Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, JapanPASJ2014-SAP016

Design of Compact THz-FEL system at Kyoto UniversitySuphakul Sikharin 1,*, Yusuke Tsugamura 1, Heishun Zen 1, Toshiteru Kii 1, Qika Jia 2
and Hideaki Ohgaki 1
1Institute of Advanced Energy Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan

2

University of Science and Technology China, Hefei, 230029, China*sukarin.supakun.78z@st.kyoto-u.ac.jp

Abstract
A development of a compact terahertz (THz)radiation source by free-electron lasers (FEL) at theInstitute of Advanced Energy, Kyoto University isongoing. The system consists of a photocathode RFgun, a focusing solenoid, a magnetic bunch
compressor, a focusing quadrupole and an undulator.
The system is designed to be simple, economical andcompact which aim to use for scientific researches orindustrial applications. The total length of the system
is less than 5 m. The radiations are generated by ultrashort and intense electron pulses which are injected toa short undulator. Interactions between electron pulseand optical field are analyzed by 1-D FEL equations.
A tracking simulation and optimization are performedby using PARMELA and General Particle Tracer(GPT) code.

INTRODUCTION
A new terahertz radiation source is under thedevelopment at the Institute of Advanced Energy,
Kyoto University. The system would consist of a 1.6
cell BNL type photocathode RF-gun, a focusing
solenoid magnet, a 4-dipole magnetic chicane bunch
compressor, a triplet quadrupole magnet and a planar
halbach undulator and a laser system. The system will
be used for energy science researches or industrial
applications. The system is designed to be simple,
economic and compact with the total length less than
5 m. The schematic view of the proposed system isshown in Fig 1.
To investigate the beam dynamics, PARMELA or“Phase And Radial Motion in Electron LinearAccelerator” [1] code and GPT or “General ParticleTracer” code [2] have been used for the simulationsof multi-particle beam dynamics from the RF-gun to
the undulator and optimization of lattice parameters.
Solenoid
Quadrupoles
Undulator
RF-gunChicaneFigure 1: Schematic view of the compact THz-FEL at
Institute of Advanced Energy, Kyoto University.

The magnets of the chicane were designed andsimulated by using computer code RADIA [3].TheFEL radiation power was estimated by the 1dimensional FEL theory by considering in a shortbunch case or called “super radiation”. The system
will be installed in the same accelerator room withKyoto University Free-Electron Lasers (KU-FEL) [4].

BEAM DYNAMIC STUDY
The electron beam dynamics of the compact
THz- FEL system has been studied by PARMELAand GPT codes. Both codes track particles in the 6dimensional phase space and include the space charge
effect. In this study, the longitudinal beam dynamics
from the RF-gun to the undulator entrance wasinvestigated by using PARMELA. Meanwhile, thetransverse beam dynamics was optimized by using
GPT. Because the undulator with vertical deflection,
which will be used for the compact THz- FEL system,
is only available in GPT.

Photocathode RF-gun
The 1.6 cell BNL type photocathode RF-gun,which was manufactured by KEK, is used to produce- 483 -

Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, JapanPASJ2014-SAP016

a high-brightness, short-bunch and low emittanceelectron beam suitable for the compact THz-FELsystem. The RF-gun performances have been studiedin [5]. The cathode plug was made from copper andloaded to the RF-gun by a load-lock system. The
improvement of the quantum efficiency (QE) of thecathode is also studying by coating the cathodesurface with Cs and Te. The photocathode RF-gun isilluminated by a pico-seconds mode-locked
Nd:YVO4 UV laser system which consists of an
acousto-optic modulator, beam position stabilizer,
two of double pass amplifiers, SHG and FHG crystal.
For simulation, the previous RF-gun model [5]was used. The accelerating voltage and bunch charge
are varied then investigate the exiting beam
dynamics. We assumed that the ratio of the fieldstrength of the half-cell and the full-cell was 1:1. The
laser profiles were assumed to be the Gaussian shapein both transverse and longitudinal. The laser
injection phase was selected at a lower phase which
provides a lower energy spread. The RF-gun andlaser parameters are shown in table 1.

MeV
9.9

A
160

9.8

140(a)

9.7

120

9.6

100

9.5

80

9.4

60(b)

9.3

40
200

9.2
9.1
02

46Time (ps)810Figure 2: Energy time phase space (a) and current
profile (b) at the RF gun exit.
3.0
2.0
1.0
0.0
-1.0

-0.3

-0.1

0.1

0.3

-2.0

Table 1: The Photocathode RF-gun Parameters
RF-Gun Parameters
RF-Gun TypeAccelerating field
Bunch charge
Solenoid field strength
Laser type
Laser longitudinal length
Laser transverse size
Laser injection phase-3.0Values
1.6 cell BNL typephotocathode110 MV/m
100 pC
1800 G
pico-seconds modelocked Nd:YVO4
rms 6.2 deg, max 12 deg
max radius 1 mm.
12 degFigure 3: Transverse phase space in x-x’ at the RFgun exit.

