I .f- 4_ L_ 197B /c/ CO2 Laser Preionisation Gary D. Spiers Center for Applied Optics The University of Alabala in Huntsville Huntsville Alabama, 35899 Final Report NASA Contract NAS8-36955 Delivery Order 70 (NASA-CR-I84424) C02 LASER N93-13797 PREIONISATION Final Report, 1990 (Alabama Univ.} 61 p Uncles G3/36 0019736 \ % % Table of Contents 1 Introduction 1 ................................................................................. 2 PreionisationReviev ...................................... 2.1 Introduction........................................ 2.2 Prelonisation requirements .......................... 2.2.1 Large electron production ..................... 2.2.2 Uniform electron production ................... 2.2.3Efficientelectronproduction............................... 2.2.4 Contaminant free preionisation ............................... 2.3 Preioniser test methods ............................................ ,,..,, 2.3.1_V chamber ................................................. IDIO_O 2.3.2Photographicstudies ........................................ 2.3.3Light_ission ...................................... ''|O', 2.3.4Microwaveinterferometry............................ 2.4Listof rec_iredequipment................................ 2.4.1_V clla_er ......................................... 2.4.2Driftfieldpowersupply ............................. 2.4.3Detectioncircuitry.................................. 2.4.4 Gas analysis ...................................................................... 2.4.5 Tile integrated photo_aphic analysis ............................................. 2.4.6 Wine resolved photographic analysis ............................................... 2.5 References .............................................................................. 3 CavityModel ............................................................................7..... 3.1 Introduction.......................................................................7..... 3.2The matrixmodel ............................................................ 7 3.3Analysis ...........................................................................I.i.... 3.4References.........................................................................i.i.. 4 LP-140DischargeCharacteristics..........................................................1.2 4.1Introduction.......................................................................1.2.... 4.1.IDischargeInitiation..........................................................1.2 4.1.2Energydeposition.............................................................1.2... 4.1.3Pulseteraination.............................................................1.2 4.i.4 LP-140Dischargecircuit ......................................................1.2.. 4.2Experimentalneasurenents...........................................................1.3 4.2.1Voltageandcurrentpulse|easure_ents..........................................1.3 4.2.2Dischargepulseanalysis ............... 13 4.2.2.1EnergyDeposition................. 15 4.2.3Sl_ar_ ......................... 16 4.3Powersupplyconsiderations............ 17 4.3.1Introduction..................... 17 4.3.2DesignCriteria.................. 17 4.3.3CircuitOutline .................. 21 4.3.4PulseTransfonmr Design 22 4.4References.........................................................................2.2.... 5 FrequencyChirp .............................................................................. 32 5.1 int_oduction.......................................................................3.2.... 5.2Analysis ..........................................................................3.2..... 5.3Modeling ...........................................................................3.2.... ii PRECEDING P_IGE BLA/_,!KNOT FILMED 5.4References................................_...........................................3.2. 6 LaserLinewidths.........................................................................3.6.... 6.1 Introduction.......................................................................3.6.... 6.2LP-140,L_/AVCO and GE/STILaserComparison.........................................3.6. 