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の検討
STUDY ON STABLE AND HIGH OUTPUT ENERGY LASER SYSTEMFOR RF-GUN AT SUPERKEKB INJECTOR張叡#, A), B), 周翔宇 A), B), 夏井拓也 A), B) , 佐藤大輔 A) , 吉田光宏 A), B)
Rui Zhang #, A), B), Xiangyu ZhouA), B), Takuya NatsuiA), B), Daisuke Satoh A), Mitsuhiro YoshidaA), B)
A)
High Energy Accelerator Research Organization (KEK)
B)
The Graduate University of Advanced Studies (SOKENDAI)

Abstract
SuperKEKB phase I commissioning finished in 2016,
according to the results of RF gun and laser systemoperation, some improvements are under doing forachieving more stable laser system. Neodymium laser
system is decided to be used for SuperKEKB phase IIcommissioning. Meanwhile, another RF gun is built forelectron generation. In order to realize reliable andcontinuous laser operation for RF gun, a backup
Neodymium solid state laser system will be built recently.
Spatial filter will be adopted for getting high laser beam
quality. On the other hand, Ytterbium laser system will beused in SuperKEKB phase III. Ethanol cooling system isunder designing to generate high laser output energy withgood laser beam quality to remove thermal effect underhigh repetition rate.

INTRODUCTION
With the aim of achieving SuperKEKB project
demands on electron beam with high charge and low
emittance, RF gun is adopted as electron generator. Since
the RF gun processes strong focusing electric field andhigh accelerating efficiency, it is possible to generate highquality electron beam for accelerator. Thence, hybrid laser
system which consist with of fiber laser and thin disklaser is used to generate UV laser for Iridium Cerium
photocathode with high quantum efficiency inside the RF
gun [1]. According to this basic guideline, an Ytterbium(Yb) hybrid laser system has been exploited and appliedfor SuperKEKB phase I commissioning. About 1.5 nC
electron is generated successfully and injected into
SuperKEKB High Energy Ring (HER) in 2016 summer.
Meanwhile, 10 days stable injection with 1 nC is also
accomplished smoothly for the first time.
According to the commissioning results of SuperKEKB
phase I, a lot of improvement should be done for lowemittance and high charge experiment in the following
phases. Stability of laser operation is the most important
issue. Thermal effect of the thin disk laser part wasserious because the combination method of Yb:YAG thindisk and heatsink. Inefficient thermal removal induced
thermal lens effect, it increased the instability of laser
system. More efficient thermal removal method should beexplored for laser operation under high repetition rate in________________________________________#rui.zhang@kek.jp

the future. Meanwhile, stable temperature of laser room isalso important for stable laser operation. Necessary
temperature management should be done. In addition, lowemittance of electron beam is very important for the later
phase commissioning. Although the RF gun design
processes strong focusing electric field, the laser beam
quality and space profile are also important to generatehigh quality electron beam. Therefore, besides theincreasing of laser pulse energy, the reshaping of laser
beam also should be explored for SuperKEKB phase II.
Spatial filter will be designed and used to increase thelaser beam quality for phase II laser system.
SuperKEKB Phase II will start from the end of thisfiscal year. With the aim of achieving the demand onelectron charge of 2 nC, an additional Neodymium (Nd)laser system is prepared and tested. By using of Nd:YAGrod laser modules, about 150 μJ UV laser is generated for
RF gun. Accordingly, 1.5 nC electron charge is gotten at
25 Hz operation [2]. Basing on the Phase I
commissioning results, laser box is built for more stablelaser operation to improve the space stability of electroncharge. Additionally, in order to get high laser beam
quality, spatial filter is under designing and will be
installed in backup line of Nd:YAG laser amplifier. At thebeginning of this year, another new RF gun was installedat 90 degree arrangement. New UV laser generation partis designed in tunnel which makes one laser system beavailable to two RF guns. Meanwhile, remote control
monitoring and measurement system will be inserted into
for real time checking green laser and UV laser beam
quality, as well as the pulse energy during commissioning.
SuperKEKB phase III operation will still use the Yb
hybrid laser system. For 25 Hz double pulses or 50 Hz
laser operation, more efficient thermal management
should be considered. Therefore, more excellent
combination method of Yb:YAG and heatsink will beadopted. In order to generate high laser output energy
with low nonlinear effect, low temperature laser system isnecessary. At the same time, it is also available to avoid
serious thermal effect. Peltier cooling laser is tested, gainof laser is higher that ambient laser operation. However,
not enough refrigerating effect as well as temperature
instability are not suitable to stable laser operation. Moreeffective ethanol cooling laser system is underinvestigation.

