Coherent Terahertz Radiation at UVSOR-II
Masahiro Katoh1, Masahito Hosaka1, Akira Mochihashi1, Miho Shimada1,Shin-ichi Kimura1, Yoshifumi Takashima2, Toshiharu Takahashi3
1

UVSOR, Institute for Molecular Science, Myodaij-cho, Okazaki, 444-8585 Japan,Graduate School of Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8603 Japan
3
Research Reactor Institute, Kyoto University, Kumatori-cho, Sennan-gun, Osaka, 590-0494 Japan

2Abstract. Development of intense terahertz radiation source is progressing at UVSOR-II, based on the mechanism ofcoherent synchrotron radiation (CSR). The terahertz CSR has successfully been produced by two methods. When thestorage ring is operated in the single bunch mode with a sufficiently high beam current, intense bursts of terahertz
radiation are emitted. Micro-structures in the longitudinal density distribution of the electron bunches created by a beam
instability may be the origin of the radiation. The duration of the bursts is typically 100 micro-seconds. The peak intensity
is 10000 times higher than that of the normal synchrotron radiation. The bursts appear chaotically or quasi-periodicallydepending on the beam current with a typical interval of 10 milli-seconds. It has been also demonstrated that the terahertz
CSR could be produced by the laser-bunch slicing method. The density modulation produced on the electron bunch by
the laser is the origin of CSR. The repetition rate of the terahertz pulses is 1 kHz, which is same as the laser repetition rate.
The intensity per pulse is 105 times higher than that of the normal SR.
Keywords: coherent synchrotron radiation, terahertz, laser, storage ring, instability
PACS: 07.85.QeINTRODUCTION
Coherent synchrotron radiation (CSR) in terahertz region can be produced by using electron bunches shorter than
the radiation wavelength. The first observation was carried out by using short electron pulses provided by a linear
accelerator [1]. Since then, terahertz CSR has been produced on many linear accelerators. On the other hand,because the length of the electron bunches circulating in the electron storage ring is typically a few cm or longer, it
had been believed until recently that the coherent synchrotron radiation could not be observed at storage rings in theterahertz region. However, in these years, terahertz CSR has been observed in many storage rings [2]. In some cases,
the rings are operated under special condition called low momentum compaction mode, to make the electron pulse
length as short as the radiation wavelength. In other cases, CSR is emitted from the electron pulses much longer than
the radiation wavelength. The origin of the CSR in these cases is micro-structure on the longitudinal densitydistribution of the electron pulse which is produced either artificially or spontaneously.
A 750 MeV synchrotron light source, UVSOR-II, is equipped with an infrared beam-line, which is the world
most powerful especially in far-infrared region [3]. In 2004, we started CSR experiments, aiming to provide intense
terahertz radiation to users through this beam-line. Soon after, we succeeded in detecting intense bursts of terahertz
radiation in the single bunch operation with very high beam current [4]. In 2005, we installed an ultra-short pulselaser which could be synchronized with the electron pulses. By using this laser, we tried a laser bunch slicing
experiment and have successfully detected terahertz CSR produced as a result of the interaction between the laser
and the electron beam [5]. In this paper, we report the recent progress in the coherent terahertz source development
at UVSOR-II.

CREDIT LINETERAHERTZ(BELOW) TO BURSTSBE INSERTEDON THEFIRSTOPERATIONPAGE OF EACH PAPER
IN SINGLE
BUNCH
EXCEPT THOSE ON PP. 139 - 142, 481 - 484, 1195 - 1197, 1206 - 1209, 1531 - 1534,
When1793the UVSOR-IIand1796 storage ring is operated in the single bunch mode with a high beam current above a certain
threshold value, which is typically 100 mA, intense bursts of terahertz radiation are emitted [4]. Terahertz radiation

CP879, Synchrotron Radiation Instrumentation: Ninth International Conference,edited by Jae-Young Choi and Seungyu Rah© 2007 American Institute of Physics 978-0-7354-0373-4/07/$23.00

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CREDIT LINE (BELOW) TO BE INSERTED ON THE FIRST PAGE OF EACH PAPERis observed at an infrared beam line BL6B by using a liquid-helium cooled InSb bolometer, which is sensitive to thewave length range from 0.2 to 3mm. The response time of the detector is several micro-second. Thanks to this fast
response, we can observe the terahertz bursts with fine temporal resolution. During this experiment, the storage ringis operated with the electron energy of 600 MeV, which is the injection energy. This is to accumulate electrons as
many as possible. The parameters of the UVSOR-II in this operating condition are summarized in Table 1.
In Fig. 2, a few examples of the terahertz bursts are shown. The duration of the bursts is typically 100 microseconds. The peak intensity is 10000 times higher than the normal synchrotron radiation observed in the same
wavelength region. This extremely high intensity strongly suggests that the bursts are CSR, although the electron
pulse length is much longer than the radiation wavelength. Micro-structure created on the electron pulse isconsidered to be the origin of the bursts. The existence of the threshold current suggests that such structure is createdby a beam instability. The bursts appear chaotically or quasi-periodically, depending on the beam current with atypical interval of 10 milli-seconds. In Fig. 2, temporal structure of individual burst is shown. There can be seen
another periodicity of a few tens of micro-seconds, which may be related to the longitudinal bunch motion [4].
(V rf= 5 5 k V

Ib = 2 0 6 m A )10 .50-0 .500 .0 50 .1
0 .1 5
T im e (s e c )(V rf= 5 5 k V

1 .5
Detector Output (a.u.)

