PROGRESS OF SuperKEKB
T. Miura#, T. Abe, T. Adachi, K. Akai, M. Akemoto, A. Akiyama, D. Arakawa, Y. Arakida,
Y. Arimoto, M. Arinaga, K. Ebihara, K. Egawa, A. Enomoto, J. W. Flanagan, S. Fukuda,
H. Fukuma, Y. Funakoshi, K. Furukawa, T. Furuya, K. Hara, T. Higo, H. Hisamatsu, H. Honma,
T. Honma, R. Ichimiya, N. Iida, H. Iinuma, H. Ikeda, M. Ikeda, T. Ishibashi, H. Ishii, M. Iwasaki,
A. Kabe, T. Kageyama, H. Kaji, K. Kakihara, S. Kamada, T. Kamitani, S. Kanaeda, K. Kanazawa,H. Katagiri, S. Kato, S. Kazama, M. Kikuchi, T. Kobayashi, H. Koiso, Y. Kojima, M. Kurashina,
K. Marutsuka, M. Masuzawa, S. Matsumoto, T. Matsumoto, H. Matsushita, S. Michizono,
K. Mikawa, T. Mimashi, F. Miyahara, K. Mori, T. Mori, A. Morita, Y. Morita, H. Nakai,
H. Nakajima, T. T. Nakamura, K. Nakanishi, K. Nakao, H. Nakayama, T. Natsui, M. Nishiwaki,J. Odagiri, Y. Ogawa, K. Ohmi, Y. Ohnishi, S. Ohsawa, Y. Ohsawa, N. Ohuchi, K. Oide, T. Oki,
M. Ono, H. Sakai, Y. Sakamoto, S. Sasaki, M. Sato, M. Satoh, K. Shibata, T. Shidara, M. Shirai,
A. Shirakawa, M. Suetake, Y. Suetsugu, R. Sugahara, H. Sugimoto, T. Suwada, S. Takasaki,
T. Takatomi, T. Takenaka, Y. Takeuchi, M. Tanaka, M. Tawada, S. Terui, M. Tobiyama, N.
Tokuda, K. Tsuchiya, X. Wang, K. Watanabe, H. Yamaoka, Y. Yano, K. Yokoyama, M. Yoshida,
M. Yoshida, S. Yoshimoto, K. Yoshino, R. Zhang, D. Zhou, X. Zhou, Z. Zong, KEK, Ibaraki, Japan
D. Satoh, Tokyo Inst. of Technology, Tokyo, Japan
Abstract
SuperKEKB is designed based on the ‘nano-beamscheme’ with the target luminosity of 8×1035 cm-2s-1. The
design of the interaction region (IR) with a very low βy*at the interaction point (IP) is discussed. The status of theinstallation and the injector commissioning ofSuperKEKB is reported.

INTRODUCTION
The previous project KEKB was an e−/e+ collider thatis used for physics experiments mainly conducted at theϒ(4S) resonance. KEKB has the highest peak luminosity
in the world, 2×1034 cm-2s-1. KEKB has been in operation
until 2010. Afterward, upgrading of KEKB has been
initiated toward SuperKEKB. The target luminosity is
8×1035 cm-2s-1, which is 40times higher than that of
KEKB. The required integral luminosity is 50 ab-1. For
the upgrade, the KEKB tunnel is being reused, and the
KEKB components are being reused as much as possible
in SuperKEKB.
Luminosity mainly depends on three parameters: thebeam currents (I), the beam-beam parameter (ξy), and thevertical beta function (β*y) at the interaction point (IP).
For achieving 40 times higher luminosity than that of
KEKB, the current I is doubled, the beam-beam parameterξy is kept almost at the same level, and β*y is reduced to
1/20 of the KEKB value, based on the ‘nano-beamscheme’. Table 1 shows the SuperKEKB machineparameters. As the nano-beam collision scheme, beams
collide at a large crossing angle, 83 mrad. Thus, thelength of the overlap region of the two beams at the IP isexpressed as σx*/φ. For realization of the small β*y, lowhorizontal emittance εx and small β*x are necessary. The____________________________________________#takako.miura@kek.jp

