Proceedings of the 13th Annual Meeting of Particle Accelerator Society of Japan
August 8-10, 2016, Chiba, Japan

PASJ2016 TUP072

TUNEABLE QUASI-MONOCHROMATIC COHERENT THZ RADIATIONSOURCE DEVELOPMENT
A. Aryshev∗ A) ,S. ArakiA) ,M. FukudaA) , Y. HondaA,B) , K. LekomtsevD)
G. NaumenkoC) , A.PotylitsynC) , M. ShevelevA) , L. SukhikhC)
N. TerunumaA,B) , J. UrakawaA)
KEK: High Energy Accelerator Research Organization, Tsukuba, JapanSOKENDAI: The Graduate University for Advanced Studies, Tsukuba, JapanC)
Tomsk Polytechnic University, Institute of Physics and Technology, Tomsk, Russian FederationD)
John Adams Institute at Royal Holloway, University of London, Egham, United KingdomA)

B)

Abstract
The motivation for developing an intensive THz sourceat KEK LUCX is coming from the growing interest to THz
radiation from various scientific communities worldwide
including chemistry, biology, genetics and many more. High
gradient photo-cathode RF gun and few tens of femto-second
laser system is used to generate a pre-bunched electron beamof a few hundred femto-seconds. We have started to investigate the production of the intense radiation beams in therange of 0.1 − 5 THz based on Coherent Smith-Purcell Radiation (CSPR) using 8 MeV electron beam at KEK LUCX accelerator. CSPR is generated when a charged particle moves
in the vicinity of a periodical pattern or grating. The gratingtype and period can be chosen to make quasi-monochromatic
CSPR spectrum. When radiation wavelength is comparable
to or longer than the bunch length it become coherent. In thisreport the status of the experiment, CSPR basic properties
and electron beam characterization will be presented.

INTRODUCTION
Intense THz radiation sources such as free-electron lasers
are widely used for different applications [1]. Nevertheless,the problem of designing of a non-expensive, compact, widerange tunable short pulse duration THz sources is not yetsolved. A few approaches to design such a source on thebasis of laser technologies and electron beam accelerators
are now extensively discussed [2]. Also there is a demand
from a users community to achieve monochromaticity ofgenerated THz radiation. Undoubtedly the usage of any sortof diffractometers and bandpass radiation filters can reduce
output power and introduce undesirable spectra distortions.
In this respect the THz source based on Smith-Purcell radiation (SPR) mechanism is promising due to monochromatic
emission described by dispersion relation:
𝜆𝑘 =𝑑 1( − cos 𝜃) ,𝑘 𝛽(1)where 𝜆𝑘 , 𝑘, 𝑑, 𝛽 and 𝜃 are the wavelength, diffraction order,
grating period, particle velocity in the light speed units andthe observation angle respectively.
∗ alar@post.kek.jp

The proposal to use coherent SPR generated by a shortelectron bunches (or by a train of bunches) as the basis ofTHz radiation sources was made by authors of the work [3].
Certainly, the important characteristic of such sources is its
monochromaticity Δ𝜆/𝜆. The relation (1) determines theradiation monochromaticity generated by infinite grating
length downstream of a finite aperture Δ𝜃:Δ𝜆sin 𝜃=Δ𝜃.
𝜆1/𝛽 − cos 𝜃(2)In the case of a finite length grating 𝑁𝑑 (𝑁 is the numberof grating periods) the SPR spectral line monochromaticity
will be defined as:
Δ𝜆1≈
.
(3)
𝜆𝑘𝑁Usage of the high diffraction order (𝑘 > 1) allows tonarrow the coherent SPR spectral line 𝛿𝜆. Hence the main
objective of this paper is to show a possibility to produce
SPR beams with a monochromaticity better than a few %choosing the high diffraction order.

EXPERIMENTAL RESULTS
The measurements were done at the Laser Undulator Compact X-ray facility (LUCX) at High Energy Accelerator Research Organization (KEK), Fig. 1. LUCX is a multipurpose
linear electron accelerator facility initially constructed as aRF gun test bench and later extended to facilitate Comptonscattering [4] and coherent radiation generation [5] experiments. It consists of RF gun [6], which was designed toproduce a multi-bunch (2.8ns separation) high quality electron beam with up to 1000 bunches, a 0.5 nC charge perbunch, and ∼ 10 MeV beam energy. This beam can be thenaccelerated to 30 MeV by the normal conductivity compact
linear accelerator [7]. Two klystrons are used for the RF
gun and accelerating structure. Also, two laser systems:
picosecond Nd:YAG and femtosecond Titanium-Sapphire
were employed for different LUCX operation modes.
Table 1 summarizes electron beam parameters in femtosecond operation mode. Short electron bunches were generated in the RF gun via photocathode illumination by thefemtosecond laser pulses with wavelength of about 266 nm.
Then electron bunches were accelerated to the energy of approximately 8 MeV in the 3.6 cell RF gun. The experiment- 1060 -Proceedings of the 13th Annual Meeting of Particle Accelerator Society of Japan
August 8-10, 2016, Chiba, Japan

