Light Sources

Observation of Transverse-Longitudinal Coupling Effect at UVSOR-II

M. Shimada1, M. Katoh2, 3, M. Adachi2, 3, T. Tanikawa3, S. Kimura2, 3,
M. Hosaka4, N. Yamamoto4, Y. Takashima4 and T. Takahashi51High Energy Accelerator Research Organization, KEK, Tsukuba 305-0801, Japan
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan3
School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI),
Okazaki 444-8585, Japan
4
Graduate School for Engineering, Nagoya University, Nagoya 464-8603, Japan
5
Research Reactor Institute, Kyoto University, Kumatori-cho 590-0494, Japan
Coherent synchrotron radiation (CSR) emits fromthe longitudinal dip-structure of radiation wavelengthscale. We have demonstrated the measurements of
CSR by the technique called ‘laser bunch slicing’since 2005 [1] and succeeded in emission of thequasi-monochromatic CSR from the bending magnet
using the amplitude-modulated laser pulse [2]. In thispaper, the turn-by-turn CSR signal is observed by the
Schottky diode detector for terahertz range with atime response of a few hundred pico-second [3]. The
detector is prepared with three different frequency
range. The UVSOR-II electron storage ring was
operated with three electron beam optics, the normal
optics, and the two low-alpha optics with different
betatron tunes of 3.53 and 3.68. Since the tune isclose to the half and third resonances, we refer to thelatter two as low-alpha (1/2) and low-alpha (1/3)
optics.
Figure 1 shows the CSR signals in two low-alphaoptics. With the low alpha (1/2) optics, as shown in
Fig.1 (a) - (c), the CSR is intense at every other turns.
In the middle and high-frequency ranges, it is intenseat the first and third arrivals. At the low-frequency
range, it could be observed up to the 11th arrival.
Although the CSR at the third and fifth arrivals is
intense, that of the first arrival is weak.
In the low-alpha (1/2) optics, the betatron tune isclose to a half integer. Thus, the intermittent CSRemission was expected to be related to the betatronmotion. To confirm this, we tried the same
experiment with the low-alpha (1/3) optics. Becauseof the limited beam time, CSR was observed only at
the middle frequency range. The result is shown in
Fig. 1 (d). The CSR is intense at every third arrival,
the first and fourth. Weak CSR is also observed at theseventh arrival, despite the absence of CSR at the
fifth and sixth.
For normal optics, although the results are notshown here, the CSR at the first arrival was detectedby all three detectors.
We have observed the transverse-longitudinal
coupling effect in laser bunch slicing by measuring
the THz CSR turn by turn with ultra-fast diodedetectors. The results were in good agreement with
the numerical simulation based on linear beam
dynamics theory.

Fig. 1. CSR signals of three diode detectors at the
low-alpha optics. Revolution time of the ring is 177
ns. (a), (b), and (c) are with the low-alpha (1/2) optics.
Sensitivity ranges are (a) 11.0 - 16.6 cm-1, (b) 7.3 11.0 cm-1, and (c) 3.7 - 5.7 cm-1. (d) is with low-alpha(1/3) optics. Sensitivity range is 7.3 - 11.0 cm-1.

Fig. 2. An example of evolution of fragments and a
dip structure in longitudinal phase space (upper) andits corresponding longitudinal density distribution(lower). The horizontal axis is an arrival order after
the revolutions[1] M. Shimada et al., Jpn. J. Appl. Phys. 46 (2007)
7939.
[2] S.Bielawski et al., Nat. Phys. 4 (2008) 390.
[3] M.Shimada et al. Phys. Rev. Lett. 103 (2009)
144802.

31

Accelerators

Measurement of Betatron Oscillation Amplitude Excited by RF Knockout
at UVSOR-II

Y. Furui1, 2, M. Hosaka1, N. Yamamoto1, Y. Takashima1, M. Adachi2, H. Zen2 and M. Katoh2
1
Graduate School of Engineering, Nagoya University, Nagoya,464-8603 Japan
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan
this is due to the non-linear effect of the sextupole
magnetic fields. The measurement agrees well withthe simulation results when the sextupole effect isincluded. This result indicates that it is essential toconsider the non-linear effects of the sextupole
magnets in the design of an RF knockout system.

