WEPLS138

Proceedings of EPAC 2006, Edinburgh, Scotland

OPERATION STATUS AND STATISTICS OF THE KEK
ELECTRON/POSITRON LINAC
Y. Ogawa#, A. Enomoto, K. Furukawa, T. Kamitani, M. Satoh, T. Sugimura, T. Suwada, Y. Yano,
K. Yokoyama, M. Yoshida, KEK, Tsukuba 305-0801, Japan
Y. Imai, T. Kudou, S. Kusano, K. Suzuki, T. Toufuku, MELCO SC, Tsukuba 305-0801, Japan
Table 1: Present beam characteristics.

Abstract
The KEK electron/positron linac has been operated
since 1982, surpassing the total operation time of morethan 120,000 hours. It presently delivers four different
beams to four different rings quite stably, even frequently
switching beam modes. Operation status and statistics isreported with the emphasis on continuing efforts invarious kinds of machine improvements, which haveensured the stable operation. Recent on-going upgrade
programs are also presented.

INTRODUCTION
The total operation time at the KEK electron/positronlinac has reached more than 120,000 hours in FY2005
since FY1982, when the first 2.5-GeV electron beam ofabout 30 mA was injected into the photon factory (PF)ring.
The linac has so far experienced extensive
renovations twice: one for TRISTAN and the other for
KEKB [1]. The required beam characteristics such asenergy, intensity and time structures have become quite
diverse as well as the operation time per year was
drastically increased at each project. Fig. 1 shows thelinac history in operation time and machine failures,
which will be analyzed in this report, relying on statistics
and our continuing efforts in machine improvements.
Table I lists the present properties of the beams delivered
to different rings, indicating that the linac operation modeis quite complicated. In Table I, KEKB LER has two
parameters in each cell, since those of a primary highintensity electron beam are also shown. The recent
requirements of the continuous injection mode (CIM) for
KEKB and the top-up injection mode for PF have pushed
further the linac to take an unprecedented operation
mode: a quasi-simultaneous injection to all rings [2].

Operation Time [hrs]Total Operation Time [x10 hrs]
Machine Failure [%]

12000

KEKB
LERPFPFARParticlee-e- / e+e-e-Energy [GeV]

8.0

4.0 /3.5

2.5

3.0

Intensity [nC]

1x2

10 / 1 x 2

0.1

0.1

# of Bunch

1x2

1x2

3-4

3-4

Repetition [Hz]10 - 50

10 - 50

2525

Injection /Day> 250

> 500

1-22

OPERATION STATUS
The typical daily operation status of KEKB is shown inFig. 2, indicating that the beam currents of HER and LER
are kept almost constant (CIM) by frequent injections,while small decays of the currents seen four times per day
reveal injection periods for PF and PF_AR. The linac
eventually undergoes full-time injections to four rings.
Presently four beam modes are switched so that allsettings of magnets, rf phases and beam timings arechanged according to the beam properties listed in Table I.
As a consequence, the minimization of a beam-mode
switching time and the realization of its reproducibility
have been among the most demanding issues for the linac
to assure a stable and reliable operation. Fig. 1 shows thatthe machine-failure rate significantly rose at thebeginning of the KEKB, but afterwards it has dropped
quite steadily. More detailed operation statistics arepresented in the following section together with somebrief interpretations, manifesting a good performance ofthe linac.

100007658000

46000

34000

22000

Machine Failure [%]Operation Time [hrs]

KEKB
HER

814000PF AR

PF AR1TRISTANPFKEKB

0

1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005

0Ring Name

FY

Figure 1: Operation history.

Figure 2: Typical KEKB daily status.

______________________________________________#yujiro.ogawa@kek.jp2700

07 Accelerator Technology
T21 Reliability, Operability

Proceedings of EPAC 2006, Edinburgh, Scotland

OPERATION STATISTICS
Fig. 3 shows a detailed operation history with timebreakdown structures, indicating a sudden increase of theKEKB injection time in FY2004 when CIM started. The
beam mode switching time, however, still accounts formore than 30 % of the yearly operation time because of
frequent mode switching, which is a major issue for ourlinac as mentioned in the previous section.
Seven-years statistics on failure rates are shown in Fig.
4; the failure includes all kinds of machine troubles, while
the injection-delayed time is a part of the failure time,causing a real injection delay, and the rf trip rate isparticularly quoted here to give a hint about the stabilityof the rf system and the accelerator sections. Failure rates
have satisfactorily diminished, indicating that the machine
availability is reaching about 98 % in FY2005.

WEPLS138

RF system
Since the reliability of an rf system is directly linked to
the linac availability, a good deal of effort has been madeespecially since KEKB started. Performances of the rfsystem, comprising 60 klystrons and modulators with
low-level rf and timing systems, have been maintained invarious aspects:•
Improve the robustness of the system itself.
•Prepare every spare, which should be easily
exchangeable when broken down.
•Predict the component failure so as to repair or
exchange in a more organized or scheduledmanner.
A typical and outstanding example of the last one,which has been utilized on a routine base, is a klystrondip test [3]. This is one of the most rapid diagnostic
methods developed at our linac for checking klystronemission performances in only several minutes. The dip
test is usually carried out for stand-by klystrons evenduring operation and for non stand-by ones in the shortperiod of maintenance scheduled every other week so thatany signs of klystron deterioration could be easily found.
For the suspected klystron, a usual emission measurement(>1 hr) is conducted and the time line of the klystronexchange is determined looking over the whole machine
operation schedule.
This maintenance criterion has
greatly helped the reduction of MTTR in the rf system.

