The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

Wavelength extension of the ISIR-FEL
toward the longer wavelength region for user experiments
Ryukou KATO, Shuichi OKUDA, Gen KONDO, Satoshi KONDO, Hiroshi KOBAYASHI,Tetsuya IGO Shin-ichi NAKAGAWA, Shoji SUEMINE* and Goro ISOYAMA
Instit~te of Scientific and Industrial Re:;earch, Osaka University;
8-1 Mihogaoka, Ibaraki, Osaka 567, JAPAN*Unicon System Co. Ltd.

Abstract

2 Accelerator and the FEL systems

The far infrared FEL based on the L-band linac atISIR, Osaka University is being modified so that the laser
wavelength can be extend to the longer wavelength region. After the wiggler and the bending magnets wereremodeled and the vacuum chambers for them were replaced , we have observed laser oscillation at wavelengths
from 21 to 126 f.Lm.

The L-band linac has the sub-harmonic buncher(SHB) system composed of two 12th and one 6th SHBs
in order to produce an intense single bunch beam. When
FEL experiments are conducted, the electron beam witha peak current of 600 rnA is injected from a thermoionicelectron gun into the SHB system, and the second 12th
and the 6th SHBs are powered to make an electron beamwith the macropulse duration of 1.8 f.LS and micropulseintervals of 9. 2 ns. Then micropulses are bunched to 20 30 ps with a pre-buncher and a buncher and the electron
beam is accelerated to 10 - 19 MeV in an accelerating
tube. The characteristics of the electron beam are listed
in Table 1.
We used an electron gun with a cathode area of 0.5
cm2 (EIMAC, Y-646B) for FEL experiments and one
with a large cathode areaof20.4 cm2 (ARCO, Model-12)
for usual operation of the linac. Recently the electron
gun usually used was changed to one with a smaller cathode area of 3 cm2 (EIMAC, YU-156), which can provide
as much current as the previous one can. We therefore
started using the gun YU-156 also for FEL experiments,
so that it is not necessary to change the electron guns.
The electron beam is transported through an achromatic bend to the FEL system. The main parameters ofthe FEL system are listed in Table 2. The wiggler is aplaner type. The magnet gap of the wiggler is variablefrom 30 to 120 mm, for which K = 0.013 - 1.472. Bychanging the electron energy from 10 to 19 MeV andthe magnet gap from 30 to 120 mm, it is possible tocover the wavelength region from 20 to 200 f.Lm with thefundamental peak, and from 6 to 67 f.Lm with the third
harmonic peak.

1 Introduction
The far infraredfree electron laser (FEL) based on
the L-band linac is now being developed at the Institute of Scientific and Industrial Re:;earch (ISIR), Osaka
University [1]. The first lasing was obtained at wavelengths from 32 to 40 f.Lm in 1994 [2, 3]. In the next
step, we are modifying the FEL system suitable for userexperiments. Since some potential users of the FEL are
interested in the longer wavelength region, we plan toextend the wavelength region up to at least 150 f.Lm.
This modification will include remodeling of the wiggler
and two bending magnets; and replacement of vacuum
chambers for the bending magnets and the mirror holders for the optical resonator. As reported previously [4],the wiggler was modified from a fixed magnet gap typeto a variable one. Then the vacuum chambers for thetwo bending magnets have been replaced with new one>with larger cross-sections in order to reduce the diffraction loss in the optical re>onator and accordingly magnet
gaps of the bending magnets have been widened. Afterthe wiggler and the bending magnets were remodeled,
we have been conducting FEL experiments. We haveobserved laser oscillation at wavelengths from 21 up to
126 f.Lm, which is the longest wavelength so far obtainedwith FELs based on RF linacs. In this paper, we willreport on the FEL and the experiments.
Table 1Main parameters of the electron beam
EnergyAccelerating frequency
Bunch spacing
Charge I bunch
Peak current I bunch
Bunch length
Macropulse length
Normalized emittance
Y-646B
YU-156

Table 2Main parameters of the FEL system
Wiggler
Nd-Fe-B
Magnet1920 mm
LengthNumber of periods 32
30 -120.mm
Magnet gap
0.013 - 1.472
K-value
Optical resonator
5532 mm
Cavity length
Radii of mirrors
3384 mm
Ml
2763 mm
M2915mmRayleigh range
3.4 mm (at 40 f.Lm)
Waist radius

