COMMISSIONING OF THE KEKB LINAC
Y. Ogawa, Linac Commissioning Group*
High Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801 JAPANwidth of about 10 ps (FWHM). After acceleration to theend of sector B (1.5 GeV), it enters into the J arc section,in which achromatic and isochronous conditions are
fulfilled. It is then re-accelerated either to the end of thelinac (8.0 GeV) or to the positron production target (3.3
GeV), depending on the operation mode. The 8-GeV
electron beam is directly transported into the ring, while
the produced positron beam is accelerated to the end of
the linac (3.5 GeV) and passes into the ring injection line
after an energy-compression system (ECS) located at thebeam switchyard. In the case of positrons, the primaryelectrons also pass through a bunch-compression system(BCS) just before the target to decrease the bunch width
so that a positron bunch-lengthening effect in thesolenoids after the target can be partially compensated,
realizing an appropriate bunch width for a positron energy
spread.
Two operation modes (8-GeV, 1.2-nC electrons and
3.5-GeV, 0.64-nC positrons) have thus been almost
established, realizing the designed beam characteristics.
For efficient commissioning, we have organized for the
first time a linac-commissioning group comprising both
members of the linac and the KEKB ring. We believethat it has made a great success in that linac beam
parameters required for ring injection are being
intensively
pursuedwhile
sharingcommoncommissioning experiences.

Abstract
The injector linac for the KEKB ring has beencommissioned step by step since last autumn, while
continuing the construction of the remaining parts as wellas ordinary operation for beam injection into the Photon
Factory. The commissioning has so far given quitesatisfactory results: (1) A single-bunched beam with acharge of about 1.5 nC for direct injection into the ring
was accelerated to the end of the linac (about 8 GeV). (2)A single-bunched beam with a charge of 6 nC for positron
production was accelerated to a positron production target(3.3 GeV). Positron beams with a charge of about 0.5 nC
have been successfully accelerated to the end of the linac(3.5 GeV) and transported to the beam line of the ringwith a proper energy spread. (3) Various kinds of beam
instrumentation have been employed and utilized for
precise beam tuning and diagnosis.

1 INTRODUCTION
The construction of the KEKB linac [1] was completed
on schedule at the beginning of May, 1998. The
commissioning, which had been carried out step by stepsince October, 1997, has entered a full-commissioning
phase at this moment so that two design beams for the
KEKB ring could be realized: full acceleration of anelectron beam (about 8.0 GeV) as well as the productionof a large quantity of positrons and their acceleration (3.5GeV).
The layout of the linac is shown in Fig. 1, comprising
two parts: a newly constructed part (sectors A, B, C and Jarc) and an upgraded part (sectors 1-5 and a beamswitchyard). A beam starting from an electron gun passes
two subharmonic bunchers (SHB1: 114.24 MHz andSHB2: 571.2 MHz) and an S-band bunching section (2856
MHz) to accomplish a single-bunched beam with a bunch
J arc

Beam

Sector-B

2 OUTLINE OF THE COMMISSIONING
Fig. 2 shows the results of commissioning concerning
the beam positions and intensity in two operation modesof the KEKB linac: an 8-GeV electron beam and a 3.5GeV positron beam. In the following sections, the beamtuning process is discussed along with the beam flow of
the linac.

Gun

Sector-A: Q magnets
B-8

B-7

C-1

B-6

C-2

B-5

C-3

B-4

C-4

B-3

C-5

B-2

C-6

B-1

C-7

A-4

C-8

A-3

1-1

A-2

1-2

Positron production target

A-1

1-3

1-4

1-5

1-6

1-7

BCS

2-1

Sector-1

Sector-C

2-2

2-3

2-4

Beam

2-6

2-7

2-8

Sector-2
PF ring

3-1

3-2

3-3

3-4

Sector-3
Beam

3-5

3-6

3-7

3-8

4-1

4-2

4-3

4-4

4-5

4-6

4-7

4-8

5-1

Sector-4

5-2

5-3

5-4

5-5

Sector-5
0

5-6

5-7

5-8

ECS

KEKB ring
100 m

Figure 1: Layout of the KEKB linac. Sectors A, B, C and J-arc are newly constructed.
*

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. T. Nakamura, Y. Ogawa, S. Ohsawa, K. Oide, K. Satoh, M. Suetake, T. Suwada

2.3 Acceleration in Sectors A and B

Electron: 8 GeV, 1.5 nC

Positron: 3.5 GeV, 0.5 nC

Figure 2: Two design beams have been achieved at theend of the linac: 8-GeV, 1.5-nC electron and 3.5 GeV,
0.5-nC positron beams.

