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Volume 44A, number 3 PHYSICS LETTERS 4 June 1973 LOWERING OF EXCITED STATE POPULATIONS OF Hel 1N A He-PLASMA UNDER THE INFLUENCE OF LASER RADiATION J. CHAPELLE, B. DUBREUIL Centre de Recherches sur Ia Physique des Hautes Tern pératures, Faculté des Sciences, F-451 00 Orl~ans, France’ and H.W. DRAWIN and F. EMARD Association EURA TOM-CEA sur la Fusion, Section Physique des Plasmas, Centre d’Etudes Nucléaires, F-92260 Fontenay-aux-Roses, France Received 26 March 1973 The number of excited helium atoms in a helium plasma decreases when it is irradiated by a CO 2 laser beam (X = 10.59 ~, power density i0 5 W/cm2). The lowering increases with principal quantum number in agreement with a theoretical model involving photoionization and gas heating. A stationary capillary type discharge was operated in The broken curves represent the relative lowering helium gas at a pressure of p 0.2 torr and a current of as a function of principal quantum number n, for each I = 25 mA. The experimental set-up is the same as de- n averaged over angular momentum states L. The lo- scribed in [1] with the only difference that the CO 2. wering is approximately constant for states of low laser beam has slightly been defocalised in order to in- principal quantum numbers (n <6) but increases con- crease the interaction volume. The mean diameter of tinuously for n > 6. the focal spot is now 0.8 mm, the mean power density We have calculated the population densitiesN~ in is l0~ W/cm 2. The increase of the gas temperature T 0 the frame of a collisional-radiative diffusion model due to laser irradiation could be measured by means [4, 5] including photoexcitation (between levels n = of a thermocouple located very close to one of the 8, 12 and n = 9,16), photoionization (for all levels orifices of the interrupted capillary. Electron density n 11) and gas heating by laser radiation. The calcula- n~ and electron temperature Te have not been mea- tions are based on an atomic He model which disting- sured. From glow discharge theory follows [2, 3] that uishes between singlet and triplet states. For n <3, in our device ~e is of the order of 10h1_ 1 ~1 2cm3 sub-states L have individually been taken into account, and kTe reaches several electron volts, for n 3 we have assumed that they are statistically The intensities of the He spectral lines emitted by populated. For the photoionization cross section, the plasma in the region of the laser focus have been Kramer’s formula for hydrogen-like states has been measured photoelectrically. A tungsten ribbon lamp used. The collision coefficients have been calculated has been used for absolute calibration of the detection on the basis of a Maxwellian velocity distribution of system. The population densities Nn,L could thus be the electrons. determined in the range of principal quantum numbers The full curves in fig. 1 show the theoretical re- 3 ~ n ~ 10 for singlet and triplet states without and sults for 1~e = 1011 and 1012 and Te = 48000 °K. The. with laser irradiation. The measurements showed a re occurs a general lowering of roughly 10% due to lowering of the excited state populations under the in- gas heating and for n ~ 6 and increasing lowering of fluence of irradiation compared to the unirradiated the population densities due to (mainly) photoioniza- plasma. Fig. 1 shows the results in the form tion. The photoionization effect is felt by levels lying Mm Nn (with laser) N~ (without laser) far below the photoionization limit (n = 11 without = . lowering of ionization energy eventually n = 10 with N N (without laser) . . . . lowering of ionization energy due to electric mictro- 201

Lowering of excited state populations of HeI in a He-plasma under the influence of laser radiation

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Page 1: Lowering of excited state populations of HeI in a He-plasma under the influence of laser radiation

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-

201

Page 2: Lowering of excited state populations of HeI in a He-plasma under the influence of laser radiation

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