COMMISSIONING OF
THE KEKB 8-GeV e- / 3.5-GeV e+ INJECTOR LINAC
N.Akasaka, A.Enomoto, J. Flanagan, H.Fukuma, Y.Funakoshi, K.Furukawa, T. Ieiri, N.Iida,T.Kamitani, T.Kawamoto, M.Kikuchi, H.Koiso, T. Nakamura, Y.Ogawa, S.Ohsawa, K.Oide,
K. Satoh, M. Suetake, and T. Suwada, KEK, 1-1 Oho, Tsukuba-shi, Ibaraki-ken, 305, Japan
Abstract

a 2856-MHz prebuncher and a buncher. The bunched electron beam is accelerated from north to south, up to 1.5
GeV by 11 accelerator modules, then deflected 180
degrees, and further accelerated in the inverse direction up
to 8 GeV by 46 accelerator modules. When the positron
beam is accelerated, a positron radiator is inserted at anenergy point of about 3.7 GeV; then, intense primaryelectrons of about 10 nC/bunch hit the radiator and emerging positrons are focused and accelerated through the restof the linac. The basic design details of the linac havebeen reported elsewhere [1, 2].
The linac has been in the process of being upgradedsince 1994. Reconstruction of the old linac and construction of the extended 17 accelerator modules were independently performed, and two linacs were combined at the endof March, 1998.
TABLE 1
Beam parameters of the KEKB e-/e+ injector linac.
Injection energy(e-)
8.0 GeV+(e )3.5 GeV(e- for e+ production)
3.7 GeV
Pulse length
single bunch
Bunch (half) width (σz)
5 ps
9Particle number (Charge) / pulse(e ) 8 x 10 (1.28 nC)(e+) 4 x 109 (0.64 nC)+(e for e production) 6 x 1010 (10 nC)
Pulse repetition
50 ppsEmittance (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%

The KEKB 8-GeV electron / 3.5-GeV positron injector
linac is almost completed and is now under commissioning. Recently, both an 8-GeV electron beam and a 3.5GeV positron beam were accelerated to the linac end. This
report describes the current status of these beams withemphasis on the newly developed beam monitors being
used for various stages of beam commissioning.

1 INTRODUCTIONKEKB includes an 8-GeV electron (e-) ring and a 3.5GeV positron (e+) ring, which are under construction after
having removed the TRISTAN accelerator. KEKB aims ata luminosity of 1 x 1034 cm-2s-1 with collisions between
1.1-A e- and 2.6-A e+. The first requirement of the linac
is to deliver full-energy beams for both rings. The second
is to increase the positron intensity to more than 0.64
nC/bunch. If the linac injects positrons with a charge of0.64 nC/bunch at 50 Hz without any beam loss, it takes
13.5 minutes to accumulate from 0 to 2.6 A (therevolution time is 10 µs). The linac beam parameters are
summarized in Table 1.
For an energy upgrade, the number of accelerator modules is to be increased from 40 to 57 and the acceleration
gain of the module is improved from 70 to 160 MeV eachby increasing the klystron modulator power, replacing the30-MW klystrons by 50-MW klystrons, and using rf
pulse compressors. For increasing the number ofpositrons, the old positron generator was moved to ahigher energy point of about 3.7 GeV from 0.25 GeV.
The new layout of the linac is shown in Fig.1.
Electron beams from the gun are bunched by 114.24-MHzand 571.2-MHz sub-harmonic bunchers (SHBs), as well asLinac Arc
AssemblyHallSector B
876New gunroom

Sector A

543245671432

PositronRadiator

1Extension
1238

12

Extended 17 units

34567

Sector 1

Sector C

1BCS

Existing 40 units

2346

Sector 2

7812345678

Sector 3

to Sector 4, 5, ECS

combined hereFig.1 Layout of the 8-GeV e- / 3.5 GeV e+ KEKB injector linac. Forty accelerator modules of the old 2.5-GeV linac were energyupgraded from about 67.5 to 160 MeV/module on the average; also, 17 new modules were added at an upstream site. The old andnew linacs were combined during March 23 to 30, 1998.

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2 COMMISSIONING
Commissioning of the extended part was initiated inOctober, 1997. The pre-injector and the accelerator modules before the 180-degree arc were tuned. During Januaryand February, 1998, the linac arc was constructed and anelectron beam was accelerated through the extended linac.
After the extended linac and existing linac were combinedat the end of March, electron beams have been accelerated
through the entire linac.

The electrons from the buncher were energy-analyzed
using a steering magnet and a beam-position monitor(bpm); their emittance was measured and matched to theacceptance of the following beam transport. Thus, about
eighty percent of the gun beam was bunched and could beaccelerated.

