Multiple excitation regenerative amplifier inertial confinement system

The invention relates to apparatus and methods for producing high intensity laser radiation generation which is achieved through an optical amplifier-storage ring design. One or two synchronized, counterpropagating laser pulses are injected into a regenerative amplifier cavity and amplified by gain media which are pumped repetitively by electrical or optical means. The gain media excitation pulses are tailored to efficiently amplify the laser pulses during each transit. After the laser pulses have been amplified to the desired intensity level, they are either switched out of the cavity by some switch means, as for example an electro-optical device, for any well known laser end uses, or a target means may be injected into the regenerative amplifier cavity in such a way as to intercept simultaneously the counterpropagating laser pulses. One such well known end uses to which this invention is intended is for production of high density and temperature plasmas suitable for generating neutrons, ions and x-rays and for studying matter heated by high intensity laser radiation.

In the field of laser technology there exists a need for high power laser 
systems. Such high power lasers have many well known areas of application 
if only the economic hurdles could be overcome. One such well known use 
will be elaborated on herein in an effort to more completely describe the 
invention, but this it not intended as the only use of the invention. 
In the field of magnetic and inertial confinement fusion, a need exists for 
high density and temperature plasmas. Such plasmas are useful in providing 
an environment for the controlled release of energy in thermonuclear 
reactions, as well as for research on the properties of plasmas and their 
interaction with electric and magnetic fields and high intensity laser 
radiation. 
In the prior art, plasmas have been generated by many different techniques, 
each possessing weaknesses which limit its applicability and/or the 
density, temperature and lifetime of the resulting plasma. For example, in 
the field of laser initiated inertial confinement fusion, experimental 
results and theoretical calculations currently indicate that a laser 
producing 200-300 TW and 200-300 KJ will be required to achieve high gain 
microexplosions and demonstrate the feasibility of net energy gain from a 
laser initiated fusion process. Present approaches to high power laser 
systems for neutron, x-ray and plasma production use master 
oscillator-driven long chains of laser amplifiers having many repeated 
optical elements to produce the desired pulses of high intensity laser 
radiation. The amplifier media employed may be coherently or incoherently 
photon pumped solid state laser media, such as Neodymium, Erbium, Thulium 
or other rare earth dopants in a glass or suitable solid matrix such as 
CaF.sub.2. The media may also be photolytic gases such as O.sub.2, 
CO.sub.2, N.sub.2 O, OCS, OCSe or Iodine. Alternatively, mixtures of gases 
such as (Ar or Ne)/Kr/F.sub.2, (Ar or Ne)/Xe/F.sub.2, or He/N.sub.2 
/CO.sub.2 maintained at a pressure on the order of one atmosphere, and 
excited by a high energy electron beam or an electric discharge may be 
employed. Irrespective of the amplification medium and excitation 
technique selected, the final amplification stage in the chain will 
account for perhaps 20-30% of the total cost of the system including the 
initial pulse generator or master oscillator. 
U.S. Pat. No. 3,723,246 to Lubin teaches and claims plasma production 
apparatus including a target production device and an electromagnetic 
devide for moving each target into the target chamber while controlling 
target position. A "tailored" laser prepulse of duration 2-3 nanoseconds 
and a "tailored" main pulse of duration 0.1 nanosecond are used to first 
vaporized the target to a fully ionized state and then to heat the 
resulting plasma. The Lubin invention passes the main pulse through each 
of several linear chains of amplifiers only once in reaching the target, 
with the prepulse being split off from the same path after passing through 
a fraction of said amplifiers. The use of regenerative configurations to 
simplify laser design is not considered. 
Regenerative amplifiers have been known since 1963 or before, but have 
never been applied in the present context. U.S. Pat. No. 3,243,724 to 
Vuylsteke teaches and claims a method for producing very short, high 
intensity laser pulses, using regenerative amplification techniques. A 
flashlamp produces population inversion in a cavity with end reflectivity 
turned off. After about 80% of the population is excited to a metastable 
state, end reflectivity is turned on and radiant energy within the cavity 
builds up by both spontaneous and stimulated emission, by extracting 
energy from the inverted excited state population. After the radiant 
energy density has increased to a peak, end reflectivity is again turned 
off and the radiation is quickly extracted from the cavity through a shunt 
path. Vuylsteke appears to contemplate only a moderately long lifetime 
upper state, such as present in a ruby rod, for example, and his 
excitation source is a flashlamp which pumps only once in a cycle. 
U.S. Pat. No. 3,597,695 to Swain et al., teaches and claims use of one or 
many passes of a laser pulse through a single amplifier placed inside a 
cavity for laser amplification. The (polarized) laser pulse is injected 
into the cavity, containing the amplifier and two mirrors spaced 
therefrom, allowed to pass one or more times through the amplifier, and 
switched out of the cavity by polarity control using a Pockels cell. The 
amplifier is excited only once. The energy output of the amplifier is 
limited by damage to the weakest optical component, the Pockels cell; and 
Swain et al., appears to contemplate only the use of relatively long 
lifetime gain media, such as Nd: glass, where multiple excitation of the 
gain medium is unnecessary. Multiple excitation of the gain medium and 
utilization of short lifetime species is not discussed. Switching 
techniques other than the Pockels cell are not described. 
U.S. Pat. No. 3,646,469 to Buczek et al., teaches and claims the use of a 
ring interferometer acting as a regenerative amplifier and being driven by 
a low power oscillator, with the amplifier gain just below the threshold 
for oscillation. The gain medium of this traveling wave regenerative 
amplifier is driven in only one direction around the ring and requires 
slaving the resonant frequency of the ring interferometer to the 
oscillator, which should have a spectrally pure output. 
U.S. Pat. No. 3,414,835 to Miller teaches and claims the use of a closed 
path optical system to cause an injected laser pulse to be multiply 
reflected from two or more surfaces and to be periodically focused and 
refocused so that the light beam passes through a transparent workpiece or 
sample many times. No provision is made for rapidly and repetitively 
amplifying the laser pulse each time it traverses the optical system or 
for injecting the target into the cavity. 
U.S. Pat. No. 3,668,536 to Michon teaches the use of a single amplifier 
with fully reflective means spaced from the ends of the amplifier, to 
reflect light back into the amplifier and cause said amplifier to respond 
as if it were several amplifier states in series. Pulse switchout means is 
also provided. Michon is forced to shorten high pulse initially, to avoid 
overlap between counterpropagating portions of the same pulse; and Michon 
does not indicate how his amplifier stages could be rapidly and 
repetitively pumped (evidently, at transit times 
.gtorsim.3.times.10.sup.-9 sec.). 
No where in the patents cited above is there suggested to repetitively 
excite a regenerative amplifier as laser pulses are injected thru it. 
Michon pumps a laser rod and then passes the laser beam through the rod 
and reflects the laser beam back through the rod without exciting the rod 
again. The use of a rod instead of discs indicates a lower energy system. 
Two separate counterpropagating pulses are not considered. The use of 
spatial filters to improve beam quality in a regenerative amplifier is not 
suggested. Prior regenerative amplifiers did not have a long enough path 
length and the required power source for repetitive pulsing of a 
regenerative amplifier. 
SUMMARY OF THE INVENTION 
The subject invention, hereinafter referred to as a Multiply Excited 
Regenerative Amplifier (MERA), uses multiple passes of one or more 
injected laser pulses through a regenerative amplifier, which may be 
repetitively excited; an optical cavity containing the amplifier; a laser 
to produce the initial pulse(s); pulse injection means to introduce the 
pulse into the optical cavity; pump means to repetitively and transitorily 
excite the amplifier; and switch means to (1) position the target, means 
in the beam path, for irradiation or to (2) remove the laser pulse(s) from 
the cavity and direct them to any one of many well known end uses of the 
laser pulse. Each pulse is caused to pass through the amplifier(s) many 
times by two or more reflective means, such as mirrors, which serve to 
return the pulse, via the same or a different path, through the 
amplifier(s). The "round trip" path length for each pulse may be quite 
long (.gtorsim.100 meters) in order to allow for repetitive pulsing of the 
amplifier medium, shaping of the laser pulse(s) or activation of the 
switch. After the pulse(s) has reached a predetermined level (usually 
energy saturation) in the amplifier it is ready to be used. One such use 
is that of a target means being irradiated to produce a high temperature 
plasma by either (1) quickly moving the target into position within the 
target chamber, located within the optical cavity or (2) optically 
switching the laser pulse out of the optical cavity so that the pulse is 
incident upon the target means in an externally located target chamber. In 
both cases for a target means and other end uses more than one 
regenerative amplifier cavity may be employed if irradiation from more 
than two directions is desired, or additional pulse pairs may be stacked 
in the regenerative amplifier. 
