GENERATION AND ACCELERATION OF LOW-EMITTANCE, HIGHCURRENT ELECTRON BEAMS FOR SUPERKEKB
M. Yoshida#, N. Iida, S. Kazama, T. Natsui, Y. Ogawa, S. Ohsawa, H. Sugimoto, L. Zang, X. Zhou,
High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japan
D. Sato, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo, 152-8550, Japan
Abstract
KEK e-/e+ linac is now in a final stage of upgrade forSuperKEKB. One of the key issues is to stably generateand accelerate a low-emittance, high charge electronbeam for SuperKEKB (a couple of single-bunched beams
with a charge of 5 nC and a normalized emittance of 20mm-mrad each).

REQUIREMENTS
The injector upgrade as shown in Fig. 2 is required for
the SuperKEKB injection due to the small dynamic
aperture and the short life time of the ring. The bunchcharge must increase up to 5 times higher than KEKB andthe lower normalized emittance is required around 20 mmmrad. Further the small energy spread of around 0.1 % isrequired in the condition of higher bunch charge for thesynchrotron injection of the HER of the SuperKEKB.
Table 1: Required Parameters for SuperKEKB
KEKB(e+/e-)

SuperKEKB(e+/e-)

Beam Energy

3.5 / 8.0 GeV

4.0 / 7.0 GeV

Bumch Charge

1.0 / 1.0 nC

4.0 / 5.0 nC

Normalized
transverse
Emittance (1)2100 / 300 mmmrad

6 / 20 mm mrad

Energy Spread

0.1 %

0.1 %

of the SuperKEKB injection:- The quasi-traveling wave side coupled cavities forthe strong focusing electric field to keep the beamside against the space charge.
- The reasonable lower maximum surface electricfield of 120 MV/m.
- The metal composite cathode (IrCe) with the longlifetime and reasonable high quantum efficiency.
- The ytterbium based laser system with thetemporal manipulation to get the minimum energyspread.

QTW-SIDE COUPLED RF GUN CAVITY
The strong focussing field is generated by the annular
coupled cavities with the narrow acceleration gap as thedisk and washer (DAW) or side coupled cavities narrowacceleration gap. Conversely the narrow gap causes along drift space as Fig.1 (a). We solved this problem touse two staggered standing wave cavities with /2 phasedifference as shown in Fig.1 (b). This acceleration field issimilar to the traveling wave from the accelerated beam.
We call this cavity as the quasi-traveling wave (QTW)[1].
This quasi-traveling wave RF-Gun can realize to obtain
both of the higher acceleration voltage and the strong
focussing field.

PHOTOCATHODE RF-GUN
The initial emittance at the electron gun is determinedby the space charge effect and the RF emittance. The
photocathode RF-Gun is required to generate such low
emittance of lower than 10 mm mrad for the high charge
electron bunch of 5 nC. From the analytical calculation,
the bunch length must be longer than 20 ps to obtain thecharge of 5 nC and the transverse emittance of 10 mmmrad.
This requirement of the electron bunch charge for theSuperKEKB is slightly higher than the standard S-band
RF-Gun. Further it is better to use same RF gun for the
positron production with the primary electron bunchcharge of 10 nC.
Thus we developed the new advanced RF gun with thefollowing specifications for the long time stable operation____________________________________________#mitsuhiro.yoshida@kek.jp(a) Normal side coupled cavities(b) Quasi traveling wave side coupled cavities
Figure 1: Structure of the quasi traveling wave cavity.

Longer bunch can reduce
space charge effect inside RF-Gun
and also CSR at J-ARC.
A-1
Chicane

B

J-ARC

4 ps

CA
10ps
Target @1-4
3.5 GeV 10nC
1 x2

KEKB obtained(e+ / e-)High charge low emittance isrequired for SuperKEKB.

A-1
RF-Gun5-10nC

SuperKEKB
required (e+ / e-)

Beam energy

3.5 GeV / 8.0 GeV

4.0 GeV / 7.0 GeVBunch chargee- e+ / e10  1.0 nC / 1.0 nC

e-  e+ / e10  4.0 nC / 5.0 nC

Beam emittance(ge)[1]

2100 mm / 300 mm

6 mm / 20 mm

1.1 GeV@DRPF

20ps
BCS

ECS

2

Due to mis-alignment:- Dispersion- Short range wakefield

34

5

Beam diagnostic station

ECS

C-bandSY2

Pulse Q-magnet & pulse steeringfor HER
7.0 GeV 5nC x 2
e y < 20 mm mradfor LER
4.0 GeV 4nC x 2
e y < 6 mm mrad

E < 0.1 %Figure 2: KEKB Injector upgrade to obtain the low emittance and the high charge for SuperKEKB injection.
The cathode cell of RF gun is most important to obtainhigh quality beam since the space charge affects the beam
size and emittance in the low energy region. The
curvature of the cathode makes the strong focussing field
against the space charge effect of the high bunch charge.
The electrode shape was optimized to reduce the surface
electric field and to supress the non-linear component
which causes the emittance growth. This RF gun consistsof 7 cavities in which 3 cavities have /2 phase difference
as shown in Fig. 3. The narrow gap of those accelerating
cavities makes the focusing force to keep the beam size.

