FEL
BL5U

Observation of CHG Spectrum at UVSOR-II
1

M. Hosaka1, M. Labat2, 3, M. Shimada4, N. Yamamoto1, M. Katoh4 and M. E. Couprie3
Graduate School of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan
2
Centre CEA-Saclay, DSM/IRAMIS/SPAM, 91 191 Gif-sur-Yvette, France
3
Synchrotron SOLEIL, Saint Aubin, BP 34, 91 192 Gif-sur-Yvette, France
4
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
the OK radiation is transported to a spectrometercoupled to a fast intensified CCD camera. Theultrashort exposure time (down to 2 ns) of this system
isolates the spectra of coherent light pulses at thelaser’s 1 kHz repetition rate from the 5.6 MHz
incoherent light pulses.
Typical examples of the CHG and the spontaneous
spectra are presented in Figs. 1(a) and 1 (b). The
CHG-FEL spectral lines around 412 nm and 266 nm,
are the second and third harmonics of the laser
wavelengths (400 nm and 266.6 nm). The measured
spectral width is ∆λ = 3 nm (∆λ = 1 nm) on thesecond (third) coherent harmonics, while spontaneousemission spectral width reaches 10 nm on both
harmonics. Additional spectral lines appear on eachside of the initial line. In the example presented inFigs. 1(c) and 1(d), when using a strong focusing, one
sideband in the shorter wavelength region of thesecond harmonics at 410.9 nm and two sidebands
grow in the case of the third harmonics.
The growth of sidebands in the spectrum isconsequence of the synchrotron motion of the trappedelectrons in the ponderomotive potential. For an
increasing laser power, the electrons experience alarger displacement in phase space, getting deeper intheir rotations cycle. For relatively low laser power,(smooth focusing case) the bunching (densitymodulation) is optimum, and the associated third
harmonic spectrum is monochromatic. As increasing
laser power (strong focusing case), the debunching(spoiling of the density modulation) starts while
sideband appears in the associated spectrum. ThePERSEO simulations reproduce the sideband growthin the case of an overbunched beam. Completed
simulation results are described in the separated paper[3].
In conclusion, we have shown that thelaser-electron interaction has to be finely adjusted toprevent the spectral structure from being spoiled by
the growth of sideband.

Coherent harmonic generation (CHG) FEL from a
relativistic electron beam has an attractive property of
producing linearly/circularly polarized short pulse
light from UV to VUV region. In the CHG-FEL
scheme using an optical klystron (OK), an externallaser source is injected inside a first undulator-themodulator-where the energy modulation of electron
beam is performed. The conversion into a density
modulation occurs in a dispersive section, andcoherent emission with strong harmonic content isproduced in a second undulator-the radiator. Recently,
we have succeeded in observing the CHG-FEL fromthe U5 optical klystron using a femtosecond Ti:Salaser [1, 2]. In this report, we describe anexperimental spectral analysis of the CHG-FEL as afunction of the seeding laser parameter. This study
serves not only for the application experiments of theCHG-FEL but also for development of the future
seeded free electron lasers.

Fig. 1. CHG FEL spectra on the (a) second and (b)third harmonics with smooth focusing configurationand on the (c) second harmonic (sideband at 410.9nm) and (d) third harmonics (sidebands at 264.2and 267.2 nm) with the strong focusing configuration.
Seeding laser power and pulse duration are 2 W and 1
pic-second-FWHM, respectively. (From ref. [3])The experiment was performed with the single
bunch electron beam at the energy of 600 MeV. The
femtosecond Ti:Sa laser at 800 nm with up to 2 W ofaverage power is focused in the middle of themodulator either with a strong focusing ofcorresponding to a Rayleigh length of ZR = 0.15 m ora smooth focusing corresponding to a Rayleigh lengthof ZR = 1.5 m. Detail of the experimental setup isgiven in the separated paper [2]. For spectral analysis,[1] M. Labat et al., UVSOR Activity Report 33(2006) 36.
[2] M. Labat et al., Eur. Phys. J. D 44 (2007) 187.
[3] M. Labat et al., Phys. Rev. Lett. 102 (2009)014801.

31

Accelerator

Development of Orbit Feedback System at UVSOR-II
1

Y. Suzuki1, M. Katoh2,1, Y. Takashima1, M. Hosaka1, N. Yamamoto1 and H. Morimoto1
Graduate School of Engineering, Nagoya University, Chikusa-ku Nagoya 464-8603, Japan
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japanremoved.

