Accelerators

Correction of Dynamic Multipoles for APPLE-II Undulator with Flat Wires
Y. Kikuchi1, 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

Introduction
APPLE-II undulator can produce quasimonochromatic light of variable polarization though
its magnetic circuit is relatively simple. Therefore, it
has been installed in many synchrotron radiation
facilities and will be installed in Central Japan
Synchrotron Radiation Research Facility underconstruction in Aichi prefecture.
APPLE-II undulator also has been installed in
UVSOR-II electron storage ring [1]. When theundulator is operated in vertical polarization modewith narrower gap than 40 mm, the lifetime ofelectron beam circulating in the storage ring
significantly decreases. The reason is considered as
dynamic multipole kicks in the undulator, whichstrongly depends on the undulator gap. At UVSOR-II,
multi-wires [2], which are glued in the upper andlower surface of undulator beam duct, are candidate
to compensate the multipole effects. In order toevaluate the validity of this method, we numericallystudied the dynamic multipoles of the undulator andexperimentally examined the correction capability ofmulti-wires.

Fig. 1. The relation between x and x’, when theundulator gap is operated with 30 mm gap andvertical polarization mode. (Blue:Without correciton,
Red:With correction).

Numerical Study on Dynamic Multipoles
In order to evaluate dynamic multipoles generated
in the undulator operated in vertical polarizationmode, we calculated the relation between thehorizontal beam injection point x and the horizontal
beam angle in undulator terminus x' using a 3D
magnetostatic code RADIA [3]. The result is shownin Fig.1. The quadrupole magnetic field is strong
within x = ±5 mm and the higher order multipole
magnetic field is strong in the other range. Expected
multipole kick with multi-wire correction is also
shown in Fig.1 as red line. Since the amplitude ofmultipole kick is comparative with that of 40-mm-gap
condition, we expected significant improvement of
the electron beam lifetime with the correction
method.

Fig. 2. Multi-wires installed in APPLE-II undulator.
Table 1. Result of correction of multipole field withmulti-wires.
Beam liftime [mA hr]
Gap [mm]
W/o correction
With correction
100
300
̆
30
100
290

Experimental Examination
Multi-wires were installed in APPLE-II undulator
in UVSOR-II (Fig. 2) and we tested the effect ofmultipole correction on the beam lifetime. The resultis shown in Table 1. When the multipoles werecorrected by the multi-wires, the electron beam
lifetime is significantly improved than that without
correction. This multi-wires will enables us to use theundulator with vertical and narrow gap condition.
Such operational condition is quite important forangle-resolved
photoelectron
spectroscopy
application of APPLE-II undulator.

[1] UVSOR Activity Report 37 (2010) 12.
[2] J. Bahrdt et al., Proceedings of EPAC08 (2008)
2222.
[3] P. Elleaume et al., Proceedins of PAC97 (1997)
3509.

31

Accelerators

Laser-Power-Dependence of Coherent THz Radiationfrom Electron Bunches Circulating in a Storage Ring

N. Yamamoto1, M. Shimada2, M. Adachi3,4, H. Zen3,4, T. Tanikawa4, Y. Taira1, S. Kimura3,4,
M. Hosaka1, Y. Takashima1, T. Takahashi5 and M. Katoh3,41
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
2
High Energy Accelerator Research Organization, KEK, Tsukuba 305-0801,Japan
3
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
4
School of Physical Sciences, The Graduate University for Advanced Studies (SOKENDAI),
Okazaki 444-8585, Japan
5
Research Reactor Institute, Kyoto University, Kumatori 590-0494, Japanin electron bunches. The dip structure becomes larger
and wider as the laser energy increases, and this leads
to an increase in the lower-frequency components.

Intensity [a.u.]

Introduction
In order to study in detail the formation of the
micro-structure in electron bunches, we have
performed laser slicing experiments at the UVSOR-II
electron storage ring. Coherent synchrotron radiation(CSR) in the THz region, which contains informationon the micro-structure, was observed under various
laser energies.
Experiments
The intensity of the THz light was measured by an
InSb hot-electron bolometer (QMC, QFI/2) and thespectrum was analyzed at the IR/THz beamline(BL6B) using an in-vacuum Martin–Puplettinterferometer (JASCO, FARIS-1), which was used tomeasure the spectra in the 2–50 cm-1 range with aresolution of 1 cm-1. Pulse processing for the CSR
measurement was done using a gated integrator (SRS,
SR250) to reduce the back ground from normal SR.
The gated integrator was triggered by a 1-kHz
repetition signal of the laser system, and a width ofabout 5 vs was chosen. In our measurements, the THz
light intensity was evaluated to be 104–105 times
larger than that of normal SR.

