THE DEVELOPING OF THE BEAM INJECTION SECTION WITH LASERSOURCE AND S-BAND ELECTRON RF GUN FOR SUPERKEKB PROJECT
Xiangyu Zhou†, Mitsuhiro Yoshida, Rui Zhang, Yujiro Ogawa, High Energy Accelerator Research
Organization (KEK/SOKENDAI), 305-0801, Tsukuba, Ibaraki, JapanAbstract
For the beam injection at Linac accelerator of theSuperKEKB project, the s-band RF gun needs to provide
low-emittance high-charge electron bunches. A photocathode RF gun system with an advanced RF cavity structure(quasi-travelling side-coupled structure, cut disk structure),a new photocathode material (Ir5Ce cathode) and a hybrid
laser system (Yb fiber and Nd:YAG hybrid system) havebeen developing in the SuperKEKB injector Linac. Furthermore, many monitors and remote controls program
were built to increase the stability of operation. By the injection system, the required charge and emittance forPhase-II commissioning were achieved.

9.1×10-4 with 219 nm laser source at room temperature,
which has the long lifetime (>LaB6).
For the laser source, an Ytterbium-doped (Yb-doped) fiber and Ytterbium-doped YAG (Yb:YAG) hybrid laser
system was development. 30 ps, mJ-class, 257 nm ultraviolet (UV) pulse was provided and injected to the photocathode [5].
The Phase I beam commissioning successfully ended inthe June of 2016. The electron beam and positron beamwith the bunch charge of around 0.7 nC is successfully delivered to the end of beam transport line.
Table 1: Requirements for Linac in SuperKEKB Phase IIElectron HER
7 GeV

Positron LER
4 GeV

Normalized emittance
γβεx / γβεy150 / 150 [μm]200 / 40 [μm]with dampingring

Energy spreadσδ0.10 [%]0.16 [%]

Bunch charge at
injection point

1.0 [nC]

0.5 [nC]

INTRODUCTION
KEK injector Linac has delivered electrons and positrons source for high energy accelerator research, particle
physics and photon science experiments for more than 30years. Then, KEKB is being upgraded towards theSuperKEKB which is an asymmetric energy of 7-GeV
electron and 4-GeV positron double-ring collider [1]. Fullenergy, high-charge and low-emittance beams are necessary in order to achieve a design luminosity of 8×1035 cm2 -1s for the study of the flavour physics of elementary particles. The injector continues to deliver various beams toSuperKEKB and light source rings such as PF and PF-AR.
In the positron beam, new Flux Concentrator (FC) andcapture section was developed for efficient positron generation. Since generated positron beam has large emittance,
new damping ring (DR) has been constructed to reducebeam emittance. Therefore, the thermionic electron gun
was reinstalled for positron beam generation with thecharge of 10 nC [2].
For the injector upgrade, the electron beams with acharge of several nC and a normalized emittance of lessthan 10 μm are expected to be generated in the electron gun.
A thermal cathode DC gun had been used for KEKB. Because the DC gun could not make low-emittance beam, aquasi-traveling wave side couple (QTWSC) RF gun which
has two side coupled standing wave field was developed[3]. This gun has a strong focusing field at the cathode andthe acceleration field distribution also has a focusing effect.
In addition another cut disk structure (CDS) RF gun wasinstalled to 90 degree injection line. These guns have beenused for electron beam study.
A multi-crystalline iridium cerium (Ir 5Ce) compound isused as a photoemitter of a high charge and low emittanceRF gun [4]. The quantum efficiency (QE) is approximately________________________________________† email address : xiangyu.zhou@kek.jp

Towards Phase-II beam commissioning, the high-charge
and low-emittance beam development is going on for thehigher requirement (Table. 1). The mainly improvement is
the laser source. An Ytterbium/Neodymium (Yb/Nd) hybrid laser system was developed. With the synthetic crystalline material Neodymium-doped YAG (Nd:YAG), highreliability of the laser source is realized. The layout of theelectron guns and laser system is shown in Fig.1. The
transport line between the laser and RF gun was restoredwith the backup line and remote control system.

Figure 1: Layout of Linac.

