STUDY OF VERTICAL BEAM BLOWUP IN KEKB LOW ENERGYRING
H. Fukuma, K. Akai, N. Akasaka, A. Enomoto, J. W. Flanagan, Y. Funakoshi, K. Furukawa,
S. Hiramatsu, K. Hosoyama, T. Ieiri, N. Iida, T. Kamitani, S. Kato, M. Kikuchi, E. Kikutani,
H. Koiso, T. Kubo, S. Kurokawa, T. Mitsuhashi, M. Masuzawa, T. Matsumoto,T. Mimashi, T. Nakamura, Y. Ogawa, K. Ohmi, Y. Ohnishi, S. Ohsawa,
N. Ohuchi, K. Oide, K. Satoh, M. Suetake, Y. Suetsugu, T. Suwada, M. Tawada,
M. Tejima, M. Tobiyama, S. Uno, N. Yamamoto, M. Yoshida, M. Yoshioka,
S. Yoshimoto, KEK, 1-1 Oho, Tsukuba-shi, Ibaraki-ken 305-0801, Japan,
E. Perevedentsev, Budker INP, 630090 Novosibirsk, Russian Federation,
F. Zimmermann, CERN, Geneva, Switzerland
Abstract
Vertical beam blowup has been observed in the KEKB
low energy positron ring (LER) since early operation. The
main characteristics of the blowup are explained bysingle-beam head-tail instability caused by an electroncloud. About 4600 solenoids were installed in the LER inorder to remove the electrons. This paper describes theexperimental study of the vertical beam blowup
emphasizing the effect of the solenoids.

bellows-NEG pump sections in the arcs. A set of threesolenoids covered a bellows-NEG pump section. The total
length of the regions where the field strength was greaterthan 20 Gauss was about 430 m. Thus about 40 % of thering circumference was covered by solenoid field as aresult of the two installations. A photograph of thesolenoids is shown in Figure 1.

1 INTRODUCTION
A blowup of the vertical beam size in both single beamand multibunch operation is observed in the KEKBLER[1]. This blowup is one of the most serious
problems limiting the luminosity of KEKB.
Observations[1,3] have shown that the main
characteristics of the blowup are consistent with themodel of single-beam head-tail instability caused by anelectron cloud which was proposed by F. Zimmermann
and K. Ohmi[2]. Many small permanent magnets, called“C-yokes”, were attached to the vacuum ducts to sweepout the electrons. Measurements of a single beam with ashort train, i.e. 60 bunches in a train, showed that the Cyokes were effective in suppressing the blowup for large
bunch spacings such as 12 rf buckets, but they were notvery effective for the shorter bunch spacing of 4 rf buckets
which is the spacing used for physics runs. Expecting thatthe electrons would be removed by solenoids more
efficiently than by the C-yokes, a large number ofsolenoids were installed in the LER.

2 SOLENOID SYSTEM
Solenoids were installed in two stages. In the first stagein September 2000, about 2800 solenoids were installed
mainly in the arc sections. The total length of thesolenoids was about 800 m. The calculated field strength
was 45 Gauss at the center of the solenoid when a currentof 5 A was applied. In the second stage of installation inJanuary 2001, about 1850 solenoids were installed in theFigure 1: Solenoids in the LER tunnel. The threesolenoids on the right side are those installed inbellows-NEG pump sections. The long solenoid on
the left side was installed in the first installation.

3 EFFECT OF SOLENOIDS
3.1 First installation of the solenoids
The effect of the solenoids on the blowup was measured
in single beam measurements. The results are summarized
as follows.
1) The threshold current of the blowup in a short train
was increased when the solenoids were excited, as
shown in Figure 2. The data in Fig. 2 is the average
vertical beam size over all bunches as measured bysynchrotron radiation interferometer. The beam size istransformed from the source point of the synchrotron
radiation to the collision point (IP).

100
half of solenoids on

5.0

Threshold current (mA)

Vertical beam size at IP (micron)

6.0

all solenoids off

4.0
3.0
2.0

all solenoids on

1.00.00

20

40608010020

solenoids off

10solenoids on

12345
Solenoid current(A)6Figure 4: Threshold current of the blowup as a functionof the excitation current of the solenoids.
2.5

Luminosity/bunch (1030 cm-2 sec-1)

20
Vertical beam size (arbitrary unit)

40

0Figure 2: Vertical beam size as a function of the beam
current. In the measurement two trains were injected on
opposite sides in the ring. Each train contained 60bunches. Bunch spacing was 4 rf buckets.

5600

Beam current (mA)

1580

solenoids on

2.0

1.5

C-yokes1.0
solenoids off
0.5

0.0

0

0.05

0.1

0.15

0.2

0.25

0.3

Ib(LER) Ib(HER) (mA2)00102030 40
Bunch

50

6070

Figure 3: Vertical beam size along a train taken by thegated camera with and without solenoids. The train
consisted of 60 bunches. Bunch spacing was 4 rfbuckets. Bunch current was 0.67 mA.
2) The vertical beam size along a train was measured by agated camera. The data shown in Fig. 3 clearly showedthat the solenoids were effective to suppress theblowup especially in the forward part of the train.
3) The vertical tune of each bunch in a train was measured
by a gated tune meter. The tune increased along thetrain then saturated. The change of the tune can beinterpreted as the focusing effect by the electron cloud.
The tune shift at saturation was enhanced about 30%
when the solenoids were turned off[4].
4) The threshold beam current at which the blowup started
was measured by the interferometer as a function ofthe solenoid current as shown in Fig. 4. It shows thatthe threshold beam current is nearly saturated at 3 A,i.e. at about a 30 Gauss solenoid field.

