FREE ELECTRON LASERS AND THEIR RADIATION
APPLICATIONS
Y. Pinhasi
The College of Judea and Samaria,
Dept. of Electrical and Electronic Engineering
P.O. Box 3, Ariel 44837, ISRAEL, yosip@eng.tau.ac.il
Abstract
Free-electron lasers (FELs) are radiation sources, utilizing
accelerated electrons, which are oscillating transverse to their
propagation axis while passing through a periodic magnetic structure
(undulator). Contrary to quantum lasers, where the operating frequency
is determined by the energy gap of the atoms of the gain medium, FELs
are tunable sources that produce intense coherent radiation across a wide
frequency range of the electromagnetic spectrum (microwave to the Xray regimes).
A free-electron laser, which utilizes an electrostatic accelerator
(EA-FEL), is characterized by high average power, potential of
continuous (CW) operation, high efficiency (tens of percents, much
higher than the capability of conventional lasers), tunability, high
frequency stability and spectral coherence. These unique features make
the EA-FEL an appropriate candidate for scientific, technological,
industrial and medical applications. The Israeli free-electron laser in the
College of Judea and Samaria, is based on a 1-6MeV tandem
electrostatic accelerator and designed to operate as a tunable source in
the millimeter and far-infrared wavelengths producing several KiloWatts of electromagnetic radiation.
The principle of electromagnetic field excitation in free-electron
laser is presented, including fundamental aspects of the interaction
between accelerated electrons and radiation. The unique features of the
Israeli EA-FEL are described, including recent theoretical and
experimental results. Utilization of millimeter and Tera-Hertz waves for
radiation user applications are also discussed.
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1. Introduction
Free-electron lasers (FEL) are high power sources of
electromagnetic radiation, utilizing accelerated electrons, which are
oscillating transverse to their propagation axis. The name free-electron
laser was chosen to distinguish this device from conventional quantum
lasers (Light Amplification by Stimulated Emission of Radiation), where
the radiation is a consequence of transitions of electrons bounded in
atomic energy states.
The foundations of FELs go back to the early investigations of
stimulated Thomson and Compton scattering carried out by Kapitza and
Dirac in 1933 [1]. H. Motz at Stanford examined in 1951 theoretically
[2] and experimentally [3-4] the radiation emitted when a fast electron
beam passes through a succession of electric or magnetic fields of
alternating polarity. Incoherent emissions in the middle of the visible
spectrum and coherent radiation at millimeter wavelengths were
produced in these experiments.
In 1957 R. M. Phillips of the General Electric Microwave
Laboratory invented the ubitron (Undulating Beam Interaction) [5-6].
The device constructed in the first experiment, used a permanent magnet
undulator in which a 70A pencil beam accelerated to 110-170KeV
passed. Both an amplifier and an oscillator were built. A maximum gain
of 13dB was obtained in the amplifier configuration at a center
frequency of 2.6GHz. The bandwidth of the amplifier's gain around this
frequency was 30%, and the saturation power was 0.9MW. The
oscillator produced 1.2MW at 10% efficiency near 2.5GHz. During the
succeeding seven years, Phillips investigated the concept of the ubitron,
which was the early version of the present day free-electron laser.
However, the full potential of this device was not recognized, and the
research work was terminated in 1964.
The interest in interaction between an undulating fast e-beam
and electromagnetic fields as related to generation of coherent radiation,
arose again when J. M. J. Madey proposed in 1971 his idea [7] of what
was later termed free-electron laser [8]. Two successive experiments
were performed at Stanford in the infrared (IR) regime. First an
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amplifier at 10.6m was announced in 1976 [9] and laser oscillation at
3.4m was obtained the following year [10]. These experiments utilized
a relativistic electron beam from a radio-frequency linear accelerator
(RF LINAC).
2. Electrostatic Accelerator FELs
While most of the facilities are based upon RF linacs, which
produce e-beam short pulses, only few projects in the world utilize
electrostatics accelerators that enable continuous wave (CW) or quasiCW (long pulse) operation. Electrostatic Accelerator FELs (EA-FELs)
are also characterized by high average power generation, high energy
conversion efficiency and high spectral purity. The property of an
electrostatic accelerator as a high quality e-beam source for a FEL is
crucial for attaining high brightness spontaneous emission radiation, as
well as high gain at short wavelengths. The unique features of EA-FELs
make them naturally fitting for a variety of applications in the present
and in the near future [11].
The first demonstration of EA-FEL operating at the far-infrared
band was made at the University of California Santa Barbara (UCSB) in
1984 [12-13]. The experiment employed a 3MeV electrostatic
accelerator, and produced 10KW over the range of 390-1,000m. This
FEL oscillator was claimed to operate in a single mode regime,
generating narrow-band radiation [14].
