OBSERVATION OF VERTICAL BEAM BLOW-UP IN KEKB LOW
ENERGY RING
H. Fukuma, K. Akai, N. Akasaka, K. Bane* , K. Egawa, A. Enomoto, J. W. Flanagan, Y.
Funakoshi, K. Furukawa, K. Hayashi, S. Hiramatsu, H. Hisamatsu, K. Hosoyama, N. Huang∗∗, T.
Ieiri, N. Iida, T. Kamitani, K. Kanazawa, S. Kato, M. Kikuchi, E. Kikutani, H. Koiso, T. Kubo, S.
Kurokawa, T. Mitsuhashi, M. Masuzawa, T. Matsumoto, S. Michizono, T. Mimashi, T. Nakamura,Y. Ogawa, K. Ohmi, Y. Ohnishi, S. Ohsawa, N. Ohuchi, K. Oide, E. A. Perevedentsev∗∗∗, D.
Pestrikov∗∗∗, K. Satoh, M. Suetake, Y. Suetsugu, T. Suwada, F. Takasaki, M. Tanaka, M. Tawada,M. Tejima, M. Tobiyama, T. Tsuboyama, N. Yamamoto, M. Yoshida, S. Yoshimoto, C. H. Yu∗∗, F.
Zimmermann∗∗∗∗, KEK, Tsukuba, Ibaraki 305-0801, Japan
S. Olsen, University of Hawaii, Honolulu, Hawaii, USA

This paper describes an experimental study of the enlargement of the vertical beam size observed in single
beam operation at the KEKB LER.

electron cloud appears as either weak or strong head-tailinstability. A beam-size blow-up will be observed as aresult of the head-tail oscillation of the instability. In thismodel the strength of the instability depends on the electron cloud density near the beam.

1 INTRODUCTION

2 EXPERIMENT

A blow-up of the vertical beam size is observed in theKEKB positron ring (LER)[1]. The beam size as a function of beam current starts to increase at a threshold beam
current and is almost doubled at 300 mA under typicaloperating conditions. Thus the blow-up is one of themost serious problems limiting the luminosity of KEKB.
The main characteristics of the blow-up observed inearly operation period are summarized as: 1) the blow-up
is a single beam and a multi-bunch effect; 2) the blow-up
has a threshold intensity which is determined roughly by(bunch current)/(bunch spacing); 3) no dipole oscillation
has been observed when the vertical chromaticity is high(5 to 8), though the chromaticity is as low as 2 and thebunch feedback system is turned off, a vertical dipole oscillation is excited; 4) the blow-up is almost independentof betatron tunes; 5) the blow-up does not depend on thepositions of the vertical masks, which are among themain impedance sources; 6) the blow-up does not dependon the vacuum pressure, especially for hydrogen, in thearc; 7) the blow-up does not depend on the excitation of
the wigglers; and 8) no blow-up is observed in the horizontal plane.
A model to explain the blow-up is proposed by two of
the authors, F. Zimmermann and K. Ohmi [2]. In theirmodel the blow-up is explained as a single-bunch instability of a positron bunch due to a large number of electrons
generated by photoemission or secondary emission. Theinstability will occur only in multi-bunch operation since
the electron cloud is built up by the successive passage ofthe bunches. The coherent dipole oscillation of positrons
along the bunch caused by the “wake” force due to theSeveral experiments were performed to examine themodel. In most experiments, so-called “C-Yoke” magnets(C-Yokes) were installed in the ring. They were attachedto the outside of the vacuum chambers to confine the electrons to the vicinity of the chamber walls. A C-Yoke
consists of two permanent magnets and a C-shaped ironyoke. The magnets are attached every 10 cm along thedrift space between ring magnets. Initially the C-Yokes
were installed only on the outer circumference of the ringand covered a 7-m long drift space downstream of every
bending magnet in the arc. Then C-Yokes were added tocover about 50% of the drift space of the whole arc andwere attached both on the inward and outward sides of thering so as to generate a quadrupole field. The polarity ofthe magnets along the chamber was reversed every 20 cm
to reduce the effects on the beam optics. The peak verticalmagnetic field was 250 G at the chamber wall.
The multi-bunch feedback was turned on in all experiments to suppress dipole oscillations.

Abstract*
visiting from SLAC, U.S.A., ∗∗ visiting from IHEP, China,
Switzerland.

