SUPERKEKB PHASE II 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
YB/ND HYBRID LASER SYSTEM FOR RF GUN IN SUPERKEKB PHASE II張叡#, A), B), 周翔宇 A), B), 熊野宏樹 C) , 豊富直之 C) , 吉田光宏 A), B)
Rui Zhang #, A), B), Xiangyu Zhou A), B), Hiroki Kumano C), Naoyuki Toyotomi C), Mitsuhiro YoshidaA), 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
SuperKEKB Phase II commissioning has finished inthis July. According to the demands for linac electronbeam, an Yb/Nd hybrid solid laser system is achieved andused for RF gun to generate qualified electron beam for
injection of High Energy Ring (HER). Compare with thelaser system used in phase I, multiple choice are preparedfor fiber oscillator and solid state amplifier stage.
Moreover, in order to realize more stable and reliablecommissioning, monitoring system and remotecontrolling system are added for long time operation. In
addition to these, improvements on synchronization RF
monitoring and temperature control also make the laserand electron beam be much more stable than before. 3.3
nC has been realized successfully by two laser lines s. 2.4
nC electron beam is measured at the end of linac for HER
injection with required emittance and energy spread. The
electron beam generated by RF gun is injected into HER
successfully in phase II commissioning.

INTRODUCTION
SuperKEKB phase II commissioning finished on 17thJuly. For achieving SuperKEKB project phase II demands
on electron beam, an Ytterbium (Yb) / Neodymium (Nd)hybrid laser system was prepared for phase IIcommissioning. As shown in Table 1, the emittance forelectron beam is not very as strict as the final
requirements of SuperKEKB project, so Nd:YAG withnarrow gain width is chosen as amplifier gain material
without spatial and temporal reshaping for laser pulse [1].
Table 1: Requirements for linac in SuperKEKB phase II
Electron HER 7
GeV

Positron LER 4
GeV

Normalizedemittance
γβεx / γβεy150 / 150 [μm]200 / 40 [μm]with damping ring

Energy spreadσδ

0.10 [%]

0.16 [%]

Bunch chargeat injection
point

1.0 [nC]

0.5 [nC]____________________________________________#rui.zhang@kek.jp

Basing on the experience of phase I and requirementsof phase II, phase II laser system for RF gun consists Ybdoped fiber oscillator, Yb-doped fiber amplifiers and
Nd:YAG rod laser amplifiers. After all the amplifiers,ultra-violet (UV) laser can be generated by utilizing twostages of second harmonic generation for Iridium Cerium
photocathode with high quantum efficiency inside the RFgun [2].
In order to guarantee smooth and continuous injection
and commissioning in phase II, higher reliability of thelaser system is necessary. Therefore, backup design are
considered and prepared for different parts of our laser
system before phase II. Firstly, three mode lock fiber
oscillators are installed, two of them are commercial
products and the other one is ANDI-type homemade
oscillator [3]. One MEMS switch is adopted to change
different seed laser for the following stages of Yb-doped
fiber amplifier. This alternative operation can realize
continuous operation if error occurred duringcommissioning. The second one, two independent
Nd:YAG rod amplification lines are achieved. After fiber
amplification part, the seed laser is divided into two parts
for the first and second Nd:YAG rod amplification line. A
delay line is inserted into the second laser line to adjust
the optical path for realizing two laser synchronous
injection into RF gun. It is optional to select one laser
injection or two laser injection according to thecommissioning requirements. Finally, new vacuumtransporting line, the precise optics controllable injection
part for the RF gun, as well as temperature monitoring
system are used in current laser system. The electron
beam with higher stability and quality can be generated
thanks to the stable and controllable laser operation.
Besides these, status monitor for mode lock oscillator andI/Q monitor for synchronization status of laser systemwith trigger RF has been adopted.
By use of the current laser system, 3.3 nC electron
beam has been generated successfully by two lasers
injection. Accordingly, about 2.3 nC electron charge areprepared for injection to HER. Meanwhile, one laser
injection mode generates 2.4 nC electron charge in RFgun and 1.5 nC at the end of linac for BT line and HER
injection. The most importing thing is the current lasersystem had been operated continuously more than 3months. It demonstrates this solid laser system is very
believable and practical.

Figure1: The overall layout of current Yb/Nd hybrid laser system for SuperKEKB phase II commissioning.

