YTTERBIUM LASER DEVELOPMENT OF DAW RF GUN FOR
SUPERKEKB
X.Zhou#, T. Natsui, M. Yoshida, Y. Ogawa, KEK, Tsukuba, JAPANAbstractA-1 unit this year. In this paper, we report an ultra-short

For obtaining higher luminosity in the SuperKEKB, thephotocathode RF electron gun with strong electric
focusing field for high-current, low-emittance beams will
be employed in the injector linac. In the photocathode RFgun, the laser source with a center wavelength of 260 nm
and a pulse width of 30 ps is required. Furthermore, for
reducing the energy spread, the laser pulse width should
be reshaped from Gaussian to rectangle structure.
Therefore, Ytterbium (Yb)-doped laser system thatprovides broader bandwidth, higher amplifies efficiency
and higher output power is employed. High pulse energy
and good stability would be expected.

pulse laser system for RF-Gun at the A-1 unit based on
the Yb-doped (Fig.1). High pulse energy and goodstability would be expected.

INTRODUCTION
SuperKEKB is a planned upgrade to the KEKB
accelerator with higher luminosity [1]. Corresponding tothe reduction of dynamic aperture and beam life, thephotocathode DAW-type RF gun for high-current, lowemittance beams will be employed in the injector linac. Inorder to generate electron beams with a charge of 5 nC
and a normalized emittance of 10 µm in the photocathodeRF gun, the bunch width should reach several tens of ps.
Recently, Ir5Ce was evaluated as cathode material of
RF Gun because of its high efficient emission (Max QE:~10-4) and long lifetime [2]. By simulation with the QE of
5×10-5, the injection laser pulse energy should be higher
than 1 mJ.
According to the simulation of emittances due to thespace-charge effect, the laser source of center wavelengthof 260 nm and pulse width of ~30 ps (FWHM) are
required. For the synchronization with the SuperKEKB,
the repetition rate of the laser is set at 51.9 MHz.
In the ref [3], when the charge distribution is uniform
in a cylinder, the energy spread of the bunch is lower than
the result of Gaussian charge distribution. For the laser
source, the pulse width should be reshaped to rectangle
structure. Therefore, the wide spectral width pulses areexpected.

Figure 1: Schematic of SuperKEKB injector linac
The photocathode DAW-type RF-Gun with Nd:YAG laser
system was already tested at the 3-2 unit [4] and the side
coupled quasi-travelling wave type RF-Gun is installed at the____________________________________________*Work supported by …
#xiangyu.zhou@kek.jp

LASER SYSTEM
For high peak power and ultra-short UV pulse
generation, the most common approach is to employ thenonlinear processes of solid-state Ti:Sapphire lasers. A
Ti:sapphire laser is usually pumped with another laser
with532 nm,
forwhich frequencydoubled Nd:YAG lasers are used. However, the low
energy-extraction efficiency and heavy maintennancelimit the application. In contrast, Nd:YAG lasers
are optically pumped using a laser diodes, that offers
high efficiency for reduction of the laser price andmaintenance costs. But, the pulse shaping of Nd:YAG isdifficult to be adjusted because of the narrow spectrum
gain.
By contrast, the fiber laser especially Yb fiber laser has
attracted attention as one of the promising practicalalternatives to usual solid-state lasers, offering highrepetition rate, low-cost and so on. Especially, thestability of the fiber laser is evidently higher than theother solid lasers with simple cooling conditions becauseof high energy-extraction efficiency of fiber. In particular,
Yb-doped glass fibers provide broader bandwidth; higher
amplification and higher output power than other fiber
amplifiers.
The thin-disk laser is a special kind of diode-pumped
high-power solid-state laser. The main difference is thegeometry of the gain medium is a thin disk, where thethickness is considerably smaller than the laser beam
diameter. The heat generated is extracted dominantly
through one end face. Substantial pulse energies are thenpossible even without using chirped-pulse amplification(CPA). Within the next few years, both fiber lasers andthin-disk lasers are expected to show significant furtherprogress.
Table 1: Characteristic of Lasers
TiNdYbType

