REJUVENATION OF 7-GeV SuperKEKB INJECTOR LINAC
K. Furukawa∗ , Y. Enomoto, M. Akemoto, D. Arakawa, Y. Arakida, H. Ego, A. Enomoto,T. Higo, H. Honma, N. Iida, M. Ikeda, H. Kaji, K. Kakihara, T. Kamitani, H. Katagiri,
M. Kawamura, M. Kurashina, S. Matsumoto, T. Matsumoto, H. Matsushita, S. Michizono,K. Mikawa, T. Miura, F. Miyahara, H. Nakajima, K. Nakao, T. Natsui, M. Nishida,Y. Ogawa, Y. Ohnishi, S. Ohsawa, F. Qiu, I. Satake, M. Satoh, Y. Seimiya, A. Shirakawa,
H. Sugimura, T. Suwada, T. Takenaka, M. Tanaka, N. Toge, Y. Yano, K. Yokoyama,
M. Yoshida, R. Zhang, X. Zhou
High Energy Accelerator Research Organization (KEK), Oho, Tsukuba, 305-0801, Japan, and
SOKENDAI (The Graduate University for Advanced Studies), Oho, Tsukuba, 305-0801, Japan
Abstract

Low emittance
RF-gun

KEK injector linac has delivered electrons and positrons
for particle physics and photon science experiments for morethan 30 years. It was upgraded for the SuperKEKB project,which aims at a 40-fold increase in luminosity over the previous project KEKB, in order to increase our understandingof flavor physics beyond the standard model of elementary
particle physics. SuperKEKB energy-asymmetric electronpositron collider with its extremely high luminosity requires
a high current, low emittance and low energy spread injection beam from the injector. The electron beam is generated
by a new type of RF gun, that provides a much higher beam
current to correspond to a large stored beam current and ashort lifetime in the ring. The positron source is anothermajor challenge that enhances the positron bunch intensityfrom 1 to 4 nC by increasing the positron capture efficiency,
and the positron beam emittance is reduced from 2000 μm
to 10 μm in the vertical plane by introducing a damping ring,followed by the bunch compressor and energy compressor.
The summary of the rejuvenation is reported.

INTRODUCTION
SuperKEKB is being commissioned as an asymmetricenergy electron-positron double-ring collider [1]. It has been
upgraded since 2010 after a decade of successful operationof the KEKB project. The Phase-1 beam commissioning
was performed in 2016 without the collision, in which thestored beam current of about 1 ampere was achieved. Withthe interaction region constructed, the Phase-2 beam commissioning has been performed since March 2018, and thefirst collision was observed [2]. The commissioning willcontinue till coming July, and then the Phase-3 beam commissioning is planned from 2019.
While the facilities in the KEKB project was reused asmuch as possible for SuperKEKB, many components werereconstructed or newly introduced in order to fulfill the advanced requirements for a 40-fold increased luminosity.

Damping
Ringe–Injector Linace+600 m

High efficiency
e+ generator

2x beamcurrent

PF
2.5 GeV

e–e+ BTPF-AR
6.5 GeVe – BT

Beam from Injector and Storage Current
SuperKEKB:
7 GeV e2600 mA
4 GeV e+3600 mA
PF:
2.5 GeV e450 mA
PF-AR:
6.5 GeV e60 mA

HER
7 GeV

SuperKEKB
3 km

LER
4 GeV

Belle II
40xLuminosity

Figure 1: Layout of electron/positron accelerator complex
with beam properties from the injector to four storage rings.
science experiments since 1982. The injector continues todeliver various beams to SuperKEKB and light source rings
as depicted in Fig. 1. The injections for light sources wereperformed even during construction for SuperKEKB since2010, and balanced scheduling between the construction andbeam delivery was considered. The construction was muchaffected by the large earthquake in 2011 because the girders were designed with the soft structure concept [3]. The
longest shutdown period of 5 months was allocated in 2017,
during which the largest scale installation was performed.
The SuperKEKB with extremely high luminosity requires
injection beams with high current and low emittance in thetransverse and longitudinal directions. As the beam lifetime at SuperKEKB is extremely short, simultaneous top-up
injections into four storage rings and a DR should be per-INJECTOR LINAC
The electron–positron injector linac at KEK has delivered electrons and positrons for particle physics and photon∗ kazuro.furukawa@kek.jp

Figure 2: A typical part of the injector at the A and B sectors.

