The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

HIGH LUMINOSITY OPERATION OF THE KEKB
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. Taw ada, M. Tejima, M. Tobiyama, S. Uno, Y. Wu t,
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 (200 1) 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].

1 PRESENT 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 accumulated
luminosity 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 luminosity 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
t visiting from IHEP, China+visiting from CERN, Switzerland

low value of the vertical beam-beam parameter of the LER
is also explained 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 design values.

2 RECENT PROGRESS
2.1

Electron 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 /cm 2 jsecjmA 2 •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. The
figure 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 with-165-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

Figure I: History 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 instability 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 and24 RF bucket spacing cases. Combining the results of thetwo experiments, it could be concluded that the solenoid
magnet wound January 2001 removed the main effect of theelectron cloud instability in the beam current region used 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 operation with 4 RF bucket spacing,
the luminosity shows saturation to an increase of the LER
beam current. In the 24 RF bucket spacing case, however,the specific luminosity stays almost at the same level evenwith much higher LER bunch currents, which is shown alsoin Fig. 3. In the region where the bunch current product islarger than 4.5 rnA 2 , 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 toex (nm)!3;//3; (m)bunch current
(rnA)

1154
(5000)#of bunches
bunch current(rnA)
bunch spacing(nsec)
bunch .length(mm@MV)ex/eyllx/VyLifetime(min@mA)
Luminosity(/cm 2 /sec)

HER
24
(18)
0.63/0.0065(0.33/0.0 10 )715
(1100)

LER
18(18)
0.59/0.0065(0.33/0.010)
845(2600)

0.62(0.22)

0.73(0.52)
8(2)6"'8@6.0(measurement)
0.069/0.053(0.039/0.052)
45.51143.58(45.52/44.08)

5.7@ 11.0(calculation)
0.048/0.030(0.039/0.052)
44.53/41.59(44.52/42.08)

160@800

300@700

4.49 X 10""(1.0 X 1034 )Table l : Present performance compared with the design.
(Values in parentheses are the design values.)

just above the half integer 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 strongstrong simulation code[6]. Results of the simulations predicted that the vertical tunes above the half integer resonance give a better luminosity than the design tunes[?]. Ac--166-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

modes existed and they need HOM dampers. Acceptable
power limit of the HOM dampers restricted 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 BEAM-BEAM PARAMETERS

Figure 2: Effect of solenoid magnets on the luminosity.

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.l.

2.3 Beam Current
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

The beam-beam parameters calculated from the luminosity
are listed in Table 1. In the calculation, we assumed thatthe vertical beam sizes of the two beams are equal, since we
use so-called "iSize feedback" system. This system aims atmaximizing the luminosity by controlling the vertical emittance of the stronger beam (usually HER). It is also assumed
that there is no beam-beam blowup in the horizontal direction, since we do not observe serious beam size blowup in
the horizontal direction. The"hourglass" effect from a finite
bunch length and degradation of the beam-beam parameters due to a finite crossing angle are also considered. Asfor the bunch length, 7mm is assumed. As is seen in Table 1, the vertical beam-beam parameter of the HER is notably low. This is also explained by the single beam blowupof the LER from the electron cloud instability. To confirm
this, an experiment with longer bunch spacing was done. In
this experiment, the bunch spacing was 24 RF buckets andno single beam blowup was observed with this bunch spacing. Beam-beam parameters obtained in this experiment islisted in Table 2 together with those in the usual bunch spacing of the 4 RF buckets. As was expected, the HER vertical beam-beam parameter with 24 RF bucket spacing ("'48nsec) is larger than that with 4 RF bucket spacing. In
this experiment, the luminosity saturated at a higher LER
bunch current compared with the case in usual operation.
This indicates that the luminosity saturation with the LER
current in the usual operation comes from the LER single
beam blowup. In this experiment, a much more serious
HER beam blowup than in the usual operation was observed
when the LER beam current increased. We had very shorttime to search parameters for suppressing the HER blowup
in the experiment. It seemed that the HER blowup could
be suppressed with more machine tuning and higher beambeam parameters could be achieved.
Another feature of the KEKB parameters is that theworking points are close to the half integer resonance as
is shown in Table 1. Particularly the horizontal tunes arevery near to the resonance. In this situation, of importance is an effect that the beta function and the emittance
are affected by the beam-beam force (dynamic-beta anddynamic-emittance). These effects have been studied byusing the SAD code. In the simulation, only the linear partof the beam-beam kick was taken into account. Fig. 4
shows a result of the simulation. In the figure, the beta
function of the LER at the IP and the horizontal emittance-167-The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

~xf~y (LER)~x/~y (HER)~x/~y (LER)~xf~y (HER)

spacing
8 nsec8 nsec48 nsec
48 nseccase 1
0.069/0.053
0.048/0.030
0.06110.033
0.075/0.038

case 20.017/0.052
0.018/0.029
O.Q18/0.033
0.028/0.035

Table 2: Beam-beam parameters with and without thedynamic- /3 effects. In the case 1, the effects of the dynamic/3 are ignored. In the case 2, we count the effects of thedynamic-/3.

Figure 4: Calculations of the dynamic beta and dynamic
emittance effects with a horizontal tune of 45.51 as function of the (nominal) horizontal beam-beam parameter. Thecalculation was done by using the SAD code. The solid line
shows a beta calculation from an analytical calculation using a simple one-turn transfer matrix.

are depicted as function of the (nominal) horizontal beambeam parameter. Also shown in the figure is the beta function which is calculated by an analytic calculation. The beta
function can be calculated by estimating a change of an onetum transfer matrix with the beam-beam kick. The analytic
calculation well agrees with that from the SAD code. As
is seen in the figure, the horizontal beta function at the IP
drastically shrinks with the horizontal tune of 45.51 and theemittance is enlarged to a large extent. What one shouldnote here is that a change of beam size in one beam dueto the beam-beam effect brings an additional dynamic beta
and dynamic emittance effect to the other beam. Therefore,
the effect should be solved consistently for the two beams.
This effect is also included in the calculation. As a result of
these changes, the horizontal beam size at the IP decreases
to a some extent and the horizontal beam-beam parameterdecreases as is also shown in Table 2. In the KEKB, there isa tendency that the closer horizontal tune to the half integer
resonance brings a higher luminosity. This tendency seems
to be explained by the dynamic beta and dynamic emittanceeffects. These effects also explain why we can reach extremely high beam-beam parameters in the usual sense in
the horizontal direction shown in Table 1.

4 FUTURE PLANS
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 spadng 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 where
the single berulll blowup in LER is not visible. We have notyet understood the reason for this luminosity degradation
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 1mppress this instability, we will install moresolenoid 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 luminosity. 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 from24om to 30om 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 of
SCC HOM dampers in HER prevents us from shorteningthat of the HER.

5 REFERENCES[1] K. Oide et all., 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),May2000.
[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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