Solid

Solid

Fiber&Solid

Energy-extraction
Efficiency

0.5%

3%

3%

Gain SpectrumBroadband

Narrowband

Broadband

CostHigh

Low

Low

Maintenance

OftenLessLess

A schematic diagram of the laser setup is shown inFig.2. The laser system starts with a large mode-area Yb-doped fiber-based amplifier system, which consists of apassively mode-locked femtosecond Yb-fiber oscillator, atransmission grating pair stretcher and two Yb-doped
single-mode fiber amplifier stages. Then the repetition
rate of the pulse is reduced to 2 Hz by an EO modulator
pulse selector. To obtain the mJ-class pulse energy, theYb:YAG solid-state thin-disk amplifiers are employed,include a regenerative amplifier and a two steps amplifier.
Deep UV pulses for the photocathode are generated byusing two frequency-doubling stages. High pulse energy
and good stability were obtained. Finally, the pulses areinjected into RF gun. In future, the laser pulse shape willbe transformed from Gaussian shape to rectangle shapeby a spatial shaping adjustment.

When the pump power is above 150 mW, the modelocking is initiated immediately by shaking the grating.
An enclosure is set up to protect the oscillator that issensitive to temperature, acoustic and wind vibration. And
the mode-locked operation was stable for all days.
The seed pulses are then pass through an isolator,
which provides the protection for the oscillator. Then, awave plate and a polarizer were employed to separate apart of seed pulse to the synchronization system. The
frequency source is supplied by KEKB with the repetition
rate of 2856 MHz. The total fiber length is about 3.4 m,
and the repetition rate is 51.9 MHz (10.38*5 MHz). Our
designs produce extremely low jitter of ~ps, with very
wide operating ranges.

Stretcher and Yb Fiber Amplifier

Figure 4: Layout of pre-amplifier.
Figure 2: Layout of Laser system.

Yb Fiber Oscillator with synchronization
A unidirectional ring cavity is employed in the passivemode-locked oscillator (Fig. 3) [5]. Highly doped Yb
fiber with a length of 30 cm was pumped by a fibercoupled pump diode delivering 250 mW at 976 nm. A
piezoelectric transducer (PZT) attached to the end mirror
after the grating pair is used to control the cavity length tolock the repletion rate with the SuperKEKB. All theoptics is laid on the breadboard (30cm×60cm), which iseasy to transport.

Figure 3: Layout of Oscillator.
The spectral bandwidth is ~14 nm, centered at 1035 nm.
And the pulse duration after the oscillator is ~200 fs
roughly with the average output power of 40 mW. Since
the unabsorbed pump power was negligibly small, thecalculated pulse energy is 0.8 nJ with good pulse quality.

Last time, a 10 m long Yb-doped fiber was employed
for amplification and stretching the pulse width. The 0.5
W output power and ~20 ps pulse width was obtained [5].
Because of the absorption of the long Yb-doped fiber, thecenter wavelength of the pulse was shift to 1060 nm. Butfor Yb:YAG Thin-disk amplifier, the main gain spectrum
is around 1037nm. Therefore, the length of Yb-doped
fiber was reduced to avoid spectrum shift, and atransmission grating pair is added before the amplifier tostretch the pulse width.
As in the Fig.4, the pair of transmission gratings with agroove density of 1740 grooves/mm is used to stretch theseed pulse to several tens ps with an efficiency of 80%.
To precompensate for the power loss in lock systemand stretcher, the pulse train is then injected into a 2 mlong 10 µm core-diameter polarization-maintaining
double-clad Yb-doped fiber with a cladding diameter of
125 µm and numerical aperture (NA) of 0.46. The Ybdoped fiber is cladding pumped by a fiber-coupled laser
diode emitting at 976 nm. At 2.8 W of pump power,
amplified pulses of 80 mW are achieved.
For higher pulse energy, the output of pre amplifier isseeded into the 2nd fiber amplifier comprised of a largemode-area polarizing double-clad photonic crystal fiber
1.2 m in length with a core diameter of 40 μm and pumpclad diameter of 200 μm. As in the Fig. 4, the structure issimilar to the pre amplifier. For stability, the amplifier isbackwardly pumped with light at a wavelength of 940 nm.
In this stage, the output power of 1.1 W.

Thin-disk Yb:YAG Amplifier
After reduction of the pulse repetition rate to 2 Hz with
an EO modulator pulse selector, such low-repetition-ratehigh-peak-power pulses could not be amplifier by fiber.
To obtain the mJ-class pulse energy, a regenerative and amulti-pass Yb:YAG solid-state amplifier is employed.

pass configuration. The conversion efficiency of 6% withthe pulse energy more than 0.6 mJ is obtained.

