Short pulse free electron laser amplifier

Method and apparatus for amplification of a laser pulse in a free electron laser amplifier where the laser pulse duration may be a small fraction of the electron beam pulse duration used for amplification. An electron beam pulse is passed through a first wiggler magnet and a short laser pulse to be amplified is passed through the same wiggler so that only the energy of the last fraction, f, (f<1) of the electron beam pulse is consumed in amplifying the laser pulse. After suitable delay of the electron beam, the process is repeated in a second wiggler magnet, a third, . . . , where substantially the same fraction f of the remainder of the electron beam pulse is consumed in amplification of the given short laser pulse in each wiggler magnet region until the useful electron beam energy is substantially completely consumed by amplification of the laser pulse.

FIELD OF THE INVENTION 
The invention relates to amplification of short pulse electromagnetic 
radiation using free electron laser techniques. 
BACKGROUND OF THE INVENTION 
The possibility of amplifying coherent electro-magnetic radiation, by 
collinear passage of the radiation and of a relativistic electron beam 
through a sequence of electric or magnetic fields of alternating polarity, 
has been recognized since the first publication by H. Motz, Journal of 
Applied Physics 22 527 (1950) on the subject. Motz considered a sequence 
of alternating direction magnetic fields, regularly spaced and 
transversely oriented relative to the common direction of travel of a 
light beam and an electron beam. Let L.sub.o be the fundamental period of 
variation of direction of the sequence of transverse magnetic fields and 
let the beam electrons move with velocity v.perspectiveto.c. The light 
beam photons will be absorbed and re-emitted by the electrons, and the 
frequency .nu. of emitted radiation will depend upon angle of observation 
.theta. relative to the common beam direction according to .nu.=v/L.sub.o 
(1-cos .theta.). For a highly relativistic electron beam and modest 
transverse magnetic field strengths, most of the radiation appears in the 
forward direction, in a narrow cone of half angle of the the order of 
.DELTA..theta.=m.sub.e c.sup.2 /F.sub.b where 
##EQU1## 
is the electron total energy. 
Motz, Thon and Whitehurst, in Jour. of Appl. Phys. 24 826 (1953), further 
considered the co-propagating light beam and electron beam in a waveguide, 
obtained some interesting general classical relativistic relationships for 
electron orbits in a spatially varying B-field, and reported the 
experimental observation of visible and millimeter wavelength radiation 
for field strengths B.perspectiveto.3,900 and 5,600 Gauss. 
In Proceedings of the Symposium on Millimeter Waves (Polytechnic Press, 
Brooklyn, 1960) p. 155, Motz and Nakamura analyzed the amplification of a 
millimeter wavelength electromagnetic wave interacting with a relativistic 
electron beam in the presence of a rectangular waveguide and a spatially 
oscillatory electric field, using a model of J. R. Pierce. The analysis 
was purely classical, and the gain was rather modest. 
Pantell, Soncini and Puthoff discuss some initial considerations on 
stimulated photon-electron scattering in I.E.E.E. Journal of Quantum 
Electronics QE-4 905 (1968). Collinear scattering, with the incident 
photon energy h.nu. being&lt;&lt;incident electron energy E.sub.e1 and periodic 
deflection of the electron beam by a microwave radiation field, is 
analyzed briefly; and a Compton scattering laser is proposed, using the 
input/output wavelength relation 
##EQU2## 
Useful gain from the device appears to be limited to the middle-high 
infrared range .lambda..gtoreq.20 .mu.m. 
Mourier, in U.S. Pat. No. 3,879,679, discloses a Compton effect laser that 
proceeds from the same principles as Pantell et al, supra. This invention, 
like that of Pantell et al, appears to require provision of an electron 
storage ring or the like for rapidly moving electrons and an optical 
cavity that is a part of the ring, for causing electron-photon scattering. 
R. M. Phillips, in I.R.E. Transactions on Electron Devices, 231 (October 
1960), used a periodic magnetic field, whose period may vary, to focus and 
axially bunch an electron beam traveling in an unloaded waveguide, 
together with a monochromatic light beam, to increase electron beam 
kinetic energy at the expense of light beam energy. The electron beam 
velocity was adjusted so that a beam electron travels one period L along 
its trajectory in the time required for the light beam (of wavelength 
.lambda.) to travel a distance L+.lambda.. The electron then senses only 
the retarding portion or only the accelerating portion of the 
electromagnetic wave. This approach converts transverse momentum, arising 
from the presence of the electromagnetic wave, into changes in axial 
momentum of the electron beam so that beam bunching occurs. Peak 
efficiency was about 10 percent for the experiments reported. 
J. M. J. Madey, in Journal of Applied Physics 42 1906 (1971), discusses 
stimulated emission of bremsstrahlung by a relativistic electron into a 
single electromagnetic mode of a parallel light beam, where both electron 
and light beam move through a periodic, transverse d.c. magnetic field. 
