Proceedings of the 2001 Particle Accelerator Conference, Chicago

PRESENT STATUS AND BEAM-STABILITY ISSUES OF THE KEKBINJECTOR LINAC
T. Suwada* , N. Akasaka, A. Enomoto, Y. Ogawa, J. Flanagan, H. Fukuma, Y. Funakoshi,
K. Furukawa, T. Ieiri, N. Iida, T. Kamitani, M. Kikuchi, T. Matsumoto, T. Nakamura, Y. Ogawa,Y. Ohnishi, S. Ohsawa, K. Satoh, M. Suetake, H. Koiso, and K. Oide, KEK, Tsukuba, Japan
upgrades for the electron gun, beam-monitoring system,
control software, and bunch-selection system.

AbstractThe KEKB injector linac was completely upgraded for
the KEK B-Factory (KEKB) project in March, 1998.
Many difficulties have been overcome during the elaborate
commissioning of the upgraded linac since the end of
1997. The 3.5-GeV positron and 8-GeV electron beams
have been injected to the KEKB rings with goodperformance. Much effort has also been continuing tostabilize the intensity and quality of the beams. Someexperimental results on the beam stability issues areshown together with the recent operation status in thisreport. A beam test on a new scheme of a two-bunchinjection was started in order to increase the positron
intensity since March, 2001.

2 OPERATION STATUS
2.1 Four Beam Modes
The injector linac has been operated in four beammodes. Two are injection modes for the electron andpositron beams for the KEKB rings, and the others areelectron injection for the PF and AR rings. The layout ofthe linac beam lines is shown in Fig.1 and the four beammodes are summarized in Table 1.
Table 1: Four beam modes of the injector linac.
KEKB/ KEKB/PFARHER
LER
ee + / eeeParticle
Energy [GeV]
83.5/3.72.5
2.5Charge [nC]1.00.6/10
0.20.2
Repetition
50502525
Inj. Time [min]
1-2
4-5
3-5
3-5Interval [hour]
1-2
1-2
242

1 INTRODUCTION
The KEK B-Factory (KEKB) project[1] started in 1994
to test CP violation in the decay of B mesons. The KEKB
comprises an asymmetric electron-positron collider with
3.5-GeV positron (LER) and 8-GeV electron (HER) rings.
In such factory machines, it is strongly required that thebeam stability of the injector linac be kept in order toachieve a short injection time for an effective
accumulation of the integrated luminosity. The
commissioning of the injector linac has continued since
the end of 1997 after the original 2.5-GeV electron linac
was upgraded in order to accomplish efficient full-energy
injection[2-4], even while continuing injection into two
photon factory rings (PF and AR).
Many practical difficulties concerning the beamstability issues have been overcome in order to keep
design beam parameters and, particularly, the goodstability of the high-current primary electron beam for
positron production. This effort is still being continued
for further improvements.
A commissioning on two-bunch injection for thepositron beam was started in order to reduce the injection
time for higher storage of the beam currents, since afurther increase of the single-bunch charge is difficult for ahigh-intensity primary electron beam from a wake-fieldinstability point of view. Such an injection scheme isexpected to halve the positron injection time, and to pushahead the KEKB experiment for an efficient accumulationof the integrated luminosity. A preliminary beam test wassuccessfully made on two-bunch acceleration after the*An electron beam with a repetition rate of 50 Hz fromthe A-Gun is bunched into a single-bunch with a bunch
width of 10 ps (FWHM) by two sub-harmonic bunchers(SHBs) operated at frequencies of 114.24 MHz and 571.2
MHz, and by a prebuncher and a buncher operated at 2856
MHz. For the KEKB, a single-bunch electron beam witha charge of 1 nC/bunch is accelerated up to 8 GeV by theentire linac from the A-Gun, and a single-bunch positron
beam with a charge of 0.6 nC/bunch is accelerated up to3.5 GeV at the linac end, while a high-intensity primaryelectron beam with a charge of 10 nC/bunch is accelerated
from the A-Gun up to 3.7 GeV at a positron target. On
the other hand, a multi-bunch short-pulse beam (1-ns FW,25 Hz) with a charge of 0.2 nC/pulse is accelerated fromthe C-Gun up to 2.5 GeV for injecting into the PF andAR rings.
A reliable software operation switch changes the four
beam modes without any degradation of the linac beam
quality[5]. The switching time was especially reduced byintroducing quick initialization of the magnet and areliable recovery scheme of device failures. Thus, thebeam modes are stably switched about 32 times/day

Corresponding author. E-mail address: tsuyoshi.suwada@kek.jp.

