KEKB PERFORMANCE
Y. Funakoshi, K. Akai, N. Akasaka, K. Bane∗ , A. Enomoto, J. Flanagan, H. Fukuma,
K. Furukawa, J. Haba, S. Hiramatsu, K. Hosoyama, T. Ieiri, N. Iida, T. Kamitani, S. Kato,
M. Kikuchi, E. Kikutani, H. Koiso, S.I. Kurokawa, M. Masuzawa, T. Matsumoto, S. Michizono,
T. Mimashi, T. T. Nakamura, Y. Ogawa, K. Ohmi, Y. Ohnishi, S. Ohsawa, N. Ohuchi, K. Oide,
E. A. Perevedentsev† , K. Satoh, M. Suetake, Y. Suetsugu, T. Suwada, F. Takasaki, M. Tawada,
M. Tejima, M. Tobiyama, N. Yamamoto, M. Yoshida, S. Yoshimoto, M. Yoshioka, C. H. Yu‡ ,F. Zimmermann§ , KEK, Ibaraki 305-0801, Japan
600

This report summarizes a recent progress of the KEKB
whose commissioning started on December 1st 1998.
Since the commissioning of earlier period was described inother papers [1][2], this report mainly deals with the commissioning from the beginning of 2000.

500

INTRODUCTION

The KEKB B-Factory is one of second generation electronpositron colliders. It has two significant features of a“very high luminosity” and an “energy asymmetric collider”. These features come from requirements of B meson physics which studies very rare processes and aims at
detecting the CP violation in the B meson system.
The design luminosity is 1 × 1034 /cm2 /sec. Other design parameters related to the luminosity also fit with thisdesign luminosity. Design beam currents are 2.6A and1.1A for the positron and electron beams, respectively. Design parameters of the horizontal and vertical beta functions at the IP for both beams are 0.33m and 10mm, respectively. Design goals of the beam-beam parameters forboth beams are 0.039 and 0.052 in the horizontal and vertical directions, respectively. Beam energies are 3.5 GeV for
Low Energy Ring (LER e+) and 8 GeV for High Energy
Ring (HER, e-). The requirement of energy asymmetry inevitably leads us to a double ring collider. From the standpoint of machine design, this double ring feature enables a“high current-multibunch” approach like synchrotron lightsources, which is vital to get to a higher luminosity. In addition to these features, the KEKB adopted a challenging
scheme of a (horizontal) crossing angle of ±11 mrad. A
motivation of the crossing angle is to simplify the IR design and to suppress effect of a parasitic collision[3].
So far the design goals above have not been fully accomplished yet. Table 1 summarizes the present performanceof the KEKB related to the luminosity together with thedesign parameters. This article describes how the present
performance has been attained and what limits it.
∗ visiting from SLAC, U.S.A† visting from BINP, Russia‡ visiting from IHEP, China§ visiting from CERN, Switzerland

28

LER Current [mA]1

HER Current [mA]

Abstract

400
300
200
100
0
800
700
600
500
400
300
200
100
0
Jan/1/1999

Jul/2/1999

Jan/1/2000

Figure 1: Histories of the beam currents.

2
2.1

PRESENT PERFORMANCE

Brief History

Fig. 1 shows histories of the beam currents. As is seenfrom the figure, there have been steady increases of thebeam currents. The increase rates, however, were not very
remarkable this year. As is described in another section,
the present beam current limitation does not come from instabilities but from some hardware heating problems. Fig.
2 shows a history of the luminosity. Contrary to the caseof the beam currents, we see a remarkable increase of theluminosity. The peak luminosity was 6.7 × 1032 /cm2 /sec
at the end of last year. The record peak luminosity of the
KEKB so far is 19.2 × 1032 /cm2 /sec which was recordedon May 29 2000. Actually, the peak luminosity almost
tripled this year. As is discussed in the next section, thisimprovement in the luminosity has been brought mainly
by squeezing beta functions at the IP and other optimization of the beam-beam effect. Some related parameters at
this record luminosity are summarized in Table 1 comparedwith the design values. As shown in Table 1, βy∗ is now
lower that the design.

Proceedings of EPAC 2000, Vienna, Austria

Luminosity [1030/cm2sec]
Daily Integrated Luminosity [pb-1]

2.2

2000

Hor. Emittance1500

βx∗ /βy∗

1000

Beam Current

500# of bunches

0

Bunch Current

100
80# of trains
Bunches/train

60

Bunch spacing
Bunch Length(calculation)40
20
0
Jan/1/1999

ξx
Jan/1/2000

ξyFigure 2: History of the luminosity.

