The 13th Symposium on Accelerator Science and Technology, Suita, Osaka, Japan, October2001

SASE-FEL IN THE FAR-INFRARED REGION BY USING A HIGH
BRIGHTNESS ELECTRO N BEAM
R. Katot, T. Igo, M. Fujimoto, Y. Nakamura, S. Isaka, T. Onishi, S. Furukawa, D. Okamoto,
S. Okuda, S. Suemine and G. Isoyama
Institute of Scientific and Industrial Research, Osaka University
8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan

Abstract
We are conducting experiments to generate selfamplified spontaneous emission (SASE) in the farinfrared region and to measure its characteristics,including peak power, dependency on the K-value, andthe spectral width, using a single bunch electron beam
accelerated with the L-band linac at ISIR, OsakaUniversity. The measured characteristics are in good
agreement with the one-dimensional model of SASE. We
have also observed the second and the third harmonicpeaks of SASE originating from ~e nonlinear harmonic
generation.

1 INTRODUC TION
Self-amplified spontaneous emission (SASE) using ahigh gain single pass free electron laser (FEL) is expected
to be one of promising approaches to realize X-ray lasers[1,2]. In this scheme, a seed of light is spontaneous
undulator radiation emitted by the electron bunch at theentrance of the undulator, and it is amplified byinteracting with the electron bunch to the saturation
power level. When SASE at the fundamental wavelength
grows up exponentially, higher harmonics also begin
developing exponentially with the gain length equal tothat of the fundamental radiation divided by n, where n is
the harmonic order [3,4,5]. This process is called thenonlinear harmonic ·generation of SASE. The thirdharmonic power is expected to reach one percent of thefundamental power in the proposed X-ray SASE-FELs.
Since the fundamental power at a few Angstrom will behigher than 10 GW, the higher harmonics will haveconsiderable power. On the other hand, in order to realize
SASE-FEL in the X-ray region, a high energy electron
beam of some tens of GeV with extremely low emittanceand a peak current higher than 1 kA, and a long undulator
over 100 m are required. The shorter the wavelength
becomes, the more severe these requirements become.
Therefore, the higher harmonics generation has attracted
much attention as a method to produce higher energyphotons.
We are conducting experimental study on SASE in thewavelength region from far infrared to submillimeterusing the L-band linac at the Institute of Scientific andIndustrial Research (ISIR), Osaka University [6]. We willreport recent experimental results including the higher
harmonic generation of SASE.

2 LINAC AND FEL SYSTEM
The L-band linac at ISIR consists of an electron gun, a
sub-harmonic buncher (SHB) system, a prebuncher, abuncher and a main accelerating tube. The gun is athermionic gun with a cathode area of 3 cm2 (EIMAC,YU-156), from which the electron beam with a peakcurrent up to 18 A and a duration of 5 ns is injected into
the SHB system for SASE experiments. The SHB system
to produce an intense single-bunch beam is composed of
two 1/12 and one 1/6 subharmonic cavities of theaccelerating frequency 1.3 GHz. Due to the velocity
modulation with the SHB system, the electron beam iscompressed to 500 ps at the entrance of the prebuncher.
The prebuncher and the buncher are travelling wave tubes
with the same frequency of the main accelerating tube.
The electron beam is bunched into 20 - 30 ps by theprebuncher and the buncher, and accelerated to 11 - 32
MeV with the main accelerating tube.

tkato@ sanken.osaka-u.ac.jp-111-

Table 1: Parameters of the electron beam and thewiggler
Electron beam
1.3 GHz
Accelerating frequency
11-14 MeV
Energy2-4% (FWHM)
Energy spread<20nC
Charge/bunch
20-30 ps
Bunch length< 1 kA
Peak current
150-200 1t mmmrad
Normalized emittance
60Hz
Repetition
Single bunch
ModeWiggler
1.92mTotal length
60mm
Magnetic period
32No. ofperiods
120-30mm
Magnet gap
0.37 T
Peak field
0.01-1.47
K-value

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

-E=12.6MeV
t.E/E=3.8%(FWHM)
Q=18.2nC

$:

3l

103

~(i) 10

3 EXPERIMENTA L RESULTS ANDDISCCUSIONS
15 7.,rm,.---··/ •2••-------------- •c.

107 ..

r.>L..

!1? 101>

r-oa.

ffi

rc.

••

J.-'

o-··o-- oo;v·• •,,..""'_.-°-At~,-,/,/-',..-.. ---7

1'...... A.zl~m0......................... 072~mt.;..:: 10°m···.

