TOWARDS RELIABLE ACCELERATION OF HIGH-ENERGY AND
HIGH-INTENSITY ELECTRON BEAMS
K. Furukawa and Linac Commissioning Groupy
High Energy Accelerator Research Organization (KEK)
Oho 1-1, Tsukuba, Ibaraki, 305-0801, Japan
Abstractof the beams.

The KEK electron linac was upgraded to 8 GeV for the
KEK B-Factory (KEKB) project. During commissioningof the upgraded linac, even while continuing SOR ring injections, we had achieved a primary electron beam with
10-nC (6:24  1010 ) per bunch up to 3.7-GeV for positron
generation. This could be classified as one of the brightest
S-band linac’s.
Since the KEKB rings were completed in December
1998, those 3.5-GeV positron and 8-GeV electron beams
have been injected with excellent performance. Moreover,
we have succeeded in switching among the high-intensity
beams for KEKB and beams for two SOR rings with sufficient reproducibility.
After commissioning of the KEKB ring started, welaunched a project to stabilize the intensity and qualityof the high-current beams furthermore, and have accomplished it while investigating every conceivable aspect.

12

The commissioning started at the end of 1997 using thefirst part of the linac just before completion of the upgradedlinac. In order to carry it, a task force called the linac commissioning group was formed, in which 7 persons from thelinac and 12 persons from the ring participated. This grouplater became a part of the whole KEKB accelerator commissioning group.
The beam was operated at the linac local control room atthe beginning. After completion of the KEKB rings theoperation rooms for the linac and the ring were mergedwith some computer network and video switch preparations. Part of the operation log-book has been recorded
electronically to facilitate communication between local
engineers and remote operators.

3

INTRODUCTION

The KEK B-factory (KEKB) project started in 1994 to
study CP-violation in B-meson decays with an asymmetricelectron-positron collider. The performance of the experiment depends on the integrated luminosity of KEKB, andhence the beam injection efficiency from the injector linac.
In order to achieve efficient full-energy injection, theoriginal 2.5-GeV electron linac was upgraded up to 8 GeV,
while enforcing the acceleration gradient by a factor of 2.5
and by extending the length of the facility by about 40%.
Because of the site limit, two linac’s with 1.7-GeV and 6.3GeV were combined using a 180-degree bending magnet
system to form a J-shape linac. Also the primary electron
beam was designed to be 10 nC per bunch to produce 3.5GeV positrons with 0.64 nC.
The upgraded electron/positron linac has been commissioned since the end of 1997, even while continuing injection to the Photon Factory (PF). We had overcome many
practical difficulties, and had already achieved most of thedesigned beam parameters[1, 2].
However, to pursue the capability of the linac and KEKB
to its utmost limit, we still continue to improve the quality

 e-mail: <kazuro.furukawa@kek.jp>
y Linac Commissioning Group: N. Akasaka, A. Enomoto, J. Flanagan,
H. Fukuma, Y. Funakoshi, K. Furukawa, T. Ieiri, N. Iida, T. Kamitani,
M. Kikuchi, H. Koiso, T. Matsumoto, S. Michizono, T. Nakamura,Y. Ogawa, S. Ohsawa, K. Oide, Y. Onishi, K. Satoh, M. Suetake andT. Suwada

COMMISSIONINGSTABILITY AND RELIABILITY

After commissioning of the KEKB ring started, we realizedthat it was necessary to manipulate the beam delicately andcontinuously in order to maintain the quality of the highintensity beams for a long term without degrading the injection performance. Thus, we have launched a project to stabilize the intensity and quality of the high-current beams.

3.1

High-Current BeamAt the beginning of the commissioning it was necessary to
make much effort to transport a 10-nC electron beam on
to the positron generation target. It was often difficult tomaintain the beam for more than one hour. Otherwise, local
bumps had to be made to cure beam instabilities, whichwere caused by a fluctuation of the accelerator equipment
and transverse wake-fields.
Such difficulties, however, were gradually resolved after
understanding the sources of the instabilities through careful beam studies, as surveillance systems were installed forrf systems and other equipment[4]. Since the commissioning had started before completing the whole linac, somepart of the accelerator equipment was not operated at the
optimum condition. The largest contributions to the instabilities came from many parameters in the pre-injector
section[3].
Thus, we had realized that it was important to study thetolerances of beams to each parameter. Table 1 shows someof those results.
Table 1: Tolerances of a 10-nC beam
Parameter
Gun high voltage
Gun timing
SHB1 (114MHz) phase
SHB2 (571MHz) phase
Buncher phase
Buncher power
Sub-booster-A phase
Sub-booster-B phase

Tolerance

 0.38 % 45. ps
 1.1 deg.
 1.3 deg.
 1.7 deg.
 0.47 % 3.5 deg.
 4.0 deg.

