Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~2918~7
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Short-Period Electron Beam Wiggler
Field of the Invention
The invention pertains to free-electron lasers
in which electrons in a linear beam are periodically
accelerated ("wiggled") perpendicular to the beam
motion by periodic transverse magnetic fields. They
radiate electromagnetic waves which are amplified and
made coherent by reflections in a resonator such as
the space between reflecting mirrors. To get high
frequencies such as infrared, the beam velocity must
be in the megavolt, relativistic range and the
periodicity of the field must be very small.
Prior Art
Periodically reversing magnetic fields have
traditionally been generated by a stack of permanent
magnets of alternating polarity. As the period gets
shorter, the magnetomotive force is reduced, leakage
flux increases and soon imposes a lower limit to the
available periodicity when generating fields across
gaps of separation usable to transmit the electron
beam.
Summary cf the Invention
An object of the invention is to provide a
magnetic beam wiggler of very short period.
A further object is to provide a wiggler of
minimum size, weight, and power consumption.
These objects are realized by forming the
periodic magnet elements as opposed rows of floating
ferromagnetic polepieces. Poles in opposite rows are
staggered in the beam direction by one-half period.
A uniform, extended, exciting magnetomotive force is
supplied from an external source, such as a solenoid
--2--
coil. The flux generated between polepieces has a
strong transverse component alternating between
polepieces of the two rows.
Brief Description of the Drawings
FIG. 1 is a section thru the beam direction of
a periodic magnet system of the prior art.
FIG. 2 is a section similar to FIG. 1 of an
alternative prior-art magnet system.
FIG. 3 is a section of a magnet system embodying
the invention.
FIG. 4 is a sketch of magnetic elements of a
free-electron laser embodying the invention.
FIG. 5 is a schematic partial section of the
magnetic and optical structure of a free-electron
laser embodying the invention.
FIG. 6 is a schematic section of an alternative
laser construction.
Description of the Preferred Embodiments
FIG. 1 (prior art) shows a simple periodic
permanent magnet (PPM) system for guiding an electron
beam in a wiggling motion. The originally linear
beam 10 passes between opposed rows 1~, 14 of
bar-shaped magnets 16 extending perpendicular to
the plane of the section and to the direction of
beam 10. Opposed pairs of magnets 16 are magnetized
in the same direction perpendicular to the beam to
produce a field 22 transverse to the beam motion.
Pairs spaced successively in the beam direction have
alternating polarity so the beam experiences an
oscillating acceleration perpendicular to the paper.
Thus, electromagnetic waves are radiated, polarized
perpendicular to the paper. Their internal
generating frequency is the forward velocity of
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the beam divided by the magnet periodiocity. At
relativistic velocity an electron is almost in
synchronism with its "own" wave, which is radiated
mostly in the forward direction. The wave frequency
received by a motionless observer is doppler-shifted
to a very high value, such as infrared. Ferromagnetic
bars 18 join the magnets of each row 12, 14 to provide
low reluctance flux return paths. It is seen that as
the magnet period is reduced, the shunt leakage flux
20 between axially adjacent magnets becomes large
compared to the useful transverse flux 22, limiting
the practical lower value of the magnet period, and
hence, the frequency generated. The high leakage
flux requires a large mass of magnetic material. Of
current interest are lasers for spacecraft where size
and weight must be kept very small.
FIG. 2 is another old scheme analogous to that
used in travelimg-wave tubes in which the magnets 16'
are magnetized in the beam direction and are separated
by ferromagnetic polepieces 24. Leakage flux 20' may
be reduced somewhat, but the magnetomotive force
available decreases with the period. No ferromagnetic
flux return is used because the fields fall off
rapidly away from the magnetic stack.
FIG. 3 is an axial section through a magnet
assembly embodying the invention. It does not use
short permanent magnets which would otherwise impose
a limit on magnetomotive force. A first row of
ferromagnetic polepieces 26 are extended perpendicular
to the paper as bars, forming a linear array
periodically spaced in the direction of beam lO".
A second row 28 forms an opposed similar array.
