Proceedings of EPAC 2002, Paris, France

OPERATION EXPERIENCE AND PERFORMANCE LIMITATIONS IN
e+e- FACTORIES
K. Oide, KEK, Oho, Tsukuba, Ibaraki 305-0801, Japan
Abstract

only 15 months after the startup of the BaBar detector. It
also exceeded its design luminosity by 50% (4.6 /nb/s) in
the fall of 2001. The integrated luminosity is approaching 90 /fb in May 2002. KEKB started the luminosity run
in 1999, and the startup speed was somewhat slower than
PEP–II’s. The main obstacle at that time was the electroncloud effects and various machine failures due to the highcurrent operation. The peak luminosity of KEKB surpassed
PEP–II’s in March 2001 and it reached 7.3 /nb/s in May
2002, which is still 30% below its design value, 10 /nb/s.
Though the peak luminosity of KEKB has been higher than
PEP–II’s since 2001, the integrated luminosities of two machines in 2001 through 2002 were roughly equal to eachother.
The ratio of the average luminosity to the peak luminosity was about 76% and 62% for PEP–II and KEKB, respectively. The strong injector of SLAC beneﬁts PEP–II prettymuch to improve the luminosity efﬁciency. Also the total running time in 2001 was longer in PEP–II by about 2
months than in KEKB.
Both machines have already produced substantial output
of particle physics as planned, including the discovery of
CP violation in the B meson system. The quick start ofthe luminosities in the two machines was remarkable in thehistory of colliders.

Since 1999 three double-ring e+e- colliders have been
operating. PEP–II and KEKB are asymmetric B–factoriesat Υ(4S), and DAΦNE is a Φ-factory. Generally speaking,the two B-factories have more or less common issues, andthe Φ–factory has somewhat different issues from the B–
factories. This paper ﬁrst describes two B-factories, thendiscusses on DAΦN later. The description on DAΦNE
owes Dr. M. Preger.

1 ASYMMETRIC B–FACTORIES
1.1

Successful Startup

Figure 1 shows the development of the peak and the integrated luminosities of PEP–II and KEKB since 1999. Thisﬁgure obviously tells the success of two machines. PEP–II
reached its design luminosity, 3 /nb/s, in the fall of 2000,

1.2

Machine Parameters

Table 1: Machine Parameters of Asymmetric B–Factories.
PEP–IIKEKB
LER HER LER HER
Energy
3.1
9.0
3.5
8.0
GeV
Circumference
2200
3016
m
Current
1.78 1.06 1.37 0.92
A
Bunches
800
1224
N/bunch
10.2
6.1
7.0
4.7
1010
Spacing
2.5
2.4m
Cross. Angle
0
22
mrad
Emittance εx
50
50
18
24
nm
βx∗
35
50
59
61
cm
βy∗
0.9
1.25 0.62
0.7cm
Hor. Size @IP
132
158
103
121
µm
Ver. Size @IP
7.9
4.5
2.8
2.8
µm
εy /εx
14
3.2
7.2
4.8
%
Bunch Length
13
12
5.3
5.5
mm
Beam-beam ξx .062 .070 .080 .074
Beam-beam ξy .056 .029 .048 .041
Luminosity
4.60
7.35
/nb/s

Lum/24
hrs
303
395
/pb

Lum/7
days
1790
2524
/pb

6666
8783
/pb
Lum/30 days

Figure 1: The increase of the integrated (upper) and peak(lower) luminosities of PEP–II and KEKB since 1999.

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Proceedings of EPAC 2002, Paris, France
Table 1 lists main machine parameters of two B–
factories corresponding to their best luminosity.

1.3

Energy Transparency

Both machines were designed with the so-called energy
transparent condition, which requires the beam currents to
be inversely proportional to the beam energy. As shownin Table 1, in the real situation, mainly due to the blow-up
of LER beam size by electron cloud, both machines violate
the transparent condition. On the one hand this veriﬁed thatthe transparent condition is not so strict to obtain the luminosity, but on the other hand, the non-transparent parameters as Table 1 was still not the optimum for the luminosity.
While the luminosity/(LER current) in Table 1 is not better
than the transparent one, the luminosity/(HER current) was
signiﬁcantly worse than that of the transparent condition.
This means that it was necessary to store HER current asmuch as possible to recover the luminosity degraded by theblow-up of the LER beam size. Thus for a design of future machines, the energy transparency will be still valid,
assuming such external blow-up being solved.

1.4

Figure 2: The dynamic β effect. This ﬁgure assumeξx =0.075 for both rings. By the focusing force of the beambeam interaction, βx∗ (bottom) shrinks as the horizontal tune
becomes close to 0.5. The emittance(upper) diverges andthe beam size (middle) stays nearly constant.

