Proceedings of EPAC 2004, Lucerne, Switzerland

KEKB PERFORMANCE
Y. Funakoshi, K. Akai, K. Ebihara, K. Egawa, A. Enomoto, J. Flanagan, H. Fukuma, K. Furukawa,T. Furuya, J. Haba, S. Hiramatsu, T. Ieiri, N. Iida, H. Ikeda, T. Kageyama, S. Kamada, T. Kamitani,
S. Kato, M. Kikuchi, E. Kikutani, H. Koiso, M. Masuzawa, T. Mimashi, A. Morita, T. T. Nakamura,
H. Nakayama, Y. Ogawa, K. Ohmi, Y. Ohnishi, N. Ohuchi, K. Oide, M. Shimada, S. Stanic,
M. Suetake, Y. Suetsugu, T. Sugimura, T. Suwada, M. Tawada, M. Tejima, M. Tobiyama, S. Uehara,
S. Uno, S. S. Win∗ , N. Yamamoto, Y. Yamamoto, Y. Yano, C. Yokoyama, M. Yoshida,
M. Yoshida, M. Yoshioka, S. Yoshimoto, F. Zimmermann†
KEK, 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
Abstract

Table 1: KEKB Machine Parameters.
June 2004
May 2003
LER HER LER HER
Energy
3.5
8.0
3.5
8.0
Circumference
3016
3016
Ibeam
1650 1220 1500 1100
# of bunches
1294
1284
Ibunch1.28 0.94 1.17 0.86Ave. Spacing
2.35
2.33
Emittance
18241824βx∗595659
58
βy∗
5.2
6.5
5.8
7.0
Ver. Size@IP
2.1
2.1
2.2
2.2
RF Voltege
8.0
14.0
8.0
13.0
νx.505 .513 .507 .512
νy.535 .582 .546 .580
ξx.113 .072 .096 .065
ξy.074 .057 .069 .052
Lifetime
152
178
127
256
Luminosity
13.92
10.57
Lum/day
0.944
0.579
Lum/7 days
6.01
3.88
Lum/30 days
24.00
12.81

We summarize the machine operation of KEKB during
past one year. Progress for this period, causes of present
performance limitations and future prospects are described.

INTRODUCTION
The KEKB B-Factory is an electron-positron double ring
collider working at KEK. Its peak luminosity surpassed
1 × 1034 /cm2 /sec in May 2003 for the first time in thehistory of colliders. This peak luminosity is also the design
value of KEKB. After this achievement, the KEKB luminosity has been still growing. In this reports, we summarize
the machine operation of KEKB mainly after the summershutdown in 2003. A status report before this was written
elsewhere [1]. Fig. 1 shows 5-year’s history of the KEKB
luminosity and beam currents. Table 1 shows present machine parameters of KEKB compared with those of about
one year ago. As is seen in the table, there are no big
changes in the machine parameters. Although the changes
are small, we made significant progress in both the peakluminosity and the integrated one with continuous efforts.
The peak luminosity increased by about 30%. On the other
hand, the daily integrated luminosity increased by about60%. The reason why the improvement in the integrated
luminosity is much larger than the peak lies in adoptionof so-called continuous injection scheme which we introduced at the beginning of this year (2004).

RECENT PROGRESS
Continuous Injection
In the conventional injection scheme, data taking by thephysics detector (named Belle) was halted during the beam
injection. After finishing the injection, Belle rose high voltage (HV) of each part of the detector and then started datataking. When the beam currents decreased to some values,
Belle halted data taking and lowered HV for the next beam
injection. The beam injection started after HV is lowered∗ visiting from NSRC, Thiland† visiting from CERN, Switzerland

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to some values. On the other hand, in the continuous injection scheme, data taking continues even during the beam
injection. The continuous injection scheme has the following four advantages over the conventional scheme. 1) We
can avoid the time loss of the beam injection and the upand-down of HV. 2) The beam currents can be kept almost
at the maximum value through the experiment. 3) We canoptimize the machine tuning parameters at around the maximum beam currents as is explained in the next subsection.
4) We can operate the machine much more stably than theconventional scheme, since basically we need not change
the machine parameters. By adopting the continuous injection scheme, an integrated luminosity per shift was increased by more than 20%.
In the continuous injection scheme, usually the beam isinjected at 10 Hz (cf. 50Hz in the conventional scheme).
After each beam injection, data taking is vetoed for 3.5
msec, which means that the dead time is 3.5% arisen from707

Proceedings of EPAC 2004, Lucerne, Switzerland

Figure 1: History of KEKB.
this veto. In the case of KEKB, the electron and positron
beam can not be injected simultaneously. At present, themode of the injector (electron or positron) is switched every 5 minutes. The continuous injection scheme was realized with preparations and trial-and-errors for more than
one year. We had to rise above some serious problems
such as malfunction of pre-amplifiers of the TOF detector
and frequent DAQ (data acquisition) errors of Belle undera high beam background. To solve the problem with thepre-amplifiers, we adapted them so that they can accept ahigher noise. The DAQ problems were overcome by upgrading the DAQ system during the summer shutdown in2003. On the other hand, we also had to make efforts todecrease the detector beam background, which was done
mainly by optimizing machine parameters.