1.0

Current: 5 mA
Laser: 322 fs

0.8

2.0 mJ
1.5 mJ
1.0 mJ
0.5 mJ

0.6
0.4
0.2
0

0

5

10

15

20

Wavenumber [cm-1]

25

Fig. 1. THz CSR spectra with different energies ofslicing laser pulse introduced.
Conclusions
The formation of micro-structure in electron
bunches by the laser slicing technique wasexperimentally studied. The THz CSR spectra were
measured for various laser energies. The measured
spectra have double-peak structure, which can be
attributed to the contributions of the first and secondturns. The spectral shape depends strongly on thelaser parameters.
The THz CSR spectra were compared withnumerical calculations. The spectral shapes agreewith the numerical calculations qualitatively, butsome discrepancies in the wave number remains.
Details of those results are described in Ref. [2].

Results and Discussion
CSR spectra shown in Fig. 1 were taken for four
different laser energies —2.0, 1.5, 1.0, and 0.5 mJ—
with same laser pulse duration of 322 fs. The
measured spectra are divided by that of normal SR in
order to exclude the sensitivity dependence of thebolometer. Each spectrum has two peaks in different
frequency ranges. The peaks in the higher and lower
frequencies correspond to CSRs emitted in the firstturn and in the second turn of the storage ring. Since
the temporal resolution of the detector is about 1 vsand the spectra were time-averaged, it cannot resolve
the emission from each turn. The measured double
peak can be explained as a multi-turn effect [1].
As the laser energy increases, especially for the
peak in the higher frequency range, the peaks shift tothe lower frequency component increases gradually.
The difference in spectral shape with different laser
energies arises from the growth of the micro-structure[1] M. Shimada, et. al., Phys. Rev. Lett. 103 (2009)
144802[2] N. Yamamoto, et. al., Nuclear Instruments andMethods in Physics Research A, in press.

32

Accelerators

Practical Introduction of Vertical Beam Position Feedback System
H. Zen1, K. Hayashi1, M. Adachi1, N. Yamamoto2, J. Yamazaki1 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
Improvements
To improve the resolution, we newly added 100turn coils to outside of the SMs. And for the power
supply of the additional coil, DC power supplies(KIKUSUI, PMC-18-2A) were introduced. We also
installed a 12 bit ADC board (INTERFACE,
PCI-3341A) for controlling the new power supplies.
The achieved resolution of the beam position with the
improved system is around 0.15 Pm. After installation,
it was confirmed that the improved system did not
make sudden orbit change.

Introduction
In 2008, an orbit feedback system was developed
for stabilizing the vertical electron beam position in
the UVSOR-II storage ring [1]. Then, it was
confirmed that the feedback system could suppress
the slow drift of the vertical beam position. In thisfiscal year, we have introduced the feedback system
to user operations with a modification of the system.
Feedback System
The feedback system consists of 24 Beam Position
Monitors (BPMs), 16 Steering Magnets (SMs) andcontrol PCs. Firstly, the electron beam orbit wasmeasured by the BPMs, which measures the beam
position at 24 points of the ring. Secondly, control
PCs calculates the applicable strength of SMs, which
minimizes the displacement of orbit. Finally, strengthof SMs is changed according to the calculated values.
This system minimizes the displacement of electron
beam along the storage ring.
Result of Test Operation
During night operations, usually done fromThursday night to Friday morning, we carried out
some test operations of the orbit feedback systembefore introducing the system to normal user
operation. A result of test operation is shown in Fig. 1.
We found that the feedback system caused sudden
orbit change shown in Fig. 1. At the same time,sudden intensity change of synchrotron radiation wasobserved in user beamlines. The origin of sudden
orbit change was determined as a lack of resolution ofSMs, whose minimum variation of the beam positionis around 4 Pm. Therefore, we made a modificationon the system to have higher resolution.

