CONSTRUCTION OF THE 8-GEV e- / 3.5-GeV e+ INJECTOR LINAC
FOR KEKB
A. Enomoto, KEK e-/e+ Injector Linac Group*, and Linac Commissioning Group**
KEK, 1-1 Oho, Tsukuba-shi, Ibaraki-ken, 305-0801, Japan

Abstract

2.1 Energy Upgrade of the accelerator module
In order to increase acceleration gradient, the rf sourceof the old accelerator modules has been upgraded from thedownstream to the upstream using summer and wintershutdown terms. For increasing the rf peak power, theklystron modulator power was increased by twice, the30-MW klystrons replaced by 50-MW klystrons, and rf
pulse compressors (SLED: SLAC energy doubler) used.
Including newly fabricated ones, 58 upgraded accelerator
modules were so far constructed and the number of theremaining modules to be installed is only one.

The KEK/PF 2.5-GeV linac has been upgraded since
1994 by upgrading the existing linac as well as extending
it towards the upstream site.
It has been almost
completed and commissioned during May and June, 1998.
This paper summarizes the construction status and thebeam performance of the new injector.

1

INTRODUCTIONKEKB includes an 8-GeV electron (e-) ring and a 3.5GeV positron (e+) ring, which has been under construction
since the TRISTAN accelerator was removed. KEKB
aims at a luminosity of 1 x 10 34 cm-2s-1 with collisions
between 1.1-A electrons and 2.6-A positrons.
One of the requirements of the injector linac is todeliver full-energy beams for both rings. The other is toincrease the positron beam intensity to 0.64 nC (4 x 109
particles) /bunch with a repetition rate of 50 Hz. This
positron intensity is ten-times as much as what the oldlinac produced, and when the beam could be injectedwithout any beam loss, it takes 13.5 minutes toaccumulate from 0 to a maximum charge of 26 µC, since
the KEKB ring has a circumstance of about 3 km.
In order to achieve these requirements, the linac has
been reconstructed and expanded [1]-[3]. For the energy
upgrade, the number of accelerator modules was increased
from 40 to 57, as well as the acceleration gain of each
module from 70 to 160 MeV. The old positron generator
was moved to a higher energy point of about 3.7 GeVfrom 0.25 GeV in order to increase the positron intensity.

2-FY1994
FY1995
FY1996
FY1997
FY1998summer winter annual
5
7
7
12
15
27
12
6
17
1
1
total
58

Among 58 accelerator modules, the SLEDs are notused in the most upstream module (pre-injector) and theaccelerator module just after the positron radiator, in order
to avoid any troubles due to electric breakdown.
2.2 High-power klystrons and modulators
The newly developed 50-MW klystrons are compact,size-compatible with the old 30-MW klystrons. They
have been satisfactorily fabricated and installed according
to the schedule described above.
Forty-seven klystron modulators were improved to
double the pulse energy and 11 modulators were newly
produced. The high-voltages applied to the klystrons weretuned so that the pulse width is 3.7 µs with ripples lessthan 0.3%. The high voltage control and the interlock
monitor are made using programmable logic controllers.
During the linac commissioning in May and June
1998, the klystrons were operated at 25 pps and the output
power was 38.6 MW in average of 55 modules with
SLEDs; and corresponding calculated no-load accelerator
gain was 164 MeV /module.

CONSTRUCTION

The KEKB injector has been constructed since FY
1994 as a five-year program. Reconstruction of the oldlinac was finished by the end of FY1996; the linac
expansion was constructed during FY1997 and the
combination with the old linac was completed by the endof FY1997 (March, 1998).

________________________________________________________________________________________________*1: K. Nakahara (head), I. Abe, S. Anami, A. Enomoto, S. Fukuda, K. Furukawa, H. Hanaki, H. Honma, H. Iijima, M. Ikeda,
K. Kakihara, N. Kamikubota, T. Kamitani, H. Katagiri, H. Kobayashi, T. Matsumoto, S. Michizono, K. Nakamura, K. Nakao,
Y. Ogawa, S. Ohsawa, T. Oogoe, T. Shidara, A. Shirakawa, T. Suwada, T. Urano, S. Yamaguchi*2: N. Akasaka, A. Enomoto, J. Flanagan, H. Fukuma, Y. Funakoshi, K. Furukawa, N. Iida, T. Kamitani, M. Kikuchi,
H. Koiso, K. Nakajima, T.T. Nakamura, Y. Ogawa, S. Ohsawa, K. Oide, K. Satoh, M. Suetake, T. Suwada

