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

Abstract

2.1 Upgrade of the existing linac

The KEKB (B-Factory at KEK) 8-Ge V injector linac isunder construction by upgrading the existing 2.5-GeVinjector as well as extending the linac towards theupstream site. This upgrade has been almost completed
and the two linacs are presently being combined. This
paper describes the construction status, with emphasis on
the newly developed beam monitors being used forvarious stages of beam commissioning.
1INTRODUCTIONKEKB includes an 8-GeV electron (e-) ring and a 3.5-GeV
positron (e+ ) ring, which has been under construction
since the TRISTAN accelerator was removed. KEKB
aims at a luminosity of 1 x 1034 cm-2s-1 with collisions
between 1.1-A electrons and 2.6-A positrons. One of therequirements of the injector linac is to deliver full-energy
beams for both rings. The other is to increase thepositron beam intensity to ten-times as much as what theold 2.5-GeV linac [1] produced; when the linac injects
with a charge of 0.64 nC/bunch at 50 Hz and without any
beam loss, it takes 13.5 minutes to accumulate from 0 to
a maximum charge of 26 µC, since the KEKB ring has acircumstance of about 3 km. The main parameters of theKEKB injector linac are given in Table 1.
In order to achieve these requirements, the linac has beenreconstructed and expanded, as shown in Fig.1. For theenergy upgrade, the number of accelerator units wasincreased from 40 to 57 with an acceleration gain of 160
MeV each; for increasing the rf peak power, the klystron
modulator power was increased by twice, the 30-MW
klystrons replaced by 50-MW klystrons, and rf pulse
compressors used.
The old positron generator was moved to a higher energy
point of about 4 GeV from 0.2 GeV in order to increase
the positron intensity.
The basic design details have been reported elsewhere [2].
2CONSTRUCTION STATUS

The KEKB project was approved in FY 1994 as a fiveyear program; the fourth year has already passed.

Since the project began, upgrading of the existing 2.5GeV linac has gradually been performed.
By the end of FY1997, of the existing 40 accelerator
units, 38 had been upgraded; 37 units were upgraded withhigher power klystrons and rf pulse compressors. In aunit just after the positron generator, the rf compressor is
not used for avoiding any disadvantage of rf breakdown.
Two accelerator units have not yet been installed: Oneunit (#C-7, see Fig.1) is being used as a temporary preinjector for the PF, because the original pre-injector wasmoved to the most upper-stream end of the KEKB
injector. The other is unit #5-8, which has been
appropriated for an energy compression system (ECS).
TABLE 1
Parameters of the KEKB e-/e+ injector linac.
[Beam]
Injection energy

(e-)
(e+)
(e- for e+ production)

8.0 GeV
3.5 GeV
3.7 GeV
Pulse length
single bunch
Bunch (half) width (σz)
5 psParticle number (Charge) / pulse (e-)
8 x 109 (1.28 nC)(e+)
4 x 109 (0.64 nC)(e- for e+ production) 6 x 1010 (10.0 nC)
Pulse repetition
5 0 pps
Emittance (2σ)(e-) 6.4 x 10-8 m.rad(e+) 8.8 x 10-7 m.rad
Energy (half) width (σE/E) (e , e+)0.125%[RF]frequency
2856 MHzaverage klystron output power
4 1 MW
pulse width
4.0 µsEnergy multiplication by rf compressor
1.8[Accelerator unit]
including four 2-m traveling-wave accelerator sections
Total number
57
Number before e+ radiator
26
Energy gain per accelerator unit
160 MeV/unit(except for the pre-injector and after e+ radiator)________________________________________________________________________________________________*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. Nakamura, Y. Ogawa, S. Ohsawa, K. Oide, K. Satoh, M. Suetake, T. Suwada

