Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, Japan

PASJ2015 WEP121

SuperKEKB 入射器における RF 電子銃用レーザーの高性能化
IMPROVEMENT OF THE LASER SYSTEM FOR RF-GUN AT SUPERKEKB INJECTOR張叡#, A), 周翔宇 A), B), 夏井拓也 A), B) , 吉田光宏 A), B), 小川雄二郎 A), B)
Rui Zhang #, A), Xiangyu ZhouA), B), Takuya NatsuiA), B), Mitsuhiro YoshidaA), B), Yujiro OgawaA), B)
A)
High Energy Accelerator Research Organization (KEK)
B)
The Graduate University of Advanced Studies (SOKENDAI)

Abstract
SuperKEKB project is aimed at the generation of electron beams with a charge of 5 nC and a normalized emittance of
10 μm at injector linac. According to these demands, an Ytterbium (Yb) laser system is selected for RF-gun. For realizing
the laser system operating under 25 Hz with double pulses or 50 Hz under single pulse operation, Yb-doped fiber andYb:YAG thin-disk hybrid laser system with a central wavelength of 259 nm and a pulse width of 20 ps is developed. 3.0
nC bunch charge has been gotten by use of current laser system. In order to achieve more stable and efficient electron
beam, improvements has been being done. In this letter, some improvements are introduced for optimizing the lasersystem.

1.

INTRODUCTION

Higher luminosity and lower emittance electron beam isrequired in SuperKEKB. The photocathode RF gun withstrong electric focusing filed for high-current, lowemittance should be adopted in the injector linac. For
generating electron beams with a charge of 5 nC and anormalized emittance of 10 μm in the photocathode RF gun,according to the simulation of emittance due to the spacecharge effect, the ultraviolet (UV) laser source with a pulse
width of several tens of picoseconds (ps) is required [1].
Furthermore, for reducing the energy spread, the laser
pulse should be reshaped to rectangle from Gaussian shape[2].
For achieving the demands on the laser source, a hybrid
laser system which consists of an Yb ions doped fiber
oscillator, Yb-doped fiber amplifiers and thin disk
Yb:YAG amplifiers. For 2 Hz repetition rate test, morethan 1 mJ UV pulse energy was obtained. As a result, theelectron beams with a charge of 5.6 nC were generated.
When the laser system was upgrade to 25 Hz, 20 mJ
fundamental laser pulse energy and 700 μJ UV pulse
energy were obtained and 3.0 nC electron beams weregotten [1, 3].
For the repetition rate of electron beam, the 25 Hz withdouble bunches and 50 Hz are requested. In order to realize
excellent thermal management under high repetition rate
operation, the laser system has been being improved.

2.

laser pulse was generated by an Yb-doped fiber ringoscillator. The generated 0.2 nJ seed laser pulse withrepetition rate of 51.9 MHz, which is synchronized with
2856 MHz trigger from accelerator. An electro-opticalfiber pulse picker is adopted for decreasing the repetition
rate of seed laser to 10.38 MHz. Then laser is amplified by
Yb fiber pre-amplifier and chirped to about 30 ps by atransmission grating stretcher. Subsequently, two stages of
Yb doped large-mode-area polarizing double-clad
photonic crystal fiber are selected as the first and main
amplifier to get strong enough pulse energy [5]. After preamplification of Yb-doped fiber and transmission grating
stretcher, double pulse with 25 Hz repetition rate isgenerated. Because thin disk laser possesses very favorable
thermal management, this configuration is used to obtainthe mJ-class pulse energy. It is worthy of being mentionedthat a novel and compact Yb:YAG regenerative amplifieris adopted which is easier to maintain and get higher
amplification. After that, four stages multi-pass amplifiers
are employed. UV pulse laser at 259 nm for photocathodeis generated by using two frequency-doubling stages andthen injected into RF gun.

CURRENT LASER SYSTEM

In the ref [4], 0.8 nC electron beams were obtained byusing of Yb laser system. By reforming the laser system
configuration, 3.0 nC electron beam was archived under 25
Hz operation.
Fig. 1 shows the layout of Yb laser system. The seed___________________________________________# rui.zhang@kek.jp- 834 -

Figure 1: Layout of current laser system for RF-gun.

Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, Japan

PASJ2015 WEP121

3.

