4,780,608
`[11] Patent Number:
`[19]
`United States Patent
`
` Cross et al. [45] Date of Patent: Oct. 25, 1988
`
`
`[75]
`
`Inventors:
`
`[73] Assignee:
`
`[54] LASER SUSTAINED DISCHARGE NOZZLE
`APPARATUS FOR THE PRODUCTION OF
`AN INTENSE BEAM OF HIGH KINETIC
`ENERGY ATOMIC SPECIES
`'
`Jon B. Cross, Santa Fe; David A.
`Creme“ Los Alamos both of N_
`Mex_
`’
`’
`The United States of America as
`represented by the United States
`Department of Energy, Washington,
`D.C.
`[21] A1791‘ NO’: 150’008
`[22] Filed:
`Jan. 26, 1988
`
`~
`
`[63]
`
`Related US, Application Data
`C°“ti““3fi‘?“ °_f 59‘ N°_- 39946! ‘‘‘-“5- 24» 1937v ab”-“'
`gonetlié
`1: adconténuatlon of 5611 N°- 317934’
`’
`an‘
`’ 3 an we ’
`Int. Cl.4 .............................................. B01D 59/44
`[51]
`[52] U.S. Cl. .................................... 250/281; 250/288;
`250/423 P; 376/122
`[58] Field of Search ............. .. 250/281, 282, 287, 288,
`250/423 P; 60/202; 376/103, 104, 122, 144,
`145; 315/ 111.81
`
`[56]
`
`References Cited
`
`Us PATENT DOCUMENTS
`4,365,157 12/1982 Uns61d et a1.
`................. .. 250/423 P
`4,582,997 4/1986 Jacquot ............................. .. 250/425
`,
`_
`Primary Exammer—-Bruce C. Anderson
`Attorney, Agent, or Fz'rrn—Samuel M. Freund; Paul D.
`Gaetjens; Judson R. Hightower
`
`_
`ABSTRACT
`[57]
`Laser sustained discharge apparatus for the production
`of intense beams of high kinetic energy atomic species.
`A portion of the plasma resulting from a laser sustained
`continuous optical discharge which generates energetic
`atomic species from a gaseous source thereof is ex-
`panded through a nozzle into a region of low pressure.
`The expanded plasma contains a significant concentra-
`tion of the high kinetic energy atomic species which
`may be used to investigate the interaction of surfaces
`therewith. In particular, O-atoms having velocities in
`excess of 3.5 km/s can be generated for the purpose of
`studying their interaction with materials in order to
`develop protective materials for spacecraft which are
`exposed to such energetic O-atoms during operation in
`low earth Orbit.
`
`16 Claims, 5 Drawing Sheets
`
`
`
`
`
`
`"7/7///An: I
`
`|.\\\\\.\\
`
`(cid:36)(cid:54)(cid:48)(cid:47)(cid:3)(cid:20)(cid:20)(cid:20)(cid:24)
`
`ASML 1115
`
`

`
`Oct. 25, 1988
`
`Sheet 1 of 5
`
`4,780,608
`
`
`
`US.Patent
`
`

`
`U.S. Patent
`
`Oct. 25, 1988
`
`Sheet 2 of5
`
`’
`
`4,780,608
`
`\\\\\\\\\\\\\\\\»...«\..\\\w/»~w\§
`
`I
`
`IIIII/’.\.
`
`N.9...‘
`
`.w\\.\\\\\_
`
`
`
`

`
`U.S. Patent
`
`Oct. 25, 1933
`
`Sheet 3 of5
`
`4,780,608
`
`1000
`
`800
`
`200400600
`
`ig.3
`
`
`
`FLIGHTTIME,ups)
`
`

`
`U.S. Patent
`
`Oct. 25, 1988
`
`Sheet 4 of5
`
`‘
`
`4,780,608
`
`600
`
`500
`
`400
`
`200300
`
`100
`
`ig.4
`
`
`
`
`
`FLIGHTTIME,t(ps)
`
`

`
`U.S. Patent
`
`Oct. 25, 1988
`
`Sheet 5 of5
`
`4,780,608
`
`500
`
`300400
`
`200
`
`100
`
`
`
`FLIGHTTIME,t(us) ig.5
`
`

`
`1
`
`4,780,608
`
`LASER SUSTAINED DISCHARGE NOZZLE
`APPARATUS FOR THE PRODUCTION OF AN
`INTENSE BEAM OF HIGH KINETIC ENERGY
`ATOMIC SPECIES
`
`This invention is the result of a contract with the
`Department of Energy (Contract No. W-7405-ENG-
`36).
