Ion Trapping Phenomenon in UVSOR Electron Storage Ring
A. Mochihashi1, M. Katoh1, M. Hosaka1, K. Hayashi1, J. Yamazaki1, Y. Takashima2, Y. Hori3
1
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan
2
Graduate School of Engineering, Nagoya University, Furo, Chikusa, 464-8603 Japan3
Photon Factory, Institute of Materials Structure Science, KEK, Tsukuba, 305-0801, Japan
We observed the dependence of vertical tune on thevacuum condition in the UVSOR ring. We changed
the vacuum conditions by turning off distributed ion
pumps (DIPs) and sputtering ion pumps (IPs), andmeasured the vertical tune with the RF-KO method.
In a series of experiments, we performed themeasurement under a multibunch condition in which
a series of 12 bunches followed by 4 empty buckets
are stored in the ring. Figures 1(a) and 1(b) show thatthe tunes changed when the averaged vacuum
pressure was changed. We also observed thedependence of the vertical tune on beam current
under the multibunch condition without changing thevacuum conditions but different two vacuum
conditions: at a low vacuum pressure (Exp. 1) and at
a high vacuum pressure (Exp. 2). Figure 2(a) shows
the change in average pressure in the ring during eachexperiment. As seen in Fig. 2(b), under themultibunch conditions the dependence of verticaltune on beam current tends to become larger withincreasing the vacuum pressure. Weak dependence insingle-bunch condition and that of horizontal tune in
the multibunch condition implies the change in thevertical tune is caused by ion-trapping phenomenon.

According to the ion-trapping theory [1], verticaltune decreases as beam current decreases, however,experiments say that the vertical tune increases withthe beam current decreases. This contradiction
suggests that the neutralization factor of trapped ionsincreases as beam current decreases; namely, thenumber of trapped ions increases as beam current
decreases. We have estimated theoretically thedependence of neutralization factor on beam current
by evaluating capture rate [2] of ions by both ananalytic method based on the classical theory of ion
trapping [1,3] and tracking method. In the calculation
we assume CO+ as a main ion species because CO isa main residual gas component in the UVSOR ring.
To reconstruct the experimental results, weconsidered not only the change in partial pressure of
CO during the experiments but also effects from C+
and O+ which come from multiple ionization of CO+.
Figures 3(a) and 3(b) show the tracking and analytic
calculations for Exp. 1 and 2 with experimental
results. The tracking calculations considering all ionsagree with the experiments qualitatively, however,they overestimate the change in vertical tune at abunch current of less than ~15 mA. One of the causesof the overestimation is estimation of multiple
ionization cross section of CO, that is difficult toestimate properly. The analytic calculations also tend
to overestimate the change in vertical tune; this isbecause stability condition can be only considered in
the analytic calculation.

Fig. 1 (a) Change in the vertical tune when vacuum
pumps were intentionally turned off. (b)Change inaverage pressure when the pumps were turned off.

Fig. 3(a) Experimental results, tracking andanalytic calculations of change in vertical tune forExp. 1 and (b) Exp. 2.
[1] R.D. Kohaupt, DESY Interner Bericht No.
H1-71/2 (1971) (unpublished).
[2] A. Mochihashi et al., Jpn. J. Appl. Phys. 44 (2005)
430.
[3] Y. Baconnier and G. Brianti, CERN Internal Rep.
No. CERN /SPS/80-2(DI) (1980) (unpublished).

Fig. 2 (a) Averaged vacuum pressure for Exp. 1,
Exp. 2 and single-bunch condition. (b) Dependence oftune on beam current in vacuum condition in Fig.
2(a).

34

Upgrade of RF Cavity in UVSOR-II Electron Storage Ring
A. Mochihashi1, M. Katoh1, M. Hosaka1, K. Hayashi1, J. Yamazaki1, Y. Takashima2
1
UVSOR, Institute for Molecular Science, Okazaki 444-8585 Japan
2
Graduate School of Engineering, Nagoya University, Chikusa-ku Nagoya 464-8603 JapanAs we have referred in previous activity report [1]we have built new main RF cavity of the UVSOR-II
electron storage ring. The new cavity can generate RF
voltage of 150 kV [1] without exchanging high-power
RF transmitter that has maximum output of 20 kW.
The cavity has been installed in a short straightsection in the UVSOR-II at shutdown period in thespring 2005, and soon has been achieved designed RF
voltage of 150kV in high-power commissioning.
Figure 1 shows a layout of the UVSOR-II
before/after the improvement of the main RF cavity.
Due to the upgrade, the RF section has been moved
from a long straight section (S7 section) to a shortstraight section (S2 section). Previous RF cavity has
been already dismounted from the ring; the site will
be able to be made good use of other devices such as
undulators in the future.

and evacuation, high-power commissioning wasstarted. The commissioning went smoothly, anddesigned RF voltage of 150 kV was achieved easily.
Now not only high-power aging but also beam test
has been proceeding to improve stability of RF fieldand vacuum condition in the cavity. After thecommissioning the cavity will be operated in users
run from May 2005.

