Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, JapanPASJ2019 FSPI003

KEK 電子陽電子入射器の現状
PRESENT STATUS OF KEK ELECTRON/POSITRON INJECTOR LINAC
張 叡 #, 明本 光生，荒川 大，荒木田 是夫，惠郷 博文，榎本 收志，榎本 嘉範，古川 和朗，東 保男，
肥後 寿泰，本間 博幸，飯田 直子，池田 光男，梶 裕志，柿原 和久，紙谷 琢哉，片桐 広明，川村 真人，
倉品 美帆，松本 修二，松本 利広，松下 英樹，三川 勝彦，三浦 孝子，宮原 房史，中島 啓光，夏井 拓也，
西田 麻耶，小川 雄二郎，邱 丰，佐武 いつか，佐藤 政則，清宮 裕史，白川 明広，杉村 仁志，諏訪田 剛，
竹中 たてる，田中 窓香，峠 暢一，矢野 喜治，横山 和枝，吉田 光宏，周 翔宇
Rui Zhang #, Mitsuo Akemoto, Dai Arakawa, Yoshio Arakida, Hiroyasu Ego, Atsushi Enomoto, Yoshinori Enomoto,Kazuro Furukawa, Yasuo Higash, Toshiyasu Higo, Honma Hiroyuki, Naoko Iida, Mitsuo Ikeda, Hiroshi Kaji,
Kazuhisa Kakihara, Takuya Kamitani, Hiroaki Katagiri, Masato Kawamura, Miho Kurashina, Shuji Matsumoto,Toshihiro Matsumoto, Matsushita Hideki, Katsuhiko Mikawa, Takako Miura, Fusashi Miyahara, Hiromitsu Nakajima,Takuya Natsui, Maya Nishida, Yujiro Ogawa, Feng Qiu, Itsuka Satake, Masanori Satoh, Yuji Seimiya,
Akihiro Shirakawa, Hitoshi Sugimura, Tsuyoshi Suwada, Tateru Takenaka, Madoka Tanaka, Nobu Toge,
Yoshiharu Yano, Kazue Yokoyama, Mitsuhiro Yoshida, Xiangyu Zhou
High Energy Accelerator Research Organization (KEK)/ The Graduate University of Advanced Studies (SOKENDAI)

Abstract
SuprKEKB project aims at a 40-fold increasement inluminosity over the previous KEKB project, in order topromote the understanding of flavour physics beyond thestandard model of elementary particle physics. Hence,injector must have the ability to generate high charge
electron and positron beam with low emittance and low
energy spread, in order to realize extremely highluminosity. The first stage of SuperKEKB Phase III
commissioning has been accomplished successfully in thisJuly. During in the spring run, stable electron beam withcomparable low emittance and low energy spread, which
generated by RF gun, is injected into the high energy ring(HER) of SuperKEKB. More than 3 months continuousand stale electron injection is realized with low injection
noise and low background. The positron beam is producedwith updated positron source and damping ring. With thepurpose of realizing stable injection, energy feedback
system is also installed in the stage. Meanwhile,simultaneous top-up injections for 4 rings is realized firstly
time on the basis of high-performance pulsed magnets. As
result, integrated luminosity is improved largely.

Unfortunately, a fire accident occurred in this April so it
took three weeks to recover. The present status ofSuperKEKB injector is introduced in this proceeding.

OPERATION AND PRESENT STATUS OFINJECTOR LINAC
In order to generate high quality electron and positron
beam for SuperKEKB, as well as improve the injection
efficiency and utilization for PF and PF-AR, improvements
and updates have been done in injector linac. High charge
electron beam with comparable low emittance is generated
by RF gun, which is driven by more powerful and stablelaser system. Long life time photocathode with higher
quantum efficiency is adopted. Updated positron capture
ensures the generation of high charge positron beam withqualified emittance and energy spread. With the assistanceof damping ring, the positron beam is injected and storedin low energy ring (LER) successfully. For realizing fourrings (HER/LER/PF/PF-AR) simultaneous injection, highperformance pulse magnets are installed. Besides these,energy feedback and beam orbit feedback, as well as
dispersion measurement and correction have been applied