`This is a Continuation of application Ser. No. 089,046
`filed Aug. 24, 1987 abandoned, which is a continuation
`of Ser. No. 817,934 filed Jan. 10, 1986, abandoned.
`
`10
`
`BACKGROUND OF THE INVENTION
`
`The present invention relates generally to a source
`for generating atomic species and more particularly to a
`laser sustained discharge atomic beam nozzle source for
`generating an intense beam of atomic species having
`high kinetic energy.
`Many types of gas discharges are used to excite mate-
`rials for analysis via atomic emission spectrosocopy.
`These discharges are produced by electric fields with a
`range of frequencies: dc arcs (constant fields), ac arcs
`and sparks (1 kHz or less), inductively coupled plasmas
`(20-50 MHz), and microwave induced plasmas (about
`2.5 GHz). All of these sources require some physical
`device to support the discharge: arcs and sparks require
`electrodes, the inductively coupled plasma uses an in-
`duction coil, and microwave plasmas employ a resona-
`tor or waveguide. Recently, free-standing continuous
`discharges have been produced by focusing the output
`of a sufficiently powerful cw-CO2 laser into inert gases,
`molecular gases and mixtures thereof at atmospheric
`pressures or above. The discharge resides near the focus
`of the laser beam independent of any physical support,
`and does not require a gas flow to stabilize the plasma as
`do some sources. Because the discharge is maintained
`by using optical frequencies (30 THz) the plasma is
`called a “continuous optical discharge” (COD). A re-
`view article entitled “Evaluation of the Continuous
`Optical Discharge for Spectrochemical Analysis,” by
`David A. Cremers, Frederick L. Archuleta, and Ronald
`J. Martinez. Spectrochimica Acta 40 B, 665 (1985),
`reivews the characteristics of such discharges as well as
`the contributions to the scientific literature thereon.
`Although cw-laser radiation ca.n maintain the continous
`optical discharge,
`the output power of such light
`sources is generally insufficient to initiate the discharge.
`Consequently, such plasmas can be initiated using con-
`ventional electrode sparks or by the spark produced by
`a focused laser pulse superimposed on the focal volume
`of the cw-laser beam used to maintain the plasma. The
`small spark plasma contains a high density of electrons
`which act as an absorbing center for the cw-laser beam.
`It is believed that at laser frequencies which are typi-
`cally above the plasma frequency, absorption occurs
`mainly via free-free transitions associated with electron-
`ion collisions (inverse Bremsstrahlung). The tempera-
`ture of the continuous optical discharge can approach
`that obtained by sparks (20,000 K. or higher) and is due
`to the penetration of high frequency optical radiation
`into the core of the plasma which is typically of the
`order of 1 mm in diameter. By comparison, at radio and
`microwave frequencies, which are below the plasma
`frequency, plasma heating occurs through direct plas-
`ma-electric field interactions characterized by much
`larger absorption coefficients. Consequently, only the
`outer layers of these plasmas are heated directly by the
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`60
`
`65
`
`2
`electric fields. The higher temperature of the continu-
`ous optical discharge is also related to its greater operat-
`ing energy density compared to more conventional
`discharges.
`The continuous optical discharge technique has been
`combined with a nozzle in “Nozzle Flow In A Laser-
`Heated Hydrogen Rocket,” by Nelson H. Kemp and
`Robert G. Root, J. Spacecraft 16, 65 (1979). Therein the
`authors described the use of a continuous optical dis-
`charge to provide the energy source to heat a working
`fluid which then expands through a nozzle,
`thereby
`producing thrust in the usual manner for space propul-
`sion. That the gas is heated in bulk by the continuous
`optical discharge can be seen by FIG. 2 thereof where
`it is shown that heating is significant approximately 3
`cm from the laser throat, and from page 66, column 2,
`where significant convective losses are discussed. No
`attention is given to maximizing the output velocity of
`the atoms produced thereby but rather to maximizing
`the throughput as is evidenced by the huge laser powers
`under consideration and the large nozzle radii contem-
`plated (10 kW to 5 MW and 0.93 to 20.8 mm, respec-
`tively).
