1
Acceleratorsand beamphysics

33

Observation of Intense Terahertz Synchrotron Radiation Produced by
Laser Bunch Slicing at UVSOR-II
M. Shimada1, M. Katoh1,2, A. Mochihashi1, S. Kimura1, T. Hara1*, M. Hosaka2, Y. Takashima2,
T. Takahashi3
1
Institute for Molecular Science, Myodaiji-cho, Okazaki, 444-8585 Japan
2
Nagoya University, Furo-cho, Chikusa-ku, 464-8603 Japan
3
Kyoto University, Kumatori-cho, Sennan-gun, Osaka, 590-0494 Japan
Coherent synchrotron radiation in terahertz region(THz CSR) can be produced from the sub-mm scale
micro-structure on the longitudinal density
distribution of the electron bunches. THz CSR isgenerated by the bunch instability [1] and the laser
bunch slicing, which is a technique to produce themicro-structures on the electron bunches. In the laser
bunch slicing, a femto-second ultra-short laser pulseinteracts with an electron bunch in an undulator andinduces the energy modulation in the bunch. As thebunch transports in the ring, the modulated electrons
are separated from the bunch and a dip is created. We
succeeded to observe high intensity THz CSR fromthe modulated electron bunch.

collected THz CSR is introduced to a liquid He
cooled InSb bolometer, which is sensitive to thewavelength region between 0.2 mm to 3.0mm.
Intensity of the terahertz pulses generated by thelaser bunch slicing is 104 – 105 times higher than thatof the normal synchrotron radiation. The detector
output for the individual pulse of THz CSR is also
shown in Fig.2. The pulse width is a few
micro-second, which is almost the same as the
temporal resolution of the bolometer.
The intensity of the pulse of THz CSR was plotted
as a function of the beam current in Fig. 3. The
intensity is proportional to square of the peak currentof the electron beam which was obtained
experimentally by using a streak camera. This resultclearly indicates that the observed THz radiation iscoherent.

Fig. 2 (Left) terahertz pulses induced by the laser
injection. (Right) detector output signal (negative) for
individual terahertz pulse.

Fig. 1 Schematic of the laser bunch slicing system.
The laser system is composed of a mode-lock Ti:Sa
laser and a regenerative amplifier as illustrated in Fig.
1. The former generates laser pulses synchronizedwith electron bunches and the latter generates intense
ultra-short laser pulses with energy of 2.5 mJ/pulse
and repetition rate of 1 kHz.
The laser pulses are introduced to the ring througha laser transport line conducted to the undulator in thefree electron laser. The laser pulses and the electron
bunches interact in the undulator polarized
horizontally and tuned at the laser wavelength, 800
nm. The streak camera was used for spatial andtemporal alignment between the laser beam and theelectron beam.
The ring was operated in the single bunch mode at
600 MeV, which is suitable to tune the undulator at
the laser wavelength, 800 nm. The THz CSR isobserved at, BL6B, which is connected to the second
bending magnet from the undulator section, and has alarge solid angle, 215 mrad x 80 mrad [2]. The*

Fig. 3 Intensity of THz CSR versus peak current ofelectron bunch. The solid line is a best-fit square
curve.
[1] Y. Takashima et al., Jpn. J. Appl. Phys. 44 (2005)
1131.
[2] S. Kimura et al., AIP Conf. Proc. 705 (2004) 416.

________

Guest associate professor from RIKEN/SPring-8

34

BL5U (FEL)

Storage Ring FEL Exceeding 1 W in the Deep UV Region
M. Hosaka1, M. Katoh1,2, A. Mochihashi1, M. Shimada1, J. Yamazaki1, K. Hayashi1,
Y. Takashima2
1
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan
2
Depertment of Material Processing Engineering, Graduate School of Engineering, Nagoya
University, Chikusa-ku, Nagoya 464-8603 Japanthe 4th power of the electron energy, higher extracted
laser power is expected at 750 MeV. The measured
threshold current for lasing is about 2 times higher
than that in the case of 600 MeV. But a higher laser
power is extracted. The observed maximum power
reached 1.1 W at a beam current of 100 mA/bunch.
During the experiment, drift of the laser power wasobserved especially at a high beam current as shown in
the figure. The power could be recovered by adjusting
the alignment with downstream mirror once again.
Therefore the power drift can be explained bydeformation of the cavity mirror due to heat-load fromsynchrotron radiation and from the FEL. The laser,
however, became almost stable after about one hour
exposure of synchrotron radiation. The FEL around230 nm was applied to two experiments on surface
physics and photo-electron spectroscopy. The FEL
extracted from the upstream mirror was transported
to the experimental stations by using aluminium
mirrors and was focused on samples by lenses. In theexperiments, they started the measurement after thelaser became stable. The FEL power around 0.5 ~ 0.2
W was actually applied. Although the experiments
were made in limited machine time, the users
succeeded in obtaining primary results.