The temporary pre-injector has a gun, a prebuncher, and amain buncher. In order to combine the extended linac
with the existing linac, and continue PF injection, thegun has been installed off the beam line with a 90-degree
bending magnet. The electron beams from the temporary
pre-injector were accelerated through the entire linac.
Beams of about 300 pC/pulse could be transmitted at agun beam of 500-pC.
All of the beam-transport magnets of the existing linac
were replaced or improved so as to accommodate higher
energy beams, adding beam-position monitors at every
location of the quadrupole magnets.
Rf conditioning was continued during May and June,
1997, in order to achieve an average rf power of 41 MW.
Some of them, which had been upgraded before thesummer of 1996, were conditioned up to this target
within 3 weeks; others took 5 weeks, and one did notachieve this goal. Since electric discharging wasfrequently observed for a faster rf phase-switching time
concerning SLED, in such cases the switching time wasloosened up to about 0.2 µs during conditioning.
Because an energy-analyzing station (EAS) at the end of
the linac, which can measure energy up to 8 GeV, had not
yet been prepared, an EAS at the end of Sector 2 was usedin order to prove the acceleration gain. The beamenergy was measured to be 2.16 GeV. Since a preinjector and 13 accelerator units were used, the average
accelerator gain per unit was more than 160 MeV, whichis the target of the KEKB linac.

The extended was been constructed and tested separatelyfrom the existing linac during FY1997.
All of the accelerator units were finished by October,1997. RF conditioning was started on October 7th, 1997.
Using
automatic conditioning software, it wassuccessfully completed from upper stream. By December
24th, the upstream 1.5-GeV part before a 180-degree linac
arc was commissioned.
The 180-degree linac arc, an achromatic and isochronous
beam-transport system, was completed at the end ofFebruary, 1998. For dispersion and isochronicity
corrections, bpms and a streak camera were effectivelyused.
3RESEARCH AND DEVELOPMENTS

There were various important contributions forconstructing the KEKB injector linac [3]-[12]. For
example, in order to replace the old klystrons while
continuing operation for the PF ring, a dimensioncompatible, compact high-power klystron was developed;
for establishing a synchronizing system between the linac
and ring accelerating frequency, a low phase-noise master
oscillator of 571.2 MHz, multipliers, and dividers werenewly fabricated; also new accelerator sections werefabricated by electroplating technology. These werereported at past international and domestic accelerator
conferences.
In this paper, progress concerning beam-instrumentation
system is described. We have used it in commissioning
the extended part of the KEKB injector, as follows:(1) Beam screens for:• rough orbit adjustment,• emittance measurement and matching with randomshutter cameras and an image-analyzing device,• spectrum measurement,(2) Beam position monitors for:• automatic search for quadrupole center,• betatron oscillation measurement,• correlation plot of emittance growth vs. beam orbit,• dispersion (Rn6, Rn66) measurements and correction
in/after the linac arc,

2.2 Extended linac
The expansion buildings were completed by January,
1996. The extended linac has a pre-injector and 17
accelerator units. These are divided into Sector A to C, as
shown in Fig.1.
In the pre-injector (unit #A-1), before a pre-buncher and abuncher of fundamental frequency, 114.24-MHz and571.2-MHz sub-harmonic bunchers (SHB) were added inorder to compress 2-ns gun pulses to single-bunch beams.
The power supplies for the SHB are 5 kW and 10 kW,
respectively.

Linac Arc
Assembly
HallSector B
876New gunroom

Sector A

543245671432

Positron
Radiator

1

Extension
123

8

1

2

Extended 17 units

345Sector 1

Sector C

Existing 40 units

67

1BCS

2

34

6

Sector 2

78

123

45678

Sector 3

to Sector 4, 5, ECS

combined here
Fig.1 Layout of the 8-GeV e- / 3.5 GeV e+ KEKB injector linac. Fourty accelerator units of the old 2.5-GeV linac were energyupgraded from about 67.5 to 160 MeV/unit in average; and, new 17 units were added at the upstream site. The old and new linacs
were combined during March 23 to 30, 1998.

