COMMISSIONING OF THE PHASE-1 SuperKEKB B-FACTORY AND
UPDATE ON THE OVERALL STATUS
Y. Ohnishi∗ , T. Abe, T. Adachi, K. Akai, Y. Arimoto, M. E. Biagini1 , M. Boscolo1 , K. Egawa,D. El Khechen2 , Y. Enomoto, J. W. Flanagan, H. Fukuma, K. Furukawa, S. Guducci1 , N. Iida,
H. Iinuma, H. Ikeda, T. Ishibashi, M. Iwasaki, T. Kageyama, H. Kaji, T. Kamitani,
T. Kawamoto, S. Kazama3 , M. Kikuchi, T. Kobayashi, K. Kodama, H. Koiso, M. Masuzawa,T. Mimashi, T. Miura, F. Miyahara, T. Mori, A. Morita, S. Nakamura, T. Nakamura,
H. Nakayama, T. Natsui, M. Nishiwaki, K. Ohmi, T. Oki, S. Sasaki, M. Satoh,
Y. Seimiya, K. Shibata, M. Suetake, Y. Suetsugu, H. Sugimoto, M. Tanaka, M. Tawada,
S. Terui, M. Tobiyama, S. Uehara, S. Uno, X. Wang, K. Watanabe, Y. Yano,
S. Yoshimoto, R. Zhang, D. Zhou, X. Zhou, and Z. Zong
KEK, Tsukuba, Japan,
1 INFN/LNF, Frascati, Italy,
2 LAL, Orsay, France,
3 University of Zurich, Zurich, Switzerland
Abstract
The SuperKEKB B-Factory at KEK (Japan), after few
years of shutdown for the construction and renovation, has
finally come to the Phase-1 commissioning of the LER andHER rings, without the final focus system and the BelleII detector. Vacuum scrubbing, optics tuning, and beam
related background measurements were performed in thisphase. Low emittance tuning techniques have also been applied in order to set up the rings for Phase-2 with colliding
beams next year. An update of the final focus system construction, as well as the status of the injection system with
the new positron damping ring and high current/low emittance electron gun is also presented.

INTRODUCTION
The SuperKEKB collider [1] is an asymmetric-energy
and a double-ring electron-positron collider. The energy ofthe electron ring is 7 GeV(HER) and the positron ring is 4
GeV(LER). The collision point is one and the circumferenceis 3 km. The target luminosity is 8×1035 cm−2 s−1 , which is
40 times as high as the predecessor KEKB collider [2]. In
order to accomplish the extremely high luminosity, a nanobeam scheme [3] is adopted. A large crossing angle and asmall beam spot size in the both horizontal and vertical direction are applied in the nano-beam scheme. The crossing
angle between two colliding beams is 83 mrad and the horizontal beam size is approximately 10 µm. When the overlap
region of two colliding beams is considered, the nano-beamscheme can exchange the coordinate x with z as:
σ̃ ∗x

=

σ̃z

=

σz ϕ xσ ∗x< βy∗ ∼ 300 µm,
ϕx

(1)
(2)

where the σ̃ ∗x and σ̃z are the eﬀective horizontal beam sizeat the interaction point(IP) and the eﬀective bunch length,∗ yukiyoshi.onishi@kek.jp

respectively. Because the geometrical reduction of the luminosity depends on the eﬀective bunch length in the nanobeam scheme, it must be smaller than the vertical beta function at the IP. The low emittance and the low beta function at the IP in the horizontal direction can realize the requirement of “hourglass condition” although the real bunch
length is long. The nominal bunch length, σz , is 6 mm in
SuperKEKB. In the case of SuperKEKB, the vertical beta
function can be squeezed down to approximately 300 µm
from this criterion.
The famous luminosity formula is modified by replacingwith the eﬀective values as:
L=

N+ N− fN+ N− f=√∗∗4πσ x σy4πσz ϕ x ε y βy∗(3)and the vertical and horizontal beam-beam parameter is expressed by
√
βy∗
1
ξy ∝
(4)
σz ϕ x ε y
ξx