Fig. 2. Photographs of additional equipments.
Left: Additional small coil around the steering magnet.
Right: Remotely controllable DC power supply.
Introduction to User Operation
The improved orbit stabilization system has been
introduced to user operations since December 2010. A
typical temporal evolution of the vertical beam
position during 36-hour operation is shown in Fig. 3.
As shown in the figure, the drift of vertical beam
position is drastically suppressed by the feedback
control. And the orbit feedback system maycontribute to suppress the photon energy variation inuser beamlines and to achieve higher reproducibilityof user experiments.

Fig. 1. Temporal evolution of vertical beam position
during a test operation of the orbit feedback system.

Fig. 3. Temporal evolution of vertical electron beam
position at the downstream of bending magnet No. 4.
[1] Y. Suzuki et al., UVSOR Activity Report 36(2009) 32.

33

Accelerators

Stabilization of the Accelerated Charge of UVSOR-II Injector
H. Zen, K. Hayashi, J. Yamazaki, M. Adachi and M. Katoh
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, JapanThe relationship between the measured RF power andthe optimum excitation current of solenoid coil was
experimentally obtained. It was found that the
optimum current had quadratic dependence on themeasured RF power.

Introduction
In fiscal year 2009, a stabilization system for
injection septum of UVSOR-II booster synchrotron
has been developed [1]. By using the system, thestability of accelerated charge in the synchrotron was
drastically improved. However, we found anothersource of fluctuation of the accelerated charge, which
was fluctuation of the RF power fed to the linac of
UVSOR-II injector. In this fiscal year, we added
feed-forward system for stabilizing the accelerated
charge of the injector.
Principle of Stabilization Method
The energy gain of an RF Linac can be described in
the formula,(1)
GEkin A P0  BI b ,where GEkin is the energy gain, P0 is the fed power tothe linac, Ib is the averaged electron beam current
injected to the Linac, A and B are constant. The best
way to stabilize the energy gain is stabilize the fed
power and beam current. However, we don’t haveremote control for the fed power. Thus we decided tovary the averaged beam current for stabilizing theenergy gain.

RF Power [Arb. Units]

0.32

Feed-Forward System
The schematic diagram of developed feed-forward
system is shown in Fig. 1. In this system, strength of
the solenoid coil between the electron gun and linac isused for changing the average beam current (Ib)injected to the linac. Amplitude of fed RF power (P0)to the linac is measured with an RF detector and an
oscilloscope. The measured waveform is sent tocontrol PC, which determines the optimum excitation
current of solenoid coil from a look-up table or an
equation.

To Booster
Synchrotron

e-BeamKlystron

12

15

18

0.80
21

Time [hh]PowerSupply

(b)

0.8
0.6
0.4
0.2
0.0
09

12

15

18

21

Time [hh]Set Value

Waveform

0.28

1.0

Electron
Gun

RF
Source

0.85

0.29

0.27
09

Solenoid
Coil

Linac

0.90

0.30

Accelerated Charge
in Synchrotron [nC]

Current
Monitor(a)

0.31

Current of Solenoid Coil [A]Results
The results of 12-hour top-up operation are shownin Figs. 2. According to the changes of RF power fed
to the Linac, the excitation current of the solenoid
coil was controlled as shown in Fig. 2 (a). Then theaccelerated charge in the booster synchrotron was
kept more than 0.4 nC for 12 hours. This value
corresponding to the injected rate of 6.1 mA/minutein the case of extraction efficiency = 30%, injection
efficiency = 70% and maximum number of injection
in one minute = 13. At the same time, the stored beam
current in UVSOR-II storage ring was kept almost
constant around 300 mA.
This feed-forward system is now always runningduring user operations for stabilizing the accelerated
charge in the synchrotron.

Fig. 2. Temporal evolution of the RF power,
excitation current of solenoid coil and accelerated
charge in the synchrotron.

ControlPC

RF Detector

Fig. 1. Schematic diagram of feed-forward system.

[1] H. Zen et al., UVSOR Activity Report 37 (2010)
35.