Thanks to a recent upgrade of the UVSOR-II storage
ring (lower beam emittance and higher peak current),an FEL gain has been enhanced much and we havesucceeded in high power lasing in deep UV region. In
2005 we replaced rf accelerating cavity system [1].
The aim is to improve lifetime of the electron beamwith higher accelerating voltage. At the UVSOR, 90.1
MHz rf cavity had been operated with a 20 kW
transmitter but the rf accelerating voltage (55 kV at
maximum) was limited by low shunt impedance ( 1M) of the former cavity. Hence the new rf cavity
was designed to have higher shunt impedance. The
new cavity was installed in the spring of 2005 and thehigh cavity voltage of 150 kV was achieved. This
upgrade is favourable to the FEL because higher
accelerating voltage leads to shorter electron bunchand higher peak current.
The lasing experiment around 230 nm was plannedoriented to users experiment [2]. Multi-layers of
Al2O3/SiO2 were also employed for cavity mirrors.
Measured round-trip reflectivity and transmission
were 98.8 % and 0.8%, respectively. The lasing
experiment was carried out with an electron energy of
750 MeV. Former UVSOR FEL experiment had
been made with an electron energy of 600 MeV.
Recently we raised the electron energy from 600 MeV
to 750 MeV, with which the storage ring is operated for
SR use. According to the Renieri limit [3], theextracted FEL power is proportional to the total
synchrotron radiation power per turn from electron
beam. Since the total radiation power is proportional to[1] A. Mochihashi et al., UVSOR Activity Report
2005 (2006).
[2] T. Nakagawa et al., UVSOR Activity Report 2006
(2007).
[3] A. Renieri, Nuovo Cimento 53 (1979) 160.

Fig. 1 Extracted FEL power and beam current as a function of time. The FEL wavelength is around 230 nm.

35Coherent Harmonic Generation Experiment
M. Labat1 ,G. Lambert 1,2, M.E. Couprie 3, Y. Takashima 5, T. Hara 4, M. Hosaka 5, A.
Mochihashi 6, J. Yamazaki 6, M. Shimada 6, M. Katoh 6
1
CEA Saclay, 91 191 Gif-sur-Yvette France
2
Synchrotron Soleil, Saint Aubin France
3
Laboratoire Aimé Coton, Orsay France
4
RIKEN SPring-8 Harima, Hyogo Japan
5
Nagoya University, Nagoya Japan
6
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan

Coherent Harmonic Generation (CHG) is a seededFree Electron Laser configuration based on storage
ring [1]. A Ti:Sa laser is focused in the first part of the
optical klystron (OK): the modulator, andsynchronized with the circulating electron bunch. The
electron beam is micro bunched at fundamental andsub harmonic wavelengths of the seeded laser. The
light emission of the electrons is then enhanced in thesecond OK part, the radiator, at the third harmonic of
the seed, i.e. 266 nm.
The intensity of the light emitted by a relativistic
electron bunch in an optical klystron can be expressedas the sum of an incoherent and a coherent term. The
coherent term equals zero for randomly distributed
electronic phases. In the case of CHG process, theinteraction between the electric field and the electrons
induces a modulation of the distribution: the intensityof the coherent emission no longer averages to zero.
For CHG experiment, the machine is operated at 600
MeV in single bunch mode. The modulator andradiator of the BL5U OK are identical and separated
by a dispersive section. The detection of the UV lightis performed using a solar blind PhotoMultiplier. In
order to observe the coherent emission at 1 kHz,
among incoherent emission at 5.6 MHz, its signal isobserved on an oscilloscope, triggered by the laser
timing system. A streak camera (Hamamatsu, C5680)
allows following the longitudinal distribution of theelectron bunch. The seeding laser is high repetition
rate (1 kHz), short pulse duration (150 fs to 1 ps)
Ti:Sa with 800 nm wavelength.
The temporal overlap in the modulator for the micro
bunching of the electronic distribution requires a
precise synchronization of the laser pulse and theelectron bunch, using a specific timing system. The
condition of synchronization is observed with thestreak camera, which temporal resolution reaches 10
ps. Spatial overlap is obtained adjusting the laser pathon the electron orbit.
A picture of the PM signal is presented in Figure 1,
illustrating the output radiation of the undulators.
Central peak corresponds to the radiation of the laser
heated electron bunch. The intensity at 266 nm is
dramatically enhanced thanks to the coherent
emission at the third harmonic of the seeding laser.