`With the advent of space shuttle flights bearing re-
`trievable specimens, a serious chemical etching of the
`craft’s surfaces has been detected along with a pro-
`nounced glow near the shuttle surfaces exposed normal
`to the direction of flight. The glow and etching have
`been correlated with O-atom density. The intensity of
`O-atoms in low earth orbit is about 1015 O-atoms/s-cmz.
`In order to simulate in-flight conditions in a ground-
`based facility, an intense source (> 10 15 O-atoms/s-cm?)
`of O-atoms having a translational velocity of approxi-
`mately 8 km/s (=5 eV), the velocity of spacecraft in
`low-earth orbit, is needed. Initial modeling of oxygen
`etching of space shuttle surfaces has shown that oxida-
`tion-resistant coatings need to be developed to increase
`the operational lifetime of critical components.
`Data are needed to model the glow and etching phe-
`nomena with the goal of developing such long-lived
`materials useful for spacecraft in low-earth orbit. Pursu-
`ant
`to this goal, useful data to be obtained from a
`ground-based O-atom source would consist of angular
`and recoil-energy distributions for both reflected O-
`atoms and reaction products, velocities of incident 0-
`atoms, and mass spectra to identify reaction products.
`The production of intense high energy,
`low mass
`(<40 amu) beams is technically difficult. The two prin-
`cipal techniques employed prior to the subject inven-
`tion use dc arcs and charge exchange, but have a num-
`ber of disadvantages. Current high intensity dc arc
`beam sources produce beam velocities of up to 4 km/s
`and require 8-12 kW of power input. The disadvantages
`of these devices are the large input energy and cooling
`requirements, instabilities in the arc due to electrode
`erosion, reduced O-atom velocities due to boundary
`layer cooling of the arc, and the high pumping speed
`requirements for the vacuum system due to the required
`high gas loads. Charge exchange sources, by contrast,
`suffer from intensity limitations at energies <l00 eV
`because of space charge defocusing. Although attempts
`to overcome this problem -using electron neutralization
`of the beam have been attempted, such sources are used
`primarily for producing large beam fluxes at energies
`greater than 100 eV. Several radio-frequency discharge
`designs exist for O-atom beam sources, but these pro-
`duce translational energies of less than 1 eV (1.6 km/s).
`
`

`
`3
`
`4,780,608
`
`4
`
`Accordingly, it is an object of the subject invention to
`provide an apparatus for generating a beam of atoms
`having high kinetic energy.
`Another object of the subject invention is to provide
`an apparatus for generating intense beams of atoms
`having high kinetic energy.
`Yet another object of our invention is to provide an
`apparatus for
`investigating the reaction of oxygen
`atoms having high kinetic energy with a target.
`Additional objects, advantages and novel features of
`the invention will be set forth in the description which
`follows, and in part will become apparent
`to those
`skilled in the art upon examination of the following or
`may be learned by the practice of the invention. The
`objects and advantages of the invention may be realized
`and obtained by means of the instrumentalities and com-
`bination particularly pointed out
`in the appended
`claims.
`
`SUMMARY OF THE INVENTION
`
`To achieve the foregoing and other objects, and in
`accordance with the purposes of the present invention,
`as embodied and broadly described herein, the appara-
`tus of this invention includes means for initiating a
`plasma in a gas containing the atomic species to be
`generated either in the form of atoms or molecules
`containing’ solely the atom of interest or molecules con-
`taining other atoms as well, means for locally sustaining
`the plasma, whereby the atoms of interest are produced,
`and means for expanding a portion of the plasma into a
`region of low pressure. Preferably, the means for initiat-
`ing the plasma include a pulsed laser having sufficient
`intentisy to generate a spark in the gas and the means for
`sustaining the plasma include a continuous output laser.
