edited
`Lean J. Rafi2£e§ms§< :4
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`:New Maxi-:9 staitge,
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`Chemicai and Laser S<:ier1;:es.‘I3i'v%sion
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`Library of Congress ?(3afa1;_jc_:a’giggéinéifnbiisatian. Data. .,
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`;>j‘§z§éisi¢;a{;, cizexnigzal, am: biological app1ications.fa..aite-:3
`Lasex.§ndu;:e-tiV,;_;1asm-as ::
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`in by any means, eiectronic or mec:h.ani::ai, including phmucopying, microfilming,
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`
`ii
`ii
`
`

`
`Céntents
`
`
`
`iii
`-xi‘
`
`$6
`
`6.}?
`
`69
`
`'35}
`
`72
`
`7.5
`T?
`
`88
`
`92
`
`93
`
`:95
`
`99.
`
`100
`
`101
`
`101
`
`105 M
`
`105
`
`110
`
`
`
`Z’5:it.f<3»§i¥;:<:3;iiz:s:1
`Ciifizsatpifin of a ’P’mpagating flasma
`Absiarptizsn Characteristics sf Heatsd Gases
`Figattzrgs sf 1?:mpagati.n_g Plasmas
`C’311B~}f}i17£1.iE3f1Si<3i1.2i} L:aser~S'u';3pori.ti:<3 ’C9m}:ms1i0n ‘Waves
`C3ne~I3i.m.en’si0naI ’La.ser~Suppcarted Detonzxtican Wave
`*One»I3ir-;1tms.i0’na} Lasensupparte-d Radiation "Wave
`j:’1‘ransi¥t'it3'n Regicms
`Radial Ex;§arz.si:3‘n
`Thermal. Co;up1i.ng.
`523.3. Other Factors‘
`Sumnraxfy
`Reierences‘
`
`»
`
`»
`
`3 Aintroduciitm to Laser Plasma. Diagnostics
`H _ Allan A. Hans; and Hector A. Baldis
`
`?»1
`3.2
`
`Introduction
`
`Introduction to Optical Diagnostics
`
`ix
`
`{Laser-Ind11c=efl..Breaixfimvnz A11-Upsdata.
`
`}§;;{;o:iu»c't:i;1n
`of Eicctrons
`.
`Eieczran €§'r{::§v£E1 in :Ga$e»s
`Lasér~I:a;&.u::.:w;'z.',Eneraizzgmwxtx of Solids ami Liqzms
`.. {fim::»aiu.s:i:iz3g 'Rem_ar_i<s
` Rc:f<~:~ren:¢:es
`
`{!(1€1iI}‘g.1§'f .P?:3s£—Br=eakt3.ox&In Fhennmena
`abexf
`Rn-at
`
`

`
`
`
`
`
`x
`
`0
`
`Contents:
`
`3.3
`
`Intmdumion to X—ray Diagnostics
`References
`_
`p
`
`4
`
`i;;;s’e'§r;S1:st.aine£1 Piasntas
`Dennis R. Keefer
`
`0
`
`4.1
`
`4.2
`4,3
`4.4
`4.5
`
`I’i1t{'0th.§¢2IiiOIi
`
`Principles cf C}pera"ti£3n.
`A31a1’ytic,aI Medfr-.}.s
`Bxperim:en.t:a1 Studies
`Ap;31,icati£3n.s of the Las-e.r~S1:.s£a;ix:ed Piasma.
`Rcfcrances
`V
`b_
`
`5
`
`i’nértiz§liy Confined. Faisinitt
`Robert L. Mcflmry anfi, Jnhn’
`
`’Sou.res
`
`‘
`
`-
`
`A
`
`.
`
`’
`
`131
`161
`
`169
`
`169
`
`1.131
`1182
`189
`196
`203
`
`287
`
`20?
`2-11
`217
`224
`227
`239
`243
`251
`2&0
`
`269
`
`2:69
`276
`283
`290
`291
`
`295
`
`295
`
`296
`302
`306
`309
`313
`318
`
`V
`
`.
`
`5.1 Histaricai Overview
`5.2
`Laser~*Fus’ion Scaiing Laws
`Car-anal Physics
`Xaay ‘Genaraiiien by L;ase'r»1’mdx3ced Plasmas
`Lam-:—'Drivan Aiziafixziii
`i%i}ydr¢:::3y:;a:x11:ic:Stabffity»gf:A¥;>IE¥£i$*¢1y I3:-iszen.:
`.1I‘m::iiaii,::sI1 U'n’.i’formi£y}'§?,r&£3_i:i1:v:‘%:1:?:m}.tS’
`Impiositm Experiments
`Rafemnces
`
`51.5
`
`5.5?
`5.8
`
`6 Lasebflasefil:8’;2.n1ici1n.i311£:t£1r‘fiifiriiziitiiiix
`
`Iaseph "R. ’Wachte’r
`
`Aspe-c:s of Scmicondmztor Fabrication
`15.1
`6.2 Applications cf Lasers in-the Semiconductor I.ndu3t1'y
`13.3
`Research Areas 0
`0
`_
`6.4 Outlook
`Refarences
`
`‘
`
`7 0 S-pectmchemical Anaiysis Using Laser P1asma.Exfiit.at:im1
`L-eon J. Radziemski and David A. Cremers
`
`7.1
`
`Review
`
`-Methods. and Properties of Analysis Using Laser 3?1asrnas
`37.2
`Analysis of Gases
`7.3
`7.4 Analysis of Bulk Liquids
`7.5 Analysis of.Partic1e.s
`7.6 Anaiysisbf Solids
`7.7 Advances in Instrumentation
`
`

