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Proceedings of IPAC2022, Bangkok, Thailand- Pre-Press Status 17-June-2022 02:30 UTC -LASER SYSTEM FOR SuperKEKB RF GUN IN
PHASE III COMMISSIONING
R. Zhang†, X. Y. Zhou, M. Yoshida KEK/SOKENDAI, Tsukuba, Japan
H. Kumano, N. Toyotomi, Mitsubishi Electric System & Service Co., Ltd, Tsukuba, JapanAbstract
In order to generate high quality electron beam with highcharge in Phase III commissioning of SuperKEKB, someimprovements have been done in Ytterbium doped fiberand Neodymium doped YAG (Nd:YAG) hybrid laser system. Spatial reshaping part for the 4th harmonic laser beamat 266 nm has been adopted to realize low emittance electron beam. In addition, for achieving continuous and stablelaser operation, position feedback system has also beenused to improve the pointing stability of laser beam. In
2021c commissioning of SuperKEKB, stable 2 nC electron
beam is generated for HER injection. Meanwhile, weachieved the best emittance results at B-sector of linac injector and BT line for comparable low injection background and higher injection efficiency. With the aim ofgenerating higher charge electron beam with good quality
in the following commissioning, a perspective towards thenext step update for current laser system is also introduced.

INTROCUTION
SuperKEKB phase III commissioning has been being
operated from 2018. Highest luminosity achieved in 2022a
commissioning is almost two times of KEKB luminosity
record [1]. Study about the bunch number of fill pattern andthe bunch current has been being done for achieving higher
luminosity and more physics run before long short down 1.
From SuperKEKB phase II commissioning, RF gun has
been used for electron beam generation source. The Ytterbium (Yb)/Neodymium (Nd) hybrid laser system is built
for high charge electron beam generation. By using of twolasers injection method, 2.3 nC electron beam was generated for SuperKEKB high energy ring (HER) injection.
Meanwhile, the highest generation charge was 5.3 nC under the full laser energy input case [2]. Basing on the operation experiences of Phase I and II, the Yb/Nd hybrid laser
system is also used for the phase III commissioning afterupgrading for the higher quality electron beam generation.
In order to generate low emittance electron for phase III
commissioning, the laser spatial distribution should be reshaped from Gaussian distribution into flat-top distribution[3]. Diffractive optics element (DOE) has been introduced
into the UV laser part before injection into the RF gun.
Thanks to the contribution of DOE, the best emittance results have been measured by wire scanner method at Bsector of SuperKEKB linac under 2 nC electron chargecondition. In addition, the discharge induced by the laser
injection decreased dramatically thanks to the applicationof DOE.

Furthermore, the laser beam pointing stability affected
the electron beam stability during phase II commissioning.
Due to the long distance from ground laser hut to the tunnel
optics system and RF gun, it is found that the temperaturefluctuation affected the laser pointing stability. We designed a laser position monitor and made a feedback system by piezo mirror mounts for the stabilization of laser
beam. It has been demonstrated that the laser pointing fluctuation has been suppressed to one-fifth of the previous
value without this stabilization system.
Beside these, electron charge feedback system has been
also developed for stable and continuous 2 nC electron
beam generation. All of these improvements ensure ourphase III operation smoothly from 2019. In this proceeding,
all the improvements mentioned above will be introduced
in details.

IMPROVEMENTS IN LASER SYSTEM
FOR HIGH CHARGE & STABLE
ELECTRON BEAM GENERATION
The ability has been demonstrated to generate highcharge electron beam for our Yb/Nd hybrid laser system
and RF gun in SuperKEKB commissioning phase II [2]. In
order to realize more stable and effective injection in phase
III, the high-quality electron beam with low emittance andhigh stability under high charge condition is necessary.
Some research and development have been done for improving the current laser system. The details and latest operation results are introduced in this part.