The capability of the EA-FEL to produce high average power
almost continuously at mm wavelength indicates that this type of FEM
can be used for heating of magnetically confined plasma for controlled
thermonuclear fusion. Such a FEM was suggested and constructed by
the Dutch FOM-Institute for Plasma Physics [15-16].
3. Fundamental of free-electron laser operation
In principle, a free-electron laser consists of an accelerated
electron beam traveling along a periodic beam deflective structure,
which forces the electrons to oscillate in the transverse direction (see
Figure 1). Many structures can be employed to create the wiggling of the
electrons. We confine the explanation here to an undulator compounded
from alternating magnets, equally spaced, with a period  W .
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Wiggler
Electron
Waveguide
Lw
Figure 1: Electron passing in a wiggler
Each electron is a moving dipole radiator emitting a wave
packet of
undulator synchrotron radiation [3], which propagates as a free-space or
as a waveguide mode. The wavelength of the radiation is given
approximately by
S 
where
W
 Z 1   Z  Z 2 ,
VZ
c
Z 
VZ - the axial electron velocity and c is the speed of light;
Z 
1
1 Z
2
Z - the axial relativistic Lorentz factor related to:
 1
Ek
mec 2
where E k is the kinetic energy of the accelerated electrons and me is
the electron mass.
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In the relativistic limit  Z  1, the radiation wavelength is
approximated by:
S 
W .
2
2 Z
Frequency
GHz
The beating of the transverse component of the radiation wave
with the wiggler-induced velocity of the electrons, creates a moving
pondermotive force, directed in the z-direction, and produces
perturbations in the axial velocity of the electrons, forming bunches in
the beam. If the pondermotive wave is slightly slower than the electron
velocity, the electrons lose energy to the wave. As a result, the
electromagnetic field is stimulated and amplification occurs.
Free-electron lasers, utilizing few MeV electrostatic
accelerators, normally operate in the millimeter and infra-red
wavelength, covering the THz frequency band. Figure 2 show a curve of
radiation frequency as a function of acceleration voltage of the Israeli
EA-FEL described in the followings.
6000
5000
4000
3000
2000
1000
0
0
1
2
3
4
5
6
7
8
9 10
Acceleration voltage [MeV]
Figure 2: Frequency vs. acceleration voltage in an EA-FEL.
4. The Israeli EA-FEL
The Israeli FEL project, which is located in the College of Judea
and Samaria, is based on 1-6MeV EN-Tandem van de Graff accelerator,
which was originally used as an ion accelerator for nuclear physics
167
experiments [17]. The machine was converted into an electron beam
accelerator and was modified to enable insertion of a magneto-static
wiggler and electron-optics focusing elements. Contrary to the EA-FEL
of UCSB, our scheme uses straight geometry for the electron beam
transport. The electron gun and the collector are installed outside of the
accelerator region. The basic parameters of the FEL are given in
Table 1.
Table 1
Parameters of the tandem electrostatic accelerator FEL
ACCELERATOR:
Electron
beam E  1 3MeV
k
energy:
Beam current:
I  1 2A
0
UNDULATOR:
Type:
Magneto-static
wiggler
Magnetic induction:
Period length:
Number of periods
planar
BW  2  3KGauss
W  4.444cm
NW  20
RESONATOR:
Waveguide:
Transverse mode:
Length:
Mirror transmission:
Curved-parallel plates
TE01
LC  3.06m
R  2  50%
The FEL is presently designed to operate in the millimeter
wavelength range [17-22] and in the future will be extended to the THz
regime. Table 2 summarizes present and future operational parameters
of the Israeli EA-FEL.
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Table 2
Operational parameters of the Israeli EA-FEL.
Present
Short – term
Long – term
Tuning range:
70 – 130 GHz
50 – 130 GHz
30GHz - 1THz
Peak intensity:
10 kW
30 kW
30 kW
Average power:
-------
1 kW
30kW
Pulse duration:
5 - 30 S
5 – 1000 S
5 S – CW
1-100 pS
Beam
dimension:
5 cm
Focusable down
to 5 mm
Focusable
down to 5 mm
Spatial
coherence:
Diffraction
limited
single mode
f
10  5
f
Diffraction
limited
Diffraction
limited
f
10  7
f
f
10 7
f
Temporal
coherence:
5. User facility and applications
The Israeli EA-FEL will serve as a radiation source for user
applications. A conceptual illustration of the radiation user facility is
illustrated in Figure 3. Radiation exposure stations of four kinds are
planned to be made available to users:
Spectroscopic stations: mostly for solid state and chemical
research. Will include cryogenic facilities down to 100 K, magnets, a
chemical hood.