`
`X3
`
`321
`
`323
`
`32'?
`
`327
`
`327
`
`331.?
`
`33.5
`
`341
`
`344
`
`345
`
`347
`
`347
`
`35!}
`
`353
`
`363
`
`365
`
`.369
`
`372 %
`376
`
`376
`
`385
`
`385
`
`386
`
`413
`
`§¥¥§€‘fl¥$
`
`'Prcgn::::.si‘s
`References
`
`
`
`’un‘t}a'n2cnia¥;fi:)f 2%u.£z2i’1y$is of Solids by.‘Las-e1?—i’t‘od1zc»etl
`3.:-15:13:13
`
`fang W. Kim
`
`=0}:-a:pt:e'r C!:rganiza'tion
`.I’11:riaii‘ueti0n
`’?ihcno.mcnalag’y cf .Lase1:- Eieatring of Condense..d~Phase
`Yargets»
`
`’t%i_v.c;.3§pe-céftriiascopy
`lhiteiisi y Zvicaswrcments and Ei'e.mcnta1..Anaiysis
`’ .Scm.mary
`Refe:ra.nc:es
`
`Laser Vap£11'i.z2itin11 far S£3’mp1e Inirodzxctmn in Atolni-cimxd
`ems »$13iE:?:tr'£¥$;:§3p:y
`cscph 3.l’I.€d.§3G13,, 133:5: Mitchell, and Nic'ho1as Nogjar
`
`.:£’3cvnvcn_ti1:j;n,3i Scfiid 8an1;;iic intr-oii'uction "fox Atomic
`8pec?t,rcs»ccpy
`Lasa.r.13gi3Iafi‘8n. r;2f’S:::fi:¥ Sampics
`'L:a.ser .A1:i1a'ticn far =Smr11r;%3i?» Iixtriccincticti in Atomic
`S';2ectr:3.:~:.m;>py
`iicizztixrzz Merits’ af Lag-at Abiatian for S‘am‘p2c: Intrcduction
`in Atomic Spectxcscopy
`-
`Laser »SQ1lIC.»{iS for ’Mass Spectrometry
`Appiicatians of Laser ’M;icr=0p'r»obc
`Appl.icatio.ns of Laser De;s»or_pti0n and Postionization
`.=Conc1usia:n
`
`References
`
`
`
`Czxrreni New Applications 0fL21ser Plasmas
`Allan A. Haves‘, David W. Forsfund, Colin J. McKinstrie,
`Justin S; Wark, Philip J. Iiargisc, Jr., Roy A. Hamil, and Joseph
`M. Kinda!
`
`10.1
`
`Introduction
`
`10.2 Applications of Lassr~P‘1asma~Gc.nerated X«Iays and
`Particles
`10.3 Lascr~P1’asma Acceleration of Particles
`
`

`
`xii
`
`_
`
`b
`
`.
`
`=£.3;:m'terat.s
`
`111.4 Laszzm-Pnisvad Power Switching
`Refierences
`
`Index‘
`
`%
`
`-
`
`1»
`
`424
`432
`
`4:37
`
`
`
`

`
`
`
`
`
`Ffiasmas
`
`{mania FL. ifieefer
`Cimter for ,£££h5':er.g§}9pff£:a¥3%2rz5
`Zkzimrisigz .9f'It?é::;2é.-mag »:,_SjIg:«;w:: Izzszimte
`'Yi£Z§::}za:é:’a, Tennessee
`
`4.1 XNTRODUCTION
`
`-first 611:»
`Plasmas created by the .ra::.imian%..fram ;£:;s:m:i .:ass::: beams
`served with the .ac'ivcnt‘ {sf “Egiani ptflsfi’ Q tchezii, ruby» iasexzs by Maker
`
`
`at 213.. (1963). Thm~:e";31Ia§m§is £32-’:’
`hey‘ gas‘ ’b.re:akdown at
`the fo-axis of 3 leans and Weft:
`file :fi1::ration ‘of’ £ha'I’aser
`..
`:3 L
`_
`..
`puisfe’. Plasmas were aim aizservesi in .- arm: {}!1‘£11(3»$T”i1.r‘fa1?}i’éS Gfitzat-eri2i1s:ir~
`radiate-d by ’h=ig’1a-gpzjwer pulsezi or »:<::cx:minu::us iaaesrs ansti “ta 11.i.”iE3j§fig 8.t6 into
`-the in<.:.i£3arj1't ‘beam at .s’t3§:£:mnic
`’Vsz;:;1’;33r”’
`1;:’vt;iI<3;;£{;ies.
`flat’: advent of
`
`:c.m_3jt:inuQ’ns, high-pézmier £2-arbfin
`‘
`i,
`_
`I
`i2i§éB1ii$A,§?BSS;i'i3.§E‘« iii} 5133?-Siifl
`a plasma in a s'teady—stata éc1:1.tiiiticm., ii; *1? i'¥§Jf§ fiwizzs of a.1assr"b¢am, 3.21:3 £116
`first ezxperimcntal observatizm -0? a “egxnzx-muons’=optic=a1 {3.iS{2}mIl'.;g%'” was 1‘.i3:~
`_ported by Generaicw. at .211. {1§7fi). This mntinuons, }aser-sustained vpiasma
`(LS3?) is often raferred to as an coniixzuzms eptical ::§i.sc,1f‘:.a.rge (COD) and it
`has a number of unique prapartias»t11?at make it an ”int::,restir:g c.a‘nd.idat’e. far
`a variety of appiicafions.
`,
`The laser-su.staim:d plasma shares many azharaateristics with mixer gas
`discharges, as explained in detail by Raizer (1980) in his coxnprehensivre. re-
`view, but it is sustained threugh .at3s0rpt’i0n of ‘power from an optical beam
`by the pm.cess.of inverse bremsstrahhmg. Since the optical frequency of the
`sustaining beam is greater than the p1as111aA-frequency, t}1’e‘beamis capable of
`propagating well into the interior ofthc. p1asma.where.iti.s absorbed at high
`intensity near the focus. This is in contrast to plasmas sustained by high»
`frequency electrical fields (n1icmwave and electrodeless discha.rges) that
`operate at frequencies below the plasma frequency and sustain the plasma
`t'hIo'ugh absorption within 21 thin "layer near theplasma surface. This funda-
`'ment21.I difference in the power absorption mechanism makes it possible to
`
`
`
`"i 69
`
`