3.1 Peak power of SASE•

0 0 ;..............

~~}IPI_.-.-----·-E-14.0MeV_t.E/E=3.6%(FWHM)
0=12.9nC

1Q·1 L-L/..___.::.•'_'---'---'------'---'------'--- '------L----'
0~

1

1.5

K-value

Figure 1: Peak power of SASE measured as afunction of K-value of the wiggler. The solid and theopen circles denote the measured data points for thebeam energy of 12.6 and 14.0 MeV, respectively.
The dashed lines were theoretical values predicted by
one-dimensional model. See text for details.
The electron beam is transported via an achromatic
beam transport line to the 32 period planar wiggler withthe period length of 60 mm. The K-value of the wiggler
can be varied from 0.01 to 1.47 by changing the magnet
gap. The main characteristics of the electron beam and thewiggler are listed in Table 1.
Light emitted by the electron beam traversing thewiggler was reflected with a downstream mirror and led
to a far-infrared monochromator in the measurement
room via an evacuated optical transport line. The highvacuum in the beam transport line and the low vacuum inthe optical transport line are separated by a 0.2 mm thick,
20 mm in diameter synthetic diamond window. Themonochromator is a cross Czerny-Turner type with aplane reflective grating. The grating is grooved with 7.9lines I mm (Thermo RGL) and its blaze wavelength is112.5 Jlm. The monochromator can be used in thewavelength region from 50 to 200 Jlm. The spectral
resolution of the monochromator is almost constant over
the range and approximately 1.5 Jlm for a slit width of 6mm. The monochromatized light was detected with aGe:Ga photoconductive detector cooled with liquidhelium.
The total sensitivity of this measurement system,including the sensitivity of the detector, the efficiency ofthe monochromator, and the transport efficiency of the
optical transport line and transmittance of the windows,
was calibrated over the wavelength range using a
blackbody radiator of a temperature 1273 K. The radiator
was placed at a position optically equivalent to the first
focal point. In order to exclude the stray light in the
monochromator, a 50 Jlm or 100 Jlm short-wavelength cut
filter was inserted between the monochromator and thedetector.

In this experiment, we used a plane mirror instead of
the plane reflective grating in the monochromator. Fig. 1
shows the peak power of emitted light as a function of theK-value of the wiggler. The filled circles show themeasured peak power for the beam of E=12.6 MeV, theenergy spread LlE/E=3.8 % (FWHM) and charge Q=18.2
nC. The open circles show the peak power for E=14.0
MeV, LlE/E=3.6 % and Q=12.9 nC. The data points andthe error bars show average values and standard
deviations, respectively, of five highest intensities inthirty successive optical pulses. In the upper case (12.6MeV) of Fig. 1, the K-value was varied from 0.67 to 1.47
and accordingly the wavelength changed from 72 to 157
Jlm. The optical power was estimated from the signal
intensity using the detector sensitivity calibrated with theblackbody radiation, transmittance of the vacuum
windows, and the pulse length assumed as Llt~Llm -I,
where L1m is the spectral width of SASE. The onedimensional (lD) theory predicts the spectral width of
SASE in the exponential growth region as0",1_A.

::::0.91 fp

(1)

fjj

where p is the FEL parameter, N the period number of the;!
wiggler. The dashed lines show the optical power
predicted by the lD theory. In these calculations, thestart-up power for SASE was calculated using Pin =3(4n) 112p 2PJN,.(ln(NJp)) 112, where N,_ is the number ofelectrons in a wavelength and Pb the electron beam power.
The functional dependence of the measured optical poweron the K-value is in good agreement with the SASEtheory.

fundamental
3rd harmonic

100120

140

160

180

200

wavelength (11 m]

Figure 2: Absolute wavelength spectra of fundamental andhigher harmonics of SASE. The open circles, the filled
triangles, and the open diamonds denote the fundamental, thesecond harmonic, and the third harmonic peaks, respectively.
The dashed lines show Gaussian distributions fitted to thepeaks. See text for details.

