SUPERKEKB PHASE III コミッショニングの RF 電子銃用
YB/ND ハイブリッドレーザーシステム
RF GUN DRIVE YB/ND HYBRID LASER SYSTEM FOR SUPERKEKBPHASE III COMMISONING張叡 #, A), B), 周翔宇 A), B), 熊野宏樹 C) , 豊富直之 C) , 本田洋介 A), B), 吉田光宏 A), B)
Rui Zhang #, A), B), Xiangyu Zhou A), B), Hiroki Kumano C), Naoyuki Toyotomi C),
Yosuke Honda A), B), Mitsuhiro Yoshida A), B)
A)
High Energy Accelerator Research Organization (KEK)
B)
The Graduate University of Advanced Studies (SOKENDAI)C)
Mitsubishi Electric System & Service Co., Ltd. (MSC)

Abstract
The first stage of SuperKEKB Phase III commissioning
has finished in this July. The electron beam generated by
RF gun is used for injection of High Energy Ring (HER).
It is the first time to realize the full and continuous
injection by RF gun at KEK. During these 4 months
operation, any problem occurred in laser system and RFgun. Thanks to the reliable monitoring system in thecurrent laser system, stable electron beam with low
emittance is provided for HER injection with comparable
low background. Meanwhile, the best laser injection
status and electron beam generation status can be foundby the precise control system. We demonstrated more than
5 nC electron charge by use of current laser system andRF gun. In order to realize high charge electron beamwith low emittance and low energy spread, we will do
temporal and spatial reshaping for laser in the following
days. Another laser system is also under investigating for
improving the quality of electron beam in the later stagesof SuperKEKB phase III.

INTRODUCTION
The new peak luminosity record, 122.9 × 1032 cm-2s-1,
is achieved during the SuperKEKB 2019 spring run [1].
The RF gun system at linac contributed a lot for thisvaluable achievement. The electron beam generated by
the RF gun is injected into HER fully and continuously.
Thanks to the comparable low emittance and low energy
spread, low injection noise and low background are very
helpful for this commissioning. Meanwhile, the RF gun
drive laser system worked 4 months without any problem
occurred, it plays an important role in smooth and longterm operation.
Basing on the operation experience of phase II
requirements of phase III early stage, the Ytterbium (Yb) /Neodymium (Nd) hybrid laser system is used [2]. Some
improvements have been done for realizing more stable
and smoother laser operation. In addition, temporary laser
beam reshaping equipment is also installed with the aim
at decreasing the emittance and instability of electron
beam. It has been demonstrated that all the improvements___________________________________________#rui.zhang@kek.jp

are effective and helpful in beam commissioning. About 1
nC stable electron beam is delivered at the end of linac forbeam transporting line and HER injection.
Although high charge is not required in 2019 spring
run, we tested high charge generation by full energy oflaser. More than 5 nC is achieved successfully, it provesthat the RF gun and drive laser system have the ability togenerate qualified high charge electron beam forSuperKEKB. About 2.8 nC electron is achieved at thelinac end. As to the high charge experiment, the emittanceand energy spread of electron beam become bad due tospace charge effect in the RF gun, as well as the wake
filed effect inside the accelerating structure.
For generating high quality electron beam for latterstages of phase III commissioning of SuperKEKB,
temporal and spatial reshaping for laser pulse will be donein the following days. All the experimental setup has been
being preparing now.

RF GUN DRIVE Yb/Nd HYBRID LASERSYSTEM FOR 2019 SPRING RUN
Yb/Nd hybrid laser system is used in phase IIcommissioning. Although the gain width of Nd:YAG laser
crystal is narrow (0.9 nm width when central wavelengthis 1064 nm), it is good enough to generate qualified
electron beam because the requirements for phase II was
not strict. 1 nC electron with 150 μm emittance wasacceptable. Similarly, the early stages of phase III
commissioning don’t require high charge and emittanceof 20 μm, so the similar Yb/Nd laser system is decided to
be used after necessary updating as transitional RF gun
drive laser system.
Figure 1 shows the of the current Yb/Nd hybrid laser
system which is used in 2019 spring run of SuperKEKB.
The overall layout of the laser system is almost the sameas the one used in phase II [2]. We used 3 oscillators forbackup of smooth operation in 2018, two are Menlocommercial products, one is homemade ANDI type. But
one Menlo oscillator is broken in 2018 winter, so thesecond Menlo oscillator is decided to be used as 1064 nm
seed laser. Except this, the other components in fiber partare the same as before. Three stages of Yb doped single

Figure 1: The layout of current Yb/Nd hybrid laser system for SuperKEKB 2019 spring run.
mode fiber (SMF) amplifiers are adopted. After the firststage of Yb SMF amplifier, one grating stretcher isinstalled to select the 1064 nm wavelength component
from the seed laser due to the seed laser is broad
bandwidth which is also used for 1030 nm laser system.
Then, a semiconductor optics amplifier (SOA) is applied
to reduce the repetition rate of seed laser from 114 MHz
to 10.38 MHz. After all the fiber amplifiers, an electricoptical module (EO) is adopted as a pulse picker toreduce the repetition rate into desired one (1-25 Hz
available).

converted and combined by a polarizer, then they are sentfrom ground laser hut to tunnel RF gun box. Anotherpolarizer is installed for dividing the two laser beams
again. After converting into the UV lasers and spatial
reshaping, the two UV laser impinge into RF gun fromtwo windows. As to the two lasers injection design,details can be checked in reference 2.
After the commissioning of phase II, we improved theNd:YAG rod laser part in 2018 summer maintenance [3].
Firstly, high dopant Nd:YAG laser crystals are installed in
the modules. Higher gain is reached under lower driving
currents. Thanks to this, lower thermal management isrealized so that the stability of laser system is improveddramatically. Meanwhile, it reduces the risk of hardware
problems under low driving current operation condition.
Secondly, in order to realize higher charge generation, thefifth amplification stage is built in the 1st laser line. 10 mJ
1064 nm laser and about 4.5 mJ green laser have been
gotten. The next, remote-control piezo mirrors are also
applied before the two lasers combination. Fine
adjustment is very necessary to correct the thermal
distortion due to the temperature fluctuation. Finally, theRF gun box optical layout is improved totally, as shownin Figure 3.