`It is also preferred that the pulsed laser is a pulsed CO2
`laser and the continuous output laser is a continuous
`output CO2 laser.
`In the further aspect of the present invention and in
`accordance with its objects and purposes, the apparatus
`hereof may also include collimating means for forming
`the atoms from the expanded plasma substantially into a
`well-defined beam, and means for analyzing products
`generated from the interaction of the beam of atoms
`with a target.
`Benefits and advantages of our invention include the
`ability to generate an intense beam of atoms having high
`kinetic‘ energy without having to heat a large volume of
`gas to very high temperatures. Only the gas which is
`expanded through the nozzle of the subject invention
`into a region of low pressure is heated. Additionally,
`since well-defined beams of the atoms of interest can be
`generated, investigations of the interaction of surfaces
`with specific atoms or molecules may be investigated by
`mass spectrometric analysis or other means. Long peri-
`ods of operation are obtainable since the interaction of
`the high temperature plasma with the nozzle surfaces
`can be minimized. In addition, the hot plasma is initiated
`and substained using focused laser radiation sources
`independent of any physical device in the region of the
`plasma such as a pair of electrodes which degrade in
`time due to the action of the hot plasma gases and may
`contaminate the atomic beam.
`Moreover, since the cw-laser beam sustaining the
`discharge can be sharply focused, and due to the small
`size of the continuous optical discharge (~l mm diame-
`ter) the plasma can be located in the nozzle very close
`to the low pressure region, thereby, permitting the at-
`tainment of very high translational velocities. A plasma
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`60
`
`65
`
`generated by electrodes, by contrast, can not be formed
`in such a small diameter nozzle.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The accompanying drawings, which are incorpo-
`rated in and form a part of the specification, illustrate
`one embodiment of the present invention and, together
`with the description, serve to explain the principles of
`the invention. In the drawings:
`FIG. 1 is a schematic representation of the laser sus-
`tained discharge nozzle source showing a pulsed TEA
`CO2 initiator laser beam, which is made approximately
`colinear with a cw-CO2 sustainer laser beam, in order to
`initiate the continuous optical discharge. Shown also is
`a movable focusing lens which is adjusted to position
`the discharge in the throat of the nozzle after the contin-
`uous optical discharge is operating stably.
`FIG. 2 shows an expanded schematic representation
`of the nozzle and the movable focusing lens for the
`subject invention.
`FIG. 3 shows xenon time-of-flight spectra obtained
`under two distinct operating conditions. Curve (a) was
`taken with the continuous optical discharge off and the
`cw-CO2 laser off, with 200 torr of xenon and with a
`room temperature nozzle. Curve (b) was obtained under
`the same conditions, but with the continuous optical
`discharge operating under the action of a cw-CO2 laser
`having an output adjusted to 60 W and a ZnSe lens
`positioned to locate the focal volume of the discharge in
`the throat of the nozzle.
`FIG. 4 shows time-of-flight distributions for two rare
`gases (c) argon with the continuous optical discharge
`off and the cw-CO2 laser off; (d) argon with the continu-
`ous optical discharge on and the plasma positioned in
`the throat of the nozzle and (e), neon with the continu-
`ous optical discharge on and the plasma positioned a
`short distance from the throat of the nozzle in the high
`pressure region.
`FIG. 5 shows time-of-flight distributions for O-atoms
`(f) and for 02 (g) with a 40% O2-60% argon mixture.
`The O-atom velocity is approximately 3.5 km/s with
`near 100% dissociation of the O2 into O-atoms as indi-
`cated by a near absence of O2 signal.
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`Reference will now be made in detail to the present
`preferred embodiment of the invention, an example of
`which is illustrated in the accompanying drawings.