• energy-stability measurement in the arc,
• fine correction of orbit distortion,(3) Streak Camera System for:• bunch tuning and monitor in the pre-injector.
• isochronicity (R56,R566) measurements and correction
after the arc.
3.1 Streak-Camera Systems
A streak-camera system has also been used in the old linacsince the 1980's to observe the bunch structure of theelectron /positron beam. At an early year, air-Cherenkov
light emitted by a beam was used as a light source into astreak camera. The system consisted of separate devices,such as an optical system, a trigger and its delay system
as well as a streak camera proper. Since the trigger
should be a low-jitter system, synchronization byaccelerating rf (6th sub-harmonic, 476 MHz) is also
required. Consequently, those systems were not so easy
to handle. It sometimes took half a day to obtain thenecessary signal. Therefore, it has for a long time beenused only for experiments.
We have been improving such a system so as to be used
for daily beam tuning in the linac, like when using anoscilloscope [12]. The developed streak-camera system
has such characteristics as:• an accuracy of 2 ps at a time-range of 0.3 ns,• an overall trigger jitter of less than 2 ps after
synchronization with 571.2-MHz accelerating subharmonics,• an automatic peak search function between a time-rangeof 5 ns (widest) and 0.3 ns.(narrowest),• a data-display repetition time of less than 1 s,
• integration of peaks arranging each center of gravitation,• remote control of the optical lens system.
Thus, one of these are being used after the pre-injector inorder to tune the bunch structure. Empirically, a chargeof more than 1 nC/pulse gives sufficient light for seeing asingle-shot signal without focusing the electron beam on
the metal plate. The other was used after the linac arc inorder to check the isochronicity of chromatic beams after
passing through the arc.
3.2 Beam Position MonitorStripline-type beam position monitors were installed in
front of each quadrupole [11]. For solving man-power
problem to develop a data acquisition system, 17 digital
sampling oscilloscopes, which are of 5 GHz, 2 ch, andcommercially available, were distributed every half of thelinac sector (typically 76.8/2=38.4 m). The signals arecombined by combiners so that each peak is notoverlapped, and introduced to oscilloscopes controlled by a
VME computer. The signal peaks are measured using theoscilloscope 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.
6SUMMARY(1) A linac upgrade was almost completed, and average
accelerator gains of more than 160 MeV per unit (20MeV/m) were obtained by a beam test.
(2) High-current acceleration and positron production are
under investigation. Streak cameras and BPMs are
effectively working for linac commissioning; a 10-nC
single bunch could be accelerated to the end of Sector B;
the linac arc has been successfully tuned. The electron-topositron conversion rate was measured.
(3) The extended and existing linac are to be combined as
the KEKB injector and fully commissioned on May andJune, 1998.
REFERENCES
1) J. Tanaka, et al., "Operation of the KEK 2.5 GeV ElectronLinac", 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.
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ACCELERATOR GUIDE FOR KEKB INJECTOR'.
4) H. Hanaki, et al.: 'LOW-POWER RF SYSTEMS FOR KEKBINJECTOR LINAC'.
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THE KEKB INJECTOR LINAC'.
8) K. Furukawa, N. Kamikubota: 'LAYERED CONTROLSYSTEM AT KEKB LINAC'.
9) N. Akasaka, et al.: 'OPERATION SOFTWARE FORCOMMISSIONING OF KEKB LINEAR INJECTOR
PROGRAMMED WITH SAD'.
10) T. Suwada, S. Ohsawa: 'RECALIBRATION OF A WALLCURRENT MONITOR USING A BEAM-INDUCED FIELD
FOR THE KEKB INJECTOR LINAC'.
11) T. Suwada, N. Kamikubota, K.Furukawa: 'NEW DATAACQUISITION SYSTEM OF A BEAM-POSITIONMONITOR AND A WALL-CURRENT MONITOR FOR THE
KEKB INJECTOR LINAC'12) Y. Ogawa, et al..: 'NEW STREAK-CAMERA SYSTEMFOR THE KEKB LINAC'.