Figure1: Schematic layout of the SuperKEKB injector linac.
___________________________________________#rui.zhang@kek.jp- 1245 -Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, Japan

PASJ2019 FSPI003

during the operation of injector linac. With all theimprovements, the new peak luminosity record, 122.9 ×
1032 cm-2s-1, is achieved during the SuperKEKB 2019spring run [1]. Meanwhile, full RF gun electron beam
injection for HER is also realized for the first time. Most
worth mentioning is the achievement of simultaneous topup injections for 4 rings for the first time in history ofinjector linac.
The schematic layout of injector linac is shown in Fig. 1.
Injector linac consists of 9 parts, they are A-Sector, BSector, J-ARC, C-Sector, 5 sectors from 1 to 5. Currently,
3 electron sources are installed at A1 injection point. One
is thermionic gun, it generates electron beam for positrontarget, PF and PF-AR. The other two are RF gun, which areused to generate the electron beam with low emittance andlow energy spread for SuperKEKB. Two bunch electron
beam operation mode is available under 50 Hz repetition
rate. The 10 nC electron beam generated by the thermionic
gun is impinged into the target that is located in 1-Sector.
The positron production target is made of tungsten becauseof its large cross section of the electron-positron paircreation and its high melting temperature. Generated
positron is captured by a high efficiency flux concentrator
and sent to the damping ring to reduce the emittance byradiation. There is a hole in the positron target for electron
pass which are used for the injection of HER, PF and PFAR. Depending on the pulse magnets in 5-Sector, theelectron beams for different ring injection can be switched
to different beam lines flexibly.

OPERATION STATISTICS (FY2018)
In FY2018, injector linac operated from April to July,after 3 months summer maintenance, the operation started
again from October. In order to prepare for the 2019 spring
run of SuperKEKB, the operation started up from January.
Figure 2 and Fig. 3 show the operation time and machine
failure. The operation time of FY2018 is 5284 hours, andthe machine failure rate is about 2.71. Compare to the dataof last year, the operation time increased while the machine
failure is comparable low. It demonstrates that all theimprovements are effective for the stable operation inphase III commissioning.

Figure 3: Failure and injection delay rate.

IMPROVEMENT AND UPDATE OF
LINAC INJECTOR
During the 2018 summer and winter maintenance,
several meaningful improvements and updates have been
done at injector linac. By right of these improvements andthe efforts of all the injector linac stuffs, some majorachievements have been accomplished.

RF Gun and Drive Laser System
The layout of RF gun and thermionic gun is shown inFig. 4. The thermionic gun is installed in the second floor
for preserving the better emittance of electron beam
generated by the RF gun. the primary RF gun with QTWSC
configuration is installed in the beam line that is applied in2018 all the year, a secondary RF gun is located in the 90degree line.

Figure 4: Layout of electron source section.

Figure 2: Operation time and machine failure.

The RF gun drive laser system used in the 2019 spring
run is almost the same as the one which was used in phase
II operation [2]. In order to realize stable and continuous
operation, the Yb/Nd hybrid laser system is updated in2018 summer maintenance. Higher gain laser crystals areused to generate higher electron charge [3]. Temporary
reshaping components are installed for low emittance.

- 1246 -Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, JapanPASJ2019 FSPI003

Meanwhile, the application of more monitors in the laser
system helps check the real-time laser status during
operation. In addition, photocathode with higher quantum
efficiency is adopted, which can be cleaned by electron
beam heating. Currently, the highest charge of 5.3 nC isgenerated successfully. Stable and continuous 1.3 nC
electron beam is achieved for 2019 spring run from the 1stday to the end without any problem occurred. Full RF gun
operation for injection of HER is realized for the first time
until now. In the following days, reshaping of laser pulse
in temporal and spatial domain will be investigated for thegeneration of stable higher charge electron beam withlower emittance and lower energy spread [4].

Pulse Magnets and Achievement of 4 Rings
Simultaneous Top-up Injections
In order to realize 4 rings simultaneous top-up injection,
the pulse magnets are applied at injector linac. In 2017,
pulsed steering magnets and quadruple magnets had been
installed from 3 to 5-Sector. In addition to this, a motorcontrolled magnet support system is developed andinstalled, as shown in Fig. 6. Position of the support ismonitored and feedbacked by six linear gauge with 10 μm
precision. Alignment and beam tuning with this system arein progress [6].

