The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

Infrared FEL lasing at Nihon University and blow up visible light
Isamu SATO, Kazuo SATO, Youichi MATSUBARA, Ken HAYAKAWA,Toshinari TANAKA, Yasushi HAYAKAWA,
Hiroyuki NAKAZAWA*, Kazue YOKOYAMA*, Koichi KANNO*, Takeshi SAKAI*, Kenichiro ISHIWATA*,
Hiroyasu INOKAWA*, Yoshihiro NAKAMURA*, Eiko HASHIMOTO*, Kazumasa FUJIOKA*,
Keisuke NAKAO*, Takuya MURAKAMI*
Shozo AMANI**, Sigeki FUKUDA**, Hitoshi KOBAYASHI**, Atsushi ENOMOTO**, Satochi OHSAWA**,
Tetsuo SHIDARA**, Seiya YAMAGUCHI**, Takuya KAMITANI**, Shinichiro MICHIZONO**, Kimichika TSUCHIYA**

Atomic Energy Research Institute, Nihon University, Narashinodai 7-24-1, Funabashi-shi, Chiba-ken, Japan*College of Science and Technology, Nihon University, Narashinodai 7-24-1, Funabashi-shi, Chiba-ken, Japan**High Energy Accelerator Research Organization (KEK), Oho 1-1, Tsukuba-shi, lbaraki-ken, Japan
Abstract
Laboratory for Electron Beam Research and Application (LEBRA) of Nihon University has proceededwith the development and research of the electron linear
accelerator (linac) toward free electron laser (FEL).
LEBRA is designed FEL to lase intense light from S!J.m
to 0.35!J.m without using of subharmonic buncher. It
uses an undulator to provide flexibility for user experiments. First lasing of FEL at LEBRA succeeded in awavelength of l.S!J.m on May 26, 2001, and it was generated by energies of 83.6MeV and electron current of
100ma. During initial tests, the intensity of FEL
reached about 108 times of light which accumulated
spontaneous emission into an optical resonator at
off-lasing. However, a saturation phenomenon within
the lasing of FEL isn't observed yet. However, we areobserving the phenomenon that visible light rises fromthe flash at the same time when it is made to do thelasing intensely in the wavelength of the 1.5 !J.ID. We report about the detailed progress of the development of
the linac and FEL.

1. Introduction
The FEL project of Nihon University was proposed
to promote research of applied science. This projectstarted in 1994, and has been requested to advanceresearches in physics, chemistry, biology, engineering,
medical science and medical treatment [ 1,2,3,4] . However, a budget of project was not enough. It wasplanned so that the accelerator is fabricated by using
the element parts supplied from the various old one to
make up lack of the fund. Fabrications of accelerator
and FEL system completed in March,1998. The firstcommissioning of linac was performed at January of1998 [5,6,7]. At that time, it succeeded in accelerating
the electron beam of 20ma in the energy of 90MeV,
and then the beam current of electron reached 220ma in
October,1998.
It confirmed that a spontaneous emission was accumulated by an optical resonator in March, 2001. It succeeded in a FEL lasing of l.S!J.m by using the electron
beam of 100ma accelerated by the energy of 83.6MeV
in May, 2001. After that, the electron beam of
83.6MeV and 40ma used at the 3rd test experiment, andthe laser strength reached about 6x107 of the accumulated light [8,20].

