FUTURE LIGHT SOURCE BASED ON ENERGY RECOVERY LINAC
IN JAPAN
Toshio Kasuga, Takaaki Furuya, Hiroshi Kawata, Yukinori Kobayashi, Shogo Sakanaka,and Kotaro SatohHigh Energy Accelerator Research Organization (KEK), Tsukuba, Ibaraki 305-0801, Japan
Norio Nakamura
Institute for Solid State Physics (ISSP), University of Tokyo, Kashiwa, Chiba 277-8581, Japan
Ryoichi Hajima
Japan Atomic Energy Agency (JAEA), Tokai, Naka, Ibaraki 319-1195, Japan
Abstract
After extensive discussions on the future light source ofthe Photon Factory at the High Energy Accelerator
Research Organization (KEK), it has been concluded thata 5 GeV energy recovery linac (ERL) should be the mostsuitable candidate to foster cutting-edge experiments, aswell as to support a large variety of user needs from VUVto X-rays. On the other hand, the Japan Atomic Energy
Agency (JAEA), which has built a low energy (17 MeV)
ERL, also proposed another 5-6 GeV ERL as a lightsource. These two institutes, with a participation of somemembers of the Institute for Solid State Physics (ISSP) ofthe University of Tokyo, agreed to promote an ERL-basednext-generation synchrotron light source in Japan. Before
constructing a 5 GeV ERL, it is necessary to develop
several critical components, such as an electron gun andsuperconducting accelerating structures, and to provetheir performance. To this end, we plan to construct a test
ERL of 200-MeV class at the KEK site. An R&D teamfor the test ERL is going to be organized in collaboration
with accelerator scientists from the other facilities, theUVSOR and the SPring-8.

INTRODUCTION
We have been investigating the feasibility of an energy
recovery linac (ERL) as a light source for several years[1-3]. After the discussions on the future light source ofthe Photon Factory, it has been concluded that a 5 GeVclass ERL should be the most suitable candidate to foster
cutting-edge experiments, as well as to support a large
variety of user needs from VUV to X-rays [4]. A
conceptual layout of a 5 GeV ERL, proposed in thefeasibility study, is shown in Fig. 1. The principal
parameters are also shown in Table 1 [1]. In order torealize this kind of light source, the development of many
components that compose the light source is necessary.
Investigations into the beam dynamics issues, for example,
the energy recovery in rf cavities, the stability of thebeams, the conservation of normalized emittance, etc., arealso essential. Therefore, construction of a test ERL
seems to be inevitable. Both the design concept and thekey technologies will be demonstrated by the test ERL.
An R&D team for the test ERL is going to be organizedin collaboration with accelerator scientists from KEK,
JAEA, ISSP, UVSOR and SPring-8. We are planning toconstruct a 200-MeV class ERL, and to substantiate theprinciple of the ERL within several years. The principal
parameters of the test ERL are also shown in Table 1.

Figure 1: Conceptual layout of 5GeV ERL.
Table 1: Principal parameters of the future light source
and the prototype.
FutureTestLSERL
Beam energy
2.5-5.0 0.06-0.2
GeV
Injection energy
5-10
5-10
MeV
Circumference
1253
68.8m
Beam current (Max.)
100100mA
Normalized emittance
0.1-1
0.1-1
mm⋅mradEnergy spread (rms)5E-5
Bunch length
1-0.1
1-0.1
ps
RF frequency
1.3
1.3
GHz
Accelerating gradient
10-20
10-20MV/m

DEVELOPMENT OF A PHOTOCATHODE
DC GUNThe combination of a DC gun and a photocathodeshould be an optimum electron source, which fulfils therequirement of ERL light sources: electron beams ofhigh-average current and ultra-small emittance. To
demonstrate the ERL-quality electron beams, we aredeveloping a photocathode DC gun at JAEA. The gun
consists of a main chamber for DC electrodes, an NEA
surface preparation chamber, and a load-lock system fortransporting a photocathode between these chambers. A
ceramic insulator of the gun and a high voltage stack of
250 kV-50mA power supply are located side-by-side in apressure vessel, holding 2-atm SF6. A high-voltage test
without a beam load has been completed successfully, andassembling of the load-lock system is in progress. We areproposing a superlattice semiconductor for thephotocathode, which realizes high-quantum efficiency
and small thermal emittance simultaneously [5]. As thefirst step of the photocathode development, we havefabricated several types of bulk samples, which are GaAs
and AlGaAs with different content of aluminum.
Measurements of quantum efficiency and life-time ofthese samples have indicated that AlGaAs has larger
quantum efficiency and longer life-time than GaAs.
Detail experimental results of photocathode and status ofDC gun are presented in an accompanying paper [6].

under ERL operations such as a beam breakup instability
due to cavity HOMs, a resistive-wall beam breakup
instability, and an ion trapping.
QM
SPACE FOR TEST UNDULATOR etc.

CHICANEBMBMSX
CRYOMODULE

CRYOMODULE

MERGER
BMFigure 2: A prototype design of the return loop for the testERL.

