Dalgarno, A.

Kallne, E.

Kallne, J.

Marmar, E.S.

Pradhan, A.K.

Rice, J.E.

DSpace@MIT

PFC/JA-84-14OBSERVATION OF CHARGE TRANSFER POPULATION OF n=2EXCITED STATES OF Ar16+ IN TOKAMAK PLASMASE. Kllne,* J. KAllne,* A. DalgarnoHarvard-Smithsonian Center for Astrophysics60 Garden Street, Cambridge, MA 02138, U.S.A.E. S. Marmar and J. E. RicePlasma Fusion CenterMassachusetts Institute of TechnologyCambridge, MA 02139, U.S.A.A. K. PradhanJILA, University of ColoradoBoulder, CO 80309, U.S.A.May 1984* Permanent Address - JET Joint Undertaking, Abingdon, OX14 3EA, ENGLANDThis work was supported by the U.S. Department of Energy ContractNo. DE-AC02-78ET51013. Reproduction, translation, publication, useand disposal, in whole or in part by or for the United States govern-ment is permitted.By acceptance of this article, the publisher and/or recipient ac-knowledges the U.S. Government's right to retain a non-exclusive,royalty-free license in and to any copyright covering this paper.Observation of Charge Transfer Populationof n=2 Excited States of Ar16+ in Tokamak PlasmasE. Kallne,* J. Kallne,* and A. DalgarnoHarvard-Smithsonian Center for Astrophysics60 Garden St., Cambridge, MA 02138, USAE. S. Marmar and J. E. RicePlasma Fusion CenterMIT, Cambridge, MA 02139, USAandA. K. PradhanJILA, University of ColoradoBoulder, CO 80309, USAResults from the first radial measurements of the n-2 to n-1 x-raytransitions of high-Z elements in deuterium tokamak plasmas are presented.The observed line intensity ratios cannot be explained by electron impactand recombination processes only, and the D + Ar17+ -> D+ + Ar1 6+ chargetransfer reaction is proposed as an additional population mechanism. Thelines emitted in the cascade following capture constitute a unique diag-nostic probe of the neutral component of plasmas and should providevaluable atomic data on the individual charge transfer rate coefficients.* Permanent Address - JET Joint Undertaking, Abingdon, OX14 3EA, ENGLAND- 2 -High resolution spectroscopy of the He-like n=2 to n=1 x-ray emissionof high-Z elements in high temperature plasmas, particularly in tokamaks,has developed into a powerful technique for determining the main plasmaparameters such as electron temperature Te and density Ne as well as theion temperature and ionic charge state distribution. Detailed ratecalculations' predict the observed relative intensities of the principaltransitions, the resonance (w), the intercombination (x and y) andforbidden (z) lines from respectively the 21Pl, 23 P2, 1 and 23S, states,and satellite lines 2, 3. The agreement between measured and calculatedline ratios confirms that excitation of the Ar1 6 + ground state, by electronimpact and dielectronic recombination, is the principal population mecha-nism. Most measurements, however, have been limited to the plasma coreregion, where the x-ray emission is brightest and most easily measured,and studies are needed to test this model throughout the plasma.We report here a systematic study of the He-like spectrum for a high-Z element (argon) in a deuterium plasma as a function of distance fromthe plasma centre along the minor radius. An earlier study of oxygen hasbeen carried out by Peacock and Summers 4 . The relative intensity oflines of Arl 6 + originating in triplet and singlet n=2 states is found toincrease strongly along the more peripheral lines of sight. The variationcannot be explained by excitation processes and we suggest that theincrease arises only in part from radiative recombination and that chargetransfer recombination, D + Ar1 7+ + D+ + Arl 6+, can be a major populationmechanism. A fraction of the plasma working gas exists as neutral atomswith the highest concentration found towards the plasma edge owing torecycling from the wall of the vacuum vessel and the plasma limiters. Inplasmas heated by neutral beam injection, charge transfer of highly- 3 -ionized atoms can significantly affect the overall power balance.Excitation by charge transfer in the visible and ultraviolet has beenseen in line emission during active H and D beam injection 5-8. Here wedemonstrate the effects of charge transfer recombination on line inten-sities of the x-ray spectrum of a high-Z element in quiescent, ohmicallyheated plasma.The experiment was performed at the MIT Tokamak, Alcator C, using aBragg crystal spectrometer. The instrument is a new, small spectrometerof