Proceedings of the 13th Annual Meeting of Particle Accelerator Society of Japan
August 8-10, 2016, Chiba, Japan

PASJ2016 TUP054

SuperKEKB 入射器における RF 電子銃用レーザー安定性と出力エネルギーの高性能化
IMPROVEMENT OF STABLE AND HIGH OUTPUT ENERGY LASER SYSTEM FOR
RF-GUN AT SUPERKEKB INJECTOR張叡#, A), B), 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), B), 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 linac. According to these demands, an Ytterbium (Yb) laser system is selected for RF-gun. 1.5 nC electron
charge has been achieved at 25 Hz for SuperKEKB phase I operation. For realizing 50 Hz laser operation and low
emittance electron beam, the current laser is not suitable. Another new laser system is under building at new laser hut.
Compare to the current laser system, new laser system is targeted at more simple, flexible and higher efficiency.
Neodymium (Nd) laser system and Yb laser system are being built for different purpose. In order to get stable and highoutput energy laser system, improvements are made based on the operation situation of current laser system.

1.

INTRODUCTION

SuperKEKB requires higher luminosity and lower
emittance electron beam. In order to fulfill these demands,
RF gun with strong electric focusing filed for high-current,
low-emittance was adopted at linac. For generating
electron beams with a charge of 5 nC and a normalized
emittance of 10 μm by use of RF gun, according to thesimulation of emittance due to the space charge effect, theultraviolet (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 bereshaped 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. The laser system was tested at 25 Hz,
20 mJ fundamental laser pulse energy and 700 μJ UV pulse
energy were obtained and 3.0 nC electron charge wasgotten [3]. During SuperKEKB phase 1 commissioning,
RF gun was selected for high energy ring (HER) injection
about two weeks, 1.5 nC electron charge was generated andinjected successfully and durably.
For the repetition rate of electron beam, 25 Hz withdouble bunches and 50 Hz are requested. As to current
laser system, delay line for double bunch operation can’tbe realized because of space limit. Aim to build a more
flexible and potential laser system, another new laser hut
has been achieved at linac. Both Neodymium (Nd) doped
laser system and Ytterbium (Yb) doped laser system has
been being developed at the new laser hut. Nd doped laser
system is a candidate for SuperKEKB phase II operation,

as for Yb doped laser system, it will be used forSuperKEKB phase III.
In order to realize effective and stable laser output, more
excellent and efficient thermal management andamplification efficiency are explored, such as new thin disklaser head, low temperature laser operation andregenerative amplifier. All these improvements will be
introduced in this proceeding.

2.

LASER SYSTEM LAYOUT AT NEW
LASER HUT

Compare to the current laser system, Nd doped laser
system and Yb doped laser system are built simultaneously
at new laser hut. One mode lock fiber laser oscillator isshared by two laser systems. For realizing backup for
SuperKEKB, four seed lasers are prepared, as shown inFigure 1.

Figure 1: Layout of laser system for RF gun at new laser
hut.

___________________________________________

Figure 1 shows the layout of laser system for RF gun at# rui.zhang@kek.jp- 995 -

Proceedings of the 13th Annual Meeting of Particle Accelerator Society of Japan
August 8-10, 2016, Chiba, JapanPASJ2016 TUP054

