PCT
`
`WORLD INTELLECTUAL PROPERTY ORGANIZATION
`International Bureau
`
`
`
`(51) International Patent Classification 3
`
`INTERNATIONAL APPLICATION PUBLISHED UNDER THE PATENT COOPERATION TREATY (PCT)
`
`
`
`(11) International Publication Number:
`W0 83/ 01349
`
`14 April 1983 (14.04.83)
`(43) International Publication Date:
`
`HOIS 3/093
`
`
`
`
`
`(81) Designated States: AT (European patent), BE (Euro-
`pean patent), CH (European patent), DE (European
`patent), FR (European patent), GB (European pa~'
`tent), JP, LU (European patent), NL (European pa-
`tent), SE (European patent).
`
`Published
`With international search report.
`
`(21) International Application Number:
`
`PCT/USS2/Ol391
`
`(22) International Filing Date: 27 September 1982 (27.09.82)
`
`(31) Priority Application Number:
`
`308,714
`
`(32) Priority Date:
`
`5 October 1981 (05.10.81)
`
`(33) Priority Country:
`
`(71) Applicant: MIAMI UNIVERSITY [US/US];
`Roudebush Hall, Oxford, OH 45056 (US).
`
`US
`
`202
`
`; 405 East Vine
`(72) Inventors: WELLS, William, E., Jr.
`Street, Oxford, OH 45056 (US). MARCUM, 8., Doug-
`las ; 604 South Main Street, Oxford, OH 45056 (US).
`DOWNES, Lawrence, Wayne ; 556 Brookfield Drive,
`Apartment 201, Fairfield, OH 45014 (US). TILTON,
`Richard, A.
`; 9027 Bogata Circle, San Diego, CA
`92126 (US).
`
`(74) Agents: BISSELL, Barry, S. et al.; Patent Administra-
`tor, Battelle Development Corporation, 505 King
`Avenue, Columbus, OH 43201 (US).
`
`
`
`L.____________.________._._____._.__.
`
`(54) Title: COLLISION LASER
`
`(57) Abstract
`
`Electromagnetic radiation (10) in a gas mixture (12)
`including helium in the X(1) state and nitrogen in the Y(1)
`state. The helium is pumped to excite a high population
`density of its atoms from the X(1) state to the X(2) state;
`and photons (15) of suitable frequency are injected into the
`mixture (12) to excite, via a three-body radiative collision of
`an atom of X(2) with a molecule of Y(1) and a photon (IS),
`a high population density of molecules of the nitrogen from
`the Y(1) state to the Y(3) state, followed by a substantially
`simultaneous return of a substantial portion of the excited
`helium atoms to the X0) state and a substantial depopula-
`tion of the Y(3) state of the nitrogen, causing the molecules
`thereof to drop to the lower energy Y(Z) state, thereby sti-
`mulating the emission from the nitrogen of two photons
`(10) at the same wavelength for each absorbed photon (15),
`and thus providing a total quantity of photon emission (10)
`with sufficient gain for amplification of electromagnetic ra—
`diation (10), and finally resulting in the depopulation of the
`molecules in the Y(2) state by autoionization.
`
`JOUTPUT
`
`+40
`
`,5
`
`
`
`-\\\\"’(\f
` "l""'l
`ELECTRON
`
`16
`l
`12
`OR
`
`
`NEUTRON
`SOURCE
`
`
`
` VACUUM
`
`
`81
`GAS
`
`HANDLING
`SYSTEM
`
`
`
`
`GILLETTE 1414
`
`l4
`
`LASER
`
`
`
`GILLETTE 1414
`
`

`

`FOR HIE PURPOSES OF INFORMATION ONLY
`
`Codes used to identify States party to the PCT on the front pages ofpamphlets publishing intematidnal ap-
`plications under the PCT.
`
`AT
`. AU
`BE
`BR
`CF
`CG
`CH
`CM
`DE
`DK
`Fl
`FR
`GA
`GB
`HU
`JP
`KP
`
`Austria
`Australia
`Belgium
`Brazil
`Central African Republic
`Congo
`Switzerland
`Cameroon
`Germany, Federal Republic of
`Denmark
`Finland
`France
`Gabon
`United Kingdom
`Hungary
`Japan
`Democratic People’s Republic ofKorea
`
`-
`
`LI
`LK
`LU
`MC
`MG
`MR
`MW
`NL
`NO
`R0
`SE
`SN
`SU
`TD
`TG
`US
`
`Liechtenstein
`Sri Lanka
`Luxembourg
`Monaco
`Madagascar
`Mauritania
`Malawi
`Netherlands
`Norway
`Romania
`Sweden
`Senegal
`Soviet Union
`Chad
`Togo
`United States of America
`
`‘
`
`
`
`

