J. Plasma Fusion Res. SERIES, Vol.5 (2002) 171-174

Properties of OV Spectral Lines in lonizing andRecombining Plasmas
MURAKAMI Izumi, KATO Takako, SAFRONOVA Ulyana I.t, KOBUCHI Takashi,GOTO Motoshi and MORITA Shigeru
National 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)

Abstract

A collisional-radiative model for Be-like oxygen ions has been constructed for OV plasma
spectroscopy. 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 line
intensity 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 plasma

1. Introduction

densities of excited states and spectral line intensities

OV spectral lines are often measured in laboratory
plasmas and used for plasma diagnostics, since oxygenis one of popular impurity materials. In plasmas of theLarge Helical Device (LHD) at National Institute forwith given electron density and temperature. Here wecompare calculated line intensity ratios of OV lines withmeasurement of LHD plasmas and discuss properties ofOV spectral lines.

Fusion Science, Japan, the OV resonance line at 630 A
is used for monitoring plasma and some OV lines aremeasured with VUV and optical spectrometers. LHD
plasmas seem to become recombining plasma at thelater phase of shots. However, most theoretical study for

2. Collisional-Radiative Model
Population densities ofexcited states are calculated

by solving rate equations with assumption of steadystate in the CRM. Electron temperature and density aregiven for each calculation. The rate equations take intoOV lines did not include recombination processes for
population kinetics and models were limited for lower

account radiative transitions, collisional excitation/deexcitation, collisional ionization to Li-like ions, anddielectronic, radiative, and three-body recombinationfrom Li-like ions.

excited 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 population

The steady-state rate equations are linear functions@200.2by The Japan Society of Plasma
Science and Nuclear Fusion ResearchC o r r e sp o ndin g aut ho r' s e - mail : mi zumi @ nifs. ac.j pt7l

Murakami l. et al., Properties of OV Spectral Lines in Ionizing and Recombining Plasmas

3. Spectral Line Intensity Ratios
3P
3.1 Triplet transitions 2s3s 35 - 2s3pof 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 componentHere we examine the triplet transitions 2s3s 3S 2s3p 3P, (J=0,1,2) at LL 2781, 2787. 2'790 A. Figure I
shows the spectra taken from the LHD experiment (shotno.10933) by the UV-visible spectrometer.

proportional to n+:n(i) = N1Q)n1+ Np(l)n* .

(1)Figure 2 shows calculated line ratios as functions ofWe call Nr(t) the ionizing plasma component, and Np(l)the recombining plasma component, respectively. The
ionizing plasma component originates from collisional
excitation from the Belike ground state, and the recom-electron density with given electron temperature inbining plasma component originates from recombination

prominent difference in the line ratios. The model I
exhibits strong electron density and temperatureionizing plasma (ionizing component only) for model 1
and model 2, respectively. The difference of atomic data(collisional excitation rate coefficients) causes

from 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 principal
quantum number and I is the orbital angular momentum

quantum number. The 2pnl levels with n > 6 are
autoionization 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 use

data calculated by R-matrix method for transitions
among 2s21, 2p21, and 2s3l from Kato et al. lll. For
other 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 by
Hullac code [0]. We use empirical formula for n > 6levels in both models.

400sso500

4s0600

Pixel numberFig. 1 OV Spectra of 2s3s 35, - 2s3p 3Pr(J=0,1,2) transitions measured in the LHD experiments.

For recombination processes, dielectronic

12recombination rate coefficients are calculated with theuse of Cowan code by Murakami et al. [l1]. These are
new data and we do not find a paper which describes thecontribution of dielectronic recombination into separate
excited states up to n = 50 precisely. Radiative
recombination rate coefficients are estimated fromphotoionization cross section by Clark et al. flZ) for

trj l,iooarz

10I

10eV 30eV l00ev300ev(0(J=2y(J=O)E !

6c'_ -c)w -a4.

2szl states with n < 5 and from hydrogenic(J=1 )/(J=0)

approximation for other 2snl levels, and three body
recombination rate coefficients are calculated as aninverse process of collisional ionization. Collisionalionization rate coefficients are calculated with Lotz
formula [l3].
Spectral line intensity ratio of transitions from i to j
and 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.