∝

β ∗x
∼ 0.003,(σz ϕ x ) 2(5)where ϕ x is the half crossing angle. Because there is anupper limit for the beam-beam parameter, both the verticalbeta function at the IP and the vertical emittance must be
small with keeping their ratio constant in order to achieve
the higher luminosity. The horizontal beam-beam parameter is almost constant and very small in the nano-beamscheme even though the small horizontal emittance andthe small horizontal beta at the IP are realized. Consequently, the dynamic beta and the dynamic emittance becomes small.
The alternative formula of the luminosity is
L∝

I ξy
.
βy∗

(6)

In order to achieve 40 times as high luminosity as the
KEKB collider, the vertical beta function at the IP issqueezed by 1/20 and the beam currents require the double
with assuming the same vertical beam-beam parameter as
KEKB(ξ y ∼ 0.09). Table 1 shows the machine parametersof SuperKEKB.
Table 1: Machine parameters(with intra-beam scattering)for the final design of SuperKEKB.

E
I
nb
C
εx
εy
β ∗x
βy∗
2ϕ x
αp
σδ
VRF
σz
νs
νx
νy
U0
τx
ξx
ξy
L

LER

HER

Unit

4.000
3.6

7.007
2.6

GeV
A

2500
3016.315
3.2
8.64
32
270

4.6
12.9
25
300

83
3.19×10−4 4.53×10−4
7.92×10−4 6.37×10−4
9.4
15.0
6
5
-0.0245-0.0280
44.53
45.53
46.57
43.57
1.76
2.43
45.6
58.0
0.00280.00120.08810.08078×1035

m
nm
pm
mm
µm
mrad

MV
mm

MeV
msec

essary to reduce beam related backgrounds. The Belle IIgroup requires the beam dose of 360 - 720 Ah.

COMMISSIONING OF PHASE-1
The subjects of the Phase-1 commissioning are• hardware check and establish a stable operation,• vacuum scrubbing,• orbit and optics tuning,• beam background study,• study of electron cloud in the positron ring(LER).
Figure 1 shows the history of 5 months commissioning inPhase-1. The total beam currents have reached 1.01 A in
the LER and 0.87 A in the HER, respectively. The averagevacuum pressure reached 10−6 Pa in the LER and 10−7 Pa in
the HER. Table 2 shows the machine parameters for Phase1.
The RF system in the LER consists of normal conducting cavities with a HOM dumped structure, ARES, to store
large beam current stably. A hybrid system of the ARES
cavities and superconducting cavities(SCC) is utilized in
the HER. The number of klystrons increases and the modification of the high-power input coupler is done in order toprovide large beam power. The most of the components arereused from those of KEKB [4], however, a relocation of the
ARES cavities was performed. The current upgrade of the
RF system makes it possible to store 70 % of the nominal
beam current as shown in Table 1.
Table 2: Machine parameters in Phase-1(without intrabeam scattering)cm−2 s−1

There are three stages for the commissioning of SuperKEKB; Phase-1, Phase-2, and Phase-3. The initial commissioning was done during Phase-1 without the final focus system before Belle II roll-in. The Phase-1 commissioning was started in February 2016 and operated until theend of June 2016 for about 5 months. The commissioning
for Phase-2 will start in November 2017 and will operate
for 5 months with the final focus system and Belle II detector. The first collision will be performed in Phase-2, however, the vertex detector will not be installed. The physics
run with the full detector in Phase-3 will start October 2018,
then the luminosity will increase gradually by squeezing thebeta function at the IP and increasing the beam currents toward the target luminosity.
The vertical emittance is one of the most important issues in Phase-1 as described above since the luminosity performance significantly depends on the coupling parameterin the nano-beam scheme. The beam-size blowup induced
by electron cloud in the LER is also an important issue inPhase-1. The study of the electron cloud can be done andconfirm mitigation techniques such as ante-chamber beam
pipes with TiN coating, and so on. Before Belle II detector roll-in, the enough process of vacuum scrubbing is nec-

