ACHIEVEMENT OF 200,000 HOURS OF OPERATIONAT KEK 7-GeV ELECTRON 4-GeV POSITRON INJECTOR LINAC
K. Furukawa∗ , M. Akemoto, D. Arakawa, Y. Arakida, H. Ego, Y. Enomoto, T. Higo, H. Honma,
N. Iida, K. Kakihara, T. Kamitani, H. Katagiri, M. Kawamura, S. Matsumoto, T. Matsumoto,
H. Matsushita, K. Mikawa, T. Miura, F. Miyahara, H. Nakajima, T. Natsui, Y. Ogawa, S. Ohsawa,
Y. Okayasu, T. Oogoe, M.A. Rehman, I. Satake, M. Satoh, Y. Seimiya, T. Shidara, A. Shirakawa,
H. Someya, T. Suwada, M. Tanaka, D. Wang, Y. Yano, K. Yokoyama, M. Yoshida, T. Yoshimoto,
R. Zhang, X. Zhou, KEK, Tsukuba, Japan
Y. Bando, SOKENDAI, Tsukuba, Japan
2.5 GeV INJECTION FOR PF/TRISTAN

Abstract
KEK electron positron injector LINAC initiated the injection operation into Photon Factory (PF) light source in 1982.
Since then for 39 years, it has served for multiple projects,
namely, TRISTAN, PF-AR, KEKB, and SuperKEKB. Its
total operation time has accumulated 200 thousand hours on
May 7, 2020. We are extremely proud of the achievement following continuous efforts by our seniors. The constructionof the injector LINAC started in 1978, and it was commissioned for PF with 2.5 GeV electron in 1982. In parallel, thepositron generator linac was constructed for the TRISTAN
collider project. The slow positron facility was also commissioned in 1992. After the KEKB asymmetric-energy collider
project was commissioned in 1998 with direct energy injections, the techniques such as two-bunch acceleration andsimultaneous injection were developed. As the soft structure design of the LINAC was too weak against the greateast Japan earthquake, it took three years to recover. Thenthe construction and commissioning for the SuperKEKB
project went on, and the simultaneous top-up injection into
four storage rings contributes to the both elementary particle
physics and photon science.

INTRODUCTION
In the experimental particle physics research in the 1970s,
there was growing expectation for the world-class Japanese
domestic collider after the successes in the world-level theoretical research. On the other hand, in the field of synchrotron radiation science, there were demands for the establishment of a synchrotron radiation research institute with adedicated accelerator. In order to realize both of those accelerator projects, a 400-m electron linear accelerator capableof 2.5 GeV direct energy injection was constructed in 1982.
Since then, the injector had been in operation for 38 years
in 2020, supporting the consecutive accelerator projectsof Photon Factory (PF), TRISTAN, KEKB, PF-AR, andSuperKEKB as depicted in Figs. 1, 3, 5 and 2. It has finallyachieved 200,000 hours of operation in May 2020. The brief
operation history would be given.
∗ kazuro.furukawa@kek.jp

At the time of the initial construction since 1978, several
university-based accelerator facilities had achieved research
successes in Japan. It was fortunate that a new linear electron
accelerator had been built to share resources for advanced
scientific research in both particle physics and synchrotron
radiation science, which were based on the accumulated
experiences at universities. The situation might be similar torecent J-PARC’s focus on both nuclear and particle physics,
as well as neutron and muon science. It seems that there
are not many accelerator projects in the world that havemultiple disciplines from the beginning. Those Japaneseaccelerators are quite unique that deliver beams for multiple
science fields.
While many researchers of synchrotron radiation researchwanted the institute independent from particle physics, theaccelerator science played a role as a bridge between those
two. One hundred sixty of 2-m-long S-band traveling-wave
accelerating structures with 2𝜋/3 mode quasi-constant gradient were installed, and driven by forty 20-MW klystrons
to achieve 2.5 GeV electron and positron beams [1]. 2.5
GeV direct energy injection was made for PF and 2.5 GeVelectron positron beams from the injector were accelerated
through the TRISTAN synchrotron chain for the 32 GeV
collision as depicted in Fig. 1.
When the synchrotron radiation facility, PF, was completed, there must be an enthusiastic response to the dedicated synchrotron radiation facility that would promoteresearch in synchrotron radiation science. The TRISTAN
project also began to take the lead in high-energy experimental physics, after Japanese theorist had contributed toTRISTAN
32 GeV/cAR
8 GeV/ce- BT

Linac delivered:for PF: 2.5 GeV e–
for TRISTAN:
2.5 GeV e–
2.5 GeV e+e+, e- Linac

PF

2.5 GeV/c

Figure 1: Injection for TRISTAN and PF.