PASJ2016 TUP072

was flat to confirm transition radiation (TR) characteristics.
The TR spectrum emitted by a single electron is assumed toSoleniod

Laser pulse

Grating
LinacScreen

RF GunBeam direction
SapphirewindowBSPMDetector

M1M2

Figure 1: LUCX beamline and experimental schematics. Abbreviations: M1 - fixed interferometer mirror, M2 - movableinterferometer mirror, BS - splitter, PM - off-axis parabolicmirror.
was performed with the beam parameters given in Table 1
except that number of micro-bunches was set to 1. The longitudinal electron bunch profile measurements method waswell described in [8]. Two Schottky barrier diode detectors
with different regions of spectral sensitivity 60 − 90 GHz
and 320 − 460 GHz were used in the experiment. Detailed
parameters list is shown in Table 1.
Table 1: LUCX Beam Parameters in fs Operation Mode and
SBD Detectors Characteristics.
Beam energy, typ.
7 MeV
Micro-bunch charge and stability 25 pC, 6 %Number of micro-bunches, max
4Bunch length, min
160 fsNormalized emittance, 𝜖𝑥 × 𝜖𝑦
1.5 × 1 𝜋mm mrad
Parameter
SBD #1
SBD #2Frequency range
60 − 90 GHz 320 − 460 GHz
Wavelength range 3.3 − 5.0 mm 0.94 − 0.65 mm
Response time
∼ 250 ps
sub-nsAntenna gain
24 dB
25 dB
Input aperture
30 × 23 mm
4 × 4 mm
Video sensitivity
20 V/W
1250 V/W
The vacuum chamber equipped with the sapphire vacuum
window which provided aperture of 145 mm. 5-axis manipulator was used for fine adjustment of the target’s positionin 3 orthogonal directions and in two rotation angles of thetarget with respect to electron beam propagation direction.
In the case of SPR geometry, the distance between grating
and the electron beam was set to 0.6 mm. The radiationspectral characteristics were measured by the Michelsoninterferometer (described in [9]) installed directly in frontof the chamber vacuum window (see Fig. 1). To generate
coherent Smith-Purcell radiation the 60 × 30 mm target wasplaced in the vacuum chamber. The back side of the target

Figure 2: TR orientation dependence measured in the range
320 − 460 GHz.
be constant within detector sensitivity bands. Therefore themeasured normalized TR spectrum was used as a spectral
eﬃciency of the entire measurement system, including spectral transmission eﬃciency of the vacuum window, detector
wavelength eﬃciency, and so on. The SPR resonances 𝑘 = 1
and 𝑘 = 5 lie within the detector sensitivity bands. Figure 2
shows the typical TR orientation dependence obtained withdetector SBD 320 − 460 GHz. The spectral measurements
by means of Michelson interferometer were done at the maxima of orientation dependence. To reconstruct spectra fromauto-correlation measurements Fourier Transform algorithm
was used.

Figure 3: SPR reconstructed spectrum in the range of 60 −90 GHz.
Two normalized SPR spectra measured in a range of 60 −90 GHz and 320 − 460 GHz are shown in Fig. 3 and Fig. 4.
Analyzing obtained spectra it is possible to find relativeline widths Δ𝜆/𝜆1 = 15% for 60 − 90 GHz and Δ𝜆/𝜆5 =- 1061 -Proceedings of the 13th Annual Meeting of Particle Accelerator Society of Japan
August 8-10, 2016, Chiba, Japan

PASJ2016 TUP072

the Ministry of Education, Culture, Sport, Science and Technology, Japan, JSPS KAKENHI: 23226020 and 24654076,
JSPS and RFBR under the Japan-Russia Research Cooperative Program (no. 15-52-50028 YaF_a), the Leverhulme
Trust through the International Network Grant (IN−2015 −012) and the European Union Horizon 2020 research andinnovation programme under the Marie Sklodowska-Curie
grant agreement No 655179.

REFERENCES

Figure 4: SPR reconstructed spectrum in the range of 320 −460 GHz.
6% for 320 − 460 GHz. As one can see the spectral peakmeasured in a range 60 − 90 GHz corresponds to 𝑘 = 1
and the spectral peak measured in 320 − 460 GHz range
corresponds to the 5th SPR resonance 𝑘 = 5.

CONCLUSION
We have investigated coherent SPR spectral characteristics both theoretically and experimentally. Our experimentalapparatus includes the Michelson interferometer with SBDdetectors. The spectral range and eﬃciency of each detector
were investigated experimentally measuring coherent transition radiation spectrum. We have compared intensities ofcoherent TR and coherent SPR in mm range measured in
identical conditions and showed that the brightness is practically comparable for both mechanisms. Our simulations
show that the intensity of the fifth order spectral line is about
two times smaller in comparison with the line intensities ofthe first and the second orders. Some adjustment of the SPRline can be achieved by the grating tilt angle 𝜃𝑔𝑟 ≪ 1.

ACKNOWLEDGMENTS
This work was supported by the Photon and Quantum
Basic Research Coordinated Development Program from

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