von Hamos geometry previously described 9 . It was mounted with apivot at the spectrometer entrance slit which allowed scanning of theradial dependence of the x-ray line emission with a spatial resolution ofabout 30 mm. The quartz crystal had a 2d lattice spacing of 6.687 A usingthe 1o11 plane. The wavelength band width was about 2.1% so either ofthe entire H- or He-like spectra of argon could be recorded. For photondetection, we used an active delay-line proportional counter 1 0 togetherwith commercially available 1-MHz electronics for time-to-digital con-version and storage of the resulting position histograms. The system wasoperated up to a data rate of about 0.5 MHz and a count rate of up to 200kHz for the strongest emission line alone; a detailed account of thedetection system will be given elsewhere. To achieve the desired countrate, the plasma was seeded with Ar, but the concentration was alwayssmall (<0.1%) having no significant effect on Zeff nor did it perturb theplasma. The plasma conditions were kept constant during the radial scanwith Te = 1.45 keV and Ne = 2.9 x 1014 cm- 3 at the centre for a plasmacurrent of 450 kA and a toroidal field of 8 Tesla. The limiter had aradius of 16.5 cm.The spatial scan measurement, spanning three decades in count rate,- 4 -was made possible only by the combination of high plasma density, highthrough-put in the spectrometer and the argon seeding. Typical examplesof spectra taken for three lines of sight, at d = -0.7, 8.3 and 11.3 cmoff the plasma axis, are shown in Figs. la, lb and Ic respectively; d isthe shortest distance along the plasma. minor radius from the plasma axisto the chord of observation. Spectra were recorded for each 20 ms of thedischarge but those shown were integrated over 200 ms to increase thestatistics; the plasma conditions were constant during this time period,with no obvious change in the He-like spectrum. In all, He-like spectrawere recorded for ten lines of sight in the range d = -7.8 to 12.8 cm andH-like spectra in the range d = -4.5 to 7.1 cm. Line fits were madeto eight readily distinguishable transitions for the He-like spectrum(shown in Fig. 1) from which we determined the following line intensityratios: G = (x+y+z)/w, K = k/w and Q = q/w. The dependence of theseratios on Te and the abundance ratio N(Arl5+)/N(Ar1 6 +) are the standarddiagnostic probes. To this list we added S = (x+y)/w which is the trip-let to singlet ratio for the 2P state. To obtain the intensity of the zline, we included a correction of -1.3 times the intensity of the line kto account for the estimated admixture of the unresolved line j 2 . Themeasured line ratios are given in Fig.2 as functions of d.The most conspicuous feature of the data in Fig.2 is the behaviourin the region d > 8 cm. Thus, S and G increase rapidly with d, followedby a tendency to level off at large d, and K and Q increase towards larged, followed by decrease at m 8 cm. From the population by excitationprocesses, we expect a monotonic increase in the line ratios G, K and Qwith decreasing Te. In the range Te " 1.4 to 0.2 keV, corresponding tothe temperaturesll at d=0 and 12.8 cm, G is indeed predicted 1 2 to increase,- 5 -but only by 25%. The ratio K shows initially the expected increase withdecreasing Te(d), but the trend is interrupted at dr 10 cm. Such effectssuggest underlying changes in population mechanisms.At the diminished temperatures, the excitation processes proceed moreslowly, while the recombination rates increase. For instance, at Te = 350eV, which is reached at d=11.3 cm, the rate coefficient for excitation ofAr16+ is 5 x 10-15 cm3 s~1, whereas that for recombination of Ar1 7 + is3 x 10-12 cm3s~1. Accordingly, the latter process is the more effectiveone when the Ar1 7+ to Ar16+ abundance ratio exceeds 2 x 10- 3 , which indeedseems to be the case, as discussed later. Some of the variations in lineintensity ratios are thus qualitatively consistent with radiative recombi-nation but also with charge transfer which must be considered. Chargetransfer recombination has a rate coefficient of about 10- 7 cm3 s-1, depen-ding weakly on Te, so that at 350 eV it is comparable to radiative re-combination, when the fractional neutral content N/Ne is 3 x 10-5.Models of the neutral deuterium distribution13 in the Alcator machineindicate No/Ne - 10~4 at a minor radius of 12 cm, toroidally close to thelimiters. In