Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

PASJ2014-SAP117

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), 夏井拓也 A) , 吉田光宏 A), 小川雄二郎 A)
Rui Zhang, Xiangyu ZhouA), Takuya NatsuiA), Mitsuhiro YoshidaA), Yujiro OgawaA)
A)High Energy Accelerator Research Organization (KEK)#, A)

Abstract
For SuperKEKB project, the electron beams with a charge of 5 nC and a normalized emittance of 10 μm are expectedto be generated in photonchathode RF gun at injector linac. Compared to the previous laser system, current ytterbium
laser system is operating in high repetition rate of 25 Hz but suffering thermal effect. In order to minimize the thermaleffect, ytterbium cascade laser design is proposed. The proposal is different from the standard ytterbium laser, 1035 nm
laser is selected as pump source. As to the output or amplified laser at 1050 nm, the quantum defect can be reduced tojust 1.5% with minimized thermal effect. Meanwhile, higher gain and less reabsorption can be obtained. At present,Yb:KGW and Yb:BOYS crystal are chosen as the gain materials. For out next step experiment, the laser properties of thecrystals at 1050 nm are tested and studied.

1.

INTRODUCTION

Higher luminosity is required in the SuperKEKB, thephotocathode RF gun with strong electric focusing filed forhigh-current, low-emittance should be adopted in theinjector linac [1]. For generating electron beams with acharge of 5 nC and a normalized emittance of 10 μm in thephotocathode 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. Furthermore, for reducing theenergy spread, the laser pulse should be reshaped torectangle from Gaussian shape [2].
As shown in Table 1, to satisfy the above conditions for
the SuperKEKB, a laser source should with central
wavelength of 260 nm, pulse width of 30 ps, pulse energyof 1 mJ is required. In addition, for the stable operation of
the SuperKEKB, the stability of the laser system is also
important. In order to synchronize with the SuperKEKB,
the repetition rate of the laser must be set at 51.9 MHz.
Table 1: Required Laser Source
Pulse Property

Required Date

Repetition rate of Oscillator

51.9 MHzCentral wavelength~260 nm

Pulse Width (FWHM)~30 ps, reshape

Pulse Energy>mJ

Spectral Width>6 nm

OthersStale, compact, removable___________________________________________# rui.zhang@kek.jp

With the aim of achieving the demands on the laser
source, a hybrid laser system which include an ytterbium(Yb) ions doped fiber oscillator, Yb-doped fiber amplifier
and thin disk Yb:YAG amplifiers. In 2 Hz repetition ratesetup, more than 1 mJ UV pulse was obtained. As a result,the electron beams with a charge of 5.6 nC was generated[3]. When the laser system was upgrade to 25 Hz, 20 mJ
fundamental laser pulse energy and 400 J UV pulse wereobtained and 0.8 nC electron beam was gotten. Because of
the thermal effect, the laser quality degradation induced thereduction of electron charge.
In order to minimize the thermal effect in high repetition
rate operation, Yb cascade laser design was proposed.
Before developing the Yb cascade laser and amplifier, it isnecessary and important to test the laser properties of Yb
doped crystals. In this paper, we introduce the Yb cascadelaser design basing on our current laser system and report
the test results of Yb doped laser crystals. Excellent
thermal management and potential application would beexpected.

2.

CURRENT LASER SYSTEM

Basing on the above requirements for SuperKEKB
project, suitable laser crystal should be selected. Yb ions
doped laser crystal possesses wider bandwidth compared
to Neodymium-doped crystal laser, so it is relatively
simple to adjust the laser pulse shape to rectangular.
Secondly, Yb-doped laser crystal has comparative long
fluorescent life time of almost 1 ms, this is very suitable
for archiving high laser output power. Thirdly, Yb doped
laser crystal has small energy level difference at room
temperature. Current commercial 970 nm laser diode canbe used as excellent pumping laser source for Yb-dopedlaser, and 1030 nm output laser can be generated. It makes- 853 -Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

