Proceedings of the 1999 Particle Accelerator Conference, New York, 1999

COMMISSIONING OF THE KEKB B–FACTORY
K. Akai, N. Akasaka, A. Enomoto, J. Flanagan, H. Fukuma, Y. Funakoshi, K. Furukawa, J. Haba,
S. Hiramatsu, K. Hosoyama, N. Huang∗ , T. Ieiri, N. Iida, T. Kamitani, S. Kato, M. Kikuchi,
E. Kikutani, H. Koiso, S.–I. Kurokawa, M. Masuzawa, S. Michizono, T. Mimashi, T. Nakamura,Y. Ogawa, K. Ohmi, Y. Ohnishi, S. Ohsawa, N. Ohuchi, K. Oide, D. Pestrikov† , K. Satoh,
M. Suetake, Y. Suetsugu, T. Suwada, M. Tawada, M. Tejima, M. Tobiyama, N. Yamamoto,
M. Yoshida, S. Yoshimoto, M. Yoshioka, KEK, Oho, Tsukuba, Ibaraki 305-0801, Japan,
T. Browder, Univ. of Hawaii, 2505 Correa Road, Honolulu, HI 96822, U.S.A.
Abstract
The commissioning of the KEKB B–Factory storage rings
started on Dec. 1, 1998. The two rings both achieved astored current of over 0.5 A after operating for four months.
The two beams were successfully collided several times.
The commissioning stopped on Apr. 19, taking a 5-weekbreak to install the Belle detector.

1 BRIEF HISTORY OF THE
COMMISSIONING
The KEKB B–Factory[1] consists of two storage rings, theLER (3.5 GeV, e+ ) and the HER (8 GeV, e− ), and the injector Linac/beam-transport (BT) system. The Linac was
upgraded from the injector for TRISTAN, and was commissioned starting in June 1997, including part of the BT
line. The injector complex was ready before the start ofcommissioning of the rings.[2]Figure 1 shows the growth of the stored currents in thetwo rings through the period of commissioning. The firststorage of the HER and LER were on Dec. 13 and Jan.
14, respectively. The initial rise of the intensity was quite
good in both rings. This figure also shows several breaks,
the scheduled one (for new-year holidays) and the unscheduled shutdowns. The total length of breaks were more than
one month. So far both rings have achieved stored currents
more than 0.5 A, which corresponds to 20% (50%) of thedesign goal of the LER (HER). The maximum current of
the HER is limited by the rf power available from the number of cavities currently installed. On the other hand, LER’s
maximum current seems to be limited by the vertical beam
blow-up due to a muti-bunch instability, as shown later.
Two major unscheduled shutdowns were experienced
during this period. One was an accident related to a false
fire alarm at the end of January, which had nothing to dowith the beam operation. Another break at the end of
February was the melt-down of a vacuum chamber in the
HER near the interaction point (IP) due to synchrotron radiation. The synchrotron radiation produced in the outgoing superconducting quadrupole (QCS) hit the chamber asa result of radiation background tuning with a large-angle
bump orbit. The chamber was replaced in 3 weeks with a
new heat-resistant design.
∗ visiting from IHEP, China.

† visiting from BINP, Russia.

0-7803-5573-3/99/$10.00@1999 IEEE.

Figure 1: The stored beam currents, in log scale, during thecommissioning are shown in HER (a) and LER (b). Several significant events are also shown. The integrated beam
current is shown in (c).
Table 1: e+ Linac/Beam Transport
Item
EnergyCharge@e+ target
Charge@end of Linac
Charge@end of BT
Repetition
Emittance

2

Design
3.5
10
0.64
0.64
50
0.25

Achieved
4
10
0.6
0.4
50
0.4

GeV
nC
nC
nC
Hz
µm

PERFORMANCE

Tables 1–4 list the main machine parameters achieved during this period, in comparison with the design values.
288Proceedings of the 1999 Particle Accelerator Conference, New York, 1999
Table 2: LER
Item
EnergyBeam current
Bunch current
Bunchesβx∗ /βy∗νx /νyRf voltage
νz @4 MV
Injection efficiency
Lifetime@0 A