Quantum mechanical and semi-classical calculations of transition rates and 
gain indicate that finite, practical gain is available in the infrared and 
visible portions of the optical spectrum. These considerations are 
incorporated in U.S. Pat. No. 3,822,410, issued to Madey for tunable 
apparatus for generation/amplification of coherent radiation in a single 
or a few closely spaced electromagnetic modes. 
Hirschfield, in U.S. Pat. No. 3,398,376 for a relativistic electron 
cyclotron maser, discloses and claims use of an axial, monoenergetic 
relativistic electron beam (E.sub.kinetic .perspectiveto.5 keV) a 
spatially-varying longitudinal magnetic field coaxial with the beam, a 
weaker, transverse periodic electric or magnetic with a resulting helical 
pitch matching that of the electron motion at the predetermined beam 
velocity and a cavity resonator with a mode frequency matching that of the 
cyclotron frequency of the resulting spiraling electrons. The apparatus 
relies upon electron cyclotron radiation and ignores any synchronization 
of electron beam and the electromagnetic beam to be amplified. 
A combination free electron laser/gas with high pulse repetition rates is 
taught by U.S. Pat. No. 4,187,686, issued to Brau, Rockwood and Stern. In 
the embodiment disclosed, the free electron laser operates at infrared 
wavelengths and the gas laser operates at ultraviolet wavelengths. The 
monoenergetic electron beam is initially bunched and accelerated to 
.perspectiveto.10 MeV kinetic energy and directed into and out of a 
multiplicity of serially arranged free electron lasers by turning magnets 
positioned at the ends of these lasers; finally, the electron beam is 
directed axially through a gas laser to utilize and convert additional 
electron beam energy to electromagnetic energy. The free electron laser 
appears to be of conventional form, utilizing fixed period magnetic fields 
to produce electron bremsstrahlung radiation and an optical resonator for 
light beam amplification. 
SUMMARY OF THE INVENTION 
An object of the invention is to provide method and apparatus for 
amplification of short laser pulses by a free electron laser amplifier, 
using no complex optics or beam switching. 
Additional objects, novel features and advantages thereof are set forth in 
the detailed description, with reference to the accompanying drawings, and 
may be realized by means of the instrumentalities and combinations pointed 
out in the appended claims. 
To achieve the foregoing objects in accordance with the subject invention, 
as broadly described herein, the method may comprise the steps of: 
providing a short laser pulse of time duration .tau..sub.L and electron 
beam pulse of longer time duration .tau..sub.B =N.tau..sub.L, where N is a 
positive integer greater than one; providing N wiggler magnets in a linear 
array; passing the electron beam pulse and the laser pulse through the 
first wiggler magnet so that the laser pulse and the last segment 
.DELTA.t=.tau..sub.L ((N-1).tau..sub.L .ltoreq.t.ltoreq.N.tau..sub.L) of 
the electron beam pulse pass through the first wiggler magnet at 
substantially the same time; delaying the electron beam pulse by a time 
interval .tau..sub.L relative to the laser pulse; passing the electron 
beam and the laser pulse through the second wiggler magnet so that the 
laser pulse and the last remaining portion .DELTA.t=.tau..sub.L 
((N-2).tau..sub.L .ltoreq.t.ltoreq.(N-1).tau..sub.L) pass through the 
second wiggler magnet at substantially the same time; and repeating the 
combination of electron beam pulse delay and simultaneous passage of laser 
pulse and last remaining portion of electron beam through a wiggler magnet 
N-2 additional times so that the laser pulse serially extracts energy from 
consecutive portions of the electron pulse and is thereby amplified 
without complex optics or beam switching as required in the prior art. 
The apparatus may comprise: a source to produce an electron beam of 
predetermined temporal duration .tau..sub.B =N.tau..sub.L where 
.tau..sub.L is the temporal duration of the laser pulse to be amplified 
and N is a positive integer (N&gt;1); N wiggler magnets, arranged in a linear 
array with adjacent magnets being spaced apart by a substantially constant 
distance, with the linear array of magnets being positioned to receive an 
electron beam produced by the source and to receive the laser pulse; and 
N-1 electron beam delay lines, one being positioned between each adjacent 
pair of wiggler magnets to delay an electron beam by a predetermined time 
interval .tau..sub.L relative to a co-propagating light wave.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A free electron laser (FEL) directly converts part of the kinetic energy of 
a high quality, relativistic electron beam into coherent amplification 
energy of a co-moving laser beam of appropriate frequency. Referring to 
FIG. 1 of the drawings, a free electron laser amplifier comprises an 
electron beam, an input laser beam and a spatially varying periodic 
magnetic field through which both the electron beam and the laser beam 
pass. The transversely directed wiggler magnetic field B.sub.w imparts to 
the electrons a component of velocity v parallel to the laser electric 
field so that the laser field E.sub.L may absorb energy from the electron 
beam continually, thus providing a laser amplification device. This 
continuous energy transfer requires that the wiggler magnet be designed so 
that it operates as a stable phase decelerator through continuous 
modification of the magnetic field period and/or strength as the electron 
beam decays. 
