The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Past, Present, and Future of KEKB
K. Oide for the KEKB Accelerator Team
KEK, Oho, Tsukuba, Ibaraki 305-0801, JapanHISTORY OF KEKB
Table 1 summarizes the brief history of the KEKB B–
Factory, the asymmetric collider for B–Physics at KEK.
It consists of two rings, the LER and the HER, for e+ at
3.5 GeV and e− at 8 GeV, respectively, and the injector
Linac.[1, 2]-2 -1

Peak luminosity (cm s )Table 1: Brief History of KEKB
Design work started; first design with
1989
1034 cm −2 s−1
1994
Approval of budget; start of construction
1995
KEKB Design Report
Sep. 1997 Commissioning of the injector Linac
started as the upgrade completed.
Dec. 1998 Start of injection to the HER
Start of injection to the LER
Jan. 1999
May. 2001 Roll-in of the Belle detector
Apr. 2001 World’s highest luminosity (then 3.4 ×1033 cm −2 s−1 ) was achieved.
Oct. 2002
Integrated luminosity reached 100 /fb as
the first machine in the world.
May 2003 Luminosity exceeded the design number
1034 cm −2 s−1 .

10 34

Peak Luminosity trends in last 30 years

KEK-B
PEP II

Date

10 33

10

CESR

32

LEP2

ISR
LEP1
DORIS II
PETRA

10 31

HERA

TEVATRON

10 30

the interaction region with the finite crossing angle, “submicro” β beam optics with 2.5π unit cell, ARES and superconducting cavities, bunch-by-bunch feedback system,
beam monitors, vacuum system, electron-cloud effect, andinjector J-Linac, etc., were so designed as to satisfy the design goal. Thus the design was comprehensively written in
KEKB Design Report.[1] The design of KEKB also fully
utilizes the existing legacies of TRISTAN, such as the tunnel, utilities, magnets, power supplies, rf system, etc.
Figure 2 shows the progress of the performance of
KEKB since Oct. 1999. Though having a number of breaks
and slumps, the overall performance has been continuously
improved toward the design luminosity. Four years werenecessary for KEKB to achieve the design luminosity. Was
it too long or not? The period of startup was specified as“100 /fb in the first 3 years” in the KEKB Design Report,
and KEKB achieved 100 /fb in October 2002, and 150 /fb
in 4 years. So the speed of startup actually satisfied theplan very well. The startup speed of KEKB was even remarkable in the history of the colliders (Fig. 1), and now
KEKB is driving the trend. Please note that the progressof luminosity in Fig. 2 corresponds to the increase of thebeam currents in some periods such as from summer 2001
to summer 2002, but in other periods luminosity improved
without the increase of the currents such as in 2003.
Table 2: Machine parameters of KEKB

DORIS

1975

1980

TRISTAN

1985

1990

1995

2000

2005
Year

Figure 1: Trends in peak luminosity of the colliders in theworld over the past 30 years. The arrow represents 1 order
growth in 5 years.
The design work of KEKB started around 1989. The
luminosity goal of KEKB was set to 1034 cm −2 s−1 fromthe beginning. As seen in Fig. 1, at the time of thestart of KEKB around 1990, the luminosity of colliders in
the world barely reached 1032 cm −2 s−1 at ISR and CESR.
Thus the goal of KEKB might look too ambitious for the
majority of the accelerator community in the world. KEKB
never compromised on its luminosity goal. On the contrary, the design goal became more and more convincing as
the design study advanced. A number of innovations weremade by intensive works in theories, simulations, and experimental studies including the high-current operation at
TRISTAN–AR. The essential elements of KEKB, such as

Current
Bunches
Bunch current
Spacing
Emittance εxβx∗βy∗Hor. Size @ IP
Ver. Size @ IPεy /εxBeam-beam ξx
Beam-beam ξyLuminosity Lum/day Lum/7 days
Lum/30 days

5/13/2003
LER HER
1.38 1.05
1265
1.09 0.83
1.8 or 2.4
18
24
59
58
0.58 0.70
103
118
2.2
2.2
4.7
2.9
.093 .068.067 .053
10.57
579
3876
12809

DesignLER HER
2.6
1.1
5000
0.52 0.22
0.6
18
18
33
33
1.0
1.0
77
77
1.9
1.9
2
2
.039 .039.052 .052
10
∼ 600