`

`.WO 83/01349
`
`_
`
`p
`
`g
`
`_.
`
`_
`
`PgT/U582/01391 _
`
`l
`
`COLLISION LASER
`
`FIELD
`
`This invention relates to methods and apparatus for
`'providing stimulated emission of electromagnetic radiation.
`
`Typical embodiments of the invention comprise laser
`
`amplifiers and laser oscillators.
`
`The invention is especially advantageous as a new
`
`type of laser amplification system, based upon a process
`of stimulated emission radiative collisions.
`The system
`
`produces a high population density of long lived excited
`
`10
`
`atoms that,
`in a three—body collision with a suitable
`atom or molecule in its ground state and with a photon of
`
`appropriate energy, results in photon emission with
`
`sufficient gain for laser amplification.
`
`The-invention typically uses the photon induced
`
`15
`
`collision between a metastable excited atom or molecular
`
`to which metastable
`species and a ground—state molecule,
`energy is transferred with high efficiency, simultaneously
`
`stimulating the emission of two photons at the same wave-
`
`length for each absorbed photon.
`
`The gain of the system
`
`depends upon an inversion of the products of the population
`densities of atomic or molecular states.
`The term
`
`"density" as used herein always means number density
`
`(unless the context shows otherwise) regardless of whether
`
`the density is of state population, or of collisions.
`
`The invention comprises a novel way to provide
`
`inversions,
`
`in that energy can be stored in one atom
`
`species in the upper laser levels, whereas the lower levels
`
`of the other atom or molecule of the collision pair can be
`
`20
`
`25
`
`:1,
`
`30
`
`depopulated as by a rapid decay mechanism. When this
`
`principle is applied to a system in which a high density
`of upper level states is populated, while at the same time
`
`the lower level is rapidly depopulated,
`
`the gain and
`
`efficiency are significant, and conditions can be realized
`
`to provide a high power, high energy laser amplifieru
`Hereinafter described in more detail is a new type
`
`35
`
`
`
`

`

`WO 83/01349
`
`PgT/U582/01391
`
`‘
`
`2
`
`of laser based on radiative collisions, with a specific
`
`gas selection, namely He~N2, as a typical embodiment.
`Gain in such a system can be described by the equation
`
`‘
`
`a
`do
`.
`.
`-‘
`[Y(2)]Ix(l)JL
`Gain ‘ EEE'-‘fiNk{[gX(2)8¥(l) {;(%)3£Yfll)l gggz) 8X<l)
`which is dealt with in detail later. For a given situation,
`
`when the numerical value of this equation is positive,
`
`amplification of a light signal can take place through
`stimulated emission.
`n
`n
`1‘1 var:
`
`1 near: Bataan vn¢m iw Mu w i
`duh.“
`“5v mm
`5
`we.
`v
`H‘iu
`
`-
`u
`A“
`1
`1
`‘1'! =4-
`n
`
`r-vn
`Ava
`a
`bahvié «3.41.15.nevavc;.LV is
`
`d.“
`ca. rvru
`.2.»
`
`10
`
`15
`
`20
`
`within a single species.
`
`The analysis hereinafter shows
`
`that laser gain can be obtained by the inversion of the
`
`product of two densities, rather than just the individual
`
`densities. This is also shown in the equation above.
`
`Such inversions are usually not possible for systems
`
`of atomic or molecular species in thermal equilibrium.
`
`’This invention provides specific means by which it is
`
`possible to produce the inversion of product population
`
`density.
`
`The typical embodiment comprising a mixture of He and
`
`N2 is described by the equation
`‘
`*
`so + HeCZBS) + 3200 -> Heals) + N2 (x;=3) 2* 73“”:
`where He(23s) represents the excited helium atoms produced
`
`in the gas mixture by the pumping source; N2(X), the
`nitrogen molecules in their initial ground state; He(llS),
`
`25
`
`the ground~state helium atom products of the 3-body
`
`collision; and N2*(x,v=3), the product nitrogen molecules
`
`remaining after stimulated emission has occurred. As
`
`seen in the equation above,
`
`to achieve gain in the system
`
`the product densities in the right—hand term of the
`
`30
`
`equation must be low.‘
`
`In the present invention, this has
`
`been unexpectedly achieved by taking advantage of the
`
`Q
`
`W
`
`fact that the N2*(X,v=3) molecule, which is still in an
`excited state with a high population density, is an auto—
`
`ionizing state with a very short lifetime.
`
`It
`
`
`
`

