The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Emission of SASE-FEL in the in the Infrared Region and Measurement of its Characteristics
Ryukou KATO, Taishi OKITA, Tetsuya IGO, Makoto KUWAHARA, Toshifumi KONISHI, RaviA. V. KUMAR,Shuichi OKUDA, Shoji SUEMINE*, Goro ISOYAMA
Institute of Scientific and Industrial Research (I SIR), Osaka University
8-1 Mihogaoka, lbaraki, Osaka 567-0047, Japan*Unicon System Co. Ltd.

We are conducting experiments of Self-Amplified
Spontaneous Emission (SASE) in the far-infrared region
using a single-bunch beam from the L-band linac at ISIR,
Osaka University. Characteristics ofthe electron beam weremeasured and it was found that the one-dimensional theoryof SASE was applicable to our SASE experiments. Weobserved strong light emitted when the single-bunchelectron beam passed through the wiggler. The intensity oflight was measured as a function of the K-value of thewiggler and it increased by three orders of magnitude as theK-value increased. The intensity variation with the K-value
agrees quite well with the SASE theory and it wasconcluded that we observed SASE in the infrared region

charge up to 73 nC. Such a high intensity beam makes it
possible to produce SASE in the infrared region. We havebeen developing an infrared FEL using the linac. In thecourse of developing the FEL, SASE was observed in 199 I
at 20 and 40 11m wavelengths using a wiggler for the FEL
and the single-bunch beam from the linac [3].
We recently began to study SASE-FEL again using theexisting FEL system and the linac. First, characteristics ofthe single-bunch electron beam were measured and then
expected performance of SASE-FEL was estimated usingsimple analytic formulas and the measured beam parameters.
We are now conducting experiments to produce SASE andto measure characteristics of the radiation. In this paper, we
report the recent progress of the SASE-FEL study at ISIR,
Osaka University.

1 Introduction

2 Experiment

The single-pass and high-gain Free Electron Laser(FEL) based on Self-Amplified Spontaneous Emission(SASE) has attracted attention as a strong candidate forrealizing an X-ray laser and there are some proposals toconstruct such facilities using linear accelerators [1, 2]. ASASE-FEL in the short wavelength region necessitates ahigh intensity and high quality electron beam and a longwiggler. Theoretical and simulation studies have beenextensively conducted on SASE, while only a limited
number of experimental studies have been performed in theinfrared region.
The 38 MeV, L-band linac at the Institute of Scientificand Industrial Research (ISIR), Osaka University isequipped with a three-stage subharmonic buncher (SHB)
system to produce a high-intensity single-bunch beam withThe linac and the FEL system are shown schematicallyin Fig. I. The electron beam with a peak current of 14 A
and a duration of 5 ns is injected from a thermoioniccathode (YU-156) with an area of 3 cm 2 to the SHB system.
After being compressed to a single bunch in the SHBsystem, the electron beam is accelerated with the 1.3 GHz
linac to an energy of 11-32 MeV. The single-bunch electron
beam is led to the FEL system through the achromatic beamtransport line comprising two bending magnets and aquadrupole triplet in between them. It is focused at thecenter of the wiggler using a quadrupole doublet, and after
passing through the wiggler it is stopped at the beam dump.
The wiggler is a 32-period, planar wiggler of the horizontal
oscillation type with the period length of 6 em. The K-value
can be varied over the range from 0.013 to 1.472.

Abstract

ISIR L-Band Linac

Fig. 1. Schematic layout of the !SIR L-band linac and the FEL system.

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The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999

Characteristics of the electron beam were measured at alocation 0.5 m downstream from the bending magnet in thestraight beam line. The bunch length was obtained bymeasuring optical transition radiation, emitted when theelectron beam passed through a stainless steal plate, using a
streak camera with time resolution of shorter than 2 ps. The
emittance of the electron beam was measured by thequadrupole scan method. The electron beam size wasmeasured as a function of. the focusing strength of thecentral quadrupole in the triplet at the exit of the linac, using
a 0.1 mm thick fluorescent plate and a TV camera. The
signal from the camera was processed using a personal
computer with a video capture board, and the beam size wasderived. The energy spectrum of the electron beam wasmeasured using a momentum analyzer magnet locateddownstream in the straight line. The total charge in thesingle bunch beam, after passing through the wiggler, wasmeasured at the beam dump with a Faraday cup.
In SASE experiments, the upstream mirror M I shownin Fig. 1 was removed and the downstream mirror M2 wasreplaced with another spherical mirror with a different
radius, so that light emitted toward the exit of the wiggler isefficiently transported through the evacuated opticaltransport line to the measurement room. The grating
monochromator has not been used for spectral analysis of
SASE light yet. Instead, the grating was replaced with aplane mirror and the spectrometer worked as a part of theoptical transport line. Light passing through a window madeof single-crystal quartz at the exit of the monochromator
was detected with a liquid-helium-cooled Ge:Ga
photoconductive detector. The detector has the time
resolution of approximately 170 ns (FWHM). The signal
from the detector was processed with a digital oscilloscope
connected to a personal computer.
3 Experimen tal results and discussionsCharacteristics of the single-bunch electron beam wereTable I
Main parameters of the electron beam and the wiggler.
electron beamenergy11-32MeV
energy spread (FWHM)
1.1-4 %
charge per bunch
22-38 nC
bunch length
20 ps
norm. emittance
150-250 1t mm mrad
peak current
1 kA
mode
single bunch
acceleration frequency
1.3 GHz
repetition
60Hz
wi fertotal length
1.92 m
magetic period
60mm
No. ofperiods
32
magnet gap
120-30 mm
peak field
0.37 T
K-value0.013-1.472