Polepieces 28 are displaced from polepieces 26 by a
half-period in the beam direction. A unidirectional
magnetomotive force is applied in the beam direction,
lZ918~7
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as by a solenoid electromagnet coil 30. It is
surrounded by a ferromagnetic sheath 32 forming a
flux return pa'ch to reduce leakage field in the
environment and provide a uniform field. In the
5 interaction space the axial field component 34 serves
to keep the beam focused but does not affect the
electrons' wiggler periodicity produced by the
transverse field components 36 alternating between
poles of opposed arrays 26, 28. The useful field
10 strength is limited only by saturation of the
ferromagnetic polepieces 26, 28, not by any permanent
magnet material. The net result is a structure of
small size, light weight and easy manufacture
which provides short periodicity unmatched by the
15 prior art.
FIG. 4 is a sketch of the magnetic components
of a beam wiggler embodying the invention.
For ease of manufacture and perfection of
alignment and spacing, the ferromagnetic polepieces
20 26, 28 are supported and spaced by interleaving
pieces of non-magnetic material. FIG. 4 il~ustrates
the magnetic part of a practical structure. Pole-
pieces 26', 28' are inserted in grooves in parallel
comb-shaped, non-magnetic support bars 38, 40 as of
25 copper, which preferably form part of the vacuum
envelope of the tube. Slots 42 can be made by
mechanical or electric-discharge machining, thus
providing accurate alignment and the uniform periodic
spacing needed for a synchronous structure, as
30 well as mechanical support and thermal cooling.
An alternative construction is a stack of
separate ferromagnetic polepices, as of iron, and
interleaved separate nonmagnetic spacers, as of
copper, the stacked parts being brazed together.
129~ 7
FIG. S is a partial-section isometric sketch
of a free-electron laser structure with optical
focusing mirrors 42 to make it part of a confocal
resonator. Mirrors 42 have central apertures 44 for
5 passage of the electron beam. Alternatively, the
undulator structure may be closed at its sides to
form a waveguide 46 carrying a transverse electric
field wave 48 polarized perpendicular to the
partial-section plane of the paper. The mirrors
10 42 partially reflect this wave 48, providing
electromagnetic feedback which makes the electron
motions, and the radiation, coherent. Alternatively,
an amplifier configuration is possible, dispensing
witn the on-line mirrorsr but providing feedback via
15 an external path, such as a waveguide or series of
external reflectors.
FIG. 6 illustrates an alternative laser
construction having a coaxial geometry. All elements
shown in cross-section are figures of revolution
20 about an axis 50. The cathode emissive surface 52
is a zone of a toroid. The electron beam 54
converges from cathode 52 to a hollow, cylindrical,
linear beam 56 which flows between periodic stacks
of ring-shaped ferromagnetic polepieces 26", 28".
25 Beam 56 is kept focused by the axial d.c. magnetic
field 58 from a solenoid magneti 30' (not shown).
The interaction is exactly the same as in the
rectangular array of FIG. 5 except that the generated
electromagnetic wave has a circular-electric-mode
30 symmetry. After passing between the magnet stacks
26", 28" the wave is radiated axially out through a
dielectric vacuum window 58. The magnetic field 58
is reduced sharply past the polepiece stacks
26", 28" so that electron beam 50 expands and is
35 collected on the enlarged surface 60 of a portion
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of the vacuum envelope where the power density is
reduced. The output section geometry i5 thus
somewhat similar to the familiar circular-electric-
field gyrotron. The electromagnetic interaction is
of course much different in that the periodic
electron motion is produced by the spatially periodic
magnetic field whereas in the gyrotron it is a
result of the cyclotron rotation in a uniform
magnetic field. The frequency limit of the gyrotron
is limited by the available magnetic field strength.
In the present laser this limitation is not present,
so much higher frequencies may be generated.
The frequency of the radiation is tunable by
varying the energy (velocity) of the electron beam
10. In a typical installation, the beam would be
energized by a linear electron accelerator (not shown)
for which means to vary the energy are well known in
the art. Like other lasers, the resonator is many
wavelengths long, so the emitted frequency will be in
one or more very closely spaced lines.
The above-described embodiment is exemplary and
not limiting. The scope of the invention is to be
limited only by the following claims and their legal
equivalents.