LATTICE AND BEAM DYNAMICSISSUES OF THE TEST ERLIn order to prove successful operations of key
components, as well as to verify our predictions ofaccelerator physics issues, we plan to construct a test ERL.
The test ERL will comprise a DC photoinjector, asuperconducting (SC) injector linac, one or two
cryomodules for the main linac, a return loop and a beam
dump. An injection energy will initially be 5 MeV, andwill be upgraded up to 10-15 MeV. The maximum beamenergy will be 60-200 MeV, depending on the number ofsuperconducting cavities in the main linac and on theiraccelerating gradient (15-20 MV/m). The maximum beam
current of 100 mA is anticipated.
An injector design [7], made by the JAEA group, wasadopted for our starting point. The design is made up of a500-kV DC photocathode gun, two solenoids,(temporarily) five three-cell cavities, some quadrupoles
and a three-dipole merger. Normalized emittances werepredicted to be approximately 1 and 0.1 mm⋅mrad underbunch charges of 77 and 7.7 pC, respectively, at the endof the merger. Further optimization of this design isunderway.
To prove successful operations of SC cavities for themain linac under high beam currents, as well as to verify
some predictions of accelerator physics issues, a returnloop is necessary. Figure 2 shows our current design. A
triple-bend achromat (TBA) lattice was adopted for twoarcs because similar lattice will be adopted to ourpractical 5-GeV ERL. Two cryomodules will finally be
installed in one of the straight sections, and a test
undulator will be installed in the other straight section inorder to evaluate the quality and the stability of emitted
photon beams. Figure 3 shows typical optical functions
calculated with a computer code SAD.
At a typical rms bunch length of 1 ps under usual beam
circulations, an intense CSR (approximately 85 keV/turnat a beam energy of 200 MeV) will be emitted along thearcs. The beam optics should then be optimized so that
small beam emittances can be preserved under the CSR.
Under the other bunch compression mode, there are suchimportant issues as i) control of R56, T566, and higher
terms for achieving shortest bunches, and ii) minimizing
emittance growth under more intense CSR. The
optimization of beam optics under both operations isunderway.
We are also investigating some potential problems

Center of Long Straight Section

Fugure 3: Typical optical functions along the return loop.
Periodic condition was assumed.

DEVELOPMENT OF ACCELERATINGSTRUCTURES
Two kinds of 1.3 GHz superconducting (SC) linacs areunder investigation, i.e., the injector linac and the mainlinac. The choice of 1.3 GHz allows us to use the SC
cavity technology that has been developed for a linear
collider [8].

Injector Linac
At a total rf voltage of 10 MV, the injector linac
delivers the RF power of 1 MW to the beam of 100 mA in
CW mode. Therefore, the most difficulty of the injector
linac is the power couplers. To reduce the power of eachcoupler, a couple of SC cavities of 2 or 3 cell structurewith double input couplers is considered as a candidate of
the injector linac. Cavity design and fabrication of a
prototype cavity will be started soon.

Main Linac
A nine-cell structure with sufficiently damped HOM
has been optimized to achieve a stable accelerating
gradient of 15-20 MV/m under the beam of 100 mA. A
large iris diameter of the cells and large beam pipes onboth ends can propagate out the HOMs from the cavity tothe absorbers, which is the way that has been successfully
used in KEKB SC cavity [9]. Furthermore, an eccentricfluted beam pipe has been proposed and adopted so as totake out the quadrupole modes [10]. Figure 4 shows thecavity with absorbers on both ends. A Cu-model and afull scale Nb cavities are under fabrication. Geometricalparameters of the cavity are listed in Table 2.

SCHEDULE
A short sketch of the development schedule of the test
ERL is shown in Table 3.
Table3: Timetable of development of the test ERL.
2006

Figure 4: A nine-cell cavity for the main linac. HOM
absorbers are on both sides.

2007

Table 2: Geometrical parameters of the 9-cell cavity.

2008

Frequency
Structure
Iris diameter
Dia. of beam pipes
R/Q
Geometrical factor
Esp/Eacc
Hsp/Eacc

1.3
980
100, 120
897
289
3.042.5

GHzcellsmmmm
Ohms
Ohms
Oe/(MV/m)

SITE
We have been planning to construct the test ERL in theexperimental hall which has been used for the cold
neutron science at the KEK Tsukuba campus [4].
However, in the design study, we found that the hall is notlarge enough to house a 200-MeV class machine. It isalmost impossible to equip VUV beamlines around it, andfurthermore it is difficult to construct sufficient radiation
shields. In the meanwhile, the KEK 12-GeV Proton
Synchrotron (PS) was closed, and the research using thePS is moving to the J-PARC project. In the near future,
the experimental halls around the PS will be available for
new activities of KEK. We are now discussing thepossibility to change the site for the test ERL. Figure 5
shows a temporary layout of the test ERL which is fit tothe east experimental hall of the PS.

2009
2010
2011

“ERL Project Office” started.
Design study, R&D (SC RF cavity, Gun).
Design study and R&D for SC RF cavity, agun, beam diagnostics etc.
Fabrication/Test of components.
Cryogenic system.
Installation (in east experimental hall of PS ?).
Commissioning, machine development.
Operation, machine development.
Construction of 5GeV ERL.

ACKNOWLEDGMENT
We would like to acknowledge the contributions ofmembers of an ERL development team, consisting of thestaff from KEK, JAEA, ISSP, UVSOR, and SPring-8.

REFERENCES[1] Study Report on the Future Light Source at thePhoton Factory —Energy Recovery Linac andScience Case—, March 2003 [in Japanese].
[2] K. Yokoya, “ERL Project at KEK”, invited talk at
APAC04.
[3] M. Shimada et al., Proc. EPAC06, p. 190.
[4] Photon Factory Activity Report 2005, Dec. 2006.
http://pfwww.kek.jp/pfacr/index.html[5] T. Rao et al., Nucl. Instr. Meth. A557 (2006) 124130.
[6] R. Hajima et al., in these proceedings.
[7] R. Hajima and R. Nagai, Nucl. Instrum. Methods
A557 (2006) 103.
[8] http://www.desy.de/~teslatdr/tdr_web/pages/[9] T. Furuya et al., Proc. 11th Workshop on RF
Superconductivity, DESY, 8-12 September, 2003.
[10] K. Umemori et al., in these proceedings.

Figure 5: A temporary layout of the test ERL which is fit to the East Experimental Hall of the 12-GeV PS.