contrast to the principal lines (w,x,y and z), the satel-lites are not affected by radiative or charge transfer recombination.The observed decrease in the line ratios K and Q is thus consistent withthe suggested change in population mechanism at d - 8 cm.We suggest that the measured intensity variations can be explainedby a combination of contributions from excitation processes and fromradiative and charge transfer recombination. There is a narrow spatialregion, between 8 and 12 cm from the plasma axis, in which the dominantcontribution switches from excitation to recombination. The chargetransfer process varies slowly with Te and its effectiveness grows rapidly- 6 -at large radii because of the increase in neutral particle density to-wards the plasma edge, and should become the major population mechanismbeyond some critical distance from the plasma axis. Indeed there mayoccur three regions dominated in turn by the three processes.The recombination of Arl 7 + relative to excitation of Ar1 6 + dependson the abundance ratio N(Arl 7 +)/N(Arl 6 +). If this ratio were given bythe ionization and recombination of ions in coronal equilibrium, it shoulddecrease rapidly with Te and hence with radius to become less than 10~4f or d> 8 cm. Our measurements of the H- and He-like Ar spectra in therange d=O to 8 cm suggest a rather moderate decrease with radius so thatN(Arl 7 +)/N(Arl 6 +) remains above 10-2 at 8 cm. We take this to mean thatin regions of large radial Ne and Te gradients, ion transport becomes afactor in the charge state balance which would, in effect, reduce therate at which the N(Arl 7+)/N(Arl 6 +) ratio would otherwise decrease withradius. The finding that radiative or charge transfer recombination isneeded to explain the data for d > 8 cm is thus consistent with the largeN(Arl 7 +)/N(Arl 6 +) ratios deduced from the relative intensities of H- andHe-like spectra measured separately in this experiment. It can be notedthat more direct information on the charge state balance will be providedby planned experiments with spectrometers of extended bandwidth (AX/X= 10%) for simultaneous measurements of the H- and He-like spectra.In the region where either radiative or charge transfer recombinationis dominant, the triplet to singlet ratio S should statistically be aboutthree, as is observed at large radii. Differences, however, can occurin the 23S1 and 23P1'2 level populations resulting from differences inthe capture distributions of the two processes. In Ar1 7 + + D + Ar16+ +D+, captures occur preferentially into states of principal quantum numbers77777-----w1- 7 -lying in the range n=7 to 11, and at the low ion velocities involved, intostates of low angular mementum k14. The high-lying states cascade downwith the emission of dipole radiation, sometimes directly to the ground1S2 150 state, but most decays pass through the excited n = 2 states.Unlike charge transfer, radiative recombination is not a resonance pro-cess, so that captures will be distributed over a range of states andorbitals starting from £=O and n=1. In order to investigate the effectof such capture differences, we have performed cascade calculations basedon the assumption of capture into the s and p states of the orbital n=9for charge transfer and statistically weighted captures into the s,p,dand f states of the orbitals n < 4 for radiative recombination. In thisway, we obtain ratios of x/y = 0.65 and 0.75, and z/(x+y) = 2.5 and 1.2,for radiative recombination and charge transfer, respectively. Thesecalculations suggest that the two population mechanisms would be disting-uishable and the observed ratios of x/y = 0.72±.1 and z/(x+y) = 0.9±.2,in the region of d = 8 to 12 cm, would favor the charge transfer mecha-nism. It is worth noting that the n=2 to n=1 transitions studied arepart of the Lyman series of transitions. The most direct manifestationof charge transfer would therefore be the spectrum of transitions span-ning the range An=1 to 10, corresponding to a wavelength range X=4.0 to3.0 A. The signature of charge transfer population would be found inthe An intensity distribution, which would also reflect the capture ratesto certain atomic states and the neutral density in the plasma. Such anexperiment, therefore, might be done by developing a spectrometer withextra large Bragg angle span, i.e. AG of about 11* in our case.In conclusion, we have presented