2.1 Electron Gun and Its Orbit Measurement
The electron gun emits a beam with an energy of 200
keV and a pulse width of 2-3 ns (FW). The beam is
immediately injected into the bunching section to form asingle bunch in the S-band structure. Since the beam isstill non-relativistic in this region, a solenoid-type
focusing system is employed against strong space-charge
forces among particles. It sometimes happens, however,that if a beam having even a slight inclination to thesolenoid field axis is injected into this region, the central
orbit may rotate in the magnetic field, causing anemittance growth. In order to prevent this kind of beam
degradation, we not only checked the transverse
components of the magnetic field, but also improved thealignment of the electron gun. As a result, the beam-orbit
rotation is considerably minimized and the beam profile
along the region becomes quite improved.

2.2 Tuning of Bunching Section
Tuning of the bunching section [2] has been carried out
mainly by observing a bunch monitor with a streak
camera (see Section 3.2) located at the end of the section.
With proper powers fed into two SHB cavities, an S-band
prebuncher and a buncher, as well as by precise tuning of
the corresponding rf-phases, we succeeded to establish asingle-bunched beam without any satellites. The bunch
widths obtained in the 1.5-nC beam and the 10-nC
primary beam for positron production are about 10 ps.
The energy spectrum is also measured at an energyanalyzing station at an energy of about 70 MeV, giving anenergy spread of about 9% (FW).

In sectors A and B, the energy must reach the J-arc
design energy of 1.5 GeV, which is realized by using 10
accelerator units with a margin of one unit. The last twounits are used as a knob for adjusting the energy in across-phase configuration so as to compensate for any
energy spread due to the slope of the sine function. In ahigh-current acceleration mode, the acceleration phase forthe rest of units is set at an off-crest angle of about 15
degrees in order to minimize the energy spread, satisfying
the J-arc energy acceptance of 2.8% (FW=4σ ).
Beam transmission from the end of the bunching
section to that of sector B is quite good, accomplishingalmost 100% without any significant loss. It has been
anticipated, however, that some problems related to wakefield effects might emerge for an intense primary electron
beam with a charge of 10 nC. In fact, we sometimes
observed some peculiar phenomena concerning beamprofiles in high-current beam acceleration, while in beam
acceleration with a small charge of 1.5 nC we found nosuch problems. Details concerning these items are
discussed in Section 4.

2.4 Tuning of the J-arc Section
The J-arc section is designed to be achromatic andisochronous [3]. The achromaticity condition is fulfilledin the following manner:• Measure the dispersion at all beam-position
monitors (BPM: see Section 3.1) along the J-arc bychanging the energy using the energy knob.
• Calculate the strengths of the quadrupole magnets
so as to simulate the observed dispersion and obtainratios to the set values.
• Set the strengths of the quadrupole magnets at thevalues calculated from the inverse ratios to the set
values.
• Measure again the dispersion and iterate the process.
For the isochronicity condition, the same procedure istaken, except for the first step, which is replaced by:• Measure the time delay with a streak camera system(see Section 3.2) at the end of the J-arc by changing
the energy using the energy knob.
In achromaticity corrections, second-order effects arealso cured by adjusting the strengths of the sextupole
magnets.
The central energy in the J-arc is maintained at 1.5 GeV
by introducing an energy feedback loop utilizing theenergy knob. As mentioned in the previous section, we
sometimes observed interesting phenomena related to adegradation of the beam quality, which made the beamtransmission in the J-arc quite low. The cures obtained byusing local bumps and adjusting the acceleration phases atsectors A and B were successfully tried, and are describedin Section 4.

2.5 Acceleration in Sectors C and 1
In the midst of sectors C and 1, the newly constructedpart and the upgraded part were joined together during thelast week of March, 1998. In order to successfully makethe prompt connection without causing any disturbances
to the ordinary operation for the Photon Factory, we had
prepared a temporary pre-injector comprising an electron
gun and a bunching section at the first section of sector 1.
Although two independent accelerators have beensuccessfully connected without any obstacles, the shortperiod for a connection has not allowed a satisfyingalignment of the accelerator components. In fact, thebeam losses in this region, which are suspected to be due
to a misalignment, have so far not been avoided. A realignment of the linac is planned for this summer.
The largest amount of charge obtained for positron
production was about 6 nC at the target, not reaching thedesign value of 10 nC. We expect a considerable
improvement after a re-alignment.

