A collisional-radiative model for Be-like oxygen ions has been constructed for OV plasmaspectroscopy. The model takes into account recombination processes as well as collisional ionization, radiative transitions, and collisional excitation/deexcitation. Two sets of atomic data are used for comparison. We obtain OV line intensities as functions of electron temperature and density. The line intensity ratios of 2s3s 35 - 2s3p \u27Pr=o.r., are measured in LHD plasmas and are consistent with our models. The line intensity ratio of 2s2p 3P - 2p"P and 2s2 tS - 2s2p rP in recombining plasma is an increasing function of temperature and one measured in the LHD plasma indicates electron temperature less than 7eV. The ratios measured in steady-state phase are larger than I and difficult to explain with the current model

GOTO  Motoshi

Izumi MURAKAMI

KATO  Takako

KOBUCHI  Takashi

MORITA  Shigeru

Motoshi GOTO

MURAKAMI  Izumi

SAFRONOVA  Ulyana

Shigeru MORITA

Takako KATO

Takashi KOBUCHI

Ulyana SAFRONOVA

National Institute for Fusion Science (NIFS-Repository)

J. Plasma Fusion Res. SERIES, Vol.5 (2002) 171-174Properties of OV Spectral Lines in lonizing andRecombining PlasmasMURAKAMI Izumi, KATO Takako, SAFRONOVA Ulyana I.t, KOBUCHI Takashi,GOTO Motoshi and MORITA ShigeruNational Instilute for Fusion Science, Toki 509-5292, JapanI University of Notre Dame, Notre Dame, IN 46556, USA(Received: 11 December 2001 / Accepted: ll July 2002)AbstractA collisional-radiative model for Be-like oxygen ions has been constructed for OV plasmaspectroscopy. The model takes into account recombination processes as well as collisional ionization,radiative transitions, and collisional excitation/deexcitation. Two sets of atomic data are used forcomparison. We obtain OV line intensities as functions of electron temperature and density. The lineintensity ratios of 2s3s 35 - 2s3p 'Pr=o.r., are measured in LHD plasmas and are consistent with ourmodels. The line intensity ratio of 2s2p 3P - 2p"P and 2s2 tS - 2s2p rP in recombining plasma is anincreasing function of temperature and one measured in the LHD plasma indicates electron temperatureless than 7eV. The ratios measured in steady-state phase are larger than I and difficult to explain with thecurrent model.Keywords:OV, plasma diagnostic, collisional-radiative model, recombining plasma1. IntroductionOV spectral lines are often measured in laboratoryplasmas and used for plasma diagnostics, since oxygenis one of popular impurity materials. In plasmas of theLarge Helical Device (LHD) at National Institute forFusion Science, Japan, the OV resonance line at 630 Ais used for monitoring plasma and some OV lines aremeasured with VUV and optical spectrometers. LHDplasmas seem to become recombining plasma at thelater phase of shots. However, most theoretical study forOV lines did not include recombination processes forpopulation kinetics and models were limited for lowerexcited states up to 2s3l levels [1-5].We have constructed a collisional-radiative model(CRM) for OV (Be-like oxygen) ions with large numberof excited states, including recombination processes foreach excited states. The CRM calculates populationdensities of excited states and spectral line intensitieswith given electron density and temperature. Here wecompare calculated line intensity ratios of OV lines withmeasurement of LHD plasmas and discuss properties ofOV spectral lines.2. Collisional-Radiative ModelPopulation densities ofexcited states are calculatedby solving rate equations with assumption of steadystate in the CRM. Electron temperature and density aregiven for each calculation. The rate equations take intoaccount radiative transitions, collisional excitation/deexcitation, collisional ionization to Li-like ions, anddielectronic, radiative, and three-body recombinationfrom Li-like ions.The steady-state rate equations are linear functions@200.2by The Japan Society of PlasmaScience and Nuclear Fusion ResearchC o r r e s p o ndin g aut ho r' s e - mail : mi zumi @ nifs. ac.j pt7lMurakami l. et al., Properties of OV Spectral Lines in Ionizing and Recombining Plasmasof the Be-like ground state population n r and the Li-likeground state population ,4+. We can obtain the excitedstate population densities n(i) as a combination of acomponent proportional to n1, and a componentproportional to n+:n(i) = N1Q)n1+ Np(l)n* . (1)We call Nr(t) the ionizing plasma component, and Np(l)the recombining plasma component, respectively. Theionizing plasma component originates from collisionalexcitation from the Belike ground state, and the recom-bining plasma component originates from recombinationfrom the Li-like ground state. The two components canbe calculated separately.In the CRM, we consider 2snlx*tL1 and2pnlzs'tL,levels with n < 6, and 2snl levels up to n = 50 as /-bundled hydrogenic levels, where n is the principalquantum number and I is the orbital angular