One object of this invention is to provide a high power laser using a 
multiply excited regenerative amplifier. 
Another object of this invention is to amplify one or more injected laser 
pulses by repetitively exciting the amplifiers in between each pass of a 
pulse thru an amplifier. 
Another object of the invention is to reduce the capital outlay of laser 
amplifier systems by using multiply excited regenerative amplifiers. 
Another object of this invention is to provide a new approach to the 
production of plasmas, neutrons and x-rays which unifies the class of 
laser amplifiers by allowing the use of gain media transitions with either 
long or short radiative lifetimes and wavelengths. Such a laser system 
could irradiate targets at 1 to 100 Hz repetition rates, and would thus be 
a suitable system for a reactor plant. 
Other objects, and advantages of this invention, will become apparent from 
the following detailed description of preferred embodiments, as 
illustrated in the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
In the laser technology there has always been the goal of obtaining high 
power laser systems for which the economic considerations are not so 
burdensome. The uses to which high power laser systems can be put are well 
understood in the field, see Lubin U.S. Pat. No. 3,723,246 which is 
incorporated herein by reference. Hereinbelow one of those well known 
uses, in an inertial confinement system as a preferred embodiment of the 
invention, will be used to more fully explain the invention. 
Considerations of laser-initiated production of high temperature and 
density plasmas from a target often begin by positing the following laser 
requirements: (1) wavelength .lambda..about.300-2000 nanometers; (2) pulse 
duration .DELTA.t.ltorsim.100 nanoseconds; (3) pulse energy (after 
amplification) .about.10.sup.5 -10.sup.6 joules; and (4) laser efficiency 
.gtorsim.1%. In designing a system to achieve these conditions, one may 
begin by assuming appropriate energy storage for single pulse extraction 
from the amplifiers (e.g., 10-100 joules/liter) and derive requirements on 
inversion density (e.g., 10.sup.16 -10.sup.17 cm.sup.-3) and gain 
cross-section (e.g., 10.sup.-19 -10.sup.-20 cm.sup.2). The radiative 
lifetime of the laser transition in this case is typically greater than 
several hundred nanoseconds, allowing the achievement of large stored 
energy densities with reasonable pumping rates. This approach places 
severe constraints on the laser medium. The excited medium must be 
particularly stable with respect to collisional and collective relaxation 
processes. Alternatively, one may begin with a prescription of gain 
crosssection (e.g., 10.sup.-16 -10.sup.-17 cm.sup.2) and derive 
requirements for energy storage (e.g., 0.1-1.0 joules/liter) and inversion 
density (e.g., 10.sup.14 -10.sup.15 cm.sup.-3). As a consequence of the 
large gain cross-section in this case, the radiative lifetime of the laser 
transition is generally less than several tens of nanoseconds. Efficient 
laser operation in this latter approach requires multiple short pulse 
excitation and pulse energy extraction on the timescale of the radiative 
lifetime. This method puts the burden on the technology, not on the laser 
medium. The subject invention unifies these two techniques by providing 
one basic approach to the design of laser produced plasma systems which 
may use short lifetime or long transitions. Where certain rare earth-doped 
glasses and other media with transitions having lifetimes of several 
microseconds or more are used, it may be necessary only to pump the 
amplifier once or a few times. However, for some applications 
time-tailored pumping may be advantageous. These rare gas halides and 
other gases with transitions having lifetimes .tau..sub.r .about.0.1-1.0 
microseconds or less are used, repetitive pumping of the amplification 
medium in a burst mode at, say, 10.sup.4 Hz or higher is employed. In this 
approach one's choice of amplification media, from among those which 
manifest laser action, is limited primarily by the achievable rates for 
repetitive pump sources. If one could devise a pump with a repetition rate 
.gtorsim.10.sup.9 Hz, possibly any laser medium could be used for the 
amplifier herein, provided a suitable cavity could be fabricated. One such 
rare earth-doped glass amplifier system is described in the patent 
application Ser. No. 868,633, filed by Emmett et al, filed on the same day 
herewith, assigned to the assignee of this application, which is 
incorporated herein by reference. This invention uses the 4f-5d transition 
of suitable rare earth dopants to produce bands of tunable radiation in or 
near the visible spectrum. 
Generically, the subject invention is directed to a laser system, per se 
with sufficient path length, which uses: (1) multiple passes of one or 
more synchronously propagating laser pulses through one or more singly or 
repetitively excited regenerative amplifiers to saturate the energy 
contained in each pulse; and (2) power conditioning, matched to the 
particular amplifier medium, either to repetitively excite the amplifier 
at rates .gtorsim.10.sup.4 Hz in a burst mode or to singly excite a long 
lifetime amplifier medium. The path length is chosen to be sufficiently 
long (possibly a kilometer or more) to allow time for the repetitive 
pulsing and laser beam modification and switching. Relaxation of the 
onerous laser medium criteria is achieved through use of an amplifier 
storage ring design, which puts the burden on the remainder of the 
technology. 
When two counterpropagating laser pulses are used for irradiating a target 
means, the target chamber containing the target may be positioned at or 
near the center of the amplifier optical cavity equidistant from the 
injection point of the two pulses. Target irradiation is then accomplished 
by either (1) quickly moving the target into position within the chamber, 
located within the amplifier cavity (hereinafter, the "particle switch" 
mode), or (2) optically switching the laser pulse(s) out of the main path 
and optical cavity so that the pulse is incident upon the target in an 
externally located target chamber (hereinafter, the "beam switch" mode). 
A preferred embodiment of the invention, employing the "particle switch" 
mode, is illustrated in FIG. 1. A master oscillator laser 12 delivers an 
initial pulse which is split into two equal parts by a beam splitter 13 
and a fully reflecting mirror 15 and is sent through isolators 25 and 27 
along paths P1 (defined by fully reflecting mirror 17) and P2 (defined by 
fully reflecting mirrors 15 and 21). The initial laser pulse at 12 may be 
delivered by any laser whose output wavelength is matched to the gain 
bandwidth of the amplifiers 33 and 35. As shown in FIG. 1, the two beams 
from the master oscillator are then injected into the cavity formed by 
mirrors 29 and 31, using two partially reflecting (.about.1% reflectivity 
at the chosen wavelength) mirrors 19 and 23. Other possible injection 
schemes, such as illustrated in FIG. 2, may be employed. It is desirable 
to inject oscillator pulses into the cavity in a discrete fashion so that 
losses are not introduced when the system is in its free-running 
regenerative mode. The revolving mirror technique has the unique advantage 
that it introduces no loss into the cavity (FIG. 2). After the oscillator 
pulses have been injected into the cavity, the mirror is moved by rotation 
out of the optical aperture before the next pulse passage occurs. The 
isolator 25 and 27 suppress any prepulse issuing from the laser 12 and 
isolate the oscillator from large amplitude pulses, which might be 
returned from the regenerative amplifier by beam splitters 19 and 23 if a 
revolving mirror is not employed. Oscillator prepulse suppression can be 
accomplished using saturable absorbers and/or fast electro-optical 
shutters. If a revolving mirror is not employed, the large pulse isolation 
requires a non-reciprocal optical element such as a polarized followed by 
a Faraday rotator. The polarizer is positioned between the oscillator and 
the Faraday rotator and rejects the return pulses. These isolation 
techniques are well known in the art of high power laser design and have 
been used to achieve 60 db and more of isolation in certain applications. 
