COMMISSIONING STATUS OF THE KEKB LINAC
Y.Ogawa*, Linac Commissioning Group (LCG)#KEK, 1–1 Oho, Tsukuba, Ibaraki 305-0801 Japan

Abstract
After entering into the full-commissioning phase of theKEKB linac in May, 1998, we have established two linac
injection modes for the KEKB ring: an 8-GeV electron
beam and a 3.5-GeV positron beam, as well as beammodes for the rings of the synchrotron light sources. All
modes are easily switched for almost one minute without
any trouble. The linac beam characteristics have been
substantially improved by introducing several fine-tuning
procedures, while key features concerning stable
operation have been pursued by employing, for instance,energy-feedback loops. We report on the present status of
the linac commissioning while especially emphasising theoperational characteristics.

these modes requires a change of at least several hundredparameters: the magnetic field strength for the beam
optics and orbit, the accelerating rf phases andacceleration/standby modes of the klystrons for the energy
and energy spread, the feedback parameters for beamstability, the beam-monitor settings for beam diagnosis
and so on.
Table 1: Beam-mode specifications

1 INTRODUCTION
The KEK electron-positron linac is the injector for two
asymmetric rings (8-GeV electron and 3.5-GeV positronrings) of the KEK B-factory (the KEKB linac is named
just after this), as well as for two different rings (2.5-GeV
electron rings) of the Photon Factory (PF). It has been
pointed out that since beam-mode switching among
various kinds of beams should be quite frequent, perfect
reproducibility of each beam could be one of the key
issues for the successful operation of the KEKB ring [1],which requires frequent and speedy injection for highluminosity collisions.
Full-commissioning of the KEKB linac [2] was started inMay, 1998, while continuing ordinary injection into thePF ring. The principal goal of the full commissioning wasto accomplish stable operation for injection to the KEKB
rings as well as to assure efficient injection to two PFrings. The beam reproducibility has been pursued mainly
regarding three aspects: the establishment of reliablebeam-mode switching, various improvements of the beam
quality, and beam-stability issues. In the following, thepresent status of the linac commissioning is reported
while placing particular emphasis on these issues.

2 BEAM-MODE SWITCHINGIn Table I, two beam modes for KEKB and two modes for
PF are listed, showing a wide variety of beamcharacteristics: a beam energy of 2.5 to 8 GeV and a beam
charge of 0.3 to 10 nC. Beam-mode switching among

Energy8.0 GeV
3.5 GeV

PF1PF2

2.5 GeV
2.5 GeV

Charge1.2 nC, single bunch
10 nC for primary e+
0.64 nC for e , single bunch
0.3 nC, multibunch
0.5 nC, multibunch

2.2 Switching Beam ModesA beam-mode-switching panel (Fig. 1) has been preparedso that all of the parameters can be easily and quickly
changed among the beam modes. For the present, we areusing two electon guns for the KEKB and PF beams,
respectively, which was inevitable because we had tocontinue injection to the PF rings even during constructionof the KEKB linac [3]. This situation makes beamswitching between the KEKB and PF beams quite
complicated, though this situation will be improved in thefuture. Since the energy ranges from 2.5 to 8 GeV and thefield strength of quadrupole magnets must be
correspondingly changed, we have introduced a simple
standardization of the magnets so as to avoid any
hysteresis effects. This procedure mainly determines anoverall time for switching of almost 1 minute. Areliability check has been carried out not only byexamining the beam characteristics along the linac, butalso by actually injecting each beam into thecorresponding ring. It has turned out that beam switching,itself, in a short period is quite reliable, although a longterm drift of the beam characteristics has been observed(see section 4).
___________________*

2.1 Beam Modes

ModeKEKB e+KEKB e

Email: yujiro.ogawa@kek.jp
N. Akasaka, A. Enomoto, J. Flanagan, H. Fukuma, Y. Funakoshi, K.
Furukawa, T. Ieiri, N. Iida, T. Kamitani, T. Kawamoto, M. Kikuchi, H.
Koiso, S. Michizono, T. T. Nakamura, Y. Ogawa, Y. Ohnishi, S.
Ohsawa, K. Oide, K. Satoh, M. Suetake, T. Suwada#appropriate intervals in each section were installed andhardware-tested to measure the beam size [5]. Recently,
the software, which nearly automates the optics matchingas well as the beam-size measurement, has been
completed and beam-tested at the beam-transport line tothe KEKB ring. The results so far obtained are quitesatisfactory and will be applied to the above-mentioned
section at the linac.