Table 1: Required and achieved parameters of injection beams, positron and electron, respectively.
StageKEKB
achieved

Phase-I
achieved

Phase-IIrequirement present

Phase-III
1st year plan final requirement

Energy (GeV)Stored current (A)Life time (min.)Bunch charge (nC)Hor. Emittance (𝜇rad)
Ver. Emittance (𝜇rad)
Energy spread (%)

3.5 / 8.0
1.6 / 1.1
150 / 200
1/1
1400 / 310-/0.13 / 0.13

4.0 / 7.0
1/1
100 / 100
0.4 / 11000 / 130-/0.5 / 0.5

4.0 / 7.0
1.8 / 1.3-/0.5 / 1
1.4 / 2.5
200 / 150
200 / 50
40 / 50
5 / 500.16 / 0.10-/-

4.0 / 7.0
3.6 / 2.6

formed by pulse-to-pulse modulations (PPMs) in order toavoid interference between three facilities: SuperKEKB,
Photon Factory (PF), and PF Advanced Ring (PF-AR).
The 600-m injector linac is composed of 60 high-power accelerating units with RF energy doublers followed by a beamswitchyard with an energy compression system as shown in
Fig. 2 [4]. The injector should meet the requirements of theSuperKEKB rings, with a small aperture at the interaction
region, doubled stored beam currents, and short expected
lifetimes. Low-emittance, high-current electrons will be delivered by employing a photo-cathode RF gun. High-current
primary electrons for positron production are generated bya thermionic gun, and then high-current positrons will be
produced using a flux concentrator (FC) and large-aperture
accelerating structures (LASs), which are then damped tolow emittance through a damping ring (DR) [5]. Design
parameters of the injection beams are listed in Table 1.

ELECTRON SOURCES
A low-emittance electron beam source and its transport
are essential to realize SuperKEKB’s nano-beam scheme forhigher collision rates. Although DR is employed to reduce
the positron emittance, cost and space restrictions makethe same solution infeasible for electrons. Thus, we havedeveloped a photo-cathode high-current RF gun.
The primary target of the gun is a bunch charge of 4 nC andan emittance of 10 mm⋅mrad to allow minor emittance blowup along the linac. Each component of the RF gun, such as
the laser, photo cathode, and cavity, was examined carefully
for stable long-term operation. A laser system with an Ybdoped fiber oscillator, a fiber amplifier, a thin-disk multipass
amplifier, and two-stage frequency doublers was installed
to examine high-power, shaped laser pulses [6]. A combination was chosen for a baseline, with an Nd:YAG laser
amplifier for higher power, Ir5 Ce cathode for longer lifetimeand reasonable quantum efficiency, and a quasi-travelingwave side-couple (QTWSC) cavity for higher accelerating
gradient and focusing (Fig. 3) [7]. A beam up to 4.4 nC
was successfully transferred to the end of the linac with thiscombination.
However, to deliver an electron beam with the required
characteristics, space-charge-effect mitigation by a longer
bunch length (30 ps) and energy-spread mitigation by a rect--/2/2
100 / 40
15 / 20
0.16 / 0.07

6/6
4/4
100 / 40
15 / 20
0.16 / 0.07

Figure 3: QTWSC cavity with a high acceleration gradient.
angular bunch shape should be manipulated carefully (temporal manipulation). Another laser station with an Yb:YAG
thin-disk regenerative amplifier or multipass amplifier will
be developed [8].

POSITRON GENERATOR
The positron beam generated in the KEKB injector wasapproximately 0.8nC × 2 bunches at 50 Hz. It should be enhanced to a dual-bunch 4 nC beam in stages. A high-charge
positron bunch will be generated with a conventional 14-mm
thick tungsten target, and will be captured by employing an
FC and LASs with velocity bunching, followed by a series ofsolenoid coils and a hundred of focusing magnets as depicted
in Fig. 4 [9]. As the generated beam emittance is large, itwill be damped by employing a DR at 1.1 GeV.
Bridge Coils

10 nC
Primary e-SolenoidDCFocusing
Magnets Spoiler

LAS Accel.
StructureTargete+beamhole

PulsedCorrector
Magnets

4 nC Injection e-FluxConcentratorPulsed
FocusingMagnets

Side view

Figure 4: Positron target with a hole beside is attached infront of FC followed by LASs and solenoid coils.

