An Intense Terahertz Radiation Source at the Compact ERL
Kentaro Harada, Photon Factory, High Energy Accelerator Research Organization (KEK),Oho, Tsukuba, Ibaraki 305-0801, Japan
Miho Shimada, UVSOR, Institute for Molecular Science, National Institutes of NaturalSciences, Myodaiji-cho, Okazaki, Aichi 444-8585, Japan
Ryoichi Hajima, ERL Development Group , Japan Atomic Energy Agency (JAEA),
Tokai, Naka, Ibaraki 319-1195, Japan(Insertion device)

Beam dump

MergerMain linac(module 2)

Main linac(module 1)

Injectorlinac
Electron gunFig. 1. Lattice of the Compact ERL.
Table 1 Tentative parameters of the test facility
Parameters (final goal)
Injection energy
5 MeV (10-15 MeV)
Injectorbeam 500 kW (1 MW)
power
Beam energy in the ~60 MeV
recirculation path(160-200 MeV)SC cavities for 9 cells x 4: single module
main linac(two modules)
Normalized
1 mm⋅mrademittance(0.1 mm⋅mrad)
Beam current
10 mA (100 mA)
RMS bunch length Usual mode : στ = 1-2 ps(Short bunch : στ = 0.1 ps)
Figure 1 shows the plan of the Compact ERL andTable 1 lists the tentative parameters. In order togenerate the CSR of the wavelength λCSR , theelectron bunch length should be shorter thanσz =λCSR
2.

For example, the electron bunch with the bunch
length of 0.2 ps (60µm) can generate the CSR of theenergy 10 meV (120 µm). In order to achieve such

1.E+18
Photon Flux Density

Extractor

2

Recirculation pathshort bunch length, however, the bunch compression
is inevitable.
For the bunch compression [1],[2], the off-crest
acceleration at the main acceleration module firstly
generates the energy gradient in the electron bunch.
Then the difference of the orbit length depending on
the particle energy at the arc section can compress thebunch length. The linear path length difference of thearc, R56, can be optimized by the quadrupoles and thesecond order effect, T566, by the sextupoles.
Furthermore, in order to suppress the emittance
growth by the CSR, Twiss parameter α at the end of
the arc section is fixed. Finally, the simulation results
show that, if the bunch charge is smaller than about
0.5nC, the bunch length can be compressed to about
0.2ps at the 65MeV beam energy [3]. The estimatedphoton flux from the bending magnet at the arc of theERL test facility is shown in Fig. 2.

[photons/sec/mrad /0.1%b.w.]

The Compact ERL is the energy recovery linac(ERL) test facility that will be constructed at KEK
Tsukuba campus as a joint project of the KEK, JAEA,
and other institutes. The Compact ERL has a great
feasibility of producing intense terahertz radiation
using coherent synchrotron radiation (CSR) from its
electron beams. Although the primary purpose of thefacility is the demonstration of the key technologies
that are essential to build ultra-brilliant new
synchrotron light source based on the ERL, thelimited user operation with the terahertz CSR is now
examined. In this presentation, we show theparameters of the Compact ERL and discuss expected
performances of the terahertz radiation.

1.E+15Enhancement by
the CSR effect

1.E+12
1.E+09
1.E+06
1.E+030.00001

0.001
0.1
Photon Energy [eV]10Fig. 2. Radiation spectrum from the bending magnetof the arc section of the Compact ERL. The beam
current is 100 mA (= 77 pC x 1.3 GHz), the bunch
length 0.2ps, and the beam energy 65MeV.
[1] M. Shimada, K. Yokoya, T. Suwada and A.
Enomoto, ”Lattice and beam optics design forsuppression of CSR-induced emittance gowth at
the KEK-ERL test facility”, NIM A 575, (2007)
315[2] R. Hajima, ”Emittance commpensation in a returnarc of an energy recovery linac”, NIM A 528,(2004) 335[3] M. Shimada, K. Harada, R. Hajima, ”Bunch
compression and the emittance growth dur toCSR”, Proc. of ERL 2007