0-7803-7191-7/01/$10.00 ©2001 IEEE.

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Proceedings of the 2001 Particle Accelerator Conference, Chicago
Sector B
B-8

B-7

B-5

B-6

C-1

C-2

180-degree
Arc e- 1.7GeV

C-3

Sector A
B-4

C-4

B-3

C-5

B-2

A-4

B-1

C-6

A-Gun (KEKB)

A-3

C-8

A-2

1-1

1-2

e + Target/e- 3.7GeV

A-1

1-3

1-4

Sector C

1-5

1-61-7

1-82-1

2-2

2-3

2-4

2-6

2-7

2-8

3-1

Sector 2

Sector 1

C-Gun (PF/AR)PF e - 2.5GeV
Beam Switchyard

3-2

3-3

3-4

3-5

3-6

3-7

3-8

Sector 3

4-1

4-2

4-3

4-4

4-5

4-6

4-7

4-8

5-1

5-2

Sector 4

5-3

5-4

5-5

5-6

Sector 5

5-7

5-8

ECS KEKB e - 8GeV KEKB e+ 3.5GeV

AR e- 2.5GeV

Figure 1: Layout of the KEKB injector linac.
for the KEKB injections, and one and 24 times/day for thePF and AR rings, respectively.

3 BEAM STABILITY ISSUES

2.2 KEKB Operation

3.1 Control of the Gun Beam

Electron and positron beams, with a charge of 0.8 and0.4 nC/bunch, respectively, are transported at the end of
the beam transport line (BT). Beam injection rates of 1.8
and 4 mA/s were recently accomplished with a repetition
rate of 50 Hz for the LER and HER rings, respectively.
Such stable high injection rates were realized by dedicatedbeam and rf feedback systems[5], and daily monitoring of
the optics matching and beam emittances by wirescanners, and fine injection tuning. Especially the energyspread of the high-intensity primary electron beam ischecked before every physics run. Typical beam
parameters are summarized in Table 2.

The control and monitoring system were reported
elsewhere[6] in detail. The present stability of the gun
beam is briefly summarized here.
The electron gun is operated at a high voltage of 200 kV
with a repetition rate of 50 Hz. It was found that under the
typical operation condition the temperature rise is about
10oC at the load resistance until the gun becomes stable,
which corresponds to a high-voltage variation of 1.5%. Aslow software feedback system was adapted in order tokeep the high voltage constant. The variation of the highvoltage is within 0.05% by this feedback, corresponding
to a gun-beam timing of less than 1 ps, which ismeasured at a wall-current monitor installed after the gun.
It is also important to keep the grid-pulse timing
constant, because the variation of this timing causes a
different bunch shape of the beam at a different rf phase. A
software feedback system is stably operated in order tokeep the grid-pulse timing within 20 ps by synchronizing
the timing with a reference rf wave of 571.2 MHz. The
phase and amplitude for the two SHBs are always
monitored using the pickup rf signals from the cavities
relative to the 2856-MHz accelerating rf wave by a fastsampling oscilloscope. The results show that thevariations of the phase are less than 0.5 and 1 degree for
SHB1 and SHB2, respectively, and the variation of theamplitude is about 1% for both the SHBs at a time scaleof one week, which satisfy the stable beam transmission
rate above 90% at the end of the buncher.

Table 2: Typical beam parameters for the KEKB
injection. The symbol “*” shows the parameters measured
at the end of the linac. Subscripts 1), 2) and 3) depict theparameters at the positron target, at the center of the J-arcline, and at the end of the sector B (E=1.7 GeV),
respectively.
e -/e +e-@BTe+@BT
Energy [GeV]
8.03.53.71)**
Charge [nC]0.8/1
0.4/0.68.01)∆E/E [%@1σ]
0.050.150.52)
γεx/γεy [mm@1σ] 0.31/0.31
2.4/2.03.5/1.83)
Injection Rate41.8[mA/s]@50 Hz(>95%)(>95%)

2.3 Operation Statistics
The total operation time was 7566 hours in FY 2000,
which was greatly pushed up due to full KEKB operation.
The net beam-injection times for the KEKB and thephoton factory rings, were 1504 and 293 hours with aloss time of less than 1%. An injection availability of thelinac greater than 99% for the four beam modes wasaccomplished.