νx

Jul/2/1999

νyBeam-beam parameters

It is shown in Table 1 that the vertical beam-beam parameter of the HER is notably low compared to the design value.
This is a consequence of the following two reasons. One
reason is that the bunch current of the LER is limited by the
hardware heating problem in usual operations. The bunch
current of the LER in Table 1 is much lower than the design. The other is a single beam blowup in the LER. This
problem is also discussed in another section.
To see how seriously these problems limit the beambeam parameter (and the luminosity), an experiment withfewer number of bunches was carried out. In this experiment, we used 188 bunches which is about 1/6 of that in
the usual operation. In the experiment, the bunch current
could be increased up to near the design, since the heatingproblem is less serious with the fewer number of bunches.
In the usual operation, the bunch spacing is 4 RF buckets (8nsec) and the LER single beam blowup is serious asshown in Fig. 5. In the experiment, the bunch spacing was
24 RF buckets and the LER single beam blowup was not
visible with this bunch spacing.
Table 2 shows a result of the experiment. ξy of the HER
with 188 bunches was much higher than that with 1069
bunches. On the other hand, ξy of the LER got smaller thanthe case of 1069 bunches. This is because the HER beamis blown up in turn in this situation. Since the LER beam
is apt to be blow up due to the beam-beam effect in theusual operation, most part of beam-beam tuning has been
devoted to suppress the beam-beam blowup of the LER. It
may be possible to suppress this HER beam-beam blowup
with more beam-beam tuning and to get even higher ξy of
the LER. With this 188 bunch operation, we obtained theluminosity of 4.04 × 1032 /cm2 /sec with almost no beam

Proceedings of EPAC 2000, Vienna, Austria

Lifetime
Luminosity
Belle CsI

LER
HER
29
30
0.7/0.007(0.33/0.010 )565
397(2600)(1100)
1069(2833)
0.53
0.37(0.87)(0.37)
1674
8(2)
5.9@9.0

6.4@5.0

0.039
0.032
(0.039)(0.039)0.036
0.018(0.052)(0.052)45.51
44.519(45.52)(44.52)44.07
42.176(44.08)(42.08)105@565 302@397
19.2 × 1032
(1.0 × 1034 )

nm
m
mA

mA

nsec
mm@MV

mim@mA/cm2 /sec

Table 1: Present performance compared with the design.
(Values in a parenthesis are the design values.)# of bunchesξx /ξy (LER)ξx /ξy (HER)
bunch current (LER)[mA]
bunch current (HER)[mA]
Luminosity [/cm2 /sec]

1069
0.039/0.036
0.032/0.018
0.53
0.37
1.92×1033

188
0.030/0.025
0.030/0.032
0.70
0.30
4.04×1032

Table 2: Result of beam-beam experiment.
tuning. This luminosity multiplied by a factor 6 is higher
than the present record of the luminosity. This means that
the LER single beam blowup and the beam current limitation from the hardware heatings limit the present peakluminosity seriously.

2.3

Specific luminosity

Fig. 3 shows a specific luminosity as function of the product of the beam currents in some typical fill. As is seenin the figure, the specific luminosity is not constant at relatively low beam currents. Since no single beam blowupof the LER is seen at the low beam current, this indicatesthat the beam-beam blowup is not negligible even at the
low currents. The tendency for the specific luminosity toincrease even at the low beam currents is reproduced by abeam-beam simulation[4].

29

28

28

1 10

27

2 10

8 10
6 10

27

2 10

150000200000250000300000350000Charge∗) (e-)
Charge∗) (e+)
Inj. rate (e-)
Inj. rate (e+)
Inj. effic. (e-)
Inj. effic. (e+)
Mode switche+ < − > e-typical
1
0.5
2
0.7
60
40

record
2.0
0.82
4.3
1.7
∼ 90
∼ 90

design
1.28
0.64
6.4
3.2
100
100

nC
nC
mA/secmA/sec
%
%

2

2

2

min.

*) at the end of LINAC
Table 3: Performance of the beam injection.