Abstract
In order to measure the betatron tune, we need tostimulate betatron oscillations. We measured
betatoron oscillation amplitudes stimulated by an RF
knockout, by using a turn-by-turn beam position
measuring technique. The results were compared withsimulations. It was found that the non-linear effect of
sextupole magnets produces a saturation effect on the
betatron oscillation amplitude.
Experimental and Calculation
The electron beam circulating in the UVSOR-II
storage ring was stimulated by the RF knockout
system. The signals from a beam position monitor
were processed by a high-speed digital oscilloscope(5GS/s). The beam position was measured turn byturn.
We simulated the betatron oscillation by transfer
matrix calculation. This simulation includes theradiation damping, the kicks by the RF knockout, andthe non-linear effect of sextupole magnets.
The kick angle of the RF knockout is based on thesimulation using electromagnetic field analysis
software, POISSON. Figure 1 shows the cross sectionof the RF knockout chamber at UVSOR-II and themagnetic field lines. In this case, magnetic field isgenerated for stimulating horizontal betatronoscillation.
The effect of sextupole magnets was included asusing the following expression:

x'n1

Fig. 1. Form of RF knockout chamber at UVSOR-II
and magnetic field lines (magenta lines).

x'n  kxn2
Fig. 2. Simulated horizontal beam position from
center of BPM chamber every turn.

where xn is beam position at n-th turn, x'n is beam
angle at n-th turn, and k is a constant proportional to
the sextupole field strengths which were determined
to make the chromaticity zero.
Results and Discussion
Figure 2 shows simulated beam position at thebeam position monitor turn by turn. After the betatronoscillation is excited very quickly within 7104 turns,
the amplitude is decreasing for the following 106
turns owing to the radiation damping andamplitude-depended tune shift produced by sextupole
magnets. Then the amplitude becomes constant.
Figure 3 shows the measured and calculated
horizontal betatron oscillation amplitude versus theelectric power fed to the RF knockout. As the electric
power is increasing, the amplitude is also increasing.
However, the amplitude tends to saturates. We think

Fig. 3. Measured and calculated horizontal betatronoscillation amplitude to power.

32

Accelerators

Distribution of Injection Signal for Synchrotron Radiation Beam Lines
K. Hayashi, M. Katoh, M. Adachi, H. Zen and J. Yamazaki
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, JapanAt UVSOR, so-called top-up operation was started
in October 2008 [1]. The status of top-up operation at
UVSOR is described elsewhere in this report. The test
top-up operation has been held every Thursday night.
It was revealed that user experiments were affectedby beam injection at some beamlines. Especially, theeffect was critical at BL6B, with FT-IR
spectrometers.
Currently, the injection bump orbit is not closed innormal injection condition, because of theinsufficiency of the current sources for the bumpmagnets. As consequence of bump leakage, largetransverse oscillation of the beam occurs at all
position of the storage ring.
Sufficient suppression of the oscillation is costly
and need time. So we decided to distribute the signalsthat indicate whether beam injection is under way ornot. Beamline users can neglect or stop acquiring dataso as to get rid of the effect of beam injection. The
signal is simple DC high/low (5V/0V) voltage,
generated at the injection control system anddistributed through a distribution board. The board isshown in Fig. 1.
At present, beam injection is held every minute,being continued at 1Hz until the beam current exceed
the desired beam current, usually 300 mA for
multi-bunch mode. We prepared for three types ofsignals: “short”, “long” and “long with prior notice”.
They are described in Fig. 2. The “long” signal
includes the whole period including all injection shots.
The “Long with prior notice” signal is similar to“long” signal, but it goes high (+5V) several seconds
before injection starts. On the other hand, the “short”
signal rise up and go down at every shot, with
controllable width.
The first application of the injection signal was done
at BL3U [2]. The result is shown in Fig. 3. The
spectrum has some spike without utilizing signals. The
spikes on the spectrum disappear employing the“long” signal, stopping the data acquisition while
beam injection is under way.
We have distributed the beam injection signal todesired beamlines. At BL6B, one of the FT-IR
spectrometers has upgraded to utilize the beam
injection signal [3]. The top-up operation is going tobe the standard operation mode at UVSOR in FY2010.