new laser hut. Three oscillators with 114.24 MHz are usedas seed laser selectively, which is synchronized with themain trigger 2856 MHz at linac. Among them, Menlo fiber
oscillator wavelength at 1030 nm has narrow spectrum so
it can’t be used as seed laser for Nd doped laser systemwith wavelength of 1064 nm. The other two homemadeANDi type oscillators are suitable to Nd laser system andYb laser system at the same time. Recently, a 10.38 MHz
fiber oscillator is being made for direct amplification in Yb
doped single mode fiber without changing repetition rate.
Seed laser is stretched into 30 ps bandwidth after atransmission grating stretcher, then amplified bysemiconductor optical amplifier (SOA) accompanied
repetition rate change from 114.24 MHz into 10 MHz.
Subsequently, seed laser entered into Yb doped single
mode fiber amplifier for both 1030 nm and 1064 nm. After
this stage, different wavelength seed lasers can be selected
and separated by an ASE rejection grating pair.
The 1030 nm seed laser is shifted into Yb:YAG thin disk
regenerative amplifier by two flip mirrors, which are used
to select 1030 nm amplification and 1064 nm laser
amplification. In order to reduce stages numbers of multipass amplifier, the regenerative amplifier is very efficientto achieve almost 106 amplification factor. It has been
demonstrated that the thin disk regenerative amplifier isavailable and effective to nanosecond and picosecond
pulse laser [4]. The last stage for Yb doped laser system iscryogenic amplifier. At low temperature, higher gain andless amplification of spontaneous emission (ASE) can begotten compared with room temperature. At room
temperature the Yb3+ ion has a quasi-three level energy
structure. Aquasi-three level energy structure means thatthe lower laser level is not entirely depopulated inequilibrium. If the lower laser level has some population,
than creating a population inversion is more difficult andrequires more pump energy. If the lower level isdepopulated in equilibrium and decays into the groundstate, then the electronic structure is called a four levelsystem. A four level system will be more efficient because
less energy is necessary for a population inversion. Inaddition, other advantages of cryo-cooling the material
include a reduction in the thermal expansion coefficientwhich leads to less thermally induced mechanical stress.
Any mechanical stress in the material will cause wave front
deterioration. Therefore, stable 1030 nm output laser withhigh beam quality can be achieved by more compact
configuration compare with current laser system [5].
By use of the two flip mirrors, the 1064 nm laser can beinjected into an Yb doped double clad fiber amplifier, thenchanged into 25 Hz or 50 Hz by an E.O. pulse picker. Acustom designed Nd:YAG rod module is used for ring
regenerative amplifier. The module can be operated under
25 Hz or 50 Hz with high amplification factor because
small rod radius and high pump power. Another identicalmodule is used for main amplifier with two passes for seedlaser.

After the final stages of amplification, 1030 nm and1064 nm laser are converted into ultraviolet lasers by twononlinear double frequency stages for injecting into RFgun. According to the plan, the 1064 nm laser system will
be used for SuperKEKB phase II commissioning, the 1030
nm laser system will be adopted for phase III because laser
pulse shaping is required for electron beam with low
emittance [1].

3.

IMPROVEMENTS OF THE 1030 NM
YTTERBIUM DOPED LASER SYSTEM

3.1Improved Au-Sn soldering Yb:YAG thin disk andcopper block composite
As to current laser system at old laser hut, laser
repetition rate has been updated from 2 Hz to 5 Hz and 25
Hz depending on the Yb:YAG thin disk and copper platecomposite, which is combined by gold tin (AuSn)
soldering [6]. Waste heat can be removed by the soldered
Yb:YAG/Cu composite, so it can realize efficient thermal
management and high repetition rate operation. However,the soldered composite must be adhered to copper heat sinkby grease, it reduces the heat removal. In addition, residual
stress is introduced by soldering. It causes the wave front
deterioration and decreases the laser beam quality.
Therefore, new copper block is designed and utilized for
new Yb:YAG thin disk laser system.

Figure 2: Achieved new Yb:YAG thin disk and copper
block soldering composite.
The achieved new Yb:YAG thin disk and copper block
soldering composite is shown in Figure 2. By replacing of
the copper plate, the copper block was used. The squareshape Yb:YAG disk with half an inch length and 1 mmthick, which is soldered onto a projection of Cu block.
Special fixture is designed for providing pressure andfixing the Yb:YAG disk and Cu block. The soldering
material is still AuSn as before because it processes many
merits. 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 ofindium-tin (InSn), this is very helpful for waste heatremoval. Secondly, the deformation of AuSn is weak onheating because its thermal expansion coefficient is- 996 -Proceedings of the 13th Annual Meeting of Particle Accelerator Society of Japan
August 8-10, 2016, Chiba, JapanPASJ2016 TUP054

comparably small. Meanwhile, it is evident from Table 1
that the thermal expansion coefficients of AuSn and copper
are almost the same. Therefore, when laser is operating, theAuSn soldering layer and copper plate can expand
synchronously to avoid fracture of the soldering layer as
much as possible. Finally, AuSn possesses high resistant tocorrosion and high creep resistance.
Table 1: Thermal Properties of AuSn, InSn, Cu, Yb:YAGand CuWMaterial(ingredientsofconstituents)

