Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, Japan

PASJ2015 WEP118

PROGRESS TOWARDS TABLETOP PRE-BUNCHED THZ FEL
Alexander Aryshev #, Masafumi Fukuda, Yosuke Honda, Konstantin Lekomtsev ,Mikhail Shevelev, Nobuhiro Terunuma, Junji Urakawa
High Energy Accelerator Research Organization, 1-1 Oho, Tsukuba-shi, Ibaraki, 305-0801, Japan

Abstract
The motivation for developing a linac-based THz source at KEK LUCX is coming from the growing interest to THz
radiation and a possibility to utilize it for soft X-ray production via inverse Compton scattering. We have proposed twoapproaches to produce the intense radiation beams in the range of 0.1-5 THz based on Coherent Smith-Purcell Radiation
and Coherent Undulator Radiation in “super-radiant” regime on a 8 MeV and 30 MeV electron beam at KEK LUCX
facility respectively. First radiation type is generated when a charged particle moves in the vicinity of a periodical patternor grating. When radiation wavelength is comparable to or longer than the bunch length it becomes coherent. Similarradiation enhancement is happening when micro-bunch period coincided with undulator period. To produce such a microbunch train of electrons a new Ti:Sa laser system for LUCX RF gun has been developed and electron micro-train has
been confirmed. In this report the status of the experiment, Ti:Sa laser system modification, CSPR and CUR basic
properties and electron beam characterization will be presented. The maximum achievable THz power from both
approaches will also be discussed.

1.

Introduction

In the last decade electromagnetic radiation in theterahertz frequency range is widely used in time-domain
spectroscopy to understand biological processes, for THz
imaging and chemical reactions [1]. Much of the recent
interest in high-brightness coherent THz light source (0.1– 5 THz) radiation associated with its ability to penetrate
deep into many organic materials without the damage
produced by ionizing radiation such as X-rays and gives abreakthrough in the rapidly expanding field of THz photon
science. Usual expectation from THz users community is ahigh brightness (high photon flux, density and shortduration), wide spectrum tunability and compactness aswell as low cost of the system. There are a few effective
ways to construct such a THz source using particle
accelerators. Most common way is to utilize high charge
bunched relativistic electron beam after compression to
produce various types of coherent polarization radiations(Transition, Diffraction and so on). In this case one needs
to maintain large accelerator complex and carefully control
initial beam parameters as well as bunch compression. As
a result it is possible to obtain high intensity radiation
pulses of a wide spectrum in exchange of the overallsystem complexity.
Another approach is to generate short electron bunches
directly illuminating an RF gun photocathode withfemtosecond laser pulses. In this case space-charge
dominated beam properties should be taken into accountand to obtain similar THz pulse intensity it is necessary toconsider several measures to enhance radiation power.
One of the most effective ways is to make radiation
emission “super-radiant”. This term was introduced awhile ago [2, and references therein] and attribute to thecase when coherent radiation emission is made by a___________________________________________# alar@post.kek.jp

sequence of electron bunches, so-called comb beam.
Indeed, the intensity of the coherent radiation is
proportional to the number of particles per bunch squared,
but for a comb electron beam (THz sequence of a several
tens fs-length electron bunches) it also multiplies by the
number of micro-bunches. This would not give much of again if there was no spectral modulation in this case. In thereality it is expected that coherent spectrum will becomediscrete and peak power will scale exponentially as anumber of electrons.
The second most effective way is to accumulate
radiation from many comb bunches or even machine cycles
in some kind of enhancement cavity. It will give further
spectral shaping and the total radiation power gain will
depend on the cavity quality factor. The proof-of-principle
experiment on radiation accumulation was alreadyperformed [3] at LUCX “FEM experiment” section, Figure
2.