`Briefly, our invention includes an apparatus for the
`generation of a high intensity beam of oxygen atoms
`having high kinetic energy. Mixtures of oxygen mole-
`cules and a noble gas are subjected to intense cw-CO2
`laser radiation, and, after a discharge is initiated by
`means of electrodes or a focused, pulsed CO2 laser
`beam, a portion of the resulting continuous plasma is
`expanded through a nozzle into 2. region of low pres-
`sure. One of the unique features of the current invention
`derives from sustaining the continuous optical dis-
`charge in the throat of the hydrodynamic expansion
`nozzle and then skimming the resulting tail flame to
`extract high velocity atom species in the form of an
`atomic beam. Translational
`temperatures as high as
`8,500 K. for a xenon atomic beam have been observed
`with Mach numbers between 5 and 17. Direct spectro-
`scopic measurements on a continuous optical discharge
`produced in xenon show that the plasma is character-
`ized by a temperature of 9,600—l0,400 K. indicating that
`
`

`
`5
`a significant fraction of the energy deposited in the
`discharge by the cw-laser is transferred to the atomic
`beam. This result is in direct contrast to the operation of
`the nozzle device of Kemp and Root, supra, where
`expected energy losses due to convection provide a
`substantial decrease in the amount of energy transferred
`to the hot exhaust gases in their calculated rocket en-
`gine. An O-atom velocity of approximately 3.5 km/s
`has been observed from an argon/oxygen mixture. Cal-
`culations show that a continuous optical discharge pro-
`duced in a helium/oxygen mixture at 8,500 K. would
`yield an O-atom velocity of approximately 8.3 km/s.
`There is evidence that the temperature of a continuous
`optical discharge in helium would exceed 8,500 K.,
`possibly attaining 30,000 K., which would yield a maxi-
`mum O-atom velocity of approximately 15.6 km/s. It
`should be mentioned that the O-atom velocity is ap-
`proximately equal to that of the expanded noble gas for
`small (up to ~40% of O2) amounts of oxygen added to
`the continuous optical discharge of the noble gas, al-
`though some reduction in translational velocity is ob-
`served as the concentration of oxygen is increased. The
`oxygen atoms generated according to the teachings of
`the subject invention are formed into an atomic beam
`and are directed at various targets of interest and the
`reaction products and scattered O-atoms emerging from
`these targets are studied as a function of angle defined
`by the incoming O-atom beam using mass spectrometry.
`It would be obvious to one of ordinary skill in the art
`after having carefully studied the teachings of the sub-
`ject invention, that other gases bearing the atomic spe-
`cies for which it is desired to generate high energy
`atomic beams may be employed. Similarly, other forms
`of plasma initiation may be used. For example, a high
`voltage electric discharge has been shown to be useful
`for this purpose. Its major drawback lies in the rapid
`destruction of such electrodes by the powerful cw-CO2
`laser beam. It is therefore preferred that plasma initia-
`tion and maintenance is best performed free of any
`physical structure. Moreover, lasers other than carbon
`dioxide may be used for the initiation and the sustaining
`of the continous optical discharge plasma. For example,
`a Nd-YAG laser has been used for the initiation step.
`Carbon dioxide lasers have been used since the output
`therefrom is readily absorbed by plasmas and they are
`available with very high power in both pulsed and cw
`operating modes. Moreover, laser heating of a plasma
`via the inverse Bremsstrahlung process varies as A2, so
`that cw-laser sources having shorter wavelengths such
`as Nd:Yag, for example, are absorbed less effectively,
`and would require substantially greater cw-laser output
`power levels to sustain the plasma. The following Table
`lists a representative sample of operating conditions for
`the continuous optical discharge nozzle source using
`different pure gases and gas mixtures.
`Turning now to FIG. 1, light from a high power cw
`laser 10 is focused by lens 12, into a region 14 in an
`appropriate gas mixture in which a continuous optical
`discharge is to be generated. Light from a pulsed laser
`16 is directed by beam mirror 18 though lens 12, by
`which it is likewise focused into substantially the same
`volume 14 as is the light from the cw-laser 10. The
`continuous optical discharge in region 14 is then ex-
`panded through a nozzle 20 having a throat 22 into a
`low pressure region 24 pumped by a forepump 26 and a
`high-throughput, low pressure vacuum pump 28. The
`position of the continuous optical discharge may be
`altered by changing the position of lens 12. Skinner
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`60
`
`65
`
`4,780,608
`
`6
`means 30 collimates the emerging atoms into a well-
`deinfed atomic beam 32 which impinges upon a target
`sample 34 supported by a rotatable sample holder 36.