NEODYMIUM LASER SYSTEM FOR
SUPERKEKB PHASE II COMISSIONING
A hybrid laser system which is made of Yb-doped fiberpart and Nd:YAG rod laser modules part has been built
for SuperKEKB phase II. The layout of this laser system
is shown in Figure 1. Seed laser is generated by a mode
lock Yb-doped fiber oscillator, then it is amplified by Ybdoped fiber pre-amplifiers. The pulse width of seed laser
is stretched into about 10 ps by transmission grating pair.
Because the spectrum of seed laser is broad, 1030 nm and1064 nm components can be selected by use of a grating
and slit. The selected two parts are injected into 1030 nm
and 1064 nm amplifier system. For phase II laser system,
the following active material is Nd:YAG, the 1064 nm
laser is chosen and amplified gradually. Changing therepetition rate into 25 Hz by using of E.O. module, 4stages of Nd:YAG rod laser modules are adopted forbuilding multi-pass amplifiers. Low gain type modules
are used for multi-pass amplifier 1 and 2. Both of themhas 2 pass configuration. The other type, high gain
module, is used for 4 pass amplifier 3 and 2 pass
amplifier 4. After all the amplifiers, about 3.0 mJ pulse
energy is achieved at 1064 nm.

Figure 1: Layout of the Nd hybrid laser system forSuperKEKB Phase II commissioning.
Make use of BBO, about 1.3 mJ 532 nm green laser isgenerated at laser hut. In order to decrease the laser
instability which is induce by air flow, a 15 m longvacuum transportation line will be built in the August.
The second BBO crystal for UV laser generation will beplace into RF gun box in tunnel. One laser source should
be used to two RF guns, so new optics system in tunnel
also will be built before this October. Details are
introduce in the following two parts.

Optics injection system for 0o and 90o RF gun intunnel
A 90o injection RF gun is installed in the beginning of
this year [3]. The layout of A1 sector at linac is shown in
Figure 2. It has been tested during this spring operation.
1.5 nC electron charge is generated successfully andaccelerated. In the future, this RF gun will be adopted as
secondary electron source for SuperKEKB project. With
the aim of realizing one laser system for two RF guns,
new optics injection system is designed and will be built
from this August.

The green laser is transported from ground laser hut tounderground tunnel by the vacuum transporting line. Thenit enters into the 0o RF gun box, as shown in the lower

Figure 2: Layout of 0o and 90o RF gun at linac. Blue
triangle zone is the optics box for 90o RF gun. Optics
system for 0o RF gun is shown in the lower right corner.
right corner of Figure 2. Before the green laser impinges
into the BBO crystal, a flip mirror is set to change thegreen laser direction for 0o and 90o RF gun. By using of
this arrangement, one laser source can be available toboth gun. Tow fourth harmonic generation setups are
installed at 0o RF gun optical table and 90o RF gun opticaltable. The blue triangle zone in Figure 2 is the 0o RF gun
optical table.
According to the previous electron generation history,
the electron charge is closely related to the laser beamquality and pulse energy. Therefore, in order to get stable
and high electron charge, remote control monitoring andmeasurement system are very important for phase IIcommissioning. Remote control flip mirror will also be
inserted into the laser transportation line for reflecting theUV laser onto energy senor to realize real time measuring.
In addition, the green laser beam profile can be monitor
by remote control beam profile monitor. During theoperating time of RF gun, the adjustment of laser systemis operable according to the measured laser beam profile.