11 12 13 14
rog n3 \cm-5\/(J=1y(J=0),,,t,,,,t,,,,1

12 13 1415

log 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 errors
are measurements of the LHD and bar is of theExtrap.

t72

Murakami l. et al., Prcpetties of OV Spectral Lines in Ionizing and Recombining Plasmas

dependence for both line ratios I(2s3s 351I(2s3s 3S1-2s-]p 3P0)

- 2s3p 3Pr1l

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 collisional
excitation rates from the ground state to 2s2p tP and 2p2
3P states. In recombining plasma, the temperaturedependence of the ratio at - 3-30 eV is caused by thatof the dielectronic recombination to 2p2 3P states.

[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. The
ratios 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 2s4l

levels by several to orders of magnitude larger
collisional excitation rates in model I than in model 2

The observed ratios in recombining phase indicate

low temperature less than 7 eV. We expect that OV lines
would be emitted in a peripheral plasma, and such low
temperature is not so surprising. However, during thecauses anomalous population distribution for 2s3p 3P J =0,1,2 states, deviating from ratios determined by thestatistical weights.
Experimental data measured from the LHD plasmas
in the Third cycle (in 1999) (circles with error bars) andthe Extrap Tl and T2 reversed field pinch experiments
at KTH in Sweden (bar) tal are shown inFig. 2. For the#1 5078 (1 .288-1

LHD 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 models
within 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 does
not reproduce the ratios. There still remains a problem
for explaining the Extrap measurement.

5 6000bEfo 4000

0200400600

8001000

ChannelFig. 3 VUV Spectra measured in the LHD experiments.

3.2 The resonance line and triplet transitions
2s2p3P - 2ptP
In this subsection we examine the line ratio of theresonance line !ls2 rS - 2s2p tP at ,t 630 A and the triplet

transitions 2s2p'3p -2p"P at M,758-762 A, which are
unresolved 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 0.184
sec. The LHD plasma was in nearly steady state with
NBI injection after pellet injection was stopped and thenchanged into recombining phase after NBI injection was.511.521t(sec)stopped.

1010T. (ev)Fig. 4 (a) Temporal distribution of intensity ratio l(2s2p15- 2s2p lP) measured in the LHD"P - 2p'3Plll(2s2Figure 4(b I shows the calculated ratios for the casesof ionizing pJasma and recombining plasma as adensity dependences are very small in the density regionof l012cm-3 - l0r5cm-3. The difference between models
I and 2 for the ratio becomes larger at higher electron

experiments (shot no.15078). (b) The calculated
intensity ratio of the same lines as in (a) for ionizing plasma (lower) and recombining plasma (upper) as a function of electron temperature formodel 2. The ratio in equilibrium plasma (1013cm'3) is also plotted (dashed line). Triangles aretemperature.

obtained by Keenan et al. [3].

function of electron temperature for model 2. Their173

Murakami l. et al., Properties of OV Spectral Lines in Ionizing and Recombining Plasmas

steady-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 by
the ADAS package [151. We will discuss about thehigher excited states.

In summary, we have constructed a collisionalradiative model and calculated OV line intensity ratios
for the resonance line and the triplet transitions.
Comparing the model calculations with measurementsof the LHD plasmas and the Extrap plasmas, we
discussed the difference caused by atomic data and theline ratios in ionizing and recombing plasma phases. We
need to improve the model with including the effect ofB-like ions for a future plan.

discrepancy in the next seition.

4. Discussion
In the LHD experiments the triplet transition line
2s2p 3PtS- 2pt'P is stronger than the resooance line 2s2rP even in steady phase. The current model
2s2p-References

seems too naive, since there would be a different
channel to populate more at 2p2 3P levels. The innershell ionization from 2s22p B-like ground state to 2s2p

[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,

3P metastable states followed by collisional excitation to

2p2 3P levels could be non negligible at T" > 30 eV in

91s (1995).
[4] T. Kato et al.,J. Phys. B 29,4241 (1996).

this electron density region.

The ion abundance of B-like oxygen ions inionization equilibrium is comparable to that of Be-like
oxygen ions at - 19 eV and is not so small at higher
temperature than 19 eV [141. OIV lines from B-like
oxygen ions are observed at the same time in the LHD$UV spectra. We need to include the effect of B-liket5l 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).
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recombination balance for all oxygen ions. Charge

30, r25 (1984).
t9l R. Mewe, Astron. Astrophys. 20,215 (19-12).

transfer process with neutral hydrogen in a peripheral
plasma could also contribute to OV line intensities andwe will need to estimate the effect as well.
The difference of atomic data causes significant
effect to the line intensity ratios as seen in Sec. 3.1. This
result 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 2s3p
3P J=0,1,2 levels would be necessary. We need moreaccurate atomic data for excitation rate coefficients fort10l 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/

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