`Reaction products 38 from the interaction of the ener-
`getic atomic beam 32 and sample 34 and non-reactively
`scattered O-atoms are analyzed by analyzer 40 at vari-
`ous angles, A, defined as the angle between the energetic
`atomic beam 32 and the path 38 of the interaction prod-
`ucts to be analyzed. The flowing gas mixture in which
`the continuous optical discharge is generated is mixed
`using flow controllers 42 and 44.
`
`TABLE
`Typical Operating Parameters for the Laser Sustained
`Continuous Optical Discharge Nozzle Apparatus
`laser
`gas
`power
`pressure
`velocity (species)
`% 02
`
`gas
`(W)
`(Psi)
`(km/5)
`(%)
`Pure Xe
`25
`30
`1.5 (Xe)
`--
`Xe + 02
`65
`30
`1.5 (O-atoms)
`~5
`Pure Ar
`250
`40
`4.2 (Ar)
`—
`Ar + 02
`600
`40
`3.5 (O——atoms)(”)
`50
`Pure Ne
`1300
`120
`6.9 (Ne)
`-—
`
`>13ooHe + 020’) <50 >12o 8.3—15.6 (O-atoms)
`
`
`
`(“Addition of large amounts of 02 to the continuous optical discharge produces a
`slight reduction in the plasma temperature resulting in a lower velocity ofthe atomic
`species.
`( )Predicted, based on work with other inert gases.
`
`FIG. 2 shows an enlarged and simplified view of the
`laser sustained discharge nozzle source. The substan-
`tially colinear pulsed initiating laser radiation and the
`cw sustaining laser radiation 46 impinge upon a ZnSe
`lens 12 and are focused into the continuous optical dis-
`charge region 14. A lens holder 48 is provided to trans-
`late the ZnSe lens along the path of the laser radiation
`in order to position the plasma discharge 14 in the
`throat 22 of the nozzle 20. The plasma is initiated away
`from the throat of the nozzle and then moved into the
`nozzle. O-ring seals 50a, b provide a gas-tight fitting for
`the lens holder in the nozzle enclosure 52. As mentioned
`hereinabove, the plasma may be initiated by means of
`high voltage electrodes placed near to the nozzle. How-
`ever, this method of initiation was found to be unaccept-
`able because of the rapid deterioration of the electrodes
`under the action of the high intensity cw-CO2 laser
`output.
`'
`FIG. 3 shows time-of-flight spectra for xenon gas
`expanded through a nozzle having a diameter of 0.1
`mm. The ordinate P(t) represents the relative intensity
`of the detected atoms. Curve (a) was taken in the ab-
`sence of a continuous optical discharge, with the cw-
`CO2 laser off and with 200 torr of xenon expanded
`though a room temperature nozzle. Time-of-flight
`curve (b) was obtained under the same conditions, but
`with the continuous optical discharge on with'the cw-
`COz laser output adjusted to about 60 W, and the ZnSe
`lens positioned to place the continuous optical dis-
`charge in the throat of the nozzle.
`FIG. 4 shows time-of-flight distributions for two rare
`gases (c) argon with the continuous optical discharge
`off and the cw-CO2 laser off; (d) argon with the continu-
`ous optical discharge on and the plasma positioned in
`the throat of the nozzle and (e), neon with the continu-
`ous optical discharge on and the plasma positioned a
`short distance from the throat of the nozzle. The\veloci-
`ties with the continuous optical discharge operating
`were 4.2 and 6.9 km/s, respectively, for Ar and Ne.
`FIG. 5 shows the time-of-flight distribution of O-
`atoms_(i) with a 40% 02-60% argon mixture. The O-
`
`

`
`7
`atom velocity is approximately 3.5 km/s with near
`100% dissociation of the 02 into O-atoms. It was found
`that 99% of the oxygen molecules were dissociated
`with this mixture of oxygen in argon as may be seen
`from the near absence of a mass 32 peak in curve (g).
