J. Plasma Fusion Res. SERIES, Vol. 5 (2002) 442445

Measurement of Bremsstrahlung Profile
with a High-Spatial Resolution on LHD
NOZATO Hideaki, MORITA Shigerur, GOTO Motoshir.
EJIRI Akira and TAKASE Yuichi
Graduate School of Frontier Sciences, The University of Tolcyo,
Tokyo 113-0033, JapanI National Institutefor Fusion Science, Toki, Gifu 509-5292, Japan(Received: 11 December 2001 / Accepted:27 August 2002)

Abstract
Measurements of the bremsstrahlung emission profile have been made in order to measure theeffective charge (Z.n) profile in Large Helical Device (LHD). The main diagnostic system consists of a84 optical fiber array, an interference filter and 84 photomultipliers, which gives a spatial resolution of 5cm at the plasma center. To evaluate the contribution of impurity line radiation, the wavelength spectrum
was measured by a visible spectrometer with a CCD camera. Analysis of the emission was performed
using data from the.two diagnostics during the quasi-steady phase of NBl-heated discharges. Based on
the profiles obtained from the two diagnostics, the effect of line radiation from the edge plasma isdiscussed. As the results, it was found that the contribution of line radiation to the total radiation fromp--4.9 to 60.9 is 20-30 Vo.

Keywords:
bremmstrahlung, effective charge, X-point, edge plasma, ergodic layer

1. Introduction

determine the contribution of line radiation based ondata from the two diagnostics and an Abel inversion
method in which the finite p-effect is taken into account.

The effective charge, (2"$, profile is used toanalyze the behavior of impurities/fuel ions and the stateof discharges [1,2]. However, reports of such
2. Experimental Setup

measurements in helical devices are very limited. A
measurement of the bremsstrahlung emission profile has
been attempted in LHD. In LHD, the sight lines viewing
the plasma center generally pass through the X-point
region. Furthermore, the main plasma is surrounded by

LHD is a helical device with a magnetic field of 3T, major and average minor radii of 3.6 m and 0.6 m,
respectively [3]. The ranges of plasma parameters areelectron density z" < 15.0x1Ore m-3, electron and ion
temperatures T"< l0 keV and Ti< 4.0 keV and stored

an ergodic region and radiation by many visible line isemitted from the low-temperature, high-density plasmaenergy We < 1.0 MJ.
The profile of visible bremsstrahlung is measured
by a system consisting of an array of 84 optical fibers
with a core diameter of 300 pm, an interference filterand 84 photomultiplier tubes (PMTs). Focusing lenses

in this region. Therefore, it is very important to evaluate
the contribution of line radiation emitted from the Xpoint and ergodic regions. For this purpose, an arraywith an interference filter and a spectrometer were used.
In this paper, we report the result of our analysis toC o r re sp ondin g aut ho r' s e - mail : no zat o @ nifs. ac@2002 by The Japan Society of Plasma
Science and Nuclear Fusion Researchjp442

Nozato H. et al., Measvement of Bremsstrahlung Profile with a High-Spatial Resolution on LHD

aq6@

3p It^.- L 5.

q
Pf?'ot;

I80.9
I60tsooI

Eo40 .ExFaa.E-:ozvc

po3m 3/O 36tl 38O aoo 1.:X)R(cn)o0.0450 500 550 600 650

Fig. 1 Parallel viewing chords, magnetic surfaces and700

llavelength (nm)the separatrix.

Fig. 2 Visible spectrum measured along a chord at .z=2.6
cm (solid line), a curve fit for the continuum level(dash-dot line), and transmission coefficient of theinterference filter (dashed line).

set in front ol each fiber make 84 parallel viewing
chords along the z-direction on slightly different two
poloidal cross-section, where z denotes the verticaldistance from the equatorial plane, and the fiber inlength of about 100 m transmits visible emissions fromthe LHD diagnostic port to the diagnostic room. Thetransmission coefficient measured at He-Ne laser
wavelength (633 nm) is about 70 Vo.The viewing chords
cover the whole geometry (zmax-znin - 900 mm) of theplasma at a horizontally elongated cross section,including the ergodic region. Figure I shows the parallelviewing chords relative to the flux surfaces. The spatial
resolution of 50 mm in the z-direction is determined by
focal length of the lens (30 mm) and the size of thefiber. The time response of the PMT is 10 ms. The
central wavelength of the interference filter is 521.5 nmwith a FWHM of 5 nm, chosen to avoid regions withstrong line emissions. The relative sensitivity among the360.8_a#oF.ta
0.6o+,U>.9P.!

o.l €=ooPqoodl

0.20.0-fl) -"10 -20 0z20 40

J60(qn)Fig. 3 Chord-integrated bremsstrahlung profile in the zdirection derived from continuum emission and84 channels was calibrated using a tungsten lamp and athe fitting curve (solid line) and the ratio of line
radiation to total radiation {dash-dot line}. Dataare taken from the spectrum shown in Fig. 2.

rotating chopper (1800 rpm). The signal cross talkbetween channels was negligible, typically less thanI Vo.
A second system is a visible spectrometer with aCCD camera. .{n array of 44 optical fibers with a core
diameter of 100 pm is arranged in the vertical direction.
The 44 parallel viewing chords observe the plasma at aslightly different toroidal angle (-0.4 degree) from theviewing chords for the array with an interference filter.
The exposure lime of the CCD camera is 160 msec andthe simultaneously observable wavelength range is frombremsstrahlung emission is proportional1'o t2, it can bewritten as follows:aPnL^ -I.89x1Oa6 n"Zn, Z,2gor!'12 -"'l-ffi)
lW'm-3'nml (1)400 nm to 700 nm.