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

3.2 Spectrum ofSASEWe have measured the wavelength spectrum of SASE
forE= 11.3 MeV, Llli/E = 3.6% and Q = 9.2 nC. The Kvalue is 1.47 at the wiggler gap 30 mm. Fig. 2 shows
wavelength spectrum of SASE in the wavelength region
from 50 to 200 J..lm measured in these experimentalconditions. The intensities were divided by the sensitivityof the measurement system calibrated by the blackbody
radiation and thus absolute values were derived in units ofJ/s/m, which stand for power in unit wavelength. Threepeaks appear around 60,90 and 180 J..lm in the spectrum.
The highest peak around 180 J..lm has been identified as
the fundamental SASE since the wavelength agrees withthe calculated value using the electron energy and the Kvalue. Since the other peaks disappeared when a 100 J..lm
short-wavelength cut filter was inserted between the
monochromator and the detector, we confirmed they were
not stray light. Because the peaks are observed only when
the fundamental peak is strong, and because they do notappear by themselves, the peaks around 90 and 60 J..lm
have been assigned as the second and the third harmonic
radiation, respectively, generated by the nonlinear
harmonic generation of SASE.
Using each peak area and the pulse duration of thesignal from the detector, 200 ns, the energy of an opticalpulse is derived to be 2.75 nJ for the fundamental peak,
73 pJ for the second harmonic, and 182 pJ for the thirdharmonic. The energy ratio of the second harmonic to thefundamental is 2.7 % and that of the third harmonic is
6.6 %. These energy ratios are much larger than thosepredicted for the X-FEL projects.

3.3 Spectral width and the gain length
Spectral widths of these peak were derived by the leastsquare fit of the Gaussian distribution function to themeasured values denoted by the dashed lines in Fig. 2; thefractional widths are 2.9% for the fundamental peak (thestandard deviation), 3.6 % for the second harmonic peak,and 5.3 %for the third harmonic peak. The spectral widthof the fundamental radiation in the exponential gain
region is given by the lD theory as Eq. (1). In the present
experimental conditions, the spectral width is calculated
to be 2.6 %, which agrees well with the measured value,2.9%.
It is predicted by theory that the gain length of the n-thharmonic is equal to that of the fundamental radiation
divided by n. Let the gain length of the fundamental, thesecond harmonic, and the third harmonic radiation beLG(l>, LG<2>, and LG<3>, respectively, the following relation
holds.

1I1

LGLG

LG-;-w :--;-(2) : --;--m- =1 : 2 : 3(2)

For the fundamental radiation, the gain length LG is inproportion to the cooperation length Lc divided by thewavelength A. Assuming this relation holds for the higher
harmonics, the gain length can be related to the fractional
spectral width, a,VA as1AAAACTA.

aA.

- o c - " " - " " - 0 " =--OJocA 'Lc ca1 c m c ALG(3)

where we assume that the Fourier transform relation holds
between the spectral width a OJ and the pulse duration Oi asaOJ·Oi ""1. Substituting the measured spectral widths, weobtain the ratios( ) = 1:1.24:1.83 ,(4)1 = 2.9: 3.6: 5.3%
11 --""T,;\:---ror---r,,-:LG~'' LG\z, LG\3,which should be compared with the theoretical prediction
given in Eq. (2). Although the measured spectral width of
the fi.n).damental radiation agrees well with the 1D theory,the spectral width ratios of the second and the third
harmonic to the fundamental radiation are smaller than
the theoretical predictions, 1:2:3, which suggests that gain
lengths of the second and the third harmonic are longer
than the prediction. It should be noted that the ratio
between the spectral widths of the second and the third
harmonic 3.6 : 5.3 (%) = 1 : 1.47 is close to the theoreticalvalue, 2 : 3 = 1 : 1.5.
The optical pulse duration of the fundamental radiation
is evaluated from the spectral width to be 7.8 ps using theFourier transform relation. The peak power of thefundamental radiation is estimated, using the energy in an
optical pulse and the pulse duration, to be 0.18 ~ 0.35 kW,
which also agrees fairly well with the lD theory, 0.16 ~2.7 kW. Since the saturation power is estimated to be 79 ~135 MW using the lD theory, the present experimental
conditions are far away from realizing power saturation ofSASE. We have, nevertheless, observed the second andthe third harmonics of SASE with the large energy ratios
to the fundamental, which indicates that bunching of theelectron beam develops considerably, because the FEL
parameter is large.

4 REFERENCES[1] The LCLS Design Study Group, SLAC-R-521 (April1998).
[2] R. Brinkmann, et al., DESY Report 1997-048.
[3] R. Bonifacio, L. De Salvo, and P. Pierini, Nucl. Instr.
andMeth. A293 (1990) 627.
[4] H. P. Freund, S. G. Biedron, and S. V. Milton, IEEE
Quantum Electron. QE-36 (2000) 275.
[5] Z. Huang and K. J. Kim, Phys. Rev. E 62 (2000) 7295.
[6] R. Kato, et. al., Nucl. lnstr. and Meth. A445 (2000)
164.

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