Feasibility Study of Generation and Observation of Far Infrared Coherent
Synchrotron Radiation at UVSOR
Y.Takashima, M.KatohA, M.HosakaA, A.MochihashiA and S.KimuraA
Graduate School of Engineering, Nagoya University, Chikusa-ku Nagoya 464-8603 JapanA)UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan
BL5U undulator (modulator) and then traveled tothe BM6. The ratio of the coherent to incoherent
radiation intensity generated from the bunch is abut 2× 105 at the wavenumber of 20 cm-1 (wavelength is0.5 mm) when the beam current is 18mA/bunch (N is2 × 1010).
In order to observe the infrared radiation, we plan to
use the InSb semiconductor bolometer at BL6. The
detector can be used to detect the radiation ofwavenumber from 5 to 50 cm-1; in the region thecoherent radiations are expected to be yielded much
more than incoherent radiations [6].

We have been considering a method for thegeneration of far infrared coherent synchrotron
radiation from UVSOR storage ring.
Far infrared coherent radiations can be emitted
when the electron bunches have density modulations[1]. In order to obtain intense far infrared synchrotron
radiations, we have been studying the feasibility togenerate coherent radiations by using the bunch slicing
technique [3,4,5].
The bunch slice can be achieved by using a laser
pulse passing together with an electron bunch throughin an undulator [5]. We tune the undulator to thewavelength of the laser, so that the energy of electrons
overlapped with the laser are modulated by theinteraction with the laser field. If the energy
modulation is several times lager than the r.m.s. energy
spread of the electron beam, the modulated electrons
are separated spatially and a dip is made in the bunchwhen the electrons pass through a dispersive section.
The modulated electron bunch with a dip can emit
coherent radiations of which wavelength is in the sameorder of the size of the dip.

Summary

We have been planning to generate and observe far
infrared coherent synchrotron radiation using thebunch slicing technique at UVSOR. A commercial
short pulse laser can be used to slice electron bunches
and make dips which are indispensable to generate
the coherent radiations. The averaged intensity of the
far infrared coherent radiation is about ten times
larger than the incoherent radiation if we use the laserof 5 kHz repetition rate and make one dip in eachbunch. We can observe the coherent radiation at BL6
using the InSb bolometer.
For further studying, we should consider methods
to improve the intensity of the radiation, for example,
how to make dips periodically in a bunch.

Experimental Equipment
We will use the existing BL5U undulator as anenergy modulator in which circulating electronsinteract with the laser pulses.
A mode-locked Ti-sapphire laser and an ultrafasst
regenerative amplifier is used to make femtosecond
laser pulses which should have enough power to makesufficient energy modulation in order to make a dip[5,6].

[1] G.L.Carr et al., Nucl. Instr. and Meth. A463, 387(2001)[2] H.Weidemann, “Particle Accelerator Physics”,
Springer-Verlag, (1993)[3] A.A.Zholents and M.S.Zolotorev, Phys.
Rev.Lett., 76, 912 (1996)[4] R.W.Schoenlein et al., SCIENCE, 287, 2237
(2000)[5] Y.Takashima et al., UVSOR Activity Report,(2001)[6] Y.Takashima et al., UVSOR Activity Report,(2003)

Intensity of Coherent Radiation
The ratio of the coherent to incoherent radiation
intensity is expressed by the following equations [1,2],P(Z ) coherent# N  f (Z ),P(Z ) incoherent

where Ȧ is the frequency and N is the number ofelectrons within a bunch. P(Ȧ)incoherent and P(Ȧ)coherent
are the power of incoherent and coherent radiations,
respectively, emitted from a electron bunch. Becauseof the large number of N (~1010), coherent radiations
can dominate over the incoherent radiations.
The beam tracking simulations has confirmed thata dip can be made in the bunch at the exit of BM6 [6].
The electrons of the width of 1 ps in a bunch weremodulated their energy up to 0.8 % in the existing

33

BL5U

The First FEL Lasing on the UVSOR-II Storage Ring

M. Hosaka, M. Katoh, A. Mochihashi, J. Yamazaki, K. Hayashi, Y. Takashima*
UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585 Japan*Depertment of Materials Processing Engineering, Graduate School of Engineering,
Nagoya University, Chikusa-ku, Nagoya 464-8603 JapanThe upgraduation of the UVSOR storage ring wassuccessfully done in Spring 2003 and the emittance ofthe electron beam has been reduced by factor 6
comparing the previous value [1]. The improved
performance of the UVSOR-II electron beam is ofgreat advantage for the free electron laser experimentand we expect lasing in much shorter wavelength.
The FEL experiment using UVSOR-II was begun in
October 2003. Before the lasing experiment wedetermined a beam trajectory for the FEL experimentby measuring spontaneous radiation spectra from thehelical optical klystron (HOK) [2]. On December 1st
the first lasing was achieved after an adjustment of
the optical cavity. The lasing wavelength was visible
region around 420 nm. Looking at an extracted FEL
power, we made more accurate adjustment of thecavity mirror. In the experiment, multi-layers of
Ta2O5/SiO2 was employed for the cavity mirrors. The
round-trip reflectivity of the cavity mirror wasevaluated by a ring-down method and the deduced
value was 99.25 %. Reducing an electron beam
current, a threshold beam current was measured andthe obtained value was 16 mA/2-bunch. Thus the FEL
gain is estimated to be 0.75 % at the electron beam
current of 16 mA/2-bunch. This value is comparedwith calculated FEL gain in Fig. 1. As seen in thefigure, although the measured gain was higher thanprevious value of the UVSOR-I, it is smaller than thecalculated one of the UVSOR-II. This indicates thatthe optimization of the FEL system has not been done
satisfactory.