value of β*y at the SuperKEKB is 0.3 mm.
As part of an additional design concept, beam energies
are changed. The LER energy is increased for longer
Touschek lifetime and mitigation of emittance growth
owing to the intra-beam scattering. In HER, as the energy
is decreased, emittance becomes lower and synchrotron
radiation (SR) power decreases. For the low SR power,
the KEKB beam chamber can be reused.
Table 1: SuperKEKB Machine Parameters
Parameters

units

LER

HER

Beam energy

GeV

4

7.007

Half crossing angle φ

mrad

41.5

Num. of bunches

2500
nm

3.2

4.6

%

0.27

0.25

Beta functions β x/β ymm

32/0.27

25/0.30

Beam currents I

A

3.6

2.6

0.088

0.081

H emittance εxEmittance ratio**

Beam-beam param.ξyBunch length

mm

6.0

5.0

H beam size σx*

µm

10

11

V beam size σy*

nm

48

62

Luminosity

cm-2s-1

8 × 1035

The estimated beam lifetimes for both rings are ~6 min,obtained without the consideration of without beam-beameffect. An effect of the beam-beam interaction to thebeam lifetime and other beam-beam related issue are
discussed elsewhere [1]. Therefore, high bunch chargeinjection beams with low emittance are required. The
LINAC has been upgraded, and the positron damping ring(DR) has been constructed anew for SuperKEKB.

UPGRADE STATUS OF MAIN RINGS
Final Focus Superconducting Magnets
SuperKEKB is based on the nano-beam scheme. Finalfocus (FF) quadrupoles have to be located as close to the
IP as possible, because beam size is strongly reduced to
the nm scale. The quadrupole magnets are, therefore,
located inside the Belle II solenoid fields of 1.5T.
Figure 1 shows the layout of the FF superconducting
magnet. Pairs of QC1 and QC2 are FF doublets. The QC
magnets are equipped with the corrector coils for
multipoles(normal/skew
dipoles,skew
quad,
normal/skew sextupoles, octupole). ES solenoids arecompensation solenoids for canceling the Belle II
solenoid field. QC1LP and QC1RP in the LER are locatedin a strong solenoid field, and space is limited. Thus, they
do not have the magnetic yokes. The corrector coils are
installed in the HER line for canceling the leakage fields
from QC1LP and QC1RP. The FF magnets have beenvery carefully designed and fabricated to minimize thehigher multipole fields which are crucially important for
the wide dynamic aperture.

The Interaction Region
The natural chromaticity of SuperKEKB is higher thanthat of KEKB. Especially, the vertical chromaticity ishigh significantly. 80% of the chromaticity is induced by
the FF. Horizontal and vertical local chromaticity
corrections (LCC) are adopted for correcting large
chromaticity near the FF in both rings as shown in Fig. 3.

Figure 3: FF optics with local chromaticity corrections.
Since large crossing angle and LCC are adopted inSuperKEKB, the beam line of ~320 m around IR has beenreconstructed completely. All components weredismantled in 2013. At present, almost all magnets havebeen installed as shown in Fig. 4.

Figure 1: Layout of the FF superconducting magnets.

Figure 4: The beam line of ~320 m around IR has beenreconstructed completely.

The Magnet System

Figure 2: Picture of left side superconducting magnets.
Figure 2 shows a picture of left side superconducting
magnets. Assemblies of all quadruple magnets werecompleted. A cold test at the temperature of 4 K was
performed for left side magnets. Satisfactory performancefor beam operation was confirmed.

For low horizontal emittance εx, the lattice wasredesigned. The LER dipole magnets of 0.89 m in the arc
were replaced with ones of 4 m, as shown in Fig. 5. In thecase of HER, beta functions and dispersions weremodified for making the emittance as small as possible
because the dipole magnets in the KEKB HER are
sufficiently long.

this date, almost all beam pipes in LER have been alreadyreplaced with new ones.
Table 2: Vacuum Chambers in LER
SectionVacuum chambers

Drift

Antechamber(Al) + TiN coating + Solenoid

Q, Sx mag

Antechamber(Al) + TiN coating

Bend

Antechamber(Al) + Groove + TiN coating

Wiggler

Antechamber(Cu) + Clearing electrode

Figure 5: The LER dipole magnets of 0.89 m in the arc
were replaced with ones of 4 m for low εx.
Figure 6 shows the wiggler magnets. Wiggler magnets
have been installed in both rings for shortening ofradiation damping time and reducting the emittance. In
LER, wiggler cycles were shortened for small dispersion.

