J. Plasma Fusion Res. SERIES, Vol.5 (2002) 167-170

Observation of Gharge-Exchange Spectra on C6*+H
in Low-Energy Gollision
KOBUCHI Takashi, sATo Kuninori, Goro Motoshi, oHyABU Nobuyoshi, KAWAHATA Kazuo,
SUDO Shigeru, MOTOJIMA Osamu and LHD Experimental Group
National Institute for Fusion Science, Toki 509-5292, Japan(Received: 1 I December 2001 / Accepted: 23 August 2002)

Abstract
The extreme ultraviolet spectra of C VI have been studied for a Neutral Beam Injection (NBI)
plasmas in Large Helical Device (LHD). A strong distortion in the population distribution over theexcited levels was observed and we conclude that is caused by charge-exchange recombining (CXR)
processes between C6* ion and recycling neutral hydrogen. Spatially resolved measurements show that
the C6+-H CXR processes take place in the plasma peripheral region in LHD. We have taken a CXR panof C VI 1s-4p line using the result of a calculation code.

Keywords:
charge exchange recombination (CXR), low-energy collision, C6*+H, collisional-radiative model (CRM),

LHD

1. Introduction

by charge-exchange neutrals reflecting on the wall
surface or by plasma bombardment. These recycling

Charge-exchange recombination (CXR) crosssections of highly-ionized ions with neutral hydrogen,which can have the order of 10-15 cm2, are one of thelargest atomic collision processes in plasmas. This CXR
process: A+q + H0 -, A+(q-l)* + H' and the subsequenttransitions (termed as charge-exchange spectroscopy(CXS)) have been intensively studied in atomic

particles may have higher energies (- a few tens eV)than Frank-Condon neutral and can deeply penetrate
into the plasma [5].
In the case of CXR in low-energy collisions, theelectron is selectively captured into a single low-excitedlevel of ions. State-selective single-electron capture
between hydrogen atom and fully-stripped ion has been
so far investigated experimentally and theoretically [7].
It is known that the principal quantum number n of theions, to which the electrons are selectively captured inlow energy collisions, is determined by the charge 4 andis irrespective of the ion species [8]. For C6* + H
collisions, the dominant level of electron capture is n =physicists and plasma spectroscopists [-3]. Especiallyas a plasma diagnostics in fusion plasma, CXS using
Neutral Beam Injection (NBI) provides capabilities ofDoppler ion temperature, plasma rotation, impurity
density and transport measurements [4].
Besides the high-energy neutral beam, there exist
neutral hydrogen having low energy (E < a few tens eV)in plasmas. These hydrogen atoms can be classified into
two kinds [5,6] as follows. (1) Cold particles by gaspuffing. Since most ofthe neutral hydrogen is produced

In this paper, we discuss CXR on C6* + H in lowenergy collision. We have carried out spatially resolved

by Frank-Condon dissociation, this energy is about 3

measurements, and it is confirmed that the reacting417).

eV. (2) Recycling particles. These are mainly produced
C o r r e sp o ndin g aut ho r' s e - mail : ko b uc hi @ nifs. ac@2O02by The Japan Society of Plasma
Science and Nuclear Fusion Researchjp167

Kobuchi T. et al., observation of charge-Exchange spectra on c6*+H in Low-Energy Collision

region of CXR processes is in the plasma periphery
region. Next, we have calculated C VI intensity

3. Experimental Arrangement

distribution from the computer code, and have comparedthem with the experimental results. Finally, we discuss amechanism occurring the CXR in the low-energy
collision.

using super conducting coils with a nominal major
radius R of 3.9 m, a minor radius a of 0.6 m andmagnetic field strength of - 3 T. The electron density n"of - 1.2 x 10ra cm-3 and electron temperature T"of - 4.5
keV have been achieved with NBI of 3 MW and pellets2. Gollisional-Radiative Model Calculation
The emission line intensity distribution of C VI

injection.

Lyman series has been calculated with the COLRAD

grazlng incidence spectrometer (Schwob-Fraenkel typecode [9]. COLRAD code is a collisional-radiative modelspectrometer), which is equipped with two multichannel
detectors. The wavelength coverage is 0.5-34 nm with aLHD is a large heliotron type device (l/m = 2ll0)

In LHD, an impurity is monitored using a 2m

code for hydrogen-like ions which solves the coupled
rate equations about the time derivatives of the excitedlevel populations. Assuming the quasi-steady-state

resolution of 0.02 nm using a 600 grooves/mm grating.

