The 13th Symposium on Accelerator Science and Technology, Suita, Osak11, Japan, October 2001

WAVELENGTH TUNABILITY OF JAERI-FEL
R. Nagait, R. Hajima, N. Nishimori, M. Sawamura, N. Kikuzawa and E. Minehara,JAERI, Tokai, Ibaraki 319-1195, JAPAN

Abstract
Wavelength tunability of JAERI-FEL has been
demonstrated by adjustment of the undulator parameter atthe electron beam energy of 16.4 MeV The wavelength
has been rapidly and continuously varied without any
readjustment of the electron beam. The demonstrated
spectral range is between 17 f.im and 23 f.im against theundulator parameter of between 0.25 and 0.70. The FEL
properties such as relative rms spectrum width andextraction efficiency are measured at each undulator
parameter. The relative rms spectrum width is estimatedfrom extraction efficiency using the universal brightness.
The measured spectrum width is compared with theestimated value for verification of the measurement.

1 INTRODUCTION
A free-electron laser (FEL) is a very useful light source
in many applications due to its potential of high power,
short pulse width and continuous wavelength tunability[ 1-4]. An important feature of FEL is the property of
rapid and continuous tunability over a wide spectral range.
The resonance wavelength of the FEL 'A is given thefollowing equation [5],(1)A-=A,(J+a/)1(2y) ,where A, is the period length of the undulator field, r is
the Lorentz factor and au is the undulator parameter.
Therefore the wavelength tunability is achieved byadjustment of the electron beam energy and/or theundulator parameter.
Recently, high power, high efficiency and ultrashrot
pulse generation was achieved at a superconducting rf
linac based FEL in Japan Atomic Energy ResearchInstitute (JAERI-FEL) [6,7]. In order to show theusability of JAERI-FEL for many applications, thewavelength has been varied by adjustment of theundulator parameter. The FEL properties such as relativerms spectrum width and extraction efficiency aremeasured at each undulator parameter. This paper reports
property of the wavelength tunability.

compressed over a long drift space, thus minimizing theemittance growth due to the bunch compression [8]. Two499.8 MHz 5-cell superconducting modules are used toPowerincrease the beam energy to 13-20 MeV
requirements limit the macropulse duration and repetition
to 1 ms and 10 Hz, respectively. The macropulse consistsof a train of short mircrobunches, hence, the optical beamThe
also consists of short, intence micropulses.
microbunches have a repetition of 10.4125 MHz with acharge of up to 0.6 nC and a width of less than 5 ps.
Behind the linac, the electron beam is led into anundulator. The undulator is a hybrid type with a period
number of 52 and a period length of 33 mm [9]. Twomirrors placed at opposite side of the undulator define theresonator. The mirrors are gold-coated to ensure highreflectivity in the wide spectral range. The distance
between the mirrors can be adjusted in order tosynchronize the circulating optical pulse with the electronbunches. Behind the undulator, the electron bunches arebent out of the resonator and dumped.
Optical resonator

Figure 1: The Layout of JAERI-FEL.
Table 1: Typical parameres of JAERI-FEL.
Kinetic energy
Average current
Bunch charge
Bunch width (FWHM)
Energy spread (rms)Normalized emittance (hor.)
Normalized Emittance (ver.)
Bunch repetition
Length of undulator period
Number of undulator period
Undulator parameter
Length of optical resonator
Rayleigh range
Mirror radii
Output coupler
Output wavelength

2 EXPERIMENTS AND RESULTS
The layout of JAERI-FEL is shown in Fig. 1. Typicalparameters of JAERI-FEL in this demonstration are listed
in Table 1. The injector consists of a 230 kV thermionictriode gun driven by a grid pulser at width of 1 ns, a 83. 3
MHz subhermonic buncher, and two 499.8 MHz 1-cellsuperconducting modules. In the injector, the electronenergy of 2 MeV andbeam is accelerated to an
rr_nagai@popsvr.tokai.jaeri.go.jp

Undulator

16.4MeV
5.3mA
0.51 nC<5 ps
1.2%

40mm-mrad
22mm-mrad10.4125 MHz
33mm
52
0.70-0.25
14.4m
LOOm
6cm4 mm ~, Center-hole
23-17 Jlm

Adjustment of the electron beam energy while
maintaining the excellent beam quality needed for FEL
oscillation is nontrivial, and involves careful adjustmentof the amplitude and phase of the rf field of the linac. Inaddition, the excitation of beam transport magnets, whichis used to inject the electron beam into the undulator, has
to be scanned simultaneously with the adjustment of theenergy. In contrast, the method of variation of the-455-

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

wavelength by adjustment of the undulator parameter
dose not suffers from these limitations. This method
allows rapid and continuous wavelength tunability. Theundulator parameter au means normalized field strength of
the undulator and is given by au=eBuA.I2mnc [5], where eis the elementary electric charge, Bu is the rms magnetic
field on axis, m is the rest mass of electron and c is thespeed of light in vacuum. The undulator is constructedfrom permanent magnets and magnetic poles, and thefield on axis is varied by adjustment of the gap between
the two rows of magnetic circuits. The undulator
parameter as a function of the gap is shown in Fig. 2. The
crosses denote measured value. The solid line is fitted tothe measured values by a function of au=C1Exp{(-g/A,j(C2+C~IA,J) [10], where C1, C2 and C3 are the freeparameters, and g is the gap length. The result of thecurve fitting, C1, C2 and C3 are 2.520, 1.215 and 1.593,
respectively. The movement of the undulator rows has ahigh mechanical accuracy in the presence of large
magnetic force. The adjustment of the undulator
parameter form 0.70 to 0.25 takes about 30 sec. The
moving speed of the row is restricted to ensure the highaccuracy.