Figure 7: Aluminium antechamber with TiN coating.

Figure 6: Wiggler magnets have been installed in theboth rings. Wiggler cycle was shortened in LER.
Most of the magnets have already been installed.
Integrated testing with power supply is in underway. The
precise alignment work of magnets will be initiated soon.

The Vacuum System
Effects of the electron cloud in the LER positron ringare more serious than at KEKB. Various countermeasuresfor preventing emittance growth owing to head-tailinstability are necessary. Table 2 shows the vacuum
chambers installed in each section as thecountermeasures.
In LER, almost all chambers were replaced withaluminium antechambers with TiN coating as shown in
Fig. 7. Specifically, in the dipole magnet section, agrooved type antechamber with TiN coating is employed.
In the wiggler section, an antechamber of copper withclearing electrode is employed, as shown in Fig. 8.
Suppression rates of electron cloud are 1/5 with theantechambers, 1/50 with the solenoids, 1/4 with thegrooves, and 1/100 with electrodes. TiN coating is 3/5 for
Cu and 1/50 for Al. The TiN coating is indispensable for
Al chamber because Al has high emission rate ofsecondary electrons. After TiN coating, the emission rateof secondary electrons is the same for Al and Cu. As of

Figure 8: Groove-type antechamber (left)
antechamber with a clearing electrode (right).

and

UPGRADE OF THE LINAC
Table 3 shows the LINAC parameters. The
requirements on the injection beam are: electrons of 5
nC/bunch and positrons of 4 nC/bunch, with low
emittance (γβεy below 20 mm⋅mrad). Positron capture
section has been upgraded for four times higher positronyields compared with KEKB, and low emittance isrealized by DR. The photo-cathode RF gun, which yields
high bunch charge and low emittance, is being developed.
This is a challenging work. The thermionic electron gun isalso employed for producing a 10 nC beam of primaryelectrons for positron production, and it is located above
the RF-gun. Two-bunch injection is performed for one
pulse at the 50 Hz repetition rate. Figure 9 shows thelayout and beam energy patterns of the LINAC. The
LINAC serves as an injector for four rings (LER, HER,PF and PF-AR of light sources). Beam modes are
switched in pulse-to-pulse for 5 rings including the DR.

Downstream of the DR, the acceleration energy depends
on the beam mode. Therefore, pulsed quads and pulsed
steering magnets will be installed for optimizing theoptics for different beam energies and for simplifying thebeam handling. For low emittance preservation, local andglobal alignment error tolerance values of 0.1 mm rms
and 0.3 mm rms, respectively, are required [2].
Beam commissioning [3] has been performed during
the upgrading. The status of the commissioning isreported as well.
Table 3: KEKB/SuperKEKB LINAC Parameters
Parameters

PositronElectron

Beam Energy [GeV]

3.5 / 4.0

8.0 / 7.0

Bunch charge [nC]

1* / 4*

1/5

Emittance γβεy[mm⋅mrad]

2100 / 20

100 / 20

Energy spread [%]0.125 / 0.1

0.125 / 0.1

Number of bunch

2/2

2/2

Figure 10: Photo-cathode Quasi-traveling wave RF gun.

(*) bunch charge of primary electron =10 nC

Figure 11: The bunch charge at the RF gun exit.

Figure 9: Layout of injector LINAC.

The Photocathode RF Gun System
A photocathode quasi-traveling wave side couple RF
gun [4] was installed as a low-emittance electron-beamsource in September of 2013, as shown in Fig. 10. ThisRF gun has a strong focusing force owing to accelerating
field. Thus, it can generate electron beams with up to 10
nC charge. For 5nC/bunch, 5 mm⋅mrad normalized
emittance is expected from simulations. For aphotocathode, an Ir5Ce cathode is employed. It has a long
lifetime and its quantum efficiency is 10-4. The laser’s
incident angle is 60degrees relative the cathode surface.
An Yb:YAG laser has been employed as a laser system.

Commissioning of the Photocathode RF Gun
The laser condition was 2 bunch×25 Hz. The RF gun
acceleration voltage was limited to 6.5 MV by breakdown(design: 13.5 MV). Figure 11 shows the bunch charge
immediately at the exit from the RF gun. At the
beginning, the bunch charge was limited by the thermal
effect. After the Yb:YAG thin-disk cooling was
improved[5], the bunch charge of 4 nC has been
maintained for a week. The laser power increased and thestability also improved.

