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

iBUMP FEEDBACK TUNING FOR KEKB ACCELERATOR
T. Aoyama, H. Iida, T. Kawasumi, T. Kitabayashi,Y. Satoh, H. Shimizu, M. Tanaka, K. Yoshii,
Mitsubishi Electric System & Service Engineering Co. Ltd.,
2-8-8 Umezono, Tsukuba, Ibaraki 305-0045, JapanandY. Funakoshi, N. Iida, H. Koiso, M. Masuzawa, Y. Ogawa, K. Oide,High Energy Accelerator Research Organization (KEK),Oho 1-1, Tsukuba, Ibaraki 305-0801, JapanAbstract
KEKB accelerator is an asymmetric electron-positron
collider, which has been designed as a high luminosity
machine. The iBump feedback system is one of thecollision tuning tools. It turned out that the luminosity isvery sensitive to the targets value of the iBump feedback.
We report an operational history of the iBump feedback
system with our trials to realize higher luminosity for theKEKB accelerator.

By controlling the horizontal offset (H-offset) of theiBump feedback system, the LER beam size and theluminosity change. If we allow the H-offset to fall below
a lower limit, the LER horizontal beam size ( a x) and thevertical beam size ( a y) uncontrollably increase and welose luminosity. We call this phenomenon Egure. As an
opposite case, if we allow the H-offset to exceed an
upper limit, the LER a x and the a y uncontrollably
decrease and the luminosity decreases again. We call thisphenomenon H azure.

1 INTRODUCTION
Construction of the KEKB accelerator [1] was finishedin November 1998 and physics experiments started inJune 1999. Since then the luminosity has beensuccessfully increasing. Up to now, we have 4.49 X 1033
cm-2 s- 1 as the peak luminosity.
To realize such high luminosity, we have applied manytuning methods for collision tuning. The iBump feedbacksystem [2] is one of such tuning methods to control theHER beam orbits at the interaction point (IP): there are
two rings in the KEKB accelerator; the high energy ring(HER) for electrons and the low energy ring (LER) for
positrons. The iBump feedback system is working tokeep an appropriate difference of the two beam positions,called offset, by making orbit bumps in the HER near theIP.
We have tried to find the best target value of theiBump feedback to reach high luminosity. However,there are many parameters that affect the luminosity, thus,
the iBump target and the luminosity depended on each
operator's tuning manner.
In this paper, we report a brief history of the iBump
feedback tuning with our trials to realize higher
luminosity for the KEKB accelerator.

2 EGURE AND HAZURE
Now, we introduce two terms, Egure and Hazure.
Figure1 is the iSize feedback monitor [3] to control theHER beam size at the IP. This panel shows a correlation
between the beam size and the specific luminosity(luminosity/number of bunches/bunch current product).

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Figure 1: iSize feedback monitor.

3 HISTORY OF THE iBUMPFEEDBACK TUNING

3.1 iBump Target Tuning
Until December 2000, the H-target value was the maintuning parameter for the iBump feedback system.
Figure2(a) shows a correlation of the H-target and thespecific luminosity. We were tuning the H-target
manually to get high specific luminosity by using thispanel. Through this manual tuning, we found that anoptimum value of the H-target unexpectedly changed
with the beam current; we call this the target-current
problem.

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close to the Egure limit (lower limit), and we must try toset the H-offset near this threshold by using informationon the· beam size and the luminosity. This. tuning is very
difficult because we must set the H-offset very near to thedangerous Egure limit below which we encounter theunwanted Egure.
Figure 2: iBump monitor.

Egure-Hazushi

Offset Up Offset Down

3.2 Programmable H-Target Changer
From December 2000 to May 2001, we used a
programmable H-target changer (PHTC) to avoid thetarget-current problem. Figure3 shows the PHTC panel.
The PHTC application refers to the LER beam current
and changes the H-target value via an H-target curvesemi-automatically: i.e., an appropriate H-target curvefor each experiment was made by manually. Although,the PHTC was successful in keeping the high luminosity,
we sometimes could not apply the previous H-target
curve to the next run without some modifications. If weuse the previous curve, the PHTC leads to Egure and/orHazure, because the condition of the · accelerator
unpredictably change at each experiment. Thus, thePHTC needed a manual real-time modification for eachrun.

Figure 4: iSize Feegback Monitor.
If an Egure occurs, we usually intentionally make theHazure state by setting the H-offset near the threshold toget the high luminosity again. we call this recovering
method Egure-hazushi, see Fig.4. FigureS shows the Hoffset tuning panels. The entry field for H-offset at IP in
the iBump FB Bump Height panel and/or the Suppress
Egure buttons are available to set the H-offset directly.