Yb/Nd DOPED HIBRID LASER SYSTEM
OF RF GUN
Based phase-I commissioning, the laser system for RF
gun consisted Yb-doped fiber oscillator, Yb-doped fiber
amplifiers and Yb:YAG thin-disk laser amplifiers.
Yb:YAG crystal has large absorption bandwidth to reduce
thermal management requirements for diode lasers, alonger upper-state lifetime, three to four times lower thermal loading per unit pump power, which is more suitable

for diode-pumping for high power diode-pumped lasers.
Yb:YAG Thin-disk regenerative amplifier system was employed in phase-I commissioning. Compared with theYb:YAG crystal, commonly used Nd:YAG rods with highoptical homogeneity, high damage threshold, consistent
performance and high processing accuracy. Although thepulse shaping of Nd:YAG is difficult to be adjusted because of the narrow spectrum gain, the structure of theNd:YAG is simple, that means reliable convenienceand less maintenance. The Emission wavelength ofYb:YAG and Nd:YAG are around 1030 nm and 1064 nm
with the bandwidth of ~3.0 ns and ~0.5 ns, which containedin the spectrum of Yb-doped fiber oscillator. Therefore, theYb-doped oscillator and Yb-doped fiber amplifier also canbe used with Nd:YAG amplification. For higher reliability
requirement of the Phase-II commissioning, Nd:YAG areappropriate candidate for laser amplifier system [6].
Therefore, the laser system upgraded, consists Yb-doped
fiber oscillator, Yb-doped fiber amplifiers and Nd:YAGrod laser amplifiers. The detail of laser part that shows inFigure 2.

Nd:YAG Amplifier Part
In order to guarantee the injection and commissioning
reliably, two beam lines were prepared. At the end of thefiber part, the signal was separate to two equal parts (firstbeam line and second beam line) by a polarizer.
The first line which has 4 stages Nd:YAG rod amplifiers
is proposed to provide the stable laser pulses with highquality. 2×79 mm Nd:YAG rod crystal are used in the firstand second stages for high amplification efficiency. APockels cell pulse picker is insert to switch the single/double bunch mode. After the Pockels cell, the third and fourthstages amplify the pulse energy up to the 4.0 mJ by usingof 4×93 mm Nd:YAG rod crystal. Then the green pulses of532 nm were generated by using a frequency-doublingstage with 5 mm thickness beta barium borate (BBO) crystal. As the Figure 3, more than 1.8 mJ second harmonic
generation (SHG) pulses was obtained with the good beamprofile and 17 ps pulse width that measured by the streakcamera.

Figure 3: Beam profile and beam width of laser source.
Different from the first line, an additional fifth Nd:YAGamplifier stage (Ф8 mm) is built in the second line to pursue higher pulse energy. The amplified pulse energy is up
to 8.0 mJ. And more than 3.8 mJ pulse energy of SHG
beam is obtained during the frequency-doubling stage.
Figure 2: Layout of laser system.

Yb-doped Fiber Part
The laser system starts with three 114 MHz mode lockYb-doped fiber oscillators. Two of them are commercial
products (Menlo system), the other one is homemade cavity with the all normal dispersion (ANDi) structure. One
MEMS optics switch is adopted to select oscillator for thefollowing amplifier stages. A transmission grating pairstretcher was employed to expend pulse to ~10 ps and select the spectrum width of 0.5 nm with the centre wavelength of 1064 nm. The weak pulses reduced repetition rateto 10.38 MHz by a semiconductor optical amplifier (SOA)
pulse picker, which was driven by a ~5 ns 100 mV electrical pulse at 10.38 MHz amplified through a high power RFamplifier. To compensate the energy loss of the gratingsand SOA, two stages Yb-doped single mode fiber amplifier
was employed. Because the amplified spontaneous emission (ASE) easy to be amplified with the weak seed pulse
by strong pump power, an additional transmission grating
was used to separate and cut the noise by a slit. After fiber
amplifier, an Electro-optic module (E.O.) adopted as apulse picker is used to pick out the 1-25 Hz signal from asequence of pulses of 10.38 MHz [7].

TRANSPORTING LINE
There is about 12 m distance from the laser clean room
to the underground RF gun. The transport line is focus toreduce the energy loss and jitter in the air. Therefore, vacuum tube was built to isolate the air turbulence between thelaser room and RF box. Two laser beams are converged bya polarizer and transported to RF gun box by one transporting line. Inside the RF gun box, the laser beams are separated again by another polarizer. The transport rate can beup to 80%. UV pulses were generated by BBO frequencydoubling stages. 450 μJ and 800 μJ are achieved by the firstand second laser line to RF gun. The optics system isshown in Figure 4. The total efficiency of the ω-4ω is morethan 10%.

Figure 4: Transporting line.
The QTWSC RF gun could be irradiated by each laser
sources with 60 degree, and another laser source works
with the backup line. Furthermore, it is optional to selecttwo laser injection with both sides. A delay line is insertedinto the second laser line to adjust the optical path for realizing two laser synchronous injection into RF gun.
Also, a flip mirror is placed in the first laser line forchanging the transmission direction to the 90 degree CDS
RF gun.