Spatial filter for backup line of Nd rod laser
amplifiers
As mentioned above, electron charge generated by theRF gun is closely related to the laser beam quality, notonly to the laser pulse energy. Meanwhile, excellent
injection laser beam quality is very helpful to generate
low emittance electron charge. Distinguished from theregenerative amplifier, amplified laser beam quality byuse of multi-pass amplifier is not good. Moreover, badthermal management of rod laser configuration also
decreases the beam quality. Therefore, spatial filter isnecessary for cleaning the amplified laser beam after all
the Nd:YAG rod laser amplifiers.
For ensuring SuperKEKB phase II commissioning, abackup line of Nd:YAG laser system will be built next tothe current laser system. High gain laser modules will be
used in this line, as well as the spatial filter. By
calculation, three types of slits and pinholes are designed,

in order to achieve exchange easily, all the slits andpinholes are manufactured on one thin tungsten plate, as
shown in Figure 3. By use of feedthrough, the proper slitand pinhole can be chosen, which is basing on theincident laser beam size. Cross shape slits are preparedfor vertical and horizontal directions beam cleaning withthe help of cylindrical lensed. The whole length of it isabout 3 m, focusing lenses are placed outside the vacuum
pipe for flexible adjustment.

amplification factor. After that, normal Yb:YAG thin disk
multi-pass amplifier will be adopted. Compare with theregenerative amplifier and loop amplifier, theamplification factor is not so high for normal multi-pass
amplifier, but big laser beam amplification is suitable tohigh energy laser amplification. It prevents fromdamaging of optical components and laser crystals, also
decreasing the nonlinear effects. The last amplifier isethanol cooling amplifier. Low temperature cooling laser
is very suitable to release the thermal effect and increase
the gain of amplifier. For testing the low temperature laser
characteristics, low temperature laser operation which isachieved by Peltier plates is tested before ethanol coolingexperiment.

Low temperature laser testFigure 3: Cross section of spatial filter for Nd:YAG laser
system. The plane shown in the figure is the laser
injection plane.
For the purpose of decreasing the vibration that isinduce by scroll pump, vacuum valve should be used.
Scroll pump only works during the vacuum level is not
good, the other time vacuum situation can be kept by thevalve to stop scroll pump for the stable laser operation.
Back up line and spatial filter building will started in thefollowing days.

By using of the compact vacuum chamber and Peltier
plates, -40 oC is achieved for laser operation. The setup isshown in Figure 5. A dichroic mirror is used to realize
face pump and seed laser face injection arrangement,
which is placed before a window of the vacuum chamber.
3.3 kW pump laser is focused on the surface of Yb:YAGdisk with an area of 8 mm x 6 mm at repetition rate of 25
Hz. The 1 mm thick Yb:YAG thin disk is soldered onto aCuW heatsink by indium soldering method [4]. Input seedlaser pulse energy is about 1.5 mJ with beam radius about
4 mm.

YTTERBIUM LASER SYSTEM FOR
SUPERKEKB PHASE III
A hybrid laser system which is made of Yb-doped fiberpart and Yb:YAG thin disk amplification part will be
explored for SuperKEKB phase III. The laser systemframework is shown in Figure 4. The seed laser and fiber
amplifier are almost the same as them of Nd hybrid laser
system. In order to decrease the thermal effect, thin disklaser configuration will be utilized. Because regenerative