6.3TheMathcadDoculent ................................................................3.6... 7 Appendix(I) ............................................................................4.0..... 8 Appendix(2) ............................................................................4.6..... 9 Appendix(3) ............................................................................5.1..... iii 1 Introduction ThisreportcoversworkcarriedoutonNASAcontractNAS8-3695d5uringtheyear1990.Thereportisdivided intothefollowinsgections:- PreiomisatioRneview:-A literaturseurveytoidentifytherequiredparaneterfsoreffectivpereioni- sationinTEACO=lasersandthemethodsandtechniquefsorcharacterisinpgreionisers. CavityIMdel:-A nunericanledeloftheLP-140cavityusedtodeterminethecauseof thetransverse modestabilitiymprovenenotbtainedwhenthecavitywasler_ened. LP-140Dis_ (_aracteristicsT:h-e|easure|enotfthevoltageandcurrentdischargpeulsesonthe LP-140andtheirsu_ent analysisresulti_inanexplanatiofnorthelowefficiencyofthelaser.An assortnenotfitas relatingtothedevelopnenotfhigh-voltagpeowersuppliesisalsoprovided.Theseitem areguidelinetshatwerepreparedforaGS_Pstudent. Freqlam_, (3irp:-A programforanalysingthefrequencychirpdatafilesobtainedwiththeliPtile andfrequencaynalyser.AprograntocalculatethetheoreticaLlIMPchirpisalsoincludedandaconparison betweenexperimenatndtheoryismade. LaserLinewidths:A-progranforcalculatintgheC02linewidtahnditsdependenceongasco_osition andpressureT.heprogralalsocalculatetshenu_erofaxialnodesunderthe_ ofthelineforagiven resonatorlength.A graphicalplotoftheresultsisprovided. Appendix(I):-Thisisa listingofthebasicprogramusedtoextracttheener_ depositiornate, impoadanceetc.fromthedischargveoltageandcurrentpulses. Appendix(2):-TwoMathcadlistings,onetocalculataerbitraryTDh,. nodesup to3,3andtheother tocalculattehefocallengthofa thicklensgiventheradiusofcurvaturoef thesurfaces.Thesecond docunentisusefulforconvertinRgOCtofocallengthandvice-versfaortheresonatorcodeinsection3. _l_w.odi(x3):-Thisisa copyofa jointpaperbetweenDr.Jaenischandmyselftobe presenteadtthe SPIEHighEner_LasersConferencienLosAngelesonJanuary24 1991. TM Duetothevarietyoftopicscoveredanoverallconclusioinsnotprovidedaseachsectionisself- contained. 1 Preionisation Revi_ 2.1 Intr(_uction This review is a copy of a documentsubmitted previously, only minor formatting changes havebeen undertaken. Theobject of the review was to provide a brief overview of the leans of obtaining quantita- tive data o, the relative merits of various preionisation schemes. Thedischarge in a Tn CO_laser can be divided into three phases, firstly preionisation, which involves the injection of electrons into the gap in which the discharge will be foned. Secondly the application of a high dV/dt voltage pulse to avalanche the preionisstier electrons up to an electron concertration suitable for maintaining a stable disclmr_. Thethird phase co,sists of the &position of energy into the discl_u_e. Duringthis final stage the voltage across the electrodes collapses to a value e_ tially independent of the driving circuit. Allthough the preionisatier pulse can be co_leted prior to the application of the avalanching pulse, it is _ore co_on to arrange the timing such that the avalanching occurs during the peak of the preionisation pulse. This ensures that a mini|urn numberof preionisation electrons are lost to attac_ent and also limits the degree of avalanching required to form the discharge. The preionisation pulse usually does not continue during the third phase, except for long pulse operation where electrons _ust be continuously injected into the discharge to _aintain its stability. For large cross-section discharges (-lOxlO cm2) the preionisation can not be generated uni- formly throughout the dischar_ gap and so to overcome this, a high preionisation density is created adjacent to one of the electrodes and a weak (non-avalanching) voltage applied across the electrodes to drift this preiomisation uniformly through the discharge volum prior to the application of the ava- lanche voltage. The preionisation of the discharge volumeis critical to the successful operation of the discharge and is therefore a vital consideration in the design of a transversely excited gas laser. 