We sometimes observed the flash of the visible lightin the test experiment of Infrared FEL. Why does suchlight occur? That process is being analyzed.
2. Progress to the FEL lasing
The first time, we have being tried such FEL thatmight produce the wavelength of from S!J.m to 0.35!J.mby using one undulator. When the undulator was badly
damaged by the radiation, we thought that the ranges of
the wavelength should be covered with two undulatorsin order to ease its radiation damage.
The ranges of wavelength for FEL were assigned respectively from S!J.m to 0.8!J.m and from 1.8!J.m to
0.35,um. The parameters of a new undulator were designed as f...w = 48mm, Nw =50 and K = 1.2. Here f...w
is one periodic length of the undulator, Nw is theperiodic number of the undulator. K is the undulator
parameter which is shown in K = eBf...w/2:n:mec, where
B is the peak magnetic field of the undulator, me is therest energy of the electron, and c is the light velocity.
The lasing wavelength f...p is defined as(1)
f...p=f...w (1+K2/2) /2y 2,
where y is relativistic factor and is given as y = (E+me)/me, and E is the kinetic energy of the electron. The
optical system was set up to lase at wavelength of
l.S!J.m. The dielectric multilayer mirrors which have thereflectivity of 99.5% was used for the reflector of the
optical resonator. For the lasing of the wavelength of
1.5(lm, the value of y is 166 from (1), therefore, theenergy of the electron becomes E=84.3MeV. In thiscase, the accelerator of our facilities can cope with thisenergy fully.
On the other hand, the electron which passes throughthe undulator radiates energy directly to the accumulated photon. The energies transferring from the electron
beam into the accumulated photon are in proportion to
the energies of photon stored in the resonator. The rateof increase of the coherent light stored in the resonatoris the gain of FEL. The gain G is specified by the
structure of the undulator and the characteristics of theelectron beam. The gain G depends on the structure of
the undulator and the characteristics of the electron
beam. If the characteristics of the electron beam don'ttake it into account, G is given as follow.
(2)
G"" f...w 2 Nw3 K2/y 3
In our case, G was 9x1o-s, and there was no inferior--117-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

ity in comparison with the example being lased byother PEL system. The high sensitivity Infrared detector(InSb:Kolmar Technologies KISDP-1-J8 40000V/Watt)
was prepared for a purpose of investigating the minuteaction of the accumulated light into the optical resonator.

Fig.1 The shape of an intensity respond ,to the accumulated light into the opical resonator (V: 2mV/div, H:
41J.s/div). The rectangle is the wave form of electron
beam (V:20ma/div) .
-~····~·-······~-,'

as the length of the optical resonator might deviate
from the value of the design. Thus, we measured theabsolute value of the distance between two mirrors into
the resonator by using the primitive ruler. After that thetest experiment of FEL was enforced continuously,
however, the symptoms of the lasing didn't appear. We
thought that the parameter contributing to the gain havegreatly deviated, then doubted the setting condition of
the undulator. But, there was no problem. Next, wereexamined all the devices which were set upon the beamline of PEL system. As that result, we found the difference of about 3mm in the gap interval on the entranceside of the undulator and the exit side. As for the difference of this interval, the magnetic field strength was
equivalent to the difference of about 20% on the entrance of the undulator and the exit. The number of 50
period of the undulator was evaluated as about 5 fromthe viewpoint of actual effect. Therefore, substantial
gain G decreased to less than 1/1000, and it is to beclear from (2). This accident was repaired at once. We
succeeded in the first lasing of PEL at the test experiment of the next day. Then, the quantity of electron
beam was the energy of 83.6MeV and the current of
about 100ma.
The intensity of the spontaneous emission accumulated into the optical resonator was measured with the
InSb detector with 1mm active area, and the output signal of this was displayed as an integral wave formshown in the figure 1. On changing the length of the
optical resonator, the integral wave form of the intensity was changing gradually, and became a wave formshown in the figure 2.
, .. ,....,........,.,.,.

''!'"'"""""""""'"' i

·t·

+

.