Fig. 1. Distribution board for injection signal.

Fig. 2. Scheme of beam injection signals.

[1] K Hayashi et al., UVSOR Activity Report 2008(2009) 35.
[2] M. Nagasaka, private communication.
[3] S. Kimura, private communication.

Fig. 3. Result of first application of beam injection
signal (O K-edge X-ray absorption spectrum for
liquid water).

33

Accelerators

Development of Turn-by-Turn BPM System at UVSOR-II

A. Nagatani1, Y. Takashima1, M. Hosaka1, N. Yamamoto1, K. Takami1,
M. Katoh2, 1, M. Adachi2, H. Zen2 and K. Hayashi2
1
Graduate School of Engineering, Nagoya University, Chikusa-ku Nagoya 464-8603, Japan
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japanof tens of microns in both horizontal and verticaldirection is sufficient for TBT measurement. A bumporbit of a stored beam and an injected beam orbit
were measured for the first time at UVSOR-II and theeffectiveness of the system was demonstrated. The
TBT system will be used for reducing the injection
effect for user experiments.

Abstract
Recently, top up operation that keeps beam current
constant began in the UVSOR storage ring. This
operation temporally affects the beam orbit during
beam injection. Therefore, a system that can measure
the temporal orbit change was required to improve
the operation. We have developed a TBT (Turn ByTurn) BPM system and succeeded in measuring abump orbit of a stored beam and an injection beam
orbit by using this system for the first time at
UVSOR-II.
TBT BPM system
The TBT BPM system consists of a high sampling
rate digital oscilloscope (5 GHz/s), a high frequency
amplifier and a waveform processing circuit between
a BPM electrode and the oscilloscope. The direct
signal from the electrode was found to be too fast tomeasure the signal peak voltage precisely under alimited sampling rate of the oscilloscope. Therefore
we have developed a simple waveform processing
circuit to make the pulse duration longer and broaden
the signal peak. The diagram is shown in Fig. 1. We
estimated the relative accuracy of the system via arepetitive measurement of a stable stored beam orbit.
Figure 2 shows histograms of distribution of
measured deviation from the averaged beam position.
Standard deviations of 10 Pm in horizontal and 50
Pm in vertical have been obtained.

Fig. 1. Diagram of waveform processing circuit.
250

200

150

Counts /binCounts/bin

200
150
100

50

50
0

100

-40

-20

0

X (Pm)

20

0
-200

-100

0

100

Y (Pm)

Fig. 2. Accuracy of this system (left shows horizontal
right shows vertical).

Measurements
We measured a bump orbit of the stored beam andan injection beam orbit using the TBT system. For themeasurement, we used B1U BPM and set theoscilloscope low pass filter at 200 MHz.
Figure 3 shows the horizontal bump orbit measured
using the system and a calculated one. The maximum
value of the amplitude was 18 mm at the BPM. As
shown in the figure, measured orbit agreed very wellwith the numerical result.
Figure 4 shows an injection beam orbit. The initial
amplitude of the betatron oscillation was 20.1 mm.
The oscillation was damped with a time scale ofmilliseconds. A vibration with a periodicity of about
20 kHz is accompanied with it.

Fig. 3. Measured bump orbit compared withnumerical one.

Conclusion
We have developed a TBT BPM system which wascomposed of commercial products like the highfrequency amplifier, the wave processing circuit andthe digital oscilloscope. We confirmed theperformance of this system. The measuring accuracy

Fig. 4. Injection beam orbit measured by TBT system.

34

Accelerators

Stabilization of the Electric Septum of UVSOR-II Injector

H. Zen, K. Hayashi, J. Yamazaki, M. Adachi and M. Katoh
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japanalgorithm by a control client PC. The PID
determination of the set value is done by a program
developed on LabView (shown in Fig. 3). The set
value is sent to the power supply via a server PC,
multi-control unit (MCU) and CAMAC modules.

