STATUS OF ERL AND CERL PROJECTS IN JAPANS. Sakanaka, H. Kawata, Y. Kobayashi, KEK, Tsukuba, Ibaraki 305-0801, Japan
N. Nakamura, ISSP, University of Tokyo, Kashiwa, Chiba 277-8581, Japan
R. Hajima, JAEA, Tokai, Naka, Ibaraki 319-1195, Japan
Abstract
Aiming at constructing a future synchrotron lightsource based on a 5-GeV energy recovery linac (ERL),
we are developing high-brightness DC photocathode guns,
gun-drive lasers, superconducting cryomodules, and 1.3
GHz high-power rf sources. We are also constructing theCompact ERL (cERL) at KEK for demonstrating therecirculation of high-brightness beams using thesecomponents. We report up-to-date status of the Japanese
ERL project.

ERL PROJECT IN JAPANSynchrotron light sources based on ERLs are expected
to bring innovation to the synchrotron radiation (SR)science [1]. We are proposing to construct a 5-GeV ERL
at KEK as a future project of the KEK Photon Factory.
The 5-GeV ERL is expected to deliver high-brightness
electron beams having normalized beam emittances of0.1-1 mm·mrad at beam currents of 10-100 mA, as well as
to produce ultra-short electron bunches having rms
lengths of about 100 fs with bunch charges of higher than
77 pC/bunch. The ERL is also expected to produce highquality electron beams for a proposed X-ray free-electron
laser oscillator (X-FELO) [2]. Typical beam parameters[3] which are required for the X-FELO at a typical photon
energy of 12 keV are: the electron beam energy of 7 GeV,
the bunch charge of 50 pC, rms bunch length of 1 ps, thenormalized beam emittance of 0.2 mm·mrad, the bunch
repetition frequency of approximately 1 MHz, and rms
energy spread of 2×10-4. We aim at attaining the abovementioned parameters by operating the same ERL as arecirculating linac [4].
The ERLs are also expected to produce both highbrightness and quasi-monochromatic gamma-rays using
the laser Compton scattering. The JAEA is proposing anapplication of ERLs for detecting radioactive isotopes
using such gamma-rays [5]. Both the ERL-based SR
source and the gamma-ray source share the common ERL
technologies. We are conducting R&D effort for the ERL
technologies since 2006.

voltage tests, the ceramic insulator with the support rod
was successfully conditioned up to the maximum voltageof 550 kV [6].
After above-mentioned high voltage test, cathode andanode electrodes, NEG pumps, and electrostatic shielding
meshes, were installed in the vacuum chamber of the gun.
After the chamber was baked, vacuum pressure in thechamber reached down to 2×10-9 Pa. Then, high voltage
conditioning of the assembled gun was carried out. In thesummer of 2010, the gun could be conditioned up to themaximum voltage of 380 kV. The maximum voltage wascurrently limited by local X-ray radiation of
approximately 30 μSv/h. As a next step, we first plan toextract beams under the current setup at a lower voltageof 300 kV, and then, we will investigate the source of theradiation. The latest status of the first 500-kV gun isreported in [7].
We also started to develop the second 500-kV gun [8].
The purpose of the second gun is to continue the gun
development after one of them has been installed in theCompact ERL. One of the guns can also serve as abackup for the other one when there happens a serious
damage in its component such as a ceramic insulator.
Both a titanium vacuum chamber and a ceramic insulator
have been produced, and vacuum tests [9] of them are inprogress.

HIGHLIGNTS OF R&D EFFORT
High-Brightness DC Photocathode Guns
To produce high-brightness electron beams having arepetition frequency of 1.3 GHz, we are developing two
500-kV DC photocathode guns. Design of the first 500kV gun started in FY2008. Schematic drawing of the first
500-kV gun is shown in Fig. 1. In order to protect aceramic insulator against electrons emitted from a supportrod, the ceramic insulator was divided into several pieces
and each piece was covered by a guard ring. Under high-Figure 1: The first 500-kV DC photocathode gun [7].

