SuperKEKB ELECTRON POSITRON INJECTOR LINAC UPGRADE FORHIGHER CHARGE AND LOWER EMITTANCE
K. Furukawa∗ , H. Ego, Y. Enomoto, N. Iida, T. Kamitani, M. Kawamura, S. Matsumoto,
T. Matsumoto, T. Miura, M. Satoh, A. Shirakawa, T. Suwada, M. Yoshida,High Energy Accelerator Research Organization (KEK), Tsukuba, Japan
Abstract
KEK electron positron injector LINAC has establishedsimultaneous top-up injections in 2019 for 5 rings ofSuperKEKB DR, LER, HER, PF ring and PF-AR as a baseof the both elementary particle physics and photon science
experiments even under a quite short beam lifetime. It improved the injection stabilities while the SuperKEKB broke
the world record of the collision luminosity of the previous
project KEKB. As the collision performance improves, thebeam-beam effect makes the dynamic aperture shrink, andthe beam lifetime reduces further. Thus, it became inevitable
for the injector to be upgraded in order to resolve the contradictory improvements of higher charge and lower emittanceof injection beams regarding beam wakefield till 2025. The
upgrade plan is described including pulsed magnets, an energy compression system, accelerating structures, girders,positron generator and so on.

INTRODUCTION
The KEK electron positron injector LINAC [1, 2] established the simultaneous top-up injections in to 5 storage ringsin 2019 in order to support both the SuperKEKB particle
collider experiment with DR, LER and HER rings [3], andphoton science experiment at PF ring and PF-AR as depicted
in Fig. 1. It succeeded in improving the efficiency of theSuperKEKB collision experiment by 237% before and after
the introduction of the simultaneous top-up injections [4].
This injection scheme became indispensable because thebeam lifetime of the SuperKEKB ring is quite short, especially at the LER positron ring less than 10 minutes in
2021. Based on this operation arrangement, the injector
LINAC gradually improves the injection performance andcontributes to the achievement of the world record of collision luminosity of SuperKEKB [5].
It has been recognized that the dynamic aperture of therings becomes small and the beam lifetime is shorter as
the beam-beam interactions becomes noticeable. While expanding the storage and injection physical aperture, it isrequired to urge the injector to enhance the injection beam
property. Straightforward multiplication of the LINAC beam
may drastically increase the injection beam emittance by the
wakefield effect in the accelerating structure. Therefore, it isnecessary to reduce the emittance in parallel. We are planning to improve the performance of the injector in line withthe progress of the improvement of collision performance instorage rings, and the outline the plan is presented including
the modification and expansion of the equipment.
∗ kazuro.furukawa@kek.jp

INJECTOR IN KEK ACCELERATORCOMPLEXIn order to realize the advanced injection into
SuperKEKB, we have been remodeling the injector
since 2011 while maintaining the injection for the two lightsources, PF ring and PF-AR. For a low emittance highcharge electron beam, we have developed an RF electron
gun that employs a combination of a quasi-travelling wave
side-coupled cavity (QTWSC), an iridium-cerium alloy
photocathode, and a high-power solid-state laser system,and promoted its stabilization [6].
The typical injection beam is limited up to 2 nC per bunchand 2 bunches at 50 Hz because of the emittance blow-up
by wakefield effect in the downstream LINAC and coherent
synchrotron radiation (CSR) effect in the beam transport.
For the high current positron beam, a tungsten target is irradiated with the electron beam from a thermionic gun, andthe positron is captured by the pulsed magnetic field of about
5 T in a flux concentrator (FC) and a long solenoid section
with magnetic field of 0.5 T. After decelerating and bunching with a large-diameter S-band accelerating structure, thepositron beam is accelerated to the damping ring (DR). Initially, there was concern that a discharge would occur in the
0.2 mm gap of FC, but the issue was solved by employing
copper-nickel alloy [7].
In order to establish the simultaneous top-up injections with pulse-to-pulse modulation (PPM), various highprecision pulsed operation devices such as pulsed magnets [8] and beam instrumentation have been developed,
and we have succeeded in realizing independent beam operation for each storage ring by developing the event-based
wide-area synchronous beam control mechanism [9].

Low emittance
RF-gunDamping
Ringe–

Injector Linac

e+

700 m
High efficiency
e+ generator

2x beamcurrent

PF
2.5 GeV

e–

e+ BT

PF-AR
6.5 GeVe – BT

Beam from Injector and Storage Current
SuperKEKB:
7 GeV e2000 mA
4 GeV e+2800 mAPF:
2.5 GeV e450 mA
PF-AR:
6.5 GeV e60 mA

HER
7 GeV

SuperKEKB
3 km

LER
4 GeV

Belle II
30x
Luminosity

Figure 1: Layout of KEK electron/positron accelerator complex.