.,
+

Fig.2 The shape of an intensity respond to the poorlasing light into the opical resonator (V: 5mV/div, H:
41J.s/div). The rectangle is the wave form of electron
beam (V:20ma/div) .
3. The document of the FEL lasing
The basic experiment for the Infrared FEL wasrestarted again in March, 2001. The spontaneous emission was accumulated into the optical resonator as
shown in the figure 1. Though we changed the lengthof the resonator, the symptoms of the FEL lasing was
not found at all. On the other hand, it had been pointed
out that the precision of the length of the optical resonator wasn't so accurate. There was such a possibility

Fig.3 The shape of an intensity from the saturated
detector respond to the violent lasing light into the
opical resonator (V: 1V/div, H: 41J.S/div). The rectangle
is the wave form of electron beam (V:20ma/div).
The spontaneous emission ejected from the front
electron beam was accumulated into the resonator, andthe energy of the back· electron beam was transferred tothe accumulated light. In other words, this is the amplification of the accumulated light, and is equivalent to
the phenomenon of the positive feedback in the opticalamplifier which included two mirror into the feedback-118-

The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

circuit. In the lasing, the intensity of this grew large
gradually by the adjustments of the accelerator parameter such as the phase, the electron beam orbit and thefocus. As the results, the output signal of the detector
exceeded 4V as shown in the figure 3, and then theamplifier of this was saturated at last. The output signal
was estimated to about 24V from the decay curve of it.

reason depends on the prebuncher that is composed ofseven cavities. That has a function which makes theelectron beam bunch and accelerate.
If the amount of charge of the electron beam issmall, the prebuncher has such ability as have bunched
the electron beam in the pulse of the subpicosecond, as
shown in the figure 6.
!

··;~T"'''·····

Progress figure of FELJ.asing
Intensity (mv>Cfttensity/Oitplt ~V/mWl

Fig.5 Upper : The shape of the intensity of the visible light (V: 500mV/div, H: 10J.Ls/div). Lower : The
shape of an intensity from the saturated detector respond to the violent lasing light into the opical resonator (V: 1V/div, H: 10J.Ls/div). The rectangle is the wave
form of electron beam (V: 20ma/div). Scale over : The
shape of an intensity from the saturated detector respond to the violent lasing light into the opical resonator (V: 2mV/div, H: lOJ.Ls/div).
Fig.4 The progress of the intensity reinforcement in
the lasing experiments. The vertical scale is theintensity of Infrared
The output signal was equivalent to about 2000 timesof the accumulated light. Through the two times test
experiment after this, the signal is strengthened asshown in the figure 4, and reached the intensity of
about 108 times of the accumulated light.
When FEL is strongly lased, we sometimes observe
the flash of the intense visible light as shown in the
figure 5 [8,18,20]. We haven't been able to do an analysis about the rare occurrence mechanism of this lightyet.
4. The FEL lasing and this meaning
The lasing experiment of the third time was tried byusing of the beam current of 40ma with energy of
86.8MeV. At this time, the diameter of the electron
beam in the center of an optical resonator was about
0.5mm. The detector was installed in the place to leavethis point about Sm. According to the measurement of
the detector, the energy of one macro-pulse reachedabout 0.3mJ/mm2 , and a peak power got over about
8kW/mm2 • Though FEL was lased even by a little beam
current, tbis lasing is based on the electronic beam of
the high density .. ~ other words, we think that it is because a bunching of the electron beam is fine. That

5. Discussion
The lasing of FEL requires the electron beam of the
big peak current of the small emittance with energy ofthe little dispersion. Therefor, there are historical details
that the high frequency electron gun was developed forthe small emittance and also the subharmonic buncher
was developed for the big electron current.
The gain is small when the beam current is poor.
Then, the long electron beam. of the duration time isnecessary to make up this. In this case, a superconductive electron linear accelerator has been used for theoperation of long pulse. Then, the lasing of FEL was a
successful in only using of these special devices. These
devices became established as a paraphernalia of thelasing of FEL. The electron linear accelerator for FELof the Nihon University isn't using the high frequency
electron gun and the subharmonic buncher as shown in
the figure 7. This broke common sense until now. Then,it became the first example in the world that the ordinary electron linear accelerator was used in a lasing of
Infrared FEL.
At present, the dielectric multilayer mirror is being
used for the optical resonator. The reflectivity of l.5J.Lm
wavelength of this mirror is about 99.5%. But, thereflectivity of 0.5J.Lm wavelength which is the 3rd highharmonics is about 15%. Therefore, it can't think that