The figure 4 shows the beam tracking simulation for 5nC beam. The normalized emittance of 5.5 mm-mrad andthe beam size of 0.4 mm are expected at the exit of the RFgun. This small beam size is obtained by the focusing
electric field without any additional magnetic field. Afterthe RF gun, the beam energy of 11.5 MeV is high enoughto supress the space charge. The required RF power is 20
MW. Also this RF gun can generate 10 nC beam for thepositron generation. The normalized emittance of 10 mmmrad and energy spread of 1 % are expected.
Figure 5 shows the acceleration mode and the coupling
mode of the regular cavity. The coupling coefficient is3%. Those 3 and 4 side coupled cavities are mounted as
90 degrees in the azimuthal angle as shown in Fig. 6.

Figure 3: Designed RF gun cavities.

(a) Accelerating mode(b) Coupling mode

Figure 5: Regular cell cavity calculation result.

Figure 4: Beam tracking simulation result.
Figure 6: Whole cavity shape.

Those two standing wave cavities are fed by 90 degreehybrid. As shown in Fig. 6, the whole cavity shape iscomplex. Thus the sliced cavity shape was machined andstacked by brazing process as shown in Fig. 7. Also thecompact 90 degree hybrid was developed. This RF gun
was installed in September 2013 replaced from athermionic cathode DC gun as shown in Fig. 8.

the laser irradiation (laser cleaning) as shown in Fig. 10left. Futher the quantum efficiency can be increased byshorter wavelength laser or the heat treatment as shown inFig. 10 right.

Figure 7: Photo of manufactured cells after brazing.

Figure 9: Quantum efficiency vs cathode lifetime.
QE×10-425

Ir5Ce@213nm
Ir5Ce@266nm

QE = 2.70×10-3

10-5

Quantum Efficiency

10-4

LaB6@266nm

20×1515

QE = 9.10×10-4

LaB6@532nm

110-6

00

Figure 8: Installed RF gun.

IRIDIUM CERIUM CATHODE
A photocathode material is a key component to obtain
the high charge electron beams using a photocathode RFgun. Generally, multi-alkali photocathodes (e.g. Cs2Te,K2CsSb) have been used as photocathode material since
they have low work function (< 3eV) and high quantum
efficiency (QE ~ 0.1). These photocathodes, however, arenot suitable for SuperKEKB injector since the lifetime ofthese photocathodes is not enough for a long-term
continuous operation. On the other hand, the metallic
photocathodes (e.g. Mg, Cu) have a long lifetime (<1year) which is enough to operate a RF injector for a year
without cathode maintenance. It is, however, difficult forthese photocathodes to generate the high charge electron
beams since they have too low QE as shown in Fig. 9.
We have tried to develop a new metal compound as aphotocathode which has reasonably high quantum
efficiency (> 10-4) and high laser durability to generatehigh charge electron beams for a long-term (> 1 year).
The iridium cerium (Ir5Ce) photocathode has areasonably high quantum efficiency (QE = 1.54 × 10-4
@266nm) and a long lifetime (>> lanthanum hexaboride :LaB6). Moreover, the Ir5Ce photocathode showed a highresistance to poisoning and it can be easily activated by×6.5×65

Ir5Ce@266nmQE = 1.00×10-3

101000
2000
3000
4000
Energy density of cleaning laser(μJ/mm2)

5000

200400600

8001000

Cathode Temperature(℃)

Figure 10: Laser activation and quantum efficiency.

YB-DOPED HYBRID LASER SYSTEM
The required energy spread of 0.1 % was achieved forthe bunch charge of 1 nC at the previous KEKB operation.
However the required bunch charge becomes 5 nC for
SuperKEKB and it induces the stronger longitudinal
wakefield. Further the shorter bunch is preferred tosupress the emittance dilution due to the transverse
wakefield from the mis-alignment of the acceleratingstructure. This stronger longitudinal wakefield induced by
the shorter bunch causes the larger energy spread.
Fortunately the energy spread at the optimum RF phasedramatically decreases in case of the uniform charge
density as shown in Fig 11.
Thus the uniform pulse shaping is required for the laser
system. It means that the chirped pulse and spectrum
shaping is required using the broadband crystal. The
Ti:Sapphire and Ytterbium(Yb)-doped crystal is thecandidate for such a broadband crystal. We adopted theYb-doped crystal to obtain the higher efficiency using thediode pumped laser since our operation repetition rate of
50 Hz is slightly high for the another crystal.
In more recent years, both fiber lasers and thin-disklasers are expected to show significant further progress.

