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CSR

Tunable Narrowband Terahertz CSR from Electron Bunch Interacted with
Modulated Laser Pulse
M. Shimada1, M. Katoh1, S. Kimura1, M. Hosaka2, Y. Takashima2, S. Bielawski3, C. Szwaj3, C.
Evain3, A. Mochihashi4, T. Hara5, T. Takahashi6
1
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan
2
Graduate School of Engineering, Nagoya University, Nagoya 464-8603 Japan
3
Université des Sciences et Technologies de Lille, Villeneuve d’Ascq Cedex㻘 France
4
Japan Synchrotron Radiation Research Institute, SPring-8, Sayo-cho Hyogo 679-5198 Japan
5
RIKEN SPring-8 Center, RIKEN Harima Institute, Sayo-cho Hyogo 679-5148 Japan
6
Kyoto University Research Reactor Institute, Kumatori-cho Osaka 590-0494 Japan
Powerful and tunable narrowband terahertz (THz)light source is expected to open a new field in biology,
chemistry, medical sciences and so on. Recently, we
have developed a new method to produce
monochromatic synchrotron radiation in the terahertz
region on storage rings [1]. In this method, a specially
tailored electron bunch emits monochromaticradiation in a uniform magnetic field. This is incontrast with the ordinary technique to produce
monochromatic radiation by utilizing periodic
magnetic fields created by undulators.
At UVSOR-II, we have already succeeded in
producing intense broadband THz emission by using
the laser slicing technique [2]. In the feasibility study
before starting this experiment, we had pointed out
the possibility to produce monochromatic radiation
by extending this technique. We had predicted that,by creating many dips periodically on an electron
bunch, we could produce quasi-monochromaticsynchrotron radiation through coherent synchrotron
radiation (CSR) mechanism.
To create many dips periodically or, in other words,
periodic density modulation, we developed a method
to produce amplitude-modulated laser pulses by using
a technique called ‘chirped pulse beating’ (see Fig. 1),
in collaboration with a French team [1]. This is arobust technique which fits the accelerator
circumstances and is easy to operate.
The amplitude modulated laser pulses are injectedto the ring through a laser transport line which wasconstructed by utilizing a part of the existing opticalcavity for free electron laser. The laser pulses interact
with electron bunches in an undulator whose
fundamental wavelength is tuned to the laser
wavelength, 800 nm in this case.
In the undulator, the laser field and the electrons
exchange their energy, and an energy modulation iscreated on the electron bunch. The energy modulationis larger/smaller where the electrons interact withlaser field of larger/smaller amplitude. After thisinteraction the electron bunch proceeds in the ring.
Since the higher/lower energy electrons go slow/fast.,
in other words, the time of flight depends on theenergy, the energy modulation is converted to adensity modulation. Electrons escape out fromstrongly modulated parts. Fig. 2 shows a result fromnumerical simulation on the formation of the density
modulation. The period of the density modulation isexactly same as that of the amplitude modulation of
the laser pulse.
The density modulated electron bunch emits CSR
at the wavelength corresponds to the density
modulation period. This can be understood by using ageneral expression of the power of the CSR;P O P0 O ( N e  N e2 f O ) ,f O³ cos 2Sz / O S z dz

2

where the P(O) is the radiation power at the
wavelength O emitted by an electron bunch, the P0(O)that emitted by a single electron, the Ne the number ofthe electrons, the S(z) is the longitudinal density
distribution of the electron bunch. If the bunch has asingle dip, the f(O) has a value close to unity for the
wavelength longer than the dip width. In this case, theCSR spectrum becomes broadband. On the other
hand, when sinusoidal density modulation is createdon the bunch, then the f(O) has a sharp peak at the
wavelength corresponds to the density modulation
period.

Fig. 1. Principle of Chirped Pulse Beating [1]. A
femto-second laser pulse is stretched and separated
into two pulses. They are merged with some delay on
one pulse.

37

Highlight
The experimental setup is illustrated in Fig. 3. The
CSR was observed at the second bending magnet
from the undulator section where the interaction
between the electron bunch and the laser pulse took
place. The THz spectrum is measured by theMartin-Puplett interferometer at the beam-line BL6B.
We succeeded in observing the narrowband THz CSR
as shown in Fig. 4(a). It was also confirmed that thelarger number of the modulation gave the narrower
line width.
Another important feature of this new technique,the ‘tunability’, was also successfully demonstrated.
Just by adjusting the position of a retro-reflector,
which is a component of ‘chirped pulse beating’optical systems, we could change the modulation
period. The radiation wavelength was found toexactly same as the modulation period as shown in
Fig. 4(b). A detectable peak was in the 12-25 cm-1
range.
Intensity of THz CSR for given laser peak power
and for the present experimental setup can be
estimated analytically as a function of the modulation
frequency. The result is bell shaped with the peakaround 20 cm-1 as shown in Fig. 5 and qualitatively
explains well the experimental results.