A
mA
m
nm
cm
cm
µm
µm
%

/nb/s
/pb
/pb
/pb

PERFORMANCE
Table 2 compares major machine parameter corresponding to the best peak luminosity, recorded on May 13, comparing to the design. While the luminosity, beam-beam pa-

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

Peak Luminosity

10.57 /nb/s

579 /pb/day

3.88 /fb/7 days
12.81 /fb/30 days
LER

HER

158.7 /fb

C- Yoke: (1) 800 m (2) 800 m Solenoid: 800 m 1250 m 1350 m

2150 m

2200 m2250 m + PM 25 m

PM 50 mFigure 2: The history of KEKB performance since 1999. The rows are (top to bottom) the peak luminosity in a day,the daily integrated luminosity, the peak stored current in the LER and the HER, and the integrated luminosity in Belle,
respectively. The integrated luminosities are the numbers recorded by Belle. The arrows at the bottom show the progressof the length of solenoid/permanent magnets to suppress the electron-cloud instability in the LER.
rameters, and the HER current were more or less equal tothe design. The LER had less current than the design, andit was compensated by the smaller βy∗ than the design.
The major differences between the best achieved andthe design are in the number of bunches (thus bunch spacing). Although the solenoid windings were so extended as
to cover roughly entire drift spaces, the effect of the electron cloud did not disappear completely. The threshold of
the vertical blowupblowup was increased to about 1.8 A
for near-4 bucket spacing. The threshold was lower for
shorter spacings. Even below the threshold, the specificluminosity per bunch still seemed to degrade for shorter
spacings. Actually, the average bunch spacing was reduced
from 4.08(=49/12) buckets to 3.77(=49/13) buckets during
this period, but in a few trials spacing less than 3.5 buckets did not give good specific luminosity. (To utilize the
2-bunch/pulse injection scheme, the bunch fill pattern musthave the periodicity of 49 buckets.) The specific luminosity with 3.5 buckets might not be much worse than 3.77
buckets, but it was not usable due to the higher heatingof the bellows at the interaction point (IP) until the summer 2003. As the number of bunches were much smaller
than the design, the bunch current was so higher than thedesign, especially in the HER. Such high current caused
higher HOM losses in all components. The ferrite HOM
absorber in the superconducting cavity (SCC) in the HER
now absorbs 10 kW/cavity, which already exceeded the de-sign and the tested power level.
As the beam current of the LER was so limited by
the electron-cloud, the ratio of the beam currents of thetwo rings was quite different from the so-called energytransparent condition (inversely of the energy ratio). The
violation of the transparent condition brought neither disastrous nor pleasant effects on the beam-beam interaction
so far.

ISSUES
Luminosity
One of the convenient expressions of a luminosity of aring collider isL≈

γ± I± ξy± RL,
2err βy∗ Ry(1)where I± , γ± , and ξy± are the stored current, the Lorentz
factor, and the vertical beam-beam tune-shift parameter foreach beam. The vertical β function at the IP (βy∗ ) was assumed to be common for both beams. We have assumed flatbeams for Eq. (1). The factors RL and Ry are geometrical
reduction factors for the luminosity and ξy , respectively,
determined by the hour-glass effect and the crossing angle.
It is known that the ratio RL /Ry does not differ much fromunity if the bunch length is shorter than βy∗ . Thus the luminosity of a ring collider is basically determined by threeThe 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

parameters, βy∗ , ξy , and I . Let us discuss issues to achieve
higher luminosity by smaller βy∗ , higher ξy , and higher I.