The fiber laser offers high efficiency, good stability, lowcost and simple configuration. And the thin-disk laser
optimizes heat removal which makes possible the highenergy pulse amplifier. We employed a hybrid system
which includes an Yb-doped fiber oscillator, amplifiers
and thin-disk Yb:YAG amplifiers.
A schematic diagram of the laser system is shown inFig.12. The seed pulse was generated by an Yb-doped
fiber ring oscillator with the repetition rate of 51.9 MHz
which is required to synchronize with our S-band LINAC
and the RF frequency of SuperKEKB ring. Then the pulse
repetition is reduced to 10.38 MHz by an EO fiber pulse
picker. After an Yb fiber pre amplifier to compensate forthe power loss in EO system, the pulse was chirped to~30 ps by a transmission grating stretcher with variableslit. An Yb-doped large-mode-area polarizing double-clad
photonic crystal fiber was employed to the first
amplification stage. Then, the pulse repetition rate wasseparated with an EO modulator pockels cell. To increase
the pulse energy, another Yb-doped LMA PCF was used.
To obtain the mJ-class pulse energy, three stages of themulti-pass Yb thin-disk amplifier were employed asshown in Fig. 13. Deep UV pulses for the photocathodeare generated by using two frequency-doubling stages.
Figure 14 shows the current pulse profile. This profile ismuch better than the normal Gaussian pulse shape. Thedazzler will be installed to obtain the flat top.

Yb:YAGLD531LD642

Figure 13: Thin-disk multi-pass amplifier.

Figure 14: Laser pulse profile.
3Gaussian
5nC 10nC

2.515nC
20nC

RF GUN COMISSIONING

σ E/E (%)

21.5

1

Square 5nC
10nC

0.515nC
20nC
00510 5nC electron
15
FWHM (ps)203015nC 25
Primary beam for
positron production

The RF gun beam commissioning was started autumn
2013. The bunch charge of 5.6 nC has been observed. TheQ-scan emittance measurements were performed using a30 mm thickness alumina fluorescent plate. Beam size
was measured as shown in Fig.15. Beam energy was 25
MeV at the plate. As a result, normalized emittance is10.7 (±1.4) mm-mrad in vertical at beam charge of 1 nC.
We have to improve the Yb laser system for the futurestable and high charge operation for the continuous
SuperKEKB injection.

Figure 11: Energy spread vs bunch length.

Figure 12: Layout of laser system.

Figure 15: Emittance measurement by Q-scan method.

EMITTANCE PRESERVATION
The transverse wakefield is main source of theemittance dilution for the high bunch charge of 5 nC after
the beam orbit is well corrected in the LINAC beamoptics. And the energy spread is determined by thelongitudinal wakefield strength. The smaller alignment
tolerance and the shorter bunch length can reduce thetransverse wakefield strength. However the shorter bunch
length causes the stronger longitudinal wakefield.
Figure 16 shows the analytical calculation of the
projected emittance in several cases of the bunch lengthand the alignment error. The bunch length of 4 ps and 10
ps are required for the alignment error of 0.3 mm and 0.1
mm respectively from Fig. 16. Since our normal bunch
length is 10 ps, we are working to reach the alighment
error of 0.1 mm for each local section which consists of 8units. However it seems very hard to keep this smallalignment error in our old support structure and thefrequent earthquake. Thus we have to consider the 4 ps
option for the alignment error of 0.3 mm.
Further the initial offset can compensate the transverse
wakefield to recover the emittance dilution. The initialoffset is taken as the two steering magnets with 90 degree
phase difference of the betafunction for each axis. Figure
17 shows the emittance optimization using the initialoffset scan. It is realized using the RF deflector to observe
the longitudinally sliced bunch shape.
The shorter bunch can suppress the emittance dilution
due to the transverse wakefield. However it causes thestronger longitudinal wakefield. The RF phase becomesdeeper and deeper corresponding to the longitudinal
wakefield strength to suppress the energy spread. Thus
the bunch length was chosen by those balances.
The initial bunch length of 20 ps generated by thephotocathode RF-Gun will be compressed to 10 ps using
the chicane at the low energy section. And the J-ARC
section is place at the energy of 1.6 GeV. We can change
this optics design to the achromatic condition fromoriginal isochronous design to obtain certain R56 tocompress the bunch as shown in Fig. 18.

Figure 17: Initial offset to optimise the projected emittance.

Figure 18: Possible R56 by achromatic design of J-ARC.

CONCLUSION
An advanced quasi-travelling wave side couple RF gunwith special metal composite cathode was installed togenerate the high charge and low emittance beam forSuperKEKB. The charge density was controlled by newYb fiber and thin-disk hybrid laser system to obtain therequired energy spread. Further the emittancepreservation is another important issue to inject the highquality beam for SuperKEKB ring.

REFERENCES

Figure 16: Analytical emittance growth due to mis-alignment.

[1] T. Natsui et al., “Quasi-traveling wave side coupled
RF gun commissioning for SuperKEKB”MOPRI033,
IPAC14, Dresden, Germany.
[2] Daisuke Satoh et al., “Development of Better
Quantum Efficiency and Long Lifetime IrCe
Photocathode for High Charge electron RF Gun”
MOPFI023, IPAC13, Shanghai, China,[3] Xiangyu Zhou et al., “Ytterbium Fiber and Disk
Laser of RF Gun for SuperKEKB” WEPME061,
IPAC14 Dresden, Germany.
[4] Mitsuhiro Yoshida et al., “Longitudinal manipulation
to obtain and keep the low emittance and high charge
electron beam for SuperKEKB injector” WEPME061,
IPAC13, Shanghai, China,.