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

PASJ2022 TUP058

DEVELOPMENT OF LASER STABILIZATION FOR SuperKEKB RF GUN
R. Zhang†1, M. Yoshida1, High Energy Accelerator Research Organization (KEK), Tsukuba, Japan
N. Toyotomi, Mitsubishi Electric System & Service Co., Ltd. (MSC), Tokyo, Japan1
also at The Graduate University of Advanced Studies (SOKENDAI), Tsukuba, Japan
Abstract
Electron beam with high charge and low emittance is required for SuperKEKB high energy ring (HER) injection.
After entering phase III commissioning of SuperKEKB,
about 2.0 nC electron beam with comparable low emittancehas been achieved successfully for injection. For continuous and stable injection and physics run, the stability of laser operation is extremely important. Due to the temperature fluctuation and long transporting line from laser hut totunnel, the laser beam pointing stability was about 50 μm
along horizontal and vertical direction. For improving thelaser pointing stability, laser beam position sensor and piezo mirror mount feedback system have been developedand applied. By use of this system, the laser pointing stability has been improved dramatically. Furthermore, it is nonecessary to adjust the laser beam position manually during the continuous injection and commissioning.

INTRODUCTION
SuperKEKB 2022ab commissioning has finished in June.
New peak luminosity record has been achieved [1]. For
continuous and stable commissioning, the stable electron
beam is crucially important. In order to generate the stableand continuous electron beam for SuperKEKB HER injection, stable laser for the electron beam generation is necessary.
As to the current laser system, we developed some partsfor improving the laser beam stability in the last few years.
For example, adoption of beam pipe for the laser beamtransporting line to separate the airflow disturbance. Except this, application of shield for laser system also improved the laser beam pointing stability to about 50 μm at
the position of virtual photocathode [2]. However, distortion of the mirror mounts and the beam transporting line isstill a serious subject for laser stability, which is inducedby the temperature fluctuation in the laser hut and accelerator tunnel.
For realizing the further improvements to the laser pointing stability, we designed a feedback system which consists
piezo-mirror mount, laser beam position sensor and software. By application of the feedback system, the laser
pointing stability has been significantly improved. Meanwhile, diffractive optical element (DOE) is also applied to
our laser system. Because of the diffractive overlap effectfor laser beam profile spatial distribution, the spatial jitteris suppressed. Thanks to these developments, the laser
pointing stability is dramatically improved, especially thevertical pointing fluctuation is suppressed to one-fifth ofthe previous value.
___________________________________________† rui.zhang@kek.jp

LASER BEAM POSITION FEEDBACK
SYSTEM
Thanks to the installation of beam pipe for laser beamtransporting line and shield for the laser system, the laser
pointing stability was improved than the previous status.
Unfortunately, the instability introduced by the temperature was still unresolvable. Firstly, our laser hut is in klystron gallery, one part of the walls of this laser hut is theexternal wall of the klystron gallery. Because of this, thetemperature inside laser hut is affected by the external temperature changes and heat influx. Although we installed thethermal insulation material to the wall, the temperaturechange inside laser hut is still not stable, as shown in Fig.
1. Associated with the temperature change of klystron gallery, temperature change inside laser hut is about 0.5 oC perday. Although the temperature change of optical table isweaker because of the installation of table shield, thechange is of 0.3 oC per day. In addition, the laser beamtransporting line is installed in the klystron gallery, thethermal distortion of fixed elements due to temperaturechange also seriously affects the laser pointing stability. In
the summer days of SuperKEKB 2020ab commissioning,
the laser position must be adjusted every day for electron
beam generation.

Figure 1: One week temperature history in the laser hut andklystron gallery at Linac.
In order to realize stable and continuous electron beamgeneration, we designed the laser beam position feedback
system. The ordinary commercial laser position feedback
monitor is available but there is one problem for our current laser system. For SuperKEKB commissioning, therepetition rate of electron beam and laser system is from 1
Hz to 25 Hz, but the commercial product is only for highrepetition laser system. Some products were tested in ourlaser system, but the response action was not accurate. For- 483 -Proceedings of the 19th Annual Meeting of Particle Accelerator Society of Japan
October 18 - 21, 2022, Online (Kyushu University)

PASJ2022 TUP058

solving this, we just bought a commercial laser position
monitor and developed the position monitor driver unit andfeedback system by ourselves. Thanks to the homemadeparts, the response action works well, the feedback flow
diagram is shown in Fig. 2.