JFY 1970
1980
1990
2000
2010
2020
Exp. Energy
8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 90Project

Injection Energy

Photon Factory

2.5 GeV

2.5 GeV

TRISTAN

2.5 GeV

32 GeV

Slow Positron

2.5 GeV - 55 MeV

0.1 - 35 keV

KEKB

8 / 3.5 GeV

8 / 3.5 GeV

PF-AR

2.5 GeV - 6.5 GeV

5 - 6.5 GeVSuperKEKB

7 / 4 GeV

7 / 4 GeV

Construct.

Injection Operation
Construct.

Injection Operation
Operation
Construct.

Operation
Injection Operation
Injection Operation
Construction

Injection

Figure 2: Electron positron accelerator projects operated by the injector LINAC.
Daily Operation Status
Red: Beam current (e–, e+)
Yellow: Collision luminosity
Green: Integrated luminosity
Pale blue: Lifetime (e–, e+)Purple: Vacuum (e–, e+)

the worldwide development of particle physics theory. It
was realized recently that the specifications of the electric
power facilities built at that time secured the expansion possibility even for the fields of nuclear physics and ultraviolet
synchrotron radiation science. Such design facilitated theupgrades towards KEKB and SuperKEKB later.

8-GeV UPGRADE FOR KEKB
Since the construction of TRISTAN, a linear collider wasexpected as the next particle physics experiment project. Instead of immediately promoting a large scale collider, however, the construction of a B Factory, KEKB, was carriedout to experimentally confirm the remaining issues of CP
violation in particle theory proposed by Makoto Kobayashiand Toshihide Maskawa. Since the injection performance
was important to achieve the integral performance as a factory, there were concerns compared to SLAC/PEP-II, which
was planning the B-factory in parallel.
The direct energy injections for the asymmetric energy collider rings were realized by an energy upgrade from 2.5 GeV
to 8 GeV by introducing SLED microwave pulse compressors and adding 50% more accelerating structures that extends the length of the injector up to 700 m in a shape of aletter ‘J’ as in Fig. 3. It was a substantial and difficult construction because the injector continued the PF injection [2].
It made maximum use of the opportunities of the longest
shutdown of nine months in 1997 during the construction.
The success of the KEKB operation was largely due tothe online operating environment of the SAD beam simulation software and the EPICS control framework which weredeveloped during the injector commissioning period [3].
KEKB and PEP-II projects developed the friendly competition, and they compared the performance of the acceleratorsin those control rooms everyday [4]. The performance ofMay.2000
Apr.2003
2 bunchesin a pulse

Feb.2005
Continuousinjection

Dec.2008
3-ring simultaneous top-up
injection with crab cavities

Figure 4: Progress of the daily operation during the KEKB
project, with the stored beam current in red and collision
luminosity in yellow.

the KEK injector was gradually improved during the projectstep-by-step [5].
The positron beam injection was enhanced by employing
dual bunch acceleration. The separation of two bunches waschosen to be 96 ns because of the frequency relationshipbetween the injector and the ring [6].
Then later, it also succeeded in continuous injection byswitching the injection every several minutes while the Bellecollision detector was in operation. In 2008, the injection
was further improved by the simultaneous top-up injection
in to three storage rings except PF-AR in pulse-to-pulse
modulation switching beams with distinct properties every
20 ms employing event-based control architecture [7] as inFig. 4.