The mirrors 29 and 31 of the optical cavity may be multilayer dielectric 
mirrors, constructed in the well-known manner to have high reflectivity 
(.about.99.8%) and low loss (.ltorsim.1%) at the laser wavelength. Such 
mirror designs have achieved high damage thresholds (10 J/cm.sup.2 for 
nanosecond pulse durations) which are also desirable in the present 
applications: it is damage threshold which limits the laser pulse energy 
that can be generated. 
The injected oscillator pulses are largely trapped between the two mirrors 
29 and 31 and caused to undergo multiple passes by reflection from 29 and 
31. As shown in FIG. 1, the center of the target chamber 39, where the 
target is to be irradiated, is positioned at the optical center of this 
cavity, formed by mirrors 29 and 31 so that the two counterpropagating 
pulses pass one another at this point. The entire optical path between the 
amplifiers 33 and 35 and target chamber 39 may be enclosed in an evacuated 
chamber, maintained at a modest vacuum (p.about.1 Torr) by conventional 
continuous pumping techniques, in order to optimize laser pulse 
propagation. Located on each side of the target chamber is a focusing lens 
37 and 38 to narrow the beam(s) (in the transverse direction) and a 
repetitively-pumped gain medium 33 and 35 (which may be pumped at a rate 
of .gtorsim.10.sup.4 Hz in a burst mode of operation) to sequentially 
build up the energy of each counter-propagating laser pulse as it passes 
through said medium. Saturable absorbers 41 and 43 (optical) prevent 
amplification of the fluorescent radiation emitted by the amplifiers. 
Spatial filters and/or optical relays 45 and 47 (also optical) are used to 
clean up phase and amplitude distortions imposed upon the laser pulses 
during amplification. These systems may be of conventional design, such as 
is shown in The University of California Report UCRL-78995 (1976) by Hunt 
and Renard. 
The saturable absorbers 41 and 43, whose use is optional, may be included 
to suppress any prepulse emitted by the laser or master oscillator 12 or 
any parasitic pulse issuing from the amplifiers 33 and/or 35. Suppression 
is not necessary but is very desirable in a high gain amplifier system 
such as used here, for two reasons. First, the passage of a prepulse or 
parasitic pulse through the gain medium many times will tend to amplify 
the disturbance and thus deplete the population inversion achieved by 
pumping, at the expense of the main pulse which follows. Second, the 
amplified disturbance may be sufficiently energetic that it can damage the 
target or otherwise degrade system performance. The principles of 
operation and design of saturable absorbers are well known in the art: see 
Thorne et al., in 45 Journal of Applied Physics 3072 (1974) and Harrach et 
al., in University of California Report UCRL-51008 (1971). 
The spatial filters 45 and 47, whose use is also optional, may be included 
to remove high spatial frequency noise from the counter-propagating 
pulses. The method of design and application of spatial filters is well 
known in the art of high power laser design as described by Glaze in 15 
Optical Engineering 136 (1976). The saturable absorbers and spatial 
filters are two examples of beam transformation apparatus used within the 
optical cavity to control the quality of the beam. Another example of such 
beam transformation apparatus is temporal pulse shaping apparatus which 
shapes each pulse as a function of time for improvement of target 
performance. Temporal pulse shaping may be carried out by saturable 
absorber or by various methods of pulse stacking, as taught and claimed by 
Thomas in U.S. Pat. No. 3,979,109 (issued Apr. 22, 1975) and by R. C. 
Harney, et al, in U.S. Pat. Nos. 4,053,763 (issued Oct. 11, 1977) and 
4,059,759 (issued Nov. 22, 1977), assigned to the assignee of this 
application. Temporal pulse shaping apparatus may be positioned internal 
to or external to the optical cavity. 
Beam transformation apparatus also includes spatial pulse shaping 
apparatus, wherein each pulse is reshaped as a function of the transverse 
radial coordinate so as to minimize diffraction, improve the lens fill 
factor or attain other worthy goals. One method of spatial pulse shaping 
uses a beam apodizer, wherein the beam is passed through a lens whose 
transmission varies with the transverse radial coordinate. This method is 
also discussed by Glaze, supra. 
The gain media used in the amplifiers can be chosen from a wide range of 
coherently pumped and flashlamp photon pumped solid state laser media such 
as Nd-, Er-, Tm- or other rare earth-doped glass, CaF.sub.2 or other 
suitable solids. The gain media may be photolytic gases such as O.sub.2, 
CO.sub.2, N.sub.2 O, OCS, OCSe, or CF.sub.3 l. Alternately, mixtures of 
gases such as (Ar or Ne)/KrF, (Ar or Ne)/Xe/F.sub.2, and other rare 
gas-halogen mixtures or pure rare gas, maintained at a pressure on the 
order of one atmosphere and excited by a high energy electron beam; an 
electric discharge or a combination electron beam-sustainer discharge may 
also be employed for pumping. The pumping of these media is optimized in 
the following manner. If .tau..sub.r is the radiative lifetime of the gain 
medium and .tau..sub.p is the pump-time for a single excitation, one seeks 
to make .tau..sub.p .ltorsim..tau..sub.r for efficient utilization of the 
medium. The pulse transit time .tau..sub.t =L/C is determined by whatever 
form of switch is employed and the length of the system, and is also 
limited by the repetition rate for the amplifier pumps 34 and 36. For 
instance, if the switch time constant is 1 .mu.sec, the cavity must be 300 
m in length. Furthermore, if .tau..sub.r &lt;&lt;.tau..sub.t, efficient 
utilization of the laser medium requires the amplifiers be excited only 
during passage of the laser pulses through them. Alternatively, if 
.tau..sub.r &gt;.tau..sub.t, the laser medium may need to be pumped only once 
over a period of time sufficient to achieve the desired level of 
amplification. In any case, consistent with parasitic and superfluorescent 
loss conditions, the gain in the medium may be adjusted, by time tailored 
pumping to effectively overcome losses and achieve laser pulse saturation 
as soon as possible (in 2-10 passes). After this, the circulating laser 
pulses efficiently extract optical energy from the amplifiers during each 
pass. Associated gain medium efficiency is limited only by the pumping 
technique: for electron beam excitation of gases, .ltorsim.40% can be 
expected; and for resonant photon pumping of solids, efficiencies 
approaching 50% are possible. 
As the two injected counterpropagating pulses grow in amplitude they 
continue to simultaneously cross at the center of the target chamber 39, 
with a period set by the distance between mirrors 29 and 31. The target 
irradiation is then accomplished by placing the target 40 in the laser 
focal region in the time interval between the (N-1)th and Nth pass, by the 
returning focused laser pulses. The target thus acts as its own optical 
short circuit or switch. More than one regenerative cavity may be 
employed, and the target can be irradiated symmetrically and 
simultaneously from two or more directions. Target irradiation rates of 
1-100 sec.sup.-1 are feasible. 
The subject invention may use one or a pair of gaseous gain media in each 
arm of the system, and the only glass components of any significance are 
the lenses 37 and 38 located at either side of the target chamber 39. 
These lenses may be replaced by spherical or off axis offaxis parabolic 
reflectors, and amplifier aerodynamic windows may be employed to eliminate 
all solid components from the laser path within the system. As mentioned 
above and discussed in more detail below, amplifiers using available high 
efficiency gaseous laser media seem most promising. However, solid state 
amplifiers could also be used. The problem of target positioning is no 
more severe here than in the case for conventional repetitive positioning 
of a target. 
The target may be a glass or plastic microballoon, with interior filled 
with hydrogen or with a deuterium-tritium mixture at many atmospheres 
pressure, such as shown by Lewkowicz in 7 Journal of Physics(d) pp. L61-62 
(1974). Other known targets developed in the field of fusion for high 
temperature plasma production and neutron or x-ray generation may also be 
used. The target chamber 39 may be sustantially a hollow sphere, 
constructed of steel, generally about 4 cm thick, of a diameter 
substantially 4 meters. The target chamber 39 will have two or more 
substantially antipodal apertures of diameter substantially two meters 
each, the apertures each containing a focusing lens 37 and 38 of focal 
length substantially two meters, to focus the collimated light to a small 
diameter at the center of the chamber. The target chamber 39 will also 
have a third, smaller aperture located on a great circle midway between 
the two lens apertures for introduction of the targets seriatim into the 
chamber. Calculations indicate that approximately one monolayer of target 
material may be deposited on the lenses interior to the target chamber for 
each target implosion. Thus, the lenses 37 and 38 would not need to be 
cleaned at least until 10.sup.3 -10.sup.4 targets have been imploded, 
depending upon the wavelength of the laser 12. These and other 
considerations will be considered in more detail. 