`

`.W083/01349 V
`
`.,
`
`_
`
`_
`
`~
`
`_
`
`y
`
`_
`
`.
`
`.
`
`PCT/US82/01391
`
`3
`
`self-destructs rapidly,
`
`thereby depleting its population
`
`and making the product density of the right-hand term
`negligible.
`BACKGROUND
`
`In 1972, Gudzenko and Yakovlenko1 described a process involving, effectively
`
`a three-body collision between atomic or molecular species X and Y, and a photon
`
`hm. Figure 1a shows the energy level for atoms X and Y that corresponds to
`
`X(l) + Hz) +fiw # X(2) + m).
`
`(1)
`
`The absorption of the photon, having an energy hm, allows a resonant two~body
`
`10
`
`collision.
`
`The production rate of state X(2) can be written
`(a
`fl%fll=otnninon,
`where [3 indicates concentration, p is the photon flux field and k is a three-body
`
`-
`
`rate coefficient for the radiative collision.
`
`If we look at kg in terms of a
`
`15
`
`normal binary collision,
`
`then
`
`I
`
`kp = <ov>,
`
`-
`
`where o is the event cross section and v is the velocity,
`
`thus (2) becomes
`
`(3)
`
`(4)
`
`‘5)
`
`20
`
`25
`
`3O
`
`n,
`
`w: <°V> mm mm.
`
`Alternately, if we look at k in terms of photon absorption,
`
`then
`
`k mm = 812,
`
`where 312 is an £instein~like absorption coefficient.
`fl§fll=n2poon.
`
`Now (2) can be written as
`(a
`
`when cast in the fonn of a collision, as in (3) and (4), the cross section
`
`0; becames a function of p,
`
`the photon flux field. when written in the form
`
`of a radiative absorption, as in (5) and (5), the Einstein stimulated absorption
`
`coefficient, 812, becomes a function of the density, [Y(2)]. The photon, hm
`does not have the energy of the difference between X(2) and X(1), but approximately
`the energy difference between X(2) and Y(2).
`A third method of describing these
`
`collisions would be the absorption of a photon by a quasi-molecule or collision
`
`complex Y(2)X(1). This model is conceptbally useful.r
`
`Harris2 has used the collisional model
`
`to describe his observations of
`
`
`
`

`

`_.wo 33/013449
`
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`__
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`_
`
`V
`
`‘
`
`,_ PCT/USSZ/Ol391
`
`4
`
`many such collisions that he and his colleagues have observed.
`
`In this work
`
`5i
`
`very large cross sections have been observed for a wide variety of collisions
`
`induced by intense photon flux fields. The largest cross section reported thus
`-13cm2.2
`4
`5
`
`have used these large cross sections to
`
`far is 8x10
`
`Harris
`
`and others
`
`propose population mechanisms for lasers.
`
`We propose a stimulated emission process based on the inverse process to
`
`(1). This process was considered briefly in reference 1.
`
`«a» + x(2) + m; + xm + m) + 2m.
`
`(-7)
`
`For this reaction, the colliding atoms are stimulated to emit a photon, (Fig. 1b),
`
`where in reaction (1) the colliding atoms are stimulated to absorb a photon.
`
`The
`
`I
`
`cross section for these two reactions,
`
`(1) and (7), are the same;
`
`thus the rate
`
`of production will be the difference between the two processes
`
`W: ko axon {no} - mm mm},
`
`The photon production rate
`
`-LLfldXZ
`dt
`
`m”)
`
`sea
`dz
`
`is the negative part of (8).
`
`If the statistical weights are included,
`
`then
`
`the gain in photon flux can be expressed as
`
`(a)
`
`(9)
`
`I
`-§£—= .
`"'
`pain — pdz
`non ‘
`
`Note that the gain of such a system depends on an inversion of the products of
`
`the densities. This provides new ways to produce inversions, for the energy
`
`storage can be in one species for the upper quasi—molecular laser level and the
`
`lower level can be deactivated in the collision partner. At high photon flux
`
`fields, a large cross section typical of Harris‘data2 would force the radiative
`
`collision tc be the chief energy pathway, making the photon production efficiency
`
`_approach the quantum efficiency. At lower photon flux fields, the energy is
`
`channeled through other processes and the efficiency would be expected to be
`
`very low. As an example helium and nitrogen are chosen as the media. Although
`
`better systems may exist, the abundance of atomic and-molecular data for helium
`
`and nitrogen makes this example useful.
`
`ll)
`
`my:
`
`10
`
`15
`
`20
`
`25
`
`oU REA 0
`one:
`
`
`
`
`14,. woo
`o.
`22? NAT10$/
`
`

