PROGRESS IN R&D EFFORTS ON THE ENERGY RECOVERY LINAC
IN JAPAN
S. Sakanaka, T. Agoh, A. Enomoto, S. Fukuda, K. Furukawa, T. Furuya, K. Haga, K. Harada,
S. Hiramatsu, T. Honda, Y. Honda, K. Hosoyama, M. Izawa, E. Kako, T. Kasuga, H. Kawata,
M. Kikuchi, H. Kobayakawa, Y. Kobayashi, T. Matsumoto, S. Michizono, T. Mitsuhashi, T. Miura,
T. Miyajima, T. Muto, S. Nagahashi, T. Naito, T. Nogami, S. Noguchi, T. Obina, S. Ohsawa,T. Ozaki, H. Sasaki, S. Sasaki, K. Satoh, M. Satoh, M. Shimada, T. Shioya, T. Shishido, T. Suwada,
T. Takahashi, Y. Tanimoto, M. Tawada, M. Tobiyama, K. Tsuchiya, T. Uchiyama, K. Umemori,S. Yamamoto
High Energy Accelerator Research Organization (KEK), Oho, Tsukuba, Ibaraki 305-0801, Japan
R. Hajima, H. Iijima, N. Kikuzawa, E. J. Minehara, R. Nagai, N. Nishimori, M. Sawamura
Japan Atomic Energy Agency (JAEA), Tokai, Naka, Ibaraki 319-1195, Japan
N. Nakamura, A. Ishii, I. Ito, T. Kawasaki, H. Kudoh, H. Sakai, T. Shibuya, K. Shinoe, T. Shiraga,
H. Takaki
Institute for Solid State Physics (ISSP), University of Tokyo, Kashiwa, Chiba 277-8581, Japan
M. Katoh
UVSOR Facility, Institute for Molecular Science, Myodaiji-cho, Okazaki, Aichi 444-8585, Japan
M. Kuriki
Hiroshima University, Kagamiyama, Higashihiroshima, Hiroshima 739-8530, Japan
D. Yoshitomi, Y. Kobayashi, K. Torizuka
National Institute of Advanced Industrial Science and Technology (AIST)
Umezono, Tsukuba, Ibaraki 305-8568, Japan
H. Hanaki
JASRI/SPring-8, Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, JapanAbstract
Future synchrotron light sources based on the energyrecovery linacs (ERLs) are expected to be capable ofproducing super-brilliant and/or ultra-short pulses ofsynchrotron radiation. Our Japanese collaboration team ismaking efforts for realizing an ERL-based hard X-raysource. We report recent progress in our R&D efforts.

INTRODUCTION
Along with mature performance of ring-based
synchrotron light sources, the energy-recovery linacs
attract our attention as very promising new light sources
which can overcome the limitations of ring-based sources.
The ERL-based light sources are expected to be capableof producing super-brilliant and/or ultra-short pulses ofsynchrotron radiation. They are also expected aspromising drivers for oscillator-type X-ray free electronlasers [1]. The ERL-based hard X-ray sources areseriously considered at the Cornell University [2], at theArgonne National Laboratory [3], and at KEK/JAEA [4].
The ERLs are also expected as intense electron drivers forcooling ion beams, and serious R&D programs are being
conducted at the Brookhaven National Laboratory [5] andat the Jefferson Laboratory [6].
We are aiming at realizing an ERL-based hard X-ray
source in Japan, and are conducting R&D efforts. We arecurrently developing such key components as: (i)photocathode DC guns which are capable of producing
ultra-low emittance, high-current beams, (ii) drive lasers
for the gun, (iii) both superconducting cavities for theinjector linac and for the main linac. We also plan toassemble these key components into the Compact ERL[7], and to demonstrate their operations.

