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10th Int. Particle Accelerator Conf.
ISBN: 978-3-95450-208-0

IPAC2019, Melbourne, Australia

JACoW Publishingdoi:10.18429/JACoW-IPAC2019-TUPGW035

A HIGHLY BRILLIANT COMPACT 3 GeV LIGHT SOURCE PROJECT IN
JAPAN
N. Nishimori†, T. Watanabe1,2, H.Tanaka2, QST, Hyogo 679-5148, Japan1
also at JASRI, Hyogo 679-5148, Japan2
also at RIKEN SPring-8 Center, Hyogo 679-5148, Japan

Abstract
A compact 3 GeV light source project capable ofdelivering the highly brilliant soft X-rays is proposed inJapan. The storage ring is designed based on a 4-bend
achromat lattice to achieve the low emittance for a small
circumference of about 350 m. The total number of 26
beamlines is available including 12 multi-pole wiggler
beamlines. The natural horizontal emittance is expected to
be around 1.1 nm.rad, and the maximum brilliance will
exceed 1021 photons/sec/mm2/mrad2/0.1% b.w. for the 1 3 keV region with a stored current of 400 mA. The
accelerator components are designed based on the studies
for the SPring-8 upgrade project. A full energy injector
linac equipped with a thermionic gun and C-band
accelerating structures developed for XFEL SACLA is
employed for the low emittance injection beam to the ring.
A future upgrade for the injector linac as an SXFEL driveris also envisioned.

INTRODUCTION

A highly brilliant compact 3 GeV light source project inJapan has progressed toward innovations in science andindustry. The light source will be constructed in Sendai,
north-east part of Japan. It will provide brilliant soft X-rays
and also widely cover the wavelengths ranging from EUV
to hard X-rays performing a complementary role to SPring8, the Japan's flagship photon source for the brilliant hard
X-rays. The target light performances are brilliance >1021
photons/sec/mm2/mrad2/0.1% b.w. and coherent ratio R =10 % at several keV photon energy. The coherent ratio isdefined as
𝑅[%] = 100,

with electron beam emittance 𝜀 , and the diffraction

limited photon emittance 𝜀 = . Here beta function atan undulator with length L is assumed to be matched to thephoton beam: 𝛽 , = 𝐿/2𝜋 [1]. The beam current andemittance are designed to be 400 mA and 1 nm.rad,respectively, to achieve the targets. The beam lifetime isdesigned to be longer than 5 hours as lifetime is crucial for
reliable light source operations even though top-up beam
injections are routinely used.
Our concept is to deliver the highly brilliant soft X-rays
with high stability and reliability to users by utilizing
advanced accelerator design and technology accumulated
at SPring-8 and SACLA for more than 20 years. Top-up
operations at SPring-8 has achieved the stability of stored

beam current better than 0.06% and the mean time between
failures to be around 200 hours. SACLA is the pioneer of acompact XFEL facility and one of the leading XFEL
facilities in the world. The new 3 GeV light facility takes
advantage of those experience and knowledge gained at
SPring-8 and SACLA to achieve the design goal. In
addition, the following issues are taken into account:extensibility to accommodate soft X-ray Free Electron
Laser (SXFEL), feasibility of reliable operation by limited
number of staffs, and tight schedule and moderate budget
for construction.
Table 1: Main Parameters of 3 GeV Storage Ring
Parameter
Beam energy [GeV]
Circumference [m]Number of cells
Long straight section [m]Short straight section [m]Betatron tune (x/y)
Natural chromaticity (x/y)
Natural horizontal
emittance [nm.rad]
Momentum compaction
factorNatural energy spread [%]Lattice functions at LSS(𝛽 /𝛽 /𝜂 ) [m]Lattice functions at SSS(𝛽 /𝛽 /𝜂 ) [m]Damping partition number(Jx/Js)Damping time(𝜏 /𝜏 /𝜏 ) [ms]Radiation loss in bends[MeV/turn]RF frequency [MHz]Harmonic number

Value
2.998
348.8432
16
5.44×16
1.6427×16
28.17/9.23-60.50/ -40.99
1.14
0.000433
0.084313.0/3.0/0.0
4.08/2.962/0.052
1.389/1.611
8.091/11.238/6.976
0.621
508.75905
592

ACCELERATOR SYSTEM
The accelerator complex of the new 3 GeV facility iscomposed of a compact storage ring with a circumferenceof 350 m and a compact 110 m injector linac in length (see___________________________________________† nishimori.nobuyuki@qst.go.jp