Consider first the highly efficient gaseous laser systems such as XeF* and 
KrF*, derived from the electrical excitation of mixtures of rare gases and 
fluorine. The means of excitation is arbitary; electron beam or electrical 
discharge could be employed. At this time, electron beam excitation is 
more attractive. This method has shown higher efficiencies and offers the 
possibility of uniform medium excitation, whereas one has the well-known 
stability problem for large volume excitation by electric discharges. For 
XeF* .music-sharp..sub.p .about.30 ns, and if the cavity transit time 
.tau..sub.t &gt;1 .mu.sec (system requirement), triode or tetrode electron 
beam sources may be employed. 
The physical size and characteristics for one example of a MERA amplifier 
which is excited by an electron beam source, are shown in Table 1. The 
total energy output from one module, denoted by E.sub.L, is related to the 
damage fluence of the material .GAMMA..sub.mat by the expression. 
EQU E.sub.L =.pi./4D.sup.2 .GAMMA..sub.mat =.pi./4D.sup.2 L.xi..eta.(1) 
Here .xi. is the medium excited stated energy density, L is the length of 
the gain medium and .eta. is the number of passes. Taking for the damage 
fluence .GAMMA..sub.mat -1 J/cm.sup.2 (a conservative value for short 
pulse width laser systems) one finds E.sub.L =30 KJ. The number of passes 
.eta. may vary, reflecting the range of stimulated emission-cross section 
.sigma..sub.s values and excited state densities in Table I. 
TABLE 1 
Design Characteristics for An Amplifier Module 
For a laser medium described by a solid right cylinder with dimension D as 
the diameter and L as the length, then the following can be stated: 
______________________________________ 
CHARACTERISTIC VALUE 
______________________________________ 
ASPECT RATIO L/D &gt;2 
ASITIC SUPPRESSION 5 
(TRANSIENT) .alpha.L = .sigma..sub.S NL 
GAIN CROSS-SECTION - .sigma..sub.S (cm.sup.2) 
10.sup.-16 -10.sup.-17 
GAIN LENGTH L (cm) 500 
DIAMETER D (cm) 200 
EXCITED STATE DENSITY N (cm.sup.-3) 
10.sup.14 - 10.sup.15 
ENERGY STORAGE - .xi. (J/l) 
0.1-1 
ELECTRON BEAM ENERGY - E.sub.B (MeV) 
1-2 
ELECTRON BEAM CURRENT DENSITY - 
1-10 
J.sub.B (A/cm.sup.2) 
______________________________________ 
The evolution of the energy fluence .GAMMA. of the laser beam, as it 
proceeds back and forth through the amplifier medium, is described by the 
Frantz-Nodvik equation 
EQU .GAMMA.=.GAMMA..sub.s 1n[1+e.sup..alpha.x 
(e.sup..GAMMA..sbsp.I.sup./.GAMMA..sbsp.s -1)], (2) 
where .GAMMA..sub.I and .GAMMA..sub.s are the initial and saturation 
fluences and .alpha. is the small signal gain. If the lower laser level is 
rapidly relaxed during the duration of the laser pulse excitation, 
.GAMMA..sub.s =h.nu./.sigma..sub.s. However, if relaxation does not occur, 
.GAMMA..sub.s =g.sub.L /g.sub.L +g.sub.u (h.sub..nu./.sigma..sub.s), where 
g.sub.L and g.sub.u are the degeneracies of the lower and upper laser 
level, respectively. For the candidate molecules KrF* and XeF*, 
.GAMMA..sub.s .perspectiveto.8 mJ/cm.sup.2 and .GAMMA..sub.s 
.perspectiveto.5.6 mJ/cm.sup.2, respectively. For high energy output, 
then, these systems must be heavily saturated: .GAMMA..sub.laser 
.perspectiveto..GAMMA..sub.mat &gt;&gt;.GAMMA..sub.s. Table 2 summarizes the 
amplifier characteristics for a XeF* system. Since the amplifier is 
designed to run at the damage limit of the mirror material, the total 
number of passes can be reduced by a factor of two if each amplifier is 
excited twice (once for each traveling wave pulse passing therethrough). 
In the amplifier design considerations hereinbefore the cavity was assumed 
to be loss-less, which is generally not the actual case. Some loss is 
associated with the intrinsic characteristics of the optical elements, and 
additional losses are associated with the mirrors, lens elements, etc. In 
an effort to evaluate these losses it is important to assess their 
deleterious effects on the performance of the present device since .GAMMA. 
.sub.laser &gt;&gt;.GAMMA..sub.s. 
If E.sub.O =.xi.Al is the saturated energy added per pass through the 
amplifier and .epsilon. is the lumped optical loss per pass, then the 
total laser output energy is 
EQU E.sub.L =E.sub.O x/(1-x)(1-x.sup..eta.), (3) 
where x=1-.epsilon.. In the limit that x.fwdarw.1 and/or 
.eta..fwdarw..infin., this expression reduces to the familiar expressions 
for a lossless or lossy cavity. One can define a figure of merit for the 
cavity as follows: 
EQU Cavity efficiency=1/.eta.(x/1-x)(1-x.sup..eta.), (4) 
which for x.fwdarw.1 is the result employed in the previous discussion. In 
FIG. 3, Eq. (4) is plotted for several values of .epsilon.. 
As anticipated, the performance of the device deteriorates markedly as 
.eta. increases (for a fixed value of x), indicating a tradeoff within the 
system. It is worth noting that for a saturated linear amplifier with a 
loss per stage given by .epsilon., an expression given by Eq. (3) is also 
obtained. Typically, .epsilon. will be 2-10%. On the basis of Table 2 and 
FIG. 3, then, powerful laser pulses can be generated using the technique 
set forth in this invention. 
TABLE 2 
__________________________________________________________________________ 
Design Characteristics for a Single Aperture XeF Amplifier 
__________________________________________________________________________ 
GAIN CROSS-SECTION Q.sub.S (cm.sup.2) - .about. 10.sup.-16 
AMPLIFIER LENGTH L (cm) - 500 
RADIATIVE LIFETIME - .tau..sub.L (NS) - 15 
AMPLIFIER DIAMETER D (cm) - 200 
PHOTON ENERGY - h.nu. (J) - 6 .times. 10.sup.-19 
TOTAL ENERGY - E.sub.L (J) - 3 
.times. 10.sup.4 
SATURATION FLUENCE .GAMMA..sub.SAT (J/cm.sup.2) - 6 .times. 10.sup.-3 
33 or 16 NUMBER OF PASSES .eta. 
INVERSION DENSITY N (cm.sup.-3) - 10.sup.14 
EXCITATION PULSE TIME .tau..sub.P (NS) - 30 
ENERGY STORAGE (J/l) - 6 .times. 10.sup.-2 
TRANSIT TIME .tau..sub.T (.mu.S) - 13 
DAMAGE FLUENCE .GAMMA..sub.MAT (J/cm.sup.2) - 1 
ELECTRON BEAM ENERGY E.sub.B (eV) - 1.5 .times. 
10.sup.6 
GAIN - LENGTH RODUCT .alpha.L - 5 
ELECTRON BEAM CURRENT DENSITY J.sub.B 
(A/cm.sup.2) - 2 - 4 
MEDIA PRESSURE - (ATM) - 0.1-1 
MEDIUM EFFICIENCY (%) - .about. 