Fig. 2. Evolution of the electron distribution in
longitudinal phase-space (upper) and their projection
onto the longitudinal coordinate (lower), (a) before
the interaction, (b) just after the interaction and (c)after traveling in the ring [1].

Acknowledgement

Fig. 3. Laser slicing system at UVSOR-II [2].

This work was supported by the Joint Studies
Program (2005–2007) of the Institute for Molecular
Science and the JSPS Fellowship Program for
Research in Japan (grant S06215), by Grants-in-Aid
for Scientific Research B15360038 and B15360039
from JSPS and by the research foundation for
Opt-Science and Technology. CERLA is supported by
the French Minist`ere charg´e de la Recherche, theR´egion Nord-Pas de Calais and FEDER.
[1] S. Bielawski, C. Evain, T. Hara, M. Hosaka, M.
Katoh, S. Kimura, A. Mochihashi, M. Shimada, C.
Szwaj, T. Takahashi, Y. Takashima (in alphabeticorder), Nature Physics 4 (2008) 390.
[2] M. Shimada, M. Katoh, S. Kimura, A. Mochihashi,
M. Hosaka, Y. Takashima, T. Hara, T. Takahashi, Jpn. J.
Appl. Phys. 46 (2007) 7939.
[3] Y. Takashima, M. Katoh, M. Hosaka, A.
Mochihashi, UVSOR Activity Report 2002 (2003)
56.
[4] Y. Takashima, M. Katoh, M. Hosaka, A.
Mochihashi, S. Kimura, T. Takahashi, Jpn. J. Appl.
Phys. 44 (2005) L1131.

Fig. 4. Typical narrowband THz CSR spectra, andtunability curve [1]. (a) Two THz CSR spectra
obtained with modulation frequencies of 16 cm-1 and24cm-1, respectively. (b) Peak frequency of THz CSR
versus laser modulation frequency.

Fig. 5. Calculated CSR intensity versus modulation
frequency [1]. The two peaks (full lines) are spectra
associated to modulation frequencies of km=400 and
km=500, respectively.

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Undulator

Independent Tuning of Variable Polarization Undulator U7
K. Hayashi, M. Katoh, J. Yamazaki, A. Mochihashi, M. Shimada
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan
Outline of U7

Result of COD correction

The variable polarization undulator, U7, wasintroduced in summer 2006 (Fig.1). It is a 3m
Apple-II type undulator. It is the 4th undulator
operational at UVSOR.

The result of COD correction is shown in Fig. 2 as
to horizontal COD in horizontal polarization mode forexample. COD without correction caused by gap
change from 100mm to 24mm is larger than 100
microns, while smaller than 10 microns withcorrection. At present, horizontal polarization mode
(gap 24mm-200mm) and vertical polarization mode
(gap 35mm-200mm) are opened for users. In verticalmode, short gap (smaller than 35mm) causes beam
life shortening, which should be improved byintroducing more sophisticated correction for
non-linear effects.

Fig. 1. Variable polarization U7 undulator.
All undulators of UVSOR can be controlled via
Ethernet from beamlines. They have several steering
magnets to cancel the closed orbit distortion (COD)
caused by pole gap change. U7 has four correction
magnets (two horizontal/vertical steerings and two
quadrupoles) at the ends of the undulator. The
controller commands DC power supply to provide
feedforward current for steering magnets, referring
both of the pole gap and the correction table in period
less than one second.

Method of COD correction
We first prepared the correction table for the local
quadrupole magnets to cancel the betatron tune shift
caused by gap change based on measurements. Then,
we prepared the correction table for the steering
magnets to compensate the COD due to gap change[1].
As the first step, COD due to the gap change wasmeasured by beam position monitors (BPM). Next,
COD caused by the excitation of the steering magnets
was measured. Then the currents of the steering
magnets were determined so as to minimize the COD.
We used least-square method, that is to find local
minimum of S given below;

Fig. 2. COD without(up) and with(down) correction
Polarization change is also allowed at fixed gap,
100mm. The COD during the polarization change
was successfully corrected to be less than 10 microns.

