25HZ SUB-MJ YTTERBIUM LASER SOURCE OF RF GUN FOR
SUPERKEKBX. Zhou#, T. Natsui, M. Yoshida, R. Zhang, Y. Ogawa, KEK, Tsukuba, JAPANAbstract
The electron beams with a charge of more than 5 nC
and a normalized emittance of less than 10 μm are
expected to be generated in the photocathode RF gun at
the injector linac of SuperKEKB. An ytterbium (Yb)doped fiber and Yb:YAG thin-disk hybrid laser systemwith a center wavelength of 259 nm and a pulse width of20 ps is developed to obtain high peak energy pulses. Asthe result, more than 3 nC and 1 nC electron weregenerated in repetition of 25 Hz with single-bunch anddouble-bunch respectively.

INTRODUCTIONFor injector linac of SuperKEKB project, more than 5nC electron beams with double-bunch is expected to begenerated in the photocathode RF gun. To high-current,
low-emittance beams generation, the laser source with 1mJ pulse energy, center wavelength of 259 nm and apulse width of 20 ps are needed [1]. Especially, thebroadband wavelength is required for the pulse shaping
control. Therefore, a Ytterbium (Yb)-doped hybrid laser
system, include an Yb-doped fiber oscillator, Yb-doped
fiber amplifiers and thin-disk Yb:YAG amplifiers isdeveloped.
For the repetition rate of electrum beam, the optional of
2 Hz, 5 Hz, 25 Hz and 50 Hz with single or double bunch
were requested. Although, more than 5 nC electron withsingle-bunch has been generated in the 2 Hz and 5 Hz [2],when the repetition rate increases to 25 Hz, the conditionof the laser amplifier system such as the thermal lenseffect is changed seriously. To correspond to 25 Hz
repetition rate, the laser was reformed.

LASER SYSTEM OF 25 HZ

Figure 1: Layout of Laser system.
A schematic diagram of the laser setup is shown inFig.1. The seed pulse with the pulse energy of 0.2 ns andspectrum of 1025-1070 nm was generated by an Ybdoped fiber ring oscillator. The pulse repetition is 51.9

MHz, synchronized with 2856 MHz trigger fromaccelerator. After an Yb fiber pre amplifier, the pulse waschirped to ~20 ps by a transmission grating stretcher witha spectral mask. An Yb-doped large-mode-area polarizing
double-clad photonic crystal fiber was employed to thefirst amplification stage. Then, the pulse repetition rate of25 Hz, double bunch was separated with two Electrooptic (EO) modulators. To increase the pulse energy,
another Yb-doped LMA PCF was used. So the pulse wasamplified to μJ-level, which was strong enough to beamplified by Yb:YAG thin-disk stage. To obtain the mJclass pulse energy, several multi-pass amplifier stages
were employed. Deep UV pulses for the photocathode aregenerated by using two frequency-doubling stages. Highpulse energy and good stability were obtained. Finally,
the pulses were injected into RF gun.

Backup of Yb Fiber Oscillator

Figure 2: Selection of Oscillator.
The seed pulse was generated by a passive modelocked oscillator with the pulse energy of 0.2 nJ at therepetition rate of 51.9 MHz (10.38*5 MHz). The spectral
bandwidth is ~50 nm, from 1025 to 1070 nm. A
piezoelectric transducer (PZT) is used to control thecavity length to lock the repletion rate with the 2856 MHz
trigger from accelerator by a synchronization system. The
mode-locked operation was stable for several months
with good pulse quality and stability.
Because the oscillator is the most important part of all
system, another backup oscillator was employed. As thefig.2, the two oscillators are set with cross polarizations
that combine by polarizer 1. Then the EO modulator isused to change the polarization of the beams between thepolarizer 1 and polarizer 2. The reflect beam of the seedpulses at polarizer 2 was chosen by the EO voltage
control.

Upgrade of Yb Fiber Amplifier stages
In the fiber amplifier stages, the improvement is focus
on the increase of the pulse quality. Two types amplifier
was set up. For low power amplification, Yb fiber withthe core diameter 4 μm same as the oscillator was used.