__________________________________________________________________________ 
10 
Design of a particle injection system involves a trade-off between laser 
system optical path length L and particle injection technology. The total 
laser system length can be minimized, without significantly increasing the 
volume required to house the system, by using a MERA ring machine 
configuration such as shown in FIG. 4. In this case, for a long system, 
the perimeter L of the ring is roughly four times the total system length. 
Insertion of the target prior to arrival of the final irradiation pulses 
requires that the particle move into the irradiation volume from a 
position r.sub..eta.-1 (where it was undamaged by earlier pulses). The 
target velocity required is then V.sub.t, given by 
##EQU1## 
for an n.sup.th power exponential intensity distribution in the focal 
plane, where E.sub..eta.-1 is the laser output energy per beam prior to 
the final amplification cycle. For a saturated system, this is nearly the 
final laser output E.sub.L .about.3.times.10.sup.4 Joules, with d.sub.t 
.about.1 mm and .GAMMA..sub.mat .about.1 J/cm.sup.2, and therefore the 
velocity required in this case is v.sub.t .perspectiveto.2.5d.sub.t 
/.tau..sub.t for n=2 (Gaussian distribution). For a laser system 4 km 
long, the injection velocity required is then v.sub.t 
.perspectiveto.1.9.times.10.sup.4 cm/sec. However, since the ratio of the 
laser pulse length (.about.1 ns) to transit time .tau..sub.t is 10.sup.-3, 
the target is essentially stationary during irradiation. If the target 
mass is 500 .mu.gm, the kinetic energy invested in the target is small: 
K.E..sub.t .about.9 mJ. 
In one embodiment targets can be positioned mechanically by placing them on 
a rotating device as shown in FIG. 5. Precise orientation of the target 
relative to the axis of rotation is achieved in this system by mounting 
prior to spin-up. Analysis of a simple system indicates that for a 500 
.mu.gm target and a laser with a 4 km path length, the spinner can be 
rotated at 60 Hz with a total energy inventment of.ltorsim.100 J. 
Deformation of the target due to the centrifugal force is probably 
tolerable. As an example, for a 10:1 aspect ratio spherical shell made of 
beryllium, the strain in the mid-plane would be .about.2.times.10.sup.-7 ; 
for other materials the strain might approach .about.10.sup.-6. This would 
not significantly perturb volumetric compression ratios of 10.sup.6 
-10.sup.8. If the target is attached to the spinner by a 100 .mu.m 
diameter joint, the stress at the joint is .about.5.times.10.sup.7 
dyne/cm.sup.2, which is much less than the tensile strength of most 
materials. 
If electrodynamic techniques, as in FIG. 6, are employed to accelerate and 
guide the particle, the acceleration voltage V.sub.ac is simply 
EQU V.sub.ac =m.sub.t 2 q.sub.t V.sub.t.sup.2, (6) 
where q.sub.t is the charge on the particle. For a given laser 
configuration, the accleration voltage is minimized by minimizing the 
target mass-to-charge ratio, m.sub.t /.sub.qt. For positively charged 
solid particles this ratio is minimized, and the limit on applied voltage 
is determined by target break-up due to electrostatic forces. The minimum 
mass-to-charge ratio is reached when electrostatically-induced stresses 
equal the tensile strength S.sub.t of the material. The minimum target 
acceleration voltage required for a spherical shell with Gaussian 
irradiation profile becomes 
##EQU2## 
where d.sub.t is shell diameter and t.sub.t is shell thickness. 
Where the outer surface of the target is a 10:1 aspect ratio beryllium 
shell. With the total target diameter and mass are 1 mm and 500 .mu.gm, 
respectively, and the laser parameters are E.sub.L .about.3.times.10.sup.4 
J and L.about.4 km, then the minimum acceleration voltage is 
(V.sub.ac).sub.min .about.3.9.times.10.sup.5 V. Particle accelerations of 
10.sup.4 -10.sup.5 cm/sec.sup.2 would not be unreasonable. Plastics and 
other metals require similar acceleration voltages. Accelertion voltages 
up to about 15 MV can be achieved with linear Van de Graff type 
accelerators. 
After acceleration, the target's trajectory will be corrected with electric 
and magnetic fields to insure accurate positioning in the irradiation 
volume. For a simple deflection system consisting of parallel charged 
plates of length L.sub.E located a distance L.sub.T from the target plane, 
an electric field E between the plates will deflect the particle a 
distance Ye, where 
EQU Y.sub.E =EL.sub.E /2 V.sub.ac (L.sub.T +1/2L.sub.E). (8) 
For L.sub.E =10 cm, E=10.sup.4 V/cm and V.sub.ac =(V.sub.ac).sub.min, the 
beryllium shell target above would be deflected Y.sub.E (cm)=0.6+0.13 
L.sub.T (cm), indicating that accurate positioning of targets with simple 
quadrupole plate configurations is simple and straightforward. An 
electrostatic target injection system similar to that described in FIG. 6 
has been developed at Lawrence Livermore Laboratory which allows delivery 
at a rate of 1 per second of targets, moving at speeds up to 
5.times.10.sup.3 cm/sec, into the focus of a lens at a distance of 3.5 m 
with an accuracy of .+-.50 .mu.m. Similar results have been obtained by 
others, as shown in Table 3. 
TABLE 3 
__________________________________________________________________________ 
TICLE ACCELERATION ACCOMPLISHMENTS 
POSITIONING 
TICLE 
D V ACCURACY METHOD REFERENCE 
__________________________________________________________________________ 
O.sub.2, AR 
25 .mu.M 
10-20 M/sec 
.+-. 5 .mu.M at 25 cm 
FLUID DYNAMIC 
C. D. HENDRICKS AND 
LIQUID T. C. ANESTOS 
DROPLETS UNIVERSITY OF ILLINOIS 
NH.sub.3 
100-150 .mu.M 
.ltorsim. 50 M/sec 
.+-. 50 .mu.M at 
FLUID DYNAMIC 
G. PORTER 
SOLID 3.5 M ACCELERATION 
ELECTRODYNAMIC 
LAWRENCE LIVERMORE 
POSITIONING LABORATORY 
H.sub.2 75-100 .mu.M 
20-100 M/sec 
-- FLUID DYNAMIC 
C. FOSTER, C. D. HENDRICKS 
SOLID ACCELERATION AND K. KIM 
UNIVERSITY OF ILLINOIS 
AND OAK RIDGE 
DIAMOND 1 .mu.M 
.ltorsim. 7000 M/sec 
-- ELECTRODYNAMIC 
J. F. VEDDER 
SPHERES ACCELERATION REV. SCI. INST., 34, 1175 
(1963) 
__________________________________________________________________________ 
In the design of a MERA laser system, as a consequence of its great length 
when operated in the "particle switch" mode, it is necessary to give 
careful consideration to the propagation of the laser radiation within the 
regenerative cavity. Due to the large aperture and the short emission 
wavelength of the MERA laser outlined here, the natural diffractive 
divergence of each pulse per pass is quite small; for XeF* (.lambda.=351 
nm), the radial expansion per pass is 
.DELTA.r.about.(.lambda./D)L.about.10.sup.-1 cm. However, several sources 
can produce laser pulse phase and amplitude distortion that may greatly 
complicate the propagation and target irradiation problem. Some of the 
more important sources are likely to be: gas flow turbulence in the 
amplifiers and beam tubes, non-uniform volumetric energy deposition, small 
scale and whole beam self focusing in solid state optical gain components 
and amplifier saturation, and shock wave generation by electron beam-foil 
loading. 
The deleterious effects of gas flow turbulence can be minimized by 
evacuating and filling beam tubes with Helium or Neon to a pressure of 
0.1-1.0 Torr and using low Mach number flows with proper conditioning in 
the amplifiers. Uniform excitation will require careful analysis and 
design. The effect of large scale nonuniformities may be compensated to 
some extent by using spatial filters and optical relay techniques, wherein 
the cavity with internal optical elements is designed to be self imaging. 
The effects of self focusing in solid state components may also be 
controlled by using this image relay technique. The propagation of highly 
saturated pulses, as where KrF* or XeF* are employed in the amplifiers, is 
a potentially serious problem; these effects can be minimized by using 
high fill factor super-gaussian radial profiles, imaging cavity techniques 
and some form of internal temporal pulse shaping. 
