Note: Descriptions are shown in the official language in which they were submitted.
RAC/RCC/gm ~ 4
Target Illumination With Multiple
Pass Free_Electron_Laser
The present invention pertains to target
illumination, and more particularly to techniques for
focusing high-ener~y pulses of electromagnetic radiation
onto a target focal region.
Hig~-ener~y pulsed laser systems have been
developed for irradiating minute targets. Such systems
generally include a laser oscillator for generating a
train of optical pulses and a series of laser amplifiers.
A single pulse from the oscillator output train is
sèlected using electro-optical switches, and the
selected pulse is then passed to the laser amplifiers.
- One or more converging high intensity pulses of electro-
magnetic radiation may be produced, and a series of
lenses and/or mirrors may be employed to focus the
pulse or pulses onto a target at the target region.
~ditional elements such as polarizers, Faraday rotators
and Pockels cells are often provided for controlled
routing of the illumination beams.
One problem presented in the various prior
target illumination systems which include refractive
transparent elements, such as lenses and laser slabs
or rods, lies in the fact that the indexes of refraction
of such elements are non-linear functions o beam
intensity. These non-linearities cause sel~-focusing
of the high-power optical beams, and can lead to damage
of all optical components and loss of focusable beam
energy. Thermal effects and distortions in transparent
optical components may also limit the repetition rate
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~fil57~
of prior ar~ laser systems. Another problem is that
the index of refraction and focal properties of the
transparent elements vary with frequency, which means
that substantial redesign and rework is required if
the illumination frequency is to be modified.
Objects of the present invention are to
provide systems and methods ~or generating one or more
beams oF electromagnetic energy and for ocusing such
energy onto a target region, which systems and methods
possess enhanced efficiency, in which the frequency
of the illumination beam or beams may be readily
adjusted without substantial alteration of system
components, which may operate entirely in a vacuum to
reduce distortions, and/or which eliminate the need
- 15 for transparent optical elements and thereby reduce
the problem of beam self-focusing and consequent
potential damage to all system elements.
A further and more specific object of the
invention is to provide a system and method for operating
a free electron laser in a pulsed mode, and for thereby
generating high powered optical pulses and focusing such
pulses onto a target region. ~~
These and other objects are accomplished in
accordance with the invention by providing an optical
resonator or cavity comprising an amplifier disposed in
a bi-directional beam path defined by a plurality of
highly reflective mirrors having a focal point for
target illumination spaced from the amplifier. In a
preferred embodiment of the invention, the amplifier
comprises a free electron laser amplifier disposed to
receive stimulating pulsed electron energy and for
emitting corresponding pulses of optical energy. The
length of the cavity beam path and the period of the
pulsed electron stimulation are coordinated such that
the previously emitted optical pulses and the electron
pulses pass through the-amplifier simulatneously.
Thus, successively generated optical pulses are cumulatively
additive in the laser amplifier. A target is injected
into the target focal region when the cumulative optical
beam has reached a desired power level.
The invention so described may be disposed
entirely within a vacuum. No optically transparent
or refract~ve elements are required. Moreover, free-
electron lasers are readily frequency-tunable in the
visible, X-ray, infrared and ultraviolet regions,
so that this invention could be used over a wide
range of frequencies.
The invention, together with additional
objects, features and advantages thereof, is more
fully set forth in the following description, the
appended claims and the accompanying drawings in which:
FIG. 1 is a schematic illustration of a
prior art free electron laser system;
FIG. 2 is a schematic illustration of a
basic embodiment of the target illumination system
provided by the present invention,
FIG. 3 is a graph useful for illustrating
operation of the embodiment of FIG. 2; and
FIGS. 4-8 are schematic illustrations of
respective alternative embodiments of the invention.
Before proceeding with a detailed description of
several embodimen-ts of the invention, it will be appreciated
that the various elements - e.g. mirrors, magnets, electron
accelerators, etc. - are illustrated schematically in the
drawing figures. Suitable structure for each of such elements,
where not described in detail herein, will be self-evident
to the skilled artisan. Similarly, suitable means for
mounting such structural elements relative to each other as
described herein will be manifest. The preferred embodiments
of the invention comprise a free electron laser, one prior
art type of which will be described in connection with FIG. l.
As noted above, the free electron laser is frequency tunable
in the infr~red, visible, ultraviolet and X-ray frequency
ranges. Thus the terms "optic" and "laser" as used herein
are not limited to light in the visible region, but encompass
at least the other electromagnetic frequency regions noted
above.