Figure 1: Dependence of radiation intensity (black dots)
and quality factor (blue dots) of the cavity for every
acquisition on the number of bunches.
At that time the first observation and investigation of the
Stimulated Coherent Diffraction Radiation (SCDR) in an- 827 -Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, Japan

PASJ2015 WEP118

open resonator as a possible mechanism for the generationof intense THz radiation beams for numerous applications
were presented, Figure 2. The detailed investigation of theradiation stored in the resonator cavity as well as the cavity
intrinsic properties was measured. We clearly observed thestimulation process as an exponential growth of radiation
power stored in the resonator as a function of the numberof bunches in the train. Nowadays we are planning toresume THz enhancement cavity study both at LUCX andcERL [4].
Combining these two techniques together it is possible
to construct tabletop THz FEL where a femtosecond comb
beam is accelerated by a RF accelerating field withgradient of the order of 50MV/m while carried on a single
RF accelerating field cycle enabling it to be accelerated to5MeV in a 7.5cm RF gun. When such a comb beam ispassed in the vicinity of a periodical structure or throughshort edge-focusing wiggler [5] it generates Coherent

Smith-Purcell Radiation (CSPR) or Undulator radiation(UR) in “super-radiant” regime if micro-bunch spacing
becames comparable with radiation wavelength which iscomparable to the grating or undulator period. Our plan is
to develop and apply an accelerator based ultra-compact
high-brightness coherent THz light source, with shortpulses of ~10MW peak power, variable frequency range
from 0.3 to 5THz, and typical energy 10uJ/pulse [6]. It wasdecided to investigate the CSPR and UR as a potential
candidate for generating intense broad-band radiation inTHz frequency range as a part of a larger THz program
launched at KEK: LUCX (Laser Undulator Compact X-rayproject) facility. The program is aiming to investigate
various mechanisms for generating EM radiation including
Undulator radiation, Smith-Purcell and other special casesof Polarization Radiation. In this report the status of theexperiment, LUCX RF Gun Ti:Sapphire laser system andComb beam generation will be presented.

Figure 2: LUCX accelerator schematics.
To establish femtosecond comb electron beam
generation the experimental setup for the observation andinvestigation of the different types of polarization
radiations was constructed at LUCX facility in KEK [7].
LUCX is a multipurpose linear electron accelerator facility
initially constructed as a RF gun test bench and later
extended to facilitate Compton scattering and coherent
radiation generation experiments. It consists of a highmode separation 3.6 cell RF gun, which was designed toproduce a multi-bunch high quality electron beam with up
to 1000 bunches, a 0.5 nC bunch charge, and 10 MeV beamenergy. This beam can be then accelerated to 30 MeV by
the normal conductivity 1 m 12–cell mode–separated linac
booster.

and femtosecond Titanium-Sapphire were employed tomake possible different LUCX operation modes. Table 1summarizes electron beam parameters usually obtained in
femtosecond operation mode of the LUCX. Figure 1 shows
LUCX beamline schematics.

Table 1: LUCX Parameters in fs Operation ModeParameterValues

Beam energy, typ.
Intensity/4 micro-bunches
Number of micro-bunches, max
Bunch length, max
Bunch length, min
Repetition rate, max

8 MeV
100pC
161 ps50 fs
3.13 train/s

Normalized emittance, εx x εy

4.7x6.5 πmm mrad

Figure 3: CPA technique: the pulse from the oscillator isstretched, amplified and recompressed to its original
duration.

Two klystrons (Toshiba E 3729 and Toshiba E3712) areused to independently feed RF gun and accelerating
structure. Also, two laser systems: picosecond Nd:YAG

To generate a sequence of femtosecond micro-bunches
the well-established Titanium-Sapphire Chirped Pulse
Amplification (CPA) echnique laser system was chosen,
Figure 3. Short pulses of a few tens femtoseconds from theoscillator are temporarily stretched to a picosecond level
before they are amplified in Regenerative Gain Amplifier- 828 -

Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, Japan

PASJ2015 WEP118(RGA) up to a few μJ. We have introduced ”pulse divider”(PD) right after the RGA so pulses are splitted andrecombined with controllable delay by the double–passMichelson interferometer. This minor modification allows
generating sequence of a spectrally chirped picosecond
pulses with variable time separation. After that they areamplified up to 2 mJ per micro–pulse by the multi-pass
Ti:Sa amplifier and re-compressed back to a few tens
femtoseconds. This is possible due to the same micro–
pulses polarizations after multi–pass amplifier. As a result
1.3 mJ of each micro-pulse energy at Ti:Sa fundamental
harmonic was available at the laser system output. Also thelaser system was extended to allow direct measurement ofa generated micro–pulse durations and its time separations
by the method based on the registration of the second
harmonic energy cross distribution produced in nonlinear
crystal. Resulting cross-correlations give calibrated
absolute temporal measurements, which is then comparedwith electron beam measurements. In case of micro-pulsed
input the cross-correlation dependence has an additional
two peaks (for two micro-bunches) and six peaks (for fourmicro-bunches) symmetric around the main correlation.
The time separation between main correlation and thesatellite peak is equal to the real time separation between
micro–pulses, Figure 4.