Fig. 1 Oscilloscope trace of the optical klystronout put radiation. Laser power=1.78 W, I=4.29mA.
Time scale: 40 ns/division.
Figure 2 shows the streak camera image of CHG
signal. This diagnostic confirms the enhancement of
the intensity at 266 nm radiated by the heated electron
bunches (see bright blue spots).

Fig. 2 Photograph of the streak camera image. Fullscales are 85 ms for the horizontal and 700 ps for thevertical axis.
First results confirmed the expected quadratic
dependency of the coherent emission with peakcurrent. An optimum undulator gap for CHG
emission was found at 40.8 mm and the signal
vanishes for gaps below 39.7 and over 42.5 mm.
Using a 1 kHz Ti:Sa laser, a Coherent Harmonic
Generation FEL configuration has been successfully
set up at UVSOR-II facility. Short (below 2 ps), UV(266 nm), coherent laser pulses are delivered.
[1] R. Prazeres, J.M. Ortega, C. Bazin, M. Bergher, M.
Billardon, M.E. Couprie, M. Velgue and Y. Petroff,
Nucl. Inst. Meth. A 272 (1988) 68-72.

36

Variation of Circumference of UVSOR-II
K. Suzumura1, M. Katoh2,1, Y. Takashima1, K. Hayashi2, A. Mochihashi2, J. Yamazaki2, M.
Hosaka1, M. Shimada2
1
Graduate School of Engineering, Nagoya University, Chikusa-ku Nagoya 464-8603 Japan
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan
A drift of the electron orbit in a synchrotron lightsource causes a drift of the light source position,
which makes harmful effects on the users
experiments. Recently, we have developed a RF-FB
system, to suppress orbit drift at UVSOR-II [1]. The
system corrects automatically the drift by controlling
the RF frequency, and its value is recorded during
user operations. The system has been working
successfully for about one year. By using theaccumulated data, we can see the variation of thecircumference of UVSOR-II in various time scales.
UVSOR-II is operated for users from Tuesday to
Friday in a week, and from AM9:00 ~ PM9:00 in aday. Fig.1 shows variations of the RF frequency andof the floor temperature of the storage ring room
during a week. During one day, the RF frequency
decreases with time. The variation amplitudes during
each day are about the same, but the absolute value
decreases gradually throughout a week. On the other
hand, the floor temperature increases with time
during a day, and the absolute value increases
throughout a week. As shown in Fig.1, the RF
frequency shows strong correlation with the floor
temperature. Fig.2 shows the variation of the RF
frequency and the floor temperature during a year.
The RF frequency varies through a year. It is higher
in winter and lower in summer. The floor temperature
also varies through a year. However, there can beseen no clear correlation with the RF frequency.
There can be seen a strong correlation between the
RF frequency and the floor temperature during a day
or during a week. This fact suggests that the variationof the circumference of the storage ring is mainly
caused by thermal expansion of the floor. Because thestorage ring components, such as magnets, are fixed
rigidly to the floor, it is considered that the storage
ring expands outward when the floor expands as its
temperature increases.
We can quantitatively validate the assumption bycomparing measured and theoretical values of the RF
frequency using the measured floor temperature. The
theoretical value can be calculated as following,f RF   f RF    t ,where fRFthe RF frequency,  the thermal expansion
coefficient of the floor, and t the measured floor
temperature variation. Each value is as following,fRF=90.1MHz, =1.0x10-5/oC for concrete andt=0.8oC in a day. As assigning these values to theexpression, the theoretical value of fRF in a day
becomes 0.72kHz. As shown fig.1, the measured
value in a day is about 0.6kHz. The theoretical value