Integrated luminosity

The daily integrated luminosity in Fig. 2 is that recordedby the physics detector(Belle). The delivered luminosity
by the KEKB accelerator is about 10% higher than therecorded one. The total integrated luminosity from the beginning of the KEKB is 5104 pb−1 as of June 15 2000.
Beam injection performance is one of the factors which
affect the integrated luminosity. Table 2 summarizes thepresent performance of the beam injection. In the usual
operation, bunch charges at the end of the injector linac(LINAC) are near its design values. Injection efficiencies
are rather poor. This is partially because the ring acceptances are severely restricted by movable masks which areused for suppressing the Belle beam background. With thepresent KEKB injection systems, it is not possible to inject
the two beams simultaneously. It takes 2 minutes to switch
LINAC from the e+(e-) mode to the e-(e+).

ROAD TO PRESENT LUMINOSITY

0.15

1.5 10**β */ β *βx / βy= 1m/10mm= 0.7m/7mm

y

33

0.1

1 10 3 3

0.05

5 10 3 2

0

Jan/1/2000

Mar/2/2000

May/2/2000

Date

Figure 4: History of the luminosity improvement.
horizontal and vertical tunes give better results. The best
luminosity in the figure is achieved with almost the sametunes as the design. The tune search in Fig. 4 was done intrial and error ways in daily tuning. The best tune foundthis way is coincided with the design tune which was determined from a strong-weak beam-beam simulation[3]. In
Fig. 4 the luminosity from a new strong-strong beam-beamsimulation[7][4] is also shown. The effectiveness of the design tunes are also confirmed by the new simulation. Moreover, the measured luminosity is roughly reproduced by thesimulation. Therefore, we have now more confidence inpredictive power of the beam-beam simulation.

3.2

Squeezing βx∗ /βy∗As shown in Fig. 4, we squeezed βx∗ /βy∗ of both rings from
1m/10mm to 0.7m/7mm in the middle of March this year.
Just after squeezing, we did not see much increase in theluminosity. However, the luminosity was raised gradually
after that and the record peak luminosity of the KEKB so
far was recorded with these beta values. At the beginningof June, βx∗ /βy∗ of both rings were successfully squeezed
to 0.5m/6mm. However, we did not see any increase of
the peak luminosity but we did see even some significant
decrease in spite of relatively intensive efforts on the luminosity tuning. We have not understood the reason forthis decrease. Since βy∗ is comparable to the bunch length,
shortening of the bunch length may be needed to raise theluminosity by squeezing the beta functions further.

Working point

A relatively extensive tune survey was done in the middleof last year. We found that a region near the design tunes
gave a relatively good luminosity. However, the design
tune itself did not bring the best luminosity last year[5].
Fig. 4 shows a history of luminosity improvement. Onlysome turning points for the luminosity jump are plotted.
Also shown in the figure are histories of the horizontal andvertical tunes for the LER. The HER tunes are more or lessthe same as those of the LER. Roughly speaking, lower

30

Luminosity [/cm 2/sec]

4 102 7

Figure 3: Specific luminosity.

3.1

LER νy -44.0

x

HER Cureent * LER Current [mA 2]

3

LER νx -45.5

Luminosity (simulation)
33

27

0
100000

2.4

Luminosity (Belle CsI)LER tunes (Distance from half or interger)

Specific Luminosity[/cm2 /sec/mA2]

1.2 103.3

IP beam diagnostics and corrections

Since the KEKB is a double ring collider, the IP beam diagnostics and corrections are much more complicated andthen much more important compared to the case of conventional single ring colliders. We have to take care of (1) IP
orbit offset, (2) (vertical) crossing angle, (3) waist points,(4) IP x-y coupling and (5) IP dispersion.
Our method to detect the IP offset is based on the beambeam scan observing the beam-beam deflection[5]. We

Proceedings of EPAC 2000, Vienna, Austria

have an orbit feedback system to maintain the zero-offset
condition once found[6]. This system makes orbit bumps at
the IP both in the horizontal and vertical directions basedon information from the BPMs of the two beams aroundthe IP. The repetition rate of the feedback is 2 or 3 seconds which is restricted by the speed of orbit measurements by the BPMs. A detection of the vertical crossing
angle is done as a part of the beam-beam deflection scan.
The beam-beam scan in the horizontal direction is done byscanning the collision point. The vertical crossing angle
is detected as an asymmetric pattern of the vertical beambeam kick during the horizontal scan[5]. A removal of thevertical crossing angle can be done by removing this asymmetric pattern. The orbit feedback system also takes careof the crossing angle condition. Coinciding the waist pointsof the two rings and the collision point is also an importanttuning item[5]. A simulation shows that the IP x-y coupling combined with the crossing angle is harmful for the
beam-beam interaction[7]. A trial to measure the IP x-y
coupling by using a pair of single-pass BPMs near the IP
is in progress[8]. In the usual operation so far, optimum
x-y coupling parameters at the IP are searched by manipulating skew quadrupoles to maximize the luminosity ina trial and error method. The IP dispersion is correctedin the usual dispersion correction process as is described
below. All these corrections mentioned above are directly
connected to the luminosity and accumulation of these corrections played an important role in increasing the luminosity.