These tolerance values were obtained to keep 90% of the
maximum beam current at the positron production target bychanging only one parameter around a good set of parameters. Software to find a correlation was used in order toacquire these data[5].
For a long, term each parameter may drift independently.
If the room temperature changes, most parameters may correlate with it. Thus, while the above tolerance values aregood references to consider the beam stability, the parameters of the equipment have to be kept within much better
limits.
In order to stabilize the equipment parameters following
the above guidelines, stabilization software, which will be
described later, was implemented as well as hardware improvements.
After such a challenging effort, we achieved a primaryelectron beam with 10-nC (6:24  1010 ) per bunch up to
3.7-GeV for positron generation, without any loss at the180-degree bending system. This could be classified as oneof the brightest S-band linac’s.

3.2

Four Beam Modes

It was anticipated that it might degrade the performance ofthe linac to switch beams between four injection modes.
After a high-current beam was achieved, we sometimes
found that the beam parameters were not optimal. Actually, the beam parameters in the four beam modes are quite
different, as shown in Table 2.
Table 2: Beam Modes of the Linac

EnergyParticle
ChargeRepetition
RefillTime
Interval

KEKB
HER
LER
8 GeV
3.5 GeVee+1.28 nC
0.64 nC(10 nC)1
50 Hz
50 Hz

PF

PF-AR

2.5 GeVe
0.2 nC

2.5 GeVe
0.2 nC

25 Hz

25 Hz

1-2 min.
1 - 2 hr.

3-5 min.
24 hr.

3-5 min.
2 - 4 hr.

5-10 min.
1 - 2 hr.

The major challenging issues here were reproducibilityof the beams in one of four modes, the reliability of switching and the switching speed to improve the integrated luminosity.
In this area, software to switch the beam modes had beendeveloped since the beginning of the commissioning. In order to accomplish the above tasks, the software was refined,
especially in the magnet initialization for the reproducibility and in recovery of the equipment failures for the reliability. It can even be re-configured easily in several aspects
by an operator. The details are described elsewhere[6].
Using this enhanced software, the loss time caused bybeam mode switching was made negligible, and the beams
became well reproduced over frequent mode switches. The
switching time for the KEKB modes became 90 to 120 seconds, which is acceptable. Thus, it is not a major issue atthe linac any more.
There are several plans for experiments that use highenergy electrons in the linac. An example is the slow
positron facility for solid-state and particle physics[7].
While the priorities of these experiments are currently low,
new beam modes for them may be added to the routine operation if it is possible to solve new switching issues.

3.3

Beam Feedback Loops

Even with the efforts on beam stabilization and reliablebeam mode switching, it was sometimes necessary to tune
the equipment parameters delicately in order to maintainsome beam parameters in the long term. Only some experts could tune the beam and it took some time.
Simple feedback loops to limit energy fluctuations of
the beams had been installed since the beginning of thecommissioning[8]. Also the same software was applied
to stabilize equipment parameters, as already describedabove. It was also applied to stabilize the beam orbits.
More than 30 feedback loops have been installed and areworking depending on the beam modes. The details aredescribed elsewhere[6].
These feedback loops have improved short-term linac
stability, and have cured long-term drifts as well.

3.4

Beam Optics

In order to reproduce the beam well under different conditions, the beam optics along the linac must be understoodwell. We have investigated several aspects to find any discrepancy between the design and the real optics.
In order to measure the beam emittance well, both the
Q-magnet-scan method and wire scanners have been used
depending on the locations. The errors in energy gain evaluations along the linac were not small, unfortunately. We
are trying to refine it using a gain derived from the rf measurement, beam energy measurement by an analyzer magnet and a longitudinal wake-field estimation.
1 3.7-GeV primary electron beam.

Using such beam information, software systems weredeveloped to match the beam optics at the fixedenergy[9] and to re-match the optics after a rf-power reconfiguration[10]. They are used daily, although it does notcover the whole linac yet, since we have several matching
points along the linac.
The effect of the transverse wake-field is not small, especially with high-intensity beams, and it degrades the beamemittance and the stability. Evaluation and reduction of thewake-field effects have been tried with some success[11].
Quadrupole wake-field effects were also observed for the
first time[12].