efficiency and spectrum width are largely depend on thedesynchronism [ 11].
Figure 4 shows mean wavelength of the measured
spectra and the resonance wavelength against undulator
parameters. The mean wavelength is slightly longer than
the resonance wavelength, which is derived from Eq. 1.
The deference of the mean and the resonance wavelengthis good agreement with the Madey theorem [12].

r .•••.•.• A:":~o;~. 1~j,. •. r;_: • • a.~oso~[1( .".~40 lr. A. .. •.... : .. ~>0.25 1
I •~.R"

2 0.8~ 0.6"'p.,~j1520"'"'I25......

130

Wavelength (f!m)

0.4

Figure 3: The spectra for various undulator parameters.

" 0.2::00.0202224262830

Gap length (mm)Figure 2: The undulator parameter against the gap. The
crosses denote the measured undulator parameter. The
solid line denotes the fitted cureve.
The variation of wavelangth is performed by theadjustment of the undulator parameter without any
readjustment of the electron beam. The spectral range isfrom 23 f!m to 17 f!m at the energy of 16.4 MeV. A
monochromater with a HgCdTe detecter is used for themeasurement of spectra of various undulator parameters.
Sensitivity of the monochromater is corrected by using
black-body radiation. The FEL radiation is coupled outthrough a 4 mm diameter center-hole in one of the mirros
and extracted to the air through a KRS5 window. The
radiation is guided via transport system in the air to themonochromater, which is installed near the output
window in the accelerator room.
For minimizing
absorption in ambient water vapor, the transport distanceis about 3m as short as possible. The monochromatersignal is averaged over the macropulse duration. The
spectra are shown in Fig. 3. There are some dips due tothe absorption in ambient water vapor. The spectra aremeasured at desynchronism of peak power.
The
desynchronism means difference between the round-trip
time of optical pulse and the repetition of the electron
beam.
The FEL properties such as the extraction

Undulator parameter

Figure 4: The mean wavelength of the measured spectra
and the resonance wavelength. The crosses and the open
circles denote measured mean wavelength and resonancewavelength, respectively.
1048~~ 36p.,~

24I

2000.8~>

<~

~

"'·
"Sl0~

0~·

m0.2s.
~~~

Undulator parameter

Figure 5: The efficiency and output power of JAER-FELagainst the undulator parameter. The open circles and thecrosses denote the measured extraction efficiency and theaverage power, respectively.

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

The extraction efficiency is estimated from energy lossof the electron beam due to the FEL interaction at thebeam dump [13]. The efficiency and output power are
shown in Fig. 5 as function of the undulator parameter.
The average output power is measured at outside of theKRS5 window for 400 j..tS macropulse operation. The
output power dependence on the wavelength differs
slightly from the efficiency due to the variation of theoutput coupling efficiency of the center-hole output
coupler [ 14].
Figure 6 shows measured and estimated relative rms
width of the measured spectra against the undulator
parameter. There is the so-called universal brightness
because the efficiency is enhanced at the expense of a
proportional specrum broadening [15]. The relative rms
spectrum width is estimated by the universal brightness
0.86=rJI(aJ./Jo), where 17 is the extraction efficiency andaJ./A is the relative. rms spectrum width. At the undulator
parameter smaller than 0.5, the measured spectrum width
is in good agreement with the estimated one. At theundulator parameter of 0.7, the measured spectrum widthis narrower than the estimated one, but the measured
efficiency consistents with the pulse width of the FELThis difference is currently underradiation [16].
investigation.

Undulator parameter

Figure 6: The measure and the estimated relative rms
spectrum width against the undulator parameter. Thecrosses and the open circles denote measured andestimated spectrum width, respectively.

3 CONCULUSION
The wavelength of JAERI-FEL has been rapidly andcontinuously varied by adjustment of the undulator
parameter without any readjustment of the electron beam.
The wavelength range is between 23 j..tm and 17 j..tm
against the undulator parameter of between 0.70 and 0.25.
The measured wavelength is in good agreement with thewavelength estimated by the undulator parameter. For thelarger undulator parameters, the extraction efficiency has
been enhanced at the expence of a proportional spectrum
broadening.

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[2] P. C. van Son et al., Nucl. lnstr. and Meth. A341
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[4] W. C. Fowler et al., Nucl. lnstr. and Meth. A429
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[5] C. A. Brau, "Free-electron Lasers" Academic Press,Inc. (1990)[6] E. Minehara et al., Nucl. Instr. and Meth. A445 (2000)
183-186.
[7] R. Hajima et al., in the Proc. of the 23rd FEL Conf.
[8] N. Nishimori et al., in Proc. of this symposium.
[9] R. Nagai et al., Nucl. Instr. and Meth. A358 (1995)
403-406.
[10] K. Halbach, J. Physique 44 CoHoe. Cl (1983) 211216.
[11] N. Nishimori et al., Phys. Rev. Lett. 86 (2001) 57075710.
[12] J. M. J. Madey, Nuovo Cim. 50B (1979) 64.
[13] N. Nishimori et al., in the Proc. of the 23rd FEL Conf.
[14] R. Nagai., J. Nucl. Sic. and Tech. 38 (2001) 15-18.
[15] D. A. Jaroszynski et al., Phys. Rev. Lett. 78 (1997)
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