Abstract
At synchrotron light sources, the drift of the orbitof the electrons in the storage ring causes a drift ofthe light source position, which affects user’sexperiments.
At UVSOR-II, an RF feedback system [1] has been
operating to suppress the orbit drift on the horizontal
plane caused by the thermal expansion of the storagering floor. However, on the vertical plane, there’s nosystem to stabilize the orbit. Here, we have developed
a feedback system by using correction magnets. It
was successfully demonstrated that the drift of theelectron orbit was suppressed.

Conclusion
We have developed the feedback system. Thissystem was successfully commissioned that orbit
suppressed on the vertical plane. The system can be
expected to contribute to high precision stabilization
at UVSOR-II in the future.

Feedback System
We have developed a feedback system to correct
the electron orbit distortion. The following is theprinciple of the system. Firstly, the system gets thedisplacement of electron orbit from a standard orbit
from the BPM (Beam Position Monitor) system,
which measures the beam positions at 24 points of thering. Secondly, the system calculates the applicablestrength of correction magnet, which minimizes thedisplacement by minimizing the followings with SVD(Singular Value Decomposition):Fig. 1. Front panel of Feedback System.

S=|R・q＋z|2
where R is response matrix calculated from the ringlattice data, q is the applicable strength of thecorrection magnets, and z is the displacement of theelectron orbit. At the last, the system transfers thevalues of correction strength to the PC controlling themagnet power supplies. The system continues tocorrect electron orbit with a feedback cycle of 10seconds.
Figure 1 shows the front panel of the system. In
this panel, the operator can select/exclude the BPM orcorrection magnet for correction and can set
previously allowable range of BPM data. If the BPM
gets false data, the system would calculate abnormalvalue. To prevent this case, the system would cancel
to transfer the values if the BPM data is out of therange previously set.

Fig. 2. Displacement of orbit before introduction of
the feedback system.

Result and Discussion
Figure 2 shows the orbit drift on the vertical plane
without the feedback, and Fig.3 shows that with thefeedback. The system was successfully working for 6hours without any errors. It was demonstrated that theorbit drift was suppressed within a few tens of
microns, and small oscillations (seen in Fig.2) wereFig. 3. Displacement of orbit after introduction of thefeedback system.
[1] K. Suzumura et al., UVSOR Activity Report 33(2006) 39.

32

Accelerator

Development of Superconducting Magnets
for Synchrotron Radiation Facility

M. Yoshida1, Y. Takashima1, M. Katoh1, 2, M. Hosaka1,
N. Yamamoto1, H. Morimoto1 and M. Torikoshi3
1Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan
3
National Institute of Radiological Sciences, Chiba 263-8555, Japan
Abstract
By using superconducting magnets (SCM), highenergy synchrotron light can be radiated. If
5Tesla(T)-SCM is introduced, hard-X ray can begenerated by even small circumstance storage ring,such as UVSOR. To apply the SCM scheme forsuperconducting bends (SCBMs), we designed thepole and coil shapes.
SCBM
The bending angle and peak field are set 12.0
degree and 5 T. The material of the Iron core and coil
is of SUY and NbTi, respectively, and the shape ofthe Iron core is C-type. The coil is cooled by 2 stages
4K-GM cryocooler. At first stage the temperature iscool down to 45K with the cooling power of 40W, atsecond stage 4.2K with 1.3W. In addition, liquid
helium vessel is also set for the accidental case, suchas power down of cryocooler. To decrease leak fields,
two field cramps are equipped at outside of coil andiron core. The schematic view of the SCBM is shownin Fig.1.

80
Fig. 2. coil shape

Fig. 3. SCBM simulation model.
Field mapping
We have calculated the field map on the beam orbit
plane from the SCBM model, as shown in Fig. 4. In
this figure, the requirement field of 5 T is obtained.

Fig. 1. The schematic view of the SCBM
To study the details of the fields and beamenvelops in the SCBM, we have used the add-in
program “Radia” [1], which is developed in ESRF.
This program enables us to calculate 3D-field
distribution with finite elements methods on
analytical calculation cord “Mathematica”.
The calculation models of the coil and iron cores
are shown in Figs. 2 and 3. In Fig. 3, the red and blue
colors indicate the coil and iron core, respectively.

Fig. 4. SCBM field map.
[1] http://www.esrf.eu/33

FEL

Stabilization of Optical Cavity of UVSOR-II Free Electron Laser
M. Koike1, M. Hosaka1, M. Katoh2, 1, Y. Takashima1, M. Adachi2,N. Yamamoto1, T. Tanikawa3 and J. Yamazaki2
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 (SOKENDAI),
Okazaki 444-8585, Japan
introduction, the FEL power in 10 minutes remains
almost unchanged

Introduction
At the UVSOR storage ring, free electron laser(FEL) experiments have been made using a helical
optical klystron. We have already succeeded in highpower lasing around 1 W in the deep UV region andthe shortest wavelength attained so far is 199nm [1].
Recently, application experiments of the FEL in thedeep UV region such as photo-electron spectroscopy
and irradiation on biological molecules have been
carried out. Although a stable FEL is favorable forthese applications, we have noticed a rapid power dropwith time when the FEL is operated with a highelectron beam current (~200 mA). This problem is
especially disadvantage for these applications which
require higher power FEL.
It was speculated that the power drop was due todistortion of a resonator mirror heated by synchrotron
radiation leading to misalignment of the optical cavity.
To prevent the power drop, we have developed afeedback system correcting automatically the opticalcavity alignment by controlling mirror angle.

