YTTERBIUM HYBRID MJ-LEVEL LASER SOURCE OF DAW RF GUNFOR SUPERKEKB
X.Zhou#, T. Natsui, M. Yoshida, Y. Ogawa, KEK, Tsukuba, JAPANAbstract
For SuperKEKB project, the electron beams with acharge of 5 nC and a normalized emittance of 10 µm are
expected to be generated in the photocathode RF gun at
the injector linac. An ytterbium (Yb)-doped laser systemwith a center wavelength of 259 nm and a pulse width of30 ps is employed to obtain high pick energy pulses.
Although, the pulse repetition of 25 Hz with doublebunch is required, more than 5 nC electron with singlebunch was generated in the 2 Hz.	 

INTRODUCTIONIn SuperKEKB project, the photocathode RF gun for
high-current, low-emittance beams will be employed in
the injector linac to obtain higher luminosity [1].
Corresponding to 5 nC and a normalized emittance of 10µm in the photocathode RF gun [2], the laser source with
1 mJ pulse energy, center wavelength of 259 nm and apulse width of 30 ps are developed.
In more recent years, both fiber lasers and thin-disklasers are expected to show significant further progress.
The fiber laser offers higher amplify efficiency, simply
constructer, good stability and low-cost. And the thin-disklaser optimizes heat remove, which made possible thehigh energy pulse amplifier. For SuperKEKB project, thelaser source structure of RF gun was designed as a hybridsystem, which include an Yb-doped fiber oscillator, Ybdoped fiber amplifiers and thin-disk Yb:YAG amplifiers.
For the repetition rate of electrum beam, the 2 Hz, 5 Hz,
and 25 Hz with double bunches were requested. Firstly,
the laser system was tested in 2 Hz. After that, the laser
was upgraded to 25 Hz.

In the ref [3], the 0.7 nC electron beam was obtainedwith Yb laser source last year. For higher energy
generation, the laser structure was reformed.
A schematic diagram of the laser setup is shown inFig.1. The seed pulse was generated by an Yb-doped fiber
ring oscillator. Then the pulse is reducing to 10.38 MHz
by an EO fiber pulse picker. After an Yb fiber pre
amplifier to compensate for the power loss in EO system,
the pulse was chirped to ~30 ps by a transmission grating
stretcher. An Yb-doped large-mode-area polarizing
double-clad photonic crystal fiber was employed to thefirst amplification stage. Then, the pulse repetition rate (2Hz, 5 Hz and 25 Hz) was separated with an EO modulator
pockels cell. For increase the pulse energy, Another Ybdoped LMA PCF was used. So the pulse was strong
enough to be amplified by Yb:YAG thin-disk stage. To
obtain the mJ-class pulse energy, a regenerative amplifier
and two multi-pass amplifier stages were employed. Deep
UV pulses for the photocathode are generated by using
two frequency-doubling stages. High pulse energy andgood stability were obtained. Finally, the pulses wereinjected into RF gun.

Reformed of Yb Fiber Oscillator

LASER SYSTEM OF 2HZ AND 5HZFigure 2: Oscillator.

Figure 1: Layout of Laser system.

A unidirectional ring cavity is employed in the passivemode-locked oscillator (Fig. 2). As the laser source
creation, the stability of seed pulse is most important. For
increase the property of the oscillator, some reforms werecompleted. Firstly, for reduce the thermal effect; a superinvar breadboard was used, which has a near zerocoefficient of thermal expansion over a wide temperature
range. Secondly, reflection gratings were replaced by apair of transmission gratings. Because of the higher
transport rate, the cavity lose was reduced, and the modelocking is easy to initiated. Finally, the piezoelectrictransducer (PZT) that is used to control the cavity length
to lock the repletion rate with the SuperKEKB, attached
to a lead block at an extra cavity. The space jitter wassharply reduced when the PZT is shaking tosynchronization.
The Oscillator was pumped by a fiber-coupled pump
diode delivering 750 mW at 976 nm. Although, the higher
pump power could create higher output power, the noise
signal such as side picks also occurs. Therefore, the pump
power was controlled below 200 mW, with the average
output power of 30 mW. The repetition rate is 51.9 MHz(10.38*5 MHz), which is supplied by KEKB with therepetition rate of 2856 MHz. The mode-locked operationand synchronization system were stable for all days.

Upgrade of Yb Fiber Amplifier stagescrystal fiber 1.2 m in length with a core diameter of 40µm and pump clad diameter of 200 µm, which is cladding
backwardly pumped by a 70 W fiber-coupled laser diodeemitting at 976 nm.
An EO modulator pulse selector was employed toreduce the pulse repetition rate. The repetition rate can be
set to 2 Hz, 5 Hz, and 25 Hz with double pulses. After
reduction of the pulse repetition rate, another Yb-doped
fiber amplifier was used for higher pulse energy obtain.
For stability, the amplifier is pumped with light at awavelength of 940 nm. Although, the laser diode pump
power could reach the 70 W in both fiber amplifiers, theDC background contains in the signal also increased,
which can be amplified in thin-disk amplifier stage. Thus,
the pump power was conduct to low level. In this stage,
the pulse was amplified to several µJ.