is close to the actual measurement value. From thisresult, there is a high possibility that the main causeof the variation of the circumference is the thermal
expansion of the floor.
It seems that there is no clear correlation between
fRF and t through a year, as shown in Fig. 2.
However, it seems to have good correlation with theoutdoor temperature. This point may be discussed
after several year operation of the RF-FB system.

Fig.1 The variation of RF frequency and floor
temperature of a week.

Fig.2 The variation of RF frequency and floor
temperature of a year.
[1] K. Suzumura et al., UVSOR Activity Report 2005
(2006), 39

37

Development of a BPM Displacement Measurement System at UVSOR-II

K. Suzumura1, M. Katoh2,1, Y. Takashima1, K. Hayashi2, A. Mochihashi2, J. Yamazaki2, M.
Hosaka1, M. Shimada2
1
Graduate School of Engineering, Nagoya University, Chikusa-ku Nagoya 464-8603 Japan
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japanis real. It is similar at the other BPM on the beamducts without water cooling trouble. However, when
we try to stabilize the orbit movement less than 10
micron, we must consider the displacements of the
BPMs. This measurement system will be integrated in
the orbit correction system.

The electron beam orbit of the UVSOR-II storage
ring is measured by 24 BPMs (Beam Position
Monitors), which are integrated on the beam ducts.
Each BPM head consists of 4 electrodes, and detects
the orbit by comparing the induced electric voltages
on the electrodes. The data are used to correct theorbit. However, there is a possibility of measurement
error caused by mechanical displacements of the
BPM heads. We have developed a system to measure
BPM displacement in real time.
The system we have developed is schematicallyshown in Fig.1. It consists of linear gauges, gaugecounter, fixing clamps and PC for data logging. The
read-out resolution of the linear gauges is one micron.
The material of the fixing clamps is aluminum. They
are fixed to iron cores of the bending magnet. With
one fixing clamp, two linear gauges can be set tomeasure horizontal and vertical displacements at once.
The data acquisition system on PC is developed byusing LabVIEW7.0. The data are stored every 5
seconds.
The temperature of the storage ring room changes
with time, because of the heats produced by theaccelerator components, such as electric power
supplies. We expected that the possibility of thermal
deformation of the fixing jigs may cause
measurement errors. We artificially changed thetemperature of a jig using a heater, to see the effect of
the thermal deformation. In the result, the error
caused by the thermal deformation was less than ten
microns for the temperature change in daily operation.
So we disregard it in this research.
We measured BPM displacements during user
operations. Fig.2 shows the BPM displacement andthe orbit position measured at a BPM called No.2
during a user operation. At this BPM, the beam
position shows a large motion of a few hundred
microns. However, the BPM position also show quit
similar motion. These data indicate that the beam
position does not move but the BPM moves. In fact,
the beam duct on which the BPM No.2 is installed
has a trouble on the cooling water channel. The
temperature of the beam duct is changed by theirradiation of SR light, as shown in Fig.3. BPM No.1
is installed on the same beam duct and its positionshows similar motion.
Fig.4 shows a BPM displacement and a beam
position at another BPM, called No.11. The BPM
displacement is much smaller than that of BPM No.2.
But, still we observed some displacement of a few 10
microns. The beam position movement is much larger
than this. Thus, we can say that this orbit movement

Fixing clamp

Fig.1 Schematic drawing of a BPM displacement
measurement system.

Fig.2 BPM displacement and orbit data measured
by BPM during a user operation day at BPM2.

Fig.3
Beam current and beam duct temperatureduring a user operation day at BPM2.

Fig.4 BPM displacement and orbit data measured
by BPM during a user operation day at BPM11.