Introduction
Top-up test runs was started in 2008. As the resultsof the test runs, the stored beam current was kept
almost constant at 300 mA (see Fig. 1). The electron
beam is injected for about 10 second every one
minute with the repetition rate, 1 Hz. As shown in Fig.
1, sometimes stored beam current gets smaller than
300 mA even with top-up injection. The beam current
reduction is due to reduction of injected beam charge
caused by reduction of accelerated beam charge in the
UVSOR-II injector. For keeping the constant beam
current, operational condition of the injector should
be stabilized. From long operational experience, wehave recognized that the voltage fluctuation of theelectric septum equipped in the injector is the mostsignificant source of the fluctuation of the accelerated
charge. In this fiscal year, we introduced a computer
based control system to stabilize the septum voltage.

Fig. 3. The program for determining the set value ofthe electric septum used for the UVSOR-II injector.

Stored Beam Current [mA]

320

Results
Demonstration experiment to check the effect ofthe developed system was carried out and the result isshown in Fig. 4. With the system, the septum voltage
was kept constant and the accelerated charge in theinjector is not so much fluctuated. While thestabilization system was turned off, the septum
voltage gradually increased and the accelerated
charge rapidly decreased. It was found that thestabilization system was really effective tocompensate the fluctuation of septum voltage. The
stabilization system is now usually used during user
operation. And then we found another source offluctuation, fluctuation of klystron output power
which is used for driving a 15 MeV linac in theinjector. The klystron fluctuation will be stabilizednear future.

300

280

260

22

24

26

28

Time [hour]3032Fig. 1. Time trend of the stored beam current in the
UVSOR-II storage ring.

Klystron
Septum
Accelerated
Output [a.u.] Voltage [a.u.] Charge [nC]

Computer Based Stabilization System
Schematic drawing of the computer basedstabilization loop is shown in Fig. 2. The pulsed
voltage waveforms of the electric septum aremeasured by an oscilloscope and the waveform data
are sent to the control PC. From the waveform and its
time trend, the set value of a high voltage power
supply for the septum is determined based on PID
Electric Septum
Electron Beam
Voltage
MonitorVoltage Signal
High
Voltage
PulserWaveform

Set ValueHigh Voltage
Power Supply

CAMAC

MCU

Control
Client PC

6
4
2
0
2.00
1.95

W/o control
1.90
0.29
0.28
0.27
15:35

15:40

15:45

15:50

15:55

16:00

16:05

Time [hh:mm]

ControlServer PC

Fig. 4. Time trend of the accelerated charge in the
UVSOR-II injector (top), peak voltage of the electric
septum (middle) and output power for the accelerator
tube (bottom) with and without the stabilization.

Fig. 2. Schematic drawing of the computer basedstabilization loop.

35

Accelerators

Single-Bunch Top-Up Operation and Single-bunch Injection at UVSOR-II
H. Zen, K. Hayashi, M. Adachi, J. Yamazaki and M. Katoh
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan

CM3

QF

Beam Current (DC Gun) [a.u.]

-0.02

6 ns

@ Exit of
DC Gun

-0.3

-0.03

-0.4

-0.04

@ Exit of Acc. Tube

-0.5
-20

-10

0

10

20

30

-0.05
40

3

Counts / 300 sec

2

10

1

10

0

10

-4

-3

-2 -1 01
2
3
Relative Bucket Number

4

Fig. 3. Result of test injection.
Single-Bunch Top-up Operation
Single-bunch top-up operation was done during
single-bunch user operation. The beam current was
kept at 53 mA for 12 hours as shown in Fig. 4. Single
bunch users were really satisfied with constant andhigh beam current.

Kicker3

0.3

60

ZS
To Main
Ring

QF
QD

QD QF

50
0.2

40
30

0.1

20
10
0

09

12

15

18

21

0.0

Time [hour]Fig. 4. Time trend of the stored beam current andinjection rate during the single-bunch top-up
operation in 16 Sep. 2009.