­( xi  u iH I H ) 2 ½
S= ¦ ®
V2 ¾
i 1 ¯ ( y i  vi I v ) ¿H
where u i , I H and xi represent horizontal COD
N BPM[1] K. Hayashi et al., UVSOR Activity Report 2003
(2002) 51.

at i-th BPM caused by the excitation of horizontal
steering magnets at unit current, excitation current ofhorizontal steering magnets and horizontal COD ofi-th BPM caused by gap change, respectively.

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FEL

Lasing below 200 nm at the UVSOR-II FEL
M. Hosaka1, M. Koike1, N. Yamamoto1, Y. Takashima1, M. Katoh2, M. Shimada2,
J, Yamazaki2, A. Mochihashi2,31Graduate School of Engineering, Nagoya University, Nagoya 464-8603 Japan
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan
3
RIKEN SPring-8 Center, RIKEN Harima Institute, Sayo Hyogo 679-5148 JapanThe UVSOR-II FEL has been recognized as a useful
tool for application experiments because of its highout-coupled average power in the deep UV region
down to 215 nm. This fiscal year, application
experiments, irradiation on bio-molecules and time
resolve experiments on a scintillator have been
performed. In the irradiation experiment, average
power of 100 mW could be maintained and in thetime resolved experiment, FEL of wavelengths of 215
nm and 240 nm was employed. In order to promote
application experiments more extensively, FEL lasing
in shorter wavelength below 200 nm is required.
The shortest wavelength attained in storage ring
FELs is 176.4 nm at Elettra, Italy [1]. They employedfluoride multilayers for resonator mirrors andin-vacuum diagnostic system. At the UVSOR-II FEL,on the other side, attainable wavelength is limited bydiagnostic system (out-of vacuum) now and therefore
we decided to demonstrate an FEL lasing just below
200 nm.
The lasing experiment was made at electron beamenergies of 600 MeV and 750 MeV. Multilayers of
Al2O5/SiO2 were employed for the cavity mirrors.
The mirrors were designed to maximize reflectivity at
198 nm. The numbers of layers for the front andbackward cavity mirrors are 41 and 51, respectively.
In order to obtain high out-coupled power from the
backward mirror, smaller number of layer was chosen
for it. Calculated round trip reflectivity using theoptical constant of Al2O5 and SiO2 is 99.1 %. After
degassing the mirror with synchrotron radiation froman optical klystron, the reflectivity was measured
using cavity ring-down technique. The measured
round-trip reflectivity was 98.5%. We suppose thatthe value is smaller than calculated one due to mirror
degradation in the process of the degassing. The firstlasing experiment was made with 600 MeV electron
beam of 100 mA/2bunch. The shortest wavelength
spectrum in the experiment is shown in figure 1. As
seen in the figure, the shortest wavelength attainedwith the mirrors was 199.3 nm although the designed
value was 198 nm. We suppose that the wavelength of
maximum reflectivity of the mirrors was shifted
toward longer wavelength due to the mirror
degradation after irradiation of synchrotron radiation,
which is often observed in other FEL experiments at
UVSOR-II. The measured threshold beam current
for the lasing was 28.9 mA/2bunch, which is almost
consistent with the FEL gain calculation.
After lasing with 600 MeV electron beam,

Fig. 1. Measured spectra of the FEL lasing at 199.3
nm. The gap length of the U5 optical klystron was
44.8 mm.
we raised it up to 750 MeV, which is favorable for ahigh power FEL operation [2]. An FEL lasing was
obtained; however, the shortest wavelength was not
shorter than 200 nm anymore. The out-coupled poweraround 202 nm was 18 mW at a beam current of 90
mA/2bunch. Since the aim of this experiment is to
examine a lasing below 200nm, we did not store highbeam current in the storage ring. However for users’
applications, we will increase the beam current (~ 200
mA/2bunch), and higher out-coupled FEL power will
be is obtained.
In conclusion, we have succeeded in an FEL lasing
below 200 nm and found no technological barrier forthe lasing. Further shortening of the lasing
wavelength, a VUV diagnostic system and cavity
mirror for the shorter wavelength is necessary.
According to a gain estimation, an FEL lasing ispossible at UVSOR-II with more than 96 % roundtrip reflectivity cavity mirrors.
[1] F. Curbis et al., Proceeding of the 27th FEL
conference (2005) 473.
[2] M. Hosaka et al., UVSOR Activity Report 2006
(2007).