Figure 2: Temporal profile of the FEL at the beam
current of 68 mA measured using a dual sweep streakcamera.
We suppose that the adjustment of the overlap
between the laser and the electron beam whose size is
reduced in the upgraduation of the storage ring is notsufficient. We are going to employ correction
magnets equipped inside HOK to maximize theoverlap.
In order to investigate characteristics of the lasing,
we measured a temporal profile of the FEL using adual-sweep streak camera. Figure 2 shows one ofobtained profiles. The shortest temporal width was 5
ps, but the stable lasing did not last more than 10msec. This is because the electron beam wassuffered by the transverse/longitudinal instability
during the measurement. The source of the instability
is supposed to the interaction with the RF cavity
system (main cavity and 3rd harmonic cavity). We aregoing to optimize the operating point of the storage
ring to suppress the instability.
In summary, we have successfully obtained the FEL
lasing with UVSOR-II, however its performance is
not satisfactory. More studies are needed for lasing inshorter wavelength.

FEL gain (%)

32

UVSOR-II[1] M. Katoh, SYNCHROTRON RADIATION
NEWS, Vol. 16, No. 6, 33 (2003).
[2] M. Hosaka et al., Proc. of the 3rd Asian Particle
Accelerator Conference (2004).

UVSOR-I
Measured(UVSOR-II)

1

Measured(UVSOR-I)
001020304050

Beam current (mA/2-bunch)

Fig. 1: FEL gain as a function of the beam current.
Measured gains for the UVSOR-I and UVSOR-II arecompared with calculated ones.

34

Improvement Plan of RF Cavity in UVSOR-II Electron Storage Ring
A. Mochihashi, M. Katoh, M. Hosaka, K. Hayashi, J. Yamazaki, *Y. Takashima
UVSOR, Institute for Molecular Science, Okazaki 444-8585 Japan*Graduate School of Engineering, Nagoya University, Chikusa-ku Nagoya 464-8603 Japan
The improvement of the UVSOR electron storage
ring[1] has been finished successfully, and atransverse emittance of the electron beam has been
decreased from 165nm-rad to 27nm-rad. Users runs
have been performed since September 2003 with highbrilliance beam optics, and since then high brilliance
SR beams have been supplied routinely for users.
Because of the decrease in the transverse emittance,
however, Touschek effect becomes prominent and acondition for a beam lifetime becomes severe; before
the improvement (H=165nm-rad) the beam lifetime
was about 1700mA·Hour at the multibunch operation,
and after the improvement the beam lifetime
decreases to 1000mA·Hour in the low-emittance
operation (H 60nm-rad in the achromatic optics). To
increase the beam lifetime and make full use of theimproved SR beams, we have planned to improve themain RF cavity of the UVSOR-II electron storagering.
The aim of the plan is to increase the beam lifetimeby increasing in accelerating voltage and easing theTouschek effect. Figure 1 shows change in theTouschek lifetime on the accelerating cavity voltage(Vc) at the achromatic low emittance beam optics(H=60nm-rad). At the present cavity voltage (Vc=55kV) the I·WTouschekproduct becomes 1650mA·H in thex-y coupling of 3%; on the other hand, the I·WTouschek
product is estimated to be 5200 mA·H at the cavity
voltage of 150 kV, as seen in the figure. Figure 2
shows calculated value of change in transmitter
power on the cavity voltage. In the calculation, theshunt impedance (Rs) of the RF cavity is assumed tobe Rs=2.2M:that is one of appropriate values fromthe calculation with Poisson/Superfish[2] for typicalre-entrant RF cavity whose resonant frequency isaround 90 MHzAs seen in the figure, the transmitter
power to generate the cavity voltage of 150 kV isestimated to be less than 20 kW, that is the maximum
power of the transmitter that is in operation in theUVSOR now. From the estimation, the target valuesof the accelerating voltage and the I·WTouschekproducthave been settled to be above 150kV and 5000 mA·H,
that is comparable to the designed parameter ofMAX-III[3].
To realize those target values, it is necessary torenew the RF cavity because the shunt impedance ofthe present RF cavity has only 0.5M:. On the otherhand, the transmitter that is in use now can be applied
to the new RF cavity because it is estimated thetransmitter has enough output power to realize thetarget value of the cavity voltage. Table 1 shows thebasic specification of the new RF cavity. As seen in
the table, the basic parameters of the RF system suchas the RF frequency will not be changed even in thenew cavity. The new cavity would be build with pure
oxigen-free-copper chamber to stabilize temperatureof the cavity easily. The target value of the shunt
impedance is 2.2M:to realize enough cavity voltage.
The cavity will be built in 2004 and installed in S2
section (B1~B2) in the spring of 2005. After theinstallation and tuning of the new cavity, the present
cavity will be removed from the ring.
Table 1. Basic specifications of present/planned
cavity.
Present
PlannedFrequency
90.1 MHz90.1 MHz
Cavity voltage
55 kV
150 kV
Shunt impedance
0.5 M:
2.2 M:
Material
SUS + CuCu (OFHC)
Cells
Re-entrant×1
Re-entrant×1
Coupler
Air-cooled
Water-cooled
Tuner
Plunger×1
Plunger×2

Fig. 1. Dependence of the calculated Touschek
lifetime on the cavity voltage.