Emittance measurement was performed with quads
scan. The beam current was 3 nC at the screen. The beam
size was measured shot-by-shot by using a camera. The
measured normalized emittance was 49 mm⋅mrad in thehorizontal and 26 mm⋅mrad in the vertical directions.
Emittance in the horizontal direction was higher than thatin the vertical direction, which was caused by the laser’s
incidence angle. As a result, high acceleration voltage ofRF gun is required for small emittance.

The Positron Capture Section
The positron capture section has been reconstructedcompletely for achieving the positron yield fourfold.
Figure 12 shows the new positron capture section. It was
installed in May 2014. The capture section consists of aflux concentrator (FC), a bridge coil, and six largeaperture S-band acceleration structures (LASs) coveredby a DC solenoid. The FC is employed for large energy
acceptance, and LASs are employed for large transverse
acceptance. A hole (φ=2 mm) for low emittance electron
beam is on axis. The positron target is off axis. The orbitsof the electron beam and of the primary electron beam for
positron production are switched by using two pulse
steering magnets upstream of the target.

Figure 12: Photo of positron capture section (left) andschematic drawing around the positron target (right).

Commissioning of Positron Capture Section
First positron commissioning was performed one monthafter the installation. Operation conditions of the FC
current, the DC solenoid current, and the acceleration
field of the LAS were not fulfilled at that time. The
charge of primary electrons was 0.6 nC. The positronyield was 30% at the end of the capture section, and 20%
at the end of Sector 2, as shown in Fig. 13. At the DR
entrance, positron yield above 40% is necessary.
Commissioning under condition of design parameters will
start during October of 2015. From the simulations
results, we expect a twofold higher positron yield.

Figure 14: The 1.1 GeV positron damping ring.

SCHEDULEThe SuperKEKB commissioning will be performed inthree stages: Phase 1, Phase 2, and Phase 3.
The first beam commissioning Phase 1 will start earlynext year. Vacuum scrubbing and basic machine tuning
will be performed at that time. A 1 nC/bunch injection
beam is required. Low emittance injection beam is notnecessary.
Phase 2 is scheduled to start in the middle of 2017. The
FF magnets and Belle II detector, except for a vertex
detector, will be installed before Phase 2. Collisionexperiment is planned, and luminosity is expected to be
the same as that of KEKB’s design. Low emittance and 2
nC/bunch injection beam is required. DR beamcommissioning, therefore, will start before Phase 2. Topup injection with pulse-to-pulse mode switching is also
necessary during this phase.
Phase 3 will start during October 2018. This is the startof the full current and full Belle II operation. At this time,
low emittance and 4 nC/bunch injection beam will be
required. We aim at a target luminosity of 8×1035 cm-2s-1.

REFERENCES

Figure 13: First result of positron beam commissioning.

THE POSITRON DAMPING RING
Construction of the DR tunnel was completed in March
2013. Figure 14 shows the 1.1 GeV positron DR.
‘Reverse-bend FODO lattice’ is employed as the DR
normal cell. The features are:
• Shorter damping time (10.8 ms).
• Large momentum aperture (1.5%).
• Low momentum compaction (0.0144).
Emittance is damped from 1400 nm to 43 nm in 40 ms.
As a current status, the installation of magnets in transportlines between LINAC and DR has almost been
completed. The installation of magnets in ring will startduring June of 2015.

[1] D. Zhou et al., “Interplay of Beam-Beam, Lattice
Nonlinearity, and Space Charge Effects in the
SuperKEKB
Collider”,
WEYB3,IPAC’15,
Richmond, May 2015.
[2] S. Kazama et al., “Emittance Preservation inSuperKEKB Injector”, MOPWA053, IPAC’15,
Richmond, May 2015.
[3] M. Satoh et al., “Commissioning of SuperKEKB
Injector Linac”, TUPTY008, IPAC’15, Richmond,May 2015.
[4] T. Natsui et al., “Quasi-Traveling Wave RF Gun andBeam Commissioning for SuperKEKB”, TUPJE003,
IPAC’15, Richmond, May 2015.
[5] R. Zhang et al., “Improvements of the Laser System
for RF-Gun at SuperKEKB Injector”, TUPWA071,
IPAC’15, Richmond, May 2015.