This XUV spectrometer is installed at the end oflevels are obtained for a siven set of electron

manifold for the vacuum pumping system of LHD. The
neutral beams do not cross the field of view of thetemperature and density.

spectrometer.

approximation, the population densities of the excitedFigure I shows the C VI line intensity ratios

To move the field of view, spectrometer is rotatedaround a pivot located 1 meter ahead from the entrance
slit. Figure 2 shows the line-of-sight for spatiallyresolved measurements using the XUV spectrometer.
The last closed flux surface (LCFS) and the ergodic
layer in vacuum magnetic field on magnetic axis
position R* = 3.75 m are shown in Fig. 2. The fields ofview for the measurements at the plasma center andedge region are also shown. In the case of center view,the line-of-sight crosses the X-points and the thick partof the ergodic layer. The line-of-sight of 500 mm height
monitors mainly the ergodic layer.

I(Ly B )/ I(Ly a) and I(Ly y)/ I (Ly B ) calculated using
COLRAD code for the plasma parameter range of n"=
1012-1014 cm-3 and T" = 10-1000 eV. Only the ionizing
plasma component, which is proportional to the densityof ground state of Be-like ion, is shown. These results
little depend on n", and in the Z" range of 100 eV to1000 eV, ?" dependence is also small: the intensity
ratios show I(Ly B ) l I(Ly a) = 0.09+0.03, I(Ly y)l I (Ly B )= 0.26+0.03. The parameters in the emission region ofC5* ions are thought to be in the above ranges, so theintensity ratios of Lyman series are expected to be
independent on both of 7" and n" in our experiment.

Divertor leg

100

I(rvv/I(LvF)
l0-r\500m highSOXMOSHolizontalx-.-f:.1

imer wall

10-2

I(Lyp)/I(Lyct)Fl
Lmt Closed Flux Surface (LCFS)

10-3

Fig. 2 The line-of-sight of spectrometer.

lol2 - lola cm3
10-4

10100

1000

T" [eVlFig. 1 Emission ratio l\LyFlll(Lya) and /(Lyy)//(Ly0 calcu-lated using COLRAD code at n. = 1012 -1014 cm-3

and[=10-1000eV.

168

Kobuchi r. et al., observation ofcharge-Exchange Spectra on ce+H in Low-Energy collision

4. Results and Discussion
4.1 A "distortion" in the regular decrement ofthe Lyman series
Typical spectra of C VI Lyman series observed

4.2 Spatially resolved intensity measurement
In order to investigate the reacting region for C6*-H
charge-exchange collision, spatially resolved intensity
measurements were tried shot by shot for C VI Lyman

with the line-oI-sight at the plasma center with exposure
time for 0.4 sec are shown in Fig. 3. Figure 3(b) shows a"distortion" in the regular decrement for Ly7 line.
Though the relative intensity of LyB to Lya is consistent
with the theoretical prediction, the intensity rutio of Lyy
toLyP reaches to 0.9, it deviates far from the expectedvalue of 0.3. The result suggests an additional electron
populating processes to n = 4 level of C5* ion.

series spectra, and an intensity distribution of C VI
series is shown in Fig. 4. A chordal radius, theperpendicular distance from the magnetic axis to thechordal view of the spectrometer at magnetic axis, wasscanned from 0 to 500 mm. The measurements weremade in a series of nominally identical NBI heatingplasma with the total stored plasma energy IVo - 0.4 MJ,

the line averaged electron density n. - 2-3 x 1013 cm-3,
the total radiation power P.u6 - 0.5 MW, magnetic field
Bt= 2.75 T and magnetic axis Ru* = 3.6 m. Solid lines in
the figure are fitted results of the measurement. The
fitted lines of I(Ly B) and I(Lyy) are within the errors ofthe measurement, but this is not the case for I(Lyu).
The relatively smaller values at 250mm might be causedby the slightly different plasma conditions, so we don't
think that they indecate a change in the emission profile.

3 1013(b)

ox

G
gq
J'(a)

rIo

2 2 tottilr
h6-iq

F>
ootlo

II,911gtr
()

II
II

EE

The spatial profiles of Lya and Lyp intensities
present hollow shapes, and both intensities steeply
decrease in the plasma peripheral region, at around 350
mm height in chord radius. On the other hand, theintensity distribution ofLyy is different from the otherlines. An intensity inversion between LyB and Ly7 isseen at around the chordal view of 450 mm height.
In order to make clear the region where the C6+-H
CXR processes predominantly take place, the Abelinversion is carried out using the fitting lines in Fig. 4.
Intensity contour is defined using the vacuum magnetic()Qll

tlt(tltttl
IItl

q00200 400 600 800

1000

pixel number
Fig. 3 Typical spectra of C Vl Lyman series.