The amplifier medium problem is actually two problems with substantially 
different time scales: (1) laser amplifier medium degradation during the 
multiple excitation cycle, and (2) laser amplifier medium clean-up between 
target irradiation events. The latter problem must be addressed in any 
usable gas laser device. However, the first problem, medium degradation 
during multiple excitation, is unique to the MERA concept and is 
compounded if the system is large. 
If the characteristic dimensions of the refractive index perturbation 
parallel and perpendicular to the radiation propagation direction are 
l.sub..parallel. and l.sub..perp., respectively, the relative density 
fluctuation which produces a deflection comparable to diffraction is 
EQU (.DELTA..rho./.rho..sub.o).sub..perp. .apprxeq.1/.beta. l.sub..perp. /D 
.lambda./l.sub..parallel. .rho..sub.s /.rho..sub.o, (9) 
where .rho..sub.o and .rho..sub.s are the amplifier operating density and 
atmospheric density under standard conditions, respectively. The 
refractive index perturbation is .DELTA..mu.=.beta..DELTA..sub..rho. 
/.rho..sub.s. The quantity (.DELTA..rho./.rho..sub.o).sub..perp. is the 
relative density fluctuation normal to the direction of propagation of the 
laser light. In the ultraviolet, the parameter .beta. is sensitive to 
particular gas species present and is smallest for Helium and Neon. In the 
estimates made in this section, it is assumed that the gas mixture 
contains a large Helium fraction; the results for Neon are similar. For a 
disturbance in the MERA system under consideration with l.sub..perp. 
.about.l.sub..parallel., the relative density fluctuation 
(.DELTA..rho./.rho..sub.o).sub..perp. is approximately 4.times.10.sup.-3 
(.rho..sub.s /.rho..sub.o). Fluctuations greater than this will begin to 
destroy beam quality. At atmospheric density this level is roughly an 
order of magnitude above what is achievable by proper fluid dynamic 
conditioning of the gas flow, and so presents no problem. However, at 
densities than atmospheric, density fluctuations are a potentially serious 
problem. For density perturbations of magnitude 
EQU (.DELTA..rho./.rho..sub.o).apprxeq.1/.beta. D/l.sub..parallel. l.sub..perp. 
/L .rho..sub.s /.rho..sub.o (10) 
the pulse will be deflected out of the system aperture. For the same 
conditions as above, (.DELTA..rho./.rho..sub.o).sub..perp. .about.14 
(.rho..sub.s /.rho..sub.o), and phase distortions of the wavefront of the 
pulse will be the limiting factor. Of course, shock-like disturbances with 
l.sub..perp. &lt;&lt;l.sub..parallel. are intolerable. A relative density 
variation across the beam that produces a phase distortion of one wave 
(.DELTA..phi..sub..perp. .about.2.pi.) is 
EQU (.DELTA..sub..rho. /.rho..sub.o).sub..perp. .apprxeq.1/.beta. .lambda./L 
.rho..sub.s/.rho..sub.o (11) 
which, for the conditions as above, become 
(.DELTA..rho./.rho..sub.O).sub..perp. .about.10.sup.-3 (.rho..sub.s 
/.rho..sub.o). This is slightly less than the condition required for pulse 
amplitude modulation, and therefore the same remarks are applicable. 
Uniform excitation of the MERA amplifier media will be essential to 
achieving low density fluctuation levels immediately prior to and during 
each multiple excitation cycle. A measure of the uniformity requirement 
can be seen by considering the rise .DELTA.T.sub.g in medium temperature 
T.sub.g during excitation, estimated by neglecting density fluctuations, 
viz. 
##EQU3## 
where n.sub.g is gas number density (cm.sup.-3), k.sub.B is Boltzmann's 
constant and .eta. is medium excitation efficiency (Table 2). For the 
present system this is .DELTA.T.sub.g .apprxeq.92.degree. K. over 33 
excitations or .DELTA.T.sub.g .apprxeq.2.8.degree. K. per excitation. Thus 
.DELTA.T/T.sub.g is .about.10.sup.-2 per excitation. If the system is to 
achieve (.DELTA..rho./.rho..sub.o).about.10.sup.-3 (.rho..sub.s 
/.rho..sub.o), a random excitation uniformity variation per excitation of 
no more than 10% and operation at atmospheric or lower pressures, will be 
required. The restriction on cumulative or coherent nonuniformities is 
more severe. 
Shock-like disturbances, launched from e-beam foils and other boundaries, 
can also seriously degrade medium quality. Where such disturbances occur, 
elimination of their effect by increasing system volume will reduce the 
excitation efficiency of the system. For a two-sided irradiation, the 
shock that comes off the foil during multiple excitation travels a 
distance 
EQU x.sub.s =M.sub.s a.sub.o .tau..sub.t .eta. (13) 
during the pulse build-up cycle, where M.sub.s is the shock Mach number and 
a.sub.o is local sound speed. If the excitation volume is increased to 
accommodate this disturbance and maintain disturbance-free operation 
during excitation in the region traversed by the pulses, then the 
excitation efficiency will decrease according to 
EQU .eta.=1-(2M.sub.s a.sub.o .tau..sub.t .eta.)/D (14) 
Efficiency .eta. is shown in FIG. 7 as a function of the number of passes 
.eta. for different shock Mach numbers. This plot should be compared with 
a similar efficiency plot, FIG. 3, for different cavity losses. Although 
this is a potentially serious problem, there is not technological reason 
why it cannot be greatly suppressed or eliminated by amplifier design. 
Following each multiple excitation sequence, the acoustic disturbances will 
propagate through the system and be convected with the gas flow. The 
magnitude of the acoustic damping problem for the large scale MERA 
amplifiers is easily evaluated. With .tau..sub.RC equal to reactor cycle 
time and neglecting gas flow effects, an initial density disturbance will 
bounce back and forth n=a.sub.o .tau..sub.RC /D times in the cavity and 
diminish in amplitude by viscous and thermal conduction damping, .sub.s, 
and by reflection (R&lt;1), reaching a final amplitude 
EQU .DELTA..rho..sub.f /.DELTA..rho..sub.i .apprxeq.R.sup.n 
e.sup.-n.alpha..sbsp.s.sup.D (n=1,2,3, . . . ) (15) 
prior to the next excitation. For the 60 Hz system described above, 
n.about.8 and .alpha..sub.s D.about.0.10/.lambda..sub.s.sup.2 (cm) where 
.lambda..sub.s is the sound wavelength or characteristic dimension of the 
density disturbance. For .lambda..sub.s .gtorsim.1 cm disturbances the 
damping is small and attenuation by reflection is essential. In this 
situation, .DELTA..rho..sub.f /.DELTA..rho..sub.i .about.R.sup.8 ; thus, 
to achieve .DELTA..rho..sub.f /.DELTA..rho..sub.i .about.10.sup.-4 
requires R.about.0.3. The combined application of boundary damping and 
convection will be required to eliminate disturbances. This points up 
another advantage of the use of Helium: high acoustic velocity, which 
facilitates boundary damping. 
Power conditioning can be one of the major concerns in the development of 
efficient lasers for fusion applications. The power conditioning system 
provides the conversion of energy from the primary source to the laser 
cavity. In power plant designs, the laser system must interface with the 
power output grid and convert this energy source to one appropriate for 
laser excitation. The magnitude of the re-circulating power is a major 
design factor, and the efficiency and reliability of the power 
conditioning system will strongly influence system performance. Similar 
design constraints will be extant for single pulse systems as the energies 
become larger. Current energy storage and transfer systems based on 
capacitors as an intermediate energy source become less cost effective as 
the magnitude of the energy stored and the power levels of transfer 
increase. 
The process of power conditioning can be considered as the transfer of 
discrete amounts of energy in shorter and shorter times. The difficulties 
in control increase with the amount of energy transfer at each step. In 
the MERA concept, the shortest time is the nominal 30 nsec required for a 
single amplifier pass. The energy deposited in a single pulse, however, is 
only a fraction of the total energy used in the amplification process. 
