Proceedings of the 2001 Particle Accelerator Conference, Chicago

KEKB PERFORMANCE
Y. Funakoshi, K. Akai, A. Enomoto, J. Flanagan, H. Fukuma, K. Furukawa, J. Haba, S. Hiramatsu,
K. Hosoyama, T. Ieiri, N. Iida, H. Ikeda, S. Kamada T. Kamitani, S. Kato, M. Kikuchi, E. Kikutani,
H. Koiso, S.I. Kurokawa, M. Masuzawa, T. Matsumoto, T. Mimashi, T. T. Nakamura, Y. Ogawa,K. Ohmi, Y. Ohnishi, S. Ohsawa, N. Ohuchi, K. Oide, E. A. Perevedentsev∗ , K. Satoh, M. Suetake,Y. Suetsugu, T. Suwada, F. Takasaki, M. Tawada, M. Tejima, M. Tobiyama, S. Uno, Y. Wu† ,N. Yamamoto, M. Yoshida, S. Yoshimoto, M. Yoshioka, F. Zimmermann‡ ,KEK, Ibaraki 305-0801, Japan
Abstract
The history of the KEKB commissioning is characterized
by struggles with three obstacles, namely the electron-cloudinstability, the beam-beam blowup and beam current limitations from many reasons. A rapid improvement in the luminosity in this year (2001) has been made by progress ineach of these three problems. This report summarizes thisyear’s progress of the KEKB accelerator, since the historyof prior days is covered by other reports[1][2][3].

1PRESENT PERFORMANCEFig. 1 shows the history of the KEKB luminosity. The
top row shows a history of a peak luminosity. As is seen in
the figure, the improvement of this year is remarkable. The
second row shows a history of a daily integrated luminosity. The third row shows a history of peak beam currents ofa day. The bottom row shows a history of an accumulatedluminosity by the Belle detector.
Table 1 shows a parameter list of the KEKB at the record
peak luminosity. This table tells characteristic features ofthe KEKB. The present KEKB is filled with a beam at every 4th RF bucket. In the design[8], the number of bunches
is 5000 which means that every RF bucket is filled with abeam. As is described below, the specific luminsotiy is decreased when the number of bunches is increased from theevery 4th RF bucket case by reducing bunch spacing. Although we tried longer bunch spacing, 4 RF bucket spacingis the best choice at the present KEKB. The other parameters are chosen under this restriction of the bunch spacing.
It is notable that the bunch currents of the present KEKB
are much higher compared with the design values particularly in the HER (high energy ring). This is also the consequence of the bunch spacing restriction. To compensate
this unusually high bunch current to some extent, the horizontal emittance of the HER is enlarged compared with thedesign. On the other hand, the LER bunch current is not
so high as the HER. The luminosity does not increase evenwith higher LER beam current. It is believed that this saturation of the luminosity with the LER current is due to thesingle beam blowup from the electron cloud instability. The∗ visting from BINP, Russia† visiting from IHEP, China‡ visiting from CERN, Switzerland

0-7803-7191-7/01/$10.00 ©2001 IEEE.

low value of the vertical beam-beam parameter of the LER
is also expained by this single beam blowup. The horizontal
and vertical beta functions at IP has been determined by atrial and error method. The vertical beta functions are much
lower than the desigan values.

22.1

RECENT PROGRESSElectron cloud instability

Studies on the nature of this instability are reported
elsewhere[4][5]. To mitigate this instability, solenoid coils
have been wound around the LER ring. Works for solenoid
winding were done three times, namely September 2000,January 2001 and April 2001. In those works, 800 m,
430m and 40m of the ring were covered with solenoid
coils, respectively. A typical length of the solenoid coils
is about 50cm, although there is some variety in length.
A typical field strength is around 45 Gauss at the centerof each solenoid when excited with a current of 5 A. Fig.
2 shows effectiveness of the solenoids. The horizontal
axis is a bunch current product of the tow beams. The
vertical axis is a specific luminosity which is defined as
a luminosity divided by a number of bunches and by thebunch current product in the unit of 1030 /cm2 /sec/mA2 .
This specific luminosity is a function of beam sizes andshould be constant when there is no beam blowup. In Fig.
2, there are three lines which correspond to cases with allof solenoid magnets on, with all solenoid magnets off andwith the solenoids of 450m wound January 2001 turnedoff. As is seen in the figure, the specific luminosity drops
drastically when all solenoid magnets are turned off. Thefigure also shows that the solenoids wound January 2001
is effective to increase the luminosity That indicates that
this year’s improvement of the luminosity partially owes
this solenoid winding.
Even with all solenoid magnets on, the specific luminosity has a beam current dependence. However, a beam-beam
blowup is included in this blowup. We need to separate itfrom the blowup. For this purpose, an experiment with alonger bunch spacing was done. In this experiment, bunchspacing was 24 RF bucket which is 6 times larger than theusual one. With the 24 RF bucket spacing, no beam blowup
is observed in a usual bunch current region. Therefore, we
assumed that the electron cloud instability is negligible with3546

Proceedings of the 2001 Particle Accelerator Conference, Chicago

Figure 1: Histoy of the KEKB luminosity.
this bunch spacing. Also in the experiment, a specific luminosity was measured as a function of the bunch current
product. This experiment was done on March 10 2001.
Fig.3 shows a result of the experiment. As is shown in thefigure, there is no big difference in the specific luminosity
between these two bunch spacing conditions at that time.
This result indicates that the effect of the electron cloud insatbility is negligible in the usual beam current range evenwith 4 RF bucket spacing. A similar experiment was done
also at the end of December 2000. At that time, there werea big difference in the specific luminosity with the 4 and 24
RF bucket spacing cases. Combining the results of the twoexperiments, it could be concluded that the solenoid magnet
wound January 2001 removed the main effect of the electron clound inatability in the beam current region usued inusual operation in the 4 RF bucket spacing case.
However, this does not means that the electron cloudinstability has been overcome completely by the solenoid
magnets. In the usual opeation with 4 RF bucket spacing,
the luminosity shows saturation to an increase of the LER
bem current. In the 24 RF bucket spacing case, however,the specific luminosity stays almost at the same level evenwith much higehr LER bunch currents, which is shown alsoin Fig. 3. In the region where the bunch current product islarger than 4.5 mA2 , the LER current was increased with
the HER current almost kept constant. In Fig. 3, the beam
current ratio of the two beam is also shown. This result indicates that the luminosity could be doubled by increasing
the LER current provided that the electron cloud instability
can be suppressed completely.