E
I
nb
εx
αp
. σδ
VRF
σz
νs
νx
νy
U0
τx

LER

HER

Unit

4.000
1.01

7.007
0.87

GeV
A

1576
1.8
4.6
2.45×10−4 4.44×10−4
7.52×10−4 6.30×10−4
7.56
12.61
4.6
5.3
-0.0192-0.0253
44.53
45.53
46.57
43.57
1.76
2.43
46
58

nm

MV
mm

MeV
msec

Vacuum Scrubbing
The vacuum system in the LER has been newly designed
in order to absorb an intense synchrotron radiation emitted
from positron beams and also suppress electron cloud effects [5]. Most of the vacuum pipes, about 93 % out of thewhole ring, were replaced with new pipes. Especially, antechambers with TiN coating were adopted in the arc section.
The vacuum pipes in the arc section are made of aluminumalloy. Most of the vacuum components were reused in theFigure 1: History of the Phase-1 commissioning. Beam current(red), lifetime(cyan), and average pressure(purple).
HER. The vacuum pipe of the HER was made of copper andthe cross section is a race track in the arc section. Therefore, the vacuum scrubbing in the LER is necessary much
more than that of the HER. Figure 2 shows an average vacuum pressure normalized by beam currents as a functionof beam dose in the LER, and that of the HER is shown in
Fig. 3. An expected photon stimulated gas desorption rate,
η in the arc section, is also shown in figures with assuming
the linear pumping speed of 0.06 m3 s−1 m−1 in the LER and
0.03 m3 s−1 m−1 in the HER.
The beam dose of 780 Ah was achieved in the LER and
660 Ah in the HER during the Phase-1 commissioning. The
pressure rise has reached 8×10−7 PaA−1 in the LER and4×10−8 PaA−1 in the HER, respectively. The pressure risein LER became approximately the same value as that of
KEKB for the same beam dose. On the other hand, the pressure rise in the HER during the beginning of Phase-1 wassmaller than that of the initial KEKB commissioning eventhough the vacuum pipes were exposed to the atmosphere
during the upgrade work of the HER. Most of the vacuum
pipes in the arc section were reused from those of KEKB.
It is found that the reused surface of the vacuum pipe cankeep the condition of the vacuum scrubbing at KEKB.

Figure 2: Pressure rise and expected photon stimulated gas
desorption rate as a function of beam dose in the LER .

Optics Tuning
The lattice for Phase-1 is the same lattice as those of
Phase-2 and Phase-3 except for the interaction region(IR).
Since there is no final focus magnet in the vicinity of the IP,
the field strengths of quadrupole magnets in the IR are adjusted so as to connect the arc lattice. The optics tuning [6]without the final focus, the solenoid field, and the local chromaticity corrections can be performed for Phase-1. The optical functions such as beta functions, dispersions, and XY couplings were measured and corrected in the LER and
HER, respectively. The beta functions are obtained by orbit
responses induced by six kinds of dipole correctors for eachx and y direction. The physical dispersions are measured byorbit displacements for the rf frequency shifts between -500

Figure 3: Pressure rise and expected photon stimulated gas
desorption rate as a function of beam dose in the HER.
Hz and +500 Hz. Note that the dispersions are physical anddiﬀerent from normal mode dispersions. In the case of the
X-Y couplings, vertical leakage orbits from horizontal orbits induced by six kinds of horizontal dipole correctors areused to correct the X-Y couplings instead of four X-Y cou-

pling parameters of r 1 - r 4 . The number of BPMs is 438 in
the LER and 460 in the HER to measure closed orbits. The
BPM gain mapping and the beam based alignment(BBA)
have been performed before the optics tuning.
In order to correct the beta functions, the measured
beta functions and phase-advance are compared with those
calculated by the model and amount of the correction offield strength for each quadrupole magnets are estimated.
Correction coils are installed for each quadrupole magnet, however, the correction was performed by familiesof quadrupole magnets during the Phase-1 commissioning.
The arc lattice adopts non-interleaved sextupole correction
scheme and two identical sextupoles are connected by −I ′transfer matrix. The dispersions and X-Y couplings arecorrected by using this specialty. The horizontal dispersions are corrected by using asymmetric local bumps in
the horizontal direction at pairs of two horizontal focusing
sextupoles(SF). The horizontal oﬀset of the sextupole generates quadrupole field, however, this quadrupole field is
localized between two identical sextupoles then only horizontal dispersions can be corrected. The vertical dispersions and the X-Y couplings are corrected by using skew
quadrupole-like correctors at the sextupole magnets. The
opposite sign of the skew quadrupole field can correct only
the vertical dispersions, on the other hand, the same sign
can correct only the X-Y couplings. Because of the noninterleaved sextupole scheme, the corrections of verticalphysical dispersions and X-Y couplings can be solved independently for each other.
Table 3 shows the results of optics corrections. Afterthese optics corrections, the vertical emittance has achieved
9 pm in the LER which was measured by an X-ray beam size
monitor. The vertical emittance is derived by ε y = σy2 / βy .
In another way, the vertical emittance can be estimated bymeasured optical functions and magnet configurations;
ε y = Cq γ 2