`
`170
`
`V
`
`Keefer
`
`gonozmw 5’£oa;c1y~sta:o;}iasII3§s,lzaving .n1aximo*m temperatures of 10',€}O'01{ or
`1nt3ro~ii1 a .sma§I soisma zooar’£h.o=foe11sof%akins», far away from any cosnfiizing
`str'ucim*o. A photo of.3.-.p1as:;na. sustained by ;a ‘iasor beam focused with a lens
`8 shown in Fig.
`‘
`"
`’
`S.»
`V
`:
`J
`iasar was
`%
`goo. Fig; 4.:i( .).;s}io's;vs schom
`region.
`”€2o‘::tiin'2oos
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`<2 been ’pr::§£%inced in a variosy of
`isosz. t_iiioXi€ioIase31's.ope'ratihg at
`"
`. 21 25 W to so-vor:a1 k~i1owat:s, Moist
`it _.
`a 12:: ozmosi, o
`air or.ia.rgo. =chamb.sxs’with.
`grovifiafi. by ix-z=a*t‘u:::-ai» ooovoction, our recent‘ oxs
`’ " £3 7o’t,_‘»?s’z;l.:. b’{:3;9%3}, Woéiio max, A{198'?’).,, and =Cros2s»ia11d’
`
`
`
`’
`
`-
`
`.
`
`
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`
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`__
`
`1733. within tho./ssustaining boam, and
`offs
`
`ow:-do. range of conditions »os~
`:o:of£;.
`1 o
`Ls.
`is .<*-.:;:m;1‘£_::::;za;4:i£>.a::s -of 122.5-or gjowor, flow, and optical :configur;a~
`
`
`
`‘
`
`.a-srnall, isolated volume
`‘til? sassaisna giiasroa
`._
`H» aggci,»
`p«or%as21ros..has siiggested 2. somber of
`sons for '£ho>1asor~sos3taioo:d plasma. Since the LSP can
`,§r£3.jr:*ogon and iho» power can. be beamed rem.o_to_1y, it has.
`operate in pox‘
`been prioposeti .t11.a;1: t;ho=co1;z!d ho us-ed for high specifimimpnlso. space
`propx§3.sion:. A nunib-or of ;7a}_3=Br.Si '12ova -deal": with this ap‘p1ic~ati0n_,. and it was
`the Subj’oot of a resisw by Gioiiib andifinior (1984). Thompson et al. (19%)
`described oxpotimonis in whioh "Laser energy was converted into €:1..eotrit;a}'
`‘energy using a -Iasor~.susitainod argon. plasma. Cromers o: 211. (’19f85) have
`suggested tho
`as a source for’ sp-ootr-ochemicai analysis and given siomo
`experimental resulss. Cross and Cremoxis (1986) have sustained plasmas in
`the throat of a .s1.naI1 I1»ozz"io to produce atomic oxygen having a directed
`‘vofocity 0f’soyera1—.k.m/sec for the laboratory study of surface interactions at
`energies and pamicie fluxes similar to those experienced by satellites in’1ow~
`garth orbit. Other applications are sugges-ted by analogy to other plasma
`devices including light sources, piasma‘ chemistry, and materials processing.
`The physical procossos that determine the ‘unique characteristics of the
`LSP will be. discussed in Sec. 4.2, and the the.ore'tica1 analyses that have been
`used to describe the»LSP will be addressed in Sec. 4.3. Expe1iin1ental results
`obtained will be prosontod in Sec. 4.4 and compared with the t’heoretic.a1
`predictions. Sec. 4.5 will consider some possible applications.
`
`

`
`La3?er—$us¥ain‘e-£1 Piasmas
`
`‘
`
`W1
`
`(1?)
`
`(3) Ph-c>t<:igfaph of a plasma sustained by a 600 W carbon dioxide iaser?
`Figure 4.1
`beam focused with a 191mm fecal length lens. (I2) Schematic representatian sh0w~
`ing how the plasma forms within. the focal vtmume.
`'
`
`

`
`“£72
`
`Keefer
`
`4.2 PRINCIPLES Q33‘ -i)}r3’ER.ATi{}N
`
`Plasmas that are c.re.at=ed 01* sustained by lasers can be g=enerated_i_n a variety
`of forms, (lcpeniiingé-<_::n"tl1b £1132-avc:£§;:ls’£ic;s cf the laser and optical ge:3me~
`
`try used ta generate ilaetm;
`' zgi1«’§:n£%f_g§I»’p13§3‘€:'c3.
`,las::.rs can gencrata plasma
`bmakdowrz dirc.{:tly :wi_ilé'i_13 a gas iljrazrfisnlis ix} 3 transient expanding glasma‘
`similar to an explesiortxg
`Image? §a§.fi.r intensiiies and longs: pulstii times,
`plasmas may be}in.iti:att3iia’£ 5:3
`.$uff_3€'€=3
`1311 ‘that; ;3rujpagam’.into the sus»
`'t'ai:zi_ng lzaeaizn €{i».S!i§}t3f3{31?§;{§ val
`itiss as:
`~
`fr-~s::s3;£;izr;x<:s.t3 detm1az;icm (LSD)
`wave or snbsonh: valocifias asia, lgfitsé
`nesd éiéxnbnstifin (LSO) wave.
`These tramient plasmas have Beef:
`sad by Rainier {I-980) anti will not
`
`be treated here,» If thie: iiaseris’
`»
`~e_'1‘a1:xv.:l i§”31€*#,»£3§[f§{tf1i5E1.3b
`:w:z;:-
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`lgeomatry, flaw, BI1£.TI’§§fi3s§Si13€T€«’aX§§§
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`be c<3n’ti_m10usly'm3§fitaifi.§:§:at‘313313531”
`11.;:.2t‘1"tI2aj_ was cf the 'bea.m. The
`inI.fens.ity that is axzailaiziliz iffimiz abfitlfltiixuoflallasslr -is insufficient to cause
`braakdtnwn in the gas, }’l1€§‘£V-€.=’?E:I‘, and an .auXiIiéa:'_y ;<'>¢;m.rc8 must be useivd. to ini-
`tiate theplasma. A sketch of 5: steady-s'tat‘.r;.la.se.r~sus’tai'ncd plasma is shown
`in Fig. 4.’i£(§). 'I.‘h;<:. _plas'm1a’~ .-m.ay"be» szz.stai;:ie£3
`Ia cmnfining chamber £6
`control the flow‘ and pr:-iizssixre or in -apex}. air wt £1 Iarg-e, chamber where the
`flow 3,5 ldete-rm-i.ne;i by thgrmm i:3u»z;3_yancy;.
`V
`,.
`In many ways, tl1£=;3a£er~s‘u;siain;3d. glasma is si’z.3:xi}.ar its direct current 0.1’
`lowfiaqtiancy -eiec;r§;}::I{;l;3*3
`anii iI1.i£i£i:3xv;avg.dilsc11m°gxi:s’tl§.at are {apex-
`.
`ated in similar .gjase-s an
`at p:a=.:ssm;:€=s. Hmyever, tits 11.33? will gamer»
`ally be; xnmte mm act: 313.6 mzaga lfiigher:maximum.te1npe:aturc: t21.a:x.n-t3ze:
`»§:.£3ntin120u.s arc s::i'm*.ce§ aizdi
`biz: :s’u::'t;;21:lI1’€:éi in it stavady sztatc well away from.
`coniaining boundaries. A fin:x}.£3afi18f£iir35I Cliiffilffintfi: in the way inwl.1ich en-
`-ergy is abscrbed by the plasma is re.s;30fl§’£fi3la .fs0_r ‘£31633-. xxiizique» charactizristics
`-of the LSF.
`-
`
`
`
`‘
`
`4.2.1 Easic Physical Prncesses
`
`In a £ili1"(3£:t current (dc) arc or in an intiulctively coupled plasma ‘(.ICI’), en-
`ergy is 'ab.s0rbecl thr0ugl1 ohmic .heating produced by. the l0s.v~fmc_;uer1cy.0r’
`direct currents flowing in the plasma. The electrical conductivity of an itleal
`plasma is. given by(Shka’1t0fsky at 211., 1966)
`
`J
`
`neg
`= M
`
`1/~—~iw
`
`in (z22+w3)
`
`4.1
`
`(
`
`)
`
`where n is thfl electron’ density, 8 the electronic charge, m the electron mass,
`to the radian frequency of the applied electric field,‘ :2 the effective collision
`frequency for electrons, andi thesquarc root of —~»‘l. In the do are (as 2 0),
`the currents are transmitted th1‘0u.gh the plasma béztween electrodes and
`
`