Fig. 2. Transmitter power for the cavity voltage.
[1] M. Katoh, in this report.
[2] J. H. Billen and L. M. Young, Poisson/Superfish
version 7.11.
[3] Å. Andersson et at., Proc. of the 8th European
Particle Accelerator Conference, p.36-38.

35

Estimation of the Impedance of UVSOR-II Electron Storage Ring
A. Mochihashi, M. Hosaka, M. Katoh, K. Hayashi, J. Yamazaki, *Y. Takashima
UVSOR, Institute for Molecular Science, Okazaki 444-8585 Japan*Graduate School of Engineering, Nagoya University, Chikusa-ku Nagoya 464-8603 Japan
In the improvement of the UVSOR electron storage
ring[1], almost all of beam ducts, especially the beamducts for straight sections, have been replaced, andmoreover, two in-vacuum undulators[2] have been
installed in the straight sections. We have measured
effective impedance of the UVSOR-II electron storage
ring by measuring threshold beam current oflongitudinal microwave instability. Another values
associated with the effective impedance have beenestimated, too.

length at the threshold current, respectively. From themeasurement of the bunch length with a streak camera,
the ratio Vth/V0 is estimated to be 2.12 in case of
Ith=70mA. From Eq. 1, the critical impedance isestimated to be Z n crit =1.2:
The effective impedance[4] Z n eff is given by­°§ V · 2 ½°
¨ th ¸  1¾ 4.2: ,n eff
n crit ®¨ V ¸°¿°̄© 0 ¹
which is larger than the measured value before theimprovement of the UVSOR[5]. The cause of theincrease in the effective impedance has not yet become
clear; it might be because of the two in-vacuum
undulators that have been installed at theimprovement.
From the longitudinal effective impedance, thetransverse broad band impedance ZT can be estimatedby the approximate relation:
2R Z,
ZT
(2)
b 2 n eff
where b is the effective radius of the beam duct. For
the beam duct which has an elliptical cross section theeffective vertical radius is written as[4]a,(3)b12^2 [1  H 1 ` 3where 2a is the horizontal width of the beam duct, and[1 and H1 are the Laslett parameters[6], respectively.
From Eq. 3, the transverse broad band impedance isestimated to be ZT=1.8×105 :m. From the broad bandtransverse impedance, the dependence of the verticaltune on the beam current can be given bydQ yr0 bcZ T,(4)
16Sef rev2 JCZ EdI bZ

Measurement
To measure the threshold current of the microwave
instability, we have stored single bunch and observed amodulation factor of the spectrum emitted from theoptical klystron. Because the modulation factor
depends on the energy spread of the electron beam, thethreshold current of the microwave instability can bemeasured by observing the change in the modulation
factor of the undulator radiation from the optical
klystron on the beam current. Figure 1 shows change
in the energy spread on the beam current by measuring
the modulation factor by Hosaka[3] in the beamenergy of 600 MeV. As seen in the figure, it is clearlyseen that the energy spread drastically increases
around the beam current of 70 mA.

where r0 is the classical electron radius, c is the speedof light, frev is the revolution frequency of the beam, Jis the Lorentz factor, C is the circumference of the ringand Z E 2S is the vertical betatron oscillation

Fig. 1. Measured value of the change in the energy
spread on the beam current in UVSOR-II.

frequency, respectively. From Eq. 4, the change in thevertical tune on the beam current by the transverse
broad band impedance is estimated to be -3.3×10-5/mA in UVSOR-II.
[1]M. Katoh, in this report.
[2]in the Proc. of 8th Int. Conf. on Synchrotron
Radiation Instrumentation (to be printed)..
[3]M. Hosaka, private communications.
[4]A. Hofmann and J. R. Maidment, LEP-Note 168(1979).
[5] A. Mochihashi, UVSOR Activity Report 2001.
[6]L. J. Laslett, PUB-616, Lawrence-Barlkeley
Laboratory.

Estimation of the Impedance
The threshold current[4] Ith of the microwave
instability is written as
3I th2

2S k bQ s20 E § V 0 · § V th ·¸ ,¨ ¸¨eD Z n crit © R ¹ ¨© V 0 ¸¹Z(1)where kb is the RF wave number, Qs0 is the synchrotron
tune, E is the beam energy, e is the electron charge,
Dis the momentum compaction factor, Z n crit is thecritical impedance[4], R is the average ring radius, andV0 and Vth are the natural bunch length and the bunch

36