`

`wo 33/01349
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`PCT/U582/01391
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`g
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`5
`
`The energy level diagram for helium and nitrogen in fig. 2 can be correlated
`to that of fig.
`l by,
`
`X(l) + Hegl‘S).
`
`X(2) + He(23$),
`
`Y(l) + N2(x,v=0),
`
`H2) » r-l‘z'ix,v=3),
`Y(3) + N:(B,v=4).
`
`and
`
`Note that the * denotes excitation to a Rydherg state near the ionization thresh-
`
`hold of nitrogen. This depresses the energy of the 8 core, v=4, “2 state to
`
`10
`
`resonance with He(23S).
`
`The reaction of interest now becomes
`
`He(23$) + N2 + We) » He + N:(x,v=3) + an».
`
`’
`
`(11)
`
`15
`
`20
`
`25
`
`"i
`
`Because the Rydberg electron is near the ionization limit (in or near the
`*
`Saha region),
`the Y(3), N2(B,v=4), state is resonant with He(23S). Also,
`the
`induced transition can be considered in the same manner as the equivalent ionic
`state.
`;The Franck-Condon factor7 shows that the transition 35385 is optimal for
`
`reaction (ll).
`
`Harris3 has derived an expression for the cross section of a dipole-quadrupole
`
`radiative collision for both strong and weak photon flux field regimes. Using
`
`this expression for the helium-nitrogen system, we get
`‘k
`*
`
`0'
`
`weak
`
`
`= w _—
`
`{ n3
`2fi2§2
`
`]
`
`[3U§§ Q¥§}2 IU§§ )2 E2
`29%
`{ZfiAwJ
`
`and
`
`*-
`
`9:
`
`aStY‘Oflg
`
`= 1, {ET} [M133in [serialize 5%.
`
`£-
`
`M...
`
`V
`
`(‘2)
`
`(13)
`
`,
`.
`(N2 9 N2 (B,v)), is determined by the energy6
`The dipole matrix element, u21,
`3?
`*
`The matrix element, u23,
`(N2 (B,v) + N2 (x, v—1)),
`and the Franck~Condon factor7.
`is weighted by the Franck-Condon factor7. The quadrupole matrix element, qu’
`(He(13S) + He(235)),
`is estimated from the equivalent singlet lifetime assuming
`electron exchange during the collision.
`The Heisskopf radius8 is no, Aw is
`
`the detuning energy (normally the energy difference'between the virtual state
`
`
`
`

`

`no 83101349
`
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`g
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`PgT/U582/01391 ‘
`
`—
`
`6
`
`and the reai state), V'is the thermai veiocity, and E is the eiectric field due
`
`to the photon flux.
`Since the Rydberg state is effectiveiy in the Saha continuum, the detuning
`
`energy. Aw,
`
`is taken to be a coiiection of the iinewidths of the three states and
`
`the bandwidth of the incoming photon fiux field.
`The vaiue of the detuning energy
`is estimated to be 2 cm'l. Figure 3 shows the iimiting case cross sections
`
`caicuiated for (11) as a function of photon flux fieid, for both the strong and
`
`A model has been deveioped for a helium-nitrogen system
`weak photon field cases.
`.
`.
`.
`.
`Ill-inn 44mg nI-ncr aar+1nn cknmn an 3"!"
`Ida “as was. »a Uaa wuexuls «souls-u u» . .5. 3.
`
`10
`
`The rate equations used in this system invoive the concentrations of He+,
`
`He} He(23s), and n; .
`The rate of He+ production is determined by the source terms involving 5,
`
`the power deposition and the N value or energy investment per ion, and metastabie—
`
`metastable ionization, He(235) + He(235) + He+ + He + e.
`
`The loss tenns invoive
`
`15
`
`charge e‘xchange,‘N2 + He+ + N; + He, and three—body conversion, Zhe + He+ e He: + He.
`The equation for He+ production is
`
`d g;
`
`= s +§me<23snz - k;[Nz][He+] — kapgemefi.
`
`(14)
`
`The coefficients are 1isted in Table 1.
`
`The He; rate equation is given by
`
`20
`
`ML enigma": - cinema] — momma?) - kaxfnzlmehmel
`
`(15)
`
`The singie source term is three—body conversion, whiie the loss terms invoive
`coiiisionai radiative recombination, He; + e + x-+‘CRR, and two— and three-body
`, +
`+
`charge transfer, N2 + He: +»N; + 2He and N2 + hez + He » N2 + 3He.
`See Tabie 1
`for the coefficients of this rate equation.
`The metastable production rate has two source terms, one dependent on the
`energy deposition and the other dependent on coilisionai radiative recombination.
`
`25
`
`LI}
`
`7?
`
`The 1055 terms depend on metastabté~metastabie ionization, two- and three-body
`Penning ionization, N2 + He(23$) » n; + He + e and N3 + He(235) + He +
`N; + 2He + e, and super=eiastic reiaxation, He(23S) + e » He + e (20 eV).
`
`ther
`
`
`
`