While an air-core pulsed coil was employed in KEKB,
it was replaced by a pulsed FC with a high field of 3.5 T
in SuperKEKB. The FC is a pulsed solenoid composed ofa primary coil and a copper cylinder with a conical holeinside. Induced eddy current flows through a thin slit to an
inner surface and generates a strong field. The achievable
field strength is mainly determined by the hole diameter andprimary pulsed current. A 2-mm hole beside the target isused to pass the electron beam to be injected into the HER.
A thermionic gun is utilized to deliver a high-charge 10 nC
primary electron beam for positron generation. Simulation
studies from the thermionic electron gun to the DR, including the FC and LASs, were performed to optimize beam
optics parameters with the target position, magnetic field,
electric field, and collimator positions that lead to higher
positron capturing ratios and lower beam loss. A target protection scheme, including a beam spoiler and a beam loss
monitor, was also developed to prevent the energy deposit
from exceeding a known limit [10].

RF SYSTEMS
Each of 60 high-power accelerating units is typicallyequipped with a pulse modulator, a S-band 50-MW klystron,
a SLED-type energy doubler, and four 2-m long quasiconstant gradient accelerating structures. Those high-power
RF modulators can operate at 50 Hz. All of them shorten thecharging time by 1 ms, in order for the injection bucket selection system to synchronize the linac and rings. One-third of
those modulators were replaced with compact inverter-type
modulators to make space for new devices.
A 60-kW driver klystron was originally employed to drivea group of 8 high power accelerating units. However, manyof units are required to operate independently in order torealize simultaneous injections. Thus, a 600-W solid-state
RF driver was developed with an FPGA-based digital IQ
modulator. They are directly connected with a master oscillator via temperature-stabilized optical links. The master
oscillator provides several RF frequencies for linac and rings
with integer relations between them.
As the beam quality is tightly dependent on the stability of
the RF system, new RF monitors were developed. The monitor is equipped with five sets of IQ detectors and fast ADCs,an FPGA, and direct optical links to the event and EPICS
control systems. About 70 monitors were installed to ensure
the adequate RF stability for the beam specifications [11].

PULSED MAGNETS
A single injector linac would behave as four independent
virtual accelerators (VAs) with hundreds of independent
parameters modulated pulse-by-pulse at 50 Hz. In KEKB,
pulse-to-pulse modulation (PPM) of injection beams was performed with moderate beam optics, and fine optics matching
was performed at corresponding beam transport lines [12].
However, for SuperKEKB injection, beam orbit and optics
management is necessary within a precision of 100 𝜇m tosuppress emittance blow-up [5].

Figure 5: Two pulsed quads and two pulsed steerings with a
newly designed girder.
Compact power supplies were developed for quadrupole
magnets with advanced design specifications of 1 mH, 330
A, and 340 V, and a 2 ms pulse width with up to 70% energy
recovery from the magnetic coils [13]. Power supplies forsteering magnets were also designed for 3 mH and 10 A.
Pulsed magnets of 30 quadrupoles and 34 steerings weremass-produced and installed during a five-month shutdown
in 2017. The pulsed magnets were examined and confirmed
to satisfy the specification of 0.1% (rms) stability (Fig. 5).

INSTRUMENTATION AND CONTROLS
Four storage rings should be filled simultaneously in topup injection mode, as previously described. To this end,
the linac should be operated with precise beam controls.
Dual-layer controls with EPICS and event-based systems
have been enhanced to support beam operation with precise
PPM at 50 Hz [12]. A VA concept was introduced to enablea single injector linac to behave as four independent VAs
switched by PPM, in which each VA corresponds to one offour top-up injections into SuperKEKB’s HER, LER, PF,and PF-AR. Each VA should be accompanied by independent
beam feedback stabilization loops to preserve the orbit for
low-emittance condition [14].
A single-shot sliced emittance measurement would bepossible, and we hope to operate on an unused bunch (astealth bunch measurement) using VA and PPM controls.

CONCLUSION
Steady progress was made in rejuvenating the injector linac. It successfully injected required beams forSuperKEKB Phase-1 and 2 commissioning. It will be further
upgraded towards Phase-3 commissioning.

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