3.4

Beam size ratio

In the usual operation of the KEKB, the LER beam is apt
to be blown up. In some situation where the LER beam
is seriously blown up due to the beam-beam effect, the luminosity is raised by enlarging the HER beam size intentionally. When we make the HER beam size be enlarged
gradually, the LER beam shrinks and the luminosity getshigher. At some optimum ratio of the vertical beam sizes,
the luminosity gets the maximum. To maximize the luminosity, it is important to look for the optimum ratio of thebeam sizes and to keep it. We have constructed a feedback
system for the purpose of keeping the ratio constant. This
feedback controls the HER beam size by making an orbit
bump, which creates a vertical dispersion around the ring,at a pair of sextupoles.

3.5

Optics corrections

Since the KEKB uses the unusual tunes which are very
close to the integer or half-integer resonance as shown in
Table 1, optics corrections are important to narrow the stopbands of the resonances. In addition to the global beta corrections, global x-y coupling corrections, global dispersion
corrections and continuous close orbit corrections (CCC)
are done in the KEKB[9]. The global x-y coupling andglobal dispersion corrections are important in the sense that
decreasing the zero-current emittance contributes to the in-Proceedings of EPAC 2000, Vienna, Austria

crease of the luminosity. An injection efficiency is also improved by these corrections. In the global dispersion correction process, the vertical dispersion at the IP is also corrected. We found that this is very important to raise the luminosity. Since the x-y coupling and dispersion corrections
are done by making orbit bumps at pairs of sextupoles, it
is very important to keep the close orbits the same. CCC
is always running during the operation with the repetition
time of 20 or 30 seconds. For the closed orbit correction,
it is important to remove offsets of BPMs. Offset measurements for all BPMs were done by using beams[10]. The
measurements were done basically by detecting changes ofclosed orbits when changing strength of quadruple magnets
beside BPMs.

4
4.1

PERFORMANCE LIMITATIONS
Beam current limitation

In the present KEKB, beam instabilities do not limit thebeam current so far as the bunch-by-bunch feedback system works well and the vertical chromaticity of the LER is
rather large. The current limitation of the present KEKB
come from heating problems of two types of hardware
components. Main problems come from the following two
types of components. One is movable masks which are vital to cut off the beam tail and suppress the detector background. The other is bellows near the IP. The first versionof movable masks has basically two serious problems[11].
One problem is that the masks have no effective damper for
trapped modes, although the Q-value of the major trappedmode amounts to several thousand. The other problem isthat sliding shield fingers of the masks are too weak andintense field of the trapped modes can easily heats up or
melts the fingers. Once the fingers are broken, the fieldof the trapped modes strays out of the chamber and resultsin additional heatings of other components such as bellows.
Fragmentations of the broken fingers can be seeds of arcing
at other places. With these original masks, we experienced
four times vacuum leaks. In the second version of the movable masks, we improved the strength of the shield fingers
and the cooling power. No damper for the trapped mode
was installed in this version. We found that the serious
synchrotron oscillations were induced due to their high-Q
trapped modes depending on the mask head position. We
gave up using this type of masks in the operation.
Based on the experiences above, we have been developing three different types of masks which solve the trappedmode problems in different ways[11]. Two types of themwere installed in the LER at the end of May and are being
tested with beams. So far the new types of masks work
well. Since the masks in the LER were replaced by the
new types at the end of May, we could increase the beam
current to some extent as is seen in Fig. 1. Before replacing the masks, we continued to use the original masks. To
lower the Q-values, the shield fingers of the masks wereintentionally removed. With the original masks, only the31

LER Beam Size at the IP [µm]σy single beamσy two beams

5.5
mons_✣5_✣5_✥✣✣✣_✤5;✥✪;✦✬
L❑m✥✣✣✣_5_✦_✤✧_✤✪_✫.❅❂❏

5
4.5
4
3.5
3
2.5
2
1.5

0

100

200

300

400

500

600

700

field at the chamber wall was 250 G. It was expected thatthe magnetic field confined the photoelectrons around thechamber wall and the beam blowup was suppressed. However, the C-York magnets brought no remarkable improvement in the blowup except for the case of a long bunch
spacing, although a simulation predicts effectiveness of the
C-York magnets. We have not yet understood the reason
why the effect of the C-York magnets is so weak. We are
now planning to install solenoid magnets instead of the CYork magnets in this summer shutdown.