1(}.19 MeV
1300 MHz
9.2 ns
2 nC
50 A2(}.30 ps
1.8 f.LS
80 ?rmm·mrad
300 1rmm·mrad-121-The 11th Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

In the wavelength region longer than 100 p,m, thediffraction loss due to the vacuum chambers in the optical resonator can not be disregarded. The vacuum chambers in the two bending magnets, which originally determined the diffraction loss, were replaced with new ones
with larger vertical sizes and accordingly magnet gapsof the bending magnets were widened to accommodate
them so that the calculated diffraction loss due to theseparts is less than that due to the vacuum chamber for
the wiggler.

0 fundamental

1.5c:

1iiClla 3rd harmonic

10.5

3 Light detection system
Laser light was taken out from the optical resonatorthrough a hole of 3 mm in diameter in the front mirror,
and led from the vacuum chamber to the air through awindow, which is either a KRS-5 plate 5 mm thick or asingle crystal quartz plate 2 mm thick, and transported
in the air to the measurement room using plane and concave mirrors coated with gold. The laser light was detected with a Ge:Be or Ge:Ga detector cooled with liquid
helium. Teflon sheets were inserted in front of the detector as an attenuator to avoid saturation of the output
signal when necessary. In some experiments, a gratingtype monochromator was used, which can monochromatize light with a wavelength shorter than 90 f.J.m. The
Ge:Be detector has the highest sensitivity around 40 "'45 p,m and the sensitivity drops rapidly as the wavelength becomes longer, while it has a cut-off filter for
light with a wavelength shorter than 20 p,m. The spectral sensitivity of the detector extends from 20 up to52 p,m. On the other hand, the Ge: Ga detector has
the highest sensitivity around 105 p,m and the sensitivity drops steeply as the wave length increases. It has
a cut-off filter for light with a wavelength shorter than
50 p,m. The sensitivity of the Ge:Ga detector ranges
from 50 to 130 p,m. For each kind of detector, we used
two detectors; one is a so-called slow detector which has
the high detection sensitivity but has the slow time response, and the other is a fast detector for measuring
the time evolution of FEL light. We have measured thetime resolution of the fast Ge:Be detector to be 170 ns(FWHM) [3], but has not yet measured that of the fastGe:Ga detector. We expect the time resolution to be approximately 100 ns. The transmission of light throughthe KRS-5 plate decreases very much above 45 rv 50 f.J.m,
while the transmission through the single crystal quartz
plate rises around 50 p,m and is high in the longer wavelength region. Therefore we used the KRS-5 window formeasurement below 50 p,m and the quartz window above50 p,m.
4 Experimental results and discussions

4.1 Oscillation experiments below 50 f.J.ID
We conducted experiments at an electron energy of
18.3 MeV. We used Y-646B as an electron gun. Laserlight coming through the KRS-5 window was directly
detected with the Ge:Be detector. The wavelength ofiaser light was determined from the electron energy andFig. 1 FEL gain as a function of the wavelength below
50 f.J.m. The data points denoted by the circles
and the squares are assigned to the fundamental oscillation and third harmonic oscillation,
respectively. See text for details.
the K-value of the wiggler. Fig. 1 shows the gain asa function of the wavelength which was derived fromthe time evolution of laser light measured with the fast
Ge:Be detector [3]. The light wavelength was varied bychanging the wiggler gap. Since the transmission of lightthrough the KRS-5 window rapidly decreases above 45"' 50 p,m and the sensitivity of the detector is practically zero above 60 p,m, we have assigned laser lightcorresponding to wavelengths longer than 50 p,m with
fundamental peak are due to the third harmonic peak.
Laser oscillation thus assigned to the fundamental peak
and to third harmonic peak are denoted by the circles
and the squares, respectively, in Fig. 1. The shortest
wavelength obtained in the experiments is 21 f.J.m withthe third harmonic oscillation.
4.2 Oscillation experiments above 50 t-tm