Table 1: Required Injector Beam Parameters
Beam
Beam energy
Normalized emittance 𝛾𝜀 𝑥/𝑦Energy spread
Bunch charge
No. of bunches/pulse
Repetition rate

Positron
4.0
100/15
0.16
4
2

50

Electron
7.007
40/20
0.07
4
2

GeV
µm
%
nC
Hz

The required beam specifications for SuperKEKB are
shown in Table 1. It is essential to manage the effects of longitudinal and transverse wakefield in the accelerating structure and CSR in the beam transport line as the beam charge
increases. At the same time, the short-term and long-term
stabilization of the equipment is also indispensable. Thus,
the LINAC upgrade is planned and being implemented inseven categories, namely; pulsed magnets, an energy compression system and movable girders for transverse and longitudinal emittance mitigation, the rf gun and the positron
capture section for enhanced beam charge, and accelerating structures and number of capacitors with PCB for aging
management.

LOW EMITTANCE PRESERVATION
The possible emittance blow-up caused by wakefield effectwith alignment errors has been evaluated and found that theerrors of accelerating structures and quadrupole magnets
should be within 0.1 mm for a short distance and 0.3 mm fora long distance for the SuperKEKB injection [10, 11]. Afterthe Great East Japan Earthquake has destroyed the alignmentby more than 1 cm, it was vigorously recovered [12]. The
beam orbit in the downstream LINAC became well organized
as the installation of pulsed magnets was necessary in orderto realize the simultaneous top-up injections [8]. However,the beam orbit deviation and optics errors in the upstream
part are quite large, and the deviations sometimes reach
1 mm or more from the center. It is considered that pulsed
magnets and possibly movable girders need to be additionallyinstalled (Fig. 2).
One of the reasons of orbit deviation was attributed tothe building distortion, and it was found that the seasonal

C3

11

18

28

38

48

Figure 3: Seasonal floor movement in LINAC tunnel.
vertical movement of seven expansion joints in the LINAC
tunnel reaches 1∼2 mm per half year as in Fig. 3 [13].
Errors in the beam optical functions, especially a dispersion function in acceleration sections, may produce anerroneous emittance growth. Thus, beam mode-dependent
optics corrections by using pulsed quadrupoles are important [14].
It was considered that the beam impedance in the transportline from LINAC to the ring is small and it should have beentransparent for beam properties. However, the longitudinalbunch structure from LINAC might induce CSR and thehigher-order magnetic fields in the bending magnets cause
XY-coupling. We will examine the emittance blow-up withincreasing bunch charge up to 4 nC, and investigate the way
to mitigate it.
The two bunches in a pulse separated by 96 ns are differentiated using the microwave timing presently [15]. However,this technique doesn’t provide fully independent tuning capability.

Pulsed Magnets
In the previous project KEKB the injection optics requirement was not strict and the optics condition for each beammode was matched in each beam transport line instead ofinside LINAC even though the beam energies were several
times different. Such strategy, however, does not work forthe beam quality in SuperKEKB because of possible orbit
deviation and residual dispersion in LINAC. Thus, hundredof pulsed orbit-correctors and quadrupole magnets were installed mainly downstream at first as described above [8].
In upstream LINAC we need to further install pulsed magnets for higher beam charge in order to overcome the charge
difference ratio of about hundred between beam modes.
Power regeneration was realized for those pulsed
quadrupoles and 80% of the electromagnetic power sent
to the coils is recovered for the next pulse.

Fast Corrector Coils

Figure 2: Example of pulsed magnets installed in the linac.

Pulsed magnets are utilized to control beam optics andorbit with independent field strength for each pulse at 20 ms
intervals, and the pulse width is about 1 ms. On the other
hand, orbits of two bunches in a pulse need to be controlled
independently by fast corrector coils. Therefore, we are de-veloping fast pulsed power supplies and coils with ceramics
embedded electrodes [16]. We expect a rise time less than
100 ns to correct one of two bunches with only 96-ns apart.

Movable Girders
As mentioned earlier, the annual and localized deviationsof the devices up to 2 mm at the expansion joints are considered to cause a large wakefield effect. Thus, we havedeveloped a movable girder with a load up to 1 t and a precision of 10 µm to remotely drive the accelerating structures
and the quadrupole magnets [17]. The beam effect will be
evaluated.