Bio-medical stations: for such applications as study of thermal
and non-thermal interaction of radiation with tissues, imaging. Will
include incubators, thermal camera.
Material processing stations: sintering of bulk and thin film
ceramics (including HTSC), surface treatment of metals, polymers. Will
include reactor chamber, sample scanning means, supply of gases, gas
exhaust.
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Atmospheric studies station: study of scattering from clouds,
aerosols, dust and particles, millimeter wave imaging, radar, wide band
communication, energy transmission. Will include a large aperture
transmitting antenna and a receiver.
In the long-term plan, it is intended to operate the FEL with a
photo-cathode injector in a pico-second pulsed mode. This will make it
possible to supply simultaneously with the mm wave pulses, also
synchronous pSec pulses in the UV, IR or THz regimes for such
applications as pump-probe experiments.
References
1. P. L. Kapitza and P. A. M. Dirac, Proc. Cambr. Phil. Soc. 29, 247
(1933)
2. H. Motz, J. Appl. Phys. 22, 527 (1951)
3. H. Motz, W. Thon and R. N. Whitehurst, J. Appl. Phys. 24, 826 (1953)
4. H. Motz and M. Nakamura, Annals of Phys. 7, 84 (1959)
5. R. M. Phillips, IRE Trans. Electron. Dev. ED-7, 231 (1960)
6. R. M. Phillips, Nucl. Instr. and Methods A 272, 272 (1988)
7. J. M. J. Madey, J. Appl. Phys. 42, 1906 (1971)
8. J. M. J. Madey, H. A. Schwettman and W. M. Fairbank, IEEE Trans.
Nucl. Sci. NS-20, 980 (1973)
9. L. R. Elias, W. M. Fairbank, J. M. J. Madey, H. A. Schwettman and T.
L. Smith, Phys. Rev. Lett. 36, 717 (1976)
10. D. A. G. Deacon, L. R. Elias, J. M. J. Madey, G. J. Ramian, H. A.
Schwettman and T. L. Smith, Phys. Rev. Lett. 38, 892 (1977)
11. A. Gover, A. Friedman and A. Drobot, Laser Focus World 95
(October 1990).
12. L. R. Elias et al., Nucl. Instr. and Methods A 237, 203 (1985)
13. L. R. Elias, IEEE J. Quantum Electron. QE-23, 1470 (1987)
14. L. R. Elias, G. Raminan, R. J. Hu and A. Amir, Phys. Rev. Lett. 57,
424 (1986)
15. P. W. van Amersfroot, W. H. Urbanus, A. G. A. Verhoven, A.
Verheul, A. B. Sterk, A. M. van Ingen and M. J. van der Wiel, Nucl.
Instr. and Methods A 304, 168 (1991)
16. W. H. Urbanus et al., Nucl. Instrum. Methods A 331, 235 (1993)
17. A. Gover, E. Jerby, H. Kleinman, I. Ben-Zvi, B. V. Elkonin, A.
Fruchtman, J. S. Sokolowski, B. Mandelbaum, A. Rosenberg, J.
170
Shiloh, G. Hazak, O. Shahal, Nucl. Instrum. Methods A 296, 720
(1990)
18. A. Abramovich, A. Arensburg, D. Chairman, A. Eichenbaum, M.
Draznin, A. Gover, H. Kleinman, I. Merhasin, Y. Pinhasi, J. S.
Sokolowski, Y. M. Yakover, M. Cohen, L. A. Levin, O. Shahal, A.
Rosenberg, I. Shnitzer, J. Shiloh, Nucl. Instr. Methods Phys. Res. A
407, 16 (1998)
19. A. Abramovich, Y. Pinhasi, Y. M. Yakover, J. S. Sokolovski, A.
Gover, Nucl. Instr. Methods Phys. Res. A 407, 81 (1998)
20. A. Abramovich, Y. Pinhasi, Y. M. Yakover, A. Gover, J. S.
Sokolovski, M. Canter, Special issue on cyclotron resonance
masers and gyrotrons of the IEEE Transactions on Plasma Science,
Vol. 27, 563 (1999)
21. A. Abramovich, M. Canter, A. Gover, J. S. Sokolovski, Y. M.
Yakover, Y. Pinhasi, I. Schnitzer, J. Shiloh, Phys. Rev. Lett. 82,
6774 (1999)
22. A. Abramovich, Y. Pinhasi, Y. Yakover, A. Gover, J. S. Sokolovski,
M. Canter, Nucl. Instr. Methods Phys. Res. A 429, 101 (1999)
171