Fig. 2 Drawing of the new main RF cavity.
Table 1cavities.

Parameters for the previous and present

Frequency
Cavity voltage
Shunt impedance
Unloaded Q
Coupling
Material
Cells
Coupler
Tuner
Inner diameter
Bore radiusFig. 1 Layout of the UVSOR-II before and after the
upgrade of the main RF cavity.
Figure 2 and Table 1 show a drawing and basic
specification of the new main RF cavity. For
comparison, the same parameters for the previous
cavity are shown in the table. Basic design of the newcavity is unchanged compared to the previous one,however, the shunt impedance and unloaded quality
factor become about 4 and 2.5 times larger. The
previous cavity was made from clad steel plates(SUS+Cu), whereas the new cavity is made from
OFHC; that makes temperature control to be easier.
To keep tuning condition stable, SUS materials are
partly used as mechanical supports in out-of-vacuum
surfaces. The new cavity has 2 movable plunger-type
tuners; one is used for auto-tuning and the other isusually fixed and can be used for detuning
higher-order-mode. These tuners have enough tuning
width for operation of the cavity (full width of 400
kHz for each tuner). Input coupler for previous cavity
was air-cooled, but new coupler has water-coolingchannel to cool especially ceramic window part.
Figure 3 shows a photograph of new RF cavity justafter installation to the UVSOR-II. After installation

Previous
90.1 MHz
55 kV
0.5 M:
8000
1.75
SUS + Cu
Re-entrant×1
Air-cooled
Plunger×1
1000mm
50mm

Present
90.1 MHz150 kV2.45 M:
20300
1.34
Cu (OFHC)
Re-entrant×1
Water-cooled
Plunger×2
1175mm55mm

Fig. 3 Photograph of new cavity covered withjacket heaters. A view from outside the storage ring.
[1] A. Mochihashi et al., UVSOR Activity Report
2003 (2004) 35.

35

BL5U(FEL)High Power FEL Lasing in the Deep UV Region

M. Hosaka1, M. Katoh1, A. Mochihashi1, J. Yamazaki1, K. Hayashi1,
Y. Takashima2, H. Nishino3, M. E. Couprie4
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 Japan
3
Entropy Control, Japan Science and Technology Agency, Toyonaka 560-0085 Japan
4
CEA/SPAM, Gif-sur-Yvette, France
300

FEL Power (mW)Power (mW)

Reflectivity (%)

I (mA/2bunch)

1000Fig. 2current.

10

20

100

30

0

Out-coupled FEL power and the beam

During the lasing experiment, we measured roundtrip reflectivity as the front mirror was irradiated bysynchrotron radiation from the helical optical klystrontype undulator. Fig. 1 shows measured round trip
reflectivity as a function of irradiation dose. As thedose increases, the reflectivity decreases quickly but
the degradation seems to be saturated at a dose morethan 100 mA hours. The averaged value of thereflectivity after the irradiation was 97.4%. With thereflectivity, the calculated threshold beam current was
25 mA/bunch and was almost consistent with themeasured threshold current of 34 mA/bunch. With
such a high threshold current, it is important to store
high beam current in the storage ring. By
optimization of the injector system, we have beenable to store a beam current of more than 100
mA/bunch. Then we could succeed in the lasing withthe mirrors and we proceeded to the application
experiment. In the application experiment, we
irradiated a biosohere with a circularly polarized FEL
and analyze a chirality of the product. During theexperiment, power irradiated on the sample wasrecorded and an example is shown in Fig. 2. As seenin the figure, the maximum extracted power was 0.28
W but it decreased as the beam current decreased.
The average power was about 0.2 W and was foundto be enough to dissociate the sample.

98

97

200
400
Dose (mA H)

200

Time (min)

99

0

200

0

100

96

300

I (mA/2bunch)

An out-coupled power of a storage ring freeelectron laser (FEL) is a critical issue for its
application experiments. Recently we planned toapply the UVSOR-FEL to an irradiation experimenton biosohere [1]. In the experiment, a switchable
circular polarized laser in the deep UV region (200 ~300 nm) is necessary. We already succeeded in lasingin the wavelength region in 1997; however theout-coupled power was too small for further
application experiment. At that time, we had toemploy high reflectivity mirrors for an optical cavity
and the transmission which is proportional to theout-coupled power was small. While since theupgrade of the storage ring (UVSOR-II), the FEL
gain has been increased by more than factor 2
because of smaller beam size and we can employ hightransmission mirrors that allow high out-coupledpower.
The lasing experiment was made is in the
wavelength region around 255 nm. Multi-layers of
HfO2/SiO2 were chosen as cavity mirrors because
they
have
the
highestefficiency(=
transmission/cavity loss) at the wavelength. Numbersof layers of the mirrors were chosen to be 19 and 15
for front and backward cavity mirrors, respectively.
We employed small number of layers for the
backward mirror because higher transmission wasneeded for the mirror from which the laser power was
extracted. The calculated round trip reflectivity is
99.3% and the transmission of the backward mirror is
0.4%.