PASJ2014-SAP117

the quantum efficiency become high comparing with theother rare earth doped laser at 1 micron wavelength. Finally,
Yb-doped glass fiber laser offers quite stable laser
oscillator with high repetition, high output power and highenergy extraction efficiency and so on. In particular, Yb
doped fiber laser provides broader bandwidth; it is good for
adjusting the seed laser wavelength in the solid stale
amplifier stages. Therefore, on the basis of the above
characteristics of Yb-doped laser, it is the most suitable
candidate for the photocathode RF gun in SuperKEKB
injector.
From the end of last century, the fiber laser and thin disklaser have attracted a great of attention to utilize for furtherprogress. The fiber laser offers high repetition rate,
excellent amplification efficiency, simple configurationand low cost. Especially, the stability of the fiber laser isevidently higher than solid state laser because of the
optimized thermal removal. In addition, the thin disk laser,which possesses very favorable thermal management,
makes the high energy pulse amplifier become possible
and available. It also ensures the high output power fromthin disk laser by using of the current commercial highpower laser diode as pump laser source because of thethermal lens effect.

Figure 1: Layout of current laser system.
The current laser system of RF gun is designed as ahybrid system, which include Yb-doped fiber oscillator,
Yb-doped fiber amplifiers and Yb:YAG amplifiers. The
schematic diagram of the laser system is shown in Figure
1. The current laser system operated in repetition rate of 25
Hz, 0.8 nC electron charge was generated. Compared withthe former laser system of 5 Hz repetition rate, the beam
charge is lower. In order to overcome the thermal effect,
we plan to apply Yb cascade laser design and test theavailability of it in advance.

3.

wavelength of output laser or amplifier laser is 1050 nm.
The layout of Yb cascade laser is shown in Figure 2.

Figure 2: Layout of Yb cascade laser design.
By utilizing the Yb cascade laser design, quantum defectreduce to 1.5%. In contrast, the quantum defects are 10.1%
and 6.7% for 941 nm and 970 nm pump lasers respectively.
This low quantum defect can realize low thermal lenseffect. Meanwhile, higher gain can be gotten benefits fromthe selection of 1035 nm pump source because it can get
rid of reabsorption effect in Yb doped laser crystals. In
addition, it is easier to manage the 1035 nm pump laserconfiguration and get higher excitation density. Because
1035 nm pump laser we achieved with excellent beamquality.
At present, we have gotten excellent and stable 1035 nm
Q-switch laser, this is a good pump source candidate forYb cascade laser. Besides, basing on our current laser
system, 1050 nm seed laser can be outputted from Yb-fiber
laser stage. Both of them ensure Yb cascade laser can bedeveloped in the following days in our lab.
3.2 1035 nm pump laser source
For obtaining excellent and stable 1035 nm pump laserfor Yb cascade laser, we built a Yb:YAG thin disk Qswitch laser. As shown in Figure 3, the Yb:YAG thin disk
with 0.5 mm thickness is attached to a copper heat sink. 4
kW quasi-CW laser diode stack of 940 nm is the pumpsource, and the pumping laser beam is focused on bothhorizontal and vertical directions. In the laser cavity, one
cubic polarizer and a one quarter wave plate are adopted tocontrol the output polarized laser. The Pockels cell is usedas the Q-switcher.

YTTERBIUM CASCADE LASER

3.1 Principle of Yb cascade laserStandard Yb doped crystal laser and amplifier are
pumped by 941 nm or 970 nm laser, because of the main
absorption peaks for Yb materials. Comparing with thestandard configuration, the Yb cascade laser design we
proposed is selected 1035 nm laser as pump source, theFigure 3: Experimental setup of the Yb:YAG thin disk Qswitch laser.
When the Q-switcher is turned on, we can get a stablepulses output laser. Figure 4 shows scalability of the output- 854 -Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, JapanPASJ2014-SAP117

pulse energy. The maximum laser output energy wasnearly 110 mJ. The inset of Figure 4 shows the output laserspectrum at central wavelength of 1035 nm, and FWHM isnearly 8 nm. By an oscilloscope, the laser pulse width ismeasured to be nearly 200 ns.

Yb:KWG and Yb:BOYS at 1050 nm which are pumped by1035 nm laser, The future experimental setup is shown inFigure 6. The 1050 nm seed laser is a commercial fiber
coupled output tunable wavelength LD laser.