Smaller βy∗
Generally speaking, a small βy∗ reduces both the physicaland the dynamical apertures of a ring. A small βy∗ simply
means a large βy at the final focusing lenses. At the present
KEKB, to make the world’s smallest βy∗ as a ring collider,βy reaches 1,000 m at one of the final focus lenses in the
HER. This requires a large aperture of the final lenses andmakes the separation of the two beam more and more difficult. The dynamic effect of a small βy∗ arises from thenonlinearity of the beam optics for chromaticity correction.
A number of ideas are applied to realize the small βy∗ inthe design of KEKB:• Finite crossing angle, 22 mrad to make the beam separation easier and simplify the arrangement of the orbits and components around the IP.
• Combination of normal and superconducting magnets
for the final focus lenses.
• Compensation solenoids to cancel the coupling effectsof the detector solenoid field.
• A large number (54 in the LER and 52 in then HER) of
pairs of sextupoles connected with −I transformation
which cancels the transverse nonlinearity.
• 2.5π unit cell of the arc lattice to optimize the locationof the sextupole pairs as well as to have good tunability in the emittance and the momentum compaction
factor.
• A local chromaticity correction system around the IP
in the LER, similar to the final focus system for a linear collider.
No beam optics works without diagnostics and corrections. First of all, all magnets and power supplies were
carefully constructed and calibrated within the margins of
the errors. The alignments of the magnets were also successful even within a limited period of construction. The
beam position monitors (BPMs) were well calibrated too.
These are the basis of the orbit/optics correction system.
KEKB routinely applies orbit/optics corrections:• Orbit correction to a reference “golden” orbit (set by
the optics correction below), every 15 seconds. The
r.m.s. residual was 30–50 µm.
• Circumference correction by chicanes (LER) and rf
frequency (HER). Circumference of KEKB drifts byabout 2.5 mm in 6 months. Though tidal effects wereseen in the drift, the major part was much slower component.
• β, x-y coupling, and dispersion corrections looking
at the orbit response on every BPM. The correctors
are the fudge factors of normal and skew quadrupoles,
and bump orbits at sextupoles. These corrections wereapplied more than once in 2 weeks. These are quite essential to enable the best operating tunes and to maintain the luminosity.
• Beam-based alignment of all BPMs. Using correction
winding of each quadrupole, the beam-based alignments was done periodically. The residual r.m.s. of
the alignment error was about 100 µm.
• Beam-based mapping of BPMs. While each BPM
were mapped at bench before installation, changes
happen in the gains of the electronics, the attenuator,
and the connectors. Thus the calibration was necessary. At KEKB an innovative method for mapping
using the beam has been developed. The result was
remarkable and even better than the mapping at thebench.
The control system of KEKB gave the basic tool of any
diagnositics and control of KEKB. It utilizes the EPICS
system as one of the largest scale in the world with about
100 input/output controllers (IOCs). The online model
SAD, linked with EPICS, is widely applied for control, diagnostics, and correction of the KEKB beam.

Higher ξy
One of the major issues in the design of KEKB was
the finite crossing angle at the IP. Though the merit of thecrossing angle was obvious for the beam separation andreduction of the parasitic crossing, many machines hesitated to use it after the bitter experience at DORIS (except
a small crossing angle at CESR). The crossing angle of
KEKB, 22 mrad, is as large as the bunch diagonal angle
σx∗ /σz , and might have enough scared conservatives. The
KEKB design justify the crossing angle as:• The magnitude of the synchrotron-betatron couplingterms arising from the crossing angle has the same
magnitude as the regular synchrotron-betatron terms
which are intrinsic to any beam-beam interaction.
Thus the crossing angle just increases the number ofterms whose effect can be avoided by choosing theoperating tunes.
• Simulations showed there are such operating tunes
that give ξy ≥ 0.05, the design value of KEKB.
• There is a backup solution, crab crossing, to restore
the effective head-on collision with crossing angle.[7]• The problem of DORIS was not the crossing angle
itself, but the tune-spread over bunches to relax thetransverse coupled-bunch instabilities, making it impossible to choose good tunes for all bunches. KEKB
did not need such tune-spread owing to the lowimpedance cavities and the bunch-by-bunch feedback.
As shown in Table 2, KEKB actually achieved the design value, ξy = 0 .05, as expected. After the completionof the design, the progress in the beam-beam simulation

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

has been remarkable. Now a strong-strong simulation becomes enough reliable at least for electron machines with
damping. Such new simulations have been confirming thesafeness of the finite crossing angle at least up to the current
magnitude of ξy .
The most important factor to determine the actual ξy is
the linear optics of two rings. Tunes, βs, and waists are
the most important parameters as well as the relative orbit
offsets between two beams. The orbit offset is managed by
feedbacks using the beam-beam deflection and beam size
response. The interferometer to measure the vertical beam
size below the diffraction limit works as an essential tool
for the orbit control. The couplings and dispersions between the degrees of freedom must be reduced as small as
possible. The optics corrections described in the previous
section are necessary but not enough to keep the best condition of collision. Thus a number of coupling/dispersion
knobs were applied to find out the optimum. The so-called
dynamic β effect was also important to increase the luminosity. It requires a horizontal tune νx very close to a half
integer. Up to now KEKB could operate νx ∼ 0.506 andνx ∼ 0.511 in the LER and the HER, respectively. Suchtunes require strict optics corrections and control of tunes.
KEKB utilizes a tune-feedback system with collision-freepilot bunches for that purpose.