MONITORING AND REMOTE CONTROLSYSTEM
Since the electron beam with high stability and quality
are required in Phase-II commissioning, the status of laser
system and RF cavity are monitored and recoded.
In oscillator, the stepping motor and piezo that adjust thefrequency in the cavity is controlled to synchronize RF frequency signal. Status monitors and I/Q monitor are employed for detect the mode-lock and synchronization.
When the RF frequency is drift, the laser frequency can beauto-adjusted immediately.
The temperature of the laser room and RF gun cavity andall the current of the power supply are monitored and recorded. Furthermore, the pulse output power of the every
stages and the pulse shaping of the every pulse pickers arealso monitored by photo diode detectors. The long term reliability of laser are recorded, and any unusual changes willbe alarmed by monitoring system.
The optics system inside RF gun box is set near the electron gun. A beam profile monitor and laser energy meterare set in the first laser line to oversee the laser status. The
real-time data can be confirmed on internet anytime. For
optimize the laser source in the commissioning study, lotsof adjustment unit were insert, such as telescope lenses distance, BBO crystal angle, X/Y stage and rotation stage. Allof these units are adjusted by fully remote control. Thus,
we can get the best condition to generate highest electroncharge during beam study.

Figure 5: monitoring and remote control panel.
Figure 5 shows the monitoring and remote control system panel by windows-PC. Thanks to the monitoring andremote control system, the all beam injection section iscontinuously working during the Phase-II commissioning.

RF GUN STUDY FOR PHASE-II
COMMISSIONING
SuperKEKB phase II commissioning has been performed since March to July 2018. The injection sector
beam study has also performed. By use of first line laser
source, 1.4 nC electron beam is generated, and the 1.1 nC
electron charge is remained at end of the Linac. The beam
orbit, energy were carried out without any significant issues. In contrast, 2.4 nC is obtained at end of the Linac byuse of the second beam line. By the both side lasers irradiation, 3.3 nC electron beam is delivered successfully. Accordingly, about 2.3 nC electron charge is prepared for injection to High energy ring (HER) of the ring accelerator.
The orbit and electron charge records are shown in Fig.6.

Figure 7: Wire scan measurement of 5 sector.

CONCLUSIONAn Yb/Nd hybrid laser source is upgrade to deliver qualified electron beam by QTWSC RF gun and Ir5Ce cathodefor SuperKEKB phase-II commissioning. Three laser oscillators and two beam lines were created. A lot of improvements of the monitoring and remote control system havebeen done to increase the reliability of the injection department. Electron charge of 2.4 nC and emittance of 50 μm
fulfilled the phase-II requirements.

REFERENCES

Figure 6: Orbit and electron charge by laser source.
The emittance of electron beam generated by RF gun isalso measured by wire scan method. At Linac sector B, theend of the RF gun, the horizontal and vertical emittanceγεx=24.958, γεy=18.640 was obtained respectively. Thenthe high bunch low emittance electron beams are transporting the Linac include the J-arc section. The horizontal andvertical emittance are less than 50 μm at the sector 5 (Figure 7). Low emittance and high charge achieved, and its
performance is enough for the Phase-II commissioning.

[1] Y. Ohnishi et al., “Accelerator design at SuperKEKB”, Prog.
Theor. Exp. Phys., 2013, 03A011.
[2] M. Satoh et al., “Commissioning Status of SuperKEKB Injector Linac”, presented at IPAC’18, Vancouver, Canada,Apr.-May 2018, paper MOPMF075.
[3] T. Natsui et al., “Injector Linac Upgrade and New RF Gun
Development for SuperKEKB”, in Proc.eeFACT’16,Daresbury, UK, Oct.2016, paper TUT2H2, pp.74-78.
[4] D. Satoh et al., “Development of Better Quantum Efficiency
and Long Lifetime IrCe Photocathode for High Charge electron RF Gun”, presented at IPAC’13, Shanghai, China, May
2013, paper MOPFI023.
[5] X. Zhou et al., “25 Hz, Sub-mJ Ytterbium Laser Source of RF
Gun for SuperKEKB Linac”, presented at IPAC’15, Richmond, USA, May 2015, paper WEPMA044.
[6] X. Zhou et al., “Developing an Yb/Nd Doped Hybrid Solid
Laser of RF Gun for SuperKEKB Phase II Commissioning”,
presented at IPAC’17, Copenhagen, Denmark, May 2017, paper THPVA047.
[7] X. Zhou et al., “Neodymium and Ytterbium Hybrid Solid Laser of RF Gun for SuperKEKB”, presented at IPAC’16,Busan, Korea, May 2016, paper THPMY041.