IMPROVEMENTS OF THE LASER SYSTEM FOR RF-GUN AT
SUPERKEKB INJECTORR. Zhang*, M. Yoshida, T. Natsui, X. Zhou, Y. Ogawa, KEK, Tsukuba, Japan
AbstractAccording to the requirements of SuperKEKB project,
the electron beams with a charge of 5 nC and anormalized emittance of 10 µm are expected to begenerated at injector linac. In order to meet thesedemands, an Ytterbium laser system has been being built.
For realizing the laser system operating under 25 Hz withdouble bunch or 50 Hz, improvements of laser system arebeing done. We obtained 3.0 nC bunch charge by use ofYb:YAG/Cu soldering composite to overcome thermal
effect. This demonstration indicates that effective andexcellent thermal management can be realized for highrepetition laser operation. Cryogenic experiment is also
introduced.

INTRODUCTION
Higher luminosity is required in SuperKEKB. The
photocathode RF gun with strong electric focusing filed
for high-current, low-emittance should be adopted in theinjector linac. For generating electron beams with acharge of 5 nC and a normalized emittance of 10 µm inthe photocathode RF gun, according to the simulation ofemittance due to the space charge effect, the ultraviolet(UV) laser source with a pulse width of several tens ofpicoseconds (ps) is required [1]. Furthermore, for
reducing the energy spread, the laser pulse should bereshaped to rectangle from Gaussian shape [2].
With the aim of achieving the demands on the laser
source, a hybrid laser system which includes an ytterbium(Yb) ions doped fiber oscillator, Yb-doped fiber amplifier
and thin disk Yb:YAG amplifiers. For 2 Hz repetition ratetest, more than 1 mJ UV pulse energy was obtained. As aresult, the electron beams with a charge of 5.6 nC weregenerated. 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
were gotten [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 beingimproved.

IMPROVEMENTS OF THE LASERSYSTEM
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.
Figure 1 shows the layout of Yb laser system. The seed____________________________________________*rui.zhang@kek.jp

laser pulse was generated by an Yb-doped fiber ringoscillator. An electro-optical fiber pulse picker is adopted
for decreasing the repetition rate of seed laser to 10.38
MHz. Then laser is amplified by Yb fiber pre-amplifierand chirped to about 30 ps by a transmission grating
stretcher. Subsequently, two stages of Yb doped largemode-area polarizing double-clad photonic crystal fiber
are selected as the first and main amplifier to get strong
enough pulse energy [5]. Because thin disk laser
possesses very favourable thermal management, thisconfiguration is used to obtain the mJ-class pulse energy.
A regenerative Yb:YAG thin disk regenerative amplifier
and four stages multi-pass amplifiers are employed. UV
pulse laser at 259 nm for photocathode is generated byusing two frequency-doubling stages and then injected
into RF gun.

Figure 1: Layout of laser system.

Au-Sn soldering Yb:YAG/Cu Composite for
AmplifierThe 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:YAGthin 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, aslisted 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, AuSn
possesses high resistant to corrosion and high creepresistance.
Table 1: Comparison between AuSn, InSn, Cu andYb:YAG
Material

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

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
surface, AnSn coating is covered as soldering material.

Figure 2: Configuration and components of fixtures for
Au-Sn soldering.

Figure 3: Accomplished Yb:YAG/Cu composite andside view under microscope.

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 disk
and copper plate composite. The deformation length ofone spring was 10 mm, so the total pressure was 1.21kg/cm2. Then the whole fixtures were placed into a
vacuum 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.

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:YAG
amplifier 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.

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 lenseffect, make laser system stable and increase the electron
charge from RF-gun, as shown in Fig. 6. It is also
potential for high repetition rate operation in thefollowing days.

The amplification ratios under different pump power
density for 1030 nm laser are shown in Fig. 8. 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. 9. For our Yb:YAG laser system, the highest gain canbe gotten for seed laser at 1030 nm.

Figure 6: Bunch charge history.

Cryogenic Experiment
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 ofseed laser. The experimental layout and setup are shownin Fig. 7. A 2 mm thick and 10 mm long square Yb:YAG
disk 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.

Figure 7: Gain test experimental layout and setup.

Figure 8: Gain test under different pump power density
for 1030 nm seed laser at different temperature.

Figure 9: Gain test for different seed laser wavelength
at 150 K under different pump power density.

CONCLUSION
Improvement of the laser system have been being done.
In order to meet the demands of SuperKEKB, lasersystem should be operated at high repetition rate. For
realizing excellent thermal management in laser system
under 25 Hz and 50 Hz repetition rate operation,
Yb:YAG/Cu composite configuration and cryogenic
experiment have been investigated and demonstrated.
Yb:YAG thin disk was soldered to a copper plate by
AuSn soldering material, effect 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 method to overcome serious thermal effectunder high repetition rate laser system.

REFERENCES[1] T. Miura, “Progress of SuperKEKB”, TUYB1, theseproceedings, IPAC’15, Richmond, USA (2015).
[2] M. Yoshida et al., “SuperKEKB injector upgrade forhigh charge and low emittance electron beam”,
TUPPD035, IPAC’12, Louisiana, USA (2012).
[3] T. Natsui et al., “Quasi-Traveling Wave RF Gun andBeam Commissioning for SuperKEKB”, TUPJE003,
IPAC’15, Richmond, USA (2015).
[4] X. Zhou et al., “Ytterbium fiber and disk laser of RF
Gun for SuperKEKB”, WEPME061, IPAC’14,
Dresden, Germany (2014).
[5] X. Zhou, “25 Hz, Sub-mJ Ytterbium Laser Source of
RF Gun for SuperKEKB Linac”, WEPMA044, theseproceedings, IPAC’15, Richmond, USA (2015).