Figure 3: Detailed operation history.

Figure 5: # of failures in subsystems (FY2005).

Figure 4: History of failure rates.
Although categorization of machine failures should bea delicate issue, a tentative classification into several
items is attempted and a recent example of # of failures insubsystems (FY2005) is shown in Fig. 5. A history ofMTTR (Mean Time To Repair = Downtime / # ofFailures) in subsystems is given in Fig. 6, showing
satisfactory improvements over the last few years. Sometypical examples that have considerably contributed tothis amelioration are discussed in the following.
Figure 6: History of MTTR in subsystems.

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WEPLS138

Proceedings of EPAC 2006, Edinburgh, Scotland

Accelerator Section

Positron Energy

The accelerator sections used in high-energy linacs,
which are usually exposed by high electromagnetic fields,
sometimes suffer unexpected rf breakdowns so that thelinac operation might be stopped. Although it is obvious
in this case that an appropriate treatment is essential for
efficient recovery, practical procedures had not been
optimized for recent severe operation conditions such as
CIM in KEKB; for instance, the rf recovery must be
carried out quite steadily without any serious operationinterruptions even during injections. Various kinds ofsophisticated auto-recovering systems have beendeveloped for this purpose, and the number of the rf
breakdown rate (rf trip rate) has decreased year by year
(Fig. 4), indicating the fact that the optimal care for rf
breakdowns is particularly crucial to a stable operation ofhigh-availability machines.

Since the length of the positron acceleration is limitedin space, it was decided that an acceleration gradient be
doubled by introducing C-band accelerator sections withappropriate rf sources and power supplies in place of thepresent S-band system. The basic R&D and the beam
acceleration test have been finished, accomplishing an
acceleration gradient of about 40 MV/m [4]. System
integration and rf aging are currently being pursued inorder to get sufficient reliability for stable operation.

Control System
The linac control system covers a broad range of items:
those directly influencing on the beam operation such as
feedback systems, and those indirectly sustaining a
reliable operation as well, which, for instance, comprisethe system redundancy.
A large number of feedback systems have been
implemented to stabilize not just the beam orbit andenergy but also the rf phases and timings; for example, afeedback of electron-gun timings is essential for stable
operation of a high-intensity electron beam for positron
production. The feedback systems, which must work
differently depending on various kinds of beams listed inTable I, are also the keys to assuring good reproducibilityof the beam mode switching.
The system redundancy is usually introduced within
limited resources of money and manpower after having
reached a certain level of reliability for each component
constituting the control system.
Various kinds ofredundant systems including file servers and networks
have been accommodated in our control system. The
emphasis of the linac, however, has been especially on notexcessively depending upon the manpower: even if nocontrol experts are present at control-system troubles, thesystem should almost automatically recover without
serious disturbances to the machine operation. The
improvement in MTTR for the control system shown inFig. 5 could represent the fruit of these efforts.

UPGRADE AND R&DSuperKEKB, a major upgrade of KEKB was proposedin 2004 and many relevant R&Ds have been carried out.
Main requirements for the injector linac are positron
beam upgrades of energy, intensity and emittance: theenergy from 3.5 to 8.0 GeV, the intensity twice as highand the emittance reduction equivalent to the present
electron beam, while items indispensable for realizationof these requirements are the system renovation of beamdiagnosis and operations. Several issues related to these
items are briefly described in the following.

2702

Positron Intensity
In the design of positron production system, a flux
concentrator would be implemented just after the positrontarget and the R&D has already started in collaboration
with a Russian group. A multi-bunch scheme has recently
been proposed to increase the total positron charge and a
preliminary experiment was conducted in the case of
three-bunch acceleration by applying a waveform
flattening technique to SLED input signals so as toequalize the energy of three bunches. The principle of thenew scheme was experimentally verified by accelerating abeam of a few nano Coulomb to the positron target [5].

Operation ModeIn SuperKEKB, a quasi-simultaneous injection isessential for efficient operation, in which, in principle, allkinds of beams must be accelerated with the same optics:
common magnet settings but different acceleration phases
for different energy. This multi-energy scheme [6] hasbeen designed and the beam test was successfully carriedout, though still requiring further machine studies such as
those concerning a common-orbit correction method.

CONCLUSIONS
Operation status and statistics of KEKB is presented,
verifying good machine availability. Recent on-going
linac upgrade programs for SuperKEKB are outlined.

REFERENCES[1] I. Abe et al., “The KEKB injector linac”, Nucl. Instr.
and Meth. A 499 (2003)167.
[2] M. Satoh, “The KEK linac upgrade for the fast beammode switch”, these proceedings.
[3] K. Nakao et al., “Results of a high-power klystrondip test in the KEK linac”, PAC’05, Knoxville, USA,May 2005, p. 3235.
[4] T. Kamitani et al., “Status of C-band accelerating
section development in KEKB injector linac”, to be
published in Proc. of LINAC’06, Knoxville, USA,
August 2006.
[5] M. Yoshida et al., “Study on high-current multibunch beam acceleration for KEKB injector linac”,
ibid.
[6] Y. Ohnishi et al., “Design and performance of optics
for multi-energy injector linac”, ibid.

07 Accelerator Technology
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