Introduction of Current Yb/Nd Hybrid Laser
SystemThe current Yb/Nd hybrid laser system is almost thesame as the laser system used in Phase II. The overall layout of it is shown in Fig. 1. Two commercial mode-lockoscillators are used for smooth commissioning, both ofthem are synchronized with the 114 MHz. One 2*1 MEMS
switch is applied to select one oscillator for the laser system operation. After one stage of Yb doped single mode
fiber (SMF) amplifier and grating stretcher, one semi-conductor optical amplifier (SOA) is used. Meanwhile, therepetition rate is changed into 10.38 MHz. An electric-optical module (EO) is used as a pulse picker to reduce therepetition rate into 25 Hz (1-25 Hz available). At the endof the fiber part, a half waveplate and a polarizer divide theseed laser into two equal parts, one is for the first Nd:YAGrod amplifier line, the other one is for the second line, bothof them have 5 stages of Nd:YAG rod amplifier.

___________________________________________† rui.zhang@kek.jp

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MC7: Accelerator Technology
T25: Lasers

- Pre-Press Status 17-June-2022 02:30 UTC -Figure 1: The layout of current Yb/Nd hybrid laser system for SuperKEKB commissioning.
To realize two laser synchronous injection for RF gun, adelay line is added in the second laser line to adjust theoptical path. After this part, two laser beams are converged
by a polarizer and transported to tunnel optics table by one
transporting line. The 532 nm green laser beams are separated again and converted into ultra-violet (UV) lasers.
Two lasers are injected into the RF gun separately from twowindows. Before injecting into the RF gun, DOEs are inserted for laser spatial distribution reshaping, this will beintroduced in the following part.
During the 2021 summer maintenance, two powerfulmodules with 10 mm diameter Nd:YAG crystals are applied in two laser lines for realizing higher laser pulse energy. Thanks to this upgrading, about 8 mJ 532 nm laserand 1 mJ UV laser energy are achieved in both laser lines.
In addition, the pulse width of 532 nm laser is measured bya streak camera, the width is about 20 ps.

Laser Spatial Reshaping for Lower EmittanceElectron Beam Generation
According to the previous simulation result, the laser
beam with flat-top spatial distribution can generate electron beam with lower emittance comparing with the caseof laser beam with Gaussian spatial distribution [3]. There
are many methods to reshape the laser beam distribution,for example serrated aperture apodizer, super Gaussianmode overlap and so on. Take into account the laser energy
loss and the complexity of setup, we decided to adopt DOE
as our reshaping component. We purchased the DOE components from MARUBUN Corporation and the maker is
SILIOS Technologies France.

Consider according to the size of photocathode and the60o incidence angle of laser beams, we designed to generate the flat-top laser intensity distribution with an elliptical
2D distribution (6 mm along long axis and 3 mm in shortaxis direction) so the cross sections of laser beams on thesurface of photocathode are round shape. Due to the fine
3D structures on the surface of DOE, it is very weak atdusts. The environment in accelerator tunnel is not clean so
we designed small vacuum chambers for the DOEs to prevent the dusts in ordinary air environment, as shown in themiddle part of Fig. 2. Although the two windows of vacuum chamber introduced optical loss, the total transmission ratio is higher than 92%.
The UV laser beam spatial distributions are shown inFig. 3. The left one is the measurement in 2020b commissioning without DOE application and the right one is in
2021b commissioning after application of DOE. It is clearthat the laser beam spatial distribution is reshaped as flattop distribution with 2D elliptical cross section as designed.
We applied the DOE components in both laser lines for twolasers injection from 2021b operation. By utilizing the flattop laser beams, the best emittance records for 2 nC electron beam has been achieved at B-sector and BT line fromthe beginning of the SuperKEKB project. The corresponding measurement results are listed in Table 1.

Figure 3: The UV laser beam spatial distributions without
and with the application of DOE.
Table 1: Wire Scanner Results at B-sector and BT Line for 2nC Electron Beam
B-sector
BT 2

Horizontal
8.57 μm
20.95 μm

Vertical
8.89 μm
17.82 μm

Application of Laser Position Sensor

Figure 2: The DOE chamber in the 2nd laser line.
MC7: Accelerator Technology
T25: Lasers

As shown in Fig. 1, after generating the 532 nm lasers inground laser hut, both of the 532 nm laser beams are sent
to the tunnel laser optics table by the transporting line. Although vacuum tube is used for all the transporting line toTHPOTK059
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Content from this work may be used under the terms of the CC BY 4.0 licence (© 2022). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

Proceedings of IPAC2022, Bangkok, Thailand

Content from this work may be used under the terms of the CC BY 4.0 licence (© 2022). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

Proceedings of IPAC2022, Bangkok, Thailand

prevent the airflow disturbance, due to the 11 meters
length, the pointing stability of laser beam was not goodenough because of the minor position drift introduced by
the temperature fluctuation. One position monitor is placedin the position of virtual cathode, the recorded one-day laser beam positions in 2019a is shown in Fig. 4(a). We cansee the horizontal and vertical fluctuations (4-σ) are about
135 μm and 150 μm respectively. This fluctuation of thismagnitude affected the stability of electron beam seriously,
frequent laser beam position adjustment and electron beamtuning were necessary before 2020a operation.