Each pass, which occurs nominally in 30 nsec,. contributes a fraction of 
the overall energy to the optical beam. This reduces the requirements on 
the final energy storage system as the system can be recharged between 
pulses. This relaxation of constraints allows new options for the power 
conditioning system. For example, a design using hot cathode guns and oil 
storage lines has been developed in the context of the MERA application 
and is the subject of a co-pending U.S. Patent application Ser. No. 
868,638 by L. G. Schlitt, filed simultaneously with this application and 
assigned to the same assignee and incorporated by reference herein. The 
Schlitt invention "stacks" and circulates voltage pulses on a transmission 
line and produces repetitive current pulses of duration 
.DELTA.t.gtorsim.25 nsec and a repetition rate of r.ltorsim.10.sup.5 
sec.sup.-1. 
The electron beam and power conditioning system are shown schematically in 
FIGS. 8 and 9 and require a power supply, an energy storage and 
conditioning module 83, an electron gun 85 and an interface 87 with the 
gain medium of the amplifier (33 or 35) on each side, for a two-sided 
system. Located on each side of the gain medium are, an anode (foil) 88, a 
foil support 89, and a cathode 91 for (each) electron gun, a high voltage 
insulator 93 and a transmission line 95. One such electron beam system is 
described fully in the co-pending U.S. Patent application Ser. No. 868,638 
by Schlitt supra. 
The primary energy storage consists of a "quasi-dc" charged oil dielectric 
transmission line. Switching and electron beam formation are accomplished 
in a thermionic tube with an active grid. The system may be modularized as 
in FIG. 9, to facilitate maintenance and minimize failure induced damage. 
The power conditioning system is capable of moderate pulse repetition 
frequencies, and in fact must be repetitively pulsed to minimize the 
impact of continuous cathode power dissipation. 
The laser medium determines most required gun properties. Voltage must be 
matched to the medium stopping power, the size is set by the required 
output energy, and the geometry must provide efficient beam energy 
deposition. Current density is determined by the required pumping rate, 
and the pulse length is limited by the inversion lifetime of the gain 
medium. 
The electron gun technology is old and well understood but for the use here 
it could present a critical design problem. Pulsed high current extraction 
is required, but grid current collection must be minimized to keep 
modulator characteristics reasonable. High voltage standoff must be 
provided for the transmission line insulator, and grid emission must be 
prevented in high electric fields. The tube must have the proper impedance 
characteristics. It must also have low inductance (preferably continuously 
matched) to permit short pulses, but must reflect pulses when the tube is 
off. 
The primary energy storage utilizes pulse stacking to minimize size and 
cost. Dispersion places an upper limit on the interpulse spacing, but this 
limit can be circumvented by increasing the size of the store or by 
state-of-the-art pulse charging from a capacitor bank between pulses. 
The parameters for a particular design for electron beam power 
conditioning, applicable to the present XeF* MERA system, are presented in 
Table 4. Foil losses require an electron energy .gtorsim.0.25 MeV. This 
prevents the use of Helium, except at higher pressures. Use of a reduced 
pressure requires reduction of electron energy or use of a higher Z gas 
such as Argon, which results in greater backscatter (12% for Ar vs. 5% for 
Ne). 
TABLE 4 
______________________________________ 
Electron Beam Power Conditioning 
______________________________________ 
energy input per amplifier 
300 kJ 
pulse length 30-100 ns 
pulse spacing 3-13.3 .mu.m 
number of consecutive pulses 
2-20 
electron energy 0.75 MeV 
geometry 2-sided, planar 
amplifier size 2 .times. 2 .times. 5 M 
beam area 10 M.sup.2 
current density 1.5 amps/cm.sup.2 
medium .95 Ne + .05 Xe 
pressure 2 atm 
beam profile uniform 
foil thickness 13 .mu.m (Nickel) 
______________________________________ 
In addition to amplifiers which utilize electron beam and electric 
discharge excitation of rare gas-halogen mixtures, photon pumping of gases 
and solids is also attractive. For instance, photolytic pumping of iodine 
compounds, such as CF.sub.3 I, C.sub.2 F.sub.5 I and C.sub.3 F.sub.7 I, 
with a KrF* laser is possible, operating at a wavelength of 1.31 .mu.m. 
Another attractive possibility is photolytic pumping of the group VI 
compounds, such as CO.sub.2, N.sub.2 O, O.sub.2, OCS, and OCSe, with rare 
gas excimer lasers such as AR.sub.2 *, Kr.sub.2 * and Xe.sub.2 *. The 
laser media emit radiation in the visible region at .lambda..apprxeq.0.55 
.mu.m. Details on these photolytic pumping schemes are discussed at length 
in U.S. Pat. No. 4,087,763, issued May 2, 1978, to E. V. George et al, 
assigned to. 
Another preferred embodiment of the invention uses a gas-solid hybrid 
amplifier system, for the following reasons. Implementation of economic 
laser fusion applications requires laser systems delivering nanosecond 
long pulses with peak powers in excess of 100 TW and at pulse repetition 
rates greater than one Hz. Identification and development of 
fluorine-based solid state host materials (characterized by relatively low 
nonlinear indices of refraction) allow the possibility of building 
Nd:glass lasers meeting application requirements with respect to peak 
power. This development notwithstanding, conventionally designed Nd:glass 
lasers utilizing these new materials will fail to meet the rigorous 
average power and efficiency requirements for two reasons: (1) low 
coupling efficiency of Xenon flashlamp radiation to the active centers (Nd 
ions) and (2) a substantial conversion of absorbed pump energy into 
thermal energy in the glass via electron-phonon processes. Coupled with 
the relatively low thermal conductivity of these dielectric materials, 
this internal energy inefficiency leads to unacceptable levels of beam 
distortion in the high pulse repetition mode of operation. The dominance 
of non-radiative over radiative processes in 1.06 .mu.m Neodymium glass 
lasers occurs because of the specific energy level structure of the 
Nd.sup.3+ ion and the use of the broadband Xenon flashlamps to pump the 
laser. Since one is not constrained to utilize either broadband flashlamps 
or the energy levels of the Nd.sup.3+ ion producing 1.06 .mu.m radiation, 
high average power, rare-earth solid-state lasers may be fabricated and 
used. 
Recently, several efficient (.ltorsim.10%), ultraviolet and visible lasers 
(rare gas excimers, metal excimers, rare gas halogen excimers, and metal 
halogen excimers) which do not store appreciable energy have been 
discovered and developed. These systems offer the possibility for the 
development of fluorine based, rare-earth doped, solid state laser media 
coherently pumped by long pulse ultra-violet and visible lasers. This 
approach is the subject of a co-pending U.S. Patent application Ser. No. 
868,633, by John L. Emmett et al, filed simultaneously with this 
application and assigned to the same assignee and incorporated by 
reference herein. An example of such a system, which uses a long pulse XeF 
laser 60 to pump a Tm.sup.3+ : CaF.sub.2 amplifier 61, is illustrated in 
FIG. 10. The flexibility of temporally and spatially tailored coherent 
pumping offers new options for the design of power limited pulsed lasers. 
The dominance of radiative over non-radiative relaxation processes in 
these media lead to very little thermal energy transfer to the solid state 
matrix, and may provide a means for achieving high average power and 
overall laser efficiency. Other particular examples of the amplifier in 
this embodiment are: (1 ) use of an XeF* laser to pump a Ce.sup.3+ :YAG 
solid laser; (2) use of an Xe.sub.2 * or Ar.sub.2 * laser to pump a 
Pr.sup.3+ :LaF.sub.3 solid laser; (3) use of XeF* to pump a Eu.sup.2+ 
:CaF.sub.2 solid laser; and (4) use of XeF* or KrF* to pump a Yb.sup.2+ 
:CaF.sub.2 solid laser. 