2.2 Preionisation requirements The mainrequirements of a preionisetion source are:- 2.2.1 Large electron production Theoretical estimates of the minixumpreionisation density required for at|ospberic pressure dis- charges range between104-106 C1-3 _2.1,m.2,2.3,2.4) and experimental verification _='s> has provided quantitative agreement of these values. The actual discharge has an electron density of 10_=-10_4 cm-_ _=.6,=._,2.. _and experimental evidenceC2.,_ has show, that initial preionisation densities above the minimumrequired value will result in lore efficient energy extraction from the discharge volume. Additionally with a larger initial electron density less avalanching is required to obtain a stable discharge and a lower initial voltage can be applied to the electrodes. 2.2.2 Uniform electron production Thepreionisatiomnustbeproduceduniformlythroughoutthedischargetopreventregionsofhigh currentdensityfromoccuring.Thisisimportantasthesehighcurrentdensityregionscanreadily degeneratientoarcs. 2.2.3 Efficient electron production WithintherequirementosftheLAWSproject,onlya limitedpowerbudgetisavailableandsothe efficiencoyfthepreionisatiopnroc_ isi|portantt,herefortehenu_er ofelectronsgenerated perunitenergyinputtothepreionisesrhouldbeashighas possible. 2.2.4 Contaminant free preionisation Thelifetimerequirementosf theLAWSprogramrequiresthatthepreionisatiosnourceshouldprovide littleornocontaminatiotnhatmaylimitthelifetimeof thelaser.As anexamplesparkpreionisa- tionhasinthepastbeenprefe_edforflowing-galsasersasitprovideslargen_rs ofelec- 2 trous,llovevairnasealedlaser_2"9"='I°t_hehi@ teweraturesparkresultsinalmst complete dissociatioonftheC02inthereqionofthesparkresultinqina shorterlifetimforthelaser thanifa diffusedischargpereionisatiosnourcehadbeenused.Additionalltyhesparkpinshavea tendencytosputtermterialontotheraindischargeelectrodeasndthelaseroptics. 2.3 Preioniser test methods Traditionallmyostpreioniserhsavebeentestedbyplacinqthemina laserandprovidedthelaserhas workedthepreionisehrasbeenconsideresdatisfactoryT.herehavebeenveryfewattempts(intheopen literaturet)oco,arepreioniserdsirectlyT.heteclmiquetshathavebeenusedtoexaminethecharac- teristicsofpreioniserasre:- 2.3.1 UIIV chamber A_ chambe<r2"6"2"g-2"Ic_hamberenablesthedirectmeasuremenotfelectronproductionfromthe preionisetro beundertakenT.ypicallay preioniseirsplacedinsidea cha_ersuchthatthe electrongseneratedarecollectebdya veryweakelectricfieldacrossa pairofelectrodesT.hese collectioenlectrodecsanbemovedtomeasurethepreionisatiofnromvariousportionsofthepreio- niser.Thedependencoefthepreionisatiodnensityon thepreionisedrrivecircuitparameterscan obviouslbyedeter|ineudsingthistechnique.Thesiqsalobtainedby thisteclmiqueforlowelectron densitieissveryslall(-nV)andconsiderablceareisrequiredinthedesiqnoftheapparatusA. dumr/electrodeisutilisedtoalloya differentiasliqnattobeobtained._,dditionalsluyitable filtersarenonellyusedtoeli|inatIeIFnoisewhicharisesfromthepreionisatiodnischarge.Asit hasbeenshownc2"6"2"_2"_'_6"2"t_hatsmellquantitiesoforganiccontalinantcsancjreatly increasethepreionisatiodnensityforuvbasedpreioniserst,oeliminatefluctuationisnmeasure- mentsextremecarelustbetakentoensurethecleanlinesosfthecha_er islaintainedB.y repeti- tivelypulsinqthepreioniseirnthecha_erthedependencoefthepreionisatiodnensityon preioniseraqecouldalsobedeteninedandaftermanypulsestheqasinthecha_ercouldbe analysedtodetermintehedegreeofdeqradatiodnuesolelytothepreioniseurndertest.However withtheadditionoflifetestingofthepreionisersa,hi@ pulseratebecomesdesirabletoallow theteststobecompletedinareasonablteim. Thiswouldrequirecoolinqandqascirculation withinthechamber.Fromtheliteraturteheeffectof thepreioniseorntheqasmixturecaneasily bedetentineadfter-100,000pulses(_orsparkpreionisers_2"9'2"_°I_f).anhourisallowedfor thistest,thisprovidesforapulserateof28Hz,whichisminilal.Asthepreioniserasreintended tobeverycleana pulserateof50Hzmaybemoresuitable. 