Peak Luminosity
The improvement in the peak luminosity shown in Table 1 has been brought by both improvement in the specific luminosity and the increase of the beam currents.
Fig. 2 shows a comparison of the specific luminosity of thepresent KEKB with that in May 2003. Two fills are shownin the figure, which gave the best luminosity so far of 13.92/nb/s (red plot) and the best luminosity before summer in2003 of 10.57 /nb/s (green plot). The fill of the red plot wasa relatively long fill for about 7 hours due to the continuousinjection. On the other hand, the fill of the green line wasthat for about one hour. From these plots and Table 1, we
can see three causes of improvement in the peak luminos-

ity . 1) The beam currents in both rings increased by about
10 %. 2) The specific luminosity increased by about 10 or
20 %. 3) The luminosity gets a peak at the highest beam
currents in the present operation.
To increase the beam currents, the following three works
were done during the summer shutdown in 2003. 1) Twomore ARES cavities were installed in HER. 2) The IR vacuum chambers were replaced with new ones which realize more cooling power against heating by SR from theIR quadrupole magnets. 3) Several vacuum components
which had troubles or would possibly restrict the beam currents were replaced with new versions (or simply removedfrom the rings), which include HER movable masks, bellows near IP, vacuum chambers of septum magnets in bothrings, an ante-chamber for beam test in LER, a vacuum
chamber of the synchrotron radiation (beam size) monitor
in HER, beam stoppers in both rings and ceramic chambersof kicker magnets in HER. Since the vacuum works above,
the HER beam current has been limited mainly by the RFpower. The two new ARES cavities brought a higher HER
current in autumn 2003 as shown in Fig. 1. At the beginning of 2004, one of superconducting cavities (SCC)
in HER was removed from the ring due to vacuum leak.
Although we had to decreased the HER beam current after
this, the HER current has been increasing by increasing RFpower for each SCC. On the other hand, the LER beam current is not limited by the RF power but by the luminosity
itself, which means that the luminosity does not increase
with a higher LER beam current than some value. We have
not yet fully understood the reason for this. We suspect708

Proceedings of EPAC 2004, Lucerne, Switzerland
the effect of the electron cloud in LER and the beam-beam
blowup of the HER beam.
As for causes of improvement in the specific luminosity,
we can list up the following four causes. The first is that wesqueezed the vertical beta functions of both rings further.
The second is fine tune survey particularly of the verticaltune of LER. The third are continuous efforts to improvemethods of the optics and orbit corrections. The fourth isinstallation of more solenoid magnets for suppressing theeffects of the electron cloud. During and after the summer
shutdown in 2003, we installed 210 more solenoid coils and50 permanent solenoid magnets.
As for the reason that the peak luminosity is obtained at
the maximum beam currents at present unlike last year, weconsider effectiveness of the continuous injection scheme.
With this scheme, we can tune the machine with almost thesame beam conditions. We experience that the beam orbits
and the betatron tunes change as function of the beam currents even without any intentional changes, the mechanismof which we still have to investigate in future.

June 03 2004
May 13 2003

Performance Limitations
One of the features of KEKB is that the number ofbunches is much smaller than the design. In the design, thenumber of bunches was assumed to be around 5000 whichmeans that every RF bucket is filled with particles (exceptfor some abort gap). In the present KEKB, the specific luminosity tends to decrease when the number of bunches
increases. This problem (bunch-spacing problem) imposesa performance limitation upon KEKB, since it is desirable
to increase the number of bunches from the view point of
avoiding hardware troubles due to HOM fields. The number of bunches is 1289 at the present KEKB. The fill pattern
is a mixture of 3 and 4 RF bucket spacing and the averaged
bunch spacing is 3.77 RF buckets ( 2.35m). To study thebunch-spacing problem, we made an experiment where 98
more bunches were added to both rings. They were placedin between 4 bucket spacing so that the averaged bunch
spacing was 3.5 RF buckets. As is shown in Fig. 3, when
we added 98 bunches, the specific luminosity decreased byabout 5 % (from red dots to blue). We also tried to add the98 bunches only in LER (positron ring). Also in this case,
we observed a degradation of the specific luminosity by
about the same amount shown in Fig. 3. On the other hand,in the case that we added the 98 bunches only in HER, weobserved no significant degradation of the specific luminosity (not plotted in the figure). This suggests that theLER beam is responsible for the degradation. A beam size
measurement using a streak camera showed that the added
bunches and bunches followed just after those bunches inLER blow up[2]. A measurement using a bunch-by-bunch
luminosity monitor showed that the specific luminosity perbunch corresponding to those bunches is degraded by about
30 %[3], which is consistent with the 5 % degradation of
the total specific luminosity. As for causes of this, we suspect effects of the electron clouds and/or combined effectsof the clouds and the beam-beam, although we need furtherstudy.

Figure 2: Progress of specific luminosity (in units of1030 /cm2 /s/mA2 ) in the past one year.

FUTURE PLANS
During the next summer shutdown, the SCC removed
from HER will come back and input couplers of two AREScavities which have now a problem will be replaced with anupgraded version. After these works, the HER beam current is expected to be raised by about 200mA. A crab cavity
for the purpose of increasing the beam-beam parameters isscheduled to be installed in each ring in the end of 2005
or at the beginning of 2006. We have a long-term plan toincrease the HER beam current up to 2A by 2007, although
the budget for this has not been approved.

REFERENCES

Figure 3: Reduction of specific luminosity with shorterbunch spacing.

[1] H. Koiso et al., “PRESENT STATUS OF THE KEKB BFACTORY”, EPAC’02, Paris, August 2002.
[2] H. Ikeda, Private communications.
[3] S. Uehara, Private communications.

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