- Pre-Press Status 17-June-2022 02:30 UTC -beam injection for HER is continued from this Januarywith comparable low injection background.

Figure 5: Current electron beam orbit.

FUTURE PLAN
Figure 4: One-day position records of the UV laser beamat virtual photocathode position without and with laser
position sensor and feedback system.
From 2020b operation, the laser position sensor andfeedback system has been applied to our laser system. Thissystem consists three parts. The first one is laser beam position monitor, which is placed behind the first mirror after
the transporting line. We use commercial laser beam position sensor and developed the homemade driver module forpulsed laser availability. The real time position datum ischanged into voltage information and record by an oscilloscope. Then all the datum is analysed in PC, the second
part of this system. After processing the real datum, theprogram will calculate the difference between the current
status and set value, then send the signal to the piezo mirror
mount and controller, which are the last part of this system.
The piezo mirror mount is used for the last mirror before
the transporting line. The laser pointing instability inducedby the minor position change is suppressed by use of thisfeedback system. As shown in Fig. 4(b), after introducing
the laser position monitor and feedback system, the horizontal and vertical fluctuations (4-σ) are about 38 μm and30 μm respectively. The vertical pointing fluctuation has
been suppressed to one-fifth of the previous value, so it is
no need to do complex laser adjustment and electron beamtuning during the continuous injection commissioning.

Latest Commissioning Results
As introduced in the former part, we have achieved thebest emittance records at B-sector and BT line thanks to thelaser beam spatial reshaping. Beside this, the continuousand stable beam injection is continued by use of the laser
position monitor feedback system. Meanwhile, one electron beam charge feedback system is also applied to ourlaser system to maintain the 2 nC electron charge generation at inside RF gun. The current electron beam orbit ofthe Linac injector and the BT line is shown in Fig. 5. One
collimator is adopted at the beginning of BT line to adjust
the energy spread for better injection. Smooth electron
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2In order to generate 4 nC electron charge forSuperKEKB requirement, it is necessary to upgrade the laser amplifier part. After 2022a commissioning, powerfulamplifier modules will be applied to generate higher laser
pulse energy. Secondly, for most effective injection forHER, low energy spread electron beam is essential. We will
carry out the laser temporal reshaping for realizing highcharge electron beam with low energy spread. Therefore,
the Nd:YAG amplification part is not good enough to fulfil
this. The new Yb:YAG laser system is under investigation.
Thirdly, the degeneration of vacuum window of RF gun
occurred during operation due to the discharge inside RFgun, better vacuum environment will be investigated fromthe next operation period.

CONCLUSION
Yb/Nd hybrid laser system is used in SuperKEKB 2022a
commissioning. For generating low emittance electronbeam, reshaping for laser spatial intensity distribution isdone by the DOE. The best electron beam emittance records under 2 nC at B-sector and BT line have beenachieved. Laser position feedback system is applied to obtain stable and continuous injection. Smooth, continuousand stable electron beam injection for HER is realized. It
will be investigated to increase electron charge and decrease energy spread by temporal reshaping in the following days.

REFERENCES[1] Y. Funakoshi, “The SuperKEKB has broken the world recordof the luminosity”, presented at the 13th Int. Particle Accelerator Conf. (IPAC’22), Bangkok, Thailand, June 2022, paper
MOPLXGD1, this conference.
[2] 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. 3663-3666.
doi:10.18429/JACoW-IPAC2019-THPGW037[3] M. Yoshida et al., “Longitudinal Manipulation to Obtain andKeep the Low Emittance and High Charge Electron Beam forSuperKEKB Injector”, in Proc. IPAC'13, Shanghai, China,May 2013, paper TUPFI004, pp. 1337-1339.

MC7: Accelerator Technology
T25: Lasers