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Fig. 1). A 4-bend achromat lattice with 16 cells is chosen
for the ring to achieve the low emittance for the small
circumference. Each cell serves a long straight section of
5.4 m for an undulator and a short straight section of 1.6 m
for a multi-pole wiggler except for the cells assigned forbeam injection, RF cavities, and beam diagnostic
instruments. In total, 26 beam lines are available for users.
Horizontal emittance is expected to be around 1.1 nm.rad,and the maximum brilliance would exceed 1021
photons/sec/mm2/mrad2/0.1% b.w. for the 1 - 3 keV region
with a stored beam current of 400 mA. The accelerator
components such as vacuum chambers, magnets andmonitors are employed from those studied for SPring-8
upgrade project [2-6]. A full energy injector linac equippedwith a thermionic gun and C-band accelerating structures
is employed to produce sufficiently low emittance beams
for efficient beam injections. The C-band system isadopted from those developed for XFEL SACLA [7,8]with some modifications. The linac is located outside of thering. In the future, it would be upgraded as an electron
driver for SXFEL. The main parameters of 3 GeV storage
ring and injector linac are listed in Table 1 and Table 2,
respectively.

Figure 1: Accelerator complex.
Table 2: Beam Parameters of 3 GeV Injector Linac
Parameter
Beam energy
Normalized emittance
Emittance at 3 GeV
Bunch charge
Bunch length
Energy spread
Repetition rate (Normal)(RF conditioning/ FEL)

JACoW Publishingdoi:10.18429/JACoW-IPAC2019-TUPGW035

Value
3 GeV
<10 m.rad<1.7 nm.rad
0.3 nC/bunch
5 ps (FWHM)
0.16 % (FWHM)
1Hz
25Hz

ON-GOING R&D
A Test Half-cell of 3 GeV Storage Ring
A test half-cell of the 3 GeV storage ring shown in Fig. 2
was constructed to study the performance of magnetic
components and to establish precise alignment procedure
MC2: Photon Sources and Electron Accelerators
A05 Synchrotron Radiation Facilitiesof the magnets. The test half-cell consists of identical two
combined-function bending magnets, different types offive quadrupole and five sextupole magnets, respectively.
The magnets are lined up on three girders.

Figure 2: A test half-cell of 3 GeV storage ring.
The combined-function bending magnets are 1.091 [m]long, and provide the defocusing quadrupole fields toreduce the horizontal emittance saving the space fordiscrete defocusing quadrupoles. The designed dipole andquadrupole magnetic fields are 0.8688 [T] and ─7.06 [T/m],
respectively. The gap is 28 mm at the center. The magnetic
field distributions along the beam trajectory and horizontal
axis are measured with a 3D Hall probe in the vendor. The
ratio of the measured quadrupole and bending fields agrees
with the calculation within 0.1 % accuracy. Although thetwo bending magnets fabricated for the present test halfcell are electromagnetic, a combined-function bending
magnet based on permanent magnets is also fabricatedusing R&D experience for SPring-8 upgrade project [9].
We will decide which type of magnet to be utilized for 3
GeV storage ring by the end of the fiscal year 2019.
The quadrupole and sextupole electromagnets are madeof a laminated iron core. The bore diameters of quadrupoleand sextupole magnets are 34 and 40 mm, respectively. The
maximum gradient of the quadrupole is 50 [T/m], and thatof sextupole is 1200 [T/m2]. The designed good fieldregion (GFR) where the field deviation from the ideal value
is within 10-3 is ± 12 mm. The magnetic field distributions
measured at the vendor with a Hall probe technique agree
well with the design. The integral multipole components ofquadrupole and sextupole magnets will be measured witha stretched wire technique.
The mechanical center of each magnet was measured witha 3D position measuring instrument at the vendor. The
center position can be relatively obtained from reference
positions at reflector holders. All the magnets are alignedwith a laser tracker by using the reflector holder on eachmagnet. Then vibrating wire measuring method [10] isused to align the magnetic center of quadrupole andsextupole magnets. The difference between the magnetic
and mechanical centers is within ± 0.05 mm.

Vacuum System
The low emittance storage ring based on multi bend
achromat lattice demands fabrication of the precise
vacuum chambers with narrow apertures carefully
designed taking account of high packing factors for the
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Content from this work may be used under the terms of the CC BY 3.0 licence (© 2019). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

10th Int. Particle Accelerator Conf.
ISBN: 978-3-95450-208-0

Content from this work may be used under the terms of the CC BY 3.0 licence (© 2019). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

10th Int. Particle Accelerator Conf.
ISBN: 978-3-95450-208-0

IPAC2019, Melbourne, Australia

magnets and photon absorbers. The design and R&D ofvacuum system addressed for the SPring-8 upgrade project[3] are employed for the 3 GeV project. Thin vacuum
chambers fitted to small magnet bores are made of stainlesssteel. Discrete pumping stations are installed close to thephoton absorbers consisting of a principal cartridge NEG
pump and a supplemental sputter ion pump for methaneand noble gases.