`The O-atom flux at a sample surface 20 cm from the
`nozzle and downstream from the skinner means was
`found to be approximately 2X 1017/s-cmz. It was also
`determined that the plasma was more readily initiated in
`the flowing noble gas, with oxygen added subsequently,
`once the plasma was operating stably. However, it is
`likely that the plasma will be initiated more easily for
`oxygen/helium mixtures with the oxygen present.
`Experiments have shown that a small decrease in the
`distance between the continuous optical discharge and
`the nozzle at short distances produces a large increase in
`the effective beam temperature. At the shortest dis-
`tance, the continuous optical discharge could be seen
`protruding through the nozzle into the low pressure
`downstream side of the nozzle. At yet shorter distances,
`the discharge was extinguished due to operation in a
`low pressure region of the nozzle and the inability to
`increase the laser power above a maximum operating
`limit.
`The cw-CO2 laser sustained discharge atomic beam
`nozzle source has produced neon velocities of approxi-
`mately 7 km/s in pure neon gas having beam intensities
`greater than 1017 neon atoms/s-cmz with the continuous
`optical discharge a short distance from the nozzle
`throat. Again the beam intensity was measured 20 cm
`away from the nozzle and downstream from the skim-
`mer means. By adding oxygen to the pure neon gas and
`moving the continuous optical discharge closer to the
`nozzle, it should be possible to produce a high intensity
`beam of O-atoms having a velocity of at least 8 km/s.
`Further, from the time-of-flight information for oxygen-
`xenon and oxygen-argon mixtures and the expected
`high temperature of the helium plasma, it can be pre-
`dicted that the present apparatus can provide O-atom
`velocities between 8.3 and 15.6 km/s for oxygen-helium
`mixtures.
`
`invention provides a
`the subject
`In conclusion,
`method for generating a high intensity beam of high
`kinetic energy atoms by expansion of a portion of a
`plasma through a nozzle into a region of low pressure
`without having to thermally heat the entire gas volume
`behind the nozzle and the entire nozzle itself to ex-
`tremely high temperatures.
`The foregoing description of one preferred embodi-
`ment of the invention has been presented for purposes
`of illustration and description. It is not intended to be
`exhaustive or to limit the invention to the precise form
`disclosed, and obviously many modifications and varia-
`tions are possible in light of the above teaching. For
`example, other gases than oxygen may be used to gener-
`ate atomic beams of other elements of interest. More-
`over, lasers other than carbon dioxide may be used to
`initiate and sustain the continuous optical discharge, the
`choice being related to the absorption of the laser radia-
`tion by the plasma. The embodiment was chosen and
`described in order to best explain the principles of the
`invention and its practical application to thereby enable
`others skilled in the art to best utilize the invention in
`various embodiments and with various modifications as
`are suited to the particular use contemplated. It is in-
`tended that the scope of the invention be defined by the
`claims appended hereto.
`What we claim is:
`
`5
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`60
`
`65
`
`4,780,608
`
`8
`1. Apparatus for generating a beam of atoms having
`high kinetic energy, comprising:
`a. means for initiating a plasma in a gas containing the
`atoms;
`b. nozzle means defining a throat portion for acceler-
`ating and expanding a portion of the plasma into a
`region of low pressure;
`c. continuous output laser means; and
`d. first focusing means for focusing the laser in a
`volume adjacent the nozzle throat to sustain the
`plasma in the focus volume effective to produce
`energetic atoms for accelerating through the noz-
`zle.
`
`2. Apparatus according to claim 1, wherein the
`plasma initiation means includes a pulsed laser and sec-
`ond focusing means for focusing the output of said
`pulsed laser into the gas adjacent the nozzle means for
`generating a spark in the gas from the interaction of the
`focused output of the pulsed laser with the gas.
`3. Apparatus according to claim 1, wherein the con-
`tinuous laser is a continuous output carbon dioxide
`laser.
`
`4. Apparatus according to claim 3, where the gas for
`plasma production comprises oxygen and a noble gas
`selected from the group consisting of helium, neon,
`argon, krypton, and xenon.