Yb/Nd DOPED HIBRID LASER SYSTEM
OF RF GUN FOR SuperKEKB PHASE II
Figure 1 shows the of the Yb/Nd hybrid laser system
which is used in SuperKEKB Phase II. The 114 MHz seedlaser is generated by an Yb-doped fiber mode lockoscillator which is synchronized with main trigger ofaccelerator. A MEMS optics switch is used to alter
different fiber oscillator. Then the seed laser is amplified
by the first Yb doped single mode fiber amplifier (SMFAmp). A transmission grating stretcher is used to selectthe proper wavelength. Then, the seed laser is amplified
by one stage of semiconductor optical amplifier (SOA)
and two stages of SMF Amp. After the SOA, therepetition rate of seed laser is reduced to 10.38 MHz from114 MHz. An electric-optical module (EO) is adopted as apulse picker to reduce the repetition rate into 25 Hz (1-25Hz available). At the end of the fiber part, a polarizer isadopted to divide the seed into two equal parts, one is forthe first Nd:YAG rod amplification line which has 4stages amplifier, the other one is sent to the second linewith 5 stages amplifier. Green lasers are generated by useof two BBO crystals in the first and second laser line
separately. To realize two laser synchronous injection forRF gun, a delay line for green laser is added in the secondline to change the optical path. After this part, two laser
beams are converged by a polarizer and transported to RF
gun box by one transporting line. Inside the RF gun box,
the laser beams are separated again and converted into
ultra-violet laser for the photocathode. The details areintroduced in the following parts.

Fiber Part of Current Hybrid Laser System
As mentioned above, three oscillators are installed in
our laser system. All of them are mode lock fiber
oscillators that can generate serval hundreds
femtoseconds laser pulse with 114 MHz. The first
oscillator is homemade ANDI type 1064 nm Yb-doped
fiber mode lock laser. It is almost the same as the one
which is used in phase I commissioning [3]. The second
oscillator is a commercial 1064 nm mode lock fiber laser,
it has very stable output and compact size. The third one
is a commercial 1030 nm mode lock laser. In order togenerate 1064 nm seed for the following Nd:YAG
amplifier stages, self-phase modulation (SPM) is adopted
to generate the 1064 nm components. The original

spectrum (red line) and generated spectrum with 1064 nm
components (blue line) are shown in Figure 2.

Figure 2: Spectrum of seed generated by SPM.
In order to realize stable and continuous laser
operation, two commercial oscillators can be controlled
by software in computer. The status of synchronization ismonitored and recorded by an oscilloscope, we canachieve the recovery as soon as possible during problem
occurred. In addition, the synchronization RF frequency ischanged during optics adjustment during ring optics
study. To avoid the oscillator stopping due to the changeof RF frequency, a feedback is used to change thestepping motor and piezo position for adjust to thefrequency change. Meanwhile, an I/Q module is added in
the RF circuits part in this May. The best situation can befound easily for the phase matching between theaccelerator RF and laser.
One MEMS switch is used to select the seed laser. After
this switch, the seed laser is amplified by one Yb SMF
Amp, as shown in the fiber part of Figure 1. One grating
stretcher is followed to serve as wavelength selector for
picking up the seed laser centred at 1064 nm with FWHM
0.5 nm. Then, the seed is injected into the SOA part to foramplification. At the same time, the SOA is act as a pulse
picker to change the repetition rate of the seed laser into
10.38 MHz. After this, the second stage of Yb SMF Ampis used. During the amplification, amplification ofspontaneous emission (ASE) appeared. In order to get ridof it, another grating pair is adopted before the third Yb
SMF Amp. At the end of fiber part, an EO module isinserted in to change the repetition rate in to the properone (1-25 Hz) which is according to the accelerator.

Nd:YAG Rod Laser Amplification Part and New
Transporting Line
Depending on polarization, the seed laser from the fiberpart can be divided into two equal parts by a polarizer.
For guaranteeing smooth and continuous injection andcommissioning in phase II, two Nd:YAG amplificationlines are built, the structure is shown in Figure 3. After
separate amplification and the second harmonic
generation, two green lasers are combined together andsent to the RF gun optics box to generate UV laser forphotocathode. One delay line is added in the second laserline to adjust the time delay for realizing two laser beams
injection mode.

BBO crystals are applied to generate green laser for
two laser lines. For the first laser line, about 2.8 mJ green
laser is achieved. On the other hand, 4.5 mJ is generated
in the second laser line. Aim to realize two laser beams
synchronous injection in RF gun, a delay line is built inthe second laser line, as shown in Figure 4. A roof mirror
is placed on a step motor stage to adjust the optical path
between the first and second line. By checking the laser
phase and accelerator RF phase, adjustment of optical
path can be done by the remote control system.
One polarizer is used to combine the first and second
laser together for transporting line from the ground laser
hut to tunnel RF gun box. According to the phase I
operation experience, vacuum tube transporting line isbuilt to isolate the air turbulence which introduces
instability of the electron beam. At the exit of transporting
line, another polarizer is placed to divide the two laser
beams again for generating UV lasers separately andinjecting into RF gun from different window.

Optics System inside RF Gun Box
After dividing by the polarizer, the two green lasers
pass though the BBO crystals to generate the UV lasers
for RF gun. 450 μJ and 800 μJ are achieved by the firstand second laser for photocathode. The optics system isshown in Figure 5.