FIG. 1 illustrates a free electron laser system
similar to that disclosed in U. S. Patent No. 3,822,410 issued
July 2, 1974 to John M. Madey which produces a continuous
series or train of low power laser pulses. A plurality of
periodic magnets 20, which constitute the laser amplifier, are
disposed in a linear array along the optical axis 22. A
continuous electron stream is routed by opposed bending
magnets 24,26 in a closed loop 28 which includes opposed
substantially linear loop portions 30,32. Loop portion
30 is coaxial with that portion of beam axis 22 which
intersects magnets 20. An electron accelerator
cavity 34 is disposed in linear path portion 32 and
is coupled to an RF generator 36 for maintaining
~3
7 ~
circulating electron energy at a desired level. Passage
of electrons through magnets 20 stimulates emission
of photon energy in a colLimated beam at a freque~cy
which is a function of elec-tron energy, spacing of
magnets 20 and magnet fiela strength. Partially
transparent mirrors 38 permit a small fraction of
the incident laser light to pass and thereby produce
a train or series of optical pulses as previously
described.
FIG. 2 illustrates a basic embodiment of the
present invention as comprising a concentric optical
resonator or cavity defined by a pair of opposed con-
cave mirrors 15,16 having a common focal point 17 on
the bi-directional beam path 19 therebetween. Con-
cerning resonant cavities generally, reference may be
had to Yariv, Quantum Electronics, Wiley ~ Sons, 2nd
Ed., Ch. 7 (1975). ~ free electron laser amplifier
14 is disposed on beam path 19 spaced to one side of
focus 17. An electron accelerator 10 produces a
periodic series of electron pulses, as opposed to a
continuous electron stream, one such electron pulse
being indicated at 12. Pulses 12 are guided along a~
trajectory 13 by suitable bending magnets (not shown)
and are thereby fed through amplifier 14 coaxially
with beam path 19. Periodic passage of electron pulses
12 through amplifier 14 stimulates emission of pulsed
photon energy in a collimated beam, which pulsed energy
then resonates between mirrors 15,16 and is focused
on each passage at 17. The length of beam path 19 is
coordinated with the repetition period of electron
pulses 12 such that successive bursts of photon radiation
5 ~ ~
in amplifier 14 are cumulatively additive to previous
pulsed emissions, the pulsed laser beam 11 traveling
on path 19 being amplified generally stepwise on
successive passes through amplifier 14 as indicated
schematically at lla-llc.
The electron pulse emission frequency (f)
of accelerator 10 in FIG. 2 is givèn by the equation
c
f = 2d tl)
where e is the speed of light in the vacuum in which
the cavity is disposed, and d is the cavity length.
For d equal to 300 meters, f would be equal -to 0.5
MHz. A linear induction accelerator triggered using
vacuum tube switches could be made to operate at this
repetition rate.
In general, the amount of amplification in
amplifier 14 on successive passes of the optical beam
will vary as a function of beam intensity and/or
amplifier magnet design. Beam collimation improves
after a number of passes through the amplifier and,
for passes on the order of a few tens or more, optical
losses (primarily at mirrors 15,16) become substantially
constant, If it is assumed that the fraction of optical
energy lost on each pass through amplifier 14 is con-
stant, which is approximately true after a number of
3 ~
passes as previously noted, then beam power P in
the optical cavity after n passes of the laser beam
through amplifier 14 will be given by the equation
P ~ Q Pi (l-~)n-i (2)
w~ere a Pi is the incremental power increase in amplifier
14 on the ith pass and ~is the fractional energy loss
pe~ pass. If it is further assumed that the increase
in optical power per pass a P is constant for all
passes, equation 1 simplifies to
p = ~ )n). (3)
6a.
11~6~7~
Equation 3 is illustrated graphically at 21
in FIG. 3. Cuxve 21 demonstrates that output laser
power P approaches an asympotitic limit 23 e~ual to
~ P/~ after a number of passes n at which the optical
gain per pass equals the losses per pass. Graph 25
illustrates the total energy generated in lase~
whereas curve 21 illustrates the actual energy available
in the optical cavity, i.e. total energy minus losses.
At n = l/S , available energy is equal to substantially
63% of total potential energy. Using high quality
dielectric mirrors with coefficients of reflectivity
on the order of 0.9995, round trip optical losses
of no more than a few tenths of a percent per pass
are incurred.