3.2 Beam Trajectory Jitter Analysis
The beam trajectory jitter quantitatively represents thebeam stability concerning the transverse wake-field effectand unexpected failures for hardware devices. An
application software [7] recording the beam trajectory
jitter was made using data from all beam-position
monitors (BPMs). The stability of the positron beam wasanalized based on a beam trajectory jitter measurement.
The trajectory jitter measured at the BPM location is
distributed on a phase space, which area can be defined as4084

Proceedings of the 2001 Particle Accelerator Conference, Chicago

mm rad]a jitter emittance. Figure 2 shows a variation of thehorizontal jitter emittance along the whole linac beamline, where the phase space is normalized by the beamenergy. The result shows that the jitter emittance of thehigh-intensity
primaryelectron
beam
grows
monotonously, except for that measured at the J-arc where
the energy jitter is added, to the positron target without
any step change, and for a positron beam the growth of
the jitter emittance is much smaller than that of the
primary electron beam. This indicates that a more careful
control of the transverse wake-field effect is required forthe primary electron beam. A new scheme which
continuously corrects the beam orbit is under way.
0.2

primary e- beam
0.175

e+ beamHorizontal Jitter Phase Space [0.150.125

J-arc line0.1

4.2 Injection Test
Bunch shapes for the two bunches were measured by an
optical transition monitor with a steak-camera systemafter fine tuning of the grid-pulser timing of the second
bunch relative to the first bunch. The result shows thateach beam was single-bunched without any observational
satellite bunches. The bunch length was 13-15 ps in
FWHM for the two bunches. A new orbit correction
method was applied to the beam, which accomplishes theaverage minimum of the two-bunch orbits. After applying
this method for a high-intensity primary electron beam,
the two bunches were successfully accelerated up to thepositron target with each bunch charge greater than 7 nC.
The charge of the two positron bunches was 0.4-0.5 nC at
the end of linac. A preliminarily injection test was
successfully performed using a special fill pattern, and it
confirmed a bunch interval of 49 buckets in the LER. We
have pushed ahead this study in order to further investigate
not only the long-term stability for two-bunch
acceleration, but also more realistic fill patterns for higher
luminocities.

0.0755 CONCLUSIONSe+ target

0.050.025

0100200300400500600

Line Length along the Linac [m]Fig. 2: Variation of the horizontal phase space based ona positron beam trajectory jitter measurement.

4 TWO-BUNCH ACCELERATIONThe KEKB injetcor linac was enthusiasticallycommissioned while it went into routine operation inDecember, 1998. The monitoring and control system of
the pre-injector were reinforced in order to stabilize thehigh-intensity primary electron beam. The averageintensity of the positron beam was remarkably increased
and stable reproducibility was achieved. In order to further
increase the positron intensity, a new two-bunch
acceleration scheme is under testing, and a further study
for long-term stability of the scheme is under way.

4.1 Generation and Acceleration Schemes

6 REFERENCES

Generation and acceleration schemes of two bunches arebriefly described, since they were reported elsewhere[8,9](see also [10]) in detail. Two successive electron bunches
are generated at the gun by firing two successive grid
pulses with an interval time of 96.29 ns, which are madeby combining from two fast grid pulsers. The bunchinterval corresponds to the period given by a common
frequency of the linac and the KEKB rings. The frequency
relation is summarized in Table 3. A beam-loading
compensation scheme for correcting the beam energy ofthe second bunch was established by adjusting the timing
based on the energy gain curve.
Table 3: Frequency relation between the linac and rings.
Multiple Frequency Period
Number[MHz][ns]
Common Frequency
1 10.385454 96.289
SHB111114.240
8.754
SHB2
5x11571.200
1.751
Linac
5x5x1 2856.000
0.350
Ring7x7508.887
1.965[1] KEKB B-Factory Design Report: KEK Report 95-7
(1995).
[2] Design Report on PF Injector Linac Upgrade for
KEKB: KEK Report 95-18, 1996.
[3] K.Furukawa, et al., Proc. LINAC2000, Monterey,U.S.A., 2000, p.630.
[4] A.Enomoto, et al., Proc. EPAC2000, Vienna,Austria, 2000, p.993.
[5] K.Furukawa, et al., Proc. LINAC2000, Monterey,U.S.A., 2000, p.633.
[6] S.Ohsawa, et al., Proc. EPAC2000, Vienna, Austria,2000, p.845.
[7] T. Suwada, Proc. the 25th Linear Accelerator
Meeting in Japan, Himeji, Japan, 2000, p.386.
[8] S.Ohsawa, et al., Proc. HEAC2000, Tsukuba, Japan,
2000, to be published.
[9] Y.Ogawa, et al., Proc. HEAC2000, Tsukuba, Japan,2000, to be published.
[10] S.Ohsawa, et al., in this conference.

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