4 OPERATION STATISTICS
With the help of the above-mentioned improvement, thelinac operation has become fairly reliable. The total operation time in FY 1999 was 7296 hours, which was greatly
increased owing to full KEKB operation[1]. The availability of the linac for injection was 99.0%, which have beenmuch improved.
The average intensity of the positron in spring 2000 was
0.62 nC, which is just less than the safety limit at the beamtransport line.

5 MORE CHALLENGES
5.1

Discharge in Accelerator Structures

The discharge in the accelerator structures at sections A1(buncher and the first normal structure) and 21 (positron
generator) became severe in March, 2000, where the beam
charge (and loss) is high and is surrounded by solenoid
coils. It was found that the discharge frequently occurred
near the trailing edge of the rf pulses.
Thus, the wave guides at these sections were re-arranged
to shorten the pulse width, and the rf-power was optimized
to the improved beams with lower voltage. Then, such adischarge decreased the rate to less than once a day.
Since it is important to understand the phenomena
deeply, a test stand for such stations was built for investigating discharge phenomena as well as for the conditioningof accelerator structures.

5.2

Two Bunch Acceleration

In order to double the positron beam charge, it is considered
to have two bunches in a linac rf pulse. Because of therf synchronization scheme between the linac and the ring,those bunches have to be separated by 96 ns at minimum2.
A preliminary study was made on this two-bunch
scheme, which produced promising results on the energy
compensation of the second bunch with a careful rf-pulse
timing control. The energy difference was estimated to be
2.5% for the 8-nC beam comparing the longitudinal wakefield with the one for a low-intensity beam. Devices for thisscheme are under preparation.
2 275th bucket in the linac and 49th bucket in the ring

6 CONCLUSIONSIn commissioning of the KEKB injector linac we haveovercome challenging issues and have accomplished a stabilization project, investigating every conceivable aspect.
The linac is providing fairly stable beams with very highavailability.
During normal operation, operators rarely change thebeam parameters. Instead, software for beam-mode switching and feedback loops takes care of them. Since the charge
limit at the beam-transport line induced by the safety reasons will be removed soon, the performance of the linac
may be more enhanced.
Throughout this improvement, we obtained valuable experiences on tolerance studies and stabilization techniqueof the timing and rf systems, especially at the buncher
section. We also gained knowledge concerning the physical phenomena of the beams particularly of an emittance
growth. They are indispensable for the design and construction of the next-generation accelerators, such as alinear collider, an FEL and an injector for super-highluminosity machines.

7

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[2] Y. Ogawa et al., “Commissioning Status of the KEKB
Linac”, Proc. PAC’99, New York, U.S.A., 1999, p.2984.
Y. Ogawa et al., “Commissioning of the KEKB Linac”,
Proc. LINAC’98, Chicago, U.S.A., 1998, p.711.
[3] S. Ohsawa et al., “Pre-injector of the KEKB Linac”, Proc.
EPAC2000, Vienna, Austria, to be published.
[4] H. Katagiri et al., “RF Monitoring System in the Injector
Linac”, Proc. ICALEPCS’99, Trieste, Italy, 1999, p.69.
[5] K. Furukawa et al., “Accelerator Controls in KEKB Linac
Commissioning”, Proc. ICALEPCS’99, Trieste, Italy, 1999,
p.98.
[6] K. Furukawa et al., “Beam Switching and Beam Feedback
Systems at KEKB Linac”, these proceedings.
[7] T. Shidara et al., “The KEK-PF Slow-Positron Facility at a
New Site”, Proc. LINAC’98, Chicago, U.S.A., 1998, p.309.
[8] K. Furukawa et al., “Energy Feedback Systems at KEKB
Injector Linac”, Proc. ICALEPCS’99, Trieste, Italy, 1999,
p.248.
[9] N. Iida et al., “Recent Progress of Wire Scanner systems
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Proc. EPAC2000, Vienna, Austria, to be published.
[10] T. Kamitani et al., “Optics Correction for Klystron Switching at the KEKB Injector Linac”, Proc. EPAC2000, Vienna,
Austria, to be published.
[11] S. H. Wang et al., “Simulations of Wake Effects on a HighCurrent Electron Beam at the KEKB Injector Linac”, KEK
Report 2000-4.
[12] Y. Ogawa, “Quadrupole Wake-Field Effects in the KEKB
Linac”, Proc. EPAC2000, Vienna, Austria, to be published.