Positron Source and Damping Ring
For the purpose of high luminosity for SuperKEKB, it isnecessary to generate positron beam with high charge lowemittance. At the same, in order to realize high injection
efficiency with low injection noise, qualified emittance iscrucial. High current positron generator and damping ringare used to generate the qualified positron beam for theLER of SuperKEKB. The positron generator is shown inFig. 5.

Figure 6: Motor-controlled magnet support system for 2
pulsed steering magnets and 2 pulsed quadruple magnets.
In 2018 summer and 2018 winter maintenance, pulsed
magnets are installed in A, B and J-arc. Especially for the24-degree merge line at A-Sector, pulsed bending magnets
are used to do pulse to pulse switching for RF gun electron
beam and thermionic gun electron beam [7]. This is thefoundation to realize 4 rings top-up simultaneous injections.
The pulsed magnets around merge point at 24-degree line
at A-Sector as shown in Fig. 7.

Figure 5: High current positron generator.
After the positron target, a flux concentrator (FC) which
provides an external magnetic field is applied to increase
the positron yield. A new FC was installed in January 2019
for the spring run of SuperKEKB. The new FC withoutwork hardened is tested at 12 kA at test bench. During thefirst stage of phase III, for stable and continuous operation,
3.5 kA is applied and there is no damage occurred until now.
Development has been being done for new FC to supress
the discharge risk by use of copper alloy and Insertion ofinsulators in the slit. Update of positron source will be donein 2020 summer maintenance [5].
The damping ring is used to optimize the emittance of
positron for better injection. Event-based control system
for injection and extraction of positron beam, energy
compress for injection, bunch compress for extraction
work well. Meanwhile, the monitoring system for
optimization of damping ring performance is stable, suchas beam position monitors, synchrotron radiation monitor
and wire scanners.

Figure 7: Pulsed magnets at the merge point of A-Sector.
Depending on all of these contributions, 4 rings top-up
simultaneous injections is achieved in 2019 spring run for
the first time, as shown in Fig. 8. While compensating thebeams with short life-time of SuperKEKB, the injections- 1247 -Proceedings of the 16th Annual Meeting of Particle Accelerator Society of Japan
July 31 - August 3, 2019, Kyoto, Japan

PASJ2019 FSPI003

for PF and PF-AR are operated simultaneously andindependently. Thanks to this, the integrated luminosity ofSuperKEKB is improved by 237% during the simultaneousinjections [8]. The stored current situation in 4 rings during
the top-up simultaneous injections is shown in Fig. 9.

arc, the dispersion measurement and correction system isdeveloped and applied. As shown in Fig. 10, before thecorrection, the dispersion is strong comparing with thedesign value. By tuning the strength of quadrupole magnets
in J-arc, the residual dispersion is well corrected
Meanwhile, a beam orbit feedback is also available. As
shown in Fig. 11, a BPM at the end of linac shows the orbit.
When the feedback is on for injector linac end, the orbit
drift of the positron beam is corrected to be about 0.1 mm,
while the previous orbit drift is about 1 mm. It is worth
mentioning that the beam orbit feedback can be applied toany point at injector linac, and it will be deployed to J-arc
sector upstream from next stage.

Figure 8: Injection orbits during 4 rings during the top-up
simultaneous injections by RF gun electron beam andthermionic gun electron beams.

Figure 10: Dispersion measurement and correction.

Figure 11: Beam orbit feedback at the end of linac for thepositron beam.

Fire Accident and Recovery

Figure 9: Stored currents in 4 rings during the top-up
simultaneous injections.

Dispersion Correction and Orbit Feedback
With the purpose of realizing stable and effective
electron beam and positron beam injections for HER andLER, some novel feedback and correction systems areapplied in this spring run [9].
Because the beam position jitter has been found that iscaused by the serious horizontal dispersion leakage from J-Figure 12: Location of fire occurred (red rectangular shows
the accelerating structure assembly room).
A fire accident occurred on 4th April 2019 inaccelerating structure assembly room, as shown in Fig. 12.
The most likely cause is puncture of plastic-case capacitors
in modulator. There is no injury in this fire accident [8].