`

`'W
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`O 83/021349
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`.
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`‘
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`. PCT/U882/01391
`
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`7
`
`RATE COEFFICIENT
`
`VALUE USED
`
`REFERENCE
`
`'
`
`3
`k1
`kg
`
`on
`
`kao
`k,1
`Sm
`kuo
`kg;
`k5
`82
`
`A'
`
`V
`
`“N:
`
`5
`
`10
`
`15
`
`_
`
`1.8 x 10"9 cma/sec
`1.2 x 10-9 cma/sec
`67.0 :5 Torr'Z/sec
`
`4.5x10
`
`-20
`
`-
`(Te/To) “ne
`
`'
`
`9
`10
`11,12
`
`+
`
`5.0X1O
`
`~27
`
`"I
`(Te/To)
`
`3
`nO cm /sec
`
`1.1 x 10-9 cma/sec
`1.5 x 10'29 cmi/sec
`S/O.56
`6.9 x 10'11 ems/sec
`2.9 X 10-” ems/sec
`7.0 x 10'11(Te)& cms/sec
`0.3 Torr-z/sec
`
`8nhvio'V/pc2
`
`2.5 x 105 cm/sec
`
`2.2 x 10'7 cma/sec
`
`V
`
`1?
`
`13
`13
`14
`15
`15
`16
`17
`
`_
`
`Einstein
`Coefficient
`
`Therma1
`Ve1ocity
`18
`
`Tab1e 1. Te= e1ectron temperature, To= p1asma temperature,
`né= e1ectron density, no= neutra1 density,
`pHe= partia1 pressure of he1ium.
`
`«1‘
`
`20
`
`25
`
`2
`
`Iosses invo‘lve threevbody conversion to mo1ecu1ar metastab1e, He(235) + ZHe ->
`He2(232) + He, spontaneousemission of a photon by a radiative co11ision,
`+ He(233) ~> N2 + He +iTm, and stimu1ated emission from a radiative co11ision,
`N
`N2 + He(23S) + ‘ficu ~> N2(x,v) + He + 2m.
`The rate equation for 1194235) is
`3
`éiflséé—éil = Sm + .7uIHeilte] - stHa<235)]= - kuoEN23[He(235)3
`- kthN2][He(23$)][HEE - ksIHe(23S)3{eJ — ezp§e£He123511
`
`-, A‘IN2]{H2(23S)] - <c‘?>{Nz][He(2is)].
`
`