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Injection

Full-Energy Injection in UVSOR-II

M. Katoh1, K. Hayashi1, J. Yamazaki1, M. Shimada1, A. Mochihashi2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan
2
Japan Synchrotron Radiation Research Institute, SPring-8, Sayo, Hyogo 679-5198 Japan1

In the previous report [1] we have mentioned
replacement of power supplies for electromagnets ofthe booster synchrotron in UVSOR; aiming at the
full-energy injection operation. Before thereplacement, the maximum acceleration beam energy
with the booster was restricted up to 600 MeV
because of specifications of the power supplies forthe electromagnet; after the beam injection we had toaccelerate the beam up to 750 MeV in the storage
ring for users operation. Such operation was notsuitable for stabilization of the beam position in thestorage ring, and moreover, it could become criticalproblem for ‘top-up’ operation which we are planningto do in the future. After the replacement and basic
tuning of the power supplies for 600 MeV operationmode, we continued development of the operating
condition for getting full-energy injection mode. In
the development we settled the 750 MeV operating
point of the booster on the extension of the 600 MeV
operation; without changing beam optics. Figure 1
shows output patterns of the power supplies for the
750/600 MeV operations. The main power supply(‘Main Coil’ in the figure) supplies all of the magnets;
bending, QF and QD magnets. Additional power
supplies (‘Sub Coil QF’ and ‘Sub Coil QD’) are also
used to control the quadrupole fields. During theadditional acceleration the QF and QD magnets
saturate because of the main power supply, so the QF
and QD output had to decrease in the acceleration
process. That means slight change in the beam optics,
but we didn’t have any serious problems in theoperation. Because of the change in the operation
pattern, a repetition rate of the booster in the 750
MeV operation is settled at 1Hz while the repetitionin the 600 MeV is usually ~3 Hz. To compensate theadditional beam loading and keep enough beam
intensity in the booster, we have adopted a patternoperation for RF accelerating voltage. To transport
the 750MeV beam from the booster to the storage
ring, we have also replaced a power supply forbending magnets in the beam transport line at the endof FY2006.
After the upgrade of the beam transport line, soon
we succeeded the full energy injection [2]. Figure 2
shows typical operation pattern of the booster
synchrotron in 750 MeV operation. The beam current
circulating in the booster was measured by an AC-CT
via an linear wave detector for RF frequency (90.1
MHz) component; in the figure the beam current
seems to change because of the change in the bunch
length during the acceleration. At present, theextraction beam current from the booster to thestorage ring is almost the same as that in the 600

MeV operation; typically it takes 3 times longer
injection time to get the same beam current in thestorage ring. In the daily users operation, however,
we re-inject the beam upon the remaining beam in the
full-energy injection operation; that shortens the time
for the injection up to almost the same as that in the
600 MeV operation actually.
Now we are planning to perform ‘top-up’ operation
in UVSOR near the future. The development the
top-up systems including hardware and control
systems are under way. Upgrade of radiation safety tocope with the continuous injection is also planning.

Fig. 1. The operation pattern of power sources forelectromagnets in the booster. 750/600 MeV patterns
are shown.

Fig. 2. A typical operating condition of the booster;
magenta: beam current from AC-CT, yellow: outputof the main power source, cyan: RF voltage frompick-up electrode in the RF cavity. The beam isaccelerated up to 750 MeV at the top of the main
power source output.
[1] M. Katoh,UVSOR Activity Report 2006 (2007) 8.
[2] K. Hayashi et. al., The 21st Jpn. Soc. on Syn. Rad.
Res. Conf. (Kusatsu, 2008) 14P004.

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CSR

Spectrum Measurement of Terahertz Synchrotron Radiation by Laser
Bunch Slicing at UVSOR-II Electron Storage Ring