After passing through ECS, a positron beam with acharge of about 0.4 nC was successfully transported to thetemporary beam dump in the midst of the injection line.
Although the beam-loss monitors installed along theinjection line showed an allowed level of radiation in thistime of operation (5 pps), it turns out that for the fullinjection mode (50 pps), the radiation level must be
reduced by almost one half. This reduction will also be
expected to be achieved with a proper tuning of the linac
and the beam-transport line.

2.6 Tuning of Primary Electrons at a e+ Target
Primary electrons for positron production pass throughBCS just before the target so as to partially compensate
the bunch lengthening of positrons occurring in thefollowing solenoids. The acceleration phases at sectors C
and 1 are set at an off-crest angle of about 30 degrees,making the phase-space orientation for BCS. Although
BCS worked quite well, fine-tuning will be carried out inthe autumn.
Since the positron yield strongly depends upon the
beam profile at the target, the beam waist was surveyed
by changing the optics before the target. The minimum
radius of the beam size was estimated to be around 1 mm,
obtaining a maximum positron production.

2.7 Acceleration in Sectors 1-5
Acceleration in this region has proceeded without any
problems for electron and positron beams. The beamcharacteristics have been measured for both beams at theend of sector 5, giving the quite satisfying results reported
in Section 5.

2.8 ECS for e+ beam
The energy acceptance required for the positron
injection line to the KEKB ring is 0.5% (FW). In order toachieve this value, the positron beam is passed through
ECS at the end of linac. An energy-compression ratio of
about 0.5 was obtained: an energy spread of 1.7% (FW)became 0.8% (FW) after ECS. According to an analysisof the data (Fig. 3), it turns out that almost 90% of thepositrons fall in the energy acceptance of the injection line.
The remaining slight difference relative to the design
value will be expected to be eliminated by fine tuning of
the acceleration phases as well as that of BCS.

Figure 3: Energy spread of positron beams compressed
into a half width by the ECS located at the end of linac,
reaching almost the energy acceptance of the ring
injection line.

3 BEAM INSTRUMENTATION
3.1 BPM Calibration
BPMs [4] installed in all quadrupole magnets have beenquite efficient tools for beam diagnosis in thecommissioning. We have performed several kinds of
BPM calibrations using a beam: measurements of theposition resolutions and offsets as well as estimations of
the calibration factors by employing a recalibration
method [5]. The obtained results are summarized inTable 1.

3.2 Bunch Monitor with a Streak-Camera
SystemAnother important progress in beam diagnosis was theintroduction of new streak-camera systems [6] for
observing a bunch structure. Three systems (at the end of
sector A and the J-arc, and just after the positron target)
have been installed, while another one is being prepared at
the end of the linac. One of the main features in the newsystem is an integration of hardware and software; all of
the necessary steps to observe streak-signals are integrated,
and the handling becomes quite simplified. Detailed
descriptions can be found in the reference.

3.3 Test of Wire Scanner
As non-destructive beam-profile monitors, wire
scanners have been tested concerning several aspects: adetection scheme and its configuration as well as aradiation-shield method. The results indicate that with asuitable configuration of the detector, wire scanners give a
sufficient signal-to-noise ratio against a strong radiation
environment. We are planning to install three sets (eachcomprising three or four wire scanners positioned atproper distances) at the end of sector B, J-arc and the linac,
respectively, so that the twiss parameters can becalculated in nearly real-time using signals from three orfour wire scanners at the same time. Details are to bepresented elsewhere [7].

effects are taken into account so that the bump can beclosed. The bump, however, sometimes failed to close theorbit, possibly because quadrupole wake-field effects
emerged when the beam profile-deformation was large.
Incorporating quadrupole wake-field effects in bumpcalculations might be necessary during the next
opportunity.

4.3 Method of Downhill Simplex
Since the beam intensity at a certain point of the linac
depends upon many parameters upstream, it may happen
that searching the optimum combination of parameters
would be practically difficult. We tried the method of
downhill simplex [8] for obtaining the maximum positronyield by changing the strength of a few upstream steeringcoils together with some acceleration phases. The results
suggest that this kind of method could be useful in somecases. A more elaborate application of the method will be
introduced in the future.