When the number of optical passes n is chosen
equal to 1/~ , power P in the optical cavity of FIG. 2
is related to electron current I by the expression
P - 0.63 I~EN (4)
where~ is the average fractional energy in electron
beam 12 converted to optical energy per pass, E is
the electron energy and N is the number of amplifier
passes per target shot. In the single-amplifier embodiment
of FIG. 2 with a single optical pulse 11 in the optical
resonator, N = n. By way of example, for beam power
P of one gigawatt at focus 17 utilizing one hundred
passes N, electron energy E of 500 MeV and a laser
efficiency ~ of one percent, an electron current
I of 3.17 amps ~ould be required, Similarly, for
a beam power of one terawatt utilizing three
hundred passes N, for the same electron energy
and an amplifier having a two percent extraction efficiency
3 3 ~5'~
7~, a peak electron current of 529 amps would be required.
This peak electron current could be reduced by reducing
the optical losses and thereby permitting a larger
number of optical passes per target shot, by utilizing
more than one laser amplifier (FIGS. 7 and 8 to be
described), by providing two optical pulses in the
cavity per target shot (FIG. 5),by utilizing more
than one optical cavity (FIGS. 6 and 7) or by
increasing the fractional energy extraction per pass
~ , In this connection, it should be noted at this
time that each of the foregoing equations (1)-(4)
will be eq~ally applicable (with an appropriate
multiplication factor) to the embodiments yet to be
described.
~ 15 After a number of passes calculated in the
manner previously described to yield an illumination
pulse of desired intensity, a target is propelled by
an accelerator 18 toward focal point 17 to arrive at
the focal point coincidentally with the focused beam~
The target may comprise, for example, a glass sphere
or shell on the order of 1 mm in diame-ter filled or
impregnated with deuterium-tritium fuel in a preferred
application of the present invention to inertial con-
finement fusion research. The target must be fired
at a velocity which is sufficiently high that the
target will intercept the illumination beam on the
intended pass, but does not intercept the beam on
the preceeding pass. For a round trip optical path 19
of a few hun~red meters in length, for example, a target
velocity on the order of 105cm/sec. is contemplated.
Accelerator 18 may comprise a magnetic accelerator, and
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the -target may be coated with or carried by a material
for conducting surface currents induced by the magnetic
acceleration field. Suitable -timing means (not shown)
couple electron accelerator 10 to target accelerator 18
for temporal coordination of beam generation and
target`injection.
In the embodimen-t of FIG. 2, the pulsed electron
energy from accelerator 10, ater passage through
amplifier 14, is preferably fed to suitable means
(not shown) for converting a portion of the total
electron energy not extracted by the amplifier into an
otherwise useful energy form. FIG. 4 illustrates an
alternative em~odiment of the invention in which the
pulsed electron stream is recirculated in a closed
lS path 28 which includes laser amplifier 14 and an
electron beam storage ring generally indicated at 41.
A magnetic switch 42 permits electron pulses to be
fed into storage ring 41 from the source accelerator
lOo A series of suitably timed low current pulses
from accelerator 10 can be used to build up a single
high current electron pulse in storage ring 41. A
pair of magnetic switches 46,48 are disposed diametrically
in ring 41 se~ectively to route the pulsed electron
stream in the ring onto loop 28, which extends through
amplifier 14 and a booster accelerator 34.
Amplifier 14 is disposed in an op-tical resonant
cavity 50 bounded by end mirrors 52,56. The curvature
of mirrors 52,56, and that of an intermediate mirror
54, are such that a target focal spot 58 is produced
in cavity 50 between mirrors 54,56. Amplifier 14 is
disposed between mirrors 52,54. When the round trip
1 J ~ ~ ~7 ~
transit time of an electron pulse in ring 41 and loop
28 is equal to the round trip time of the laser pulse
in cavity 50, a laser pulse and an electron pulse
will pass through the amplifier simultaneously as
pre~iously described, with the electron stream trans~
ferring energy cumulati~ely to the laser beam on
each pass. When laser beam power has reached a
desired level, as previously discussed in connection
with equations (2)-(4), a target is injected to inter-
cept the laser beam at focal point 58. Suitabletiming means lnot shown) coordinate switches 46,48
with target injector 18.
Closed loop electron paths of the type
illustrated in FIG. 4 are preferred in accordance with
~ 15 the invention over open-type paths of the type illus-
trated in FIG. 2. Ho~ever, the extraction of energy
from the electron pulses on each pass through the laser
amplifier tends to cause a spread in the energies
of the individual electrons on each pass, which in
turn will effect amplifier gain on succeeding passes.
In the open loop system of FIG. 2, exciting electrons
are only passed to amplifier 14 one time, and hence
electron spread on succeeding passes is not a problem.
