EXPERIMENTAL VERIFICATION OF SELF-AMPLIFIEDSPONTANEOUS EMISSION IN THE FAR-INFRARED REGION
R. Kato, R. A. V. Kumar, T. Igo, M. Kuwahara, T. Konishi, M. Fujimoto,Y. Nakamura, S. Isaka, S. Mitani, S. Okuda, S. Suemine and G. Isoyama,ISIR, Osaka University, Mihogaoka, Ibaraki, Osaka 567-0047, Japan

Abstract

2 EXPERIMENTAL

We are conducting experiments to generate selfamplified spontaneous emission (SASE) in the farinfrared region and to measure its characteristics, such as
peak power, dependency on the K-value, and spectral
width, using a single bunch electron beam accelerated
with the L-band linac at ISIR, Osaka University. The
measured characteristics are in good agreement with theone-dimensional model of SASE. We have also observed
the second harmonic peak of SASE originating from thenon-linear harmonic generation.

1 INTRODUCTION
Self-amplified spontaneous emission (SASE) is a highgain single pass free electron laser (FEL). In this scheme,a seed light is spontaneous undulator radiation emitted
by the electron bunch at the entrance of the undulatorbecomes, and it is amplified by interacting with theelectron bunch to the saturation power level. Since no
optical resonator is used, it is expected to be one ofpromising approaches to realize X-ray lasers. TwoSASE-FEL projects are proposed in the wavelength
region of a few angstroms [1,2].
SASE-FEL in the X-ray region requires a high energy
electron beam of some tens of GeV with extremely low
emittance and a peak current higher than 1 kA as well as
a long undulator up to 100 m. Before constructing such
big facilities, a lot of experimental studies are necessary
together with theoretical and simulation studies. Several
SASE experiments have been conducted in the infrared
region, but most of these are qualitative studies. SASEFEL experiments in the short wavelength regions beganquite recently; the APS group at ANL has observed
SASE at 530 nm [3] and the TTF-FEL group at DESY
has reported observation of SASE at 109 nm [4]. In orderto understand physics of SASE, quantitative
experimental studies are necessary and they are going tobegin.
We are conducting experimental study on SASE in theinfrared region up to 200 µm using the wiggler for theconventional FEL and the 38 MeV, L-band linac at theInstitute of Scientific and Industrial Research (ISIR),Osaka University [5]. In this report we will report recent
experimental results including wavelength spectra ofSASE.

The L-band linac is equipped with a three-stage subharmonic buncher (SHB) system composed of two 1/12
and one 1/6 SHBs in order to produce an intense singlebunch beam with charge up to 91 nC/bunch. For thesingle-bunch operation mode, the electron beam with apeak current up to 28 A (typically 18 A in ourexperiments) and a duration of 5 ns is injected from a2
thermoionic gun with a cathode area of 3 cm (EIMAC,YU-156) into the SHB system. After being compressed
to a single bunch, the electron beam is accelerated to 11- 32 MeV with the 1.3 GHz accelerating tube. Theelectron beam is transported via the achromatic beamtransport line to the FEL system. The wiggler is the 32
period planar type with the period length of 60 mm. The
K-value can be varied from 0.013 to 1.472. The main
characteristics of the electron beam and the wiggler are
listed in Table 1.
In SASE experiments, the upstream mirror for theoptical resonator of the FEL was removed and thedownstream mirror was replaced with a mirror with anappropriate focal length to collect SASE. Light emitted
by the single bunch beam passing through the wiggler
Table 1: Parameters of the electron beam and thewiggler
Electron beam
Accelerating frequency 1.3 GHz
Energy11-14 MeV
Energy spread
2-4 % (FWHM)
Charge/bunch< 20 nC
Bunch length20-30 psPeak current< 1 kA
Normalized emittance
150-200 π mm mrad
Repetition
60 HzModeSingle bunch
Wiggler
Total length
1.92 mMagnetic period
60 mm
No. of periods
32
Magnet gap
120-30 mm
Peak field
0.37 TK-value
0.013-1.472