3.4 Wall Current Monitor: Charge Calibration

5 BEAM CHARACTERISTICS

Although the quantity of charge is one of the key
parameters, the calibration is not a simple task, especiallyin the case of a very short pulse, like a single-bunch beam
(10 ps) in the S-band linac. We employed two methods: a
relative calibration utilizing a signal from the beaminduced field observed at the end of accelerator section as
well as an absolute one using a Faraday cup installed in
the straight branch line at the entrance of the J-arc. Details
will be presented elsewhere.

The beam characteristics obtained during thecommissioning are listed in Table I, compared with thedesigned target values. As far as the beam intensity at theend of linac is concerned, the target values have beennearly realized for electron and positron beams, though aslight increase of positrons may be required. The singlebunch purity is practically 100%, while the bunch lengthis not in agreement with the design value, which does notseem to be a serious problem. The emittance growthobserved in sectors A and B for an intense beam might be
reduced by the effective use of local bumps or the methodof downhill simplex. The energy margin for ring injection
is rather poor at this time of commissioning, but can beconsiderably improved during the next commissioning
period. The energy spread of both beams falls at almost
the energy acceptance of the injection line.

4 FINE TUNING OF INTENSE BEAM
4.1 Acceleration-Phase Tuning
The acceleration phase for an intense beam has been set
at an off-crest angle of about 15 degrees in sectors A and
B for achieving an energy acceptance of the J-arc (Section
2.3). On the other hand, it has been about 30 degrees offcrest in sectors C and 1 for BCS (Section 2.6). The
maximum beam transmission in these regions, however,
was obtained by even a finer adjustment of theacceleration phases in sectors A and B, so that they wereslightly shifted in the inverse direction relative to eachother. In this way, the beam profiles in the J-arc weregreatly improved, suggesting the existence of transverse
wake-field effects, which are clearly shown in the next
section. A detailed analysis will be carried out during thenext commissioning period.

4.2 Orbit Tuning
Local bumps have been introduced somewhere insectors A and B, depending upon the beam state toimprove the beam transmission in the J-arc and sector C.
In calculations of a bump, dipole transverse wake-field

6

CONCLUSIONS

The first commissioning of the KEKB linac was
successfully completed, reaching two major goals: 8-GeV
acceleration of electron beams and 3.5-GeV accelerationof positron beams with sufficient quantity and quality.
High-quality hardware completion of the machine wasalso verified and confirmed through commissioning,satisfying the beam characteristics required for the KEKB
ring.

7 ACKNOWLEDGEMENTS
The authors would like to thank to all of the staff
members of linac for the high-quality completion of theconstruction. They also greatly appreciate various formsof support by the project leaders, Profs. K. Nakahara andS. Kurokawa.

Table I: Evaluation/Score of the KEKB Linac Commissioning
Beam Type

Target Value
Beam @ Gun
Charge*1
Pulse Width (FW)Beam @ A1
Charge*1
Satellite Bunch
Bunch Width (FWHM)
Beam @ B8
Charge*1
Transmission (A1-B8)
EnergyEnergy Spread (±2σ)
Energy Stability (p-p)
After Feedback
Beam @ C1
Charge*1
Transmission (J-Arc)
Bunch Width (FWHM)
Beam @ e+ Target
Charge*1
Transmission (C1-17)
Transmission (A1-17)
EnergyBeam @ 21_45
Charge*1
e+ Conversion (/GeV)
Beam @ 23_43
Charge*1(e-)
Transmission (21-23)
Beam @ 58
Charge*1
Transmission (23-58)(e-)
Transmission (A1-58)(e-)
Transmission (21-58)
e+ Conversion (/GeV)
Energy(e-)
Energy Spread (FW)Beam @ ECS
Charge*1
Transmission (ECS)
EnergyEnergy Spread (FW)
Norm. Emittance (Bmag)
Injectorx(×10-6)y(×10-6)B8x(×10-6)y(×10-6)
C1
x(×10-6)y(×10-6)23x(×10-6)y(×10-6)57
x(×10-6)y(×10-6)
Orbit
Deviation(rms)∆x∆yVariation∆x∆y
BPM
Resolution
Offset*2
Repetition Rate
*1
Under calibration, *2Calibrated