I5,y
.
Jx I2

(7)

The normal dispersions can be derived by X-Y coupling
parameters and transfer matrix between neighboring two
BPMs in the model. The vertical emittance of 8 pm in the
LER is obtained by this estimation which is consistent with
the X-ray measurement. On the other hand, the measured
vertical beam size in the HER was 30 µm for βy = 7.6 m,
which corresponds to the vertical emittance of 120 pm. Thisvalue seems to be too large because the optics tuning is thesame level between the LER and HER as shown in Table
3. A simulation assuming a misalignment of the sextupoles
which reproduces the measured optical functions provides
the vertical emittance of 5 - 20 pm. We consider that thecalibration issues of X-ray monitor still remains. The vertical emittance is indirectly estimated to be 6 pm by using themeasured optical functions in the HER.

Electron Cloud
Electron cloud is a serious issue for e+ e− storage-ringcollider. We have tried to suppress the eﬀect by using

Table 3: Results of the optics tuning in Phase-1. X-Y
coupling∗ refers an average value of rms(∆y)/rms(∆x) induced by six kinds of horizontal dipole correctors. Dispersions are physical variables in the table.
Items
Coupling strength
X-Y coupling∗
Hor. dispersion
Ver. dispersion
Hor. β function
Ver. β function
Hor. tune
Ver. tune

SymbolLER

HER| C − | (×10−3 )rms(∆y)/rms(∆x)
rms(∆η x )rms(∆η y )rms(∆ β x / β x )rms(∆ βy / βy )∆νx (×10−4 )∆νy (×10−4 )

1.2
0.9 %
8 mm
2 mm
3%
3%
2
5

2.0
0.6 %
11 mm
2 mm
3%
3%
5
1

solenoid field at KEKB with a circular vacuum pipe formany years. In SuperKEKB, ante-chambers with TiN coating are adopted in the arc section to suppress the electron
cloud in the LER. Figure 4 shows the electron current measured by the electron monitor for various vacuum chambers used at KEKB or SuperKEKB. The measured electron
current indicates the electron density around the beam orbit. When the region of the lower beam current(<400 mA)
is considered which dominates photoelectrons, the antechamber suppress photoelectrons significantly comparedwith the circular vacuum pipe. In the region of the highbeam current(> 400 mA), the secondary electrons are dominate and it is found that the TiN coating can suppress thesecondary electrons very well. The mitigation of electron
cloud by using the ante-chamber with TiN coating has been
confirmed in Phase-1. However, nonlinear pressure risefor the beam current and beam-size blowup were observedin Phase-1. It is found that the nonlinear pressure rise iscaused by multiplication of electrons at the aluminum bellows chamber without TiN coating. The bellows chamberswith a length of 0.2 m locate every 3 m in the whole ringand the number of bellows is 800. In order to suppress themultiplication of electrons, permanent magnets generating
solenoid field were installed and the pressure rise was reduced significantly.
Figure 5 shows the beam-size blowup as a functionof total beam current for several rf-bucket spacing with600 bunches. It is found that the threshold of the beamsize blowup depends on the rf-bucket spacing, namely,
the electron density increases as the rf-bucket spacing decreases. Even though the ante-chamber with TiN coating and solenoid magnets at the aluminum bellows wereadopted, we still observed beam-size blowup above 900 mA
in the case of 3 rf-bucket(6 ns separation) spacing and 1578
bunches in total. Test of nano-beam scheme will be able tobe performed in Phase-2 since the beam-size blowup can besuppressed until 1 A when 4 rf-bucket spacing is adopted.
We need more solenoid field at the drift spaces such as
the ante-chamber to suppress the beam-size blowup when

we try to achieve the design beam current with the design
bunches in Phase-3.

