The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Present Status of KEKB
Shin-ichi KUROKAWA
KEK, High Energy Accelerator Research Organization
Abstract
KEKB, a high-luminosity, asymmetric-energy, two-ring,
electron-positron collider aimed at B-physics was completed
by the end of November 1998 and the commissioning of theaccelerator took place from December pt to April 19th with
the BELLE detector waiting outside the interaction point.
The maximum stored current reached 514 rnA in the electron
ring and 542 rnA in the positron ring. BELLE detector thenhad been rolled in to the interaction point and the operationof the full KEKB started on May 24th. After the firstsuccessful beam collision on June 1st and znd with aluminosity of 1.5 x l0 30cm-2s- 1 the luminosity of KEKB
increased step by step and finally reached 3 x 1032cm-2s-1 on
August 4th a few hours before the start of the long summer
shutdown. The integrated luminosity from June -August run
amounted to 25/pb. During the summer shutdown we will
modify the vacuum system near the interaction to reducebeam background to BELLE. The operation will resume inmid October and continue until the end of June 2000 withsome break around New Year.

1. Introduction
By the end of November 1998, the construction of KEKB
was completed and on December l't commissioning started.
KEKB[l,2] is a high-luminosity, asymmetric-energy,
two-ring, electron-positron collider in a 3-km tunnel, whichhoused TRISTAN, a 30-GeV x 30-GeV single ring electronpositron collider. At KEKB, 8-GeV electrons and 3.5-GeV
positrons are stored in separate rings, circulate in opposite
directions, and collide at one interaction point (IP), which
the BELLE detector surrounds. The electron ring is called
high-energy ring (HER) and the positron ring low-energyring (LER).
The cross section of the tunnel is large enough to enable aside-by-side installation of LER and HER (see Fig. 1). The
two rings change their inner and outer position at the IP andat a crossover point opposite to the IP, where HER and LER
have different heights so as to avoid collisions of electronsand positrons. An interchange of the inner and outer
position of the rings is necessary to make the two rings havethe same circumference. Electrons and positrons are directly
injected into the KEKB rings from the linac. Figure 2 shows
the schematic layout of KEKB.
The design luminosity of KEKB is 1034cm 2s- 1• Thisluminosity is 250-times as large as that of TRISTAN andmore than one-order of magnitude higher than the maximum
luminosity ever achieved by existing electron-positron
colliders.
In order to obtain a high luminosity, we should increase
the beam currents in the rings and squeeze the beams at theIP as much as possible. At KEKB, we need to store 1.1 A in-Fig. 1 Two rings installed side-by-side in the tunnel.

..

Fig. 2 Shematic layout of KEKBHER and 2.6 A in LER and make the horizontal and verticalbeam sizes at the IP 90 f!ID and 1.9 f!ID, respectively. The
main issue is, therefore, how to store such large currents inthe rings and at the same time maintain stable collisions
between electron and positron beams. At KEKB, the beam

1 --

The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

is distributed to 5000 bunches with a bunch spacing of 59em.
KEKB adopts a finite-angle crossing scheme, where theelectron and positron bunches collide at ± 11 mrad. In this·scheme bunches are naturally separated after a collision. Apair of superconducting final-focus quads is used to squeeze

the beams. Anti-solenoids in front of each quad effectively
cancel out the solenoid field of the BELLE detector. Crabcrossing scheme is a fallback option and superconducting
crab cavities are being developed.
The main parameters of the KEKB rings are summarizedin Table 1.

Table 1. Main Parameters of KEKB.
LERRing
3.5
EnergyE3016.26Circumference
c1x1034
Luminosity
L:!:11
Crossing angle()X
I 0.052
0.039
Tune shifts
Sxf S
0.33 I 0.01
Beta function at IPf3*)f3*y
2.6
Beam current
I0.4
Natural bunch lengtha;,
7.4x10-4
Energy spreadad0.59Bunch spacingsb
3.3x1010
Particles/bunch
1.8x10-8; 3.6x10-10
Emittance
Ex/ By0.01-0.02
tune
Synchrotronv,
45.52 I 45.08
Betatron tune
vxf'~>J;1 x 10-4 - 2x 10-4
MomentumaP
compaction factor
o.81 t ; 1.stt
Energy loss/tum
Uo
5-10
RF voltage
vc508.887
RF frequency!RF5120
hHarmonic number
43t 123tt
LongitudinalrE
damping time
2.6t 14.stt
Total beam power
PB
2.1t; 4.ottRadiation powerPR
0.57
HOM power
PHOM
16.3
Bending radius
p0.915
Length of bending[BMa ett without wigglers tt with wigglers

HER
8.0GeVm

1.1Aem6.7x10-4mm

46.52 I 46.08

3.510-20

MeVMVMHz

23

ms

4.0
3.8
0.15
104.5
5.86

MWMWMWmm

2. Hardware Systemby+ 5.1 mm in LER and+ 5.4 mm in HER. The plus signmeans that the ring is longer than the design value.