Primary Electron Beam : 10 nC @ Target
Positron Beam : 0.64 nC
Target Value
Achieved Value
Score

Electron Beam : 1.2 nC
Achieved Value

Score

1.5
2.0

nC
ns

2
1.8

nC
ns

A
A

13
2.0

nC
ns

~ 14
2.8

nC
ns

A
B

1.4
<2
5

nC
%
ps

1.9
0
10

nC
%
ps

A
A
B

> 10
<2
16

nC
%
ps

11
~0
11

nC
%
ps

A
A
A

1.2 nC100%
1.5 GeV
±1.4
%
< 0.2
%

~ 1.8
~ 95
1.5 (Max 1.8)
±0.58

nC
%
GeV
%

A
A
A
A

> 10
100
1.5
±1.4
< 0.2

nC
%
GeV
%
%

~ 9.5
~ 86
1.5 (Max 1.8)
±0.75
fast
0.1
drift<
0.2

nC
%
GeV
%
%
%/h

A
A
A
A
A
A

~ 1.8
~ 100

nC
%

A
A

> 10
100
16

nC
%
ps

~ 9.0
~ 95
18

nC
%
ps

A
A
C

> 10
100
100
3.5

nC
%
%
GeV

~ 6.0
~ 67
~ 57
3.3

nC
%
%
GeV

C
C
C
B

(e+)

>2

nC

~ 1.5
~ 7.6

nC
%/GeV

A

(e+)

> 0.6
30

nC
%

~ 0.8
~ 53

nC
%

A
A

(e+)

> 0.6
100

nC
%

~ 0.7
~ 88

nC
%

A
A

~ 1.8 %/GeV
3.5
GeV

~ 47
%
~ 3.5 %/GeV
3.5
GeV
±0.8
%

A
A
A

> 0.6
80
3.5
±0.25

nC
%
GeV
%

~ 0.5
~ 70
3.5
±0.4

nC
%
GeV
%

B
A
A
B

60
60
< 1600
< 750

m
m
m
m

m
m

m
m
m
m
m
m
m
m
m
m

A
B
A
B

1600
1600

89 (1.0)120 (1.1)
770 (1.3)
760 (1.5)800 (1.6)
820 (1.2)
5600 (6.5)
6600 (16)2000 (1.3)
2300 (1.6)

< 0.1
< 0.1
< 0.1
< 0.1

mm
mm
mm
mm

0.3
0.3
±0.2
±0.2

mm
mm
mm
mm

B
B
B
B

< 0.1
< 0.1
50

mm
mm
pps

~ 0.1
0.1~0.2
5

mm
mm
pps

A
A

1.2
100

1.2
100
100

nC
%

nC
%
%

8.0+0.8 GeV

1.6

nC

1.5
94
79

nC
%
%

8.0+0GeV

A
A
B

A
(e+)

60
60
< 1600
< 750

m
m
m
m

1100
1100

m
m

< 0.1
< 0.1
< 0.1
< 0.1

mm
mm
mm
mm

< 0.1
< 0.1
50

mm
mm
pps

71
66
230
500

(1.2) m(1.2) m(1.6) m(1.8) m

A
A
A
A

(e+)
(e+)
(e+)
(e+)
< 0.1
< 0.1
< 0.1
< 0.1

5

8 REFERENCES[1] A. Enomoto et al., “Commissioning of the KEKB 8+
GeV e / 3.5-GeV e Injector Linac”, Proceedings of
EPAC98, Stockholm, June 22-26, 1998.
[2] S. Ohsawa et al., “”, Proceedings of APAC98, KEK,
March 23-27, 1998, in press.
[3] T. Kamitani et al., “Optics”, Proceedings of APAC98,
KEK, March 23-27, 1998, in press.
[4] T. Suwada et al., “BPM”, Proceedings of APAC98,
KEK, March 23-27, 1998, in press.

mm
mm
mm
mm

pps

A
A

B
B

[5] K. Sato et al., “Recalibration of Position Monitorswith Beams”, Proceedings of PAC95, Dallas, USA,May 1-5, 1995.
[6] Y. Ogawa et al., “New Streak-Camera System for the
KEKB Linac”, Proceedings of APAC98, KEK,
March 23-27, 1998, in press.
[7] Y. Funakoshi et al., in preparation.
[8] J. Flanagan et al., “Downhill Simplex Method”,
presented in KEKB mini-workshop, KEK, July 21-23,
1998.