ELECTRON GUN
An electron gun to generate electron beams of highbrightness and high-average current is one of the mostessential components for the ERL light source. Weemploy a photocathode DC gun for the ERL. A 250-kV,50-mA gun has been developed at JAEA. The gun has
been almost assembled and the first photo-current was
obtained from a cathode of NEA-GaAs. Apparatuses forbeam measurements are under installation. We plan tomeasure the transverse emittance by a double-slit
configuration and to measure the temporal profile with adeflecting cavity. The design of these apparatuses isdescribed in a previous paper [8]. From particle tracking
simulations, the normalized transverse emittances are
expected to be 0.59 and 0.11 mm!mard for bunch chargesof 77 and 7.7 pC, respectively.
In addition to the development of the 250-kV gun, we
started to build a 500-kV gun, which will be installed in
the Compact ERL. A multiple-stacked cylindrical ceramic
for a high-voltage insulator is designed to support 500-kV
DC voltage. Studies on photocathode physics and ultrahigh vacuum to achieve long-life cathode operation are
also underway.

operation. We designed a new 2-cell cavity and made a
prototype as shown in Figure 2. The injector linac
consists of three 2-cell cavities, each of which has double
power couplers to reduce the power per coupler, as wellas to keep a symmetric field configuration around thecoupler ports. Main parameters are listed in Table 1.

DRIVE LASER

Table 1: Parameters of the SC cavity for the injector linac.
Goals at the Compact ERL are shown in parentheses.

Drive lasers for the photocathode DC gun are underdevelopment. The requirement for the laser system is atunable wavelength around 800 nm with an average
power of 15 W, a pulse width of 10-20 ps, and arepetition rate of 1.3 GHz. An ytterbium(Yb)-doped fiberamplifier (YDFA) system is most promising [9,10]
because of its scalability to high average power [11]. The
tunable wavelength-conversion can be realized with theoptical parametric amplifier which is pumped by afrequency-doubled YDFA. The required average power
for the YDFA is above 200 W. The National Institute ofAdvanced Industrial Science and Technology (AIST) hasa laser technology for an average power of 30 W at arepetition frequency of 80 MHz with pulse widths of afew picoseconds. Based on this technology, we started todevelop a master oscillator and a power amplifier system
at AIST in collaboration with ISSP and KEK. We havealready succeeded in oscillating a 118-MHz Yb-doped
fiber laser oscillator. Figure 1 shows an output of thedeveloped oscillator. We are challenging to upgrade its
repetition rate up to 1.3 GHz.

Figure 2: A developed two-cell SC cavity for the injector.

Cavity type
Number of cavities
Total
acceleratingvoltage
Beam current
R/Q
Cavity loss
Coupler power
Qext

SUPERCONDUCTING CAVITIESInjector Linac
An injector linac for a 5-GeV-class ERL should
accelerate electron beams of 100 mA up to the beamenergy of about 10 MeV without energy recovery. Hencea power of 1 MW has to be transferred to the beam in CW

MV100
200
5.8 (1.5)
170#2 (85#2)
1.7#105 (8.4#104)

mA"WkW

Main Linac
A 9-cell cavity with sufficiently damped higher-ordermodes (HOM) was designed for the main linac. To reduce
the transverse impedances of its dipole modes, thediameters of both irises and beam pipes of the cavity arechosen to be large. Furthermore, one of the beam pipes
has a mode converter called “Eccentric Fluted Beam pipe(EFB)”, in which the quadrupole modes can be partly
transformed into dipole modes, and then, can propagate
out through the beam pipe [12,13]. The principal
parameters are shown in Table 2.
Table 2: Parameters of the 9-cell cavity for the main linac.
Frequency
Accelerating gradient
RF power per cavity
R/Q
Esp/Eacc
Hsp/Eacc

Figure 1: Output of a developed 118-MHz Yb-doped fiber
laser oscillator.