Conclusion
We have developed a feedback system to stabilize
the FEL power by changing mirror angles. The system
was successfully commissioned.
As future plan, we aim more accurate correction bycontrolling RF frequency to synchronize therevolution frequency with the FEL round trip
frequency.

Feedback System
Figure 1 shows the outline of the feedback system.
First, transmitted FEL power through a cavity mirror ismeasured by a photodiode, is processed and is send toa personal computer. Then the software on thepersonal computer decides the direction of a mirror
to change. The principle of changing the mirror angle
is based on comparison of FEL powers before andafter the change of the mirror angle. If the FEL power
after changing the mirror angle is higher than one
before changing it, the software decides to change themirror angle to the same direction again. On the other
hand, if the FEL power after changing the mirror angle
is lower than one before changing, it decides to change
to the opposite direction.
Finally, the software transfers the determined value
to a stepping motor controller, and the mirror angle ischanged adjusting a gimbal by the step motor. Thisroutine action is made every 2 seconds and it controls
upstream and downstream cavity mirrors.

Fig. 1. Block diagram of the feedback system.

Fig. 2. FEL power variation before and after
introduction of the feedback system.

[1] M. Hosaka et al., UVSOR Activity Report 35(2008) 40.

Result and Discussion
Figure 2 shows the FEL power variation at 230nm
before and after introduction of the feedback system. Itis clear that the FEL power has been stable and therapid FEL power drop has been suppressed. Before
introduction of this feedback system, FEL power
decreases by a half in about 10 minutes. After the34Injection

Top-Up Test Operation at UVSOR
K. Hayashi, M. Katoh, M. Adachi and J. Yamazaki
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, JapanOne purpose is keeping radiation at storage ring low,
cutting off the electrons slipping off the central orbit
which will be lost in storage ring. Another purpose isprotection of magnet poles of in-vacuum typeundulators.

Introduction
Keeping the beam current constant withintermittent injection (so-called top-up operation) has
been realized at several synchrotron radiation
facilities. With top-up operation, we can provide not
only higher photon flux but also constant flux tobeamlines. It is expected that the stability of the lightbeam also improves, mainly owing to temperaturestabilization of the accelerator and beamline
components. Thus, top-up operation has been one of
the most important plan at UVSOR.
To achieve top-up operation, we had alreadyupgraded the power sources for the booster
synchrotron and the beam transport line. With thisupgrade, 750MeV (full energy) injection had beenenabled. To ensure radiation safety, we had installed alead shielding wall around the storage ring.
In fiscal 2008, we prepared:
1. An interlock system to ensure the integrated charge(electron number) lower than a specified limit, and a
trigger managing module to automatically keep thebeam current to a fixed value.
2. Two copper slits in beam transport line.
Soon after obtaining permission of Ministry ofEducation, Culture, Sports, Science and Technology
in October 2008, we began top-up test operation.

Fig. 2. A slit located at beam transport line.
Status of Top-Up Operation
An example of beam current trend is shown in Fig.
3. Short time fluctuation due to beam loss in every
interval is about 1.5mA, which corresponds to 0.5
percent of stored beam. Some dips in the graph are
due to decrease of the injection efficiency, whichshould be settled in future improving the stability ofaccelerator components.

Injection Charge Measurement and Automatic
Injection SystemInjection charge measurement is necessary forradiation safety. With this system (Fig.1), we measure
the weekly integration of injected charge (electronnumber) in top-up mode. If the integration goes overthe limit, high voltage for the beam transport magnets
will be turned off. Injection module automaticallyturn on/off the injection trigger to keep the beam
current to a fixed value. Electron charge is measured
using Integrated Current Transformer (Bergoz Co.).

Fig. 1. Charge integration and automatic injection
module (left), Integrating Current Transformer (right).
Fig. 3. A beam current trend in top-up test operation.
One minute interval, up to 15 seconds injection.
The data was taken every 12 second.

Slits at Beam Transport Line
Two slits which utilize 60mm thick copper blocks
were introduced to the beam transport line (Fig. 2).

35