CM2
Solenoid Coil

Accelerator
Tube
CM1

-0.2

10

Main Cavity

Septum

QD

-0.01
5 ns

Injection Rate [mA/sec]

Kicker1

EnergyAnalyser-0.1

Fig. 2. Short bunch train generated by the Linac.

Stored Beam Current [mA]

QF
QD

Kicker2

0.00

SeptumFast Kicker

QD
QF

0.01

0.0

Time [ns]SF QF QD
QD
QF

0.1

Beam Current (Acc Tube) [a.u.]

For stable injection of the electron to a certain
bucket, firing timing of the gun is generated from 90
MHz RF signal and 1 Hz trigger signal. Test injection
and purity measurement was done. The result isshown in Fig. 3. It was confirmed that the bunch
purity more than 500 was achieved by only thesingle-bunch injection.

Introduction
The Single-Bunch (SB) top-up operation isstrongly required at UVSOR-II, because of quite
short beam lifetime with the operational mode. For
the usual SB operation, we used to inject fourbunches and undesired three bunches were eliminatedby an RF Knock Out (RF-KO) [1]. With that injection
scheme, electron loss in the storage ring is large andradiation is high. High radiation environment is notgood for top-up operation. The best way to reduce theradiation is inject single electron bunch into thestorage ring i.e. no bunch elimination in the storage
ring. In fiscal year 2009, we achieved single-bunch
injection and single-bunch top-up operation by only
modifying electron gun and trigger system for thegun.
Single-Bunch Injection
A schematic drawing of the UVSOR-II injector isshown in the Fig. 1. For multi-bunch injection,
electron bunch train with the energy of 15 MeV andthe macro-pulse duration of 1.4 Ps is generated by the
70 keV DC Gun and Accelerator Tube. A part ofbunch train is injected to the booster synchrotron andaccelerated up to 750 MeV.
Since the frequency of the acceleration cavity ofbooster synchrotron is about 90 MHz, i.e. the bucket
length is about 11 ns, single-bunch circulation andacceleration in the booster synchrotron could beaccomplished if we could generate short pulse train(pulse duration of 5 ns) at the Linac. The DC Gun has
already equipped a grid-pulser for short pulse
generation. The electron bunch waveform which is
generated by the short pulse grid pulser is shown in
Fig. 2. We succeeded to generate 5 ns pulse trains.

70 keV DC Gun[1] A. Mochihashi et al., UVSOR Activity Report 30(2003) 39.

Fig. 1. Schematic drawing of UVSOR-II injector.

36

37

Light Sources

Feasibility Study of Ultra-Short Gamma Ray Pulse Generation by Laser
Compton Scattering in an Electron Storage Ring

Y. Taira1, 2, M. Adachi2, 3, H. Zen2, 3, T. Tanikawa3, M. Hosaka1, Y. Takashima1, N. Yamamoto1,
K. Soda1 and M. Katoh2, 3, 1
1
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
2
UVSOR Facility, Institute for Molecular SciencG, Okazaki 444-8585, Japan3
School of Physical Sciences, The Graduate University for Advanced Studies, Okazaki444-8585, JapanThe collision of laser photons with free relativistic
electrons results in laser Compton scattered (LCS [1])gamma rays. The scattered gamma rays are intense,
quasi-monochromatic, tunable in energy, narrow inangular spread, and highly polarized. They are usefulfor applications such as nuclear physics, generation ofpolarized positrons, nondestructive inspection, andelectron beam diagnosis. We aim to generate
ultra-short gamma ray pulses with sub-picosecond
width through laser Compton scattering technology,
and explore the applications. The size of an electron
bunch circulating in a storage ring is a few
centimeters in the longitudinal direction, a few
hundreds of microns in the horizontal direction, and a
few tens of microns in the vertical direction.
Therefore, the ultra-short gamma ray pulses may begenerated by injecting femtosecond laser pulses fromthe perpendicular direction into the electron beams,
because the interaction time between the electron
beams and the laser will be shortened.
We carried out the head-on collision experimentbecause it was much easier to realize the collision
between the laser and the electron bunch. For the
90-degree collision, it is necessary to adjust thetiming in the picosecond range. An existing opticalport for FEL at was used for the head-on collisionexperiment. The laser and detector system used in thisexperiment can be applied to the 90-degree collisionexperiment. The pulse width, the pulse energy, andfrequency of the laser were 55 fs (rms), 2.0 mJ, and 1
kHz, respectively. The laser was 10 mm in diameter,
which made aligning the laser beam and the electron
beam easy. The power of the laser at the collision
point was estimated at 0.25 W from the attenuation at
the mirrors. The LCS gamma rays are detected by a
NaI scintillator. In front of the detector, a collimatoris placed to restrict the angular acceptance. The
collimator is a lead block 10 cm thick with a hole 5
mm in diameter. For energy calibration of the detector,
137
Cs and 60Co are used. A gate signal synchronizedwith the laser injection is sent to the MCA. The noisefrom bremsstrahlung gamma rays is reduced to 1/20.
By adjusting the laser timing, the laser and electron
can be collided at an arbitrary place in the straightsection. We set the collision point 7.6 m from thedetector. The beam current was around 1 mA, much
lower than the normal operating condition, to avoid