Table 4: HER

DesignAchieved
3.5
3.5
GeV
2.6
0.54
A
0.52
2.3
mA
5000
1024
33/1
100/1.0cm
45.35/44.41 45.27/44.18
8
4.8
MV
0.01180.0110
100
80
%
14
>8
hour

Item
EnergyBeam current
Bunch current
Bunchesβx∗ /βy∗νx /νyRf voltage
νz @8 MVσz @0 mA
Injection efficiency
Lifetime@0 A

Design
8
1.1
0.22
5000
33/1
44.53/42.20
20
0.01195.6
100
45

Achieved
8
0.51
4
800
100/1.1
44.65/42.1
90.0114
5.6
80
>8

GeV
A
mA
cm
MV
mm
%
hour

Table 3: e− Linac/Beam Transport
Item
EnergyCharge@end of Linac
Charge@end of BT
Repetition
Emittance

2.1

Design
8
1.2
1.2
50
<0.1

Achieved
8.5
1.2
1
50
0.06

GeV
nC
nC
Hz
µm

Lattice

The commissioning started with a βy∗ = 2 cm lattice, andlater switched to βy∗ = 1 cm at the time shown in Fig. 1.
No degradation in injection efficiency, maximum current,
or lifetime was observed in either ring with the βy∗ = 1 cm
optics. As for βx∗ , only 100 cm has been tried so far for
either ring.
The ring lattices have been understood for the basic parameters such as tunes, β-functions, dispersions, etc.[3]The betatron tunes have been searched to optimize the injection efficiency, maximum current, beam profile, and thenoise in the pilot detector at the IP, BEAST. So far the tworings have been operated at different betatron tunes, since
they have been optimized independently. They will be adjusted if necessary for the beam-beam effect. Those tunes
are also different from the design tunes which were optimized by beam-beam simulation. The simulation suggests
a point, (0.52, 0.08), which is very close to the integer orhalf-integer resonances. It has, however, been difficult toaccess such a location with the current size of the betatronstop bands (∼ 0.05).
One of the discrepancies in the lattice between themodel and the measurements is the differences of the betatron tunes as shown in Tables 2 and 4. They are bigger in the LER, and the calibration of the superconducting quadrupoles at the IP, which are common to both LER
and HER, relative to the iron quadrupoles is under investigation. The super- and normal conducting magnetic measurements were done independently.[4] An analysis using
single kicks of the corrector dipoles all around the ring wasdone.[5] It shows a mismatch of the β-functions in bothrings of 20–50%, while the measured β ∗ s are sufficiently

close to the model. It is speculated that the mismatch of thebetas should bring an error in chromaticities, which currently differ from the model by a factor of 2–5. The higher
order chromaticity is also somewhat different from that of
the model.
Though such differences are seen, the measured transverse dynamic aperture is at least larger than the physicalaperture. The longitudinal acceptance of the LER is about±1.5% which is smaller than the model (±2.5%). This canbe a source of injection background in the LER, which is a
few times higher than that in the HER.
The x-y couplings of the rings are roughly corrected byvertical bump orbits at strong sextupoles, looking at the effect of a single kick in one plane on the orbit in the otherplane.
So far no problem has been seen related to the alignmentof the rings. The circumferences of the rings are different
from the design by about 5 mm, and their relative difference was only 0.3 mm. The offset of HER’s circumference
was corrected by choosing the rf frequency (including the
Linac), then the LER’s was adjusted to the HER’s using
chicanes. In the initial stage of commissioning, a turn-byturn beam position/phase monitor (TBTM) played an important role in detecting the path length via phase oscillation.