34

Accelerators

Stabilization of Pulsed Kickers for Creating Injection Bump Orbit
in UVSOR-II Storage Ring
H. Zen, K. Hayashi, J. Yamazaki, M. Adachi and M. Katoh
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan

Introduction
Injection of UVSOR-II storage ring is done withthree pulsed dipole magnets (kickers) and creatingbump orbit near the injection point. The height andtiming of those kickers have large drift. Such large
drift changes the amplitude of bump orbit, whichcauses decreasing of injection efficiency or electron
beam dump. For stable top-up operation of
UVSOR-II, the amplitude of bump orbit should bestabilized. Therefore a feedback system to stabilize
the pulsed kickers was developed.

Main Trigger
90 MHz RF

Bunch
Selector
Timing of this signal isvaried for stabilization.

Set Valueof Delay

DG535

Discharge
Trigger

ControlPC

Trigger
SignalWaveform

Bump
KickerOscilloscope

Monitor
SignalSet Value of Charging Voltage

Fig. 1. Schematic diagram of the developed feedback
system.

Feedback System
Figure 1 shows the schematic diagram of thedeveloped feedback system. By using an oscilloscope,
the excitation current signals of the bump kickers aremeasured as shown in Fig. 2. The oscilloscope istriggered with an external trigger which generated bya digital delay generator (DG535). We set a thresholdlevel to determine the timing of the kicker andrecognized that the operational timing of the kicker isthe timing when the current waveform crossing thethreshold level as shown in Fig. 2. And the pulse
height of the kicker is recognized as the maximum
value of the waveform as shown in Fig. 2. In thecontrol PC, measured timing and amplitude arecompared with ordered timing and amplitude which
are given by machine operators. Based on PID
algorithm, the control PC calculates the optimum
delay and charging voltage of the kicker. Thosevalues are sent to DG535 and power supply of the
kicker.
Since the oscilloscope and DG535 have 4 channels,
three bump kickers are controlled by one oscilloscope,
one DG535 and one control PC at the same time.

Measured Height

Measured Signal [V]86Threshold420Measured Timing
-2

0

1

2

3

4

Time [Ps]Control Value [Arb. Units]

1000

7.0

(a)
980
6.5
960
6.0
940
920
09

12

15

18

5.5
21

Measured Pulse Height [V]Fig. 2. Typical waveform of excitation current of apulsed kicker.

Time [hh]
2.1

1.95
1.90

(b)
2.0

1.85
1.9
1.80
1.8

1.75
1.70
09

12

15

Time [hh]18

Measured Timing [Ps]

Delay of Trigger Signal [Ps]

Results
Figure 3 shows the result of feedback control. By
properly changing the control value of charging
voltage of power supply of the kicker and delay oftrigger signal of the kicker, we succeeded instabilizing the pulse height and operational timing of
the kicker.
Though the height and timing of all kickers arestabilized, the injection efficiency of the UVSOR-II
ring is not so stable. To keep high injection efficiency,
we should slightly vary the strength of steering andfocusing magnets of beam transport line between thering and injector. Counter measures for thefluctuation will be developed near future.

1.7
21

Fig. 3. Temporal evolution of operational condition ofa pulsed kicker, (a) control value of charging voltageof the kicker and measured pulse height, (b) delay ofdischarge trigger signal and measured operational
timing of the kicker.

35

Accelerators

Timing Stabilization of Extraction Kicker of UVSOR-II Injector
H. Zen, K. Hayashi, M. Adachi, J. Yamazaki and M. Katoh
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan

Introduction
From this fiscal year, we have started top-up
operation of UVSOR-II storage ring during all user
operation. In usual operation, 14 buckets of
UVSOR-II storage ring are filled by electrons andremained 2 buckets are empty in order to prevent
instability caused by ion trapping in the ring [1]. For
keeping 2 empty buckets during long time top-up
operation, we developed a feedback system forstabilizing the timing of extraction kicker of
UVSOR-II injector.

Main Trigger
90 MHz RF

Bunch
Selector
Set
Value

DG535

Timing of this signal isvaried for stabilization.

Discharge
Trigger

ControlPC
Waveform

Trigger
SignalFast
KickerWaveform

Oscilloscope

Fig. 1. Diagram of the timing feedback system for the
fast kicker.