`

`wo 83/01349
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`PCT/U882/01391
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`-
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`8
`
`Coefficients for equation (16) are also defined in Table 1.
`
`'
`
`a
`
`The ionized molecular nitrogen rate equation is
`+
`igzh szfie+3W21 + kutueilml + kntneiluuliael
`
`+ kuoinZ‘SHE-‘lzl + kn[He(235)BLNz][He] t A'EHE(23$)3[Nz}
`
`5
`
`+ ’<o‘9>[H9(235)3[Nz] ~ mums].
`
`(W)
`
`All the source terms have been defined previously and the loss term involves
`
`dissociative recombination, h; + e a N: + N.
`in Table 1.
`
`The rate coefficients are given
`
`Finally, a charge balance eouation is used to conserve the system’s
`charge.
`>
`
`10
`
`These equations have been solved in steady state for power depositions
`between 200 Wcm3 and
`2 Mw1cm3 in a mixture of one atmosphere of helium and
`
`various percentages of nitrogen. The data presented here is for a mixture with
`
`1% nitrogen.
`
`15
`
`The gain for the preceding system is calculated from equation (10).
`It
`_should be emphasized that the N;{x,v=3) state is autoionizing19 and has a lifetime
`of about 1
`0'10 seconds. This makes the product density,{He(115)][N;(x,v=3)],
`negligible since the lower levels self-destruct. The calculated gain is shown
`
`in fig. 4.
`
`The decrease in gain at the higher photon flux fields is due to the
`
`20
`
`high destruction rate of helium metastables,
`
`the effects of which are shown in
`
`fig. 5. The calculated efficiency (ratio of radiative power to power deposition)
`is shown in fig. 5 and saturates near the quantum efficiency of 15%.
`
`It should be emphasized that, although the gain for the system is large
`
`(see fig. 4), significant energy loss due to superradiance will not be a problem
`
`;
`
`255
`
`due to the low efficiency at small photon flux fields. Significant energy
`
`extraction will only occur in the direction of the incoming oscillator beam, since
`
`the intensity of the photon field determines the cross section for radiative
`collisions.
`
`_
`
`Further analysis and experimental results are
`
`30 presented in the section on carrying out the invention.
`
`and in Figures 8-16.
`
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`we 83/01349
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`PCT/US82/01391‘
`
`DISCLOSURE
`
`Typical apparatus according to the present invention,
`for providing stimulated emission of electromagnetic
`radiation, comprises means for containing a mixture
`
`in the X(l) state, and a second gas,
`including a first gas,
`in the Y(l) state; means for pumping the first gas to
`
`excite a high population density of its atoms or molecules
`
`from the X(l) state to the X(2) state; and means'for
`injecting photons of suitable frequency into the mixture,
`to excite, via a three—body radiative collision of an
`
`atom or a molecule of X(2) with a molecule of Y(l) and a
`
`photon, a high population density of molecules of the
`second gas from the Y(l) state to the Y(3) state, followed
`by a substantially simultaneous return of a substantial
`portion of the excited first gas atoms or molecules to
`the X(l) state and a substantial depopulation of the Y(3)
`‘state of the second gas, causing the molecules thereof to
`
`drop to the lower energy Y(2) state,
`thereby stimulating
`the emission from the second gas of two photons at the
`same wavelength for each absorbed photon, and thus pro—
`viding a total quantity of photon emission with sufficient
`gain for amplification of electromagnetic radiation, and
`finally resulting in the depopulation of the molecules
`
`in the Y(2) state by autoionization.
`
`10
`
`15
`
`20
`
`25
`
`The pumping means typically comprises external means
`
`. for bringing the first gas from the X(l) state to an
`excited metastable state X(2).
`Such means may comprise
`means for bombarding the gas with electrons, which
`typically comprises external means for generating an
`electron beam and directing the beam through a thin foil
`
`window to strike the mixture, or it may comprise means for
`
`directing neutrons to strike fissionable material to
`produce energetic fission product particles to strike and.
`thereby ionize a portion of the mixture and thus to
`produce electrons that bombard X(l) atoms or molecules
`in the mixture. Other typical external means comprises
`
`30
`
`35
`
`v)‘
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`PCT/U882/0139l
`
`lO
`
`electrical discharge, radiation,
`means.
`’
`
`thermal, or chemical
`
`Typical photon injecting means comprises an external
`
`source of radiation at a frequency substantially resonant
`
`with one of the transition frequencies of the second gas.
`
`Typically the external source is a laser} such as a
`
`tunable dye laser of power sufficient to produce a high
`
`density of three-body collisions.
`
`The containing means may comprise a resonant Optical
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`into the mixture to provide self—sustained oscillation.
`
`Typically the first gas comprises metastable atoms
`
`or molecules and the second gas comprises molecules with
`
`15
`
`an ionization potential less than the excitation energy
`of the atoms or molecules of the first gas.
`The X(l)
`
`state typically is the ground state.
`
`in which the
`The first gas typically is helium,
`X(l) state is He(lls) and the X(2) state is He(235).
`
`20
`
`25
`
`30
`
`The second gas typically is nitrogen,
`
`in which the
`
`the Y(2) state is N2* (X, v=3),
`Y(l) state is N2(X),
`where * denotes excitation to a Rydberg state near the
`
`ionization limit, and the Y(3) state is N2* (B, V=4).
`Typically the'first gas comprises either a noble
`
`gas, such as helium, neon, or argon; or nitrogen.
`
`The
`
`second gas typically comprises nitrogen, oxygen, carbon
`
`monoxide, carbon dioxide, nitric oxide, uranium hexa-
`
`fluoride, or heptafluoroiodopropane.
`
`The gain varies directly with the product of the
`
`population density of the atoms or molecules in the X(2)
`
`state multiplied by the population density of that
`molecules in the Y(l) state minus the product of the
`
`population density of the atoms or molecules in the X(l)
`
`state multiplied by the population density of the
`
`molecules in the Y(2) state. Typically the depopulation
`
`35‘
`
`of the molecule in the Y(2) state to lower nonresonant
`levels by autoionization is rapid and substantially
`
`u
`
`m,
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`

`

`we 83/01349
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`-7 .
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`PCT/U582/01391
`
`ll
`
`complete, so that the product of the pOpulation density
`of the atoms or molecules in the X(l) state multiplied
`
`by the population density of the molecules in the Y(2)
`
`state is negligible,
`
`(because [Y(2)] is approximately
`
`zero), and thus the gain is substantially proportional
`
`to the product of the population density of the atoms or
`molecules in the X(2) state multiplied by the population
`
`density of the molecules in the Y(l) state.
`
`The auto-
`
`ionization is substantially complete in a time less than
`
`10
`
`the radiative lifetime.
`
`DRAWINGS
`
`Figure l is an energy level diagram showing
`
`radiative collisions for (a) absorptive and (b)
`
`stimulated emission processes.
`
`15
`
`Figure 2 is a graph of potential energy against
`
`internuclear distance for N2. Note the vibrational
`
`expansion of the B-state of the ion.
`
`Figure 3 is a graphical representation of cross
`
`section of a dipole-quadrupole radiative collision
`
`20
`
`25
`
`30
`
`35
`
`against photon flux as calculated by Eg. 12 (weak field)
`and Eq. 13 (strong field)3 for the helium-nitrogen system.
`
`Experimental results are also plotted in Figure 3.
`
`Figure 4 is a graph of predicted gain against photon
`
`flux for 1% N2 in 700 Torr of helium for power depositions
`of 200 W/cm3 to 2 MW/cm3 under the influence of lO KW/cm2
`to 100 MW/cm2 photons.
`Figure 5 is a graph of predicted metastable (He(235))
`densities against photon flux for 1% N2 in 700 Torr of
`helium for power depositions of 200 W/cm3 to 2 MW/Cm3 under
`the influence of 10 KW/cm2 to 100 MW/cm2 photons.
`
`Figure 6 is a graph of predicted efficiency against
`
`photon flux for 1% N2 in 700 Torr of helium for power
`depositions of 200 W/cm3 to 2 MW/cm3 under the influence
`of 10 KW/cm2 to 100 MW/cm2 photons.
`Figure 7 is a schematic, partly sectional, View of
`typical apparatus according to the present invention.
`
`BUREA U
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`