1122

2.1 Formation of electron cloud
The average beam size at various bunch spacings wasmeasured by the interferometer[3] as a function of beamcurrent. As shown in Figure 1, without C-yokes thethreshold intensity scaled as Ib/sb, namely the charge density of the beam, where Ib and sb are the bunch current andthe bunch spacing respectively. With C-yokes the beam
size scaled as Ib/sb Ib . In models of the single bunch instability due to the electron cloud, the growth rate is proportional to ρ Iba where ρ is the density of the electron
cloud and a is a number depending on models[2,7]. The∗∗∗

visiting from BINP, Russia, ∗∗∗∗ visiting from CERN,

Proceedings of EPAC 2000, Vienna, Austria

observation is consistent with this picture if the densityof the cloud is proportional to that of the beam (“partialneutralization”). The reason why the scaling changed
before and after the installation of C-Yokes is notunderstood yet. C-Yokes may change the shape of “wake”
function due to the electron cloud.

era[5].
The
data
shows that the size
gradually
increases
along the train, andthen almost saturates(See Fig. 7).
To measure the decay length of theblow-up a test bunch
was injected immediFigure 3: Build up of the electron ately after a train. Thecloud (total charge per meter) as a beam size of the test
function of time in sec for threebunch was measured
different bunch populations. Thebunch spacing is 4 rf buckets[4]. as a function of the
train-to-bunch spacing. The data show that the decay length of the blow-up is
about 12 rf buckets when the bunch spacing of the train is
4 rf buckets. Furthermore the beam size of two trains
with a train-to-train gap was measured to observe thebuild up or decay of the cloud over two trains. As shownin Fig. 4 while the beam size of the first train started toblow-up at the 7th bunch, in the second train the 2ndbunch already showed a sizable blow-up which can beinterpreted to mean that the density of the cloud generated
1.1e+100.5e101.5e102.5e10

1e+10
9e+09
8e+09
7e+09
6e+09
5e+09
4e+09

6.0

2e+09
1e+09

Without C-Yokes

5.0

00

4.0
3.0
2.0
bunch spacing in rf buckets
4812
16

1.0
0.0

0

0.02 0.04 0.06 0.08 0.1 0.12
bunch current/bunch spacing(mA/bucket)

0.14

With C-Yokes

5.04.0
3.0
bunch spacing in rf buckets
481216

2.0
1.0
0.00

0.05

0.1

0.15

0.2

bunch current2/bunch spacing(mA2/bucket)

Figure 1: Blow-up of beam size in various
bunch spacing with and without C-Yokes.
0.20

Tune shift/charge density((mA/rf bucket)-1)

simulation after the passage of 60th bunch
0.15
0.10
0.05
0.00
bunch current=0.21 mAbunch spacing
in rf buckets
346810203040
Bunch id50

6070

Figure 2: Vertical betatron tune shift along thetrain for four different bunch spacings of 3, 4, 6 and
8 rf buckets. The tune shift is normalized by thecharge density of the beam (Ib/sb).
To obtain information on the cloud build up thevertical betatron tune of each bunch was measured along
the train by a gated tune meter. As shown in Fig. 2, 1)
the tune increases along the train and almost saturates atabout 20th bunch, 2) the tune shift is almost proportional
to the charge density of the beam, and 3) (saturated tuneshift) / (charge density) is about 0.12. These observations
are consistent with a simulation[2,4,6] for a field free
region though many C-Yokes were installed at the time ofthe measurement. An example of the simulation is shownin Fig. 3.
Another indication of the build up of the cloud is theobservation of the vertical beam size by fast gated cam-Proceedings of EPAC 2000, Vienna, Austria

5e-08

1e-07

1.5e-072e-07

2.5e-073e-07

3.5e-074e-07

4.5e-07

5e-07

20
15
10
5
gap of 32 buckets
00-0.05
0

Vertical beam size (arbitrary unit)

Vertical beam size at IP (micron)Vertical beam size at IP (micron)

3e+09

6.0

10203040
Bunch id506070

Figure 4: Beam size over two trains measured by
the fast gated camera. Train-to-train gap which is
not shown in the figure, is 32 rf buckets.
by the preceding train is quickly reestablished in the second train. Fig. 5 shows the simulation results with andwithout magnetic field. Again, the above measurements
are consistent with the simulation for a field free region,
though C-Yokes were attached for these measurements.
Fig. 6 shows the blow-up before and after the first installation of the C-Yokes. Though the blow-up was improved for a spacing of 12 rf buckets, the improvement
for a spacing of 4 rf buckets was not remarkable. Also
there was not much improvement after the second
installation of the C-Yokes although the simulationshows that the density of the electron cloud decreases by afactor 10 with the quadrupole C-Yoke configuration ascompared to the field free case[6].