Figure 5: Luminosity per bunch with and withoutsolenoids. The horizontal axis is a product of the bunch
current of LER and HER (High Energy Ring). The datawith C-yokes is also shown. The fill pattern was that usedduring physics running. The number of bunches and thebunch spacing were 1153 and 4 rf buckets respectively.
The effect of the solenoids was also confirmed by themeasurement of the luminosity. As shown in Fig. 5 theluminosity per bunch increased by a factor of 1.6 when
the solenoids were excited. The luminosity per bunchwith C-yokes is also shown in Fig. 5. The data withsolenoids and with C-yokes almost coincided below 0.15
mA2. It is seen that the saturation of the luminosity withsolenoids starts at higher current than it does with Cyokes. Whether this improvement comes from thereplacement of the C-yokes with the solenoids or not is
not clear because the beam-beam effect, which depends onmachine parameters and beam tuning, also affects theluminosity.

3.2 Second installation of the solenoids
Fig. 6 shows the luminosity per bunch before and after
the second installation of the solenoids. The data shows

3.0
2.5
2.0
1.5

Before installation of additional solenoids(2000/12/26)

1.0
0.5
0.0

0

0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
2

Ib(LER) Ib(HER) (mA )Figure 6: Luminosity per bunch before and after thesecond installation of solenoids. The data denoted by “x”
was taken between the first and second installations of thesolenoids. The horizontal axis is the product of the bunch
current of LER and HER. The fill pattern was that used in
the physics run. The number of bunches was 1153 andbunch spacing was 4 rf buckets.
that the saturation of the luminosity at 0.2 mA2disappeared after the second installation. At present we do
not know how much the additional solenoids contribute tothis improvement because other machine parameters suchas tunes and beam tuning also affect the luminosity. The
vertical tunes in both rings were changed between the two
measurements from just above integer to above half
integer. The effect of the change of the tunes on theluminosity has not been clarified yet. We will soon
compare the luminosity with and without the additionalsolenoids to measure their effect more clearly.
The behavior of the luminosity is also given in
separate papers [5,6].

4 REMAINING BLOWUP
Though it was confirmed that the solenoids and Cyokes were effective to decrease the blowup, the blowup
still remains. As seen in Fig. 3 the beam size starts toincrease at the 30th bunch even with the solenoids on.
Furthermore there is an indication that the effect offorward bunches on the blowup of rearward bunches does
not disappear even at relatively long range. Fig. 7 shows
the vertical beam sizes of two trains which were separated
by 120 rf buckets from each other. In Fig. 7 the beam
size along the second train increases faster than along thefirst train and the beam size at the tail in the second trainis larger than that in the first train.
The remaining blowup may be caused by the electrons
which are not removed yet or by unknown mechanisms.
We have a plan to install more solenoids both in the long

Vertical beam size (arbitrary unit)

After installation of additional solenoids(2001/04/07)

20

Vertical beam size (arbitrary unit)

Luminosity/bunch (1030 cm-2 sec-1)

3.5

20

solenoids off

15
105

The inter-train gap of120 rf buckets is here.

0020406080
Bunch

100

120

1406080Bunch

100120140solenoids on

15105002040

Figure 7: Effect of train spacing on the blowup. Bunch
spacing was 4 rf buckets. Bunch current was 0.81mA.
Data was taken between first and second installation ofsolenoids.
straight sections and in the arc sections to see how muchthe blowup is reduced by further winding of the solenoids.

5 ACKNOWLEDGEMENTS
The authors would like to acknowledge the members of
KEKB accelerator group and Belle group for help to wind
the solenoids.

6 REFERENCES[1] K. Akai et al., “COMMISSIONING OF THE KEKB
B-FACTORY”, PAC’99, New York, Particle
Accelerator Vol.1, 288-292(1999).
[2] F. Zimmermann and K. Ohmi, “Head-Tail Instability
Caused by Electron Clouds in Positron Storage
Rings”, Phys.Rev.Lett.85:3821- 3824(2000).
[3] H. Fukuma et al., “OBSERVATION OF
VERTICAL BEAM BLOW-UP IN KEKB LOW
ENERGY RING”, 1122, EPAC2000, Vienna, June2000.
[4] T. Ieiri et al., “MEASUREMENT OF BETATRONTUNE ALONG BUNCH TRAIN IN THE KEKBLOW ENERGY RING”, in these proceedings.
[5] K. Oide et al., “STATUS OF KEKB B-FACTORY”,
in these proceedings.
[6] Y. Funakoshi et al., BEAM-BEAM EFFECT AT
THE KEKB”, in these proceedings.