Figure 3: Structure of the Nd:YAG rod laser amplification
part of current laser system.
2 mm diameter Nd:YAG rod laser modules are used inthe first and second stages of the first and second laserline for high amplification efficiency. Vertical cavity
surface emitting laser (VCSEL) pumped commercial
modules are selected because they has high gain, long life
time and stable operation. Before the third amplification
stage, a Pockels cell pulse picker is used to set single
bunch mode or double bunch mode. After the Pockels
cell, the seed laser is amplified by the third and fourthstages by using of Ф4 mm modules. Different from thefirst line, an additional firth stage (Ф8 mm) is built in thesecond line to generate higher pulse energy for the RF
gun by only one laser line.

Figure 4: Delay line part and transporting part in current
laser system.

Figure 5: Optics system inside the RF box.
Compared to the optics system inside RF box, a lot ofimprovements has been made for more efficient laser
injection into RF gun. Firstly, we installed 16 remotecontrol actuators for laser fine adjustment by fully remotecontrol. They can be used to adjustment the telescope
systems, angles of the BBO crystals, the rotation of wave
plates, UV laser horizontal and vertical positions, as wellas the rotation angel of the mirrors before RF gun
injection windows. Thanks to these, we can get the best
condition to generate highest electron charge during RF
gun study. Secondly, a beam profile monitor and laser
energy meter are set in the first laser line to check thelaser status. All the real-time data can be confirmed oninternet anytime. The third, one remote controlled flip
mirror is placed in the first laser line for changing thetransmission direction to the 90 degree CDS RF gun
which is installed in 2017 [4]. Two lasers inject into RF
gun from two opposite windows and impinge into
photocathode with 60o degree as shown in Figure 4. In thefuture, both one laser injection mode and two lasers
injection mode will be used for different purpose, such as
SuperKEKB HER injection and PF or PF-AR injection.

including the BT part is shown in figure 8. At the end oflinac, about 1.5 nC electron charge is transmitted into theBT line for injection into HER, but the charge reduces toabout 1.0 nC due to loss. The study about the beam loss inBT line is under investigating, and this is the mostimportant task in the future.

RF GUN STUDY FOR SUPERKEKBPHASE II COMMISSIONING
SuperKEKB phase II commissioning has been being
done from this march. In order to generate qualified
electron beam by RF gun, the laser beam and RF gun
study has been done. The laser beam monitoring system
are also used to check the long term reliability.
By use of current laser system, 3.3 nC electron beam isgenerated successfully by use of two laser beams
injection. Accordingly, about 2.3 nC electron charge isprepared for injection to HER. Meanwhile, one laser
injection mode also generates 2.4 nC electron charge inRF gun and 1.5 nC at the end of linac for BT line andHER injection. The orbit and electron charge records areshown in figure 6.

Figure 7: Wire scan measurement of electron beam atlinac 5 sector.

Figure 8: Linac and BT line electron beam orbit for HER
injection in phase II commissioning.

CONCLUSION

Figure 6: Orbit and electron charge diagrams. Above: twolaser beams injection mode at 5 Hz; Below: one laser
beam injection mode at 5 Hz.
The emittance of electron beam generated by RF gun isalso measured at linac sector 5 by wire scan method, theresult is shown in figure 7. We can see the horizontal andvertical emittance are 50 μm and 50 μm respectively.
Both of them fulfilled the requirements of phase II forelectron beam.
The electron beam generated by RF gun has been
injected into HER successfully. The orbit diagram

An Yb/Nd hybrid laser system is built to generated
qualified electron beam for SuperKEKB phase IIcommissioning. A lot of improvements have been done toincrease the electron charge and decrease the emittance,
as well as enhance the long term reliability of laser
system.
To guarantee smooth and continuous injection andcommissioning in phase II, three mode lock laser
oscillator are installed. We also achieve two laser beamline for RF gun. One laser beam injection mode and twolaser beams injection mode can be selected flexibly.
Feedback between the laser and RF trigger is added formore solid operation during phase II commissioning.
Meanwhile, another new RF gun is installed at linac A1

section. Besides these, the remote control opticaladjustment parts for laser system are adopted to realize
high electron charge generation efficiency.
By use of this hybrid laser system, 3.3 nC electron
beam is generated successfully by using of two laser
beams injection mode. And about 2.3 nC electron charge
is prepared for injection to HER for SuperKEKB phase IIcommissioning. One laser injection mode also generates
2.4 nC electron charge in RF gun and 1.5 nC at the end oflinac for BT line and HER injection.

REFERENCES[1] M. Yoshida et al., “SuperKEKB injector upgrade forhigh charge and low emittance electron beam”,
in Proc. IPAC’12,
New Orleans, USA, paper
TUPPD035.
[2] R. Zhang et al., “Improvements of the laser
system for RF-gun at SuperKEKB injector”,
in Proc. IPAC’15, Richmond, USA, paper
TUPWA071.
[3] X. Zhou et al., “Developing an Yb/Nd doped Hybrid
solid state laser of RF Gun for SuperKEKB Phase IIcommissioning”, in Proc. IPAC’17, Copenhagen
Denmark, paper THPVA047.
[4] R. Zhang et al., “Study on stable and high output
energy laser system for RF-gun at SuperKEKBinjector”, in Proc. PASJ’17, Sapporo, Hokkaido,
paper WEP117.