55

All the accelerator modules had been conditioned up to
about ten-percent higher peak power, and the trip rate was
low during the beam commissioning, ex., 0.31
times/klystron.hour during June 7 to 13. Since theoperation started, two klystrons has been replaced owing
to trouble in the output window and the gun cathode,respectively; and 4 more klystrons are to be removed
during this summer shutdown in order to tune thefocusing field again.

needle using solenoid. The radiation from the SLEDs is
sufficiently low and the shielding is not necessary, though
there are several SLEDs with relatively higher radiation.
The tuners and the detuners are also well working.
2.4 Accelerator sections

2.2 Low-power rf systems
The low-power rf systems have been extensively
changed for the KEKB injector [4].
In order tosynchronize the linac beam with the ring rf buckets withina requested precision of +-30 ps, the common factors
between the linac and the ring rf frequencies were searchedunder the condition that the linac frequency was fixed andconsidering the ring rf bandwidth. The highest common
factor was consequently chosen to be 10.38545 MHz,
which is the 275 (11 x 5 x 5) -th subharmonic of the linac
rf as well as the 49 (7 x 7) -th subharmonic of the ring rf(508.8872 MHz). This means the linac beam and the ring
rf bucket are synchronized at 10.38545 MHz.
The subharmonic bunchers (SHBs), which produce
single bunch beams from the gun beam pulses with awidth of about 2ns, are to be operated at the fifth (571.2
MHz) and the 25-th subharmonic (114.24 MHz). For theformer rf, a 10-kW solid-state amplifier was newly
fabricated, however, for the latter, an old amplifier using
vacuum tubes was still utilized with slight improvement.
The frequency of a master oscillator was chosen to be
the fifth subharmonic (571.2 MHz). Using an opticalfiber cable, the linac fundamental rf (2856 MHz) istransferred from a main-booster to sub-booster stations
located at the upstream of every 8 accelerator modules.
For the SLED system, the sub-booster rf drive
systems were improved so as to inverse phase at the latterpart of the rf. The switching time is changeable. During
the rf conditioning, it was set around 200 ns in order toavoid the rf breakdown.
Sub-booster klystrons were replaced by newly
developed 60 kW klystrons in order to drive 8 klystrons
instead of 4 klystrons in the old linac case. These
klystrons are water-cooled. During the commissioning,
the correlation between the rf phase shift and the coolingwater temperature was studied and measured 1 deg /0.1
deg.C. The temperature stability for the rf systems has
been improved to be around 0.2 deg. by changing coolingtour fan switching to continuous control of a 2-way valve.
2.3 SLEDs
So far 55 SLEDs has been operated. These SLEDs
are of two-hole coupling type and are modified in order tofacilitate handling in the existing linac: the tuner withsmooth adjustments with the necessary resolution (2 kHz
in resonant frequency); the drive mechanism of the detuner

About seventy 2-m sections, which are 2π/3-mode
traveling-wave structures, were fabricated using electroplating method. One of them was tested up to 36 MV/m
(21 MV/m is operational), where any limitation was notfound. By the linac commissioning, all of the newsections were conditioned without any problem up to
more than the operational gradient.
An accelerator section used just after the positronradiator is one meter long; the input coupler has two
symmetric coupling holes in order to reduce electric fieldaround the coupler and the input waveguides. This section
is operated in the solenoidal magnetic field of 0.5 T andfed by an rf of about 30 MW, 1 µs without a SLED,
producing an acceleration gradient of 17 MV/m. Rf
conditioning for the accelerator sections used in themagnetic field was carefully performed. Consequently the
klystron trip due to electric breakdown was hardly
experienced during the commissioning for the positron
beam in June. This was a drastic change from the fourmeter sections which had been used in the old positronsource.
2.5 Pre-injector
The KEKB injector requires the pre-injector to produce
single bunches with a bunch length of about 10 ps(FWHM) and a charge of more than 10 nC. The gun isable to emit pulsed beams with a pulse width of about 2
ns (FW) and a peak current of about 10 A. In order tocompress the gun beam to the single bunch beam, a114.24-MHz SHB and a 571.2-MHz SHB were installed
before the 2856-MHz bunchers. The gun beam energy
was increased up to 200 kV (100 kV in the old electron
linac) for suppressing space charge effect on the basis of acomputer simulation. The pre-injector showed a good
beam performance in the commissioning.
2.6 Positron production
The main improvement for the positron source was toreplace 4-m accelerator section by shorter sections. In the
KEKB injector, two 1-m sections and two 2-m sections
were installed and rf pulses with shorter width are to befed. This improvement was very effective for stable
operation of the positron beam. The electron-to-positronconversion rate normalized by the incident electron energy
was about 3.3%/GeV at the end of the linac.
2.7 Beam Instrumentation and control
One of the great progress of the KEKB injector fromthe old injector is to have developed a beam instrumentation system based on beam position monitors (BPMs)