Experiments were conducted at electron energies of
14.0 and 14.4 MeV in order to make the wavelength long.
We used not only Y-646B but also YU-156 as an electron
gun in these experiments. Laser light coming throughthe quartz window was detected directly or through themonochromator with the Ge:Ga detector. With thecombination of the quartz window and the Ge:Ga detector, the detectable range of light was from 50 to 130
p,m. Fig. 2 shows the gain as a function of the wavelength which was also derived from the time evolutionof laser light measured with the Ge:Ga detector. The
wavelengths of the data points denoted by the diamonds
were measured with the monochromator and those of theother points were derived from the electron energy and
the K-value of the wiggler. The electron energy and theK-value were calibrated with the wavelengths measured
with the monochromator. The gain values denoted by
the circles were obtained with the electron gun YU-156,
which is usually used in operation of the linac and thosedenoted by the squares and the diamonds were obtainedwith Y-646B. Values of the normalized emittance of the-122-

The lith Symposium on Accelerator Science and Technology, Harima Science Garden City, 1997

tA
til! iI i A!.

0YU-158;14.0M&I/

aY-646B;14.4M&I/

QY-646B;14.4Mall(monochro)

AY-6468;13.0M&I/

Ay: I ilii'9't f1, iji1I ;

jI!

II .

iAf;!

I 1•

Fig. 2 FEL gain as a function of the wavelength above 50 1-£m. The gain values denotedby the circles were obtained at an electron energy 14.0 MeV using the electron
gun YU-156 and the other points were obtained at respective energies using theelectron gun Y-646B. The wavelengths of the data points denoted by the diamonds
were measured with the monochromator. The dotted line is the spectrum of theblack body radiation measured in the air with the monochrom ator and the Ge:Ga·detector. See text for details.
electron beams from YU-156 and Y-646B were measured
to be 300 and 80 1l'mm·mrad, respectively, as shown in
Table 1. Although they are appreciably different, themeasured values of the gain &e not very much different
in both cases except for the gain value at 86.3 p,m. The
dotted line in Fig. 2 shows the spectrum of light emitted
by a black body radiator at 1300 K which was measured
up to 90 l-£m wavelength with the same monochrom ator
and the detector. The vertical scale for the spectrum
is arbitrary units. The spectral shape is mostly determined by absorption of light by the water vapor in the air
though it also includes smooth wavelength dependencesof efficiency of the detector and that of the monochromator. It is apparent from the figure that FEL light we canmeasure in this wavelength region is considerably limited
due to the absorption of light in the transport line bywater vapor in the air. At the end of these experiments,
we decreased the electron energy down to 13 MeV andobserved lasing at wavelengths of 129 "' 131 p,m. The
gain and the time spectrum at 129 p,m are shown by thetriangle in Fig. 2 and in Fig. 3, respectively.
2~~~--~-r~~~-r--~~~
c 1.5::I€..!!!.1ii:5

0.5

0 o~~~~~~~~4~~~~~

time (tJ.S)

Fig. 3 The time spectrum of the laser light from theFEL operated at the wavelength 129 l-£m measured with the fast Ge:Ga detector.

5 Conclusion
In the experiments after the remodeling of the wiggler, the following results were obtained. We obtainedlasing from 32 beyond 126 1-£m with the fundamenta l oscillation and from 21 to 25 l-£m with the third haJ"ffionicoscillation. The FEL gain was measured to be "' 120% at maximum with the fundamenta l oscillation and tobe rv 160 % with the third h&monic oscillation. In thewavelength region above 50 1-£m, the measured gain wasalmost constant independent ly of the wavelength and thebeam emittance. We plan to confirm these experimenta l
results in following experiments.

Acknowled gement
This work was supported in part by a Grant-In-Ai dfor Scientific Research from the Ministry of Education,
Science, Sports and Culture of Japan.

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Yamamoto, S. Suemine, T. Okada, S. Takeda, K.
Tsumori, T. Hori, Nucl. Instr. and Meth. A 368(1995) 248.
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Yamamoto, S. Suemine, T. Okada, S. Ishida, T. Yamamoto, S. Takeda, K. Tsumori, T. Hori, Nucl. Instr. and Meth. A 358 {1995) 244.
[3) S. Okuda, S. Ishida, G. Isoyama, Y. Honda, R. Kato,Nucl. Instr. and Meth. A 375 {1996) 329.
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