Meltingpoint(ºC)

Thermalconductivity(W/m/K)

Thermalexpansioncoefficient(10-6/K)

Au80Sn20

280

58

16In50Sn50

118

3420

Cu-

396

16.4

Cu10W90-190

6.3

Cu15W95-200

7.0

Yb:YAG-

6.8

6.8

(a)

(b)

(c)

(d)

new one can achieve good thermal management and low
residual stress for stable and high quality laser output.
Recently, new heatsink material is investigated. As
listed in Table 1, Cu15W85 and Yb:YAG have the smallest
difference for thermal expansion coefficient. It is very
necessary to think about the different of thermal expansion
coefficient for laser active material and heatsink, because
the synchronous expansion can protect the Yb:YAG disk
from fracturing, as well as the intactness of soldering layer.
3.2 Low temperature laser operation
At room temperature, Yb3+ ion has quasi-three levelstructure, so it has high laser threshold. As decreasing as
the temperature, the lower level is depopulated inequilibrium and decays into the ground state, so theelectron structure becomes four level structure. Low laser
threshold and high gain can be obtained. Therefore, the lowtemperature laser operation is important. In addition,excellent thermal management can be realized for low
temperature laser operation. As shown in Figure 4, acompact vacuum chamber is design for low temperaturelaser. Two pieces of Peltier plates are adopted as coolingdevice, the soldering Yb:YAG/Cu composite can also beused in the vacuum chamber. The achieved lowest
temperature is -36 oC under full power operation of Peltierplates.

Figure 3: Reflected laser beam of a fundamental HeNelaser for optical measurement about soldering quality andthermal management. (a) and (b) are for old soldered
composite, (c) and (d) are for new one.
Soldering quality and thermal management are tested for
new soldering composite by use of an optical experimental
setup. For comparison, test results of old design are also
shown in Figure 3. As seen in Figure 3 (a), the reflectedfundamental pattern helium neon laser beam is changed
seriously, it demonstrates that the residual stress is strongthat is introduced by soldering. As to (b), the reflectedbeam is tested under 3 J pump energy, it changes a little sowaste heat is removed efficiently. About the soldering
composite, the residual stress is much weaker than the oldone, as shown in (c). Under 3 J pump, the reflected test
laser beam is changed, but the distortion is not serious.
Therefore, by comparing with the old soldering design, theFigure 4: low temperature laser vacuum chamber.
In order to test the amplification factor under diferenet
temperature for seed laser with different wavelength, ahomemade tunable fiber laser is used. As shown in Figure
5, the seed injects into the chamber from left side, thenimpinges into the Yb:YAG disk from side face. Pump laser
emitted from laser diode stack is focused onto the front
surface of the Yb:YAG disk, the active area is about 12.7mm x 4 mm. By meticulous alignment, amplified seed laser
can be measured during pump duration by a pin photodiodeby filtering the powerful pump laser, then we can get theamplification factor by comparing with the seed laser
without pump. The output power of seed laser is 10 mW,so accurate amplification can be tested below saturation.

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Proceedings of the 13th Annual Meeting of Particle Accelerator Society of Japan
August 8-10, 2016, Chiba, Japan

PASJ2016 TUP054

LD stack, active area is about 5 mm x 5 mm after acylindrical lens and a dichromatic mirror. Calculated beam
size inside the cavity is shown in Figure 8.

Figure 5: Experimental setup for low temperature laser
amplification factor test.

Figure 7: Regenerative amplifier cavity configuration.