Laser diode (LD) pump was coupled with thewavelength-division multiplexing (WDM) coupler, which
compact the construction. For high power amplification,large-mode-area (LMA) polarizing double-clad photonic
crystal fiber (PCF) with a core diameter of 40 μm wasused. The signal and pump pulse focus into Yb fiber bylens in different direction. And dichroic mirrors wereused to separate signal and pump beams. The isolators
were set between the each amplifier to avoid the pump goback to signal.

Figure 3: Yb-doped fiber pre amplifier.
In the low power amplification, the weak seed pulse
makes the amplified spontaneous emission (ASE) easy to
be generated. The ASE noise will be amplified during
each amplifier, which reduces the conversion efficiencyof the second harmonic generation (SHG) stage. Foramplify the seed pulse effectively, one LD was separated
to 3 parts with the output power of 15%, 35%, 50%,respectively. Seed pulse was amplified step by step (Asfig3). All the 3 amplifiers were coupled each other withno free space, which reduced energy loss.
The pair of transmission gratings with a groove densityof 1740 grooves/mm is used to stretch the pulse to several
tens ps with gratings distance of 1.5 m. For matching thegain area of Yb:YAG disk and Yb-doped fiber, a spatial
slit was performed for spectral shaping between thegrating pair and reflector, where the spectrum is spatiallyseparated with a parallel beam. The spectrum of the signal
was reformed to near rectangle shape with the bandwidthof ~4 nm at center wavelength of 1033 nm. And atelescope lens was employed to reduce the beam size ofthe signal before the stretcher, which sharp the side ofpulse shaping.
The output of stretcher is seeded into the fiber amplifier
by a PCF fiber of 1.2 m, cladding backwardly pumped bya 70 W fiber-coupled laser diode emitting at 976 nm. The
output power of LD was controlled in low level to inhibit
the ASE noise.
Last time, the signal pulses of 51.9 MHz repetition rate
were reduced to 10.38 MHz firstly by an additional EO
fiber pulse picker. Because the energy loss of the fiber
EO modulator is more than 90%, the amplify efficiency
declined seriously with ASE noise occur. Therefore, apulse selector consist of two EO modulator was employed
to reduce the pulse repetition rate to 25 Hz with double
bunch of the distance of 100 ns.
After pulse picker, the signal of 25 Hz was amplifier byanother PCF Yb-doped fiber amplifier. In this stage, thepulse was amplified to several μJ pumped with light at awavelength of 940 nm.

Redesign the Thin-disk Amplifier
When the pump repetition rate increases to 25 Hz, thecondition of the laser amplifier system is changed
seriously in the thin-disk amplifier stages. Firstly, thethermal lens effect is occurred when the signals passthrough the Yb:YAG crystal, which can be compensatedby lens and telescope system. Secondly, the coolingrequirement of the crystal is exceeding increased because
the higher repetition rate of the pump pulse. The
temperature change of the thin-disk crystal let the noiseand background signal increased obviously, which affect
the quality and amplify efficiency of the laser source.
The disk thickness was reduced to 0.5 mm to increase
the heat remove from the crystal and avoid the thermallens effect. Thermal conductivity was improved by a newsoldering component, with the Au-Sn layers on top
surface (Au-80% and Sn-20%) [3].

Figure 4: Thin-disk multi-pass amplifier with cavity.
For 2 Hz and 5 Hz repetition rate amplification, theregenerative thin-disk amplifier can provide effective
amplification after fiber amplifier. But for 25 Hz double
bunch amplification, the output energy was unstable dueto the lower gain condition. The structure of theregenerative cavity is too complex to compensate thermallens, especially the ghost pulse was difficult to reducefrom first bunch due to the limited extinction ratio of thepockels cell. Therefore, a multi-pass amplifier inset into acavity was designed to replace the regenerative amplifier.
As the fig 4, the inject signal beam with the verticalpolarization was reflected by polarizer 1, and amplified
10-15 times pass through the Yb:YAG thin-disk crystal.
The polarization of the amplified pulse was turn tohorizontal by the wave plate, which cause the amplified
pulse pass through the two polarizers and amplified again.
After the two loops, the pulse went out the cavity withvertical polarization by the reflection of polarizer 2. A 2.4
kW laser diode, is employed as the pump source, withoblique injection into the Yb:YAG thin-disk crystal witha spherical mirror and a plano-convex cylindrical lens.
Although more than 30 pass amplification was occurredas the regenerative amplifier, the stability and quality wasincreased, because no pockles cell used in the cavity.
After the cavity amplification, the simply multi-pass
amplifiers were used for higher amplification. The
structure of the multi-pass amplifier is shown in Fig5.
The max of 6 pass amplification can be performed in a

multi-pass amplifier. For compensate thermal lens effect,
double lens was used in front of the thin-disk crystal.
The pump source was increased to 9.6 kW, with 4 laser
diode setting around the Yb:YAG crystal. After the 4
multi-pass stages amplification, more than 5 mJ output
pulse energy was obtained.