ENERGY SPREAD MITIGATION
For the issue of longitudinal emittance we originallyplanned to accelerate longitudinally rectangular-shaped
beam from the RF electron gun [6]. Ytterbium-doped YAG
fiber and thin disk laser with an excellent frequency response
was developed to provide a shaped laser pulse. However, thehigh current beam more than 1 nC per bunch was unstable.
Therefore, a neodymium-doped YAG laser has been
adopted to achieve a higher beam current. As the time response is limited for longitudinal shaping, only a Gaussian
shape is available and as a result, the energy spread is expected to greatly expand at 4 nC per bunch.

Energy Compression System
An energy compression system for positrons with magnetic chicane and accelerating structures was constructedfrom the beginning as the energy spread is large because of
the bunch compression system after the damping ring. As
mentioned above, an energy compression system became
necessary for high-current electrons. Therefore, we are planning to construct an energy compression system (ECS) by
using R56 in the beam transport line. As the distance fromthe power modulator is large, circular S-band waveguides
will be applied to suppress the power loss.
Since the origin of the emittance growth in the beam transport line hasn’t been identified, we are carefully discussing
the interference between the ECS design and possible effects such as CSR. It is possible that the longitudinal bunch
structure has to be manipulated in order to suppress CSR,
for which we could utilize pulsed quadrupoles that will be
installed.

HIGHER BUNCH CHARGE
If we can achieve low emittance and low energy spreadbeams, we expect to be able to inject into SuperKEKB rings
with the condition shown in Table 1. Depending on the
progress of collision operation and the beam lifetime in the
future, however, it may be necessary to enhance the beam
charge further especially for positrons such as 6 nC per bunch
as a future possibility.
Several possible optimizations could be applied in thepositron capturing section. Presently, the transverse distance
between the target and the hole to pass electrons is 3.5 mm,

and electrons are at the center of accelerating structure behind in order to suppress the wakefield. If the distance could
be shortened, positrons would be closer to the center. The
orbit inside of the long solenoid section had not observed because of electron and positron pairs and the tight equipment
arrangement. Recently, a new beam position monitor thatcan distinguish generated positrons from electrons that is
only 150 ps apart [18] and the orbit could be controlled with
newly installed pulsed correctors. The beam optimizationsin these areas are being considered.

REACHABLE ENERGY
The collision experiment at SuperKEKB is usually performed with energy that generates Υ(4S) state of B mesons,but an experiment at Υ(6S) state with 440-MeV higher isanticipated.
150 accelerating structures of total 230 in LINAC are
40 years old with initial designed gradient of 8 MeV/m butwith 20 MeV/m now. As they are degraded by long-timedischarges and cooling water leaks, the accelerating gradientbecomes lower and we cannot reach Υ(6S) state. Therefore,
we are replacing 16 accelerating structures before 2023 as inFig. 4, in order to realize the experiment at Υ(6S) state [19].
We will determine whether further replacements are necessary or not.

Figure 4: New S-band accelerating structures installed in
the tunnel.
The capacitors in the high-power klystron modulators are
more than 30 years old, and may contain small amount of
Polychlorinated Biphenyls (PCBs). It is necessary to replacethem all as they need destructive inspections for PCBs. We
are planning to replace about 700 capacitors with ceramic
containers in several years in order to maintain the voltage.

CONCLUSION
The KEK electron-positron injector LINAC will be further upgraded in seven categories for lower longitudinal andtransverse emittance and higher beam charge as well as thecountermeasures against aging components. While no insurmountable difficulties are foreseen in the construction, adeliberate operational plan would be constructed soon.