2.2 Single bunch characteristics of the blow-up
To prove the single bunch nature of the blow-up a test
bunch was injected immediately behind a train. The beamsize of the test bunch was measured at several bunch cur-1123

2.5e+12

2.5e+11
4 bucket spacing, no field, 14-bunch trains, 32-bucket gap
at moment of bunch passage

4 bucket spacing, symmetric configuration
at moment of bunch passage

The increase of vertical betatron tune
and beam size along the train, thedecay time of the blow-up and themeasurement of the beam size over
two trains are consistent with thesimulated build up of the density of theelectron cloud for a field free region,
though a large number of C-Yokes
Figure 5: Simulated electron density near the beam per cubic meter as awere attached on the chambers duringfunction of time in sec. Left: without magnetic field, right : with quadrupole
the measurement. The reason why themagnetic field[6].
C-Yokes are not very effective is notclear yet. Recently, a nonlinear pressure rise, which is
Bunch spacing = 4 rf buckets
8suspected to arise from multipacting, was observed in thewithout C-Yokes
6straight sections and in the drift spaces without C-Yokes
with C-yokesin the arc. Thus, a large number of electrons in the re2e+122e+11

1.5e+121.5e+11

1e+12

1e+115e+115e+100

05e-081e-07

1.5e-072e-07

2.5e-073e-07

3.5e-07

02e-08

4e-08

6e-08

8e-08

1.2e-07

1.4e-07

1.6e-07

1.8e-07

2e-07

4
200

100
200
Beam current(mA)

300

Bunch spacing = 12 rf buckets

8without C-Yokes
with C-yokes

6
4
200

100
200
Beam current (mA)

300

Figure 6: The blow-up before and after the first
installation of the C-Yokes. The ring was filledwith 4 trains each of which contained 60 bunches.
rents of the test bunch. The bunch current for all otherbunches was kept constant. The results showed that thebeam size of the test bunch increased when its bunch current increased, which demonstrates that the blow-up is asingle bunch effect.
The effect of the vertical chromaticity on the blow-up
was measured by the fast gated camera. If the blow-up iscaused by the head-tail instability it should be sensitive tothe chromaticity. Fig. 7 shows that the blow-up along thetrain became weaker when the chromaticity was increased.
The effect was confirmed by the interferometer. Observed
beam size dependence on the chromaticity could be consistent with either strong or weak head-tail instability[2,7].
This question requires further studies.

3 SUMMARY
The blow-up of the vertical beam size observed in
KEKB LER is a single bunch effect only appearing in
multi-bunch operation as predicted by the F. Z. and K.O.
model.
The blow-up was weakened at high vertical chromaticity, which could be consistent with either strong or
weak head-tail instability.

1124

1e-07

Vertical beam size (arbitrary unit)

Vertical beam size
at IP (micron)Vertical beam size
at IP (micron)015

10Qy'-0.1
1.3
2.3
7.3
11.9

5005

10

1520
Bunch id2530

35

Figure 7: Beam size along the train observed by thefast gated camera in various chromaticities.
Diffraction effect is not corrected.
maining field-free chambers may be responsible for thepersistent blow up.
At present there is no clear experimental evidence
against the F. Z. and K.O. model in which the blow-up iscaused by a single bunch instability due to the electroncloud.

REFERENCES[1] Y. Funakoshi et al., “KEKB Performance”, in theseproceedings.
[2] K. Ohmi and F. Zimmermann, “Head-Tail Instability
Caused by Electron Cloud in Positron StorageRings”, KEK Preprint and CERN-SL-2000-015 (AP)(May, 2000).
[3] T. Mitsuhashi et al., “Optical Diagnostics Systemfor KEKB B-Factory”, in these proceedings.
[4] F. Zimmermann, “Electron-Cloud Studies for theLow Energy Ring of KEKB”, CERN SL-Note-2000004 AP (2000).
[5] J. Flanagan et al., “High-Speed Gated Camera
Observations of Transverse Beam Size Along Bunch
Train at the KEKB LER”, in these proceedings.
[6] F. Zimmermann, “Electron Cloud at the KEKB Low
Energy Ring: Simulations of Central Cloud Density,
Bunch Filling Patterns, Magnetic Fields, and Lost
Electrons”, CERN-SL-2000-017 (AP) (May, 2000)[7] E. A. Perevedentsev, "Single-Bunch Instability
Caused By Photoelectron Cloud", to be published.

Proceedings of EPAC 2000, Vienna, Austria