`
`Lasebfiustalned Plasmas
`
`'
`
`-.
`
`A 173
`
`the size of the plasma is determined by the ‘size and spacing of the alvactrocle
`and the c-:mfi.r:in:g boundaries.
`In the ICP, the currents are i.nduce€:i.
`inta
`the plasma fmm alzernaiing »cur,rents ficnwirxg in a surro:undi.ng .so1enoidal
`
`<:;c3ii.. The are is sag;
`inmgl
`' "thin a container that deztermines the plasma
`dian1eter, ‘whereas fine Ian
`of the "plasma
`determined by the length (iii
`U
`the sblenaid.
`'I‘l1€*-.,I'{T3P apergtes 2;; frgguencies wfsll belflw the Vplasma frequency
`
`p
`
`.........
`
`_» 1;’?
`
`::..—~
`
`W?
`
`4
`
`.
`
`7
`
`
`
`9
`p
`< >
`~4.,2
`
`whe1'e..¢9 i.sé-fin: ppiérmittiviiijg ef fi?ae~$pa<:e:.. In this frequemiy range, the 616:8-
`irama:g‘na1ie.’fieljdT, Clims inoi ’pr¢a{p'.a1gata gss a ‘wave. xvit’hi‘n the ;11.agma,’bnt*is
`.gg:~;¢;;n;x_ate;i, agan ¢”a.ne$i;enti»wave (Holt and Hask-en, 1965) aver dis-tagnces
`
`(‘#3)
`
`is iaihespe-e:i;of1ighz:.» ’l§hu.s, tfmaplasma is su.s~tai.ne::ii by e:w;,rgy»Aab~
`-whet-3»
`scsrbeti within a_.small..ia;,:.e.r near its miter surface. that preduccs‘ a r‘athm*:flalt
`temperature ’pr;afi§l;e w“'“
`thgpllavsma and llimits the maximum tzmgaera.
`t11r&:$ that ::a:i“be<3§3taines3.,
`V
`-
`The freq-ugncy of the pptieaz fields (23 'I‘_¥~Iz for the 1&6 gm ca;r'b*c>n_. éiax»
`ide Zascr} nsgedlbr-{ha
`is greater than the plasma frequency, and there~
`fare the .i'nei’d’en'£ laser. ¥:>ea.m nan -propagate well into tl1el'nterim‘ before
`it is s:i,g£xifi<3anft}l,y »a.;bs-Qrbtzd’ ’t}n;‘QiIg.h the _pIOG6SS Of .invarse ‘nremsstralxluzlg
`(Shkarofsky at 211., 1966)‘ Sine-e the facusi'ng‘ of the law: beam pmducsd
`by a lens or ‘mirmr is essentially preserved as the "beam propagates into the
`plasma, very largeJfie}z:l. strengths may beproduced within the plasma near
`the beam focus. It is £12833 large field strengths that lead ta peak tcinperas
`tures in the LS1’ that are geneirally greater than those obtained with either»
`dc arcs or the K31’ and make it possible to sustain a small Volume of plasmla
`lnear the focus, ‘wel'l away from any confining walls.
`Inverse bremsst.1'ahlung— is a process in which the plasma electrons ab~
`sorb photons from the laser 1363111 during inelastic collisions with ions, neuw
`trals, and othsr electrons. The collisions betwew electrons and ions are
`‘the _domlnant process for the LSP and the absorption coefficient is given by
`(Shkamfsky et al., 1966)
`
`W
`
`“‘
`
`(La)
`
`ICT
`
`(
`
`we 3n.S.0G 1~e'”“’/"T
`.l
`-.
`
`Fzw/kT
`
`>
`
`V
`1.4.
`
`O )
`
`