Figure 1: Beam-mode switching panel.

3 IMPROVEMENTS OF BEAM QUALITY
3.1 Software Feedback Loop
Some software feedback loops have been introduced inorder to stabilise the gun energy, the powers of two subharmonic bunchers (SHB) and the beam energy and theorbits at the J-arc section (1.5 GeV) and at the end of thelinac, while hardware feedback loops are implemented forthe rf phase-lock system in the SHBs. Fig. 2 shows anexample of the feedback panels [4]. The time constant of
the software feedback is over several seconds. As a result,the medium-period drift during beam injection was greatlyimproved. A sudden large variation of the quantities,
however, which is discussed in the next section, can not
be cured by this feedback, simply because the dynamic
range is limited by various boundary conditions.

3.2 Wire Scanners for Matching
There are several regions where optics matching isnecessary: at the exit of the bunching section, before/after
the J-arc section and at the end of the linac. In order tofacilitate and automate this matching, sets of a wire
scanner system comprising at least three monitors at

Figure 2: Example of the feedback panels. (upper: control
panel, lower: status log)

4 STABILITY ISSUES
The beam-stability issues in our linac are somehow
complicated in that we must accomplish stable operationof different kinds of beams repeating a frequent beamswitching. A simple classification concerning the stability,
such as long- or short-term variations, is not clear, since
only one beam can not be continuously observed formonitoring. It may happen that the status of someequipment changes, even during beam switching. In thiscase, for instance, it is not easy to identify the sources ofvariations by observing just the status of different beams.
In this connection, various efforts to search for the sourcesof variation have been pursued through deriving
information from the status of not only the beams, but also
all equipment.

Another example of a new monitoring system is a fast
BPM detection system [7], in which data-taking at a beam
repetition rate of 50 Hz can be realized. A FFT-analysisof the data taken at a point of large dispersion in the J-arc
section suggests the existence of energy variations, rather
than simple orbit fluctuations, in the upstream sections. A
statistical treatment, such as a correlation analysis, for alldata taken by the above-mentioned system is being
applied in order to figure out what is happening in thelinac.

4.1 Search for Sources of Orbit Variation bySingular Value Decomposition (SVD)

Element of eigen vector

The technique of singular value decomposition (SVD) [6]
was applied to search for the sources of the orbit variationfrom the exit of the bunching section to the two upstream
sectors of the linac. The matrix comprises time sets of therow vectors consisting of the signals of many beamposition monitors (BPM) along the linac. Figure 3 shows
an example of the orbit patterns corresponding to thelargest singular value. It seems that the sources of orbitvariation originate just at the beginning of the linac. We
have tried to identify them by checking the corresponding
equipment, but have not yet succeeded.

Figure 4: Observed phase variation of the first klystron forthe bunching section (upper).

1
0.8
0.6

5 CONCLUSIONS

0.4
0.2
0-0.2
-0.4
-0.6
-0.8
-105

10

15

Spatial Pattern (Horizontal)

20

BPM number

Figure 3: Example of the orbit patterns corresponding to
the largest singular value by the SVD method.

4.2 Reinforcement of Stability-Monitoring
SystemIn order to identify the sources of variation, a stabilitymonitoring system is being prepared. One of the mostessential systems is an rf phase-monitoring system. For
the moment, this system works only in three upstream
sections, but has already been shown to be a quite
powerful tool for stability monitoring. For instance, it has
turned out that the phase of the first klystron for thebunching section sometimes varies by a few degrees (Fig.
4), possibly due to the environmental temperature.

Two linac injection modes for the KEKB rings and thosefor the Photon Factory rings have been established.
Beam-mode switching among four kinds of beams has
been successfully verified by injecting a beam into eachring, employing various beam-tuning procedures,including feedback loops. The operation experience so far
indicates that there are some stability issues to be figuredout in order to maintain a high-quality beam. Manyefforts are being made to identify the sources of variations.

6REFERENCES[1] K. Oide, in these proceedings.
[2] Y. Ogawa, “Commissioning of the KEKB Linac”, to be publishedin Proc. of LINAC98, Chicago, August 23-28, 1998.
[3] A. Enomoto, “Commissioning of the KEKB 8-GeV e- / 3.5-GeV e+
Injector Linac”, Proc. of EPAC98, Stockholm, June 22-26, 1998.
[4] K. Furukawa et al., in these proceedings.
[5] Y. Funakoshi et al., in these proceedings.
[6] Y. Ohnishi et al., private communication.
[7] T. Suwada, private communication.