IMPROVEMENTS OF THE CURRENTLASER SYSTEM

Au-Sn soldering Yb:YAG/Cu composite for thindisk amplifiers
The difficulty in controlling thermomechanicaldistortions has been one of the most important factors forpreserving high beam quality and developing high average
power solid-state lasers. By comparing with the 0.8 nC
electron beams result we reported in the ref [4], 3.0 nC
electron beams were gotten by using of Yb:YAG thin disk
and copper plate composite. Waste heat generated in laser
active disk can be removed to the copper plate effectively.
Due to effective thermal removal in regenerative amplifier
and multi-pass amplifiers, the amplified laser pulse became
more stable and efficient.
Gold-tin (AuSn) was selected as soldering material forbonding Yb:YAG thin disk and copper plate. It possesses
several advantages. Firstly, it has high thermal
conductivity and low thermal expansion coefficient, as
listed in Table 1. The thermal conductivity is high for AuSn
comparing to that of indium-tin (InSn), this is very helpful
for waste heat removal. Secondly, the deformation of AuSn
is weak on heating because its thermal expansion
coefficient is comparably small. Meanwhile, it is evident
from Table 1 that the thermal expansion coefficients ofAuSn and copper are almost the same. Therefore, whenlaser is operating, the AuSn soldering layer and copper
plate can expand synchronously to avoid fracture of thebonding layer as much as possible. Finally, AuSn
possesses high resistant to corrosion and high creepresistance.

coating is covered as soldering material.

3.1

Figure 2: Configuration of fixtures for Au-Sn soldering ofYb:YAG/Cu composite.
The Au-Sn soldering fixtures are shown in Fig. 2. Threepieces of spring with force constant of 0.05 Kg/mm areused to provide pressure onto the Yb:YAG thin disk andcopper plate composite. The deformation length of one
spring was 10 mm, so the total pressure was 1.21 kg/cm2.
Then the whole fixtures were placed into a vacuum
chamber for soldering process at 310 ºC about 10 minutes.
The accomplished Yb:YAG/Cu composite is shown in Fig.
3. From side view of the composite under a microscope,
we can see that thickness of the Au-Sn soldering layer isabout 16 μm compare to the 0.5 mm thick Yb:YAG thindisk.

Table 1: Comparison Between AuSn, InSn, Cu andYb:YAG
Material(ingredientsofconstituents)

Meltingpoint(ºC)

Thermalconductivity(W/m/K)

Thermalexpansioncoefficient(10-6/K)

AuSn(80:20)

280

58

16InSn (50:50)

118

3420

Copper-396

16.4

Yb:YAG-

116.7

Figure 3: Accomplished Yb:YAG/Cu composite and sideview under microscope.

The Yb:YAG thin disks used in our laser system have0.5 inch diameter and 0.5 mm thickness. Yb-ions dopant is
10 a.t. %. Anti-reflection (AR) coating is covered on thetop surface for 940 nm pump laser and 1030 nm seed laser.
On the bottom surface, high reflection (HR) film is coatedat 1030 nm. Outside the HR film, gold (Au) coating iscovered for metalizing the Yb:YAG thin disk. As to thecopper plate, it has the same diameter as the Yb:YAG thindisk and thickness of 2.0 mm. On the top surface, AnSn

Figure 4: Optical measurement experimental setup.
For testing the thermal effect, an optical experimental
setup was built, as seen in Fig. 4. A TEM00 mode operating
He-Ne laser was enlarged and divided into two equalbeams. One was impinged onto the Yb:YAG amplifier
which is pumped by 5 Hz 600 μs LD laser, the other wasreflected by HR mirror. Finally, two beams were collinear- 835 -

Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, JapanPASJ2015 WEP121

and overlapped, wave front interference could bemonitored by CCD camera and PC.

stack. Reflected measuring laser is measured and analyzed
by wave front camera. By comparing the unpumped
wavefront and pumped one, we can monitor the thin disk
deformation caused by thermal effect. The measured
results are shown in Figure 8.

Figure 5: Optical measurement results for thermal effect.
(a) and (b) are measured by use of old design; (c) and (d)are for Yb:YAG/Cu composite.
Thermal effect measurement was operated under 1 J
pulse energy of LD pump. As seen in Fig. 5, (a) and (b) arewave fronts for old design without pump and with 1 J pumpaccordingly. As to old design, the Yb:YAG thin disk wasadhered to heat sink directly. From (a) and (b) we can see
the wave front distorted under pump because thermal lenseffect occurred seriously. On the opposite, measurementfor Yb:YAG/Cu composite showed excellent thermalmanagement. It is evident that both the wave front without
pump in Fig. 5(c) and the wave front under 1 J pump in Fig.
5(d) are almost unchanged, although some stress wasintroduced by soldering. Yb:YAG/Cu composite have beenused in every stages of our laser system. The waste heat
could be removed efficiently, it is necessary to reduce thethermal lens effect, make laser system stable and increase
the electron charge from RF-gun, as shown in Fig. 6. It isalso potential for high repetition rate operation in thefollowing days.