Extraction Scheme and Problem
A fast kicker and an extraction septum are used forextracting electron bunches from UVSOR-II injector,
750 MeV booster synchrotron. At first, the fast kicker
suddenly changes the electron beam orbit in theinjector. Then 4 bunches in 8 circulating bunches are
successfully injected to the extraction septum. Thosebunches are bent in the septum and transported to thering. For filling 14 buckets of the storage ring, thetiming of the fast kicker is properly controlled.
Although the trigger signal sent to the fast kicker iswell controlled, the operational timing of the kicker
has long term drift of larger than 40 ns, whichcorresponds to time interval of 4 buckets. To reducethe drift, a feedback system was developed.

3.0

Voltage [V]

2.5
2.0
1.5
Threshold Level

1.0
0.5
0.0

Measured Timing
0

50

100

150

200

250

Time [ns]Fig. 2. Way to determine the operational timing of thefast kicker.
120

Time [ns]Developed Feedback System
The diagram of the feedback system is shown in
Fig. 1. By using an oscilloscope, the excitation
current signals of the fast kicker are measured asshown in Fig. 2. The oscilloscope is triggered withthe external trigger which generated by a digital delay
generator (DG535). We set a threshold level todetermine the timing of the kicker and recognizedthat the extraction timing of bunches is the timing
when the current waveform crossing the thresholdlevel as shown in Fig. 2. In control PC, the extraction
timing is compared with the ordered timing which is
given by machine operators. The control PC
calculates the optimum delay of DG535 based on PID
algorithm, and sends commands to set the timing
delay of DG535 to the calculated delay.

Histogram

100

~3 ns

80

60
00

02

04

06

08

Time [hh]Fig. 3. Temporal evolution of operational timing offast kicker with developed timing feedback system.
(a) w/o feedback

Results
Figure 3 shows the result of feedback control. The
fast kicker timing was stabilized around 90 ns, which
was the ordered timing of this operation. Then thetiming jitter was around 3 ns in FWHM. Figures 4
show the waveforms of bunch pick-up signal after
12-hour top-up operation. As shown Fig. 4 (a), allbuckets were filled by electrons without the feedback
system. In contrast, we succeeded in keeping the 2
empty buckets with the feedback system as shown in
Fig. 4 (b).

(b) with feedback

Fig. 4. Photographs of an analog oscilloscope
showing an electron bunch pick-up signal after
12-hour top-up operation without (a) and with (b) thetiming feedback for the fast kicker.
[1] J. M. Byrdt et al., SLAC-PUB-7389 (1996).

36

Light Sources

Experiments on the Narrowband Coherent Synchrotron Radiation

M. Hosaka1, Y. Takashima1, C. Evain2, C. Szwaj2, S. Bielawski2, M. Shimada3,
A. Mochihashi4, Y. Takahashi5, S. Kimura6 and M. Katoh6
1
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
2
Université des Sciences et Technologies de Lille, Villeneuve d’Ascq Cedex, France
3
High Energy Accelerator Research Organization, KEK, Tsukuba 305-0801, Japan
4
Japan Synchrotron Radiation Research Institute (JASRI), SPring-8, Sayo-cho,
Hyogo 679-5198, Japan
5
Research Reactor Institute, Kyoto University, Osaka 590-049, Japan
6
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
Narrowband terahertz coherent synchrotron
radiation can occur from an electron bunch whoselongitudinal distribution is sinusoidaly modulated. We
have already demonstrated this by creating themodulation using interaction inside an undulator
between the electron beam and laser pulse containing
a sinusoidal amplitude modulation: the resulting
narrowband terahertz spectra were observed using a
Martin-Pupplett Fourier transform far infra-red
spectrometer [1]. In this paper, we report a detailstudy on the emission, focusing on the efficiency
dependence on modulation (or resulting coherent
synchrotron radiation) frequency and laser power.
The experimental setup is the same as that we usedpreviously [1], but we simplify the laser pulse
shaping system. Instead of using a pair of gratings toachieve dispersion, we extract the pulse of ourregenerative amplifier just before the compressor, andsend it to a Michelson interferometer. The major
improvement over the previous system is higher pulse
energy and wider tuning range of the modulation
frequency.
To investigate the efficiency dependence of thecoherent terahertz emission on the modulation
frequency, we recorded the emission intensity as afunction of the modulation frequency by changing theMichelson interferometer position. At the same time,
we measured the spontaneous synchrotron radiation