2.3.2 Photographic studies Bothti|e-avexaq<e_d'_'-2"_°a_ndtimeresolve<d2"_°'_'_I'_'_photoqraphisctudieshavebeen used tostudypreionisatioannddischargfeormtion.Thetimeaveragedstudiesalthou@of limitedvalue _illshowreqionsofveryhi@ currentdensity.Timeresolvedp_otoqraphpyrovides_uchmoreinfor- mationandcanbeusedtofollowthedischargdeevelopmenatndenablesflawsintheunifomitytobe easilyseen.Thetechniqureequirestheuseofa streakcameraorpreferablya framingcamerawitha resolutio-n10-20nS. 2.3.3 Light Emission Manyofthepreionisatiosnchemesrelyontheproductionofuv li@t whichisusedtopreionisethe dischargevolume.Measurementhsavebeen_adeoftheli@t productiofnromthesepreioniserussinga photodiod_e='x_>,howeverthedisadvantagoefthistechniqueisthatthecorrelatiobnetweenli@t outputandelectrondensity,althou@probablyvalid,hasnotbeendemonstrated. 3 2.3.4 Microwave interferometry Earlyattemptstomeasuretheelectronconcentrationisna discharguesedanX-bandmicrowave interferomet(e=r'=3_,howeverthesensitivitoyfthedevicewasinsufficienatndthetechniquewould appeartohavebeenabandoned. of thetechniqueosutlinedabovethe_ chamberisthemostimportantforthequantitativceomparison ofpreionisersT.hetimeaveragedphotographiysrelativeleyasyto accomplisha,lthoughideallya large formatcamerashouldbeusedtoensureadequateresolutionT.hetimeresolvedphotographpyrovidesthe onlymeansofstudyingthedischargfeormtiononthepreionisetrherebyenablingtransient_hotspots_, whichfrequentliyndicatepotentialfailurelocationst,obe detected.Themicrowaveinterferometecran bediscountedduetoitslimitedsensitivity. 2.4 List of required equipment Theequipmenitdeallyrequiredforthecharacterisatioofnthepreionisertshereforeconsistsofthe followinigtem.Itshouldbenotedthatonlyageneraldescriptionisgivenatthispoint. 2.4.1 UHV chaLber AnUHVchamberassemblyconsistinogfcollectioenlectrodesp,ortsforsupplyinggasandtoenable gassamplingS.uitablefeedt]Eouc_fsortheelectricaclormectionwsillbe requiredA. largeportto whichthetestpreionisearssemblycanbefixedanda largetransparenwtindowdirectlyopposite thisportto enablephotoqEaphoyfthepreionisatiodnisc3arge.Inorderto facilitatmeovementof thecollectioenlectrodesr,otaryfeedthroughmsustbeprovidedifmanualadjustmenitsenvisa_. Alternativelayndpossiblypreferabliyfautomatidcatacollectionisrequireda pairofstepping motorsandtheirassociateedlectronidcriversisrequired.Iflifetestingisenvisagetdhengas coolingandrecirculatiofnansmustalsobesupplied, 2.4.2 Drift field power supply Thedriftvoltagerequiredobviousldyependsontheelectrodespacingbuttypicallyavoltageof -0.3Vcm-ltor-rIisusedgivinga driftvoltageof -2kV foran 8cmgapatatmospheripcressure. Thusa0-5kV dcpowersupplyshouldsuffice. 2.4.3 Detection circuitry Thisconsistsprimarilyofa differentiaalcamplifierandHF filters(whichmaybe includedinsome amplifiers)T.raditionalltyheoutputfromtheamplifierhasbeenfedtoanoscilloscopaenddis- playedsothata photograpohf thesignalcanbeobtained.Thisobviouslylendsitselftodigitisa- tion. 2.4.4 Gas analysis Iftheeffectofthepreioniserosnthegaschemistryisto bestudieda massspectrometewrillbe required. 2.4.5 Time integrated photographic analysis A suitablelargeformatcameraisrequired. 2.4.6 Time resolved photographic analysis A suitablehighspeedframingcameraisrequired. The above list only covers the equipaent required to analyse the electron production from the various preionisers. Thus in addition to this list suitable circuitry for driving the resI_ctive preionissrs would be required together with high voltage and hi@ current probes for their electrical analysis. 2.5 References 2.1)Pall_r,A.J.;"Aphysicalnodalon the initiationof atnospheric-pressureglow discharges";Appl. Phys.Lett.;25; 138 (1974) 2.2)Karnyushin,V.N.,A.N.Xalov& R.I.Soloukhin;"Influencoef preionisationconditionson thedevel- opnentof a bo[(x_eneoudsischargeingases";Soy.J. Quant.Electron.;8; 319 (1978) 2.3)Levattex,J.I.& Shao-ChiLin;"Necessaz,/conditionfsorthe holKx}eaeousformationof pulsedava- lanchedischargesat highgas pressures";J. Appl.Phys.;51;210 (1980) 2.4)Gavrilov,P.T.,J. Limpoucb,G. Loncar,K. Masek& M. Vrbova;"Necessaryconditionsforthe form- tionof self-sustaineddischargein TEA_D= lasers";J. Phys.E: Sci.Instmm.; 20;906 (1987) 2.5)Suzuki,S.,Y. Ishibashi;M.Obara& T. Fujioka;"Dependenceof laseroutputon initialphotoelec- trondensityinTEA CO2laser";Appl.Phys.Lett.;36;26 (1980) 2.6)Babcock,R.V.,I. Liber|an& W.D.Partlow;"Volu|epreionisationfrombaresparks";IEEEJ. Quant. Electron.;12;29 (1976) 2.7)Kaminski,W.R.;"Coronapreionisationteclmiqueforcarbondioxidelasers";_ ReportB2R- 98071-02;UnitedTeclmologyResearchCentre (1982) 2.S) Kline,L.E.