SLOW POSITRON AND OTHERS
BelleKEKBHER
8 GeV/cPF-AR
6.5 GeV/ce- BT

Linac delivered:for PF: 2.5 GeV e–
for PF-AR: 3 GeV e–
for KEKB: 8 GeV e–
3.5 GeV e+e+, e- Linace+ BTLER
3.5 GeV/cPF

2.5 GeV/cFigure 3: Injection for KEKB, PF and PF-AR.

The injector facility usually contributes by injecting
beams into the downstream storage rings. However, there
are cases where experiments are performed using directelectrons. In the past at KEK LINAC, there have been experiments at the beam dump for a search for an axion likeparticle, beam irradiation calibration of tile fiber calorimeter detectors for the SSC project, positron beam channelingexperiments, and positron production using electron channeling radiation. As a result of the positron generation experiment, a crystal target was actually applied to the positron
generation for KEKB injection in 2006, and the positrongeneration efficiency was successfully improved by about
25% [8].
Another ongoing project is the research experiments formaterial science and particle physics by slow positrons. Between injections to the TRISTAN project slow positrons
were generated at the end of LINAC. While the experimental setup had to be moved twice due to the change in thebeam operation of the main LINAC, now a dedicated 50
MeV linac is used for slow positron experiments sharing thesame resources as the main LINAC [9]. It is valuable toproduce scientific results within the LINAC building.

SUPERKEKB AND SIMULTANEOUS
TOP-UP INJECTION
After the KEKB project was completed in 2010 the injector, whose girders were built with the concept of a flexiblestructure, suffered a serious damage due to the Great East
Japan Earthquake in 2011. While light source injections
were recovered in 3 months, it took three years to start thebeam test for SuperKEKB as the alignment was severely
deteriorated. Actually, the alignment requirement becamemuch tight in order to suppress the wakefield effect in the accelerating structure. While the light source injections weremaintained even during the upgrade towards SuperKEKB,
the longest construction period of five months was secured
in 2017.
The energy enhancement was the main subject for theprevious KEKB injection upgrade, on the other hand theSuperKEKB project required quality improvements of theinjection beam for the electron positron collision with thenovel nano-beam scheme [10] as in Fig. 5. A beam emittanceof about 20 mm.mrad was required with a charge of 4 nC perbunch. The positron capture section was reconstructed [11]in order to increase the positron charge employing a flux
concentrator pulsed solenoid, large aperture S-band structures (LASs), long DC solenoid magnets and a hundred ofquadrupole focusing magnets. A damping ring was newly
constructed to damp the positron emittance. For the electron beam, a new high-current and low-emittance RF gun
was developed with an iridium-cerium photocathode and aquasi-travelling-wave side-coupled cavity (QTWSC) [12].

Belle II

Beam from Injector and Storage Current
SuperKEKB:
7 GeV e2000 mA
4 GeV e+2800 mA
PF:
2.5 GeV e450 mA
PF-AR:
6.5 GeV e60 mA

3 km

PF-AR
6.5 GeVFigure 6: Yearly operation hours of the injector LINAC.

CONCLUSION
The KEK electron-positron injector LINAC has been able
to achieve 200,000 hours of operation while supporting experiments in multiple fields of photon science and particle
physics.

REFERENCES

LER
4 GeV
SuperKEKB

HER
7 GeV

More than hundred pulsed focusing and corrector magnets
were newly installed [13]. Pulsed low-level RF controls werealso developed. Based on those equipment upgrades [14],the simultaneous top-up injections for four storage rings ofSuperKEKB HER, LER, PF ring and PF-AR were realized
in 2019, which contributed to the considerable improvementof experimental efficiency [15].
The continuous operation of the electron-positron injector
has supported the history of advanced research at KEK byproviding beams for various projects as shown in Fig. 2.
At 8:50 a.m. on May 7, 2020, 200,000 hours of operation
was accomplished as shown in Fig. 6, since the PF injection operation started in 1982. Authors are very proud tohave reached the milestone of 200,000 hours of operation
by inheriting the operation results accumulated by our predecessors. Although no special event was held due to theeffects of COVID-19, a toast was made with morning coffee
via video conference. In the statistics, the partial failure rate
indicates that a certain device has failed but the beam injection operation was still possible due to redundancy, and theinjection stop rate indicates that the failure was so seriousthat the beam injection was impossible. At the beginning ofeach program, the failure rate increases before understanding the equipment characteristics, but it tends to stabilize as
countermeasures are taken.

e– BTe+ BT

e–e+PF
2.5 GeVHigh efficiency
e+ generator

Injector Linac
700 m

Damping
Ring

Low emittance
RF-gunFigure 5: Injection for SuperKEKB as well as PF and PFAR.

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