pile-up in the detector. The maximum energy andintensity of the LCS gamma rays under theexperimental conditions are 13.1 MeV and 460
photons s-1 mA-1, respectively.
Figure 1 shows the measured spectra. We compared
the measured and calculated gamma ray spectra. The
response of the scintillator detector was calculated by
the EGS5 code [2]. Curves in Fig. 1 show thecalculated spectra. The measured spectral shapes agree
well with the calculation when the intensity of thecalculated spectra is multiplied by 0.73, while thespectral shape measured with the collimator does notagree well with the calculation. This disagreement ispresumably due to the misalignment of the collimator.
If the collimator is assumed to be shifted by 1.7 mm in
the horizontal direction, the measured spectrum agrees
well with the calculation.
In the near future, we will carry out a 90-degreecollision experiment using another view port forvertical injection.

Fig. 1. Measured spectra of the LCS gamma rays.
Red points are measured without collimator; blue ones
are with collimator. Error bars are 1 of statisticalerror.
[1] J. Stepanek, Nucl. Instr. and Meth. A 412 (1998)
174.
[2] H. Hirayama et al., SLAC-R-730 (2005).

38

Light Sources

Top-Up Operation of the UVSOR-II Free Electron Laser

H. Zen1, M. Adachi1, T. Tanikawa1, 3, N. Yamamoto2, J. Yamazaki1, M. Hosaka2,
Y. Takashima2 and M. Katoh1
1
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
2
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
3
The Graduate University for Advanced Studies, Okazaki 444-8585, Japan
Introduction
At the UVSOR-II, a free electron laser (FEL) has
been developed. The FEL has various good features
such as intense, monochromatic, short pulse duration,
variable polarization and wavelength tunability.
However, the long term stability of the FEL is reallypoor because of the time varying nature of stored
electron beam current. There are two main effects ofthe time varying beam current. One is the variation ofheat load on the resonator mirror and resulting
deformation of the mirror. This effect is quite rapid asshown in Fig. 1. To compensate the deformation,
frequent mirror adjustments are required formaintaining the laser power. The other is the variationof the laser gain. As shown in Fig. 2, the laser power
gradually decreased as the beam current decreased.
Keeping the stored beam current by top-up
operation of the storage ring is a solution for those
problems. In this fiscal year, we demonstrated FEL
lasing during top-up operation of the storage ring.

Demonstration of FEL Top-Up Operation
The FEL lasing during top-up operation wasdemonstrated. The main operational parameters arelisted in Table 1. As the result of top-up operation, we
succeeded in keeping the stored beam current andFEL power as shown in Fig. 3. The FEL power was
kept around 110 mW more than one and half hour.
During this FEL top-up demonstration, VUVirradiation experiments were carried out by usergroup of Yokohama National University.
Table 1. Main operational parameters
Electron
Beam Energy
750 MeVBeam
Beam Current
130 mA/2-bunch
Emittance
27 nm-rad
Energy Spread
4.2 × 10-4
Bunch Length
108 ps
OpticalPeriod Length
110 mmKlystronNum. of Period
9+9Num. of Disp.
95
Polarization
HelicalGap
43.9 mm
FEL
Cavity Length~13.3 mPulse Rate
11.26 MHz
Wavelength
215 nm

FEL wavelength = 210 nm

500
400
300

MirrorAdjustment

200

MirrorAdjustment

0

0

1

2

3

4

5

6

7

8

Time [min.]Fig. 1. Laser power decrease due to rapid heat load
variation on the resonant cavity.

