FREE-ELECTRON LASERS
AND THEIR RADIATION APPLICATIONS
Y. Pinhasi
College of Judea and Samaria, Ariel, Israel
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 X-ray
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 Kilo-Watts 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 freeelectron 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 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. 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  Z 
Z 
W
2
 Z 1   Z  Z
VZ
( VZ is the axial electron velocity and c is the speed of light) and
c
1
1  Z
2
is the axial relativistic Lorentz factor related to:
 1
Ek
mec 2
where Ek is the kinetic energy of the accelerated electrons and me is the electron
mass. The curve in figure 2 describes the wavelength as a function of acceleration
energy. In the relativistic limit  Z  1, the radiation wavelength is approximated by:
S 
W
2
2 Z
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
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than the electron velocity, the electrons lose energy to the wave. As a result, the
electromagnetic field is stimulated and amplification occurs.
Wavelength
micron
10000
1000
100
10
1
0.1
1
10
100
Acceleration voltage [MeV]
Figure 2: Wavelength vs. acceleration voltage in an FEL.
3. ELECTROSTATIC-ACCELERATOR FELs
Free-electron lasers, utilizing few MeV electrostatic accelerators, normally
operate in the millimeter and infrared wavelength, covering the THz frequency band.
Figure 2 shows a curve of radiation frequency as a function of acceleration voltage of
the Israeli EA-FEL described in the followings.
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 quasi-CW (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].
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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].
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 (shown in Figure 3), which
was originally used as an ion accelerator for nuclear physics 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 and the radiation
frequency as a function of acceleration energy is drawn in Figure 3.
Figure 3: The Israeli Electrostatic-Accelerator Free-Electron Laser.
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Table 1: Parameters of the tandem electrostatic accelerator FEL.
ACCELERATOR:
Ek  1 3MeV
I0  1 2A
Electron beam energy:
Beam current:
UNDULATOR:
Type:
Magnetic induction:
Magneto-static planar wiggler
BW  2  3KGauss
W  4.444cm
Period length:
Number of periods
NW  20
Waveguide:
Transverse mode:
Curved-parallel plates
RESONATOR:
TE01
LC  3.06m
R  2  50%
Length:
Mirror transmission:
Frequency
GHz
10000
1000
100
10
1
0
1
2
3
4
5
6
7
8
9 10
Acceleration voltage [MeV]
Figure 4: Frequency vs. acceleration voltage in an EA-FEL.
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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. 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
Diffraction limited
Diffraction limited
single mode
f
10  7
f
f
10  7
f
Temporal
coherence:
f
10 5
f
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 4. 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 nonthermal 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.
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.
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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.
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