2 cell310 (5)

1.3
15-20
208973.042.5

GHzMV/mkW"Oe/(MV/m)

Fabrication and surface treatment processes were tested[14,15] on two single-cell cavities named C-single and Esingle, which are shown in Fig. 3. C-single has the same
cell shape as that of the central cell of the 9-cell structure.
E-single has the shape of the end cell which is equippedwith beam pipes for the 9-cell cavity. Figure 4 shows theQ-E plot obtained from vertical cold tests. Both thecavities satisfied the specification of 20 MV/m with theunloaded-Q of 1!1010.
Development of a new power coupler for the main linac
is in progress. Two ceramic disks of HA997 are used atthe both warm end and cold ends of a 60-" coaxial
antenna coupler. Figure 5 shows a schematic drawing of
our test stand for the rf-windows.

photons/s/mrad2/0.1%b.w. [16] at the photon energy ofabout 10 meV (~2.4 THz) from compressed bunches
having pulse widths of 59 fs (rms) at the beam energy of
155 MeV. The Compact ERL can also provide ultra-short
X-ray pulses having the pulse widths of about 110 fs.
With the 90° Compton-backscattering configuration, we
anticipate a photon flux of 3.5#103 photons/pulse/3%b.w.
[7] at the beam energy of 60 MeV, the bunch charge of
100 pC, and the laser pulse of 10 mJ/pulse.
ChicaneStraight section

Arc 1(Bunch compression)

Arc 2
MergerExtractor

Figure 3: A sketch of the KEK-ERL 9-cell cavity (upper),and two test cavities, C-single (left) and E-single (right).

Main
acceleration
module 2Mainacceleration
module 1

Beam dump

Injectorcavity
Solenoid, buncher

Electrongun

Figure 6: Plan of the Compact ERL.
Table 3: The principal parameters of the Compact ERL
and the ERL-based future light source.

Figure 4: Result of the vertical tests of single-cell cavities.

Beam energy
Injection energy
Path length
Beam current
Charge per bunch
Normalized emittanceBunch length (rms)Compact
ERL
0.065-0.2
570
10-1007.7-770.1-1
0.1-3

Lightsource
5~101253
10-1007.7-77
0.1-1
0.1-3

GeV
MeV
mmA
pC
mm!mradps

ACKNOWLEDGMENTS
We are very grateful to Profs. M. Tigner and S. Gruner,
and to the members of the Cornell Laboratory forAccelerator-based Sciences and Education (CLASSE) forvaluable collaboration.

REFERENCES
Figure 5: Schematic drawing of a coupler test stand.

DESIGN OF THE COMPACT ERL
We have completed a conceptual design [7] of theCompact ERL, which is a test facility planned to be built
at KEK. The plan of the Compact ERL is shown in Fig. 6.
Table 3 shows the principal parameters of the Compact
ERL, together with those of the ERL-based future lightsource. The primary purpose of the Compact ERL is todemonstrate reliable operations of the key components, aswell as to investigate accelerator physics issues that are
critical to build the ERL for the light source.
The Compact ERL can also be used as: (i) an intense
terahertz-radiation source using the coherent synchrotron
radiation (CSR), and (ii) an ultra-short or a high-intensity
X-ray source using Compton backscattering with laser
pulses. We anticipate a photon flux density of about 1017

[1] K.-J. Kim et al., FEL 2007, TUBAU05.
[2] G.H. Hoffstaetter et al., PAC’07, p. 107.
[3] M. Borland et al., PAC’07, p. 1121.
[4] T. Kasuga et al., PAC’07, p. 1016.
[5] V.N. Litvinenko et al., PAC’07, p. 1347.
[6] R.A. Rimmer et al., PAC’07, p. 2493.
[7] R. Hajima et al. (ed.), KEK Report 2007-7/JAEAResearch 2008-032 (2008) [in Japanese].
[8] N. Nishimori et al., Proc. ERL07 (to be published).
[9] H. Tomizawa for LAAA, LAAA Note/ERL-Report003, 2006 [in Japanese].
[10] J. Itatani and Y. Kobayashi, “Laser for ERL photocathode”, proposed to KEK-JAEA ERL project,2007 [in Japanese].
[11] F. Röser et al., Opt. Lett. 30 (2005) 2754.
[12] K. Umemori et al., APAC2007, p. 570.
[13] M. Sawamura et al., PAC’07, p. 1022.
[14] K. Umemori et al., in these proceedings, MOPP159.
[15] H. Sakai et al., in these proceedings, MOPP153.
[16] K. Harada et al., Infrared Phys. Tech. 51 (2008) 386.