Figure 4: Feedback system working process for adjusting
the laser beam position to the target value area.

DIFFRECTIVE OPTICAL ELEMENT

Figure 2: Feedback flow diagram.

From 2020c commissioning, DOE has been applying incurrent laser system for achieving spatial reshaping of laserbeam. According to the simulation results, it is possible toget high quality electron beam with low emittance and low
energy spread by use of flat-top spatial distribution [3].
Therefore, the DOE are selected to reshape the originalGaussian spatial distribution. The principle of DOE is diffractive optics principle by the combination of lens and designed optical plate with micro-configurations. Every parton the surface of DOE can refract the laser beam that impinges on it into designed area at the imaging plane. Thenthe summation of the image is the patten which we want toget. For our case, we designed an ellipse flat-top distribution according to the shape and incident angle for the photocathode, the experimental result is shown in Fig. 5.

Figure 3: Laser beam position sensor.
The laser beam position information is recorded by thelaser position sensor, which is installed behind the firsthigh reflection mirror after the 11-meter-long laser beamtransporting line, as shown in Fig. 3. Then the signal is gotten by the sensor driver unit and sent to the oscilloscope.
After tracking the voltage information by PC and comparing to the setting value, drive signals are sent to piezo-mirror mount driver then the piezo-mirror mount can be adjusted until the current laser beam position is adjusted tothe target value area, the feedback system working process
is shown in Fig. 4. We apply the piezo mirror mount to thelast reflection mirror before the laser transporting line. Thisfeedback system was installed during the SuperKEKB
2020c commissioning. In the past two operations in summer, the laser pointing instability induced by the temperature change and thermal distortion has been compensatedwell, there is no need to do the additional manual adjustment as before. Additionally, a novel piezo mirror mountwith wide range of adjustable displacement has been applied in the feedback system from 2021c commissioning.
It can realize more flexible adjustment and compensationfor our laser system.

Figure 5: Reshaping for laser beam spatial distribution by
the DOE and lens.
What we didn’t expect was the application of DOE is
also effective to the laser beam stabilization. The analysis
revealed that thanks to the overlap function of the DOE andlens, the jitter of reshaped laser beam profile at the imaging
plane is suppressed. In addition, the input laser beam size
is much larger than the size of final beam profile. The divided laser beam by DOE is focused on the image planethrough the lens, the determined optical path is helpful todecrease the instability.

EXPERIMENTAL RESULTS
One laser beam monitor is placed in the position of virtual cathode, the recorded one-day laser beam positions in
2019ab commissioning is shown in Fig. 6(a). We can see
the horizontal and vertical fluctuations (4-σ) are about 135
μm and 150 μm respectively. This fluctuation of this magnitude affected the stability of electron beam seriously, frequent laser beam position adjustment and electron beam- 484 -Proceedings of the 19th Annual Meeting of Particle Accelerator Society of JapanOctober 18 - 21, 2022, Online (Kyushu University)

PASJ2022 TUP058

tuning were necessary before 2020a operation. Thanks tothe application of laser beam position feedback system andDOE, the laser pointing stability record is shown in Fig.
6(b) which is measured in 2021c commissioning. 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.

Figure 6: One-day position records of the UV laser beamat virtual photocathode position without and with laser position feedback system and DOE.

CONCLUSION
Laser position feedback system and DOE are applied incurrent laser system to realize stable and continuous electron beam generation for SuperKEKB commissioning.
Thanks to these developments, the laser pointing stability
is greatly improved. There is no need to do manual laser
beam adjustment and electron beam tuning during continuous operation even the temperature fluctuation exists.

ACKNOWLEDGEMENTS
The authors also thank to Mr. K. Kawakami and Mr. H.
Kumano 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, June 2022, pp. 1-5.
doi: 10.18429/JACoW-IPAC2022-MOPLXGD1[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.

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