Figure 5: Yb:KGW and Yb:BOYS crystal samples.
When the 1050 seed laser is impinged the Yb doped
crystal which is pumped by 1035 nm laser, the output
power of amplified 1050 nm laser can be gotten by

Poutput = Pin exp( gL),
Figure 4: Output pulse energy of Yb:YAG Q-switch laser
versus the pump energy. Inset: the spectrum of output laser.
By utilizing the Q-switch laser, we can test the opticalproperties of the Yb-doped laser crystals, such as theabsorption coefficient at 1035 nm and the emissioncharacters at longer wavelength.
3.3 Laser properties of Yb:BOYS and Yb:KGW
For realizing Yb cascade laser, Yb3+:KGd(WO4)2(Yb:KGW) and Yb3+:Sr3Y(BO3)3 (Yb:BOYS) are selected
as the candidates, the laser properties of them are illustrated
in Table 2 [4]. From the table we can see the Yb:KGW
crystal with high stimulated emission cross section, highabsorption efficiency and low laser threshold. Yb:BOYS
crystal possesses boarder tuning range and longer lasingwavelength. Figure 5 shows the picture of the processed
crystal samples used in our experiment.

(1)where g is the gain coefficient and L is the length of crystal.
We adjusted the 1035 nm and 1050 nm lasers to overlap
and pass through the Yb doped crystal along the length
direction. Then they were separated each other by apolarizer. In order to eliminate the fluorescence emission,
long propagation distance was necessary before theamplified seed laser was measured by a photodiode.
Comparing with the Q-switch laser pulse duration, theupper laser level lifetime of Yb ions is much longer, so we
can measure the 1050 nm laser power variation by anoscilloscope during this steady-state duration.

Table 2: Laser Properties of Yb:KGW and Yb:BOYSProperties

Yb:KGW

Yb:BOYSAbsorption wavelength (nm)

981

975

Peak absorption cross section(10-21 cm2)120 (E ǁ a)

σ 7.3
π 9.4

Fluorescent lifetime (ms)

0.75

1.1

Room temperature laser
wavelength (nm)

1026

1066

Emission cross section at laser
28
wavelength (10-21 cm2)

2

Emission bandwidth (nm)2060

Tuning range (nm)1020-1070

1017-1086

Mean refractive index2.03

1.75

Figure 6: Experimental setup for testing the small signal
gain coefficient Yb:KWG and Yb:BOYS at 1050 nm.

Figure 7: The pump laser spot burning hole on a film paper
under microscope.

It is very important and valuable for the later cascade
experiment to test the small signal gain coefficients ofFor calculating the estimated small signal gain
coefficients of Yb:KGW and Yb:BOYS, we used a film- 855 -Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

PASJ2014-SAP117paper to get the burning hole of the pump laser forobtaining the beam diameter, the measured pump laser spot
size is shown in Figure 7. Then according to formula (1)and crystal parameters, the estimated small signal gain
coefficients of Yb:KGW and Yb:BOYS are 0.116 cm-1 and0.105 cm-1 respectively. We are going to test the accurate
small signal gain coefficients in the next step experimentby the experimental layout which is shown in Figure 6.

4.

CONCLUSION

For obtaining high current, low emittance beams in theSuperKEKB project, the current hybrid laser system
operated in repetition rate of 25 Hz and 0.8 nC electron
charge was generated.
To overcome the thermal lens effect, Yb cascade laser
configuration is proposed. This design can minimize thethermal effect in high repetition rate laser system and gethigher gain.
Yb:KGW and Yb:BOYS crystals were selected as theproper candidates for cascade laser design. The estimatedsmall signal gain coefficients at longer wavelength werecalculated. We will measure the accurate values in our next
step experiment.

REFERENCES[1] M.Yoshida, et al., “Longitudinal manipulation to obtain andkeep the low emittance and high charge electron beam forSuperKEKB injector”, Proceedings of IPAC 2013, Shanghai,
China, May 13-17, 2013.
[2] T. Natsui, et al., “Quasi traveling wave side couple RF gunfor SuperKEKB”, Proceedings of IPAC 2013, Shanghai,
China, May 13-17, 2013.
[3] X. Zhou, et al., “Ytterbium fiber and disk laser of RF gun for
SuperKEKB”, Proceedings of IPAC 2014, Dresden,
Germany, Jun 15-20, 2014.
[4] S. Chenais, et al., “Spectroscopy and efficient laser action
from diode pumping of a new broadly tunable crystal Yb3+Sr3Y(BO3)3,” J. Opt. Soc. Am. B 19, (2002) 1083.

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