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 emittanceof 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 and Yb:YAG thin-disk hybrid laser system with a central wavelength of 259 nm and a pulse width of 20 ps isdeveloped. 3.0 nC bunch charge has been gotten by use of current laser system. In order to achieve more stable andefficient electron beam, improvements has been being done. In this letter, some improvements are introduced for
optimizing the laser system.

1.

INTRODUCTION

Higher luminosity and lower emittance electron beamis required in SuperKEKB. The photocathode RF gun
with strong 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 RFgun, according to the simulation of emittance due to thespace charge effect, the ultraviolet (UV) laser source witha pulse width of several tens of picoseconds (ps) isrequired [1]. Furthermore, for reducing the energy spread,the laser pulse should be reshaped to rectangle fromGaussian 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 mJfundamental 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 torealize excellent thermal management under highrepetition rate operation, the laser system has been being
improved.

2.

Fig. 1 shows the layout of Yb laser system. The seedlaser 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 stagesof 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 obtain the mJ-class pulse energy. It is worthy of being
mentioned that a novel and compact Yb:YAG
regenerative amplifier is adopted which is easier tomaintain and get higher amplification. After that, fourstages multi-pass amplifiers are employed. UV pulse laser
at 259 nm for photocathode is generated by using two
frequency-doubling stages and then 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.
____________________________________________#rui.zhang@kek.jp

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

3.

IMPROVEMENTS OF THE CURRENTLASER SYSTEM

3.1

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 highaverage power solid-state lasers. By comparing with the0.8 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 heatgenerated in laser active disk can be removed to thecopper plate effectively. Due to effective thermal removal
in regenerative amplifier and multi-pass amplifiers, theamplified 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 deformationof 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, AuSnpossesses high resistant to corrosion and high creep
resistance.

surface, AnSn coating is covered as soldering material.

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

Table 1: Comparison between AuSn, InSn, Cu andYb:YAGMaterial(ingredientsofconstituents)

Melting
point(ºC)

Thermalconductivity(W/m/K)

Thermalexpansioncoefficient

AuSn(80:20)280

58

16InSn (50:50)

118

34

20

Copper-396

16.4

Yb:YAG-

116.7

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

(10-6/K)The Yb:YAG thin disks used in our laser system have
0.5 inch diameter and 0.5 mm thickness. Yb-ions dopant
is 10 a.t. %. Anti-reflection (AR) coating is covered on
the top surface for 940 nm pump laser and 1030 nm seedlaser. On the bottom surface, high reflection (HR) film iscoated at 1030 nm. Outside the HR film, gold (Au)coating is covered for metalizing the Yb:YAG thin disk.
As to the copper plate, it has the same diameter as theYb:YAG thin disk and thickness of 2.0 mm. On the top

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 twoequal beams. One was impinged onto the Yb:YAGamplifier which is pumped by 5 Hz 600 µs LD laser, theother was reflected by HR mirror. Finally, two beams

were collinear and overlapped, wave front interference
could be monitored by CCD camera and PC.

by 4 kW LD stack. Reflected measuring laser is measured
and analyzed by wave front camera. By comparing theunpumped 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)are wave fronts for old design without pump and with 1 Jpump accordingly. As to old design, the Yb:YAG thindisk was adhered to heat sink directly. From (a) and (b)we can see the wave front distorted under pump because
thermal lens effect occurred seriously. On the opposite,measurement for Yb:YAG/Cu composite showed
excellent thermal management. It is evident that both thewave 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 was introduced by soldering.
Yb:YAG/Cu composite have been used in every stages of
our laser system. The waste heat could be removed
efficiently, it is necessary to reduce the thermal lens effect,
make laser system stable and increase the electron charge
from RF-gun, as shown in Fig. 6. It is also potential forhigh repetition rate operation in the following days.

Figure 7: Measuring experimental setup of this diskdeformation.

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
the laser amplifiers and checking the quality ofYb:YAG/Cu composite. It is also a good tool for tracing
the unstable part 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 deformationis very 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 isadopted as measuring laser. After beam expanding, it
impinged on the Yb:YAG/Cu composite which is pumped

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

seed laser. The experimental layout and setup are shownin Fig. 9. A 2 mm thick and 10 mm long square Yb:YAGdisk was placed into a vacuum chamber, which is pumpedby 5 Hz 600 μs laser at 940 nm. A cryocooler head wasalso inside the chamber. Seed laser was tunable andimpinged 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 efficientlaser output energy. It will be adopted in every stages ofthin 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 inFig. 11. For our Yb:YAG laser system, the highest gain
can be gotten for seed laser at 1030 nm.

Figure 11: Gain test for different seed laser wavelength at
150 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[1] R. Zhang, et al., “Improvements of the Laser System for
RF-Gun 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 Gunfor 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.