Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 WEP051

LASER SYSTEM FOR SuperKEKB RF GUN AND
ITS MULTI-FUNCTIONALIZED APPLICATION
R. Zhang†1, X.Y. Zhou1, M. Yoshida1, High Energy Accelerator Research Organization (KEK), Tsukuba, Japan
N. Toyotomi, H. Kumano, Mitsubishi Electric System & Service Co., Ltd. (MSC), Tokyo, Japan1
also at The Graduate University of Advanced Studies (SOKENDAI), Tsukuba, Japan

Abstract
Electron beam generated by RF gun is used forSuperKEKB high energy ring (HER) injection. Basing on
the commissioning experience before, stable and continuous injection has been realized successfully. In order togenerate high charge electron beam, upgrade of Nd:YAGlaser amplification modules and amplification of powerfulmodules have been adopted in our laser system. Meanwhile,with the aim of generating high charge beam with lowemittance, essential spatial reshaping of UV laser has beeninvestigated. Diffractive optical element (DOE) is set in
UV laser part for studying and operation. The best emittance measurement results in B-sector and BT part havebeen achieved for better HER injection. Finally, we also
realized the multi-functionality of the 2nd laser line for
HER electron beam generation and photocathode cleaning
during SuperKEKB commissioning.

INTRODUCTION
SuperKEKB 2022ab commissioning has finished in June
2022. New peak luminosity record has been achieved [1].
For generating high quality electron beam with high charge,
we achieved some upgrades from 2020c commissioning.
Although the current Yb/Nd hybrid laser system cannot realize pulse reshaping (pulse stacking method) in time domain, it possesses the stability and simplicity for stable andcontinuous commissioning [2]. Hence the Yb/Nd hybrid laser is still adopted as the RF gun driven laser system togenerate electron beam for SuperKEKB commissioning.
In order to generate 2 nC electron beam by use of current
RF gun, the increasement of laser pulse energy is the firstthing that should be upgraded. New powerful amplifier
module as booster amplifier is accomplished in-house.
Meanwhile, due to the problem of product life, some newpurchased amplifier modules are installed in the laser system. About 700 μJ laser energy at 266 nm can be gotten by
one laser line. Beyond this, two lasers incidence method isused for electron beam generation, so 2 nC electron beamgeneration is an achievable job. As the electron charge increases, the space charge effect becomes stronger. Excellent emittance result was gotten under 1 nC case, as to 2 nCor higher electron beam, it is difficult to achieve low emittance without additional development. According to thesimulation results, it is possible to get high quality electron
beam with low emittance and low energy spread by use offlat-top spatial distribution [3]. Therefore, the DOE are

selected to reshape the original Gaussian spatial distribution to flat-top one. With all these upgrades, the best emittance results of 2 nC electron beam are realized at B-sector
and BT line by wire scanner method in SuperKEKB operation history.
On the other hand, the 2nd laser line is reformed in 2021
summer maintenance for realizing the multi-functionality.
As the application of DOE in laser system, the cleaning
function of laser for photocathode became impossible because the UV laser telescope system cannot be installed.
With the aim to keep the cleaning function of our laser system during operation, double-layer laser construction is investigated. Thanks to this design, we can achieve simultaneously the laser path with DOE for reshaping and theother path with telescope for laser cleaning.

CURRENT LASER SYSTEM AND RECENT UPGRADES
Current laser system is based on the Yb-doped fiber oscillator, Yb-doped fiber amplification part and Nd:YAG
rod amplification part, as shown in Fig. 1. Two 114 MHz
commercial Yb-doped fiber oscillator are installed. One oscillator is selected by a MEMS switch. The spectrum of oscillator is wide band from 1010 nm to 1070 nm, one gratingis inserted for selecting 1064 nm component for Nd:YAGamplification part. The following semi-conductor opticalamplifier (SOA) is for changing the 114 MHz repetition
rate into 10.38 MHz, which is designed for SuperKEKB
double bunch operation with 96 ns time interval. The 10.38
MHz laser pulse train at 1064 nm is amplified by threestages of Yb-dope fiber amplifier. Two EO modules areused in our laser system, one is for changing the 10.38
MHz repetition rate into 1~50 Hz, the other one is used toreduce the background noise for solid-state laser amplifiers.
After the Yb-doped fiber amplification part, the seed laser is divided into two equal parts, both are sent into twoamplification lines separately. Both of the amplificationline has 5 stages Nd:YAG rod amplification modules forreaching 10 mJ laser pulse energy. The first two stages use
diameter of 2 mm Nd:YAG rod, the 3rd and 4th stage use
4 mm one. The last stage, boost amplifier stage, uses diameter of 8 mm Nd:YAG rod for decreasing nonlinear effect.
After the second stage amplifier, one Pockels cell is used
to choose one bunch operation or two-bunch operation for
SuperKEKB HER injection.