At the above conditions, the beam at the solenoid
magnet exit has the average energy of 9.4 MeV, theRMS energy spread of 0.104 MeV, the bunch length
at FWHM of 5 ps, the peak current of 125 A. Theenergy time phase space and current profile at thesolenoid exit are shown in fig.2 and fig 3.

Magnetic Chicane
The magnetic chicane bunch compressor is a new
designed component which consists of 4-rectangularH-type dipole electromagnets. The chicane creates

an energy dependent path inside where the path of anelectron with higher momentum is shorter than anelectron with lower momentum. Consequently, theelectron bunch become shortens in the longitudinal
which lead to the peak current getting higher. Theenergy slits are added between 2nd and 3rd magnet toremoved undesired electrons which do not contributeto the FEL radiation.
The magnet has the geometry length of 75 mm
and the distance between magnets is 125 mm. The
deflection angle of the chicane can be varied from 0
to 30 degree which corresponds to the 1 st ordermomentum compaction (R56) of 0 to -130.1 mm. Theoptimized chicane parameters for the beam at theRF-gun condition in the table 1 are shown in thetable 2.

- 484 -Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, Japan

PASJ2014-SAP016

Table 2: The Optimized Magnetic Chicane Parameters
Parameters

ValuesMagnetic chicane
Type

4 dipole magnet
H-Type rectangular
21 degree-51 mm.

Defection angle
1st order momentum
compaction (R56)
Energy filter (MeV)
Peak magnetic field
Coil current

rms beam size (mm)
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.50.0min 9.3, max 9.57
0.158 T
1886 A-turns(a)Chicane

0.5

1.0

1.5

Figure 4: Energy time phase space (a) and current
profile (b) at the undulator entrance.
y (cm)
0.5
0.3
0.1
-0.1

0.1

0.3

0.5

-0.3
-0.5

2.0

position (m)

2.5

3.0

3.5

Figure 7: RMS beam size by GPT simulations.
Undulator and THz-radiation
The THz-radiations of the compact THz-FEL aregenerated by injecting an ultra-short electron bunch
to the undulator. Each individual electron moves
sinusoidal and emits the radiation in almost the samephase or called “super radiation”. In this case, theelectron bunch length has to be significantly shorter
than the undulator resonance wavelength. The superradiation power of the ultra-short bunch can be
estimated by the 1-dimensional FEL theory [6].
݈௕ ଶ ଷܾ) ߩ ܲ௘ߣ௦where Psr is the super radiation power, lb is the bunchlength, Os is the resonance wavelength, b is thebunching factor ܾ = ห‫ ݁ۃ‬௜ఏ ‫ۄ‬ห where ș is the phase ofthe electrons inside the FEL radiation wave, ʌ is theFEL parameter and Pe is the power of the electronbeam.
For the compact THz-FEL, the undulator is theplannar halbach type with the deflection in thevertical. The undulator has 10 periods with the period
length of 70 mm and adjustable gap. At the gap of 40mm, the peak magnetic field is 0.275 T, the rms
undulator parameter is 1.27, the resonancewavelength is 244 μm. From the beam parameters at
the entrance of the undulator, the FEL parameter is0.035 and the estimated FEL radiation power is about0.21 MW.

(b)-0.3

Triplet UnUndulatorRF-gunܲ௦௥ < (4ߨ-0.1 -0.5stdy

Solenoid

0.0

At the undulator entrance, t he el ect r on bunch
length at FWHM becomes 0.14 ps and the bunching
factor is 0.68. The peak current is up to 400 A andthe rms energy spread is 0.63%. The rms beam size
in horizontal and vertical is 0.2 mm and 1.2 mm. Theenergy time phase space and the current profile at theexit of the undulator entrance are shown in fig.4 andfig.5.

stdxx (cm)Figure 5: Particle distributions in transverse x-yplane at the undulator entrance.
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Proceedings of the 11th Annual Meeting of Particle Accelerator Society of Japan
August 9-11, 2014, Aomori, JapanPASJ2014-SAP016

CONCLUSION AND OUTLOOKThe beam dynamics of the compact THz-FEL has
been investigated by numerical simulation. Theelectron bunches at the undulator entrance haveaverage energy of 9.41 MeV, energy spread of 0.63%,
bunch length of 0.14 ps and peak current of 400 A.
Such a beam will provide the super radiation powerof 0.21 MW for the wavelength of 244 μm.
REFERENCES
[1]
[2]
[3]
[4]
[5]
[6]

L.M. Young and J.H. Billen, “PARMELA,” LANLTechnical Note LA-UR-96-1835, 2002.
S.B. van der Geer, M.J. de Loos, Proc. 1998 Eur. Part.
Acc. Conf., Stockholm (1998).
http://www.esrf.eu/Accelerators/Groups/InsertionDe
vices/Software/Radia
H. Ohgaki et al., J. Korean Phys. Soc., 57 No.2, 344348, (2010).
H. Ohgaki et al., Proceeding of FEL 2007, 390-393,(2007).
Q. Jia, Personal communication, Jun 30, 2014.

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