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`we ass/01349
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`PCT/U882/01391
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`12
`
`Figure 8 is a graph of potential energy against
`interatomic distance for Het2351-N2 and He(lls)-N2*(B,v=4)
`after Richardson and Setserzo,
`A
`
`Figure 9 is a block diagram of an experimental
`
`apparatus and a data acquisition and reduction system
`
`used in experiments concerning the present invention.
`
`Figure 10 is a graph of percent absorption at 3889
`
`Angstroms against time for experimental He—N2 plasmas.
`
`Figure 11 is a_graph of emission at 3914 Angstroms
`
`against time shoving stationary afterglow of pulsed E-beam
`
`discharge.
`
`'
`
`Figure 12 is a graph of emission at 3538 Angstroms
`
`against time showing stationary afterglow of pulsed B—beam
`
`.
`
`discharge.
`
`Figure 13 is a simplified schematic view of flowing
`
`afterglow apparatus used in the experiments.
`
`Figure 14 is a spectrogram of the nitrogen product
`emission from about 3000 to 5240 Angstroms for first system
`
`10
`
`15
`
`negative of nitrogen excited by the triplet helium
`metastable.
`
`20
`
`Figure 15 is a spectrogram of positive ion nitrogen
`
`emission from about 3500 to 3600 Angstroms.
`
`Figure 16 is a bar graph of nitrogen emission at 3537,
`3538, and 3539 Angstroms one microsecond into the afterglow.
`Time resolution is 200 nanoseconds, and the statistical
`
`error is about 4 percent.
`
`.CARRYING OUT THE INVENTION
`
`Referring now to Figure 7, typical apparatus
`
`4/
`
`according to the present invention, for providing
`
`stimulated emission of electromagnetic radiation 10,
`
`comprises means ll for containing a mixture 12 including
`
`a first gas,
`
`in the X(l) state, and a second gas,
`
`in the
`
`Y(l) state; means 13 for pumping the first gas to excite
`
`a high population density of its atoms or molecules from
`the X(l) state to the X(2) state; and’means 14 for
`
`injecting photons 15 of suitable frequency to the mixture
`
`25
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`30
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`we 83/01349
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`,
`
`13
`
`12,
`
`to excite, via a three-body radiative collision of
`
`an atom or a molecule of X(2Y with a molecule_of Y(l) and
`
`a photon 15, a high population density of molecules of
`
`the second gas from the Yil) state to the Y(3) state,
`
`followed by a substantially simultaneous return of a
`
`substantial portion of the excited first gas atoms or
`
`molecules to the X(l) state and a substantial depopulation
`
`of the Y(3) state of the second gas, causing the molecules
`thereof to drop to the lower energy Y(2) state,
`thereby
`stimulating the emission from the second gas of two
`
`10
`
`photons 10 at the same wavelength for each absorbed photon
`
`15. and thus providing a total quantity of photon emission
`
`10 with sufficient gain for amplification of electromagnetic
`
`radiation 10, and finally resulting in the depopulation of
`
`15
`
`the molecules in the Y(2) state by autoionization.
`
`The pumping means typically comprises external means
`
`13 for bringing the first gas from the X(l) state to an
`
`excited metastable state X(2).
`
`Such means may comprise
`
`means 13 for bombarding the gas with electrons, which
`
`20
`
`typically comprises external means 13, such as an electron
`
`gun, for generating an electron beam 16 and directing the
`
`beam 16 through a thin foil window 17 to strike the
`
`mixture 12; or it may comprise means 13, such as a fast
`
`burst nuclear reactor, for directing neutrons lB-to
`strike fissionable material, such as 3He added to the gas
`
`25
`
`mixture 12, or solid material 19 placed on the wall 20 of
`
`the container ll, to produce energetic fission product
`
`particles to strike and thereby ionize a portion of the
`mixture 12 and thus to produce electrons that bombard
`
`3O
`
`X(l) atoms or molecules in the mixture 12. Other typical
`
`external means 13 comprises well known electric discharge,
`
`radiation,
`
`thermal,or chemical means.
`
`Typical photon injecting means comprises an external
`
`source 14 of radiation 15 at a frequency substantially
`
`35
`
`resonant with one of the transition frequencies of the
`
`second gas. Typically the external source is a laser 14,
`
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`WO 83/91349
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`mommy/01391
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`14
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`such as a tunable dye laser 14 of power sufficient to
`
`‘;
`