Position inside cavity (mm)Figure 8: Beam radii in cavity shown in Figure 7.

Figure 6: Amplification factor under different temperaturefor different wavelength seed laser.
As seen in Figure 6, the amplification factor at low
temperature is much high than that at room temperature.
For instance, the amplification factor at 1030 nm,
absorption peak for Yb:YAG, is almost 5 times higher thanthat at ambient temperature. It demonstrates that more
efficient amplification can be realized by this compact low
temperature laser design. At the same, the thermal effect ismuch weaker than the current laser operation.
3.3 Regenerative amplifier
For nanosecond and picosecond pulse laser, regenerative
amplifier is an effective candidate because it has very highamplification factor, our cooperator has achieved very
stable and effective regenerative amplifier successfully,
and the parameters of their laser system is similar with our
new laser system design [4]. Therefore, it is necessary toadopt a regenerative amplifier as a preamplifier after fiberpart.
A regenerative cavity has been designed, as shown inFigure 7. The seed laser comes from the fiber passes thecombination of beam splitter, Faraday rotator and halfwave plate, then enters into the amplification cavity.
During Pockels cell on, the seed laser pulse can be trapped
into the cavity and amplified many times. Two identicalsets of telescope are places in the cavity for getting parallel
beam and compensating the thermal lens effect of Yb:YAGdisk. The Yb:YAG/Cu soldering composite is used whichis introduced in the above parts. Pump source is a 10 kW

The regenerative amplifier has been built and tested, theamplification of seed laser inside the cavity is monitored
by a PD, which is behind a mirror for testing the leak laser,
as shown in Figure 9. Amplification build up process inside
the cavity can be seen clearly. However, output energy is
not enough until now due to alignment and instableenvironmental problems. The experiment is under doing
for achieving ~mJ amplified laser for cryogenic laser
amplifier.

Figure 9: Amplification build up process inside the cavity.

4.

CONCLUSION

Another new laser system is being built at another newlaser hut based on the operation situation of current lasersystem. For realizing the demands of follow SuperKEKB
phases, the new laser system should possesses more
flexibilities. For example, it should have more space fordouble bunch operation layout and laser pulse shaping.
Therefore, improvements of the laser system have been- 998 -Proceedings of the 13th Annual Meeting of Particle Accelerator Society of Japan
August 8-10, 2016, Chiba, Japan

PASJ2016 TUP054

being done.
As to the solid part of new laser system, new soldering
Yb:YAG disk and copper block composite is made forgetting high beam quality output laser and low thermaleffect. Meanwhile, this new soldering laser head is also
used for low temperature laser operation. A compact
vacuum chamber is designed, Peltier plates are adopted ascooling device. Theoretical analysis and experimental
study indicate that the amplification factor is higher thanthat at room temperature. In addition, regenerative
amplifier is designed and under building. In brief, all theefforts are being done for obtaining stable and high output
energy laser system for RF gun of SuperKEKB.

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[2] T. Natsui et al., “Quasi-Traveling Wave RF Gun and BeamCommissioning for SuperKEKB”, Proceedings of IPAC
2015, USA, Richmond, May 3-8, 2015.
[3] X. Zhou et al., “Ytterbium fiber and disk laser of RF Gun for
SuperKEKB”, Proceedings of IPAC 2015, Germany,
Dresden, Jun 15-20, 2014.
[4] M. Chyla et al., “Optimization of beam quality and opticalto-optical efficiency of Yb:YAG thin-disk regenerative
amplifier by pulsed pumping”, Opt. Letts., vol. 39, pp. 14411444, 2014.
[5] M. Divoky et al., “Overview of the HiLASE project: highaverage power pulsed DPSSL systems for research andindustry”, High Power Laser Science and Engineering, vol.
2, e14, pp. 1-10, 2014.
[6] R. Zhang et al., “Improvements of the Laser System for RFGun at SuperKEKB Injector”, Proceedings of IPAC 2015,
USA, Richmond, May 3-8, 2015.

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