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 inthis July. During in the spring run, stable electron beamwith comparable low emittance and low energy spread,which generated by RF gun, is injected into the highenergy ring (HER) of SuperKEKB. More than 3 months
continuous and stale electron injection is realized withlow injection noise and low background. The positron
beam is produced with updated positron source anddamping ring. With the purpose of realizing stableinjection, energy feedback system is also installed in thestage. Meanwhile, simultaneous top-up injections for 4rings is realized firstly time on the basis of highperformance pulsed magnets. As result, integrated

luminosity is improved largely. Unfortunately, a fire
accident occurred in this April so it took three weeks torecover. The present status of SuperKEKB injector isintroduced 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 bymore powerful and stable laser system. Long life time
photocathode with higher quantum efficiency is adopted.
Updated positron capture ensures the generation of highcharge positron beam with qualified emittance and energy
spread. With the assistance of damping ring, the positron
beam is injected and stored in low energy ring (LER)
successfully. For realizing four rings (HER/LER/PF/PFAR) simultaneous injection, high performance pulse
magnets are installed. Besides these, energy feedback andFigure1: Schematic layout of the SuperKEKB injector linac.
___________________________________________#rui.zhang@kek.jp

beam orbit feedback, as well as dispersion measurement
and correction have been applied during the operation ofinjector linac. With all the improvements, the new peakluminosity record, 122.9 × 1032 cm-2s-1, is achieved during
the SuperKEKB 2019 spring run [1]. Meanwhile, full RF
gun electron beam injection for HER is also realized forthe first time. Most worth mentioning is the achievementof simultaneous top-up injections for 4 rings for the firsttime in history of injector linac.
The schematic layout of injector linac is shown inFigure 1. Injector linac consists of 9 parts, they are ASector, B-Sector, 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 beamfor positron target, PF and PF-AR. The other two are RFgun, which are used to generate the electron beam withlow emittance and low 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 thetarget that is located in 1-Sector. The positron production
target is made of tungsten because of its large crosssection of the electron-positron pair creation and its highmelting temperature. Generated positron is captured by ahigh efficiency flux concentrator and sent to the damping
ring to reduce the emittance by radiation. There is a holein the positron target for electron pass which are used forthe injection of HER, PF and PF-AR. Depending on thepulse magnets in 5-Sector, the electron beams fordifferent ring injection can be switched to different beamlines 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 2019spring run of SuperKEKB, the operation started up fromJanuary. Fig. 2 and Fig. 3 show the operation time andmachine failure. The operation time of FY2018 is 5284
hours, and the machine failure rate is about 2.71.
Compare to the data of last year, the operation timeincreased while the machine failure is comparable low. It
demonstrates that all the improvements are effective forthe stable operation in phase 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 in
Figure 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 withQTWSC configuration is installed in the beam line that isapplied in 2018 all the year, a secondary RF gun islocated in the 90-degree 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.

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 the1st day to the end without any problem occurred. Full RF
gun operation for injection of HER is realized for the firsttime until now. In the following days, reshaping of laser
pulse in temporal and spatial domain will be investigated
for the generation of stable higher charge electron beamwith lower 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
motor-controlled magnet support system is developed andinstalled, as shown in Figure 6. Position of the support ismonitored and feedbacked by six linear gauge with 10 μm
precision. Alignment and beam tuning with this system
are in progress [6].

Positron Source and Damping Ring
For the purpose of high luminosity for SuperKEKB, it
is necessary to generate positron beam with high charge
low emittance. At the same, in order to realize highinjection efficiency with low injection noise, qualified
emittance is crucial. High current positron generator anddamping ring are used to generate the qualified positron
beam for the LER of SuperKEKB. The positron generator
is shown in Figure 5.

Figure 6: Motor-controlled magnet support system for 2pulsed steering magnets and 2 pulsed quadruple magnets.

Figure 5: High current positron generator.
After the positron target, a flux concentrator (FC)which provides an external magnetic field is applied toincrease the positron yield. A new FC was installed inJanuary 2019 for the spring run of SuperKEKB. The newFC without work hardened is tested at 12 kA at testbench. During the first stage of phase III, for stable andcontinuous operation, 3.5 kA is applied and there is nodamage occurred until now. Development has been being
done for new FC to supress the discharge risk by use ofcopper alloy and Insertion of insulators in the slit. Updateof positron source will be done in 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.

In 2018 summer and 2018 winter maintenance, pulsed
magnets are installed in A, B and J-arc. Especially for the
24-degree merge line at A-Sector, pulsed bending
magnets are used to do pulse to pulse switching for RFgun electron beam and thermionic gun electron beam [7].
This is the foundation to realize 4 rings top-up
simultaneous injections. The pulsed magnets aroundmerge point at 24-degree line at A-Sector as shown inFigure 7.

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 Figure 8. While compensatingthe beams with short life-time of SuperKEKB, theinjections for PF and PF-AR are operated simultaneously
and independently. Thanks to this, the integrated
luminosity of SuperKEKB is improved by 237% during
the simultaneous injections [8]. The stored current
situation in 4 rings during the top-up simultaneous
injections is shown in Figure 9.

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

Because the beam position jitter has been found that iscaused by the serious horizontal dispersion leakage from
J-arc, the dispersion measurement and correction system
is developed and applied. As shown in Figure 10, before
the correction, 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 theorbit. When the feedback is on for injector linac end, theorbit drift of the positron beam is corrected to be about
0.1 mm, while the previous orbit drift is about 1 mm. It isworth mentioning that the beam orbit feedback can beapplied to any point at injector linac, and it will be
deployed to J-arc sector upstream from next stage.

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].

Figure 12: Location of fire occurred (red rectangularshows the accelerating structure assembly room).
A fire accident occurred on 4th April 2019 inaccelerating structure assembly room, as shown in Figure

12. The most likely cause is puncture of plastic-casecapacitors in modulator. There is no injury in this fire
accident [8]. The injections for 4 rings had to be stopped
until the recovery on 22nd April for 3 weeks. Although
this is no direct damage to injector linac, carbon sootwhich are generated by the burnt everywhere could
prevent high voltage operation of 60 high-power pulsed
modulators, and high-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. After the clean-up, low-power and highpower test are performed successfully, and beam test also
finished subsequently on 23rd April. Under the premise ofensuring that everything was ready, the injection for HERand LER started, and collision mode re-started on 26th.
PF light source injection was started as originallyscheduled on 7th May.

SUMMARY
Injector linac achieved the injection task successfullyfor the 2019 spring run of SuperKEKB and the operation
for PF and PF-AR. Full time continuous and stableelectron beam injection for HER is realized by laser andRF 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. Applicationof 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 underdoing for more flexible adjustment. Novel copper alloy
FC will be applied from 2020 to realize more stable andeffective operation for the positron source. Furthermore,
the feedback 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 thenear future.

REFERENCES[1] Y. Ohnishi et al., “Start of phase 3 commissioning at
SuperKEKB”, 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 SuperKEKBproject”, 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 24degree beam merger line”, presented at PASJ’19,
Kyoto, Japan, July 2019, paper FRPH034, thisconference.
[8] K. Furukawa, “Fire and safety consideration at KEKelectron/positron injector linac”, presented at
PASJ’19, Kyoto, Japan, July 2019, paper FRPI027,
this conference.
[9] Y. Seimiya, “Emittance preservation”, KEK,Tsukuba, Japan, the 23rd KEKB Accelerator Review
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