`
`374
`
`’
`
`,§(e»efe»r
`
`where 5. is P1anck’s canstzmt divided by 271*, k B0i§zm.an.:1’s c»£3n»S,iaIifs 8I1€3;T
`-the temgm.r.amre of the electrons. ‘T113 faster G is the Gaugnt factor 322:! the
`Eactor 3:59 ‘isngiven by
`
`
`A. 3
`3
`16;.
`.22
`
`. L‘
`.
`.
`.3133 "~'-’—" *“** nflfin
`3 H1303’
`.47n°.9
`
`112
`
`I
`3kT_.j
`
`.
`
`‘
`
`I
`
`(4-5).
`’
`
`facifliis. a.
`T where 2,’ is the ionic cha:rge and n,;,. the ion -density. The
`mec1;anic»ai£ currantinn to the -::.1as.si.c.ai theary,-_’an£i axiensive téixhies
`have} been given by Karzas and Latter
`Fiifif f
`‘
`"
`"
`"
`the phntnn energy is ’mm.:.h lass than than
`’
`bracketmzi term in
`(4.4) is m%:a:ri}y .in<3e-nail \.
`
`c.n=effit*:ient:"is ’e:ssen’t§aIiy *pi?(}p’D2‘ti£3Iia51 "tn nu: sqnnm. :3.
`.
` I
`sang’ "ha ‘beam.
`_
`.
`size (313 the
`win Ciepend on sencrai féc-‘ms
`g:;n.metry,,1a5er*pmver-, and n}}sarp.ti»n’n cnefliciant. *
`nf. the-..1aser ‘imam as is; prapagratas ~.~2i:thin the
`;i:9gi-van by e€:r"s.1z.inr
`
`
`
`til
`
`_
`
`“*"*‘ “iii
`
`A
`
`..
`
`(.42-E5)
`
`?.EE3‘
`: ai>sa:n~~
`’
`- where s is the: distance along the iocai di.tex;z.iczn nf ‘pin
`.
`tizanifiength lfnr is a .d0minant».iengxh scaia f:c:r’t.}'?as‘3.
`,,
`. 288 it ,ci.ntér’.
`jnas .
`‘the’ r:§'is_t;ance.-oyggr whigan the’p»nwnz.,is absGx*beéi fr-pm {?h’e“b¥3{am., Ft3r'tFhis’}-."=na’r
`2:
`:59-n,»t"n.¢ dimension nftna nigimamparaznre ;é2£¥2sn:éb§i:z_.‘
`"<2:
`fiifasma
`
`alnng the laser b.3am'wi1i in-, of the nrder’ n£ ;h:¢;»a¥3snr;):t:»nn inngiiz. Aiihnfiigh
`it is {he n'bsorption3.engx11 that determines ‘tlixfsilangtii inf-thn»:p}a.s£na»a1:nn;g the V
`Imafm fixigs, :1. is the kzser beam cI.iameter_1;hat. dnierxniizxes HIE: gafasma ’di33.I !LE-’¢'
`tar; ’I‘i1e’;31zisn1a expands to fl}! the begin ;:n.n’e .néh”ti:r:e it
`an}: in absmb
`pawns, "then rapifiiy c¥az;1‘e=a:ses in teniparaiurit m:tsi.<§e tines: ’?:se.am ti1mu1_gTh
`thertznai <:Q.nd11cti0na.nd radiative: kiss .gnachan.isms..
`The pnsition‘ 9f the
`‘relative to the facai paint is £:1‘iiic.a1'in deierminw
`ing its ntructure andfiie range of para.mcters fnr whicih it {tan be .maiI’3taine€I.
`When the plasma is initiated near the beam foam, it prnpagatea into. the
`sustaining beam and seeks a stable. _pnsition. The pnsitidn Bf stabiiity will be
`1ocat»e»d‘where the imam intensity is ‘just stnjfficinnt that~.1:.ha ‘abs+:n“ba»x;i power
`will baiance thclosses due to convection, thermal canfiuctinn, and.therm.a1
`radiation. A _number of factors cmnbine to determine this pnsiticm of sta-
`'bi1iiy.inc1nding the transverse pmfile of the incident beam, t.hefoca1.leng-th
`and aberrations of the focusing lens or mirror, the piasma bpressura, and the
`incident flow velocity (Keefer at 211., 1986; Walla at 211., 1987).
`The power per unit volume that is absorbedby the plasma is given by
`
`
`
`P == cu’
`
`V
`
`(4.7)
`
`

`
`Laeer~$usta’ined Plasmas‘
`
`3?5
`
`where I is the local irzadiaznee of the laser beam. Since I depends on -the
`transverse profile of the "incident beam a’sIwe¥.I as ‘the focal length and aber~
`’rations of the lens, these -chamet-erisiies will .:influenc-e £i1e’Ioca’ti0n within
`
`the focal region at
`the.
`inimm:n.sus1:ai.ni::;gi’n.tensity is located. For
`example, for a small flnnmbexh '“n;s,. the.::i:z2:ensitg; decreases rapidly with hp
`cre.asin_g .dist.arfxce fro::'n..1l’1fie»’focus;’an€¥»the p.ia‘$nf2.a will .s’1;abi.iize:near the focus.
`‘For -a larger effnumber system., ii1;s,i':».,i.:3'e=:1$?i£y’ tiecreaees 138$ rapidly and the
`plasma will ;stabi.1i2e .a£.2§:-position fill,
`awe:
`$6.. ..
`oezzfi. I’:i<’3‘e=ez¥., for
`
`-
`gszgzifieienily 1ofxgf’*foc.aI.1e13§glEh§f :an;t.;1 be
`s:
`p=oWer3 §z1ae:nas..h:eve’be=en 0b~
`’
`’
`”
`'
`’
`v2§aeier1;.i1fi§3£})»a,s “’I.a:seIfzs;;pp;Q2*tec3 com-
`
`
`
`
`
`,
`b
`’
`:
`The. detaiieazi ;~r;;>.a.t.i,,2::.l
`’la5tfons betweendme opiieaigésgemg, 9.
`
`'
`
`”
`
`sizitaiis »3§,§’14E*.e.t*€$1?I2:i:§1fi1l“.:s:_17'¥._3r§It.1I1%:'i.nterxe~
`’
`’
`-
`.2. m‘g»ee-.-_,.§. epressefeief
`
`convection, condu-ction,
`1 11:’
`» 1
`
`and thermal radiation; '.F§.1e poo.
`’
`"
`63.133 zteia’
`E: tcH;13e.fo-cal point
`’ :§sx2e 1.
`at. which the plasma stabiliz ’
`
`a s::ah.Ie -;:‘iaxm:a
`a.nd.flow .fT«:3f.r.
`Most of theeairiy exp moo
`chambers or in f,:pen»air,w?E1efe»t11’ " xii.
`by the effects of thermal buoy
`‘
`
`the pr.esear:e .and laser
`.
`pressure w11.er;:»i:»was I}fi3Esib}g’tG .
`- These. -experiments
`.
`.
`eraiov et »a1., 1972;; ’Ko.zlo‘v ee a‘{., 1494:
`f¥31r”l:z£>3:%h5};z1serl}:§osver and
`vindicated that there were np}::»’er ami loxé.-tea‘
`.
`.
`pressure at which the LS? =eoi:}€i be silfitixifxeéié
`Generalov et al. (1972) suggested that the u}1pe,2*l.imi,t.for p-owe’: was a re-
`sult of forming the LS? with‘ a hori'zonta1b.ea.m. In this geomet:ry, thermal.
`buoyancy ‘induces a flow iransvers-e to the =o}§t::ieal axis. The imiuced flow
`carries, the plasma up and out of the beam 'w§1en.h.iglf;er laser poxver causes
`the plasma to stabilize farther from the focus. They were u‘nab1.eyto estab-
`lish an upper power limit when the experiment was operated xzrith the beam '
`propagating vverticallyl upward. Kozlov et_;a'1. (1974) deyeloped a [radiative
`model for the LS? and explainerd the upper power limit on the basis that
`V the plasma must stabilize close enough to-the ‘focal point. that the geomet~
`ric increase of laser beazn-i1}'ten.sity going. into the plasma was greater than
`the loss of intensity due to absorpfi-on. They speculated that the failure of
`Generalov et al. (1972) to observe this limit in a vertical beam was due to
`rapid extinction and reignition of the plasma.
`It is clear from the experiments of Generalov et a1. (1972) that flow can
`have a large effect on the range of Apxfessure and "laser power that will support
`
`