this correlation effectively gives scale of the horizontal
image size in RF degrees, what in turn can be recalculated
to the time scale. Our measurements show that the responsetime of the Cs2Te photocathode is of the order of 250 fs.
Detailed report already submitted for publication in peerreviewed journal and will be published soon.
To demonstrate comb beam generation possibility wehave acquired four micro–bunch electron density
distribution measured at MS3G DMQ screen by RF zerophasing technique, Figure 5. Image clearly shows four 250
fs bunches separated by approximately 500 fs what is
consistent with Figure 4. Right now repetition rate of thiscomb is limited by Ti:Sa laser system and can not be fasterthan 10 Hz. In order to overcome current limitation one
needs to consider new CPA laser system (presumablyfiber-based) with micro-bunching capability.

Figure 5: Four micro–bunch electron density distribution
measured at MS3G DMQ screen by RF zero-phasing
technique.

Figure 4: Four laser micro-pulse cross-correlation.
For successful accelerator operation in femtosecond
mode a well-established on-line diagnostics and control ofthe laser beam and electron beam are needed. To confirm
electron beam parameters the quasi–ballistic electron
optics was designed. This optics makes 5 mm horizontal
dispersion at the MS3G 300 μm–thick DMQ screen which
is located beyond BH1G bending magnet after the linac
booster. Calibrating transverse electron density
distribution by BH1G dipole magnet current change wehave measured initial electron beam center–of–massenergy is equal to E = 8.25 ± 0.002 MeV and energy spread
is dE = 18.8 ± 0.2 keV, i.e. dE/E 0.2%.
The rms electron bunch length is measured by the zerophasing technique [8]. To estimate bunch length thecorrelation of the RF phase with image centroid shift onMS3G screen was measured. The linear approximation ofNevertheless our result further widens the potential ofdesigning a table-top tunable THz FEL based on superradiant coherent radiation. With no doubts space-chargeeffects play a fundamental role in preservation of thetemporal structure of the comb electron beam and limits
the maximum achievable beam charge. The further workon higher charge per bunch is desired for strong THz
radiation generation. It can be done by increasing of the RF
gun acceleration gradient and optimization of the laser spot
size at the photocathode. Also detailed comparison of zerophasing measurements with these based on THz spectral
measurement system (Michelson interferometer forspectrometry of intense broadband radiation in THz
frequency range and bunch shape reconstruction [9]) is
foreseen in nearest future. The work on super-radiantundulator radiation properties simulation and its
comparison with CSPR is also ongoing.
Further details on the project progress will be publishedin successive papers.

- 829 -Proceedings of the 12th Annual Meeting of Particle Accelerator Society of Japan
August 5-7, 2015, Tsuruga, Japan

PASJ2015 WEP118

Acknowledgements
Authors would like to thank A. Araki for valuable help,
useful discussions and support of the LUCX accelerator
operation and maintenance. This work was supported byPhoton and Quantum Basic Research Coordinated
Development Program from the Ministry of Education,Culture, Sports, Science and Technology, Japan and JSPS
KAKENHI grant numbers 23226020 and 24654076.

References
J.L. LaRue, T. Katayama, A. Lindenberg, et.al. Phys.
Rev. Lett. 115, 036103 (2015).
D. Li, K. Imasaki, et.al, Proc. FEL2006 Conf.
TUPPH049, 2006.
A. Aryshev, S. Araki, M. Fukuda, et al., Nucl. Instr. andMethods in Phys. Res. section A, 763, 424 (2014)
Y. Honda, et.al, Proc. Part. Acc. Soc. Japan, WEP022,2015.
S. Kashiwagi, et.al., Phys. Rev. ST Accel. Beams 12,120703 (2009).
A. Aryshev, et.al., Proc. Part. Acc. Soc. Japan, SUP020,2013.
M. Shevelev, A.Aryshev, to be published
D. X. Wang, G.A. Krafft, C.K. Sinclair, Phys. Rev. E
57, 2283 (1998).
M. Shevelev, A. Aryshev, et. al., Nucl. Instr. andMethods in Phys. Res. section A, 771, 126 (2014).

- 830 -