2.1 Tuning of the pre-injector
The first phase of pre-injector tuning was to transport
a gun beam of 200 keV at the central trajectory from thegun to the exit of the buncher using a solenoid field of
about 0.1 T. By improving the assembling error of thegun cathode, it was proved that the beam position at thebuncher entrance did not move when the beam energychanged from 100 to 200 kV. Though the beam trajectory
to the buncher was improved, it was also found that thisposition might not be completely in accord with thebuncher center axis, because it was necessary to apply asteering coil at the entrance of the buncher. This point has
not yet been improved.
In order to smoothly tune the buncher system and toproduce a single-bunch beam, it was very effective to use
a streak-camera system. We have been improving it inorder to use it for daily beam tuning in the linac, likewhen using an oscilloscope. Optical transition light(OTR) emitted by electrons from a metal plate, which iseasily inserted into the beamline, is utilized as the lightsource to the streak-camera. The trigger-delay and synchronization systems were assembled in the streak camera
proper, and became controllable by a personal computer.
The developed streak-camera has an accuracy of 2 ps, andsuch necessary functions as an automatic peak search,
integration of the peaks arranging each center of gravitation, and remote control of the optical lens system.
Thus, one is installed after the pre-injector.
Empirically, a charge of more than 1 nC/pulse gives
sufficient light for seeing a single-shot signal without
focusing the electron beam on the metal plate.
Figure 2 shows the bunch width of the electron beamwith a charge of about 10 nC. The gun beam has a full
width of about 3 ns (an initial phase width of 120 degrees
at the entrance of the 114.24-MHz SHB). By this SHB, it
is bunched to 1 ns (an initial phase width of 200 degrees
at the entrance of the 2856-MHz prebuncher).
Wemeasured the single-bunch purity and tuning tolerance ofthe buncher components. For a high-current gun beam of
10 nC /pulse, the phase tolerance for the 114-MHz SHB
is quite severe (several degrees) at an input power of 10kW, which is the maximum operating power. The fullwidth at half maximum (FWHM) of the bunch width wasmeasured to be around 10 ps.

10.7 ps

Fig. 2 Bunch width at a charge of about 10 nC.

2.2 Beam transport with a 180-degree arc
The linac arc is an achromatic and isochronous beamtransport comprising 6 dipoles, 7 quadrupoles, and 6sextupoles. The energy acceptance is 3% at full width.
The bpms were very important for tuning the beamtransport system. In the new linac, bpms were installed in
front of each quadrupole. In order to solve the manpower
problem to develop a data- acquisition system, 17 digital
sampling oscilloscopes (5 GHz, 2 ch, and commerciallyavailable) were distributed every half (typically 40 m) of
the linac sector. The signals from bpms are combined bycombiners so that the signals from different bpms do not
overlap each other, and are introduced to oscilloscopes
controlled by a VME computer. The peak-heights aremeasured using the oscilloscope functions and analyzed by
the VME. The position signals for one beam pulse aremeasured by a beam-trigger signal distributed to themonitor station. The position information from all bpms
is presently renewed every 1.4 seconds.
The arc was tuned using a low-current beam of 2
nC/bunch, of which the energy width was 0.2%. The
dispersion pattern in the arc and the dispersion error after
the arc were measured by bpms while changing the beamenergy. The isochronicity was measured by a streakcamera at the exit of the arc. In both measurements, theenergy dependencies were measured up to the second order,
and the errors were corrected using sextupoles.

2.3 Acceleration of an 8-GeV electron beam
The first acceleration of a 2-nC electron bunch to 8
GeV was achieved on April 27th, 1998, by employing 54
accelerator modules. The average gain of the module wasmore than 150 MeV. Due to three accelerator modules,
which have not yet been installed, and several modules
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which have not been sufficiently conditioned or phased, asufficient energy margin was not obtained.

2.4 Tuning of a high-current primary beam
The most difficult issue was to accelerate a highcurrent electron beam. The beam aspect changed when thecharge exceeded 5 nC/bunch. We could accelerate morethan 10 nC/bunch before the linac arc; however, so far, amaximum of 7.5 nC/bunch has been obtained to thepositron radiator. The emittance was measured by thewaist-scan method with a quadrupole and a beam screen.
The typical normalized emittances were ~0.1 mm after thepre-injector (70 MeV), ~0.3 mm at the 500-MeV point,
and 0.6~1 mm before the arc. Such emittance growthdepended on the charge and the beam orbit. At highcurrent, the beam has two cores, which have different
transverse/longitudinal positions, and energies. These twocores have already been observed after the buncher section.
The growth of the transverse position difference of thecores has been observed in the vertical direction, and isdependent on the orbit. The energy difference has been
observed in the arc. In order to suppress the emittance
growth, bumps of about 2 mm are tested (Fig.3).

2.5 Tuning of the positron beam
As of June 17, a maximum positron current of 0.75
nC/bunch was transported to the end of the linac with a
primary electron beam of about 3 GeV and 6 nC/bunch.
This electron-to-positron conversion ratio slightly
exceeded what we had expected for the design value (10-nC
e- to 0.64-nC e+ at the linac end); at present, neither theprimary nor positron beams are sufficiently stable, and thereproducibility is not very good. The one-sigma

normalized positron emittances were 5.7 mm in thehorizontal direction, and 3 mm in the vertical. The
positron energy was more than 4 GeV, and the energy
width 0.5% at full width.

4SUMMARY(1) A linac electron energy of 8-GeV was achieved. Inorder to obtain a sufficient margin, two modules will beadded this summer and incomplete rf conditioning andphasing will be improved.
(2) A positron intensity near to the design value has beenachieved. However, the stability and reproducibility issues
should be further investigated.
(3) Studies will still be conducted regarding the highcurrent primary electron beam.

REFERENCES
1) I. Sato, et al. edited, “Design Report of PF Injector Linac
Upgrade for KEKB” (in Japanese), KEK Report 95-18.
2) A. Enomoto, “Upgrade to the 8-GeV Electron Linac forKEKB” , Proc. of LINAC96, Geneva, Switzerland, 26-30
August, 1996, pp.633-637.
3) 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.
4) Y. Ogawa, et al.: "NEW STREAK-CAMERA SYSTEM FOR
THE KEKB LINAC", Proc. of APAC98, Tsukuba, Japan,
23-27 March, 1998, to be published.

mGunDump

Arc (1.5 GeV)

BCS

Positron radiator

ECS

Fig. 3 Beam-position monitor display. The vertical axes indicate the horizontal displacement (mm) vertical displacement(mm) and charge/bunch (nC) from top to bottom. The horizontal axis indicates distance along the linac.

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