RF-Gun cavity, cathode and experimental results will beshown in the ref [4].

UPGRADE OF LASER SYSTEM TO 50 HZWhen the pump source repetition rate increases from25 Hz, the heat in the Yb:YAG crystal cannot bedissipated effectively, which cause the thermal increase.
Although the crystal thickness and soldering components
was improved, the thermal conductivity is insufficientwith 50 Hz pump condition. The amplify efficiency andbeam quality decline seriously.
In contrast, Nd:YAG crystal are also pumped using alaser diodes, that offers less thermal lensing, more
uniform gain than Yb:YAG. Although the pulse shapingof Nd:YAG is difficult to be adjusted because of thenarrow spectrum gain, the Nd:YAG is an appropriate
candidate for 50 Hz double bunch amplification.
Therefore, the Nd:YAG simplify and stabilize laser
system without pulse shaping will be tested in next step.

Figure 5: Thin-disk multi-pass amplifier.
Two piece of 5 mm thickness beta barium borate(BBO) crystal are used to generate ultraviolet (UV)
pulses by using two frequency-doubling stages at a center
wavelength of 259 nm. Because of the damage threshold
limitation of the thin-disk crystal and optical mirrors, thelaser pulses cannot be amplified more than 5 mJ reliably.
Although the energy of fundamental beam is lower thanbefore, the ASE pulses and background noise wereinhibited. The SHG conversion efficiency of the twostages was up to 60% and 30% respectively. And the final
UV pulse energy of 0.8 mJ was obtained in 25 Hz.
The distance of laser beam transmits from the laserroom to the RF gun is about 15 m by several reflectmirrors. For UV pulses with center wavelength of 259
nm, the transmitting energy loss was up to 50%.
Therefore, the last SHG stage was set up near the RF gun.
The energy loss of the visible beam with center
wavelength of 517 nm was reduced to 5%.

Laser Injection at RF Gun

Figure 7: Spectrum separation by grating pair.
The Emission wavelength of Nd:YAG is around 1064
nm with the bandwidth of ~0.5 ns, which contained in thespectrum of Yb-doped fiber oscillator. Therefore, the Ybdoped oscillator and Yb-doped fiber amplifier also can beused with Nd:YAG amplification. As the Fig.7, the 1064
nm part of the signal pulse was separated by gratings atthe stretcher stage. A additional Yb-doped fiber amplifierand Nd:YAG amplifier will be employed (Fig.1).

CONCULUTION
For SuperKEKB project, an Yb-doped hybrid laser
source is developed. The laser system contains with a Ybdoped fiber-based oscillator, large mode-area Yb-dopedfiber amplifier, thin-disk Yb:YAG multi-pass amplifiersystems and two SHG stages. Some improvement wereperformed to reduce the background noise and increase
the quality of the pulses, which cause the SHG
conversion efficiency is up to 60% and 30% with 2ω and4ω respectively. More than 3 nC electron beam is
obtained with 25 Hz single-bunch.

REFERENCES
Figure 6: Charge history of RF-Gun.
The UV laser pulses are injected into the cathode in RF
gun with the angle of 60°. As a result of RF gun beammeasurement, more than 3.0 nC beam generation fromthe gun was achieved in 25 Hz, single bunch (Fig.6). The[1] T. Miura et al., TUYB1, IPAC15, Richmond, USA,proceedings of this conference.
[2] X. Zhou, et al., WEPME018, IPAC14, Dresden,
Germany, 2014.
[3] R. Zhang, et al., TUPWA071, IPAC15, Richmond,USA, proceedings of this conference.
[4] T. Natsui et al., TUPJE003, IPAC15, Richmond,
USA, proceedings of this conference.