4103102101

E=12.6MeV∆ E/E=3.8%(FWHM)
Q=18.2nC

157 µ m107 µ m1010127 µ m

72 µ m

E=14.0MeV∆ E/E=3.6%(FWHM)
Q=9.2nC

070 µ m

-1

0.5

1

1.5

K-value

Figure 1: Peak power of SASE measured as afunction of K-value of the wiggler. The solid andthe open circles denote the measured data points forthe beam energy of 12.6 and 14.0 MeV,respectively. The dashed lines were theoreticalvalues predicted by one-dimensional model. See
was reflected by the downstream mirror and led to a farinfrared monochromator in the measurement room via anevacuated optical transport line. The monochromator is across Czerny-Turner type with a plane reflective grating.
The grating is grooved with 7.9 lines / mm (Milton Roy)and its blaze wavelength is 112.5 µm. The
monochromator can be used in the wavelength region
from 60 to 190 µm. Spectral resolution is dependent on
the measured wavelength and the slit width, and it isabout 1.5 µm in this wavelength region for the slit widthof 6 mm. The monochromator is evacuated with a rotary
pump through the optical transport line. The
monochromatized light is taken out from themonochromator through a 2 mm thick single-crystalquartz window, and led through a short vacuum pipe to a
Ge:Ga photo conductive detector cooled with liquidhelium. The Ge:Ga detector has the highest sensitivity
around 105 µm and the sensitivity drops steeply on thelonger wavelength side, while it decreases slowly on theshorter wavelength side. The sensitivity is not zero evenbeyond 150 µm, though it is very low. Therefore we canmeasure the optical signal around 180 µm if the intensityof light is very high. The sensitivity on the shortwavelength side is limited to 50 µm by the shortwavelength cut-off filter in the detector.

3 EXPERIMENTAL RESULTS ANDDISCCUSIONS
3.1 Dependency on K-value
We conducted SASE experiments in the wavelength
region from far infrared to submillimeter, in which very
high gain of SASE is expected, and observed strong
infrared light emitted by the single bunch beam when it

passes through the wiggler. In this experiment, we used aplane mirror instead of the plane reflective grating in themonochromator. Fig. 1 shows the peak power of emitted
light as a function of the K-value of the wiggler. The
filled circles show the measured peak power for thebeam of E=12.6 MeV with charge of Q=18.2 nC and theenergy spread of ∆E/E=3.8 % (FWHM). The open
circles show the peak power for E=14.0 MeV with
Q=9.2 nC and ∆E/E=3.6 %. The data points and theerror bars show average values and standard deviations,
respectively, of five highest intensities in thirty
successive optical pulses. For the upper case (12.6 MeV)
in Fig. 1, the K-value was varied from 0.67 to 1.47 andaccordingly the wavelength changed from 72 to 157 µm.
The optical power was estimated from the signal
intensity using the detector sensitivity calibrated withblackbody radiation, transmittance of the vacuum-1
windows, and the pulse length assumed as ∆t≈∆ω ,where ∆ω is the spectral width of SASE in theexponential growth region. The dashed lines show theoptical power predicted by the SASE theory. In thesecalculations, the start-up power for SASE was calculated
1/2 2
1/2using Pin = 3(4π) ρ Pb/Nλ(ln(Nλ/ρ)) , where ρ is the fel
parameter, Nλ the number of electrons in a wavelengthand Pb the electron beam power. The functional
dependence of the measured optical power on the Kvalue is in good agreement with the SASE theory.

3.2 Wavelength spectra
We have measured wavelength spectra of SASE for E= 11.96 MeV, ∆E/E = 2.08 % (FWHM), and Q = 15.5
nC. By changing the wiggler gap from 30 to 33 mm, theK-value is decreased from 1.47 to 1.26 and consequently
the wavelength of the fundamental peak varied from 173
to 141 µm. Figure 2 shows wavelength spectra of SASE
measured in these experimental conditions. The
intensities are not corrected with detector sensitivity andgrating efficiency, and therefore they are preliminary.

intensity [arb. units]

peak power per pulse [W]100.8

gap=30mm

31mm

0.6
0.40.2080

32mm
33mm
100120140160

wavelength [ µ m]

180

Figure 2: Wavelength spectra of SASE measured forsome wiggler gaps. The dashed lines were sums of aGaussian fit for the main peak and a straight line forthe background. See text for details.