1.6

The search for good betatron tunes would have been very
difﬁcult for a double-ring collider that have four tunes, if
simulations did not exist. Beam-beam simulations, either
strong-weak or strong-strong, indicate that best luminosity
is achieved just over an integer or half-integer horizontal
resonances. The half-integer resonance is preferable because the orbit distortion will be more stable than the integer one. KEKB has been systematically pursuing suchtunes, starting from 0.56 in 1999, 0.52 in 2000, 0.514 in
2001, 0.510 in 2002. The luminosity actually became better as the horizontal tune came closer to the half integer.
This was basically the same result as the prediction of thesimulations.
The access to the half integer was made possible by reducing the width of the stop band with optics corrections.
At KEKB, optics correction including β, coupling, dispersion corrections have been done regularly in every 2 weeks
with an online model by SAD. The residual β-beat is suppressed to below 5% by the correction. Such a correction
system is also under development at PEP–II, and whosetune has been set closer to half-integer, 0.52, recently.
A possible explanation of the superiority of half-integer
is give by the so-called dynamic β effect. The beam-beamfocusing force at the IP changes the beta functions and theemittances for particles at the core where the beam-beamforce is nearly linear. This effect becomes stronger at an integer or half-integer resonances. Figure 2 shows the effectof the dynamic β, emittance, and beam size as functions of
the horizontal tune above the half-integer. For instance, βx∗
is squeezed down to 25% at νx = 0.510 by the dynamic
β effect. It is interesting that the horizontal emittance isincreased as νx comes close to 0.5, resulting that the horizontal beam size at IP stays nearly constant for the tunechange. We assume both rings have same tunes. Therefore
although the luminosity is not directly affected by the tune,
the effective beam-beam parameter, which is inverse of theemittance, will be reduced as the tune comes close to thehalf-integer. The nominal horizontal tune shift in Table 1

Crossing Angle

One of the big differences between two machines is the
crossing angle at IP. KEKB (as well as DAΦNE) has 22
mrad horizontal crossing angle. This angle is roughly equal
to the bunch diagonal angle σx /σz = 20 mrad. Up to now,
no serious effect related to the crossing angle has been noticed at KEKB. With the crossing angle their beam-beamtune-shift parameters have exceeded 0.07 and 0.04 in horizontal and vertical planes, respectively. When KEKB was
designed, strong-weak beam-beam simulations justiﬁed thecrossing angle, and later strong-strong ones conﬁrmed that.
The experimental results basically agree with these simulations. The crossing angle will help shorter the bunch spacing that is necessary to increase the beam current further.

1.5

Access to Half Integer

Maintaining Collision

Number of methods have been applied to collide two
beams and maintain high luminosity:• 4D Orbit feedback measuring beam-beam kick by the
IR BPMs (KEKB). Care was needed when the BPMs
are moved within a ﬁll by the temperature change at
the IR.
• Dithering method with a fast luminosity monitor to
optimize the orbit (PEP–II). At KEKB the limited acceptance of the monitor makes it hard to utilize.
• Flip-ﬂop control using either the dispersion bump or
horizontal offset of two beams (KEKB).
• Controlling the betatron tunes during a ﬁll looking at
the pilot bunches (KEKB: off-collision, PEP–II: oncollision).

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Proceedings of EPAC 2002, Paris, France

Figure 3: An example of the β correction in KEKB LER. Left is before correction showing the β and phase beats in x andy planes. It was corrected (right) reducing the residual within about ±5% after a few iterations. This correction is done by
an online model with SAD. A turn around time for the measurement/correction is about 5 minutes (by H. Koiso, et al.).
is as high as 0.075, but the effective value is much smaller
due to the dynamic β effect. This should contribute to relax
nonlinear effects of the beam-beam interaction.
Squeezing β ∗ near the half-integer is only valid in thehorizontal plane. The reason is that βy∗ has been alreadysqueezed down to the bunch length, and further reductionof βy∗ just increases the hour-glass effect and does not contribute to the luminosity at all. This is also consistent withthe result of simulations.

1.7

Optics and the orbit around the ring is also important tosqueeze β ∗ s, reduce εy and x-y coupling, and access to thehalf-integer resonance. The correction must be as local as
possible to avoid higher order effects. As a nature of thecollider, these machines have strong sextupoles distributed
around the ring, and the control of orbit at sextupoles has
been very important to keep the beam optics as ideal as
possible. The orbit at sextupoles can be used to knobs tocorrect dispersions, x-y coupling, and β errors as done in
KEKB (may be applied at PEP–II also). The local chromaticity correction system applied in LERs of both PEP–II
and KEKB was successful to reduce the strength of the arc
sextupoles to stabilize the optics.
The beam-based alignment of all BPMsdone in KEKB
were also necessary to ensure the optics correction scheme.