`
`
`
`me L-SP cs:;::§d}_2»e susta.i-nan are val
`._
`_»
`b
`oinIy:.£;:u‘ the gmiicziiar.-expnrimsntal’ gjeomotry»»nsen to obtain them.
`
`
`
`when {ha pins
`iizb ‘ iasma -
`
`
`
`
`{bait :t}1;&’3¥6i' absn:rb:e=d from £335. beam, given byis balanced.-’
`fi1Bic£¥n1?I3Cii?§’F£§', con§’nc'§i9w,» .a‘nd rafiiation Ioiss-es. -Sinoes, in generai, "the ‘in,
`
`
`' _ T, the 1333222,. the plasma. will a.<:ijust.._;i‘z"2 size, shag
`
`
`
`
`
`
`a £331.31-:’:.. .9? bonnrbhsbzunii fiansitinns,» msnitin-Vg in lino rac}ia»£inn and absnr
`tion, =an£1;fr£:c:e?ii:onn£i amzi free-afrae transfisionsbthat resin}: in .cz3'n£:in'nuzn rad}
`3’iiQI}’-£{X1§.‘&}3S{}3f13I
`0%; §I1e.opzicaI§_y thinportion of the spnctrnm, ‘chi
`rmi.1.annn xvii: : Sfibxigiy ;i?::$i:rbaii by the plasma or sn1‘r0i3fidin§,¥ niééim.
`regions and wii} fiimpiy swap»: from the plasma. {Ether Iportions of tho sp€:C~
`trnm will
`sirongiy absorbed, resulting in a transport of energy within thxi
`plasma. In the oniiizaiiy thick1'i1niI:,_this resu.1‘ts;in a diffusive energy trans“
`port that is _-similar to thermal conciucnoxz, but may be signifmantiy larger.
`Detailed ca1cui.ations. of the LS? (Ieng and Keefer, 1986) indicate that this
`raé3.ia*tive transport is a donxinant factor in the determination of the strut:
`turn and posi.t.io.n of the LSP. In particular, it is the radiative transport that
`’ determines the ternperature gmdiont in the upstream .front of thcplasma,
`thereby determining the position in the beam for which convection losses
`are balanced by -absorption.
`The position of stability for the LSP also depends on the plasma px'eS~
`sure, The absorption coefficientis a strong function of plasma density, as
`soon from Eq. .(4.4).. If the pressure is incroased and the absorption <;oe:ffi«
`cient increases, then the plasma can absorb more power from the beam and
`will move away from the focus to a lower intensity region in the beam. At the
`
`
`
`b
`:
`
`‘W6
`
`Keefer
`
`Fiastxnas sust-a.i.nsd in the free jot issuing from a nozzle have
`a stabio
`bean studied by Gerasimenko at 31. (198.3) who n1ea.sured the discharge
`*2vn’vn wincigy nZo:1,g ma bean: andxanbges for the existencn bf a steaciybtate’
`
`d;‘is's»;:h
`..Rn;:::$n£E’»
`’
`}3§.*’;{1tS’hi'1_Vs‘3 been noncixxcted in confined tubes
`
`=s.vhare=»£.0i1:ced nnnvaction :1 :ni'nat€-,d, {be flow (Wane et a1.,
`:£9{87_). It was
`.:f01unr?: ihaifi-n addition. to power and ;3rass11r:a, both the flow and optibcai ge-
`"
`”
`' avg’-:2:’pr0fQz3ndinflnen'c-e on the <:'I1aractt31'isiicsVofti1n =
`
`
`
`
`
`