Redesign the thin-disk Amplifier
Figure 3: Pulse picker and stretcher
Last time, the signal pulses of 51.9 MHz repetition rate
were reduced to 2 Hz with an EO modulator pulse
selector. The duration of pulse-to-pulse was ~ 20ns,
which is short than the range of EO modulator possible
pulse duration. The side pulses were remained before
and after the main pulse. Therefore, an additional EO
fiber pulse picker was employed to reduce the repetition
rate to 10.38 MHz firstly. Then pulse-to-pulse duration
was expanded more than 100 ns, which is easy to shieldside pulses. Because the energy loss of the fiber EO
modulator is more than 90%, an Yb fiber pre amplifier
was set to compensate for the power loss. The pair oftransmission gratings with a groove density of 1740
grooves/mm is used to stretch the pulse to several tens pswith an efficiency of 80%.
The gain spectrum of Yb:YAG is around 10 nm centerat 1037 nm. By contrast, the pulse that be generated byYb fiber has wide spectrum from 1020 nm to 1080nm.
For match the gain area of Yb:YAG disk and Yb-dopedfiber, a spatial slit was performed for spectral shaping
between the grating pair and reflector, where thespectrum is spatially separated with a parallel beam (Fig.
3).

Figure 4: Yb-doped fiber amplifier
The output of stretcher is seeded into the fiber amplifier
by a large-mode-area polarizing double-clad photonic

The thickness of the Yb:YAG thin-disk crystal is 2mm, which not fit for high energy capacity. To obtain themJ-class pulse energy, a regenerative and a multi-pass
Yb:YAG solid-state amplifier was employed last year.

Figure 5: Thin-disk regenerative amplifier
25Hz, 2 bunches means the duration of the two bunches
is about 100 ns. Consider the response speed of thepockels cell (~20 ns); the total cavity length of the one
round trip is about 36 m. The improvement of theamplifier started with restructures the ring cavity to alinear cavity. Thus, the cavity length was reduced to 9 m.
The two Pockels cells were used as the control theentrance and exit times of the cavity respectively (Fig. 5).
Usually, the thin desk crystal acts as an end mirror
within the laser resonator. Although the HR coating wasperformed in back surface of crystal, the multiple
reflections occurred in front surface. As the fig. 5, another
0°high reflection mirror was used as an end mirror. And
the signal beams pass through the thin-disk crystal twotimes in one round trip, which increases the efficiency of
the regenerative amplifier.
A 2.4 kW laser diode, be employed as the pump source,
oblique inject into the Yb:YAG thin-disk crystal with aspherical mirror and a plano-convex cylindrical lens. The
pulse energy was increase to more than 1 mJ, and themulti-pass amplifier was used for higher amplification.
For low repetition rate pumping as 2 Hz and 5 Hz, thethermal lens effect can be ignored. After a telescope
collimator, parallel light beam inject into a 2-pass
amplifier and 1-pass amplifier (Fig. 6).

Figure 6: Thin-disk multi-pass amplifier
The pump source is increase to 9.6 kW, with 4 laser
diode setting around the Yb:YAG crystal. After the 2stage amplifier, more than 20 mJ output pulse energy isobtained.
Deep UV pulses are generated by using two frequencydoubling stages. Two piece of 5 mm thickness beta
barium borate (BBO) crystal are used to generate UVpulses at a center wavelength of 259 nm. Although theenergy of fundamental beam is strong enough, the noise
pulses and DC background exist in the signal pulses. The
final UV pulse energy of 1.0 mJ was obtained in 2 Hz,
and 0.6 mJ obtained in 5 Hz with the total SHG
conversion efficiency of 5%. Fig. 7 shows the UV pulse
shape in the time domain.

structure is too complex to maintain, which tie thestability of the all system down. Therefore, we try to use
other amplifier replace the regenerative amplifier. For
increase the fiber amplifier stages, the stretcher was set
between the oscillator and 10 MHz pulse picker. Thus,
the pulse energy of fiber stages is strong enough toamplify directly by thin-disk multi-pass amplifier stages.
After fiber amplifier, 3 multi-pass stages were performed.
For compensate thermal lens effect, double lens was usedin front of the thin-disk crystal. For improve the heatremove of the crystal, the disk thickness was reduce to0.5 mm.

Figure 8: Layout of 25 Hz laser system
More than 20 mJ fundamental pulse energy wasobtained. For noise and background problem, the UV
pulse was reduced to 400 µJ, and 0.8 nc electrum beam
was obtained.

CONCULUTION

Figure 7: UV pulse profile

Laser injection at RF Gun
The UV laser pulses inject into the cathode in RF gun
with the angle of 60°. As a result of RF gun beammeasurement, 5.6 nC beam generation from the gun wasachieved in 2 Hz. The RF-Gun cavity, cathode andexperimental results will be shown in the ref [2].

UPGRAD OF LASER SYSTEM TO 25HZWhen the repetition rate increases to 25 Hz, thecondition of the laser amplifier system is changed
seriously. First of all, the thermal lens effect cannot beignored in the thin-disk amplifier stages. Secondly, thenoise and background signal is obviously increased,
which affect the quality of the laser source.
A schematic diagram of the restructure laser setup of
25 Hz is shown in Fig.8. Although the regenerative thindisk amplifier can provide effective amplification, theIn SuperKEKB project, for obtaining high-current, lowemittance beams in the photocathode RF gun, the UV
laser source with 1 mJ, 30 ps was developed. The hybrid
laser system contains with a Yb-doped fiber-based
oscillator, large mode-area Yb-doped fiber amplifier andthin-disk Yb:YAG amplifier systems.
In 2 Hz repetition rate setup, more than 1 mJ UV pulse
was obtained. As the result, the electron beams with acharge of 5.6 nC was generated.
For overcome the thermal effect in 25 Hz repetition
rate, the laser was reformed. 0.8 nC beam generation isachieved by the new laser source.

REFERENCES[1] T. Miura et al., MOPRO001, IPAC14, Dresden,
Germany, proceedings of this conference.
[2] T. Natsui et al., MOPRI033, IPAC14, Dresden,
Germany, proceedings of this conference.
[3] X. Zhou, et al., WEPME061, IPAC13, Shanghai,
China, 2013.