Understanding Optics

Understanding the beam optics was a key issue toachieve the high luminosity. The beam optics around the
IP is especially important, because the optics around the
IP is quite irregular due to the low-β insertion and the xy coupled components. At PEP–II, number of measurements were done to analyze the beam optics around IP. As
PEP–II does not have a compensation solenoid to cancel
the BaBar’s solenoid, the handling of the skew components
were quite important and not easy. By the help of the compensation solenoid, the beam optics at the IP was relatively
easier in KEKB. The x-y coupling was corrected locallyby the skew quadrupoles near the IP, as an integrated partof the optical correction system of KEKB. Even with suchcorrection methods, there were a lot of unknown behaviorof the orbit and optics remaining around the IP. To solve theissue, more BPMs are necessary not only for every quads,
but also for skew quadrupoles.

1.8

Electron Cloud

The LERs of the asymmetric B–factories were the ﬁrst
machines to show the electron-cloud effect, especially thesingle-bunch blow-up. The coupled-bunch effect of theelectron cloud was ﬁrst seen at the Photon Factory around1990, and was already known at the design stage of theB–factories. The growth time of the coupled bunch effectwas estimated to be below the damping rate by the bunchby-bunch feedback system in both machines. Such estimation has been veriﬁed experimentally in KEKB, and thecoupled-bunch effect was actually not the critical issue.
The more striking effect of the electron-cloud was the3Proceedings of EPAC 2002, Paris, France
single-bunch effect. It was actually predicted by F. Zimmermann and T. Raubenheimer around 1997, well before
the start of the B-factories, for the bunch compressor of the
NLC as well as the fast-ion effect. Unfortunately it was notwell noticed by the B-factory people until the blow-up of
the LER beam was observed in their rings. The blow-up
of the vertical beam size in LER was observed in March
1999 at KEKB, and was explained as a single-bunch effect of the electron-cloud in October 1999 by F. Zimmermann. Sooner or later the blow-up was also observed at
PEP–II. After that a number of measurements were doneusing synchrotron light image/interferogram, gated camera, gated tune meter, gated luminosity monitor, electroncloud detector, etc. Those measurements basically support
the prediction by the single-bunch theory and the simulations.
There are two sources of the electron cloud. One is
the photo-electron and the other is the multipacting withthe secondary electron at the chamber wall. The ﬁrst one
dominated at KEKB since the vacuum chamber is a simple
cylinder, while PEP–II has an antechamber. This difference was one of the factors to make the startup of KEKB
slower than PEP–II until the solenoid was fully installed in
2001. The major cure of the electron cloud was the externalsolenoid ﬁeld both in PEP–II and KEKB. Both machines
have wound solenoids with 20 to 50 Gauss covering morethan 95% of the ﬁeld-free region. As the result the electron
cloud is not the critical issue at least up to the present beam
current with 4 bucket spacing.

these measurements are sensitive to the cloud far fromthe beam core, the cloud at the beam core might besmaller than these measurements.
• If there still remains clouds somewhere in the ringwith solenoids, it must be in the magnets. If it is true,
special method other than solenoid will be necessary.
Also a trapping effect by quad and sextupole ﬁelds has
been predicted by L. Wang, but its effect on the beam
is not known yet.
• Horizontal blow-up was seen at PEP–II, even stronger
than vertical. The mechanism is still unknown. It is
not observed at KEKB at all.
• The bunch-by-bunch luminosity indicates strong dependence along a train at PEP–II. Also an effect ofmini-gap was strong at PEP–II. These are very weakat KEKB.
• Observations suggested a mixed effect of the electroncloud and the beam-beam interaction as predicted by
E. Perevedentsev, et al. Even below the thresholdof the single-beam blow-up, the luminosity was degraded by the electron cloud.

Figure 5: The blowup of the horizontal(upper) and thevertical(lower) beam sizes at PEP–II LER. Effect of thesolenoid is seen in both planes (by A. Kulikov, et al.).
Figure 4: The blowup of the vertical beam size at KEKBLER for various lengths of the solenoid. With the current
conﬁguration the blowup is not seen any more up to 1.4 A.
(by H. Fukuma, et al.)