new measurements of the x-ray lineemission of He-like Ar from both central and peripheral regions of a------------- 8 -tokamak plasma. We find that the principal lines of the spectrum forperipheral plasma regions (r/a > 0.5) are dominated by population throughradiative and charge transfer recombination because of two factors: thetemperature in this region is low, so as to disfavour population by coreexcitation of n=2 Ar1 6 + states; the abundance ratio N(Arl 7 +)/N(Arl 6 +) isfound to be enhanced which is believed to come from radial ion transport.We argue that charge transfer will become the supreme population mode forthe principal lines of the He-like spectrum beyond some critical radiusbecause of the rapid increase of the neutral concentration No/Ne towardsthe plasma edge. Our spectra for d > 10 cm are consistent with dominantcharge transfer population. The results illustrate that the technique issufficiently efficient to record fractional neutral concentrations of10-4 to 10- 5 at Te = 200 - 500 eV and we suggest that dedicated studies ofhigher An transitions should offer even greater sensitivity. The spectro-scopy of He-like ions can thus be used for the diagnosis of neutralsin tokamak plasmas (from both passive and active sources) and it can alsobe of interest for atomic rate studies.AcknowledgementsWe gratefully acknowledge the continuous support we have enjoyedfrom Dr. R. R. Parker and the Alcator staff and the enthusiastic work byScott Magoon on computer programming.This experiment was supported by the U.S. Department of EnergyContract DE-ASO2-76ET53052 and EA77A01-6010 (4ith the National Bureau ofStandards) and Division of Chemical Sciences Contract DE-AC02-76ER02887.- 9 -References1. A.K. Pradhan and J.M. Schull, Astrophys. J. 249, 821 (1981), andA.K. Pradhan, ibid 263, 477 (1982).2. L.A. Vainshtein and U.I. Safronova, At. Data. Nucl. Data Tables 21,49 (1978)3. A.H. Gabriel, Mon. Not. Roy Astron. Soc. 160, 99 (1972).4. N.J. Peacock and H.P. Summers, J. Phys. B. 11, 3757 (1978).5. D.E. Post, L.R. Grishman and R.J. Fonck, Physica Scripta T3, 135(1983).6. R.J. Fonck et al, Phys. Rev. Lett. 49, 737 (1982).7. R.C. Isler, Phys. Rev. Lett. 38, 1359 (1977).8. R.C. Isler and E.C. Crume, Phys. Rev. Lett. 41, 1296 (1978).9. E. KAllne and J. KAllne, Physica Scripta T3, 185 (1983).10. J. K9llne, E. Kallne, L.G. Atencio, C.L. Morris and A.C. Thompson,Nucl. Inst. Meth. 203, 415 (1982) and to be published.11. R. Gandy, private communication.12. E. KAllne, J. KAllne and A.K. Pradhan, Phys. Rev. A 28, 467 (1983).13. M.J. Greenwald, C.L. Fiore, N. Sharky and D.P. Schissel, Bull. Am.Phys. Soc. 26, 976 (1981) and C.L. Fiore private communication.14. R.K. Janev, Physica Scripta T3, 208 (1983) and R.K. Janev, D.S. Belicand B.H. Bransden, Phys. Rev. A 28, 1293 (1983).- 10 -Figure CaptionsFig.1 The He-like spectrum of argon recorded during 200 ms of singleplasma discharges for three different lines of sight through theplasma centre (a), and through points of d = 8.3 (b) and 11.3 cm(c) off from the centre. The low data points in (a) are shownas a thick line. The abscissas give the actual counts recordedwithout correction for variations in the Ar concentration.Predicted wavelengths (Ref.2) are indicated relative to theresonance line (w). Key to the letter symbols (Ref.3):w, 1S2 1S -1s2p 1 P1 ; x, 1s2 1S0-1s2p 3p2 ; y, is2 IS -1s2p 3 1q, 1s22s 2S 1/2-is(2s2p1P)2p 3/2; r, 1s22 2S1/2-1s(2s2p 1P)2p1/2;a, 1s 2 2p 2 P3/ 2 - 1s2p 2 2 P3/ 2 ; k, 1s 2 2p 2 p 1 / 2 -1s2p 2 2 D3/2;z, 1s2 1 S-1s2s 3S1 ; j, 1s22p 2p3/2 - 1s2p 2 2D5/ 2'Fig.2 Results on the line intensity ratios, S-(x+y)/w, G-(x+y+z)/w,K=k/w and Q=q/w for different lines of sight given by the distancefrom the plasma centre. The errors shown are statistical andthe arrows show the position of the plasma limiters.Ix (.)0805.49- tI . a.2400q I ~:~*~ \~_____ .z..360 480Position [Channel No]FIGURE 13.96 4.cALCATOR 0805.37w x y q r a k z(j)- -ii-J 464p150010005001000805.40-,*0 -050L)InC0(-) 5080 220 34060460HJ1I.40204220 -340220 3404.02.006. .0~60G~4.0~2.0K0.4-0.20.2-0.1 +e-10 0 10d (c m)FIGURE 2

Observation of charge transfer population of n=2 excited states of Ar 16+ in tokamak plasmas

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Observation of charge transfer population of n=2 excited states of Ar 16+ in tokamak plasmas

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