Figure 2: Structure of the Nd:YAG rod laser amplification
part of current laser system.
After the fiber stage, a beam splitter is adopted todivide the seed laser into two equal parts for Nd:YAG rod
amplification part, as shown in Figure 2. The applicationof two parallel laser lines has many advantages. Firstly, it
is possible to realize two laser beams injection into RF
gun for higher charge generation and better emittance ofelectron beam. Secondly, each laser line can be used forbackup line when problem occurred. This insures smoothand continuous commissioning of SuperKEKB. Everylaser line has 5 stages of Nd:YAG rod amplifiers, a delayline is installed in the 2nd laser line for adjusting the laser
phase and 2856 MHz RF accelerating phase. Pockels cellpulse pickers are inserted before the 3rd amplification
stage for achieving double laser bunches or singe bunch
operation. In the following stages, double bunch electron
beam will be used for injection and collision, we will use
the temporal pulse width of pulse pickers to operate.
Achieving all the amplification, about 10 mJ 1064 nm
lasers are generated separately. The green lasers areFigure 3: Top view of the RF gun box optical layout.
In order to realize comparable low emittance ofelectron beam, temporary spatial reshaping apertures areused for the UV lasers. A remote-control motorized wheelwith 12 sorts of apertures is designed to reshape the UVlaser beam. In addition, two CCD cameras are installed at
the positions of virtual cathode. Together with theaperture, the UV laser beam profiles can be monitoredwith the aim of finding the best laser injection status forphotocathode, as shown in Figure 4. Besides this, twointernet laser energy meters are set after the apertures, realtime laser status can be monitored anytime for deceasing
the injection noise and background. All of these
improvements are helpful to achieve electron beam withlow emittance and low energy spread. Most worth
mentioning is the RF gun drive laser system had worked
for 4 months continuously without any problems, and theelectron beam generated by RF gun is used for HER
injection full spring run. This achievement is being hailed
as milestone in phase III commissioning of SuperKEKB.

Figure 6. 2.8 nC electron charge is achieved successfullyat the end of linac. Additionally, 5.3 nC electron charge isgenerated in RF gun by scanning the phase of 2856 MHz
RF wave. Due to the strong space charge effect in RF gun,as well as the wake filed effect in accelerating tube, theemittance of high charge electron beam became worse.
This is an important issue that needs to be studied andresolved in the follow days.

Figure 6: Electron charge orbit for high charge test.

Figure 4: UV laser beam profiles of the 1st and 2ndline at the position of virtual cathode.
In order to investigating the pointing stability of laser
beam, one laser beam position monitor is placed at thevirtual cathode position. We collected more than 60,000
datum of laser beam pointing, the spatial fluctuations
along x and y direction are about 150 μm, as shown inFigure 5. In the following days, we will study therelationship of laser pointing stability and electron beamstability, and make feedback system for more stableelectron beam generation.

As to the 2019 spring run of SuperKEKB, high charge
electron beam is not required. Compared with the electroncharge, better quality of electron beam and long-term
stability is more important. Therefore, about 1 nC
electron charge is generated with lower emittance for 4
months injection. The electron beam orbit is shown in
Figure 7. Accordingly, the horizontal and verticalemittance of B-sector are 12.6 μm and 11.2 μm
respectively, which is measured by wire scanner. At 5sector, the emittances are 65 μm and 58 μm.

Figure 7: Linac and BT line electron beam orbit for HER
injection in 2019 spring run.

CONCLUSION
Figure 5: Laser beam pointing stability at the positionof virtual cathode.

RF GUN STUDY RESULTS OF SUPEREKB
2019 SPRING RUNHigh charge test has been done by use of full energy of
two lasers, the orbit and electron charge are shown in

By improving the Yb/Nd hybrid laser system after
phase II commissioning, more stable and higher pulse
energy RF gun drive laser system is achieved. It is used in2019 spring run of SuperKEKB for 4 months fullcommissioning. Any problem occurred during this run forour laser system and RF gun. We achieved continuousand stable electron beam injection into HER with lower
injection background and noise, which is generated by RFgun with low emittance. Higher electron charge
experiment is also investigated by use of full laser energy.
2.8 nC electron charge is transported to linac end
successfully. In addition, more monitoring setups areinstalled in current laser system for accomplishing thebest status for electron generation and smooth operation.
In the following days, laser reshaping for temporal andspatial distribution will be done to improve the quality ofelectron fundamentally. We are planning to use pulse
stacking technology for generating electron beam withlow energy spread and low emittance to meet therequirements of SuperKEKB phase III later stages.

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., “Yb/Nd hybrid laser system for RF
gun in SuperKEKB phase II”, in Proc. PASJ’18,Nagaoka, Nigata, Japan, August 2018, pp. 763-767.
[3] R. Zhang et al., “Hybrid Yb/Nd Laser System for RF
Gun in SuperKEKB Phase II and Phase III
Commissioning”, in Proc. IPAC’19, Melbourne,
Australia, May 2019, pp. 3663-3665.