momentumquantum number. The 2pnl levels with n > 6 areautoionization states and we do not include them to theCRM. The energy levels and transition probabilities arecalculated by Cowan code [6] for levels with n < 6, andby hydrogenic approximation [7] for higher levels.For collisional excitation rate coefficients, we usedata calculated by R-matrix method for transitionsamong 2s21, 2p21, and 2s3l from Kato et al. lll. Forother transitions among z < 6 levels, we use 2 data sets:Model 1 uses data by Sampson et al. [8] fot 2lnl'-2p31"transitions and Mewe's empirical formula [9] for othertransitions. Model 2 uses collision strength calculated byHullac code [0]. We use empirical formula for n > 6levels in both models.For recombination processes, dielectronicrecombination rate coefficients are calculated with theuse of Cowan code by Murakami et al. [l1]. These arenew data and we do not find a paper which describes thecontribution of dielectronic recombination into separateexcited states up to n = 50 precisely. Radiativerecombination rate coefficients are estimated fromphotoionization cross section by Clark et al. flZ) for2szl states with n < 5 and from hydrogenicapproximation for other 2snl levels, and three bodyrecombination rate coefficients are calculated as aninverse process of collisional ionization. Collisionalionization rate coefficients are calculated with Lotzformula [l3].Spectral line intensity ratio of transitions from i to jand from p to q is obtained as I(i + j)ll(p -> q) =A,(ij)n(i)/(A,(p,q)n(q)), where A,(lj) is radiativetransition probability for transitions from I toj.3. Spectral Line Intensity Ratios3.1 Triplet transitions 2s3s 35 - 2s3p 3PHere we examine the triplet transitions 2s3s 3S -2s3p 3P, (J=0,1,2) at LL 2781, 2787. 2'790 A. Figure Ishows the spectra taken from the LHD experiment (shotno.10933) by the UV-visible spectrometer.Figure 2 shows calculated line ratios as functions ofelectron density with given electron temperature inionizing plasma (ionizing component only) for model 1and model 2, respectively. The difference of atomic data(collisional excitation rate coefficients) causesprominent difference in the line ratios. The model Iexhibits strong electron density and temperature400 4s0 500 sso 600Pixel numberFig. 1 OV Spectra of 2s3s 35, - 2s3p 3Pr(J=0,1,2) transi-tions measured in the LHD experiments.(J=1 )/(J=0)trj l,iooarz I10eV 30eV l00ev300ev(J=2y(J=O)E ! -'_w -a4.(J=1y(J=0),,,t,,,,t,,,,11210(06cc)11 12 13 14-5\rog n3 \cm /12 13 14 15log n. (cm ,Figu.2 Line intensity ratios, (J=2)/(J=0) and (J=1)/(J=0)of 2s3s 35 - 2s3p 3P transitions (see Sec. 3.1 ), as afunction of electron density in ionizing plasma formodel 1 (a) and model 2 (b). Circles with errorsare measurements of the LHD and bar is of theExtrap.t72Murakami l. et al., Prcpetties of OV Spectral Lines in Ionizing and Recombining Plasmasdependence for both line ratios I(2s3s 351 - 2s3p 3Pr1lI(2s3s 3S1 - 2s-]p 3P0) [shown as (J=2)/(J=0)] and I(2s3s35, - 2s3p 3P,;/I12s3s 35, - 2s3p 3po; lshown as (J=l)/(J=0)1, but the model 2 shows smaller dependences. Theratios in recombining plasma (recombining componentonly) have larger dependences in model 1 than in model2, similarly to ones in ionizing plasma.Escape from 2s3p 3P6 to upper levels such as 2s4llevels by several to orders of magnitude largercollisional excitation rates in model I than in model 2causes anomalous population distribution for 2s3p 3P J =0,1,2 states, deviating from ratios determined by thestatistical weights.Experimental data measured from the LHD plasmasin the Third cycle (in 1999) (circles with error bars) andthe Extrap Tl and T2 reversed field pinch experimentsat KTH in Sweden (bar) tal are shown inFig. 2. For theLHD measurements, the electron density andtemperature are nearly constant during exposure time(0.6-1 sec.) and plasmas are expected to be in steadystate. The LHf) measurements agree with both modelswithin error birrs. The model 1 indicates low electrontemperature of 10-30 eV. Weak temperature dependenceof the model 2 does not specify the temperature region.As for comparison with the Extrap measurement, themodel I indicates -30 eV which is lower than expectedby Kato et al. [4]. On the other hand, the model 2 doesnot reproduce the ratios. There still remains a problemfor explaining the Extrap measurement.3.2 The resonance line and triplet transitions2s2p3P - 2ptPIn this subsection we examine the line ratio of theresonance line !ls2 rS - 2s2p tP at ,t 630 A and the triplettransitions 2s2p'3p -2p"P at M,758-762 A, which areunresolved in the LHD VUV spectra taken with theVUV multi-channel spectrometer (Fig. 3). Figure 4ashows the temporal distribution of the intensity ratio forthe shot no.150'78 in 1999. The exposure time was 0.184sec. The LHD plasma was in nearly steady state withNBI injection after pellet injection was stopped and thenchanged into recombining phase after