Volume44A,number3 PHYSICSLETTERS 4 June1973

LOWERING OF EXCITED STATE POPULATIONS OF Hel 1N A He-PLASMAUNDER THE INFLUENCE OF LASER RADiATION

J. CHAPELLE, B. DUBREUILCentredeRecherchessur Ia PhysiquedesHautesTernpératures,FacultédesSciences,F-45100 Orl~ans,France’

and

H.W. DRAWIN andF. EMARDAssociationEURATOM-CEAsur la Fusion, SectionPhysiquedesPlasmas,

Centred’EtudesNucléaires,F-92260Fontenay-aux-Roses,France

Received26 March1973

Thenumberof excitedhelium atomsin a helium plasmadecreaseswhenit is irradiatedby a CO2 laserbeam(X =

10.59 ~, powerdensity i05 W/cm2). Thelowering increaseswith principalquantumnumberin agreementwith a

theoreticalmodelinvolving photoionizationandgasheating.

A stationarycapillary type dischargewas operatedin The brokencurvesrepresentthe relativeloweringhelium gasat a pressureofp 0.2torr and a currentof asa function of principalquantumnumbern, for eachI = 25 mA. The experimentalset-upis the sameasde- n averagedoverangularmomentumstatesL. Thelo-scribedin [1] with theonly differencethat the CO

2. weringis approximatelyconstantfor statesof lowlaserbeamhasslightly beendefocalisedin orderto in- principalquantumnumbers(n <6) butincreasescon-creasethe interactionvolume.The meandiameterof tinuously for n > 6.thefocal spotis now 0.8mm, themeanpowerdensity We havecalculatedthepopulationdensitiesN~inis l0~W/cm

2.The increaseof the gas temperatureT0 theframe of a collisional-radiativediffusion model

due to laserirradiationcould be measuredby means [4, 5] includingphotoexcitation(betweenlevelsn =

of a thermocouplelocatedvery closeto oneof the 8, 12 andn = 9,16), photoionization(for all levelsorificesof the interruptedcapillary. Electrondensity n � 11) andgasheatingby laserradiation.Thecalcula-

n~andelectrontemperatureTe havenotbeenmea- tionsare basedon an atomic He modelwhich disting-sured.Fromglow dischargetheory follows [2, 3] that uishesbetweensingletandtriplet states.Forn <3,in our device~e is of theorderof 10h1_1 ~1

2cm3 sub-statesL haveindividually beentakeninto account,andkTe reachesseveralelectronvolts, for n � 3 we haveassumedthat theyare statistically

The intensitiesof the He spectrallines emittedby populated.Forthe photoionizationcrosssection,theplasmain theregionof the laserfocushavebeen Kramer’sformulafor hydrogen-likestateshasbeenmeasuredphotoelectrically.A tungstenribbonlamp used.Thecollision coefficientshavebeencalculatedhasbeenusedfor absolutecalibrationof the detection on the basisof a Maxwellianvelocity distribution ofsystem.The populationdensitiesNn,L couldthusbe theelectrons.determinedin therangeof principalquantumnumbers The full curvesin fig. 1 showthe theoreticalre-3 ~ n ~ 10 for singletandtriplet stateswithoutand sultsfor 1~e= 1011 and 1012 andTe = 48000°K.The.with laserirradiation. Themeasurementsshoweda re occursa generalloweringof roughly 10%due toloweringof the excitedstatepopulationsunderthe in- gasheatingandfor n ~ 6 and increasingloweringoffluenceof irradiationcomparedto the unirradiated thepopulationdensitiesdue to (mainly) photoioniza-plasma.Fig. 1 showsthe resultsin theform tion. Thephotoionizationeffectis felt by levelslying

Mm Nn (with laser) — N~(withoutlaser) far belowthephotoionizationlimit (n = 11 without= . lowering of ionizationenergy eventuallyn = 10 with

N N (withoutlaser) . . . .

lowering of ionizationenergydueto electricmictro-

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Volume44A, number3 PHYSICSLETTERS 4 June1973

I I I 4 I~ 0‘ure effect ~[%)

—~ Temperat I~fl Temperature effectN0 •6P +—. 4P

10 6 - ——~~~—---- ~ 0=3—10 — ~ 0 ..4— —— — _v’;:i~’~5 n=3 , 55S .4D ~ 6

1OI/4~7p•6S io~iI8 HELIUM TRIPLET SYSTEM—20 — Iii •7S — —20 — —

HELIUM SINGLET SYSTEM —0—0-

1e ~ cm-3 THEORYJ,~J — 0e iO11cm~d ii~I —~—~- 0e o10~2cm~hi —°——---°—30 — 9

—v—c— n~.iO12cm-~ THEORY- —30 1/ --+---+-Mean volues,Experim -

4V1 Individual ~010.45, .6P vaLues Experim. -_ 12,1

—40 — / -40 — ii—.,--——-+- Mean values

on=1O

—50 - 181% for n.h n~ii Hfor n~12

—60 I I —600 0.05 0.1 i/~2 0 0.05 0.1

Ionization ii~2L..._Ionization

limit Limit

Fig. 1. Relative lowering of thepopulationdensitiesof singletand triplet states,asa functionof ionization energy

in unitsE~=13.58eV.

fields). This is a consequenceof collisional-radiative vidual collisional-radiativeprocessesis different for thecouplingbetweenall levels.The theoreticalresultsare two cases.in principal agreementwith themeasurementsnotonly for the relativevalues~.iV

0/N~ but also for theabsolutevaluesN0. The observedeffectcanby no

Referencesmeansbe explainedby a rise of theelectrontempera-ture duringthelaserirradiation, sincethis would leadin this specialtypeof plasmato an increaseof the ex- [1] B. Dubreuil, P. RansonandJ. Chapelle,Phys.Lett. 42A(1972)323.cited statepopulations.We mentionthat thereis a [2] G. Francis,Handbuchder physik,Vol. 22 (SpringerVerlag,

certainrelationbetweenour resultsandthoseobtained Heidelberg,1956)p. 53—208.by Kaplafkaet al. [6] on an afterglowplasma. [3] A von Egel, IonizedGases,2nd Ed. (University Press,Lon-

The numericalevaluationof the rate equationsshows don, 1965).[4] H.W. Drawin,M. Fumelli andG. Weste,Z. Naturf. 20a

further that for the samelaserpowerdensity the lower- (1965)184.ing effect is generallymuchlargerfor transientrecom- [5] H.W. Drawjn, Z. Physik 186 (1965)99.

bining (afterglow)than for stationarydiffusion-domina- [6] J.Kaplafka,H. Merkelo andL. Goldstein,AppI. Phys.Lett.ted plasmas,sincethe relativeimportanceof the mdi- 19 (1971)197.

202