Figure 4: Electron current as a function of beam current in
the LER.

compressed into 10 ps by using a chicane. We also prepareda thermionic gun to be used for primary electrons to generate positron beams and also for the injection to the HER. We
stacked two pre-injectors of the rf gun and of the thermionicgun vertically to switch injection from them. The rf gun islocated at the same level as the main linac, on the other hand,
the thermionic gun is on the upper deck. The vertical transfer line is built to merge electron beams from the thermionicgun to the main linac. The rf gun was operated for 10 days
and the condition was stable [7]. The normalized emittanceof 20 - 30 µ m has been achieved by the rf gun. The charge
intensity was 1.7 nC measured just after the rf gun. Further
modifications for the rf gun will be prepared for the injection in Phase-2 and Phase-3.

UPGRADE STATUS FOR PHASE-2
Major upgrade issue between Phase-1 and Phase-2 is aconstruction of the final focus system. The final focus system consists of four quadrupoles, 16 corrector coils, 4 cancel coils for the HER to compensate leakage field from the
LER, and compensation solenoid for each side of the IP.
These magnets are superconducting magnets and have ironor permendur yokes except for the most closest magnets to
the IP in the LER. The left side of the final focus system has
been installed in the IR and we have measured and adjusted
an alignment of the magnets. The fabrication of the final
focus system for the right side will be finished and installed
within this year.

CONCLUSIONS
Figure 5: Beam-size blowup(vertical) in the LER.

Injector Linac
Low emittance and high intensity electron and positron
beams are necessary for the injection to the LER and HER.
A flux concentrator and 10 large aperture S-band accelerating structures were installed to increase positron intensity for the LER. The huge emittance of the initial positron
beam will be decreased by a dumping ring(DR) at 1.1 GeV.
Since the DR is under construction, the positron beam is injected directly to the LER bypassing the DR during Phase1. We operated the flux concentrator with the half of thenominal current since the head part was damaged due tolarge electric discharge last year. The reason of the damage comes from that the hardening process was abbreviated.
The charge intensity of positron beams was 0.6 nC at the endof the linac for the 7 nC primary electrons. The modified
version of the flux concentrator will be installed next year.
A photo-cathode rf gun is used to make a low emittanceand high intensity electrons for the HER injection. The rf
gun consists of Ir5 Ce as the cathode and a Yb doped solidstate laser with a hybrid system of fiber-based amplifies
and Yb:YAG thin-disk multi-pass amplifies for Phase-1. A
quasi-traveling wave side-coupled cavity is adopted to accelerate an initial electron beam. The bunch length of 25 ps is

The vacuum scrubbing was successfully done and the low
emittance tuning was performed during Phase-1. The vertical emittance of 10 pm which is the target value at this stage
has been achieved in the LER. The vacuum system, the RF
system, and the beam instrumentation worked without any
serious trouble. The fabrication and field measurements ofthe final focus system and the construction of the positron
damping ring will be completed next year. The first collision will be provided in the end of 2017. The SuperKEKB
project keeps on going.

REFERENCES[1] Y. Ohnishi et al., Prog. Theor. Exp. Phys. 2013 03A011 (2013).
[2] T. Abe et al., Prog. Theor. Exp. Phys. 2013 03A001 (2013).
[3] “SuperB Conceptual Design Report”, INFN/AE-07/2, SLACR-856, LAL 07-15, March 2007.
[4] T. Abe et al., Prog. Theor. Exp. Phys. 2013 03A006 (2013).
[5] Y. Suetsugu et al., Proceedings of IPAC2016, Busan, Korea,
May (2016), paper TUOCB01.
[6] Y. Ohnishi et al., Proceedings of IPAC2016, Busan, Korea,
May (2016), paper THPOR007.
[7] M. Satoh et al., Proceedings of IPAC2016, Busan, Korea, May(2016), paper THPOY027.