___________________________________________† rui.zhang@kek.jp- 668 -

Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 WEP051

Figure 1: The layout of current Yb/Nd hybrid laser system for SuperKEKB commissioning.
Because all the Nd:YAG rod amplification modules
were purchased about 10 years ago, some of them fail frequently during the operation due to the performance decay.
From 2021 we started to replace the old modules by new
products for smooth and stable electron beam operation.
For increasing the laser pulse energy to be able to generate
2 nC electron beam, the 8 mm diameter Nd:YAG modules
are made in-house as boost amplifiers. By upgrading, about
10 mJ laser pulse energy at 532 nm is obtained in Nd:YAG
amplifier line 1, and 8 mJ in Nd:YAG amplifier line 2. Finally, 750 μJ and 700 μJ 266 nm UV laser are separately
generated for electron beam generation.

without DOE application and the right one is in 2021b
commissioning after application of DOE. The laser beam
spatial distribution is reshaped as flat-top distribution with
2D elliptical cross section as designed. We applied theDOE components in both laser lines for two lasers injection
from 2021b operation. By utilizing the flat-top laser beams,
the best emittance records for 2 nC electron beam has beenachieved at B-sector and BT line from the beginning of theSuperKEKB project. The corresponding measurement results are listed in Table 1.

Figure 3: Reshaping for laser beam spatial distribution by
the DOE and lens.
Table 1: Wire Scanner Results at B-sector and BT Line for
2 nC Electron Beam
Figure 2: Installed DOE and compact vacuum chamber experimental setup in current laser system.
As introduced before, the space charge effect becomesstronger under higher electron charge situation. It has detrimental effects to the quality of electron beam. We investigated the application of DOE for our laser system from2019. Considering the space in RF gun optics box, combination of lens with focusing length of 1 m and DOE plateis decided finally. The reshaped laser spatial distribution isdesigned as flat-top pattern with elliptical 2D shape at theimaging plane, which is the surface of photocathode. Thelong axis of ellipse is 6 mm along the vertical direction andshort axis is 3 mm in the horizontal direction. All of theseare decided by the incidence angle of laser with respect tothe photocathode. Due to the surface micro-configurationof DOE plate, it is very weak at dust. A compact vacuum
chamber is manufactured to provide a clean environment
for the DOE plate, as shown in Fig. 2. Total loss of thissetup is measured to be about 8% for the 266 nm laser. To
the best of our knowledge, this is the first time to apply theDOE for UV laser reshaping in the world.
The UV laser beam spatial distributions are shown in Fig.
3. The left one is the measurement in 2020b commissioning

B-sector
BT 2

Horizontal
8.57 μm
20.95 μm

Vertical
8.89 μm
17.82 μm

MULTI-FUNCTIONALITY OF THE 2NDLASER LINEOwing to the limited available space, the installation ofDOE in the 2nd laser line was delayed. On the other hand,laser cleaning function of the 2nd laser line was decided tokeep. It is impossible and incompatible to fulfil both functions at the same time by only one laser path layout. As tousual electron beam generation mode, the input laser beam
size for DOE is larger than 10 mm (FWHM), and the final
beam shape is ellipse with 6 mm long axis and 3 mm shortaxis. But for laser cleaning mode, the final beam size on
the surface of photocathode should be small for high beam
intensity. For solving this problem, double-layer laser construction is proposed. The block diagram of double-layer
layout for the 2nd laser line is shown in Fig. 4. Because
there is not enough space for building another laser pathfor the installation of DOE, two remote control flip mirrors
are introduced to switch the laser propagation to the direction perpendicular to the optics table. In this way, the newlaser path for DOE can be built directly above the current- 669 -Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 WEP051

laser path that is for laser cleaning mode. These two flip
mirrors are synchronized for both in and out. When they
are inserted in, the 266 nm UV laser is reflected to the upper layer, and firstly expanded by a telescope then sent to
the lens and DOE combination. Oppositely, the laser is in
the lower layer as always and passes reduction telescope tobe smaller beam size for photocathode cleaning when thetwo flip mirrors are out. One part of the double-layer experimental setup is shown in Fig. 5.

CONCLUSION
Upgrades of Yb/Nd laser system have been achieved
from 2022 summer. By exchanging the Nd:YAG rod amplifier modules, higher laser pulse energy is realized forgeneration of 2 nC electron beam for SuperKEKB HER injection. In order to improve the quality of electron beam,
DOE plates are installed in current laser system for getting
flat-top spatial distribution of laser beam. We haveachieved the best emittance measurement results for 2 nC
electron beam in SuperKEKB history. In addition, doublelayer configuration for the 2nd laser line is applied for
multi-functionality. The 2nd laser can be used for electron
beam generation and photocathode cleaning during commissioning.

ACKNOWLEDGEMENTS
Figure 4: Block diagram of double-layer layout for the 2ndlaser line.

The authors also thank to Mr. M. Ishitsuka and Mr. K.
Kawakami for their kind supports.

REFERENCES[1] Y. Funakoshi et al., “The SuperKEKB has broken the world
record of the luminosity”, in Proc. IPAC’22, Bangkok, Thailand,June2022,pp.1-5;[2] R. Zhang et al., “Yb/Nd doped hybrid solid laser of RF gun
and beam com-missioning for phase III of SuperKEKB”, inProc. IPAC’19, Melbourne, Australia, May 2019, pp. 36633666;
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, pp. 1337-1339.

Figure 5: One part of the double-layer experimental setup.
Blue line shows the laser for electron beam generation bypassing the DOE; the green line shows cleaning mode laser.
By virtue of this method and upgrades of laser system, 2nC electron beam with low emittance has been achieved,
the electron beam orbit at Linac and BT line is shown in
Fig. 6. Smooth and stable 5 months long 2022a commissioning is achieved after these upgrades and developments
by current laser system.

Figure 6: The electron beam orbit for SuperKEKB HER
injection in 2022a commissioning.

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