spectrum using the same setup in order to take into
account the frequency dependence of the detecting
system. The relative efficiency is deduced dividing
the frequency response by the spontaneoussynchrotron radiation spectrum. The result ispresented in Fig.1. The broad 20 cm-1 peak structureagrees well to theoretical calculations based on a
slowly-varying envelope approximation [2]. The low
frequency emission (around 3-5 cm-1) is attributed toemission after long transport of the storage ring (onefull turn and more).
To investigate the efficiency dependence thecoherent terahertz emission on the laser power, we
attenuated the laser using a half-wave plate followed
by a polarizer. By rotating the half-wave plate, we
obtained the dependence of terahertz power withincident laser power [Fig. 2]. This dependence
appears to be quadratic with good approximation. The
quadratic scaling is generally expected in the limit of
low laser powers [2]. However, at higher laser power
departure from the scaling law is theoreticallyprospected [2]. As a next step of this study, we aregoing to examine the behavior with higher laser
energy.

Fig. 1. Ratio between CSR and spontaneous SR
versus modulation wavenumber. The blue solid lineis theoretically deduced form factor.

Fig. 2. Terahertz energy versus laser pulse energy.
The blue solid line is quadratic law fit.

[1] S. Bielawski, et al., Nature Physics 4 (2008) 390.
[2] C. Evain, et al., Phys. Rev. ST Accel. Beams 13(2010) 090703.

37

Light Source

Generation of Energy-Tunable Gamma Rays via Inverse Compton
Scattering in an Electron Storage Ring

Y. Taira1, M. Adachi2,3, H. Zen2,3, T. Tanikawa3, N. Yamamoto1,
M. Hosaka1, Y. Takashima1, K. Soda1 and M. Katoh1,2,3
1
Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan3
School of Physical Sciences, the Graduate University for Advanced Studies, Okazaki444-8585, Japan
ring, 90.1 MHz. The wavelength and the frequency ofthe laser were 800 nm and 1 kHz, respectively.
Gamma rays were detected by an NaI scintillator(1600 cm3), and the absorption energy was measured.
Experimental data of the absorbed energy spectrum
for various collision angles are shown in Fig. 1.
Maximum gamma-ray energy and intensity varied ina range from 4 to 9 MeV and from 350 to 850
photons s-1, respectively. It was successfully
demonstrated for the first time that the energy of the
gamma rays could be tuned by changing the collision
angle.

Gamma rays in the MeV-region can be generated
via inverse Compton scattering of laser photons witha relativistic electron beam of sub-GeV or GeV
circulating in an electron storage ring. This
technology is called laser Compton scattering (LCS)[1]. LCS gamma rays are tunable in energy,
quasi-monochromatic, intense, and high polarized,
and are used in nuclear science, positron generation,and electron beam diagnosis.
The methods available for tuning the energy of
LCS gamma rays include 1) selecting electron beamenergy, 2) selecting the laser wavelength, 3) choosing
the scattering angle of the gamma rays using acollimator and an absorber, and 4) controlling thecollision angle between a laser and an electron beam.
Since LCS experiments were usually carried out atelectron storage rings operated with fixed electronenergy, method 1) cannot be applied to generate LCS
gamma rays of tunable energy.
Ohgaki et al. [2] performed analytic calculations
and computer simulations on the energy spread andintensity of LCS gamma rays using methods 2) - 4)
for a 1.4-GeV electron storage ring. The energy
spreads obtained using methods 2) and 4) (1 %) were
smaller than the spread obtained using method 3)(7-9 %), and the intensity obtained using method 3)(~ 105 photons s-1) was the highest of the three.
However, the intensity obtained from method 4) is
expected to be increased to ~ 106 photons s-1 when amode-lock pulse laser was used. Thus Ohgaki et al.
concluded that method 4) with a mode-lock laser waspreferable for generating gamma rays of continuous
tunable energy using a laser of fixed wavelength with
fixed electron energy.
However, method 4) has never been studied
experimentally because LCS gamma rays are usually
generated via head-on collision between the electron
beam and the laser by injecting the laser through an
optical port used for extracting synchrotron radiation.
To generate gamma rays of tunable energy using
method 4), we installed an optical port in the electron
storage ring UVSOR-II which a laser can be injectedfrom the vertical direction with an adjustable collision
angle.
Laser pulses of 2.0 W power were provided by a
Ti:Sa laser system (COHERENT, LEGEND-HE)
synchronized with an RF frequency of the storage

