam Acceleration", these proceedings.
[9] S. Fukuda, et al., ¦Performance of the Rf-Source for the KEKB
Linac§, these proceedings.
[10] H. Matsumoto, et al., ¦High power test of a SLED system withdual side-wall coupling irises for linear colliders§, N.I.M. A33(1993) 1-11.
[11] S. Yamaguchi, et al., ¦High-Power Test of Traveling-Wave
Type RF Pulse Compressor§, Proc. of the 1995 Particle Accelerator
Conference, Dallas, U.S.A., March 10-15, 1995, pp.1578-1580.
[12] S. Yamaguchi, et al., "Design of the Input and Output RF
couplers of the Accelerator Structure for the KEKB Injector§, theseproceedings.
[13] A. Enomoto, et al., "Performance of the Upgraded Positron
Generator at KEK", The third European Particle Accelerator
Conference, Berlin, Germany, March 24-28, 1992, pp.524-526.
[14] J. Choi, et al., "Bunching System of the KEKB Linac ", theseproceedings.
[15] Y. Ogawa, et al., "Wake-Field Issues Concerning the KEK BFactory Injector Linac", Proc. of the 1994
Internal Linac
Conference, Tsukuba, Japan, August 21-26, 1994, pp.535-537.
[16] T. Suwada and H. Kobayashi, ¦Development of Beam-Position
Detection Electronics for the KEKB Injector Linac§, Proc. of the 7th
Beam Instrumentation Workshop, Argonne, U.S.A., May 6-9, 1995.
[17] T Ogawa, et al., ¦Improvement of the Alignment System of the
KEK 2.5-GeV Electron Linac§, Proc. of the 1995 Particle
Accelerator Conference, Dallas, U.S.A., March 10-15, 1995,
pp.2087-2089.
[18] N. Iida, private communication.
[19] T. Kamitani, private communication.