& L.J.Denes;"Investigationsof glowdischargeformationwith voltmepreionisation";J. Appl.Phys.;46;1567 (1975) 2.9)Norris,B.& A.L.S.S|ith;"SealedstableTEAC02 lasers";Universityof St. Andrews;Scotland; AnnualReportforCVDResearchProjectRUI6/3 (1977) 2.10)Smith,A.L.S.& B. Norris;"Limitingprocessesin sealedphotoionisationTEACO= lasers";J. Phys. D: Appl.Phys.;Ii;1949(1978) 2.11)Yamabe,C., T. Matsushita,S. Sato& K. Horii;"Parametricstudiesof uv preionisationin TEACO= laser";J. Appl.Phys.;51;898 (1980) 2.12)Scott,S.J.& A.L.S.Smith;"Ultravioletphotoionisationin transverselyexcitedatnosphericCO= lasers";J. Appl.Phys.;64;528 (1988) 2.13)Scott,S.J.;"Exporimentalinvestigationson an X-raypreionissrtestbed";J. Appl.Phys.;64; 537 (1988) 2.14)Harchetti,R.,E. Penco,E. _dillo & G. Salvetti;"Optinisatioonf corona-dischargephotoioni- sationsourcesforCO= lasers";J. Appl.Phys.;54;5672 (1983) 2.15)Harchetti,R.,E. Penco& G. Salvetti;"Anew typeof corona-dischargephotoionisationsourcefor gas lasers";J. Appl.Phys.;56;3163 (1984) 2.16)Scott,S.J.& A.L.S.Smith;"Sourcesof photoionisationin transverselyexcitedatmosphericCO= lasers";Appl.Phys.Lett.;41; 783 (1982) 2.17)Scott,S.J.& A.L.S.Smith;"Identificationof theoriginsof photoionisationin CO, TEA lasers"; Appl.Phys.B; 33; 1 (1984) 2.18)Richardson,H.C.,K. Leopold& A.J.Alcock;"LargeapertureCO= laserdischarges";IEEEJ. Quant. Electron.;QE-9;934 (1973) 2.19)Hasson,V. & H.M.VonBer_ann;"Spatialcontrolof pulsedhigh-pressurepre-ionisationstabilised glowdischarges";J. Phys.E: Sci.[nstrlm.;13;632 (1980) 5 2.20) Theop]_nis, G., _ V. Hasson_"X-raypreionisod laser develop|ent'; IVCO_verett l_esearch I_boratory Inc.;SummryReport (1985) 2.21)}larchettRi.,,E.Penco& G.Salvetti;"UltravioplreetioniseCdO=TEAlaserwithhiqhoutputpower densityutilisingnonconventionaellectrodperofile";IEEEJ.Quant.Electron.Q;E-18;170(1982) 2.22)Sakai,Y.,M. Takahasl_Si.,S_ka_to&H.Tagashira;"C0T2EAlaserdischar_develop|entA- hi_h-speed-caerianvestigationJ".;Appl.Phys.;50;647(1979) 2.23)Sequin,H.J.J.,J.Tulip&D.NcKen;"UVphotoionisatidoennsitymasure|entsinTEAlasers"_Appl. Phys.Lett.;23;344(1973) 6 Cavity _)del 3.1 Introduction The transverse _de structure of the laser output pulse is important for obtaining a good far-field beam profile. Theinitial beamprofile of the LP-140 laser bad considerable transverse lode instability _''_ . Asche_tic of the LP-140cavity is shownin figure 3.1. Jaeniscb _''*_ found that when the cavity length was extendod by increasing the seperation between the grating and the lens, the transverse node stabil- ity i_roved. TheDodel discussed here was developed to provide an explanation for this improvement. CATHODES L2 I1 c ) C ) ( ) C • _H2 ' -'e""ll"'--'u , VIIIIIII/,/////J//////////I//////AI • _//_/////___////////_COMMOANNODE Vz/_ • Grating • r////////////////////////////////A I IV L1 ( } [ } c ) ( _ / M3 Grating extension Figure 3.1. TheLP-140 cavity layout. 3.2 The matrix model ThenodelisbasedontheworkofSie_nn_'=_,andhasbeenimplenenteadsa Mathca(d''_ doctment.In thismodeleachoftheopticalelementsinthecavitycanbeexpressedasa 2x2matrix.By co_iningthe matricesa singlematrixfortheoverallopticalsystemcanbeobtained.Fromthismatrixthema_ifica- tionandFresnelnumbersoftheresonatocranbe deterlinod. Thesimplegeonetricamlagnificatioofntheresonatorvarieswiththecomplexcurvature,q of theinput wavefrontandforlasingactiontooccurtheWnvefrontnustbecapableofreproducinigtselfoverlany roundtrips.Thisneansthattheconplexcurvatureof thewavefrontafteronecompleteroundtripl_t returntoitsinitialvalue.Thisconditiocnonstrainsthemagnificatiotnoa singlevalueandit is thisvaluethatiscalculatebdytheprogram. Ratherthanprovidea detailedtheoreticatlreatiseofthe|odel,thereaderisreferredtothe book(,.=)bySieq_ whichprovidesacomprehensivaeccountofthetheory.TheMathcaddocunent follows:- 7