2.2

Orbit Fluctuation

One of the biggest issues, unanticipated at the design stage,is fluctuations of the closed orbit. Two kinds of fluctuation, both in the vertical direction, have been observed sofar. The first one is a slow drift. The vertical closed orbits of the rings drift randomly by typically 1 mm/hour at
the IP quads, or 0.2 mm/hour in the arc section. The drifts
in the two rings are highly correlated and the magnitudes
are roughly equal. Orbit analysis locates the source at thequadrupoles at the IP, but it was hard to distinguish whether
one or all of them are responsible. Also, a correlation withthe ambient temperature around the IP is observed (fig. 2).
The amount of drift translates into an offset at the IP of
5 µm/hour, whereas the design beam size is 1.4 µm. The

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Proceedings of the 1999 Particle Accelerator Conference, New York, 1999
26.6

orbit
correction

beam tuning

8

26.4

4

26.2

0

26
no current

-4

-8

25.8
air-conditioner
turned on

25.6

-12
Mar/11/99 12:00:00

Concrete shield temperature [ºC]

Vertical BPM reading @ QCSL [mm]

12

25.4
Mar/11/99 18:00:00

Mar/12/99 0:00:00

Figure 2: The change of orbit (upper trace) and the temperature of the concrete shield structure (lower) at the IP.
These data were taken on Mar. 11 for about 20 hours, afterswitching off the air-conditioner in Tsukuba hall aroundthe IP. The air-conditioner was turned on in the middle.
Though orbit correction was applied several times during
this period, a clear correlation between the orbit and thetempereture is seen.
drift is also large enough to increase the background anddegrade the injection efficiency. To cure the drift a continuous closed-orbit correction system has been applied toboth rings to maintain the orbit at the beam-position monitors(BPMs), and has been so far found effective.
The second kind of orbit fluctuation is a vertical vibration of the orbit at about 14 Hz. The amplitude of the vibration is about 100 µmpp in the arc, and continues withconstant amplitude for hours or days. Sometimes it stops
for hours and then restarts. Both rings have similar vibrations, but they are not synchronized and their frequencies are slightly different. Orbit analysis points to a source
around the IP, but it has not yet been identified. This vibration can make the two beams miss each other verticallyby a few sigma. A collision feedback responding rapidly
enough will be necessary.

2.3High Intensity

The LER is equipped with 12 ARES copper cavities, andthe HER 6 ARES and 4 superconducting cavities. So far
the number of rf cavities installed in the rings is enough tostore about half of the design intensity. The HER has already reached this limit. Both rf systems have been working quite well and the down time due to rf has been quite
small.
At the initial stage of commissioning, the baking of thevacuum chamber is the most important task in any electronpositron storage ring. Figure 3 shows how the photodesorption coefficient has improved as the integrated current has increased. These curves basically fit the expec-

Figure 3: The photo-desorption coefficient (lower trace)and the beam current (upper trace) versus the integrated
beam current for HER (a) and LER (b). The vertical dashedlines indicate the timings of the conditioning of the NEG
strip modules.
tation very well, so the baking of the chambers has beensuccessful so far.
The beam lifetime is so far determined by the vacuum
pressure. The lifetime extrapolated to zero current is longer
than 8 hours. It matches the design numbers shown in Table 2 and 4 to within the accuracy of the extrapolation. The
lifetime of the LER is decreased at high current when thevertical masks are closed. This is due to a blow-up of thevertical size caused by a mutibunch instability, described
below.
An abrupt decay of the lifetime has been observed in
the HER below 200 mA. A possible explanation is dusttrapping. Spikes in the background have also been reportedby the BEAST, which are correlated to the presumed dust
events. An interesting phenomenon noticed in the BEAST
was a ”micro-dust event” which looks similar to a larger

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dust event but is too small to be evidenced in the lifetime.
These dust events decrease at higher current.