M. Shimada1, M. Katoh1, S. Kimura1, M. Hosaka2, Y. Takashima2, N. Yamamoto2,
A. Mochihashi3, T. Hara4, T. Takahashi5
1
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan
2
Graduate School of Engineering, Nagoya University, Nagoya 464-8603 Japan
3
Japan Synchrotron Radiation Research Institute, SPring-8, Sayo Hyogo 679-5198 Japan
4
RIKEN SPring-8, Sayo Hyogo 679-5148 Japan
5
Kyoto University Research Reactor Institute, Sen-nan Osaka 590-0494 Japan
Laser bunch slicing is a technique for producing
intense coherent synchrotron radiation (CSR) in
terahertz region by formation of a sub-millimeter dip
on electron bunches. The intensity of CSR isenhanced at the wavelength longer than thelongitudinal size of the dip. We have already beensuccessful in observation of THz CSR signal train
synchronized with the 1kHz laser trigger signal [1].
The intensity of the THz CSR is 104 – 105 times
larger than that of the normal SR and proportional tosquare of the peak current of the electron beam.
The experimental setup is shown in Fig.1. A Ti:Sa
laser pulse with sub-pico sec duration propagatingfrom the upstream of an undulator interacts with anelectron bunch under a resonance condition. Then theelectron changes its energy and its orbit after thebending magnet. This is a process of dip formation.
Fig. 2 shows a schematic of the dip formation withvarious pulse durations. From the numerical results,
you can see that a laser with shorter pulse duration
makes a sharper dip. It means that the high frequency
component of the THz CSR is enhanced with theshorter laser pulse duration.
THz CSR is gathered with large acceptance 215
mrad x 80 mrad at beamline BL6B installing a magic
mirror [2]. A Martin-Puplett interferometer is placedin front of the bolometer for spectrum measurement.
A gated integrator is used for reducing thebackground noise.

shown in Fig.3. We demonstrated that the THz CSR
spectrum depends on the laser pulse duration.
High-frequency component of the THz CSR is more
intense in the case of shorter laser pulse duration.
This result is well explained by the numericalsimulation.

Fig. 2. Numerical simulations of longitudinal phasespace (upper) and electron bunch density distribution(lower) in dip formation process. (a) : Before laserinteraction, (b) : just after laser interaction, (c) :after passing through bending magnets, (d) : same as(c) but with longer laser pulse duration.

Fig. 3. Spectra of THz CSR obtained with various
laser pulse durations, 160 (red), 320 (green), 530(orange), and 710 fs (blue). For comparison, thespectrum of the normal SR obtained with the same
system is also plotted but the intensity is normalized.
[1] M. Shimada et al., UVSOR Activity Report 2006
(2007) 34.
[2] S. Kimura et al., Infrared Phys. Technol. 49(2006) 147.

Fig. 1. Schematic of laser bunch slicing system in
UVSOR-IIThe experimental results of THz CSR spectra are42

Radiation

Radiation Monitoring around Accelerator Using Nuclear Emulsion
Y. Taira, K. Morishima, K. Hamada, T. Nakano
F-lab, Department of Science, Nagoya University, Nagoya 464-8602, Japan
Nuclear Emulsion is a radiation detector, originallyused for cosmic ray experiment, and high energy
elementary particle experiment.
The structure of Nuclear Emulsion is that 45
micron emulsions applied both faces of 200 micron
TAC bases. This type of Nuclear Emulsion is called“OPERA film”.
Tracks of Charged particles in Emulsion read as
3-D information, which consists of position, angle
and depth of tracks. The resolution of track is 0.05
micron for position and 0.3 mrad for angle. And
scanning of tracks is done automatically, which speed
is 5 min/cm2 [1].
To monitor radiations of scattering around UVSORStorage Ring, We set up detectors during operation(Fig. 1: circled red ring). The structure of the detector
is 20 emulsions, 24*24 mm2, stacked andvacuum-sealed in laminated sheet. Emulsions canmeasure angle, position, momentum, flux and particle
ID (electron or gamma ray) of arrived radiations.
The angle distribution of the detector is Fig. 2.
Electrons (include from gamma ray) arrived fromright side to the detector. We checked blue print,
confirmed electrons arrived from quadruple magnet
and bending magnet. The structure of angle
distribution is due to the matter of magnets.
Momentumiscalculated
from
multipleelectromagnetic scattering. Scattering is inversely
proportional to momentum.
From this experiment, Nuclear Emulsion is very
useful to measure leak radiations emitted fromaccelerator during operation.
Radiation monitoring using Nuclear Emulsion has
a great possibility to expand. For example, first thedetector for gamma ray CT. Second, monitor neutron
at Heavy ion accelerators (RIKEN), Nuclear reactors
and heavy particle cancer treatment facilities.

Fig. 2. Angle distribution of electrons that momentum
more than 100 MeV/c.
[1] T. Nakano, Proceedings of International Euro
Physics Conference on High Energy Physics(Budapest, Hungary, 2001) 1218.

Fig. 1. Set up backward from quadruple and bending
magnet.

43