Figure 7: Measuring experimental setup of this disk
deformation.

Figure 8: Wavefronts from Yb:YAG/Cu composite. (a)unpumped; (b) pumped.
By analyzing the curvature radius of wavefront fromYb:YAG/Cu composite, the instantaneous deformation ofthin disk can be checked. This is very important andnecessary for controlling the thermal effect occurred in thelaser amplifiers and checking the quality of Yb:YAG/Cucomposite. It is also a good tool for tracing the unstablepart in all stages of amplifiers.
3.3

Cryogenic experiment

Figure 6: Electron bunch charge history.
3.2 Precise measurement of thin disk deformation
For obtaining excellent and stable output laser for RFgun, the real time measurement of thin disk deformation isvery important. A compact experimental setup wasdesigned for measuring the thermal deformation in ourlaser system, as shown in Figure 7.
A pigtail output coupling fiber laser at 976 nm is adopted
as measuring laser. After beam expanding, it impinged on
the Yb:YAG/Cu composite which is pumped by 4 kW LD

Figure 9: Gain test experimental layout.
In the aim of realizing more stable laser system andeffective thermal management, cryogenic experiment is agood candidate. We also tested gain of the Yb:YAG disk
under different temperature and different wavelength of- 836 -Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, JapanPASJ2015 WEP121

seed laser. The experimental layout and setup are shown inFig. 9. A 2 mm thick and 10 mm long square Yb:YAG disk
was placed into a vacuum chamber, which is pumped by 5
Hz 600 μs laser at 940 nm. A cryocooler head was also
inside the chamber. Seed laser was tunable and impinged
into Yb:YAG disk at Brewster angle. The gain was tested
by photodiode (PD) and oscilloscope.

composite configuration and cryogenic experiment havebeen investigated. In addition, instantaneous measuring
setup was built for realizing more stable and efficient laser
output energy. It will be adopted in every stages of thin disk
amplifier in the future.
Yb:YAG thin disk was soldered to a copper plate by
AuSn soldering material, effective thermal removal wasachieved under 25 Hz laser operation. As a result, 3.0 nC
bunch charge was generated successfully.
As to the final aim of 50 Hz laser operation in thefollowing days, cryogenic test was done. It is a good
available candidate to overcome serious thermal effect inhigh repetition rate laser system.

REFERENCES

Figure 10: Gain test under different pump power density
for 1030 nm seed laser at different temperature.
The amplification ratios under different pump power
density for 1030 nm laser are shown in Fig. 10. The gain
becomes higher with the decreases of temperature. Highgain can be obtained by use of cryogenic laser operationcompared to current room temperature experiment. Highgain also can be realized under high pump power density
due to low thermal effect under cryogenic environment. In
addition, the amplification ratios for different wavelength
seed laser were tested at 150 K, as shown in Fig. 11. For
our Yb:YAG laser system, the highest gain can be gotten
for seed laser at 1030 nm.

[1] R. Zhang, et al., “Improvements of the Laser System for RFGun at SuperKEKB Injector”, Proceedings of IPAC 2015,
USA, Richmond, May 3-8, 2015.
[2] M.Yoshida, et al., “Longitudinal manipulation to obtain andkeep the low emittance and high charge electron beam forSuperKEKB injector”, Proceedings of IPAC 2013, China,Shanghai, May 13-17, 2013.
[3] T. Natsui et al., “Quasi-Traveling Wave RF Gun and BeamCommissioning for SuperKEKB”, Proceedings of IPAC
2015, USA, Richmond, May 3-8, 2015.
[4] X. Zhou et al., “Ytterbium fiber and disk laser of RF Gun for
SuperKEKB”, Proceedings of IPAC 2015, Germany,
Dresden, Jun 15-20, 2014.
[5] X. Zhou, “25 Hz, Sub-mJ Ytterbium Laser Source of RF Gun
for SuperKEKB Linac”, Proceedings of IPAC 2015, USA,Richmond, May 3-8, 2015.

Figure 11: Gain test for different seed laser wavelength at150 K under different pump power density.

4.

CONCLUSION

Improvement of the laser system have been being done.
For getting better thermal management in laser system
under high repetition rate operation, Yb:YAG/Cu- 837 -