Figure 4: Layout of Yb hybrid laser system forSuperKEKB phase III commissioning.
amplifier can realize high amplification factor, at the sametime, the amplified laser beam quality is better due to themode selection of regenerative amplifier cavity. Then, athin disk loop amplifier will be built due to it has highFigure 5: Experiment setup of Peltier cooling 3 passmulti-pass amplifier stage.
3 passes amplifier is built for testing the amplification
factor. Laser pulse energy for every pass is shown in
Figure 6. After 3 pass amplification, 11.6 mJ laser pulse
energy is achieved. This pulse energy is almost same as
the energy of laser system which is used in SuperKEKB
Phase I, but pass number of multi-pass amplifier is muchless [4]. Obviously, the amplification factor is higher at
low temperature operation, so less amplifier stage can beadopted to reduce the size of laser system and improve
the stability. In addition, it is very helpful to decrease theunwanted nonlinear effect inside the Yb:YAG crystal byusing of bigger pump size and seed laser beam size.
Although the low temperature operation can be realized
by the Peltier plates, the temperature stability is aquestion. With the change of temperature, the amplified
laser pulse energy changes. It causes the electron chargegenerated by RF gun to fluctuate. In addition, the coolingcapability of Peltier plate is not enough for higher laser
amplification.

Figure 6: Laser pulse energy of the Peltier cooling multipass amplifier under 3.3 kW pump laser.

Ethanol Cooling Laser Amplifier
According to the amplification results of low
temperature experiment, it is known that the lowtemperature laser operation is very efficient. Comparing
with cryogenic laser experiment, amplification factor oflow temperature is not so high, but the gain spectrum oflaser crystal is broader than that at cryogenic temperature.
It is suitable to do laser pulse shaping for generating
electron beam with low emittance. Therefore, we willbuild a low temperature laser amplifier for SuperKEKB
phase III.
Due to the insufficient cooling capability of the Peltiercooling plate, Stirling refrigerator is chosen for low
temperature laser operation. Because the freezing point of
ethanol is low than -100 oC, it will be used as
refrigeration fluid. Operation temperature will be set from-100 oC to -80 oC, this temperature is enough for our low
temperature laser experiment. Moreover, the gain
spectrum width is not narrow. The experiment setup
design is finished, as shown in Figure 7. Homemade
Yb:YAG/CuW laser head will be placed into a smallchamber. This experiment will be started and tested in thiswinter, it is also the final amplification for generatinghigh charge electron beam during SuperKEKB phase III
commissioning.

Figure 7: Designed experimental ethanol cooling laser
amplifier.

CONCLUSION
A new hybrid laser system have been being done forSuperKEKB phase II commissioning basing on theoperation results of SuperKEKB phase I. It consists of Yb
doped fiber laser and Nd:YAG rod laser module multipass amplifier part. About 3 mJ 1064 nm laser isgenerated, 200 μJ UV laser will be used for RF gun.
Another new RF is installed as the secondary electron
source for phase II operation. Accordingly, new laser
optics system for two RF guns will built to realize one
laser system application for two guns. In addition, inorder to generate electron charge with low emittance,spatial filter will be inserted after all the Nd:YAG rod
laser amplifiers.
Furthermore, low temperature laser amplifier is tested
for getting high laser pulse energy, which is available toour homemade new Yb:YAG disk laser head. Compare to
the ambient and cryogenic laser operation, it is more
efficient with broader gain spectrum. As to SuperKEKB
phase III operation, this is very suitable to do laser pulse
shaping for obtaining high charge electron with lowemittance. An ethanol cooling laser amplifier is design
and will be built at the end of this year.

REFERENCES[1] M. Yoshida et al., “SuperKEKB injector upgrade forhigh charge and low emittance electron beam”,
in Proc. IPAC’12,New Orleans, USA, paper
TUPPD035.
[2] X.Y. Zhou et al., “Yb fiber and Nd:YAG hybrid laser
system of RF gun for SuperKEKB's phase-IIcommissioning”, presented at PASJ’17, Sapporo,
Hokkaido, this conference.
[3] T. Natsui et al., “RF gun development forSuperKEKB”, presented at PASJ’17, Sapporo,
Hokkaido, this conference.
[4] R. Zhang et al., “Developing an Yb/Nd doped Hybridsolid state laser of RF Gun for SuperKEKB Phase IIcommissioning”, in Proc. IPAC’17 CopenhagenDenmark, paper TUPAB056.