`
`
`
`§;,=a»s.er~'S'usta’¥ne»d Ptasmas
`
`' 1??
`
`861% '’£i1116. the :p.iasrz1a:3ength aiong the .beam deer-.a.a-ses b.ez:aus-e {‘.!f the. de:~«
`£;1‘£:as’e in absarptimx Iength, 13111 the d.iamet=e.r i.n»creasa3 ta iii} the .Im*ger :c.m;s’s
`secti;3.n 0f the beam, Thus, far the s.ame1ase.r beam stars:-niititizis, a ihig?he.r-
`
`px‘t:ssuir?e
`wii} stzififiizc
`a, 'po.i_nt farther away frmn the ma: gixaint arzci
`have 3zsmailer.}engt11~'1o~C%1am;:1er ratio ’tha.rz 3 }-msv;:r:pr£:ssu.rc
`.
`,1-z’1cide21tiase’r pzpwer, as wail as t:i1ef/nuznber and absrraiirsms cf the fez)
`Qusixlg c3pt;ies,» will also .ir’:.fiuenc»e the ;3;3si,£iz3n. at which the
`St:ab.i}izes
`iihe .2bsa’m., Fmm {me 'f0r:agoi1:1g <3ii5£:us.£i€31}., it is clear {hat as the}:-e.a.m
`paw: is increassgiv, the piasma xvii! mme up t11e’bean1 away ifmm the fiizezal
`iyciintg. Thia distsance that it mmzes. -is tisterminegi by ‘the .f1n.umb=:;£ (zajtia mi
`
`‘ ”
`’
`‘
`is in tiéhe: 33€:a::I1.Ciii£ti1x€t'£e2f:i'I1£:i£i81:1tb£3:1fllfiffi-izizfiizfig
`”
`”
`.
`nee vtjhe rate:»of‘,ch,a11ge»~in beam 3?nte’i15‘i’i§'
`.
`_-
`mi
`
`:22 at: incmasein ffnumbe-r; Lens ai:er;ra§:ions:.can-.aisc>..§1arska
`axis {:1
`Va _e:£:.é‘t c;f11p}asma p=o-aitkan (iieefmf at 31., 198$}. Tn part;icu13r, Wfh:fi1'ifii1 an-~
`2i1Iia't Eaea-:2; frem a’n.n.nst;.abie1a¢ser <>:si:.i*_£3at:>r
`:5Ei}s:u$:€:g€§
`133:3 sghericai Ivans,
`'it..pr£:§-Ch:-ccs an am’nu’iar’pmfQc,us region. befimr reachirxgfhéfm1aI.p?t3i.Iz£;, and
`_ the
`’
`t_§i1i$ ragion 'n1.ay’136»su.ffi:£:im1‘i’to »sz3.st.aiz1 an zmI1uiar" 2131.3;
`
`».::1;£ize: z:»'bs;é:rvat:iAn.:1.s :c}is<msscd. above, it is cigar ti1.ai’£h*a;$%:>sii»i£§I: 0f
`the: plasma reélaiive in ‘the £003} paint has a: ;)}:‘=Gf£}1i}:1d»E§ff{i3{fi’(}1‘i’ii1f3fp1§}8ma
`risticsi. Aime ’upjpi:f.1im.iis Qf:at3%t*.i¥i2y fgr bath i1.asr:r= -ptczwer ml»
`,;;ha.ra:.--tie
`
`@173
`V
`a}i§§’=i§€£r's
`that {ha piasn1.a fiecizmes xxzmzible whan it ixnavasvma
`far’fm?11_1;
`fi>~Ga¥. paint. This may be due to the fat-‘xi, as ggreposedihyfbiitfizfiizv
`at 32. :(3‘§§?¥-Q, ihmas ‘iihepiasma .maves sufiicifen£1y’f.ar‘a’wa3?
`fiiia-£’.:u:s:..,
`i}§e- ztaigte af infiease of the beam .ime;nxity in ihsa :iir_e.céti<2h a:.f‘2;::c>pagi1atie£n
`’ zsyim-s sma.}Ier. 8’ir:c.;e the. temperexture €if't}1€f
`pi.a:srr:a .m::,st 3 "
`ease.-tax:-ttxe
`
`hr-mm grgipiagates into the upgsmzarzi edge itixietpj-’Iasmai_ .:ifiiensi:y*I0f-the
`’bt:am’.mnst 3715:: increase. At some paint, ii1e;dmsre.as’e -of i}za.’b»eam iI1.;tz3n,s.iiy
`due-ti} »abé;f:§rpti0n is greater than the incrieéases due in focfisizig, Sc! the piasnaa
`Esecomeéa xmstabie and extingui-s'hcs. Recent <:.a1cn}atit:ms by Jéng. gm :1, Keefmr
`(198?a},i1mvav€.r,
`indicate: that there may exist 1.00211 're:gi£3-213. =withi2: {ha LS?
`where .the b-eam intensity c§:1cre.asAes'as it penetraies the piiasma.
`. Aconsidarabie xziegtee of control of the structure and position Bf ihe:’LSP
`can be gained though both optical geometry and flow, in Vadditian ft) .1353:
`p-mver anti. ’prcss;ure. Utilization of thss-e additional parameters maisze-.5 ‘it,
`p’0ssib1t:’to successfuliy 0pc:ra'te the LSP ever a wider range of exptzriniemtal
`conesiiiierzs, enabling a wider range of potential appiications.
`
`
`
`'
`
`13.2.2. Flasnux Characteristics
`
`Lasemustained plasmas have been operated in 21 variety of molecular and
`rare gases at pressures from 1 to more than 200 atm. The resulting plasmas
`have characteristics that are similar to are plasmas operated at similar pres-
`
`

