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 emittanceof 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 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. 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
was gotten [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 and injected 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,___________________________________________# rui.zhang@kek.jp

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 thindisk laser 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 is shared by two laser systems. For
realizing backup for SuperKEKB, four seed lasers areprepared, as shown in Figure 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new laser hut. Three oscillators with 114.24 MHz areused as seed laser selectively, which is synchronized withthe main trigger 2856 MHz at linac. Among them, Menlofiber oscillator wavelength at 1030 nm has narrowspectrum so it can’t be used as seed laser for Nd doped
laser system with wavelength of 1064 nm. The other two
homemade ANDi type oscillators are suitable to Nd laser
system and Yb laser system at the same time. Recently, a10.38 MHz fiber oscillator is being made for direct
amplification in Yb doped single mode fiber withoutchanging repetition rate. Seed laser is stretched into 30 ps
bandwidth after a transmission grating stretcher, thenamplified by semiconductor 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 and1064 nm. After this stage, different wavelength seedlasers can be selected and separated by an ASE rejection
grating pair.
The 1030 nm seed laser is shifted into Yb:YAG thindisk regenerative amplifier by two flip mirrors, which areused 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 efficientbecause less energy is necessary for a population
inversion. In addition, other advantages of cryo-coolingthe material include a reduction in the thermal expansion
coefficient which leads to less thermally induced
mechanical stress. Any mechanical stress in the material
will cause wave front deterioration. Therefore, stable
1030 nm output laser with high beam quality can beachieved by more compact configuration compare withcurrent laser system [5].
By use of the two flip mirrors, the 1064 nm laser can beinjected into an Yb doped double clad fiber amplifier,then changed into 25 Hz or 50 Hz by an E.O. pulse picker.
A custom 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 forseed laser.
After the final stages of amplification, 1030 nm and1064 nm laser are converted into ultraviolet lasers by two
nonlinear double frequency stages for injecting into RF
gun. According to the plan, the 1064 nm laser system will
be used for SuperKEKB phase II commissioning, the1030 nm laser system will be adopted for phase III
because laser pulse shaping is required for electron beamwith low emittance [1].

3.

IMPROVEMENTS OF THE 1030 NM
YTTERBIUM DOPED LASER SYSTEM

3.1

Improved 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 heatsink by grease, it reduces the heat removal. In addition,
residual stress is introduced by soldering. It causes thewave front deterioration and decreases the laser beam
quality. Therefore, new copper block is designed andutilized 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 mm
thick, 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 thatof indium-tin (InSn), this is very helpful for waste heat

removal. Secondly, the deformation of AuSn is weak onheating because its thermal expansion coefficient iscomparably small. Meanwhile, it is evident from Table 1
that the thermal expansion coefficients of AuSn andcopper are almost the same. Therefore, when laser isoperating, the AuSn soldering layer and copper plate can
expand synchronously to avoid fracture of the soldering
layer as much as possible. Finally, AuSn possesses highresistant to corrosion and high creep resistance.
Table 1: Thermal Properties of AuSn, InSn, Cu, Yb:YAG
and CuW
Material(ingredientsofconstituents)

Melting
point(ºC)

Thermalconductivity(W/m/K)

Thermalexpansioncoefficient(10-6/K)

Au80Sn20

280

58

16

In50Sn50

118

34

20

Cu

-

396

16.4

Cu10W90

-

190

6.3

Cu15W95

-

200

7.0

Yb:YAG

-

6.8

6.8

(a)

(b)

(c)

(d)

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 opticalexperimental setup. For comparison, test results of old
design are also shown in Figure 3. As seen in Figure 3 (a),the reflected fundamental pattern helium neon laser beam
is changed seriously, it demonstrates that the residual
stress is strong that is introduced by soldering. As to (b),the reflected beam is tested under 3 J pump energy, itchanges a little so waste heat is removed efficiently.
About the soldering composite, the residual stress is much
weaker than the old one, 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 oldsoldering design, the new one can achieve good thermal
management and low residual stress for stable and highquality 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 andheatsink, because the synchronous expansion can protect
the Yb:YAG disk from fracturing, as well as theintactness 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, thelow temperature 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 be
used in the vacuum chamber. The achieved lowest
temperature is -36 oC under full power operation of
Peltier plates.

Figure 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 thefront surface of the Yb:YAG disk, the active area is about
12.7 mm x 4 mm. By meticulous alignment, amplified
seed laser can be measured during pump duration by a pin
photodiode by filtering the powerful pump laser, then we
can get the amplification factor by comparing with theseed laser without pump. The output power of seed laser
is 10 mW, so accurate amplification can be tested below

saturation.

effect of Yb:YAG disk. The Yb:YAG/Cu soldering
composite is used which is introduced in the above parts.
Pump source is a 10 kW LD stack, active area is about 5
mm x 5 mm after a cylindrical 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
than that at ambient temperature. It demonstrates that
more efficient amplification can be realized by thiscompact low temperature laser design. At the same, thethermal effect is much weaker than the current laser
operation.
3.3

The regenerative amplifier has been built and tested,
the amplification of seed laser inside the cavity ismonitored 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 instable environmental problems. The experiment isunder doing for achieving ~mJ amplified laser forcryogenic laser amplifier.

Regenerative amplifier

For nanosecond and picosecond pulse laser,
regenerative amplifier is an effective candidate because it
has very high amplification factor, our cooperator has
achieved very stable and effective regenerative amplifier
successfully, and the parameters of their laser system issimilar with our new laser system design [4]. Therefore, itis necessary to adopt a regenerative amplifier as apreamplifier after fiber part.
A regenerative cavity has been designed, as shown in
Figure 7. The seed laser comes from the fiber passes the
combination of beam splitter, Faraday rotator and halfwave plate, then enters into the amplification cavity.
During Pockels cell on, the seed laser pulse can betrapped into the cavity and amplified many times. Two
identical sets of telescope are places in the cavity forgetting parallel beam and compensating the thermal lensFigure 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 laser
system. 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
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 thermal
effect. 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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for SuperKEKB”, Proceedings of IPAC 2015, Germany,
Dresden, Jun 15-20, 2014.
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