REFERENCES[1] M. Akemoto et al., “The KEKB injector linac”, Prog. Theor.
Exp. Phys, vol. 2013, p. 03A002, 2013. doi:10.1093/
ptep/ptt011[11] Y. Seimiya et al., “Investigation of beam variation andemittance growth simulation with both misalignments andthe beam jitter for SuperKEKB injector linac”, in Proc.
IPAC2017, Copenhagen, Denmark, 2017, pp. 1304–1307;
doi:10.18429/JACoW-IPAC2017-TUPAB005[2] K. Furukawa et al., “Rejuvenation of 7-GeV
SuperKEKB injector linac”, in Proc. IPAC2018,
Vancouver, Canada, 2018, pp. 300–303. doi:
10.18429/JACoW-IPAC2018-MOPMF073[12] Y. Okayasu et al., “Survey report for KEK tsukuba campus injector”, in Proc. PASJ2021, Matsuyama, Japan,
2021, pp. 618–621; http://www.pasj.jp/web_publish/pasj2021/proceedings/PDF/WEP0/WEP006.pdf[3] K. Akai et al., “SuperKEKB collider”, Nucl. Instrum. Methods A, vol. 907, 2018, pp. 188–199. doi:10.1016/j.nima.
2018.08.017[13] M. Tanaka et al., “Measurement of floor movementin the KEKB injector LINAC tunnel (2)”, in Proc.
PASJ2015, Tsuruga, Japan, 2015, pp. 891–894;
https://www.pasj.jp/web_publish/pasj2015/
proceedings/PDF/WEP1/WEP137.pdf[4] K. Furukawa et al., “Advanced acceleration mode switchingfor simultaneous top-up injection at KEK electron/positron
injector linac”, in Proc. PASJ2020, Matsuyama, Japan,
2020, pp. 1–6. https://www.pasj.jp/web_publish/pasj2020/proceedings/PDF/WEOO/WEOOP01.pdf[5] Y. Ohnishi et al., “SuperKEKB operation using crab waist
collision scheme”, Eur. Phys. J. Plus, vol. 136, 2021, p. 1023.
doi:10.1140/epjp/s13360-021-01979-8[6] M. Yoshida et al., “Generation and acceleration of lowemittance, high-current electron beams for SuperKEKB”,
in Proc. LINAC2014, Geneva, Switzerland, 2014, pp. 21–25;
https://epaper.kek.jp/LINAC2014/papers/
moiob03.pdf[7] Y. Enomoto et al., “A new flux concentrator made of Cu alloy
for the SuperKEKB positron source”, in Proc. IPAC2021,
Campinas, Brazil, 2021, pp. 2954–2956; doi:10.18429/
JACoW-IPAC2021-WEPAB144[8] Y. Enomoto et al., “Pulse-to-pulse beam modulation for 4
storage rings with 64 pulsed magnets”, in Proc. LINAC2018,
Beijing, China, 2018, pp. 609–614; doi:10.18429/
JACoW-LINAC2018-WE1A06[9] K. Furukawa et al., “New event-based control system forsimultaneous top-up operation at KEKB and PF”, in Proc.
ICALEPCS2009, Kobe, Japan, 2009, pp. 765–767; https://epaper.kek.jp/icalepcs2009/papers/thp052.pdf[10] H. Sugimoto et al., “Design study on KEK injector linac
upgrade for high-current and low-emittance beams”, in Proc.
IPAC2012, New Orleans, USA, 2012, pp. 1206–1208; https://epaper.kek.jp/IPAC2012/papers/tuppc021.pdf[14] Y. Seimiya et al., “Low emittance beam transport for e − e+ LINAC”, in Proc. eeFACT2018,
Hong Kong, China, 2018, pp. 126–130; doi:
10.18429/JACoW-eeFACT2018-TUPAB02[15] K. Furukawa et al., “Beam feedback systems and BPM readout system for the two-bunch acceleration at the KEKB
Linac”, in Proc. ICALEPCS2001, San Jose, USA, 2001,
pp. 266–268; https://epaper.kek.jp/ica01/papers/
WECT006.pdf[16] C. Mitsuda et al., “Accelerator implementing development of ceramics chamber with integrated
pulsed magnet for beam test”, in Proc. IPAC2019,
Melbourne,
Australia,
2019,
pp. 4164–4166.
doi:10.18429/JACoW-IPAC2019-THPTS027[17] S. Ushimoto et al., “Development of motorized movable structures for accelerator girders and magnet supports”, in Proc. PASJ2021, Takasaki, Japan, 2021, pp. 422–
426. https://www.pasj.jp/web_publish/pasj2021/
proceedings/PDF/TUP0/TUP007.pdf[18] T. Suwada et al., “First simultaneous detection of electron
and positron bunches at the positron capture section of theSuperKEKB factory”, Sci. Rep. vol. 11, 2021, p. 12751. doi:
10.1038/s41598-021-91707-0[19] H. Ego et al., “New S-band accelerating structure
for the KEK electron and positron injector LINAC”,
in Proc. PASJ2021, Takasaki, Japan, 2021, pp. 130132. https://www.pasj.jp/web_publish/pasj2021/
proceedings/PDF/WEOA/WEOA04.pdf