Higher Current
Higher luminosity requires higher current anyway. Highcurrent is the toughest issue related to the wide range of thebeam dynamics and the accelerator technology. Here wejust list up issues on high current experienced at KEKB.
Synchrotron light KEKB has already accumulated 1.5 A
in the LER (in regular operation, the maximum was 1.8 A),
and 1.1 A in the HER, and these currents are not less thanmany light sources. If one counts the size of the ring, thestored charge is well higher than them and even comparable
to the charge per pulse at the J-PARC. The energy losses ofthe synchrotron light are 2.6 MW in the LER at 1.5 A and4.4 MW in the HER at 1.1 A. The synchrotron light hasenough energy density to melt down the vacuum chamber
if improperly hit it. Such accidents were experienced at
KEKB a few times near the IP and the wiggler sections.
High vacuum The average pressure in the KEKB beam
pipe is maintained below 100 nP a with the stored beam.
It was achieved by non evaporative getter pumps (NEGs)
with 200 /s pumping speed placed in every 1 m along thebeam pipe. Such good vacuum is necessary to reduce thebackground for the detector, to increase the beam lifetime,
and to avoid the fast-ion instability.
Beam loss The stored high current at KEKB has enoughpower to melt, evaporate, and destruct the beam pipe andother components if it hits them directly. Such accident
happened at KEKB many times as the result of a big changeof orbit due to beam instabilities and/or trip of rf system. A
system, consisting of beam loss monitors, rf phase detectors, transient data loggers, abort kckers, etc., have beendeveloped at KEK to prevent such accidents by detecting
the beam loss as early as possible and to abort the beam
immediately. Fragile components such as the movable collimators needed a number of improvements since the beginning of KEKB operation after experiencing such beamlosses.
Electromagnetic resonances The accelerating cavities
of KEKB were so designed that all higher order or parasitic resonances escape or damp via wave guides, slots,
dampers, and absorbers. As the result, no instability, either longitudinal or transverse, has been arisen from rf cavities up to the design current. Resonances at other components such as bellows are avoided in the design, but occasionally resonances start from tiny distortion and grow up
catastrophically toward destruction. Such destructions happened a few times per year. Anomalous resonant couplingof bellows or pumping slots via TE-like modes, which usually do not couple to the beam, occurs near the collimatorsthat break the symmetry to excite such modes.
Acceleating mode The accelerating mode of an rf cavity, which is the inevitable resonance, has a problem forhigh current storage. The beam loading voltage at KEKB
is huge enough to cancel all acceleration if the cavities are
tuned at the center of the resonance. Thus the detuningof the accelerating mode is necessary, but if the detuning is large, the coupled-bunch modes nearby the accelerating mode becomes unstable. To avoid such instability,
KEKB chose a passive stabilization scheme, which reduces
the amount of detuning by increasing Q values of the accelerating mode. The normal conducting ARES cavity[3]was so designed that the Q value is one order higher thanusual cavities according to the huge storage cavity. The superconducting cavity naturally has a very high Q value.[2]Both cavites has been working successfully until now, andtheir stabilities are sufficient. The trip rate of the rf system itself is roughly once per day. The passive stabilization scheme does not need a sophisticated feedback system
involving the rf and the bunch-by-bunch feedback.
Non-resonant modes Even after the removal of all resonances, wake fields with frequencies higher than the cut-offof the beam pipe (HOM) remain as a problem. Such modes
become more important for shorter bunch length. The heating power of such modes roughly scales as
W ∝

IN
,
σz

(2)

where N is the number of particles/bunch. It is interestingthat the scaling (2) is very similar to the expression for luminosity, Eq. (1), since σz ≤ βy∗ is necessary. This meansthat for a given environment, a high luminosity means ahigh heating due to the non-resonant modes. This will impose a very strict constraint on the future upgrade of themachine.
Beam instability Instabilities caused by electromagnetic
wakes have been well studied and more or less understood according to the development of the beam dynamics