600

Fig. 1 Roundtrip reflectivity of the cavity mirrors as
a function of irradiation dose.

[1] H. Nishino et. al., in this issue.

36

BL6B

Obsevation of Intense Far-Infrared Synchrotron Radiation at UVSOR-II

Y. Takashima1, M. Katoh2, M. Hosaka2, A. Mochihashi2, S. Kimura2 , T. Takahashi3
1
Graduate School of Engineering, Nagoya University, Chikusa-ku Nagoya 464-8603 Japan
2
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan
3
Research Reactor Institute, Kyoto University, Kumatori-cho, Osaka, 590-0494 Japan
We have observed intense far-infrared synchrotron
radiation at the beam line BL6B [1] of UVSOR-II
storage ring [2] operated in single bunch modes. The
wave length region of the far-infrared radiations
which are detected by using an InSb bolometer isbetween 0.2 mm and 3.0 mm. The duration and theperiod of the bursts are about 200 µsec and 10 - 15msec, respectively. Each burst consists of several
quasi-periodic micro-bursts whose periods are about
30 µsec. The peak intensity of the bursts is about
10000 times larger than the intensity of synchrotron
radiations in the same wavelength region observed innormal multi-bunch modes. We expect that the burstsof the far-infrared radiation are “coherent synchrotron
radiations”.

the bursts are about 10000 times larger than theintensity of the ordinary synchrotron radiations of thesame wave length at the same average beam current.

Summary
We observed bursts of far-infrared synchrotron
radiation in the wave length region between 0.2 mm
and 3.0 mm at BL6B of UVSOR-II. The bursts weregenerated when the beam current exceeded a certain
threshold current. We expect that the bursts arecoherent synchrotron radiations caused by density
modulations of electrons in a beam bunch.

Lock-in Amp. output (a.u.)

100

Experiments and Results
We measured the bursts of far-infrared synchrotron
radiation at the BL6B beam line constructed for using
the synchrotron radiation of infrared region [1]. The
electron beam energy was 600 MeV in our
measurements.
We used an InSb hot electron bolometer (Infrared
Laboratories, Inc) to detect the far-infrared radiations.
The detector was sensitive to the wavelength region
between 0.2 mm and 3 mm. The response of thedetector was several microseconds.
Fig. 1 shows the average intensities of the
far-infrared radiations in the single bunch mode (dots)
and the multi-bunch mode (gray line) with theaverage beam currents. The far-infrared radiations
passing thorough a chopper (100 Hz) were detectedby the InSb bolometer. The output signals from thebolometer were inputted in a lock-in amplifier. In themulti-bunch mode, the intensity of the far-infrared
radiations is proportional to the beam current. In the
single bunch mode in lower beam current, the averageintensity is also proportional to the beam current andis the same as it in the multi-bunch mode. However,when the beam current is over 140 mA, the intensityof the far-infrared radiations is suddenly increased.
There are also increases of the intensity at around 80
mA.
Fig. 2 shows the time structure of the far-infrared
radiations at the beam current of 178 mA measured
by using an oscilloscope. The radiations are generated
as periodic bursts at this beam current. The period of
the bursts is about 10 – 15 msec.
The time structure of a typical burst at 201 mA is
shown in Fig. 3. The pulse width of the burst is about
200 msec in which there were several micro-bursts of
the width of about 30 msec. The peak intensities of

10

1

30

40

50

60 70 80 90100

200

Beam current (mA)

Detector Output (a.u.)

Fig. 1 Average intensity of THz-radiaion.
(Vrf=55kV Ib=178mA)

2
1.5
1
0.5
0
-0.5

0

0.05

0.1

0.15

0.2

Time (sec)Fig. 2Time structure of THz-bursts.

Detector Output (a.u.)

1.5 (Ib=201mA)
1
0.5
0
-0.5
0.0098Fig. 30.01
0.0102Time (sec)0.0104

Time structure of a burst.

[1] S. Kimura et al., AIP Conf. Proc. 705.
[2] M. Katoh, Journal of Japanese Society for
Synchrotron Radiation Research, 17(1) (2004)10.

37