Amplifier 14 in FIG. 2 may comprise a simple linear
periodic magnet array of the typ shown in the above-
referenced U. S. Patent 3,822,410, or may comprise a
simple periodic magnet helical array of the type described
in PhYsical Review Letters, Vol. 36, page 717 (1976).
In the closed loop system of FIG. 4, however, account
should be taken of electron spread for peak efficiency.
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One technique is to design amplifier 14 to have reduced
sensitivity to electron spread as is discussed in Smith
et al, Stanford University Report No. HEPL 830, August
1978. Another technique is to provide a modifiea
5 amplifier such as a multiple stage ampli-Eier to extract
energy from the recirculating pulsed electron stream
while minimizing electron energy spread. Either or
both of such techniques may be utilized withou-t departing
from the scope of the invention.
FIG. 5 illustrates ~7et another modified
embodiment of the invention wherein two optical pulses
are produced in a cavity 79 using a single recirculating
electron pulse. The resonant cavity 79 is defined by
a pair of opposed coa~{ial concentric mirrors 80,82
15 having a common focal point 84 at the target region.
In the modification of FIG. 5, the round trip transit
time of electrons in loop 28 is equal to one-half of
the round trip transit time of an optical pulse between
mirrors 80,82. Thus, two high energy optical pulses
20 are produced in cavity 79 which intersect at target
~ocus 84 on every pass. The target injected at 84
thus will be illuminated by e~ual intensity beams from
opposite directions.
FIG. 6 illustrates a modi~ication to the
25 err~bodiment of FIG. 5 wherein a second concentric resonant
cavity 79a is defined by the opposed concave mirrors
80a,82a and has a target focus at 84 coincident with
the :Eocu5 of mirrors 80,82~ A second free electron
laser amplifier 14a is disposed in cavity 7ga. The
30 closed loop electron path 28a is e~panded to excite
both amplifiers 14 and 14a to illuminate a target at 84
7 ~
uniformly in four quadrants. It will be understood
that an electron storage ring and switches (40,~8
in FIG. 5) will be provided but are not shown in FIG.
6. AS a modification, amplifiers 14,14a may be energized
by separate coordinated electron loops coupled to
common or separate electron storage rings.
FIG. 7 illustrates a further modification
wherein free electron amplifiers 14a-l~b are disposed
in respective legs of the beam paths defined by
mirrors 80,82,80a and 82a, one between each of such
mirrors and the common focus 84. Each of the amplifiers
is fed by an associated secondarv electron loop 28a-28d
which are respectively connected to a primary closed
loop 86 by the bi-directional magnetic switches 46a-46d.
~ 15 Each secondary electron loop includes an associated
accelerator 34a-34d~ Additional accelerators 88 are
provided in primary loop 86. FIG. ~ illustrates a
further modification of the invention wherein a folded
resonant cavity is defined at either end by the con-
cave reflectors 90,9~. A pair of intermediate concave
re~lectors 94,g6 are disposed coaxially with respective
end reflectors 90,92 and are directed toward each
other to form a common focal spot 98. Additionally,
mirror pairs 90,94 and 92,96 cooperate to form regions
therebetween of reduced beam diameter and in which a
pair of free electron laser amplifiers 14a,14b are
disposed.
Although the present invention has been
: described in detail in connection with several alternative
embodiments thereof, it will be appreciated that the
invention is susceptible to any number of additional
modifications and variations. For example, it is not
essential for the present invention for the free
electron laser ampli~iers illustrated in the various
drawing FIGS. to comprise linear or helical arrays
of magnets. As a modification, such magnets may be
replaced by means for generating electromagnetic
pump fields of suitable geometry. Similarly, in
accordance with the present invention in its broadest
application, the lasing means may comprise other than
a free electron laser. For example, dual gas lasers
have been constructed comprising a first gas for ab-
sorbing outside energy and assuming a metastable
state having a relatively long lifetime and a second
gas for absorbing a fraction of the energy stored in
-- 15 the first gas and actually performing the lasing ~unction.
The various multiple pass optical resonant cavity con-
figurations illustrated in FIGS. 2 and 4-8 may be used
with such dual gas lasing means for allowing enhanced
energy extraction from the energy storing first gas.
Thus, in accordance with the present invention in
its broa~est aspects, an optical resonant cavity is
provided with at least one sharp optical focus at a
target region. Lasing means are disposed in the resonant
cavity for cumulatively pumping optical energy into a
pulse resonating in the cavity. M~ans are provided
for injecting a target into the target region to be
illuminated by the cumulative optical pulse after
such pulse has attained a desired energy level.
T~e invention claimed is:
13~