The 14th Symposium on Accelerator Science and Technology, Tsukuba, Japan, November 2003

until the design of KEKB. As mentioned before, the design of KEKB basically suppresses all electromagnetic instabilities below the radiation damping without feedback,and it has been so verified in the actual operation. Therewas some discrepancy, however, in the longitudinal singlebunch impedance and the HOM loss in the measurement
and the design. Though it has not yet confirmed quantitatively, additional impedances surely exist at the collimators and the coherent radiation in the wigglers. An idea
to suppress a bunch lengthening is to make the momentum compaction factor negative, and it has already been
tried at KEKB successfully. The resistive wall effect, especially the tune shift due to the non-circular cross section of
the beam pipe in the HER, has been a problem, since theamount is big enough (∼ 0.02/A ) to get out of the optimum tune for the luminosity without the tune-feedback as
the stored current decays.
Fast ion instability One of the modern instabilities observed at KEKB besides the electromagnetic ones is the
fast-ion instability in the HER. The transverse bunch-bybunch feedback has been quite effective to suppress it.
Therefore this instability is practically less problem for theoperation except when the vacuum pressure or the tuningof the feedback is bad.
Electron cloud The most serious instability at KEKB
has been the electron-cloud instability at the LER. Though
the coulped-bunch instability due to electron cloud had
been known blowupexperimentally at Photon Factory,[4]and theoretically explained[5] before KEKB Design Report. The coupled-bunch effect was actually observed at
KEKB and the bunch-by-bunch feedback was again very
effective to suppress it as planned. The more serious one
was, however, the single-bunch instability by the electron
cloud[6] to cause the blowup of the vertical beam size beyond a threshold current. Although such single-bunch effect had been predicted before the start of KEKB operation,
but not payed enough attention by the KEKB members.
Since the frequency range of the single bunch effect is
as high as 30 GHz, it is not possible to suppress themby a feedback. Then the only possible cure was to apply
weak magnetic field at the beam pipe to prevent the cloud
to come close to the beam. Such an effort already started
in Nov. 1999 by attaching a number of permanent magnets
on the beam pipe for about 800 m. Though these magnets
were actually as effective as solenoids, it was hard to confirm. The solenoid windings replaced the permanent magnets since Oct. 2000, then their effects were experimentallyconfirmed easily. The solenoid windings were extended toabout 2,300 m before summer 2003 (Fig. 2), to cover thenearly entire free space of the LER.
Though the solenoids were so extended, the electron
cloud instability did not remove completely. The threshold current for 4 bucket spacing increased from 0.4 A (nosolenoid) to 1.8 A (2,300 m). This means that about 20%of the cloud still remains somewhere in the ring, including
the magnets, but their location has not been identified.

Injection Storing high current requires a strong injector.
The injector linac was commissioned one year earlier than
the rings, and has been operated with good stability sincethen. One major breakthrough on the injection is the success of the two-bunch injection scheme for positrons. By
choosing the bunch separation to 49 ring rf bucket, it waspossible to accelerate two positron bunches in the same rf
pulse. The energies of the bunches were equalized by adjusting the timing of the rf pulse taking the SLED wave
form into account. The two-bunch injection has been regularly used for the operation. The injection rate was actually
doubled and achieved higher numbers than the Design Report.

FUTUREKEKB has various upgradability toward 10 to 50 times
higher luminosity than the current design. For instance:• By introducing a crab crossing with crab cavities, thebeam-beam parameter can reach 0.1 to 0.25.
• Smaller βy∗ down to 3 mm is possible by redesigning
the final quadrupoles and the compensation solonoids.
• Higher beam current up to 9.4 A in the LER and
4.1 A in the HER is possible by new beam pipe, newbellows, more cooling system, more rf systems withimproved ARES (LER), couplers, HOM absorbers(SCC), etc.
• Charge switch of the rings with C-band system and adamping ring in Linac will help the needs of positrons
and relax the electron-cloud effect.
The present tunnels, facilities, magnets, power supplies
will be still usable for the upgrade. For an e+ e− B–factorywith the luminosity range 1035 − 1036 cm −2 s−1 , upgrading
KEKB will be the cheapest solution.

ACKNOWLEDGMENT
The authors togethor with the KEKB Accelerator Team
thank to all people who have ever supported KEKB during
the design, construction, and operation.

REFERENCES[1] KEKB Design Report and the real time information of KEKB
are available at http://www-kekb.kek.jp.
[2] Progress until 2001 is summerized in Nucl. Instrum. Meth.
A499, 2003.
[3] Y. Yamazaki and T. Kageyama, Part. Accel. 44:107-127,
1994B[4] M. Izawa, Y. Sato, and T. Toyomasu, Phys. Rev. Lett.
74:5044, 1995.
[5] K. Ohmi, Phys. Rev. Lett. 75:1526-1529, 1995.
[6] K. Ohmi and F. Zimmermann, Phys. Rev. Lett. 85:3821-3824,
2000.
[7] K. Oide and K. Yokoya, Phys. Rev. A40:315, 1989.