140

140

130

130

120

120

110

110

100

0

20

40

60

80

100

Stored Current [mA / 2-bunch]100

FEL Average Power [mW]

Laser Power [a.u.]

600

Time [min.]Fig. 3. Time trend of the FEL average power andstored beam current during a FEL top-up operation.
[1] M. Hosaka et al., UVSOR Activity Report 29(2002) 45.

Time (min.)Fig. 2. Laser power decrease due to FEL gain
decrease caused by beam current decrease [1].

39Light Sources

Coherent Harmonic Generation in VUV Region at UVSOR-II

T. Tanikawa1, M. Adachi1, 2, H. Zen1, 2, M. Hosaka3, N. Yamamoto3, J. Yamazaki2, Y. Taira3and M. Katoh1, 2
1
The Graduate University for Advanced Studies, Okazaki 444-8585, Japan
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
3
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
Experimental Parameters
Table 1 shows parameters of the electron beam, theOK and Ti: Sapphire laser in this experiment.

Introduction
At the UVSOR-II, electron storage ring, coherent
light source developments based on laser seeding
techniques are in progress. In the previous results,
generation of deep ultra-violet (UV) coherent
harmonic (CH) with variable polarization by using afemto-second laser and an optical klystron (OK) has
been demonstrated[1, 2].
Based on the successful results, the coherent
harmonic generation (CHG) in a shorter wavelength
region has been aimed. For the purpose to measure it,a vacuum ultra-violet (VUV) spectrometer has been
constructed. In this experiment, the spectra of CH in
VUV region have been successfully observed[3].

Table 1. Experimental parameters.
Beam Energy
600 MeV
Beam Current
20 mA/1-bunch
Emittance
18 nm-rad
Energy Spread
3.4 × 10-4
Bunch Length
114 ps
OpticalPeriod Length
110 mmKlystron Number of Periods
9+9
Number of Dispersion
45
Ti:
Central Wavelength
801 nmSapphire Pulse Energy
2.05 mJ
Laser
Pulse Duration
1.3 psElectron
Beam

Design and Construction of VUV Spectral
Measurement System
At the UVSOR-II, the spectral measurement of
CHG has been performed by utilizing a spectrometerfor visible and deep UV light (C5904, Hamamatsu
Photonics). In order to measure VUV CH, the new
VUV spectral measurement system has been
constructed.
Figure 1 illustrates configuration of the new one,which is directly connected to the storage ring at
downstream of OK. The VUV spectrometer covers
the wavelength range of 50-300 nm limited by aconcave replica grating (2400 grooves/mm, Pt coated,
4.5 of F number). It is Seya-Namioka configurationof 64 degree of input-output angle and compatible
with an ultra-high vacuum environment. An electron
multiplier tube is used as the photo detector, whose
wavelength range is below 200 nm.

Experimental Results
The CHG up to 9th order (89 nm) has been observed.
In Fig. 2 the spectrum of the 5th harmonic of SE(Spontaneous Emission) and CH are shown. The dataclearly demonstrate a bandwidth of CH becomesmuch narrower than that of SE.

Fig. 2. Spectra of SE (blue line) and CH (red line) at
the 5th harmonic.
[1] M. Labat, et.al., The European Physical Journal D,
44, Number1 (2007) 187-200.
[2] M. Labat, et.al., Proceedings of FEL 08, Korea,
2008.
[3] T. Tanikawa, et.al., Proceedings of FEL 09,
England, 2009.
Fig. 1. Photo of VUV spectral measurement system.