Efficient (.ltorsim.10%) long pulse UV and visible pump laser systems of 
sufficient beam quality can be developed. Implementation of a hybrid laser 
system requires a design which minimizes the linear and non-linear phase 
and amplitude propagation distortions inherent to solid state laser media 
and also maximizes the pump conversion efficiency. The linear stress and 
thermally-induced distortions set limits on high average power design. For 
instance, the phase distortion .DELTA..phi..sub..perp., across the beam 
due to a corresponding temperature variation .DELTA.T.sub..perp., 
neglecting mechanical and thermally-induced stresses, is: 
##EQU4## 
where L is the length of path traversed. For most solid state media 
(1/n).sup..differential.n /.differential.T and 
(1/L).differential.L/.differential.T are .about.10.sup.-6 -10.sup.-5 
/.degree.C. As an example, for CaF.sub.2 at .lambda.=315 nm Eq. (16) may 
be written simply 
EQU .DELTA..phi./2.pi..apprxeq.0.6 L(cm) .DELTA.T.sub..perp. (.degree.C.), 
(16') 
which imposes stringent constraints on thermal uniformity, reflected in 
pump beam uniformity, gas flow cooling requirements, and overall thermal 
environment control. The nonlinear distortion is due primarily to 
intensity-induced, self-focusing phenomena which limits the peak power 
achievable in a single pulse. The accumulated nonlinear phase distortion 
is the well-known B integral, viz 
##EQU5## 
where k=2.pi./.lambda. and the nonlinear index contribution is n.sub.2 
=.upsilon.I. If a CaF.sub.2 matrix is used, .upsilon. is 
.apprxeq.1.7.times.10.sup.-16 cm.sup.2 /W. 
Parasitic oscillation and fluorescent amplification can depress overall 
system efficiency. In practice, parasitic oscillation is a more serious 
problem than superfluorescence and limits the small signal gain-beam 
diameter or amplifier length product to be .alpha.D&lt;n where n.about.3-5. 
The restriction on beam diameter can be removed by using a mosaic 
structure with index matching to a suitable absorbing medium positioned 
between individual amplifier elements, as shown in co-pending U.S. Patent 
application Ser. No. 868,644 by John L. Emmett, filed simultaneously with 
this application, assigned to the same assignee and incorporated by 
reference herein. The index matched absorbing medium acts to suppress 
parasitic laser amplifier emissions that would otherwise propagate in a 
non-longitudinal direction so as to deplete the amplifier inversion 
density, without seriously affecting longitudinal amplification of laser 
radiation. Cascaded laser systems, as compared to regenerative systems, 
place the minimum constraints on medium quality since each element is 
traversed only once by the amplifying laser pulse. However, these systems 
are much less cost effective and therefore consideration of regenerative 
techniques is justified. 
The propagation and extraction considerations presented for XeF* above have 
been used in a preliminary design study of the XeF* laser 60 pumping a 
Tm.sup.3+ (0.1% mole):CaF.sub.2 hybrid system shown in FIG. 10. The 
results for two different regenerative amplifier designs, with different 
assumptions concerning the saturation characteristics of the solid state 
medium, are presented in Table 5. In each case, two possible system clear 
apertures are considered: a kilojoule class 20 cm diameter system and a 
100 kilojoule class 200 cm diameter system. The 200 cm diameter system 
employs a disc having a mosaic structure composed of a number of 20 cm 
diameter elements for parasitic suppression. The disks are given to be 
circular, anti-reflection-coated and oriented normal to the direction of 
propagation of the laser light. The B integral and thermal phase 
distortions were calculated using Eqs. (16') and (17). The maximum thermal 
phase distortion is estimated by assuming that the temperature variation 
across the disk equals the temperature rise, 8 or 16.times.10.sup.-3 
.degree. C., in the medium due to the laser energy deposition. Since this 
distortion probably develops over a time scale much longer than the laser 
excitation and extraction time, induced thermal effects are not expected 
to affect single pulse operation. However, from Eq. (16'), it is clear 
that the thermal environment of the disks prior to excitation must be very 
stable. Based on the B integral values presented in Table 5, this system 
will probably not require spatial filtering. The representative point 
designs presented in Table 5 are encloraging evidence of what is possible 
with single-pulse, power-limited, solid-state hybrid laser systems. Owing 
to the low disc temperature rise per pulse, one also expects that gas 
cooling techniques will allow operation of solid-state hybrid lasers on an 
average power basis. 
TABLE 5 
__________________________________________________________________________ 
Summary of the Design Characteristics for an XeF Laser Pumped 
Tm.sup.3+ (0.1% mole): CaF.sub.2 Disk Amplifier 
__________________________________________________________________________ 
Gain Cross Section .sigma..sub.s (cm.sup.2) - .about. 10.sup.-19 
Amplifier Diameter D(cm). - 20* or 200** 
Radiative Lifetime .tau..sub.L (.mu.s) - 20 
Number of Disks - 5 
Photon Wavelength .lambda.(A) - 4550 
Disk Thickness (cm) - 4 
Pump Fluence .GAMMA..sub.p (J/cm.sup.2) - 8.8, 17.6 
Amplifier Medium Length/Pass L(cm) - 20 
Saturation Fluence .GAMMA..sub.s (J/cm.sup.2) - 2.2, 4.4 
Number of Passes .eta.- 5 
Input Energy E.sub.I (J) - 0.01, 1.0 
Excitation Pulse Time .tau..sub.p (ns) - 200 
Output Energy E.sub.o (kJ) - 1.6*, 2.7*, 160**, 
Transit Time .tau..sub.L (.mu.s) - 3 
270** 
Output Fluence .GAMMA..sub.o (J/cm.sup.2) - 4.6, 8.7 
Total B. Integral (rad.)- 1.9, 3.4 
Gain-Length Product .alpha.L - 4 
Disk Temperature Rise/Pulse (m.degree.C.)- 8, 16 
Inversion Density (cm.sup.-3) - 2 .times. 10.sup.18 
Max. Thermal Phase Distortion (waves) - 
0.5, 1.0 
__________________________________________________________________________ 
*20 cm aperture 
**200 cm aperture, 20 cm mosaic element aperture 
***Assume .DELTA.T .about. 8, 16 m.degree.C. 
Another embodiment of the invention, the beam switch mode, does not require 
that the target be accelerated into the path of the beam. Referring more 
particularly to FIG. 11, optical switches 71 and 73 are positioned in the 
main beam path to simultaneously deflect each counter-propagating pulse 
out of said main path and cavity to be perpendicularly redirected by fully 
reflecting mirrors 75 and 77 to an externally positioned target chamber 39 
containing the target 41. In the inactivated state, each optical switch 71 
and 73 passes the laser pulses through undeviated in direction, or 
deviated by a fixed and controlled amount. With the optical switch 
activated, the laser pulse is now deflected in direction by a (new) fixed 
amount so as to pass through focusing lenses such as 37 and 38 and thus 
initiate laser implosion at the target. The optical switches are activated 
between the (N-1).sup.th and N.sup.th pass of a laser pulse through the 
laser amplifier(s), after the pulse has reached saturation intensity. 
In this embodiment, one may use a single amplifier, as shown in FIG. 11, or 
more than one such amplifier; one advantage of the use of a single 
amplifier being lower capital and electric cost. A second advantage is 
that the gain media excitation need only be synchronized to the 
(preferably simultaneous) passage of one or two or more 
counter-propagation pulses. In practice, this may require that the single 
amplifier be positioned symmetrically in the beam path, for example as 
shown in FIG. 11. 
The optical switch 71 and/or 73 may be any of a large variety known in the 
prior art. See, for example, the survey of beam deflection techniques in 
V. J. Fowler and J. Schlafer, 54 Proc. I.E.E.E. pp 1437-44 (1966). More 
particularly, one may use the well-known deflection of polarized light 
which is incident on a multilayer dielectric-coated polarization splitter. 
The multiple excitation regenerative amplifier may be used to produce high 
energy laser pulses to find ranges, heat materials, join materials, 
implode laser fusion targets, and the like. 
Although the preferred embodiments of the present invention shown and 
described herein have been directed in use in an inertial confinement 
system, it will be apparent that modification and variation may be made 
without departing from what is regarded as the subject matter of the 
invention.