Lyd (xl0-r)
40"Lya(xo.r)

f-i:o;

3zovNormalized Minor Radius

0 100 200 300 400 s00 600

Fig. 5 Radial profiles of observed emission intensity ofLya, LyB and Ly7. These are calculated using Abelinversion method. Ly7"" has been estimated fromheight [mm]Fig. 4 Spatially resolved distribution of Lyman seriesspectra.

the relation between LyB and Ly7 on CRM.
t69

Kobuchi T. et aI., Observation of Charge-Exchange Spectra on C6*+H in Low-Energy Collision

surface in magnetic axis position Ru* = 3.6 m. In theoutside region of LCFS (p > 1.0), it is simply assumed+ H system. We try to reproduce the measured
distribution in decrement of C VI Lyman series

that a closed magnetic surface still exists. Figure 5shows the Abel-inverted radial emission profiles ofemission from CORLAD code with CXR process.
Assuming that the condition from CORLAD code withLyman series. The radial profiles of Lya and LyB havethe same shape and peak position at normalized miner
radius p = 0.9. The peak position of LyTis located at
outside those of Lya andLyB.
We have tried to pick up the CXR component(LyTc) from the measured Ly7 intensity. We define
I(Lyyc) as follows:

CXR process is taken, we may understand a mechanism
in which the CXR occurs in low-energy collisions inI(LvYc) = I(LvT) - 0.26 x I(Lvb,detail.

In order to extend the discussion, we have toinvestigate a wide range of plasma parameter and otherelements. We expect to observe the same charactersabout other elements, oxygen etc. If other elements arealso observed with the same characters, the(1)This assumption is reasonable for our experiment be-measurement of such as a distinctive feature may be
used for the observation of a behavior and density ofrecycling particles by comparing the results of a modelcause the electron temperature is a few hundreds eV atcalculation with the CXR process at more central region.

where the value of 0.26 is the normal ratio (see. Fig. 1).

the ergodic layer and the peak of the radial profile of

Acknowledgements

C5* exists at the inside the ergodic layer. The right-handside of Eq. (1) represents the difference between theWe are grateful to technical staffs for their help in

measured Lyy and the intensity calculated with CRM.
The emission profile of Lyygt is also shown in Fig. 5.
This differs from the emission profile of Lya and Ly B,operating the LHD, NBI, ECRH and diagnostics. And
we also wish to thank for the other members of LHD
group.

and peak position is located at outside those of Lyu andReferences

LyB. The emission profile of Lyygy suggests the reacting region of the CXR processes.

[l] R.C.Isler, Phys. Rev. Lett.38, 1359 (1977).
[2] R.K. Janev, D.E. Belie and B.H. Bransden, Phys'
Rev. A 2E,1293 (1983).
t3l R.C. Isler, Plasma Phys. Control. Fusion 36, l7l

5. Gonclusion
We have shown the characteristics of the CXR ofcarbon ions in low-energy collisions and have found
CXR spectrum which originates from the low-energy
collision on C6* + H system. Using the spatiallyresolved measurements and Abel inversion technique,the reacting region in the low-energy collision on C6* +H system is found at peripheral region. Comparing theresults of COLRAD code with the relative intensity ofLyB, we estimate CXR component in the measured Ly7line intensity.
In order to investigate a behavior of recycling
particles and the influence to plasma confinement, wemust take a plasma parameter condition in which theCXR process occurs in the low-energy collision on C6*(1994\.

t4l K. Ida and S. Hidekuma, Rev. Sci. Instrum 60,876(1989).

t5l Y. Nakashima et al., J. Phys. Soc. Jpn. 52, 4166
fl983).

t6l H. Takahashi et al., J. Phys. Soc. Jpn. 44, 1363
(1978).

t7l N. Toshima and H. Tawara, NIFS-DATA-26.
t8l M. Kimura et al., J. Phys. Soc. Jpn. 53,2224(1984).

t9l N.N. Ljepojevic, R.J. Hutcheon and J. Payne, Computer Physics Communications 44,157 (1987).

t'to