`produce a high density of three-body collisions.
`
`In typical embodiments,
`
`the first gas is helium and
`
`' the second gas is nitrogen.‘
`
`The containing means 11 may comprise a resonant
`
`optical cavity 21 with mirrors 22,23 for injecting some
`
`of the emitted photons 10 back into the mixture 12 to
`
`provide self-sustained oscillation.
`
`’
`
`A typical embodiment of the invention comprises a
`
`direct nuclear pumped laser. As shown in Figure 7,
`
`within the container or laser cell 11 is positioned a
`
`cylinder 26 which is coated on the inner wall 19 with
`
`the source of ionizing radiation, a fissionable material.
`
`The cell ll is typically made of quartz, while the
`
`cylinder 26 typically comprises aluminum, coated either
`
`with boron—lo, uranium-235, or other fissionable material.
`
`The length of the aluminum cyclinders 26 is generally
`
`determined by the dimensions of the source of radiation
`
`to which the coating 19 is subjected.
`
`The cell 11, and
`
`in particular the aluminum cylinder 26,is connected to a
`
`'gas-handling vacuum system 27. such as via a stainless
`
`steel vacuum line 28.
`
`The gas-handling system 27 is used
`
`to introduce the gas 12 into the cylinder 26,
`
`to maintain
`
`the proper pressure of the gas 12, and to evaculate it
`
`when desired. Monitoring of the gas pressure in the cell
`
`ll may be done by an ion gage 29 mounted internally to the
`
`laser cell 11.
`
`The laser cell 11 is placed adjacent to a
`
`source of neutron flux 18 such as a nuclear reactor 13.
`
`m
`
`The laser cell ll is provided with windows 31,32, cut at
`
`either end at Brewster’s angle and is placed between the
`
`mirrors 22 and 23.
`
`The mirror 23 is the output mirror
`
`and, as such, has a reflectivity less than that of the
`
`other mirror 22.
`
`The reflectivities of the mirrors 22,23
`
`should be on the order of 95% or better to prevent losses
`
`,while the power is building up in the‘laser active medium
`
`For example,
`
`the mirror 22 might have a reflectivity of
`
`10
`
`15
`
`20
`
`25
`
`3O
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`WO 83/013439
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`15
`
`99.8% while the mirror 23 might have a reflectivity of
`
`99.5%.
`The apparatus may be operated as a quasi steady—state
`laser or as a pulsed laser.
`In quasi steady-state with a
`
`sufficient neutron flux provided,
`
`the lasing action may
`
`be made continuous. With pulsed operation,the gas is
`
`introduced into the tube 26 by the gas—handling system 27
`
`and, afterward the lasing pulse is outgassed by the
`
`gas—handling system 27.
`
`10
`
`In embodiments of the invention where the particle
`
`source 13 comprises an electron gun or other direct source
`of electrons 16,
`(or where a gas such as 3He is used as
`the fissionable material),
`the coating 19 typically is
`
`omitted.
`
`In embodiments serving as laser amplifiers,the
`
`15
`
`mirrors 22,23 typically are omitted.
`
`In embodiments used
`
`as oscillators,
`
`the external photon source 14 typically
`
`is omitted. Where the laser 14 is employed, a hole 33 is
`
`provided in the mirror 22 for the photons 15 to pass
`
`through.
`A typical method according to the invention, for
`providing stimulated emission of electromagnetic radiation
`10 in a mixture including a first gas, in the X(l) state,
`
`and a second gas,
`
`in the Y(l) state, comprises pumping,
`
`via the electron or neutron source 13,
`
`the first gas to
`
`excite a high population density of its atoms or molecules
`from the X(l) state to the 2(2) state; and injecting, via
`
`20
`
`25
`
`the laser 14, photons of suitable frequency to the
`
`mixture 12,
`
`to excite, via a three-body radiative
`
`collision of an atom or a molecule of X(2) with a
`
`30
`
`molecule of Y(l) and a photon 15, a high population
`
`density of molecules of the second gas from the Y(l) state
`
`to the Y(3) state, followed by a substantially simultaneous
`
`return of a substantial portion of the excited first gas
`
`atoms or molecules to the X(l) state and a substantial
`depopulation of the Y(3) state of the,second gas, causing
`
`35
`
`the molecules thereof to drop to the lower energy Y(2)
`
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`WO 83/01349
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`7
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`16
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`state,
`
`thereby stimulating the emission from the second
`
`gas of two photons in at the same wavelength for each
`absorbed photon 15, and thus providing a total quantity
`
`w
`
`?
`
`of photon emission 10 with sufficient gain for amplifica-
`
`tion of electromagnetic radiation 10, and finally resu