38

Touschek Lifetime Measurement with a Spurious Bunch in Single-bunch
Operation in UVSOR-II Electron Storage Ring
A. Mochihashi1, M. Katoh1,2, M. Hosaka2, Y. Takashima2, Y. Hori3
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan
2
Graduate School of Engineering, Nagoya University, Chikusa, Nagoya 464-8603 Japan
3
High Energy Accelerator Research Organization (KEK) Oho, Tsukuba 305-0801 Japan1single-bunch impurity N1/N0 the Touschek lifetime of
the main bunch can simply be written as [4]
N 
1
d
(1)
 log 1  .
 T ( N 0 ) dt  N 0 

We have developed a method to measure theTouschek beam lifetime of an electron storage ringusing spurious bunches in single-bunch operation bymeasuring changes in the single-bunch impurity overtime. To measure a spurious bunch and the main bunch
simultaneously, we use a photon counting method with
sufficient dynamic range and response time. We
demonstrate the method by measuring the Touschek
beam lifetime in the UVSOR-II electron storage ring.
We find that the Touschek beam lifetime dominates thetotal beam lifetime in UVSOR-II in the usual vacuum
condition. In SuperACO[1], a method to measure theTouschek lifetime by storing two bunches whose
intensities are different has been developed. The ‘two
unequal bunches method’ is very effective formeasuring the Touschek lifetime if it is assumed that
the Touschek lifetime is inversely proportional to thenumber of electrons in the bunch. The need for theassumption can be avoided by preparing two buncheswith extremely different bunch charges. If theTouschek lifetime of the weaker bunch of the two isvery long compared with that of the more intensebunch, it is possible to measure the Touschek lifetimeof the intense bunch without making any assumption
about the dependence of the Touschek lifetime on thebunch charge. However, it is difficult to make twobunches that have extremely different bunch charges
when the bunch charge of the intense bunch is small.
To make two bunches with extremely different bunchcharges, with the more intense bunch having a small
bunch charge, use can be made of the phenomenon inwhich extremely weak bunches compared with themain bunch can be generated automatically insingle-bunch operation in electron storage rings [2,3].
The spurious bunch charge is extremely small
compared with the main bunch regardless of the size ofthe main bunch charge, and can be generated
automatically. Even though a spurious bunch can growcontinuously during a measurement, it is possible tocancel the effects of the growth and measure theTouschek lifetime of the main bunch precisely using
only the intensity of the main and spurious bunches,
without requiring calculated beam or storage ringparameters. We consider the change with time in thenumber of electrons in a main bunch N0 and that in the
RF bucket immediately following the main bucket N1.
We assume the Touschek lifetime of the spurious
bunch is much longer than that of the main bunch
because of N0>>N1. When the growth of the spurious
bunch in one Touschek lifetime period of the mainbunch T(N0) can be neglected compared with the

If the assumption for the growth rate of the spurious
bunch is not valid, it is still possible to measure theTouschek lifetime by repeating the measurement witha different spurious bunch charge with the same mainbunch charge. For spurious bunch charges N1 and N1*(N1>N1*) and main bunch charge N0, T(N0) can also
be written simply as N  N1* d1.
 log 1(2)
 T ( N 0 ) dt  N 0 
By considering the single-bunch impurity value in
UVSOR-II, we adopted Eq.(2) to measure theTouschek lifetime in UVSOR-II. Figure 1 shows
measured decay-rate and the Touschek beam lifetimein normal and poor vacuum conditions. To make thepoor vacuum condition we turned off all of the ion
pumps and heated a part of the beam ducts of thestorage ring. In the figure, the decay-rate lifetime
decreased in the poor vacuum condition but the
Touschek lifetime still kept the same value as thenormal vacuum condition. The vacuum pressure
estimated from the beam lifetime agreed with theresult from the vacuum gauge measurement.

Fig.1 Measured decay-rate and Touschek lifetime innormal/poor vacuum condition in single-bunch
operation.
[1] J. C. Besson et al., Commissioning of SuperACO,
ORSAY Report RT-88-01,(1988) 34.
[2]T. Kasuga et al., Jpn. J. Appl. Phys. 28(1989) 541.
[3]T. Obina et al., Nucl. Instrum. Methods Phys. Res.
A 354 (1995) 204.
[4]A. Mochihashi et al., Nucl. Instrum. Methods Phys.
Res. A 572 (2007) 1033.

39