56

which were installed at every location of quadrupole
magnets [5]. The BPMs were widely utilized to tune linac
beam in such cases as orbit correction and dispersion
correction of the linac beam transport system.
The BPMs comprises stripline-type beam position
monitors and associated signal analyzing systems. For
solving man-power problem to develop a data acquisition
system, 17 digital sampling oscilloscopes, which are of 5
GHz, 2 ch, and communicated with a VME computer,were distributed every half of the linac sector (typically
38.4 m). The signals are combined by combiners so thateach peak is not overlapped and their peaks are measured
using the oscilloscope functions and analyzed by VME.
The position signals for one beam pulse are measured by abeam-trigger signal distributed to the monitor station.
The position information from all BPMs is renewed every
1.4 second at present.
The other important monitor is a streak-camera system
to observe the bunch structure of the linac beam [6]. The
streak-camera system was also used in the old linac.
However, it was not so easy to handle, because it
consisted of separate devices, such as an optical system, a
trigger and its delay system, a synchronization circuit
between the trigger and the rf to reduce trigger jitters, aswell as a streak camera proper.
Further, air-Cherenkov
light emitted by a beam was used as a light source into a
streak camera; therefore, it has for a long time been used
only for experiments.
Newly developed streak-camera systems are more
simply arranged, computer controlled, and they utilize
optical transition light (OTR) emitted from a metal mirror
which can be easily inserted to the beamline. Thus four
streak-cameras were installed after the pre-injector, after
the linac arc, after the positron radiator, and the linac end.
So far they were frequently used at the pre-injector as areal time bunch tuner.

3

BEAM PERFORMANCE

The linac commissioning was carried out as scheduledin May and June and the results near to the design wereobtained as follows:electronpositron
dsgn achvd
energy (GeV)8>8charge (nC/bunch)
1.2
1.5
energy width (% FW) 0.5 ---emittance (1σ, mm)
1.1
----

dsgn achvd
3.5>4
0.64 0.6
0.5
0.8
1.6
2.3

A remarkable feature of the linac commissioning wasuse of the beam instrumentation system and SAD(strategic accelerator design) computers in various stagesof the beam tuning. Especially it was indispensable totune high-current electron beams for positron production
while suppressing emittance growth.

4

SUMMARY(1) A linac upgrade was almost completed. A total energyof more than 8 GeV and average module gain of more than
160 MeV (20 MeV/m) were obtained.
(2) A positron beam near to the design quality has beenachieved.
(3) To achieve a complete performance in the KEKB
commissioning, the remaining issues, such as the linacalignment check, replacement of the old devices, andpreparation for the 50-Hz operation, is to be still
continued during this summer.

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) I. Sato, et al. edited, “Design Report of PF Injector Linac
Upgrade for KEKB” (in Japanese), KEK Report 95-18.
3) A. Enomoto, “Upgrade to the 8-GeV Electron Linac for
KEKB” , Proc. of LINAC96, Geneva, Switzerland, 26-30
August, 1996, pp.633-637.
4) H. Hanaki,et al., "Low-Power RF Systems for the KEKB
Injector Linac", Proc. of APAC98, Tsukuba, Japan, 23-27
March, 1998, to be published.
5) T. Suwada, N. Kamikubota, K.Furukawa: "NEW DATAACQUISITION SYSTEM OF A BEAM-POSITIONMONITOR AND A WALL-CURRENT MONITOR FOR THE
KEKB INJECTOR LINAC", Proc. of APAC98, Tsukuba,
Japan, 23-27 March, 1998, to be published.
6) Y. Ogawa, et al.: "NEW STREAK-CAMERA SYSTEM FOR
THE KEKB LINAC", Proc. of APAC98, Tsukuba, Japan,
23-27 March, 1998, to be published.

2.8 Linac Alignment
All the buncher system are operated in the Helmholtz
coil of about 0.1 T. In this low energy region, thealignment of the equipment was carefully checked; bychanging the gun beam energy, the position change on
beam screens was investigated. However the alignmenthas been still checked in this summer shutdown, because
it was essential to use steering coils before the buncher
during the commissioning.
The linac alignment has not been sufficient at present.
One of the reason is the girder slide rollers weresuperannuated. They have been replaced during thissummer shutdown. And the linac alignment will beselectively improved upstream of the positron radiator inorder to increase the primary electron beam intensity.

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