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July 31 - August 3, 2019, Kyoto, Japan

PASJ2019 FSPI003

The injections for 4 rings had to be stopped until therecovery on 22nd April for 3 weeks. Although this is nodirect damage to injector linac, carbon soot which aregenerated by the burnt everywhere could prevent highvoltage operation of 60 high-power pulsed modulators, andhigh-precision operation of the injector facility. The partial
clean-up is carried out for more than two weeks, the rest
part will be done in the 2019 summer maintenance. Afterthe clean-up, low-power and high-power test are
performed successfully, and beam test also finishedsubsequently on 23rd April. Under the premise of ensuringthat everything was ready, the injection for HER and LER
started, and collision mode re-started on 26th. PF lightsource injection was started as originally scheduled on 7thMay.

SUMMARY
Injector linac achieved the injection task successfully forthe 2019 spring run of SuperKEKB and the operation for
PF and PF-AR. Full time continuous and stable electron
beam injection for HER is realized by laser and RF gun
system. New FC is also working well without any problem
occurred. Together with the damping ring, 1 nC positron
beam is obtained at the end of linac. Application of pulsed
magnets laid a solid foundation for the 4 rings
simultaneous injection. With these efforts, 4 ring
simultaneous top-up injections has been achieved
successfully for the first time at KEK. Meanwhile, many
flexible feedback systems are applied for achieving morestable and high-quality injections. Although a fire accident
occurred in April, all the stuffs of injector linac worked
together to overcome difficulties. The injector linac
restarted to work after 3 weeks clean-up.
In order to generate high charge electron and positron
beams with low emittance and low energy spread for thelatter stages of SuperKEKB phase III commissioning,some improvements and updates have been being done
from 2019. Temporal and spatial reshaping for the RF gun
drive laser system will be done to get high quality electron
beam. In addition, new laser system is under doing for
more flexible adjustment. Novel copper alloy FC will beapplied from 2020 to realize more stable and effective
operation for the positron source. Furthermore, thefeedback system will be deployed at J-arc section for low
dispersion and stable beam orbit. All of them will greatly
improve the performance of injector linac in the near future.

project”, presented at PASJ’19, Kyoto, Japan, July
2019, paper WEPI023, this conference.
[4] Y. Honda et al., “Temporal shaping of laser pulse forphoto-cathode gun utilizing coherent pulse stacking”,presented at PASJ’19, Kyoto, Japan, July 2019, paper
FRPI028, this conference.
[5] Y. Enomoto, “Positron source”, KEK, Tsukuba, Japan,
the 23rd KEKB Accelerator Review Committee, July
2019.
[6] Y. Enomoto, S. Sasaki and S. Ushimoto, “Motorcontrol of magnet support frame 1”, presented at
PASJ’19, Kyoto, Japan, July 2019, paper THPH025,this conference.
[7] T. Kamitani et al., “Design of the pulsed dipole magnet
for KEK electron/positron injector linac 24-degree
beam merger line”, presented at PASJ’19, Kyoto,
Japan, July 2019, paper FRPH034, this conference.
[8] K. Furukawa, “Fire and safety consideration at KEKelectron/positron injector linac”, presented at PASJ’19,
Kyoto, Japan, July 2019, paper FRPI027, thisconference.
[9] Y. Seimiya, “Emittance preservation”, KEK, Tsukuba,
Japan, the 23rd KEKB Accelerator Review
Committee, July 2019.

REFERENCES[1] Y. Ohnishi et al., “Start of phase 3 commissioning atSuperKEKB”, presented at PASJ’19, Kyoto, Japan,July 2019, paper FSPH008, this conference.
[2] R. Zhang et al., “RF gun drive Yb/Nd hybrid laser
system for SuperKEKB phase III commissioning”,
presented at PASJ’19, Kyoto, Japan, July 2019, paper
WEPI022, this conference.
[3] X. Zhou et al., “Upgrade of electron beam generation
system for Phase-III commissioning of SuperKEKB- 1249 -