Proceedings of the 25th Linear Accelerator Meeting in Japan (July 12-14, 2000, Himeji, Japan)[13P-26]

SIMULATION STUDIES OF THE PROPOSED FAR-INFRARED FELAMPLIFIER AT ISIR, OSAKA UNIVERSITY
Ravi A.V. Kumar*), Ryukou Kato and Goro Isoyama
Institute of Scientific and Industrial Research, Osaka University,
8-1 Mihogaoka, Ibaraki, Osaka 567-0047, JAPANAbstract
Computer simulation studies are currently underway for the proposed FEL amplifier experiment to be carried out
using the existing 38 MeV, L-band linac based FIR FEL system at the Institute of Scientific and Industrial Research(ISIR), Osaka University. This paper describes the time dependent and time independent simulation results using
Genesis code. Dependence of FEL amplifier characteristics are investigated such as power spectrum and profile,frequency spectrum and FEL bunching parameter on parameters like beam current, input power and interaction length.
The operating current regime and input seed laser powers required for optimum amplification are given. The code has
also been used to compare SASE experimental data with simulation results with a reasonably good correlation.

1.

INTRODUCTIONThe Free-Electron Laser (FEL) has been deemed tobe the ‘next generation light source’. With the extensive
research and development in the field of single pass,
Self Amplified Spontaneous Emission (SASE) FELs,
the accessible wavelength of the FEL has now been
extended far beyond the UV region. The SASE being
the best suited configuration because ofproblems
pertaining to use of optical elements at shorter
wavelengths.
SASE was first observed here at the Radiation
Laboratory of the Institute of Scientific and Industrial
Research (ISIR), Osaka University in 1991 during thecourse of developing the infrared FEL using the 38
MeV, L-band linac [1]. Detailed experimental study on
SASE is currently being conducted using the wiggler for
the infrared. Although strong SASE has been observed,
it is not possible to go into the saturation regime under
the present experimental conditions due to the limited
number of wiggler periods (32). It was proposed that theabove mentioned FEL system could be used to conduct
single-pass FEL amplifier experiments using an injected
seed laser source, which would help understand thephenomenon of SASE in the strong signal regime. It isalso necessary to know the behavior of the system andits dependencies on various experimental parameters
before actually conducting the experiment. The onedimensional (1-D) theory does offer an insight into thebehavior, but due to the fact that many factors are often
neglected in 1-D estimations, the results may notcorrelate exactly with the experimental observations.
Hence it is essential that a more detailed, realistic______________________________________________________________________________________________________________________________________*)

Ravi.A.V. Kumar, (81)-06-6879-8487,
E-mail : ravi25@sanken.osaka-u.ac.jp
On leave from Institute For Plasma Research,Bhat, Gandhinagar, India, 382 428

simulation is carried out, taking into account as many
experimental parameters as possible. This paper
attempts to study the effect of the beam current, input
seed laser power and interaction length on the FEL
amplifier experiment.
2.

GENESIS 1.3 - THE SIMULATION CODE

Ever since the design and building of FELs started,different simulation codes [2] have been developed,
each one tailored to describe the phenomenon of FEL
occurring in different regimes. Genesis has evolved
from an earlier 3-D FEL simulation codes TDA [3], andthen TDA3D. Genesis, like its predecessors, solves theparaxial FEL equations with the approximation of slow
varying amplitude of the radiation field. Also, in
Genesis, time dependent simulation as well as
simulation of single-pass SASE has been added [4].
In the time dependency studies, to avoid thelimitations imposed by the CPU time and available
memory, the code does not simulate the bunch as awhole, but divides it into thin time samples of length of
one radiation wavelength each, equally distributed overthe simulated time window.
In the proposed experiment, high-intensity, single
bunch e-beam of charge up to 73 nC and a pulse length
in the range 9-50 ps from the 38 MeV ISIR linac istraversed through a 32 period Halbach type, permanent
magnet (Nd-Fe-B) planar wiggler of 60mm period. The
wiggler gap can be adjusted from 30 to 120 mm andhence the wiggler constant K can be varied from 1.47 to0.013. A seed laser source at appropriate wavelengthand power would be injected along the axis of the beamand this would be amplified by the single- pass FEL
process. The simulation parameters are given in Table.1,−355−

Table.1. The parameters used in the simulationand they are obtained from real-time, measured datafrom on-going SASE experiments.
3.