Fig. 1. Experimental data of the energy spectrum foreach collision angle listed in the figure. Intensity wasstandardized by the beam current values.
Measurement data of every 100 channels weresummed. Error bars in the vertical and horizontal
directions are statistical error and energy calibration
error, respectively.
[1] J. Stepanek, Nucl. Instr. and Meth. A 412 (1998)
174.
[2] H. Ohgaki, et al., J. Nucl. Sci. and Tech. 44(2007) 698.

38

Light Sources

Saturation Phenomena on Coherent Harmonic Generation at UVSOR-II

T. Tanikawa1, M. Adachi1,2, H. Zen1,2, M. Hosaka3, N. Yamamoto3, Y. Taira2,3, J. Yamazaki2
and M. Katoh1,2,3
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, Japanequals a multiple of the harmonic wavelength,
interference produces peaks in the Fouriercomponents, and hence in the CH intensities.
In summary, we have demonstrated that CHG is a
useful probe for investigating the formation andevolution of the microbunching caused bylaser-electron interactions, which contributes to thenext-generation seeded free-electron lasers [4].

Coherent harmonic generation (CHG) is a method
for producing high-order harmonics of a laser light byusing a relativistic electron beam. This electron beaminteracts with the laser in a sinusoidal magnetic fieldof an undulator, resulting in microbunching of theelectrons at the laser wavelength and in the emissionof coherent harmonics (CHs) with a fully coherence,
an ultrashort pulse, and a variable polarization [1].
As results of measuring CH spectra by using avacuum ultraviolet spectrum measurement systemconstructed in last year at UVSOR-II [2], they havebeen observed up to 9th harmonic. Figure 1 shows theintensities of the 5th, 7th, and 9th CHs as functions of
the peak laser power. Figure 2 shows the intensity of
the 5th CH over a wide range of peak laser powers.
Dots in Fig. 1 and Fig. 2 represent measured value. In
Fig. 1, the CH intensities increase with increasing thepeak laser power, reaching a maximum at a value ofthe power that decreases slightly with the order of theCH. In Fig. 2, the CH intensity oscillates after the
maximum.
The dashed and solid curves in Fig. 1 and 2
represent the results of a one-dimensional analytical
calculation based on a simple equation for the form
factor at each harmonics [3]. The calculation ondashed curve assumes that the longitudinal
distribution of the laser is uniform, and that on solid
curve assume that the distribution is a Gaussian. As
shown in Fig. 1, the calculations reproduced theobserved variation in the width and position of theintensity peak for the three harmonics. The peak
position shifts to lower peak laser powers withincreasing order of the CH. Fig. 2 shows that
calculations also reproduced several other peaks
observed in the deep saturation regime qualitatively.
To understand these results, we carried out aone-dimensional particle tracking simulation of thebunching process in the saturated regime. In Fig. 1, in
the case of higher-order CHs, the Fourier componentof a longitudinal distribution on the electron beam islargest near the optimal bunching condition, where
the bunching is sharpest. However, for lower-orderCHs, the Fourier component is largest when theelectrons are slightly overbunched because of thecontribution of more electrons. For this reason, thelower-order CHs reach their maximum intensity atlarger peak laser power. In Fig. 2, the experiments
and simulations show several peaks appearing beyond
the first maximum, which can be explained by an“additive interference” of the double-peak structure
shown in Fig. 3. When the separation of the peaks

Fig. 1. Measured intensities of the 5th (red), 7th (green),and 9th (blue) CHs plotted as functions of the peak laser
power.

Fig. 2. Measured intensity of the 5th CH plotted as afunction of the peak laser power.

Fig. 3. Simulation result of the bunching process on the
overbunching condition in the OK.
[1] M. Labat et al., Phys. Rev. Lett. 101 (2008)
164803.
[2] T. Tanikawa et al., UVSOR Activity Report 37(2010) 40.
[3] P. L. Csonka, Part. Accel. 11 (1980) 45.
[4] T. Tanikawa et al., Appl. Phys. Exp. 3 (2010)
122702.

39