2.4

Instabilities

Single bunch currents can be stored much at higher than thedesign value with sufficient margin. No disastrous instability or bunch-lengthening has been seen.
The LER and HER have been operated in multi-bunchmode mostly with 10 ns bunch spacing. Both rings havea bunch gap of about 10% for the rise of the abort kicker.
Usually the HER has 4–7 more gaps (i.e., bunch-trains) for
clearing ions.

for this instability. Without the feedback, usually the tailsof the bunch-trains are lost. Dependences on the tunes, orbit, synchrotron motion, bunch fill pattern, etc., are underinvestigation.

2.5

Injection

The injector linac has already achieved its design performance, at least for the short term.[2] There remain a number of issues related to long-term stability and repeatability. A difficult problem is switching the same linac between
four modes (KEKB e− , KEKB e+ , PF, PF-AR) which needdifferent settings of rf, magnets, and even different guns.
Further improvement in the injector is necessary.

3 COLLISION
Table 5: Parameters of the multi-bunch collision in Mar.
1999. Beam sizes are obtained from the beam-beam scanin Fig. 5, assuming equal sizes for both beams.
Current
Bunchesσx /σyLifetime
Estimatedluminosity

Beam Current (mA)
Figure 4: The vertical beam size of the LER measured by
the synchrotron radiation interferometer as a function of thebeam current. The bunch-by-bunch feedback was turned
off.
Several multi-bunch instabilities have been observed inboth rings, but none of them has an identified source yet.
The LER sees an excitation of longitudinal oscillation,with its strength depending on the betatron tunes. Witha particular choice of betatron tunes, the LER accumulated
440 mA without bunch-by-bunch transverse feedback.[6]At bad tunes, the oscillation starts at about 10 mA, but atthe best tune, only at 350 mA or higher. At bad tunes, vertical oscillation was observed simultaneously, so the bunchby-bunch feedback was effective in such cases. The threshold depends on the vertical closed orbit, vertical dispersion,and vertical chromaticity. It is unknown what the relation
is between the longitudinal and the vertical instabilities.
An important phenomenon in the LER is a blow-up of
the vertical beam size at I ≥ 350 mA (Fig. 4), observed by
the synchrotron light monitor,[7] using both direct imaging
and interferometry. The observed vertical size, translated
to emittance assuming the design βy , reaches εy /εx >∼ 1 atI = 500 mA. Though the beam size increases as the current, no vertical oscillation was seen. Among suspectedsources of the blow-up are vertical dispersion and synchrotron oscillations, but these are not yet confirmed. Thisvertical blow-up eventually limits the stored current by hitting the vertical masks in the arc. No pure-transverse instability has been seen in LER up to 500 mA.
In the HER, there is a strong vertical instability over
100 mA. The bunch-by-bunch feedback is very effective

LER HER
65
13
200
200
197/4.0
200
40

µm
min.

1.7×1031

cm−2 s−1mA(a) horizontal scan(b) vertical scanFigure 5: The beam-beam deflection in (a) horizontal and(b) vertical planes, during the multi-bunch collision in Mar.
1999. Lines are fitted curves for Gaussian bunches.

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Proceedings of the 1999 Particle Accelerator Conference, New York, 1999
Collision of the two beams has been tried three times,[8]once per month since February. In February the first collision was done with single bunches, and horizontal andvertical beam-beam deflections were clearly observed. The
horizontal deflection was detected by changing the RFphase of the LER, looking at BPM readings of the HER.
Due to the finite crossing angle at the IP, the change in RF
phase shifts the longitudinal collision point and the horizontal separation of the two beams. The measured horizontal offset between the beams is cancelled by use of horizontal dipole correctors. The vertical deflection was measured
by scanning a vertical bump orbit in one ring, looking at the
BPM signals of the other ring. It was interesting that there
was a vertical offset between the two beam of 0.73 mm at
the IP.
In March, a multi-bunch collision was tried with the parameters shown in Table 5. The resultant deflection curves
are shown in Fig. 5. There was also a successful singlebunch collision, and the vertical offset remained the sameafter 50 days from the previous single-bunch collision.
There was also a correlation between the beam-beam deflection and Bhabha events in the BEAST detectors. Unfortunately the orbit drift was not cured then, so it was hardto obtain a real luminosity for substantial duration.
In April there was a third trial of the collision to establish conditions with high current. This time, the orbit vibration in the LER at about 14 Hz strongly disturbed thevertical deflection signal. The vibration had disappeared
fortuitously during the collisions in February and March.
Also the lifetime of the LER was shortened during the vertical scan when the two beams became closer than about
30 µm. It is not known whether this loss of lifetime is related to the vibration or not. Another suspected cause of
the loss of lifetime is the blowup of the vertical beam size
at high current.
Optimization of the collision conditions such as tunes,
orbits, beam sizes, etc. will be necessary after the roll-in of
Belle.