Gun-Drive Lasers
Negative electron affinity (NEA) photocathodes of theguns are driven by lasers. To produce electron beams of
100 mA, drive lasers for the guns should produce laser
pulses having an average power of 15 W with a repetition
frequency of 1.3 GHz at a typical wavelength of 530 nm,
assuming quantum efficiency of the NEA surface to be1.5%. To optimize conflicted requirements from low
beam emittance and high quantum efficiency, it isdesirable that the laser wavelength can be tuned between
500 to 800 nm. We chose a MOPA (Master Oscillator andPower Amplifier) type laser using ytterbium fibers.
In our strategy, we first prepare a reduced drive laser
which is needed to test the guns up to the beam current of
10 mA. An average power of 1.5 W is required at 530 nm.
We have prepared a 1.3-GHz, 100-mW laser system byassembling commercially available laser oscillator and afiber amplifier. By reinforcing its fiber amplifier, anaverage power will be upgraded to 1.5 W in FY2010.
Concurrently, we plan to develop full-featured drive
laser within several years. At present, we could
demonstrate to produce an average laser power of 10 W
(150 nJ/pulse) with a reduced repetition frequency of 85
MHz at a wavelength of 1035 nm [10]. Introducing thislaser to a LBO crystal, we could produce the second
harmonic (518 nm) light of 4.8 W. A 1.3-GHz fiber-laser
oscillator using an active harmonic mode-locking method
is also under development.

Superconducting Cavities for the Injector
To pre-accelerate 100-mA beams from a buncher cavity
up to the beam energy of 10 MeV (or 5 MeV in cERL),
three two-cell superconducting (SC) cavities are used.
Using two input couplers per cavity, each coupler should
transmit an input power of approximately 170 kW.
Average accelerating gradient of about 14.5 MV/m isrequired. Higher-order modes (HOMs) are damped using
improved TESLA-type HOM couplers.
We have produced two prototype cavities. Figure 2
shows the second prototype cavity where five loop-type
HOM couplers and two input-coupler ports are equipped.
Under tests in a vertical cryostat, this cavity achieved thefield gradient of slightly higher than 40 MV/m without
connecting HOM pickups [11]. Tests with HOM pickups
are underway. We have also produced two prototype input
couplers. These couplers were tested up to an average rf
power of 26 kW (peak power: 130 kW). An improvement
in some heating locations which were found during thistest is under way. Design of a cryomodule which canhouse three two-cell cavities is in progress.

Figure 2: The second prototype cavity for the injector.

Superconducting Cavities for the Main Linac
In the main linac of the 5-GeV ERL, beams areaccelerated by a large number of nine-cell cavities.
Accelerating gradient of 15-20 MV/m is required. HOMs
are damped using ferrite absorbers in the beam pipe.
We have produced two prototype cavities. Under thelatest test on the first prototype cavity, we could achieve
an accelerating gradient of 25 MV/m. We have also
produced prototype input windows. Under high-power
tests, the prototype windows could be tested up to an
average power of 27 kW in CW mode. Developments of aHOM-absorber assembly and of a cryostat which canhouse two nine-cell cavities are in progress. Recent
development in the main SC cavities is reported in [12].

RF Sources
We have been developing 1.3-GHz rf sources for theERL since 2007 [13]. Each two-cell cavity of the injector
is driven by a 300-kW klystron while each nine-cellcavity of the main linac is driven by a 20-30 kW source.
We have developed a 300-kW CW klystron (ToshibaE37750) for the injector. Under high power tests, thisklystron could produce the maximum output power of
305 kW under a cathode voltage of 49.5 kV and a beam
current of 9.75 A. We have also developed a 150-kW
circulator and the other waveguide components. Usingthem, the prototype input couplers for the injector weretested under high power.
For the main linac of the cERL, we prepared two rf
sources: a 30-kW induction output tube (IOT), VKL-9130
from CPI, and a 35-kW klystron, E3750 from Toshiba.
We will evaluate these sources and make a choice for thefuture ERL project. We are installing one 300-kW
klystron, two IOTs, and two 35-kW klystrons for thecERL. Figure 3 shows the 300-kW klystron which wasinstalled in the East Counter Hall.
The rf voltages in the SC cavities should be stabilized
very precisely. Typical stability requirements for thefuture ERL are 0.01% (rms) for amplitude and 0.01
degrees (rms) for phase. To control rf voltages very
precisely, we are developing a digital low-level rf (LLRF)
system [14]. Our temporary goals at the cERL are 0.1%
(rms) for amplitude and 0.1 degrees (rms), respectively.