Detector Output (a.u.)

1 .5

10 .5
0-0 .50 .2

Ib = 1 7 8 m A )00 .0 50 .1
0 .1 5
T im e (s e c )0 .2

FIGURE 1. Typical temporal structure of the terahertz bursts observed at higher beam current (left) and lower (right). Terahertz
component of the normal synchrotron radiation is continuously emitted between the bursts but in the background level.

Detector Output (a.u.)

1 .5(V rf= 5 5 k V

Ib = 2 0 1 m A )10 .50-0 .50 .0 0 9 80 .0 10 .0 1 0 2
T im e (s e c )0 .0 1 0 4

FIGURE 2. Typical fine temporal structure of a burst.
TABLE 1. Main Parameters of UVSOR-II during CSR experiments
Parameters
ValuesElectron Energy
600 MeV
Circumference
53.2 mNatural Emittance
17.4 nm-rad
Natural Energy Spread
3.4 x 10-4
RF Frequency
90 MHzNatural Bunch Length
3.1 cmSynchrotron Frequency
14.4 kHzLongitudinal Damping Time
19 msecBending Radius
2.2 m

TERAHERTZ RADIATION BY LASER BUNCH SLICINGThe very high intensity of the bursts described above is very attractive for some applications. However, theoccurrence of the bursts can not be predicted even when they appear quasi-periodically. To control the terahertz
radiation, we have tried the laser bunch-slicing technique [6], in which a part of electron bunch is sliced out as a72
Downloaded 22 Feb 2007 to 133.48.165.19. Redistribution subject to AIP license or copyright, see http://proceedings.aip.org/proceedings/cpcr.jspresult of the interaction between the laser and the electron beam. If we use a sub-pico-second laser pulses, we cancreate micro density structure of sub-millimeter scale on the electron pulse. This structure would be the origin of thecoherent terahertz radiation.
This technology can be realized by using an undulator whose fundamental wavelength can be tuned to the laser
wavelength and an external short pulse laser synchronized with the RF acceleration of the storage ring. In 2005, aTiSa laser, which can be synchronized with the RF acceleration of UVSOR-II, was installed. The UVSOR-II storage
ring is equipped with a free electron laser (FEL) [7]. The undulator used for FEL can be tuned to 800 nm, thewavelength of the TiSa laser. Basic parameters of the laser and the undulator are summarized in Table 2 and 3. It isestimated that the laser intensity is sufficiently high to produce energy modulation whose amplitude is comparable tothe momentum acceptance of the ring [5].
The experimental setup is schematically shown in Fig. 3. The light ports for the optical cavity of FEL can be usedto introduce the TiSa laser pulses to the ring. The laser pulses interact with electron bunches in the undulator. As theresult of the energy exchange between laser field and electrons, a part of the electron bunch suffers a large energy
modulation. As the bunch is proceeding in the ring, a dip, which is as short as the laser pulse width, is created on thebunch. This micro-structure is the origin of CSR, which is observed at an infrared beam-line located downstream of
the undulator section.
TABLE 2. Main Parameters of TiSa Laser
ParametersValues
Wave Length
800 nmPulse Energy
2.5 mJ
Repetition Rate
1 kHz
Pulse Width
130 fsec to ~1psec
TABLE 3. Main Parameters of Undulator during the CSR experimentParameters
ValuesNumber of Periods
21Period Length
110 mmPole Length
2.35 mPolarization
LinearDeflection Parameter
6.18

FIGURE 3. Schematic drawing of the laser bunch slicing experiment at UVSOR-II73
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FIGURE 4. Terahertz pulses produced by the laser bunch slicing (left) and the temporal structure of individual pulse.

We have observed intense terahertz pulses which were synchronized with the laser injection as shown in Fig. 4.
The measurements were done for a beam current much lower than the threshold beam current of the spontaneous
terahertz bursts described in the previous section. The intensity per pulse is 105 times higher than that of the normal
synchrotron radiation. The time response of the detector is not fast enough to resolve the temporal structure ofindividual pulse. The intensity of the terahertz radiation was measured versus beam current. The result showed thatthe intensity is proportional to square of the peak current of the electron beam. This clearly indicates that theobserved radiation is CSR. More details of the experimental results will be described in a separated paper [5].

SUMMARY AND CONCLUSION
Intense terahertz radiation source based on the coherent synchrotron radiation mechanism is successfully
progressing at UVSOR-II. We have succeeded in producing intense terahertz radiation by two methods. Intensebursts of the terahertz radiation can be produced just by operating the ring in single bunch mode with a high beam
current above a certain threshold value. Intense terahertz pulses could also be produced by the laser bunch slicing
method. In both cases, coherent synchrotron radiation is produced by electron bunches much longer than theradiation wavelength. Micro structures on the longitudinal density distribution are the origin of coherent radiation.
The peak intensity is extremely high as compared with that of normal synchrotron radiation. Next step is to increase
average intensity, which is currently limited by the bursting interval or the laser repetition rate.

ACKNOWLEDGMENTS
The authors wish to give thanks to the engineers of the UVSOR facility for their helps during the experiments. A
part of this work was supported by Grant-in-Aid for Scientific Research (B15360039) of Japan Society for thePromotion of Sciences

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