3.3 Spectral width

relative spectral width (rms)

The spectral width of the fundamental peak wasderived by the least square fit of a Gaussian function forthe peak and a straight line for the background to themeasured spectrum as shown by the dashed lines in Fig.
2. Fig.3 shows measured standard deviations of thefundamental peaks and the theoretical spectral width
predicted by the one-dimensional model. The calculated
spectral width increases slightly with the wavelength

0.05
0.04

intensity [arb. units]

Since the sensitivity and the efficiency have peaks at 105µm and 112.5 µm, respectively, we thought that a peakin the longer wavelength side has higher power.
Two or three peaks can be seen in each spectrum inFig. 2. The wavelength of the highest peak in each
spectrum agrees quite well with the calculated
wavelength using the electron energy and the K-value
corresponding to the gap quoted in the figure. Thesepeaks are, therefore, identified as fundamental peaks ofSASE. The wavelength of the rightmost peak in Fig. 2(a) is 88 µm and it is precisely half the wavelength of thefundamental peak. The peak has been observed only forthe larger K-value. It is theoretically predicted that the nth harmonic of SASE grows more rapidly than thefundamental peak with a gain length equal to that of thefundamental SASE divided by the order of harmonic, n[6]. This peak disappeared when a short-wavelength cut
filter of 100 µm was inserted between themonochromator and the detector, while the fundamental
peak survived. These results exclude a possibility that it
may be due to stray light of the fundamental peak. Thus
the peak has been identified as the second harmonic of
the fundamental SASE peak, which presumably
originates from non-linear harmonic generation of SASE.
The intensity of the second harmonic peak to the-2
fundamental peak is roughly estimated to be from 10 to-3
10 using the intensity ratio in the spectrum together
with the detector sensitivity and the monochromatorefficiency.

10.80.6

λ =80.9 µ m
σ λ =1.30 µ m
σ λ /λ =1.61%

0.40.2060

708090wavelength [µ m]100

Figure 4: Wavelength spectra of second harmonicpeak of SASE. The solid circles denote the measured
data points. The dashed lines were sums of a
Gaussian fit for the main peak and a straight line forthe background. See text for details.
from 2.2 to 2.3 %. These values are slightly smaller thanthe average value of the measured spectral width 2.5 %.
If we take into account a contribution of the energy
spread of the electron beam, 0.9 % (rms), the theoreticalvalues would come closer to the measured values. Fig. 4
shows the second harmonic peak of SASE. The relative
spectral width of this peak is 1.6 %. By assuming theFourier limit, ∆ω×∆t≈1, the pulse length of thefundamental peak and the second harmonic peak are
1.44 ps and 1.13 ps, respectively. These lengths areshorter than the pulse length of the spontaneous
undulator radiation, N×λ = 17 ps, where N is the periodof the wiggler, λ the fundamental wavelength. The
difference shows that the peaks did not originate fromthe spontaneous emission.

4 CONCLUSIONS
We
have
quantitatively
measured
SASE
characteristics, such as peak power, dependency on theK-value, and spectral width, in the far-infrared region.
The good agreement between the experimental results
and the one-dimensional model of SASE shows that theessential process of SASE mechanism is easy to appear
in the far-infrared region.

0.03
0.02REFERENCES
1-D m odel

0.010100

120

140160wavelength [µ m]180

Figure 3: Relative spectral width of fundamental
peaks of SASE. The solid circles denote themeasured data points. The dashed line shows thetheoretical value predicted by the one-dimensionalmodel. See text for details.

[1] The LCLS Design Study Group, SLAC-R-521(April 1998).
[2] R. Brinkmann, et al., DESY Report 1997-048.
[3] S. V. Milton, et. al., Phys. Rev. Lett. 85 (2000) 988.
[4] J. Andruszkow, et. al., Phys. Rev. Lett. 85 (2000)
3825.
[5] R. Kato, et. al., Nucl. Instr. and Meth. A445 (2000)
164.
[6] H. P. Freund, S. G. Biedron, and S. V. Milton, IEEE
Quantum Electron. QE-36 (2000) 275.