Magnet system
The total number of magnets installed in the tunnel
amounted to 3400, 1700 main magnets (dipoles, quads andsextupoles) and 1700 small steering magnets. We measured
the magnetic fields of all magnets before installation andfound that all of the magnets satisfied the requirement.
Alignment of these magnets was performed by using
SMART systems, which can measure 3-D positions of themagnets with 10 ppm accuracy. For example, the standard
deviations and averages of the position error perpendicular
to and along the beam direction at arc sections of the rings
were less than 0.14 mm and 0.04 mm, respectively. This
good accuracy of the alignment lead to the result that thecircumference of the rings differed from the design values

RF system
A high current stored in a KEKB ring requires singlemode cavities with high stored energies in order not to
excite coupled-bunch instabilities. Two types of cavities
are used at KEKB: a normal-conducting cavity, called
ARES, and a superconducting, single-cell, single-mode
cavity, SCC. The final number of cavities in the rings is 20
ARES in LER and 12 ARES and 8 SCC in HER. 12 ARES
have been installed in LER and 4 ARES and 4 SCC in HER
by the start of the commissioning.
ARES is a three-cell cavity system where an acceleratingcell is resonantly coupled with a large energy-storage cell
via a coupling cell in between.

-2 -The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

A superconducting cavity can be operated at a highaccelerating field having a large stored energy, and isimmune to high beam loading.
The performance of thecavities were measured first in a vertical cryostat and thenin a final horizontal cryostat. All of these cavities showedthat they satisfied the requirement of KEKB (1.5MY/cavity).
These ARES cavities and superconducting cavities werebeing operated stably from December 1998 to August 1999
and accelerated up to 514 rnA in HER and 542 rnA in LER.
During these operation the total number of breakdowns ofsec was about 10.
Vacuum system
Except for some vacuum ducts around the IP and thecrossover point, all vacuum ducts of KEKB are made ofcopper. The number of ducts amounts to 2000 and theducts are connected to each other by a piece of shielded
bellows. The photodesorption coefficients of these copper
ducts decreased, as planned. By August 4th, the integrated
stored currents in LER and HER reached 130 and 75 Ahand the photodesorption coefficient 11 was reduced down to10-5• No serious heating of vacuum components wasobserved.
3. Commissioning
The commissioning of KEKB started on December 1stand continued until April 19th[3]. BELLE was not rolled in
the interaction point. Without the BELLE detector,
cancellation of the BELLE solenoid was unnecessary andexcitation of anti-solenoids was lowered to the 10% level.
A small detector for the commissioning, BEAST, wasinstalled at the IP to measure the beam background ..
The first storage of the beam in HER was on December
13th, whereas that in LER took place on January 14th.
During the commissioning period, the beam currents wereincreased step by step and reached 514 rnA in HER and 542
mAin LER.
The commissioning started with fJ •y= 2 em lattice, andlater this value was reduced to the design value of fJ •y= 1em in both rings. No degradation in the injection efficiency,maximum current, or lifetime was observed. Single-bunch
currents higher than the design values (4 rnA in HER and2.3 rnA in LER compared to the design values of 0.22 rnA
and 0.52 rnA, respectively) were achieved without any
instabilities or serious bunch lengthening. Transverse multibunch instabilities were observed in both rings. Bunch-bybunch feedback systems were effective for these
instabilities.
Collisions of the two beams were first tried in February
by making single-bunch electrons and positrons collide.
The horizontal and vertical beam-beam deflections wereclearly observed. In the case of a finite-angle crossing
scheme, the horizontal distance between the electron andpositron bunches was changed by changing the RF phase ofLER. The change in the RF phase in one ring shifts thelongitudinal collision point, which simultaneously changes
the horizontal separation of the beams. The vertical-deflection was measured by scanning a vertical bump orbit
in one beam and measuring the kick of the other beam.
In March, collisions of the beams were tried again
between multi-bunch electrons and positrons.
Theestimated luminosity calculated from this beam-beam
deflection data was 1.7 x 1031cm-2s-1•
The third trial of collisions was not successful. Afterextensive study we found that the LER beam had a 0.4 Hz
vertical oscillation caused by a small leakage field from the12 Ge V Proton synchrotron: DC power cables connecting
power supplies in a hut to magnets of the PS and a dummymagnet housed in the hut produced a leakage field of 20
mGauss over 10 m in the KEKB tunnel below. We
cancelled out this field by installing an additional current
loop near the source; remaining oscillation of the beam wasreduced to 0.1-0.2 J.tm at the IP.
The operation of the full KEKB started on May 24th.
After the first successful beam collision on June 1st and 2nd
with a luminosity of 1.5 x 1030cm-2s-1 the luminosity ofKEKB increased step by step and finally reached 3 x
1032cm-2s-1 on August 4th a few hours before the start of thelong summer shutdown. The integrated luminosity fromJune -August run amounted to 25/pb. Figure 3 shows theincrease of the luminosity from June to August. Beam size
monitors on the basis of double-slit interferometry werehelpful to tune the beams for collision[4].
During the summer shutdown we will change one
aluminium vacuum duct near the IP with a copper duct toreduce back-scattered X-rays that enter into BELLE. The
operation will resume in mid October and continue until theend of June 2000 with some break around New Year.

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References[1] KEKB B-Factory Design Report, KEK Report 95-7,
August 1995.
[2] S. Kurokawa, "Present Status of KEKB Project",
Proceedings of EPAC-98, Stockholm, June 1998,
pp.123-127.
[3] K. Oide, "Commissioning of KEKB", to be publishedin the Proceedings of PAC-99, New York, March 1999.
[4] T. Mitsuhashi, in these Proceedings.

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