NBI injection wasstopped.Figure 4(b I shows the calculated ratios for the casesof ionizing pJasma and recombining plasma as afunction of electron temperature for model 2. Theirdensity dependences are very small in the density regionof l012cm-3 - l0r5cm-3. The difference between modelsI and 2 for the ratio becomes larger at higher electrontemperature.In ionizing plasma, naively the resonance line isalmost always stronger than the triplet transition line.The temperature dependence of the ratio is caused bydifferent temperature dependences of the collisionalexcitation rates from the ground state to 2s2p tP and 2p23P states. In recombining plasma, the temperaturedependence of the ratio at - 3-30 eV is caused by thatof the dielectronic recombination to 2p2 3P states.The observed ratios in recombining phase indicatelow temperature less than 7 eV. We expect that OV lineswould be emitted in a peripheral plasma, and such lowtemperature is not so surprising. However, during the0 200 400 600 800 1000ChannelFig. 3 VUV Spectra measured in the LHD experiments.5 6000bEfo 4000.5 1 1.5 21t(sec)10 10T. (ev)Fig. 4 (a) Temporal distribution of intensity ratio l(2s2p"P - 2p'3Plll(2s2 15 - 2s2p lP) measured in the LHDexperiments (shot no.15078). (b) The calculatedintensity ratio of the same lines as in (a) for ioniz-ing plasma (lower) and recombining plasma (up-per) as a function of electron temperature formodel 2. The ratio in equilibrium plasma (1013cm'3) is also plotted (dashed line). Triangles areobtained by Keenan et al. [3].#1 5078 (1 .288-1173Murakami l. et al., Properties of OV Spectral Lines in Ionizing and Recombining Plasmassteady-state phase the ratios are larger than I, whichcannot be reproduced by our CRM for ionizing plasmaor equilibrium plasma in which ionization equilibrium isassumed with ion abundance ratia n+lnt calculated bythe ADAS package [151. We will discuss about thediscrepancy in the next seition.4. DiscussionIn the LHD experiments the triplet transition line2s2p 3P - 2pt'P is stronger than the resooance line 2s2tS - 2s2p rP even in steady phase. The current modelseems too naive, since there would be a differentchannel to populate more at 2p2 3P levels. The inner-shell ionization from 2s22p B-like ground state to 2s2p3P metastable states followed by collisional excitation to2p2 3P levels could be non negligible at T" > 30 eV inthis electron density region.The ion abundance of B-like oxygen ions inionization equilibrium is comparable to that of Be-likeoxygen ions at - 19 eV and is not so small at highertemperature than 19 eV [141. OIV lines from B-likeoxygen ions are observed at the same time in the LHD$UV spectra. We need to include the effect of B-likeions in the CRM with a model for ionization/recombination balance for all oxygen ions. Chargetransfer process with neutral hydrogen in a peripheralplasma could also contribute to OV line intensities andwe will need to estimate the effect as well.The difference of atomic data causes significanteffect to the line intensity ratios as seen in Sec. 3.1. Thisresult also indicates the importance of higher excitedstates in a CRM even in a ionizing plasma. In order toexplain the Extrap measurements with large ratios, somemechanism to make population anomaly between 2s3p3P J=0,1,2 levels would be necessary. We need moreaccurate atomic data for excitation rate coefficients forhigher excited states.In summary, we have constructed a collisional-radiative model and calculated OV line intensity ratiosfor the resonance line and the triplet transitions.Comparing the model calculations with measurementsof the LHD plasmas and the Extrap plasmas, wediscussed the difference caused by atomic data and theline ratios in ionizing and recombing plasma phases. Weneed to improve the model with including the effect ofB-like ions for a future plan.References[1] T. Kato et al., At. Data Nucl. Data Tables 44, 133(l990).t2l F.P. Keenan et al., Astropbys. J.382,349 (1991).t3l F.P. Keenan et al.,Mon. Not. R. Astron. Soc.276,91s (1995).[4] T. Kato et al.,J. Phys. B 29,4241 (1996).t5l E.E. Doschek et o/., Astrophys. J. 518, 909 (1999).[6] R.D. Cowan, The theory of atomic structure andspectra, University of Califomia Press (1981).t7l U.L Safronova et al., Physica Scripta 47, 364(l993).[8] D.H. Sampson et al., At. Data Nucl. Data Tables30, r25 (1984).t9l R. Mewe, Astron. Astrophys. 20,215 (19-12).t10l A. Bar-Shalom et al., Phys. Rev. A 38, 1773(1988).[11] I. Murakami et al., NIFS-DATA-66 (2001).[2] R.E.H. Clark et al., At. Data Nucl. Data Tables 34,415 (1986).[13] W. Lotz, Astrophys. J. Suppl.14,207 (1967).l14l M. Arnaud and R.Rothenflug, Astron. Astrophys.Suppl. 60,425 (1985).I I 5 ] URL=http://adas.phys. strath.ac.uk/r74

Properties of OV Spectral Lines in Ionizing and Recombining Plasmas

Properties of OV Spectral Lines in Ionizing and Recombining Plasmas

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