40

Light Sources

Electric Field Detection of Coherent Synchrotron Radiation

I. Katayama1, H. Shimosato2, M. Bito2, K. Furusawa2, M. Adachi3, 4, M. Shimada5, H. Zen3, 4,
S. Kimura3, 4, N. Yamamoto6, M. Hosaka6, M. Katoh3, 4 and M. Ashida2, 7
1Interdisciplinary Research Center, Yokohama National Univ., Yokohama 240-8501, Japan
2
Graduate School of Engineering Science, Osaka University, Toyonaka 560-8531, Japan
3
UVSOR Facility, Institute of Molecular Science, Okazaki 444-8585, Japan
4
GraduateUniversities for Advanced Studies (SOKENDAI), Okazaki 444-8585, Japan
5
High Energy Accelerator Research Organization (KEK), Tsukuba 305-0801, Japan
6
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
7
PRESTO JST, Tokyo 102-0075, Japan

Coherent terahertz synchrotron radiation (CSR) is avery promising light source for nonlinear terahertz
spectroscopy because it may potentially haveultrahigh power and it can be combined with thenormal synchrotron radiation. We recently
demonstrated generation of CSR by applying the laser
bunch slicing technique [1], which is first invented
for X-ray generation. However the detection of theelectric field of CSR from laser bunch slicing has notbeen realized until now because the laser and themeasurement port are far separated (20 m in the caseof UVSOR). In this work, we show that the use of theseed oscillator for probe and 24 m optical fiber fordelivering it to the measurement port are useful androbust techniques to observe the electric field of the
CSR. These techniques can be applied to the
far-infrared
spectroscopy
using
CSR
and
non-destructive characterization of the electron
bunches with longitudinal density modulations [2].
Figure 1 shows the setup for the CSR measurement.
Probe pulse for terahertz detection is divided from theseed oscillator and is coupled to the large mode-area
photonic-crystal optical fiber that delivers the pulses tothe detection port. Positive chirping due to the longfiber is compensated by a compressor. The pulse
duration after the compressor is about 180 fs. The
probe goes through the optical stage with a long
delay-line and is focused on the electro-optic crystal(ZnTe). The polarization rotation due to the CSR
electric field is detected with electro-optic sampling
method.
The rest of the seed oscillator is amplified and issent to the storage ring. The laser pulse interacts withthe electron bunch at the undulator. The spacial andtemporal overlaps are confirmed by monitoring theundulator radiation and the laser at the monitor
station. As the result of the laser-electron interaction, apart of the electron bunch is strongly
energy-modulated and a dip is created on the bunch as
it passes through two bending magnets.
The inset of Fig. 1 shows the detected electric fieldof the CSR waveform. This waveform can be obtained
repeatedly, that means the CSR is very stable and is notfrom the chaotic CSR The oscillation of the terahertz

Fig. 1. Experimental setup for the electron bunchslicing. Whole laser setup is synchronized with the
RF frequency. We used the seed oscillator for
probing the electric field of coherent synchrotron
radiation.
field is detected with the period of 2.5 ps. In the inset
of Fig. 1 we also show the Fourier transformation thatindicates the peak frequency is about 0.4 THz. This
spectrum is in good agreement with the FTFIR
measurement of the same radiation at the same port asis reported in the reference [1,3].
In summary, we have demonstrated that the CSR
generated with the laser bunch slicing technique has aphase-locked waveform. The use of the electro-optic
sampling method and the 24 m long fiber enables us torealize a robust technique for electric field detection
even if the distance between the laser and thedetection port is far. The use of seed oscillator forprobe also has strong advantage since we can choose
the nearest pulse to the CSR for detection within the 11
ns range (90 MHz). Therefore, the technique we usedin this study is promising for detecting the electricfield of novel coherent synchrotron radiations.
[1] M. Shimada et al., Jpn. J. Appl. Phys. 46 (2007)
7939-7944.
[2] S. Bielawski et al., Nat. Phys. 4 (2008) 390-393.
[3] M. Shimada et al., Phys. Rev. Lett. 103 (2009)
144802.

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