RESULTS & DISCUSSION

In this paper, the influence of three basic parameters viz.,the beam current, interaction length and the input seed
power on the output power and frequency spectrum arestudied.
Figure 1 shows the dependence of the of the output
power on the peak beam current. It can be clearly seenthat there are two distinct regimes of current indicatedby the two distinct slopes. The output in the lower
current regime (slope ~ 1.9 ± 0.2) may not be wholly
due to the SASE but due to a major contribution fromcoherent spontaneous emission which has a square
dependence on the number of electrons. But at higher
currents (slope ~ 4.9 ± 0.15), the SASE process isdominant which can also be inferred from the plots of
the FEL bunching factor which is a measure of thespatial modulation of the electrons in the wiggler (Fig.2).
The bunching factor is not very well defined until thecurrent approaches the second regime. The temporal

7Output power (W)10410Input Power
1µW
10mW
1W100W

110-2

10100

1000
Peak current (A)Fig.1. The dependence of the output power on beam
current-2

10Ipeak= 2.5kA
310-3

10Ipeak= 1.0kA
Ipeak= 0.5kA-410Ipeak= 0.05kA-51002040Time (ps)

21080

60

Fig.2. The dependence of the bunching factor onbeam current
profile of the electron bunch is also indicated in thegraph (dashed line).
The dependence of the output power on the input
seed power is shown in Fig.3. The shot noise power ofthe system is given by [5]Pshot ≈3 4π ρ 2 PbæN öN λ lnç λ ÷è ρ ø,

(1)

where ρ is the FEL parameter, Pb is beam power and Nλis the number of electrons in one wavelength. Thispower is typically in the 10-3 to 10-2 W range for theparameters used in this simulation. It can be seen thatfor input powers less than the shot noise power, theoutput is more or less constant. Thus, it is important tokeep the input seed laser power above the shot noise
power level in order that the system effectively acts as
an amplifier of the input seed laser.
Figure 4 shows the power spectrum at different input
seed laser powers. It can be seen that the power
spectrum for higher input power is narrower as

Ipeak = 1kA

105103Current (A)FEL Bunching factor

12.42 MeV
2.18 %
1.472
6.0 cm32166 µm
150 π mm mrad
93 π mm mrad
3.3 mm
1.1 mm
0.96 m
5.64 mm
0.6 m
1506.0 mm
4096

Output power (W)Beam energy
Energy spread (HWHM)
Wiggler K parameter
Wiggler period
No. of periods
Simulation wavelength
Normalized Emittance (x)Normalized Emittance (y)e-beam radius (x)e-beam radius (y)Optical beam waist location
Optical beam waist radius
Raleigh length
No. of slices
Current length
No. of particles

Ipeak = 0.5kA

110Ipeak = 0.1kA
Fig.3. The dependence of the output power on input
seed laser power-1
10 -7-4-1
210101010Input power (W)Fig.3. Dependence of the output power on input
seed laser power

-4

Experimental power (a.u.)

Pin= 100W

10

Pin= 1WP(W)-110

Pin= 1mW-410

1.0Simulation
λsim = 166.02±0.03 µm
σsim = 3.02±0.07 µm

0.20
0.16

1.5

2.0
ω(THz)

Experimentcompared to the noise-startup.
The saturation of the system is studied as a functionof the interaction length as well as beam current andinput laser power, and it is shown in Fig.5. Saturation
occurs in the region of 60 periods when the peakcurrent is 1 kA and input power is 1 W while it occurs
at about 130 when current is 500 A and input power is1W. Hence it is experimentally possible to approach
saturation by controlling all of these three parametersviz., interaction length, beam current and the input
power.
Genesis was also used to check the validity of thecode and also to correlate the experimental data of thecurrently ongoing SASE experiments. Figure 6 shows
the preliminary results of one such comparison ofexperimental SASE power spectrum with simulation.
The experimental peak wavelength as well as bandwidthof emission are comparable to the simulation. It isplanned to carry out such comparisons in more detail inthe near future.

-43x10-42x100.12-41x100.080λ0(1D) = 160.8 µm

155

2.5Fig.4. The dependence of the power spectrum
on the input power-44x10λexpt = 167.8±0.24 µmσexpt = 3.02±0.8 µm

150-7100.245x10160 165 170
Wavelength (µm)

175

180

Fig.6. Comparison of the SASE experiment andsimulation
4.

CONCLUSIONSThe time dependent 3-D simulations for the proposed
FEL FIR amplifier experiments are underway using
Genesis code. Dependencies of the output power onbeam current, input power as well as interaction length
are presented. In order that amplification actually occurs,
it is essential that the input power be kept well above
the shot noise power and beam currents above thethreshold value above which the SASE contribution tothe output is predominant. The code has also been used
to correlate on-going experimental SASE data.
5.

ACKNOWLEDGEMENTS

One of the authors (Ravi A.V. Kumar) is indebted toGakushin for the post-doctoral fellowship awarded tohim and to Monbusho, Government of Japan, for theaward of Grant-in-aid for research.
REFERENCES

Output power (W)10108610410

210

1 kA, 1µW
1 kA, 1W
0.5 kA, 1µW
0.5 kA, 1W

0050
100
150
200
Interaction length (Periods)

Fig.5. The dependence of saturation on N at different
input powers and beam current.

Simulation power (W)2[1] S. Takeda et.al., IEEE Trans.Nucl.Sci. NS-32,3219,(1985).
[2] T.M Tran and J.S. Wurtele, Physics Reports, 195, 1,(1990).
[3] T.M Tran and J.S. Wurtele, Comput. Physics.Commun.
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[4] S. Reiche, Nucl.Instr.And Meth. A429, 243, (1999).
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