4

DETECTOR BACKGROUND

The beam background at a detector is always a serious
problem for a good and clean experiment. In an experiment with a high current collider like KEKB, it is one of
the biggest concerns in two respects: radiation damage ofthe detector material; and high occupancy of the detector
signal due to beam background.
To monitor the beam back ground during commissioning, the BEAST detector was installed around the IP chamber. It consists of 24 PIN diodes, 53 propotional counters,
10 CsI counters, 72 BGO counters and other dosimeterssuch as MOSFETs. Background optimization has been
made by tuning the masks in the arc and at the IP, injection
orbit and phase, matching wih the transport line, and soon. By a series of optimizations, the background at injection, which was the dominant source of background at theearly stage of commissioning, is now comparable with thebackground due to stored current. The background duringstorage mode is almost decided by the vacuum level inside
the beam pipe and, therefore, has been gradually improved.
For the HER beam, the background measured now isnearly propotional to the beam current and not quadratic.
This implies that background reduction due to vacuum improvement is reaching its limit. By energy spectral measurement of background, SR can be excluded as the dominant source of the current background. The dependenceof dose on radial distance from beam indicates a somewhat
uniformly distributed background component, which may
be generated away from the IP. One candidate source is a
hot spot down stream of the IP where several beam loss
monitors exhibit large activity. Further investigations arenecessary to identify and extinguish it. The LER on theother hand still exhibits a quadratic dependence of background on beam current and, therofere, the backgroundis decreasing along with vacuum improvement. Together
with the fine tuning of several parameters mentioned above,
LER background in storage mode has also been reduced
much and is better than the one in HER.
Although the current level of background is still higher
than expected at the design stage of the BELLE detector,
it becomes just possible to operate the BELLE detector inthis background environment with a reasonable life time
due to radiation damage. Further improvement is expectedby the installation of additional arc masks for both the LER
and the HER and reinforcement of shielding in front of thedetector.
The authors thank S. Anami, K. Egawa, K. Endo, E.
Ezura, T. Kageyama, K. Kanazawa, T. Katoh, T. Kubo, T.
Furuya, T. Honda, S. Isagawa, S. Mitsunobu, H. Mizuno,
K. Nakahara, H. Nakanishi, H. Nakayama, R. Sugahara, S.
Takeda, Y. Takeuchi, K. Tsuchiya, Y. Yamazaki, all members of the KEKB accelerator group, F. Takasaki, all members of the Belle & BEAST Collaborations, H. Sugawara,
Y. Kimura, M. Kihara, T. Shibata, M. Kobayashi, and all
directors of KEK, for supporting the commissioning of
KEKB.

5

REFERENCES[1] KEKB B–Factory Design Report,(1995).

KEK–Report–95–7,[2] Y. Ogawa et al, in these proceedings.
[3] H. Koiso et al, in these proceedings.
[4] For the iron magnets, see M. Masuzawa and K. Egawa et al,in these proceedings.
[5] Y. Ohnishi et al, in these proceedings.
[6] E. Kikutani and M. Tobiyama et al, in these proceedings.
[7] J. Flanagan and T. Mitsuhashi et al, in these proceedings.
[8] Y. Funakoshi et al, in these proceedings.

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