1.9

Acceleration of High Current

The asymmetric B-factories were also successful justin storing high currents. PEP–II has achieved maximum
current higher than 2 A, and KEKB 1.5 A also. These
stored currents are even higher than usual synchrotron lightsources. It is remarkable that the high current was achieved
with complicated beam optics at the IP with number of xy coupled components. Light source machines may have
undulators with short gaps to make injection difﬁcult, but
the gap of the vertical masks of B–factories are also very

Though the effect of solenoid was so remarkable, it is
not clear yet the problem was completely solved or not.
Number of issues are remaining:• Measurements by gated-tune and the coupled-bunch
growth rate show that there still remains 1/4 to 1/2 ofthe original cloud even with solenoids (KEKB). Since4Proceedings of EPAC 2002, Paris, France
narrow (for instance, ±3 mm for KEKB).
The most difﬁcult problem to accelerate such high current was the stability of the beam with the rf cavities. PEP–
II and KEKB took different approach at this issue. PEP–
II’s rf cavity is a very compact single-mode damped cavity. To compensate the heavy loading of the beam, PEP–II
developed a sophisticated feedback system controlling the
klystron phase for each bunch together with the bunch-bybunch longitudinal feedback. In this sense PEP–II took avery active control system. KEKB chose the opposite direction: passive stabilization. KEKB’s two types of cavities, the ARES copper cavity and the superconducting cavity(SCC), both have very high stored energy to reduce thebeam loading small enough. As the result, KEKB does notneed special feedback system for the longitudinal plane.
It only uses a slow 0 and -1 mode feekdbacks. Due tothe heavily damped impedances of these cavities, KEKB
does not need bunch-by-bunch feedback for the longitudinal plane (it is necessary for the transverse planes to suppress the electron-cloud, the fast-ion, and the resistive-wallinstabilities).
Though their choices were opposite, both rf schemes
worked as expected up to the present stored currents. Remaining issues are:Figure 6: The Layout of DAΦNE (by M. Preger).

2.1

Table 2 shows the present performance of DAΦNE withKLOE and DEAR detectors, respectively. With the luminosity collected in April shifts, the capture of kaons in Argon atoms clearly observed.
Table 2: Present Performance of DAΦNE
KLOE
DEAR
Energy510
MeV
Bunches
47 + 47 45 + 45
Current
1+1
0.8 + 0.8
A
Emittance1µm
βx∗ /βy∗
400/4cm
Luminosity
51
46
/µb
Int Lum /day
2.5
1.1
/pb
Lum Lifetime
20
30
min.

• The bunch-by-bunch longitudinal phase difference issigniﬁcantly higher in PEP–II.
• The availability of the SCC at KEK is limited by theregulation for the He refrigerator imposed by the government. It is very hard to extend the running time
longer than 40 weeks per year.

1.10 Summary• Rapid startup, good coopetition. Sufﬁcient for theplanned physics.
• Many schemes worked as expected = victory of theaccelerator technology and the beam dynamics. Electron cloud was serious, but cured by solenoid up tosome extent.
• But, no big breakthrough for future! Is Super B beyond 100 /nb/s to be possible only by a brute force(=higher current, more power, more cost)?

2

Present Performance

2.2

Short Term Plans• Deliver > 300 /pb to KLOE.
• Further improve luminosity performance and signalto-background ratio in order to observe and measure
the properties of kaonic hydrogen in DEAR (∼ 40 /pb
required).
• During a long shutdown (from November 2002 to January 2003) install new interaction regions for KLOE
and for a new magnetic detector (FI.NU.DA.,aimed at
the study of hypernuclear physics on IP2), with modiﬁed optic and supports in order to decrease the IP
β-functions, optimise background rejection and provide variable quadrupole rotation to operate at different magnetic ﬁelds (from 0 to maximum) in thesolenoids.

DAΦNE

DAΦNE is a high luminosity electron-positron collider
designed as a Φ-factory for the production of a high rate of
K-mesons from the decay of the F resonance at 1.02 GeV
CM. It consists of two rings on the same horizontal plane,crossing at an angle of 25 mrad in two interaction regions(IR). Up to 120 bunches can be stored in each ring The
ﬁrst IR hosts a magnetic detector (KLOE) mainly aimed
at the study of CP violation. In the second IR the DEAR
experiment studies the properties of kaonic atoms, namely
atoms where kaons are captured in the inner shells in placeof electrons.

The author thank Drs. M. Preger, R. Holtzapple,
J. Seeman, M. Sullivan, H. Fukuma, Y. Funakoshi, H.
Koiso, S. Stanic, M. Suetake, and members of PEP–
II/DAΦNE/KEKB groups for useful information.

5