LER Beam Current [mA]

Figure 5: Current dependence of the LER beam size (single
beam and two beams).
fill pattern of the 4 RF bucket spacing could be used in theusual operation. All other bucket spacing resulted in serious heating of the masks.
After the heating problems of the movable masks weresolved, the beam current limitation comes from the heatingof the IR bellows due to the higher order mode. This heating is sensitive to the bunch length and bunch currents. Dueto this problem, we can not shorten the bunch length of theLER. During this summer shutdown, the IR bellows will
be replaced with the new type which improves its coolingpower thoroughly.

4.2

LER single beam blowup

Fig. 5 shows a current dependence of the LER beam size
both in the single beam and the two beam cases. The
beam sizes in the figure is values at the IP which is translated from the measurement point by using the design optics. Every 4th RF bucket was filled with a beam except
for a 10% abort gap and some train gaps in both cases.
As we can see from the figure, there is a serious beam
blowup dependent on the beam current even in the single
beam case. The present understanding for this single beam
blowup based on phenomelogical observations and somebeam studies[12] is that the blowup is induced by a single bunch instability in an environment of a photoelectron
cloud[13]. Since the photoelectron cloud is built up by the
successive passage of bunches, the blowup occurs only inthe multibunch operation. From the chromaticity dependence of the beam blowup, the strong head-tail instability
seems to be responsible for the beam blowup[12]. To curethis single beam blowup, we installed a large number of socalled “C-York” magnets in the LER. A C-York magnet iscomposed of two button-shaped permanent magnets and a
C-shaped iron york. The number and configuration of the
C-York magnets have been upgraded step-by-step. In thepresent configuration, the C-York magnets are attached on
the vacuum chamber in every 10cm of the drift space in thearc section (they cover about 50% of the whole arc). The
magnets are installed both inside and outside of the chamber so that they generate quadrupole field. To cancel out
the effect of the field seen by the beam, the polarity of themagnets was inverted in every 20cm. The vertical magnetic

32

4.3

Beam-beam blowup

The beam-beam blowup has been considerably improvedby the method described in this article. However, there is
still some beam-beam blowup even at relatively low beam
currents as is seen in Fig. 3 and 5. Also in the strongstrong beam-beam simulation, the blowup at the low beam
current is reproduced even if there is no optics errors[4].
The simulation also shows that the luminosity is increased
even with this beam-beam blowup by using lower verticalemittance. This may suggest that we should go for a lower
emittance optics.

4.4

Detector beam background

After several modifications at the summer 1999, the background level in the BELLE detector has been somewhat
comfortable. Thanks to steady improvement of vacuum
condition, substantial background rate can be kept withintolerable level while the total beam current of the KEKB
has gradually been increased since the last October. Recent daily dose in a typical collision operation amounts 600
rad nearby the silicon vertex detectors (SVD) and an occupancy in the inner most layer of the SVD is as small as 3
Currently a potential problem for a higher beam current operation is a large leakage current and high occupancy ratein the inner three layers of the cental drift chamber. They
are expected to be reduced by a new mask to be installed in
QCS in this summer.

5

REFERENCES[1] K. Oide et al., PAC’99, New York, March 1999.
[2] K. Oide et al., Factories 99, Tsukuba, September 1999.
[3] KEKB B-Factory Design Report, KEKReport957, June 1995.
[4] M. Tawada et al., in these proceedings.
[5] Y. Funakoshi et al., Factories 99, Tsukuba, September 1999.
[6] M. Masuzawa et al., in these proceedings.
[7] K. Ohmi, in these proceedings.
[8] Y. Ohnishi et al., in these proceedings.
[9] H. Koiso et al., in these proceedings.
[10] M. Masuzawa et al., in these proceedings.
[11] K. Kanazawa et al., in these proceedings.
[12] H. Fukuma et al., in these proceedings.
[13] K. Ohmi and F. Zimmermann, CERN-SL-2000-015 (AP),May 2000.

Proceedings of EPAC 2000, Vienna, Austria