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Figure 5: H-offset tuning panels.

Figure 3: Programmable H-Target Changer.

3.3 H-Offset Direct Tuning
In May 2001, we started to tune the H-offset valuedirectly instead of the H-target. Figure 4 shows the iSize
feedback monitor panel. If we decrease the H -offset, thebeam size increases and then the luminosity alsoincreases. However, if we allow the H-offset to fall
below the lower limit, an Egure occurs and we lose theluminosity. If we manually increase the H-offset, both thebeam size and the luminosity decreases, and when the Hoffset exceeds the upper limit, we will encounter aHazure. Therefore, the best tuning point exists at very

By using the H-offset direct setting method, thenumber of Egure and Hazure phenomenon are
successfully decreased. Although some operators could
keep the H-offset near the threshold and could master theEgure-hazushi procedure, most operators were not able
to do the good tuning. The main reason for this problem
was a lack of a clear guideline for the H-offset tuning.

3.4 Guideline for H-Offset Setting
To improve the above problem, we sought bettermethods for the H-offset tuning. Finally, we found that akick angle of the ZHQC2LE (KRB), which is one of thesteering magnets near the IP, is related to the H-offset.
The KRB of the ZHQC2LE is synchrono1,1sly varying
with the H-offset value. This steering magnet, ZHQC2LE,-308-The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

is controlled by the continuous COD correction system[4].

Figure 6 shows the KRB, the H-offset and theluminosity. This figure says that there is a possibility ofthe existence of the best ratio between theZHQC2LE:KRB and the H-offset. Indeed, if the ratio ismuch different from. an optimum value, Egure or Hazure
usually occurs and we loose the luminosity. Here we notethat the peaks of the H-offset in Fig. 6 came from beaminjections.

H-OFFSET

To avoid such a bad situation, we have constructed autility tool called the H-offset easy feedback. This tool
keeps the appropriate ratio between the KRB and the Hoffset automatically. The H-offset easy feedback control
panel is shown in Fig. 8.

ZHQC2LE :KRB

Figure 8: H-offset easy feedback panel.

Figure 6: Relation between the KRB of the ZHQC2LE,
the H-offset value and the luminosity.

3.5 H-Offset Easy Feedback System
Now we have a guideline for direct tuning of the Hoffset. As we mentioned before, there is a correlation
between the KRB and the H-offset. To realize highluminosity, an operator should change the H-offset
according to the variation of the KRB: i.e., if the KRB isincreasing then an operator should increase the H-offset.
Mter we applied this guideline, the number of the Egureand the Hazure phenomena has been drasticallydecreased.
Although, this tuning guideline is quite clear and easy
to understand for operators, it is not the best method
because it requires operator's constant attention. The
variation of the KRB is very fast so an operator must
keep his eyes on the monitor panel and he must change
the H-offset value very quickly. Thus; an operator had noway to tune the other accelerator parameters if hechanged the H-offset manually. Figure7 shows the KRBOffset relation monitor and the H-offset direct tuningpanel.

Owing to this tool, the operator's task became very
short for the H-offset tuning. Mter an operator tuned theappropriate H-offset value at the beginning of theexperiment only once, the H-offset easy feedback toolchanges the H-offset value to keep the very stable highluminosity instead of the operator. As a result, now wehave time for tuning the other accelerator parameters.

4SUMMARY
Higher luminosity of the KEKB accelerator has beenpossible with the progress of iBump feedback tuning. To
realize a stable iBump feedback, we have continuously
improved the iBump tuning method. Now, we have aguideline for the iBump feedback which is useful to keep
the high luminosity without Egure or Hazure.
We continue to make efforts to reach the designedluminosity, 1034 cm-2s- 1, for the KEKB accelerator.

5 ACKNOWLEDGMENTS
The authors are grateful to the KEKB commissioning
staff for useful comments and discussions to tune theiBump feedback system.

REFERENCES[1] "KEKB B-Factory Design Report", KEK Report 957, 1995.
[2] M.Masuzawa,et.al.,"IP ORBITAL FEEDBACK FOR
COLLISION TUNING AT KEKB", EPACDO, June
2000.
[3] N.iida,et.al.,''Luminosity Optimization by Controllinga Beam Size Ratio at KEKB", Proceedings ofHEACC200 I, March 200 I.
[4] K.Oide,et.al.,''COMMISSIONING OF THE KEKB
B-FACTORY", in Proceedings of Factories'99, KEK
Proceedings 99-24, pp.12-16, September 1999.

Figure 7: KRB-Offset relation monitor (left) and H-offset
direct tuning panel (right).

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