Laser injection at RF Gun
Then the laser beam injects into the Ir5Ce cathode in
RF gun with the angle of 60 degree. As a result of RF gun
beam measurement, 0.7 nC beam generation from the gun
was achieved. This beam can be transported to the end ofJ-linac. The RF-Gun cavity, cathode and experimental
results will be shown in the ref [6].

Next Upgrade of the laser system

Figure 5: Thin-disk Yb:YAG.
Yb:YAG
crystal,apromisinglaser-active
materials, has a much larger absorption bandwidth tomitigate thermal management requirements for diodelasers, a longer upper-state lifetime and lower thermalloading per unit pump power, which have demonstrated
significant potential for application in compact, efficient,
diode-pumped laser systems.
For such high energy amplification, thin-disk laser isthe best way to heat removal. The cooled end face has adielectric coating which reflects both the laser radiationand the diode pump radiation. The gain crystal also acts
as an end mirror within the laser resonator.
Although the thin-disk laser heads is not fit for highenergy capacity lasers due to small thickness, it can beused for regenerative amplifiers. The relatively small gainof the thin disk can be compensated with a larger numberof resonator round trips. In this step, a 2.4 kW (1.4 J)
laser diode, is employed as the pump source, inject withoblique incidence into the Yb:YAG thin-disk crystal by aplano-convex cylindrical lens.
For higher amplification, the final amplifier wasconstructed as a purely 2-pass multipass amplifier(without a resonator and optical switch). The pumpsource is increased to 10 kW, with 4 laser diode setting
around the Yb:YAG crystal, and 10 mJ output pulse
energy is obtained. The damage surface and unregularly
beam profile limit the overall gain (1%) and requires acarefully optimized setup in order to preserve a highbeam quality.
Deep UV pulses for the photocathode are generated byusing two frequency-doubling stages. The nonlinear SHG
crystal of beta barium borate (BBO) is chosen because of
its higher UV transparency. UV pulses at a center
wavelength of 259 nm are generated in the same single-

In the amplifier stages, the pulse was stretched fromseveral hundreds of fs to several tens of ps. For chirped
pulse, the pulse shaping in the time domain can becontrolled by adjusting the spectral shape of the pulse. Itis also an important reason that the Yb laser was used,because the wider spectral width pulses could begenerated compared with the others. A spectral range
adjustment system will be constructed before the RF gun
to reshaping the pulse shape.
To increase the stability and power of the laser source,
the output of fiber and thin-disk amplifier should beincreased. Improve the center wavelength from 940 to976 nm and amplifier structure of the pump source will
help to increase the amplification energy-extraction
efficiency. Furthermore, the jitter of the synchronizedsystem should be reduced to hundred fs level.

CONCULUTION
For RF electron gun in the SuperKEKB, the laser
source with a center wavelength of 259 nm and a pulse
width of 30 ps is developed. The laser system starts witha large mode-area Yb-doped fiber-based oscillator, which
generates 51.9 MHz seed pulses with center wavelengthof 1035 nm and synchronized with KEKB trigger. The fspulse is stretched until several tens ps, and amplified by
Yb-doped fiber amplifier. Then pulses are reduced to 2
Hz of repetition rate by EO pulse picker. To obtain themJ-class pulse energy, a Yb:YAG regenerative and 2pass solid-state amplifier is employed. Deep UV pulses
for the photocathode are generated by using two
frequency-doubling stages. Beam generation of 0.7 nC isachieved by the laser source.

REFERENCES[1] T.Higo et al., TUPPR005, IPAC12, Louisiana, USA,2012.
[2] T. Natsui et al., TUPPD057, IPAC12, Louisiana,USA, 2012.
[3] K.J. Kim, Nucl. Instr. and Meth. A 275 (1989) 201.
[4] M. Yoshida et al., TUPPD035, IPAC12, Louisiana,USA, 2012.
[5] X.Zhou, et al., WEPPD056, IPAC12, Louisiana,USA, 2012.
[6] T. Natsui et al., TUOCB103, IPAC13, Shanghai,
China, proceedings of this conference.