`
`113
`
`-
`
`-Keefer
`
`are usualiy somewhat higher
`sures, but the peak tampcraiures in the
`than ‘these for the comparable am, Ratiimon "from the plasma can be a Sig-
`nificant fI‘actioI1 of the total! paws: ,iz?:pI.1.2, a:n.ci .r21:3.ia£ion 1:ransp.r;:2rt plays 2.-
`major £0.13 in £:‘etez:minin_ ghe structure of tha plasma. Continuum —abs0r-p—
`{ion processes are ofyam?-tzifziar,1’3n“zp0.1*ta:zce in these plasmas since thepower
`to sustain the piasma is aiasarbazi -through, these. me-cham‘-sms,
`The» .caniin1mm ahsargzzizan pxtmrzss =i;:sz:31w;s both bmzn.d.~fxe»e trans.itio1}.s
`('p}1»0t0iDnizatit}n) and fi'.6t3».f£'efi transiftions (invarsa brfimsstrahimig) in
`whirsfh phmxmjs are absorbad. frram the»»3.a4sa.r1?::eam. The .fr<w-free. transitions
`inmiva =~‘:}ec&;roz1 caiiisicsns with Eicms, other éeiectrizéns, and.neutr:a1 particles
`gfihikarafsicy 2: .31., 3;’9‘§S-;;; £3r.i:e:m, ’19t54)..
`:éi9mi'nan:t absarption process
`
`for the: };S?’is £1:-:s:ugh ;m'iiiaion.§ ihcéimen
`.
`.. as a-nciiens, .ar1d‘t,.ha a_5smp~
`-tiozx ccmfiicient far this pxocrasa: is» given by Big.
`For the usual case in
`the 15?, km <<::;ic'1’~and the ~a'?bs£33.fpii<:;I1’ is; appraximatifiy‘ j;3m;3ortiana} to the
`sq’na’r=::-*, of the 1as.€.r- xvavgisxzgtiigg
`to
`’$irs3z:g,'xvaveier;=gtIa tiepsnéerigms,
`mi. «sf the. r;6'§3=$:rté:£§. e?£P€:i.m.:an'ta1 xii-:;s';'i3I},;tsf£}i*’ thfi LS,P”hav'a» been ebtainad us«
`ing thfi 10.6 mm w.avek°,;ngfl1 carbon ciiflxiiie laser. Si-nae the Iength 592136
`for the plasma is sf the ’:::xrder of {he»absorpt:i<3n,1enfgtIh;, the length ref ‘the
`bpiasma and .1113 ptiw-er .rc:q§::k-ed tD’sus'ta1”31i.tw0n1d be axpeetcd in increase
`dramaticaiiy .for'sh::::rtjer xsiiair-e1;::1:gth_ lasers Cmjgrgntiy, -thé anfiiy other iasezrs:
`-ziiat are fikeiy eanciidams ta suszaiz: £;ti1I;t1iz'21§€n:s1 g:1ajs::2as¢-aria» the iiyfirsgen
`€11‘ denteri'umfi1u:«:rid.e ahamicai iasers mat Qperate at waveiengzjhs (if 3’ to
`4 ;.sm..
`'
`'
`'
`.
`Tfismzal radi;atiQ13’.is’i§i1l6 czi thfi :rxI£3s:.’.i_x:;pi3:’ta:x;t c'ha:;acteris:ics of the
`LS1’. Thezmai -rad_.i.a1:io1n I0s.t.ffG2i£1.’£iif:~;’3IaF:§'ti:a can acccmnt fer neariiy :31:
`the ;;=aw::r absaribed by tiihe 'p3’as1fna wizcn thce flow :1’1.ro_ug?h the piasma is
`smaii ant}. viii} amount for :a s.ig1.2,if‘want f.mciie'n of absorbed power even
`when the canvectiva. 353.8335 are-’ large. The thermai :rad.iatican. consists of
`continuum radiation resuiting from _rec0xI£binati0n (free.-‘bound transitions)
`and bramsstrahhxng (free-fr-ea tr-ansitions’) as well as line radiation (’boun<i~
`buund transitiims). Caicuiazian of this radi.atic:n is straightforward, aI~
`though rather tetiieus, when the plasma is in 10031 thermodynamic equ§~
`librimm QLTE) ‘{Griem, 1964).
`‘Local thermeadynamie equiiibriurm is as»
`tablished when the eiectmn collisional rate proccsses dominate the pro-
`cesses of radiative decay and recembination. When LTE is e~st.ab.1ished
`in the plasma, the density -in specific quantum states is the same as a sys~
`team in complete thermal -equilibrium having the same total density, tern»
`perature, anti chemica1v.c_ompositi0n.
`It should be amphasjzecl that this
`dues not imply that the radiation is similar ta a biackbody at the plasma
`temperature. In general, thus spectrum of the radiaticm from the plasma
`will have a complex structure: consisting of the superposition of relatively
`narrow spectral lines and a continuum having a complex. spectral struc-
`ture.
`‘
`
`

`
`s.ss
`S55
`
`§ ,
`
`Laser-Sustaine-ti Piaamas
`
`179
`
`
`
`.
`
`_
`
`The absorption: c-oc1ficient.in tho plasma‘ depends on the wavcicngtli, and
`for thc o'i:r.avi.o1ci."portion of tho spec'tr’um below the wavelength of tho rcso»
`nance’Iin.cs (-trxansiti
`:13 in ‘wing the groom state), the r.ac:’iia2_io.n is si'mrLg.iy
`
`absoirocd by 233:: pk:
`’
`_
`the coo1cr.su.:roxmdi.ng gas. This rcsnits in a
`szrozzjg raciiativc 'ira,os{3ort”'mecha’nis’m that is imgortant in :?:etcm1.i.o;in_g the
`st:ru.c.:it:i::'e ’of“t]he p§asma.. Qitr.-an, 'rad;iative tr-ansport for strongly .absor3:in_g
`gases is :moda:%§.cd as a -ziiffosivc energy trans-port simiiar to thc.nna1..conduc~
`izion.
`£}1o’.3£rooIg’iy ioni2ed.rcg§on5 -o:f"thc piasma, the r.adiativc 1r;ansport_.is.
`many fimos larger moo the ’i=mr.ins§c ihcrmzii conduction and is the. dofifinaxzt
`hjcat~transf:cr
`This -is cspcciaiiy tone in the ugsiroam rcgzoxz of
`»whcro
`et;e~;:1:p:»ar§i£=21r[o grafiiont is iiargc, mad .ra£i.ia£io.x_1 transoott
`fiaot
`sfzozwcf
`i‘£?:371Q$;:ifi$ of»-the inciident "flow.
`titoioiigorcwavcioogth region above the resonance transi:ion_s, flrxs..ab~
`,,
`sorption of'ii1e.,rafii.at’iofl by ciao piasma anti the surrounciing gas is much.
`smafilcrt. TI:1c=abscrptionIczxgihforthis radiation is after; largo -com
`6
`
`{some cha'rac’teriisiice::Ii.o:zen;sio11s -of the 'pIa§m3,, and -muc11»o£’thc,rao1a_,:oo
`ESL >963.
`rcgiozi of ti;he.:spectrm:r2, tho ’pias"ma may be considered g:sp~
`/zioaiiy thin, anti iftho piasfmax is in ’L'.E‘B, than the €:sc:aping:;’:a3;iiatiOn eanfbo
`’us':eo ‘to characterize the tempcratutc within the LS? {£{e=e:£cs at 31,, ’1?§86;
`
`987).
`
`
`mic §??i‘§f1zii.1 to-o piasma is far from miform, as
`
`mod
`to o*::£.’aiji1 {ho oxp»orime:nta3.3 tcmperatoresshown
`in Figs. 43-2, 4.4, and 4:.}(}..i,s: =:¥cscribc<3, in Ci-otaii i’z'1S»oc.. 4.4.2). Ti1:is.’§igo.m
`shows an is:ot%za:m4:;pic:t of me» ta:n;::«cr.at::umc czzzcemred in .an.LS?sz.:sa:a ct:
`in 2,5 aim of =’ gozxizy .a.o&rbon’c1iox-idc laser »o§;er*a£’ing at 3. wiavelcogfix
`19,5 cm. ’f1?i1e’,;5i=asi:aa Iozigifh, ago diameter, as ciotcrminccl by the ::o;.5o§K
`"is-otimrm, are 311 311:1 émto’, rocpectivcly. flo