Figure 3: A 300-kW klystron in the East Counter Hall.

THE COMPACT ERL
DesignTo demonstrate the generation and recirculation ofhigh-brightness beams using our R&D products, we areconstructing the Compact ERL at KEK. Initial goals ofthe cERL are the beam energy of 35 MeV, the beam
current of 10 mA, and the normalized emittance of 1mm·mrad. The cERL will initially comprise a 5-MeV
injector, a main linac with a single cryomodule having
two nine-cell cavities, and a single return loop.
After its commissioning, the cERL can be upgraded intwo ways. By increasing the number of cryomodules, thebeam energy can be upgraded up to 125 MeV. By
installing the second return loop, the beam energy can bedoubled; this also allows us to investigate the beam
dynamic issues concerning the two-loop ERLs. Figure 4
shows our final planned configuration of the cERL while
an initial configuration will be simpler.
We are designing the beam optics for both the initial
single-loop configuration (beam energy: 35 MeV) [15]and the double-loop configuration (245 MeV) [16].
Figure 5 shows an example of the beam optics where weassumed initial conditions at a merging point to be (βx, αx,βy, αy) = (47.1 m, 1.65, 21.5 m, 5.52) and the maximum
beam energy to be 245 MeV. Further matching between
the injector and the recirculation loop is in progress.

for the cERL which includes: 1) clearing the hall, 2)
refurbishing the building, and 3) refurbishing the coolingwater system and the electric substation. Figure 6 shows
the interior of the East Counter Hall after its
refurbishment.
We also installed a liquid-helium refrigerator systemhaving a cooling capacity of 600 W (at 4 K) and a part ofrf sources. These systems are under tests.
According to our current schedule, we will construct a
radiation shield during FY2011. At the same time,magnets, vacuum chambers, and the other components
will be produced. An injector cryomodule will be finishedand tested. During FY2012, most of the components ofthe cERL will be installed. We will then startcommissioning at the end of FY2012.

Figure 6: The inside of the East Counter Hall after its
refurbishment.

REFERENCES

Figure 4: Planned layout of the cERL.

Figure 5: Example of beam optics for the two-loop cERL.

Construction Status
The cERL is being constructed in the East Counter Hallat KEK. During FY2009, we prepared the infrastructure[1] For example, D. Bilderback et al., New J. Phys. 12(2010) 035011.
[2] K. -J. Kim, Y. Shvyd’ko, S. Reiche, Phys. Rev. Lett.
100 (2008) 244802.
[3] R.R. Lindberg and K.-J. Kim, PAC09, TU5RFP049.
[4] S. Sakanaka et al., IPAC’10, pp. 2338-2340.
[5] R. Hajima et al., Nucl. Instrum. Methods A 608(2009) S57.
[6] R. Nagai et al., Rev. Sci. Instrum. 81 (2010) 033304.
[7] N. Nishimori et al., in these proceedings, THP111.
[8] M. Yamamoto et al., Proc. 6th Particle Accelerator
Society Meeting, 2009, pp. 860-862 [in Japanese].
[9] M. Yamamoto et al., Proc. 7th Particle Accelerator
Society Meeting, 2010, WPES124 [in Japanese].
[10] I. Ito et al., IPAC’10, pp. 2141-2143.
[11] S. Noguchi et al., IPAC’10, pp. 2944-2946.
[12] K. Umemori et al., in these proceedings, TUP006.
[13] S. Fukuda et al., IPAC’10, pp. 3981-3983.
[14] T. Miura et al., IPAC’10, pp. 1440-1442.
[15] K. Harada et al., t Proc. 7th Particle Accelerator
Society Meeting, 2010, THPS002 [in Japanese].
[16] M. Shimada et al., Proc. 7th Particle Accelerator
Society Meeting, 2010, WEPS029 [in Japanese].