RF System
The radiation loss per turn in 3 GeV ring is estimated tobe 1.1 MeV where 0.62 MeV is loss in bends and the rest
is loss in insertion devices. An RF power of 440 kW is thus
required for compensating the radiation loss of 400 mA
beam. To make beam lifetime longer than 10 hours, an RF
accelerating voltage of 3.6 MV is required. Installation
space of an RF accelerating system is limited to one of the
5.4 m long straight sections, because most of the straightsections need to serve as spaces for insertion devices. A
new type of compact TM020 cavity has been developed toaccommodate the space limitation [11]. A higher ordermode (HOM) excited in the cavity causes coupled-bunch
beam instability and needs to be damped. The new cavity
has ferrite HOM dampers at the node of the fundamental
frequency of 508 MHz inside. Thus, the new cavity does
not require external waveguides or pipes along the beamaxis to extract HOM power. The total cavity length along
the beam line can be short. Four new cavities will be
installed in the long straight section and the accelerating
voltage per cavity is 0.9 MV for 3.6 MV acceleration. The
shunt impedance of the new cavity is estimated to be 6.8
M and RF power of 120 kW per cavity is required for 0.9
MV acceleration. In total, the klystron RF power of 920
kW is required for stored beam current of 400 mA.
We have developed a technique to weld ferrite HOM
dampers on the copper cavity surface. The high-power testof the new HOM-damped cavity is underway and will befinished by this autumn.

Injector Linac
The injector system is designed to be compact, costefficient, robust and capable of driving SXFEL. The highgradient C-band accelerator technology developed at
SACLA/SPring-8 [7,8] is employed to achieve 3 GeV fullenergy acceleration. Twenty units of C-band accelerator
system are installed in the linear accelerator tunnel. The
unit consists of two C-band 2 m accelerator structures fed
by a 50 MW pulsed klystron. The RF power from the
klystron is compressed by an RF pulse compressor (SLED)by a factor of 4 to supply 80 MW peak power to eachaccelerator structure. Some of the C-band accelerating
components such as the accelerating structure itself, SLED,and waveguide vacuum window are modified from their
original designs or newly developed in terms of cost
efficiency and robustness. The high-power tests for thosecomponents will be complete in 2019. The RF phase andamplitude are precisely controlled by a compact digital
low-level RF system based on Micro-TCA.4 technology.
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A conventional 50 kV dc gun equipped with acommercially available gridded thermionic cathode (CPI
Y845) is employed as an electron source instead of a 500kV pulsed gun with a CeB6 cathode used to generate lowemittance beam at SACLA [12]. This is because the 3 GeV
storage ring does not require such small normalized
emittance as 1 mm.mrad as shown in Table 2 and theinjector system consisting of the 500-kV pulsed gun israther complicated [13]. Instead, a 238 MHz cavity is used
to accelerate the 50 keV beam to 500 keV. A 476 MHz
buncher cavity followed by 2 m S-band accelerating
structure provides 0.4 nC bunch charge with 5 ps FWHM
to the 80 m long C-band accelerator. An electron gun test
stand was built to evaluate the beam quality of our 500 keVelectron source consisting of a thermionic 50 kV dc gun
and a 238 MHz RF accelerator [14].
The electron beam parameters of 3 GeV injector linac
listed in Table 2 are obtained from a particle tracking
simulation (PARMELA) in a similar way as those for
SACLA [13]. For the future SXFEL, the electron source
will be replaced with a new one capable of generating a
low emittance beam.

Ring Beam Injection System
The beam injection system of the storage ring needs
careful designs for stable operation of the next generation
light source having a narrow dynamic aperture. A renewed
off-axis beam injection system has been developed for
efficient, stable and transparent beam injection together
with the capability of top-up operation [15]. The injection
system consists of a windowless beam transport, invacuum pulse septum magnet, a pair of twin kickers having
identical magnet characteristics up to a few tens kHz. The
system allows a small injection amplitude of around 4 mm
and transparent beam injection during the top-up operation.
The vacuum pressure less than 10-7 Pa is achieved for the
prototyped in-vacuum pulse septum magnet. The detailed
studies of the magnetic performance of the pulse septum
magnet and the twin kickers are in progress and will befinished by the end of the fiscal year 2019.

SUMMARY
Accelerator system design for the 3 GeV light source
project in Japan was completed by taking advantage ofexpertise acquired in R&Ds for the SPring-8 upgrade
project. Some remaining R&D programs on accelerator
components should be finished on schedule by the end of
the fiscal year 2019.

ACKNOWLEDGMENTS
We would like to thank all the members of 3 GeV
accelerator working group at QST, JASRI and RIKEN for
their contributions to the 3 GeV compact light source
project.

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JACoW Publishingdoi:10.18429/JACoW-IPAC2019-TUPGW035

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Content from this work may be used under the terms of the CC BY 3.0 licence (© 2019). Any distribution of this work must maintain attribution to the author(s), title of the work, publisher, and DOI

10th Int. Particle Accelerator Conf.
ISBN: 978-3-95450-208-0