As indicated schematically in FIG. 2, the subject invention allows one to 
efficiently amplify a short laser pulse in a FEL amplifier, even if the 
laser pulse time duration is only a small fraction of the time duration of 
the electron beam pulse, without resorting to high speed electron beam 
switches (which are wasteful of power) or complex laser pulse stacking 
optics. One begins with a laser pulse of time duration .tau..sub.L and a 
relativistic beam pulse of time duration .tau..sub.B =N.tau..sub.L, where 
N is a positive integer greater than one. One then arranges n wiggler 
magnets (11, 15, 19, . . . ) and N-1 electron beam delay lines (13, 17, . 
. . ) in an alternating pattern and a substantially linear array. 
FIG. 3 shows the temporal relationship of the electron beam energy content 
and the laser pulse intensity as electron beam and laser beam enter the 
first wiggler magnet. The first portion .DELTA.t.sub.1 =(N-1).tau..sub.L 
of the relativistic electron beam pulse is allowed to pass through the 
first wiggler magnet 11 with no laser pulse present. The last portion 
.DELTA.t.sub.2 =.tau..sub.L of the electron beam pulse passes through the 
wiggler magnet 11 in timed relationship with the passage of the laser 
pulse so that the laser pulse extracts energy (only) from this last 
portion of the electron beam according to FEL principles. 
The temporal relationship of the electron beam energy and the laser pulse 
intensity as these two pass through the second wiggler magnet 15 is shown 
in FIG. 4. The electron beam then passes through a first electron beam 
delay line 13 and is delayed by a time interval .tau..sub.L relative to 
the laser pulse (undelayed), and the electron beam and laser beam then 
pass into a second wiggler magnet (FIG. 2). As the electron beam has been 
delayed by a time interval .tau..sub.L relative to the laser beam, the 
first portion .DELTA.t=(N-2).tau..sub.L of the remaining electron beam 
pulse passes through the second wiggler magnet with no laser pulse 
present; and the last energetic portion .DELTA.t.sub.4 =.tau..sub.L of the 
electron beam pulse passes through the magnet 15 together with the laser 
pulse (also of time duration .tau..sub.L) so that once again the laser 
pulse extracts energy (only) from this last energetic portion of the 
electron beam. 
As indicated in FIG. 5, the process of electron beam delay-laser 
pulse/electron beam interaction within the wiggler magnet, is then 
repeated an additional N-2 times until each segment of length 
c.DELTA.t=c.tau..sub.L of the electron beam pulse has been substantially 
absorbed in amplification of the laser pulse, producing an electron beam 
energy pattern at the output of the N.sup.th wiggler magnet substantially 
as shown in FIG. 5. With this approach, one can reduce the temporal 
duration .tau..sub.L of the pulse to be amplified to 20 nanoseconds or 
less. 
The wiggler magnets may be of conventional design, with wiggler magnetic 
fields B.sub.w of 1-5 kiloGauss and associated wiggler wavelength of 
perhaps .lambda..sub.W .perspectiveto.1-100 cm; all N such magnets should 
be substantially identical. The length of each such magnet will be 
determined by other considerations such as desired conversion efficiency. 
The electron beam delay lines may each utilize a combination of magnetic 
fields (between consecutive wiggler magnets) to divert the electron beam, 
direct it through a circuitous path to introduce the appropriate time 
delay, and return the electron beam to the longitudinal axis of the next 
wiggler magnet for further interaction with another segment of the optical 
beam to be amplified. One possible problem here is that the "tired"0 
electrons (those that have previously interacted most strongly with the 
optical beam and hence have suffered the largest kinetic energy decrease) 
will move along an arc with a smaller instantaneous Larmor radius (for 
constant magnetic field) than will a beam electron with higher kinetic 
energy. However, maintenance of phase relationships among the diverted 
beam electrons is apparently not a problem. 
No complex optics such as pulse stacking are required with this approach, 
only passive electron beam optics are used for the wiggler magnets and 
electron beam delay lines, and no power-hungry beam switching is used. Two 
disadvantages of this system are that N wiggler magnets are required, 
which must be temporally synchronized, and that the laser radiation itself 
must propagate through the N wiggler magnet regions. 
Although the preferred embodiment of the invention has been shown and 
described herein, variation and modification may be made without departing 
from what is regarded as the scope of the invention.