2.2

Working pointAt the beginning of February 2001, vertical tunes of bothrings were moved from just above the integer resonance toεx (nm)βx∗ /βy∗ (m)bunch current
(mA)

LER
18(18)
0.59/0.0065(0.33/0.010 )
880(2600)

1154(5000)# of bunches
bunch current(mA)bunch spacing(nsec)
bunch length(mm@MV)ξx /ξyνx /νyLifetime(min@mA)
Luminosity(/cm2 /sec)

HER
24(18)
0.63/0.0065(0.33/0.010 )715(1100)

0.76(0.52)

0.62(0.22)
8(2)5.5@6.0(calculation)
0.064/0.046(0.039/0.052)
45.51/43.58(45.52/44.08)
160@800

5.7@11.0(calculation)0.052/0.029(0.039/0.052)
44.53/41.59(44.52/42.08)
300@700

4.32 × 10(1.0 × 1034 )33

Table 1: Present performance compared with the design.
(Values in parentheses are the design values.)

just above the half interger resonance. Before the change,
the tunes are very near to the design values. These design
tunes were determined by strong-weak beam-beam simulations. Recently, we newly made a tune survey with a stronstrong simulation code[6]. Results of the simulations predicted that the vertical tunes above the half interger resonance give a better luminosity than the design tunes[7]. Ac-3547

Proceedings of the 2001 Particle Accelerator Conference, Chicago
modes existed and they need HOM dampers. Acceptable
power limit of the HOM dampers resticted the HER beam
current. In the new version, a masking function is realized
by deforming the vacuum chamber itself. Since there is notrapped mode, they need no HOM dampers. After replacing
the masks, we could increased the HER beam current andthe luminosity also increased. The luminosity jump in theend of April as shown in Fig. 1 was brought by this beam
current increase.

3

Figure 2: Effect of solenoid magnets on the luminosity.

24 bucket spacing

4 bucket spacing
24 bucket spacing

Figure 3: Comparison of the luminosity with different
bunch spacing.
cording to those results, we changed the vertical tunes. Just
after changing the tunes, the luminosity did not increase somuch. However, the stability of the beam operation was improved due to less large orbit drifts. In the middle of March,
the fractional part of the LER (horizontal and vertical) tunes
were changed from (0.52,0.60) to (0.51,0.58). Although theamount of the tune change was small, the luminosity showssome jump as is shown in Fig.1.

2.3

The most natural way to increase the luminosity in the current situation of the KEKB is to increase the number ofbunches. As is mentioned above, the present filling scheme
is basically 4 RF bucket spacing. Although we have tried 3
RF bucket spacing several times, every time the specific luminosity with 3 RF bucket spacing was worse than that with
4 RF bucket spacing even in the beam current region wherethe single beam blowup in LER is not visible. We have not
yet understood the reason for this luminosity degaradation
with 3 RF bucket spacing.
At the present KEKB, the luminosity is not limited by
the beam current limitations in the sense that the luminosity
does not increase with a higher HER or LER beam current.
With a higher LER beam current, the luminosity saturation
comes from the single beam blowup from the electron cloudinstability. To suppress this instability, we will install moreselenoid coils in the LER in this summer. As for a longterm plan, we have a plan to replace vacuum chambers in
the ARC section with ante-chambers. Also we have a plan
to exchange the electron and positron beams between thetwo rings, which means that the positron beam current canbe decreased for the same luminoisty. With a higher HER
beam current, the luminosity saturation comes maybe fromthe beam-beam blowup of the LER beam. To mitigate thissituation, we will try to increase the HER emittance from
24nm to 30nm shortly. Another possibility for the luminosity improvement is to shorten the bunch length of the twobeams. We are preparing for a measurement of the LER
bunch length and for shortening it, since HOM heating ofSCC HOM dampers in HER prevents us from shorteningthat of the HER.

Beam Current

4

In the history of the KEKB, the beam currents have been
limited from many reasons which include the detector beam
background. Among those, the most serious limitation has
come from tolerance of several hardware components to ahigh beam current. We have solved those hardware problems mainly by replacing hardware components in question with the new ones with which the problems were fixed.
Most recently, the HER beam current was limited by theHOM power limit of movable masks for protecting theBelle detector from the beam background. In the middleof April, we replaced the movable masks with those of anew version. In the old version of the masks, some trapped

FUTURE PLANS

REFERENCES[1] K. Oide et al., PAC’99, New York, March 1999.
[2] K. Oide et al., Factories 99, Tsukuba, September 1999.
[3] Y. Funakoshi et al., EPAC 2000, Viena, June 2000.
[4] H. Fukuma et al., HEACC 2001, Tsukuba, March 2001.
[5] K. Ohmi and F. Zimmermann, CERN-SL-2000-015 (AP),May 2000.
[6] K. Ohmi et al., Phys. Rev. E49 751, 1994.
[7] Y. Wu, HEACC 2001, Tsukuba, March 2001.
[8] KEKB B-Factory Design Report, KEKReport957, June 1995.

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