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The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001

the flash of the visible light is FEL lasing of the 3rdhigh harmonics. The forward radiated energy from theelectron beam travelling in a single pass through theundulator with the strength parameter k grows exponentially. If the parameter of K depends on the magnetic nonlinear field of the undulator strongly, the 3rd highharmonics of FEL will be enhanced. The high harmonics of FEL will form microbunch structures into theelectron beam. This is consistent with predictions of
Self-Amplified Spontaneous Emission (SASE) . At thattime, we shall observe the flash into the visible light.
6. The troubles and the developments
It became clear in September, 1999 that a permanent

magnet of the undulator that has sustained a radiation
damage[l1,12]. We determined to make the new
undulator used in the wavelength in the range from
Spm to 0.8pm. A new undulator was delivered in
March, 2000 [13]. The high harmonics spectrum of the
spontaneous emission was measured in June, 2000
[14]. As for the time of the first operation, the output
pulse of the klystron has been narrowed by the durationof less than 10!-ls because of the discharge of that
output-window. We executed the test experiment to extend to 20!-ls the pulse duration of the klystron.
The output-window of the klystron was damaged into
the experiment one after another. The one more highfrequency window was installed in the wavegiude connected to the output window of the klystron [ 16] .
However, this high frequency window was neverdamaged though the same amount of high frequency
electric power passed through the output-window of the
klystron and this high frequency window. We got a hintfrom the above mentioned fact, and installed two new
ion pumps near the output-window, and tried to discharge the gas radiated from that surface rapidly.
The long pulse operation of the klystron became possible as that result [16,17]. The operation of the accelerator was stopped for five months from July, 2000
until December for an experimental hall building extensively. Accelerator operation was resumed in February,
2001. The value of G, which took an emittance, a peakcurrent, and so on into account, have to be bigger thanthe loss coefficient of the reflector. The development ofa low emittance and an intension of a peak current were
executed in order to improve the character of electron
beam [17,19]. A new cathode of a small diameter was
prepared to make the emittanc of the electron beamsmall, and the electric power of the buncher was increased for the intension of a peak current [ 8] .
Electron beam to be used in FEL was expected to
produce high quality photon sources. We proposed suchprojects as spontaneous emission of ultraviolet and aparametric X-ray source in 1999 [ 12]. We are hoping
for that challenging application research will be promoted by using these photon sources at near future.
7. Conclusion
We were proceeding with the improvement of theaccelerator for the past six years. Such troubles as wellas the pulse transformer, the backdiode circuit, the insulated transformer of klystron heater and the corrosionof cooling, and so on became extinct as that result. The
stability of the accelerator was improved by the introduction of the stabilization power supply, the repair ofstrengthening of the focus system, the arrangement of
the accelerator structure, the strengthening of the vacuum system around the klystron high frequency window,
the improvement of the waveguid union defect, thephase compensation of the high frequency amplifier, thefine tuning of the thyratron, and so on. The long pulse
operation of the klystron for the short pulse and thesmall emittance of electron gun were attained. The advance of the accelerator progressed steadily, and theperformance of the electron beam was improved remarkably. The measurement system was prepared withthe monochrome meter, the streak camera, and the highsensitivity CCD camera. It became possible that the
spectrum of the spontaneous emission and a bunchedelectron beam were measured in real time as that result[9,10]. Because the permanent magnets sustained radiation damage, the short wavelength FEL became use
impossibility [ 11] .
So, we introduced the undulator that the length of theperiod was two times. The experiment environment of
the FEL lasing was prepared. Then, we made theInfrared FEL be lased by the construction of the accelerator which breaks the common sense of FEL.
However, FEL is still unstable [ 14,16], and it can't
be 'used for the joint use experiment. It is necessary toenhance the stability of the accelerator.
Acknowledgements
The authors would like to acknowledge helpful discussions with members of the FEL research community.
Works have been supported by Nihon University andHigh Energy Accelerator Research Organization andmuch research organization and many enterprises.
The laser lasing is the biggest research result in the
joint research on the Nihon University and the high energy accelerator research organization. The authors are
deeply grateful to the members of the high energy accelerator research organization who supported this project. Furthermore, this project has been supported by the
Y.Sezai Nihon University president and the K.Kojima
Atomic Energy Research Institute head especiallystrongly. The authors appreciate heartily to both leaders.
References[1] I.Sato, et al., Proc. of 22nd LAMJ, 104-106 (1997).
[2] K.Hayakawa, et al., Nucl.Instr.and Meth. A375, ABS25-26(1996).
[3] T.Tanaka, et al., Nucl.Instr.and Meth. A407, 11 103-104(1998).
[4] T.Tanaka, et al., KEK Proceedings 98-10 Nov. 722-724,
1998 A.(APAC98)[5] T.Tanaka, et al., Proc. of 23rd LAMJ, 25-27 (1998).
[6] I.Sato, et al., Proc. of 23rd LAMJ, 22-24 (1998).
[7] K.Hayakawa, et al.,Proc. of 23rd LAMJ, 166-168 (1998).
[8] I.Sato, et al., Proc. of 26th LAMJ, 30-33 (2001).
[9] K.Yokoyama, et al., The 12th Sym. on Accel. Sci. andTech., Riken,Japan, 473-475(1999).
[10] H.Nakazawa, et al., The 12th Sym. on Accel. Sci. andTech., Riken,Japan, 394-396(1999).
[11] I.Sato, et al., The 12th Sym. on Accel. Sci. and Tech.,
Riken,Japan, 37-39(1999).
.
[12] Y.Hayakawa, et al., The 12th Sym. on Accel. Sci. andTech., Riken,Japan, 391-394(1999).