`5. Apparatus for investigating the reactions of oxygen
`atoms having high kinetic energy with a target, com-
`prising:
`a. means for initiating a plasma in a gas containing
`oxygen;
`b. nozzle means defining a throat portion for acceler-
`ating and expanding a portion of the plasma into a
`region of low pressure;
`c. continuous output laser means;
`d. first focusing means for focusing the laser in a
`volume adjacent the nozzle throat to sustain the
`plasma in the focus volume effective to produce
`energetic atoms for accelerating through the noz-
`zle;
`e. collimating means for forming the oxygen atoms
`from the expanded plasma into a substantially well-
`defined beam; and
`f. means for analyzing products generated from the
`interaction of the beam of oxygen atoms with the
`target.
`6. Apparatus according to claim 5, wherein the
`plasma initiation means includes a pulsed laser, said
`apparatus further comprising means for focusing the
`output of the pulsed laser adjacent the nozzle means to
`generate a spark in the gas from the interaction of the
`focused output of the pulsed laser with the gas.
`7. Apparatus according to claim 5, wherein the gas in
`which the plasma is produced comprises oxygen and a
`noble gas selected from the group consisting of helium,
`argon; krypton, and xenon.
`8. Apparatus according to claim 7, wherein the prod-
`uct analyzing means includes a mass spectrometer and
`means for measuring time-of-flight.
`9. Apparatus for generating a beam of atoms having
`high kinetic energy, comprising:
`a. means for initiating a plasma in a gas selected from
`the group consisting of helium, neon, argon, kryp-
`ton, and xenon;
`b. nozzle means defining a throat portion for acceler-
`ating and expanding a portion of the plasma into a
`region of low pressure;
`c. continuous output laser means;
`
`

`
`4,780,608
`
`10
`energetic atoms for accelerating through the noz-
`zle;
`e. means for introducing oxygen gas into the plasma
`to produce energetic oxygen atoms;
`f. collimating means for forming the oxygen atoms
`from the expanded plasma into a substantially well-
`defined beam; and
`g. means for analyzing products generated from the
`interaction of the beam of oxygen atoms with the
`target.
`13. Apparatus according to claim 13, wherein the
`plasma initiation means includes a pulsed laser, said
`apparatus further comprising second focusing means for
`focusing the output from the pulsed laser adjacent the
`throat of the nozzle means to generate a spark from the
`interaction of the focused output of the pulsed laser
`with the gas.
`14. Apparatus according to claim 13, wherein the
`continuous output laser is a continuous output carbon
`dioxide laser.
`
`15. Apparatus according to claim 14, wherein the gas
`in which the plasma is produced comprises oxygen and
`a noble gas selected from the group consisting of he-
`lium, neon, argon, krypton, and xenon.
`16. Apparatus according to claim 15, wherein the
`product analyzing means includes a mass spectrometer
`and means for measuring time-of-flight.
`It
`It
`It
`Ii
`It
`
`9
`d. first focusing means for focusing the laser in a
`volume adjacent the nozzle throat to sustain the
`plasma in the focus volume effective to produce
`energetic atoms for accelerating through the noz-
`zle; and
`e. means for introducing a gas containing the atoms
`into the plasma for producing energetic atoms.
`10. Apparatus according to claim 9, wherein the
`plasma initiation means includes a pulsed laser and sec-
`ond focusing means for focusing the output of the
`pulsed laser into the gas adjacent the nozzle for generat-
`ing a spark in the gas from the interaction of the focused
`output of the pulsed laser with the gas.
`11. Apparatus according to claim 9, wherein the con-
`tinuous output laser is a continuous output carbon diox-
`ide laser.
`
`12. Apparatus for investigating the reactions of oxy-
`gen atoms having high kinetic energy with a target,
`comprising:
`a. means for initiating a plasma in a gas selected from
`the group consisting of helium, argon, krypton, and
`xenon;
`
`b. nozzle means defining a throat portion for acceler-
`ating and expanding a portion of the plasma into a
`region of low pressure;
`_c. continuous output laser means;
`d. first focusing means for focusing the laser in a
`volume adjacent the nozzle throat to sustain the
`plasma in the focus volume effective to produce
`
`10
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`60
`
`65