measured over the energy range from 32 down to 11 MeV.
They are listed in Table 1, together with the main
parameters of the wiggler. The normalized emittance fN isapproximately 200 1t mm mrad and the relative energy
spread is 1.1-4 %, depending on the electron energy. A
typical energy spectrum of the electron beam is shown in
Fig. 2. Charge in a single bunch was measured to be 22-38
nC. The peak current calculated from the charge per bunch
and the measured bunch length of 20 ps exceeds 1 kA.
According to the one dimensional theory of SASE [4],the power of coherent light grows exponentially as PsAsE
ex:: exp[z/Lgain], where z is the longitudinal distance along
the wiggler and Lgain is the power gain length. It is given by
Lgain = A.v/4 .J3 7tp, where p is the FEL parameter. The onedimensional formula is applicable when the following
conditions are fulfilled; (a) Exory< N41t, (b) Lgain< LR, and(c) ajy < p, where Exory is the horizontal or the verticalemittance ofthe electron beam, It the wavelength of SASE,
LR the Rayleigh length, and ajy the energy spread of theelectron beam.
The condition (a) states that the transverse crosssection of the electron beam should be included in that ofthe photon beam and it is tested in Fig. 3. The solid circles
and the solid triangles are measured emittance of theelectron beam. The curves with the K-values show A-/47t
calculated for three different K-values of the wiggler. The
dash-dot line shows the beam emittance calculated for thenormalized emittance of200 1t nm rad. The inequality issatisfied for the larger K-value below 20 MeV. The
condition (lb) states that light should be amplified before itspreads out due to diffraction. Substituting LR = 4nt:/31It into
the inequality (b), one obtains the inequality to be compareddirectly with experiment as 1tLgain/47t~ < £, where ~ is thebetatron function. It is tested in Fig. 4. The solid circles andthe solid triangles are the measured emittance and thecurves show the left side of the inequality for three K-values.
The condition (b) is apparently fulfilled. The condition (c)states that the energy spread of the electron beam should beless than the bandwidth of saturated light, which is equal tothe FEL parameter p, and tested in Fig. 5. The solid circles
are the measured energy spread and the curves show
5-4<U3peak=15.2Me V(!I.E/E):J-E~FWHM= 2.8%>.

( f)

cQ)

2c

5Fig 2.

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1015energy (MeV)
Energy spectrum of the electron beam.

20

The 12th Symposium on Accelerator Science and Technology, Wako, Japan 1999•u

~ 10 '·,',,,~EEvt

K=1.472"'. '•,,:·-..... -~o-.::::::

detector efficiency
correctedhoriz.emittancevert. emittance·-.....<·----.,. , '·---..=.

0.7Q)'::::::::::::::_::------...

2EQ)•0• •SASE000()•c2:t::::'..----------Iii •(.)•@1)

coherent-........

20

1 0. 2energy [MeV]Fig 3.

Fig 6.

0.5

2030

energy [MeV]Fig 4.

Measured beam emittance and the condition(b). See text for details.

Gi 0.02a;------....---!.,_E~

Clla.
...Jw

0.01

LL·-·:·:=;~!==~=:==~=--. -------------------·•1....
... - •.
•••••

relative energy spread (r.m.s.)

20

1.2

1.4

1.6

Light intensity as a function of the K-valueof the wiggler. See text for details.

We conducted experiments to observe SASE at electron
beam energies lower than 15 MeV and larger K values of
the wiggler, because gain is expected to be higher and theone-dimensional conditions are fulfilled. We have observed
the strong light emitted by the single bunch beam when
passing through the wiggler. Figure 6 shows the lightintensity measured as a function of the K value of thewiggler. The electron energy was 12.6 MeV and the energy
spread was 3.8 %. The charge in a bunch was 18.2 nC. The
K value was varied from 0.67 to 1.47 and then thewavelength changed from 72 to 157 j.lm. The open circles
are intensities measured with the detector. Since thesensitivity of the detector changes considerably in the above
wavelength region, the measured intensities are correctedusing the spectral sensitivity of the detector and theintensities after correction are shown with the solid circlesin Fig. 6. The intensity of light increases by three orders ofmagnitude for the K-value ranging from 0.67 to 1.47. The
solid curve shows a fit of the SASE theory, where thenormalization factor of the formula is adjusted to fit theexperimental data. The agreement between the theory andexperiment is excellent. The dotted line in Fig.6 is thecalculated intensity of incoherent or coherent synchrotron
radiation from the wiggler. The functional dependence ofthe light intensity on the K-va1ue is quite different from thatof SASE. Therefore it is concluded that we have observedSASE in the infrared region.

tf

100.8

K

Measured emittance of the electron beam andthe condition (a). See text for details.

•.__..__._._........._,_._..__.__.._._........._,_....._..__.................._._.....__.

0.6

30

energy [MeV]

References
Fig 5.

Measured energy spread of the electron beamand the condition (c). See text for details.

[1]

calculated FEL parameters for different K values. It can beseen that the inequality is satisfied for the larger K value inthe entire energy region shown in Fig. 5. As a conclusion,
most of the conditions are fulfilled in our experimental
conditions, which means that the one-dimensional model isapplicable to our SASE experiments. The maximum gain
calculated using the theory exceeds 1 X 10 5 at an electronenergy of 15 MeV and the K-value of 1.472.

[2]
[3]

[4]

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