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The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October 2001[19] K.Kanno, et al., Proc. 26th LAMJ, 73-75 (2001).
[20) Y.Hayakawa, et al., to be accepted, The 23rd International FEL Conference, Darmstadt, Germany (2001).
Notice LAMJ :Linear Accelerator Meeting in Japan[13] K.Hayakawa, et al., Proc. 25th LAMJ,56-58 (2000).
[14] Y.Hayakawa, et al., Proc. 25th LAMJ, 358-360 (2000).
[15] K.Hayakawa, et al., Proc. 26th LAMJ,285-287 (2001)[16] T.Sakai, et al., Proc. 26th LAMJ, 222-224 (2001).
[17] K.Yokoyama, et al., Proc. 26th LAMJ, 231-233 (2001).
[18] T.Tanaka, et al., Proc. 26th LAMJ,246-248 (2001).

Pulse duration time for bunched beam
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-+-4MW---sMW·A··5.5MW......__ 5.75MW

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0.1

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Input Power for Prebuncher (KW)

Fig. 6 The pulse duration time of the electron beam shows a result of a calculation of beam tracing whichmakes the both input electric power of the pre-buncher and the buncher a variable.

125MeV Electron Linac of LEBRA in Nihon University
FC3FC2FC1
1 ~ ml ~M FEL

G :Gun
PB : Prebuncher
Q : Quadrupole Focus: Dummy LoadD
KM : Klystron Modulatoru : Undulator

ML
B
PS
DC
BM
BD

: Magnetic Lens
: Buncher: Phase Shifter: Directional Coupler:Bending Magnet:Beam Dump

FC :Focus CoilAL : 4m Accelerator: Attenuater
AT:KlystronK: MirrorM
PXR : Prametric Xray

Fig.7 The layout of the 125 MeV electron linear accelerator and FEL system and Prametric Xray generator.

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