3. Experimental Results

where the units of ks, tti, Ts and )" are m-3, m-3, eV andnm, respectively. Since the background noise of theFigure 2 shows a typical spectrum at x=),.9 grlr'
taken with the visible spectrometer, when the lineaveraged density was 5.5x101e m-3. Since theCCD detector is less than a few counts per pixel and isnegligible, the continuum emission can be fitted by acurve. as shown with the dashed line. The bremsstrah443

Nozato H. et al., Meastrement of Bremsstrahlung Profile with a High-Spatial Resolution on LHDlung emission profile obtained in this way is shown inFig. 3. The asymmetric profile of bremsstrahlung originates from the vertical asymmetry in the flux surface
geometry at the poloidal cross-section of the measurement. Considering the transmission coefficient of the interference filter, the fitted curve of the continuum emission and the measured spectrum, the fraction of line radiation to the total radiation can be derived. This is also
shown in Fig. 3. From z = 40 cm to z = 40 cm, the ratio of line radiation to the total radiation is estimated to

it is important to check the change of the profile with anoise of the measurement system and difficult to

be 2O-3O 7o.

direction is zero at z = 55 cm (p=l.tZ), the point is usedas the boundary condition. To obtain the stableemissivity profile, an effective time resolution of 5 msec
at maximum is needed by a time-averaged operation,
which corresponds to 50 points due to a sampling timeof 0.1 msec. As known in Fig. 4, the convergence of theprofiles is saturated from more than 50 points. The three
profiles are individually measured by plasma of a same

In order to obtain the radial emission profile fromthe line-integrated measurements using Abel inversion,

s 600ievaluate an error bar in conjunction with the calculationof Abel inversion and a measurement noise. The radial
emissivity profile was calculated using only lower halfof the plasma, because emission from the edge region isvery strong in the upper half of the plasma (see Fig. 3).
This is probably caused by line emissions from the highdensity, low temperature region near the inboard Xpoint. In this Abel inversion, since the signal in the z-condition, which have the electron density of n"=.: 5000

2.0x101e 1m3;. The bremsstrahlung profile measured
using the array with the interference filter is presented
by the dashed line in Fig. 5, compared to the total
radiation profile and the bremsstrahlung profile deducedE{m3m20009(m80(nfrom the visible spectrometer. These profiles areobtained assuming that the boundary of emission islocated at p=I.|. The emission increases at the plasma
centet (p30.2) in all cases. It suggests that the emission
from the vicinity of the X-point is strong and the- 70O
3 600iI"***3"-fuwff:: s000i".

E ro003000,00$l20.

accumulated error in Abel inversion would appear insideFig. 4 Convergence of each radial emissivity profile
based on time-averaged results with interference

filter measurements after Abel inversion.
Additional peak

3.0

E$iE?tfr*fio

1.0FEVOLfo@Poo--IItI,E€
2.0>d9LII-----==+:-----"\ _Total

2.0

jd'.tgt _,Ets>opo>oJ 0.0
0.4

0.6

0.8

1.0

6.0:.=
oo\1.0'=R-,=3.75ml\_ - -u@FBremsstrah I ung
0.2@@oEOu!

!

0.0
0.0

Rax =3.6m

ri

1.2P

Fig. 5 Radial emission profiles measured with interference filter and bremsstrahlung, and total radiation

profiles measured with visible spectrometer.
Emissivities are obtained from Abel inversion.

0.00.00.20.4

0.60.81.0

1.2p

Fig. 6 Comparison of the bremsstrahlung profiles deduced from interference filter measurement between R"*=3.6 m (n"=2.3x101s m'3) and 3.75 m(n"=2.0x10t' m-3) configurations. Areas indicatedby A and B denote integration of the additionalemission at p(0.3.

Nozato H. et al., Measvement of Bremsstrahlung Profile with a High-Spatial Resolution on LHD

4. Summary
Visible bremsstrahlung emission has been

p=0.2 becaust: the local emissivity is calculated fromthe edge of tht: plasma to the center by turns to solve amatrix equalion. In the measurement with theinterference fitter, the profile has more radial structure
than the measurement with the spectrometer. The reason
for this difference probably arises from the line radiation
contribution. A more complete calibration and moredetailed invesr.igation are necessary. In LHD plasmas,
the volume oI the private region depends on the axis
position R"* [4,5]. Path-lengths of the private region in
the R* - 3.6 m and 3.15 m configurations are 13 cm andmeasured by two diagnostics in NBl-heated dischargesof LHD. The contribution of line emissions wasestimated to be 20-30 Vo by separating the continuum

emission in the visible spectrum measured by thespectrometer. The aim of the diagnostic using theinterference filter is to study the transport ofimpurity in
combination with impurity pellet injection within thetime-scale of several millisenonds and to complete 2"6
profile measurement. In the near future, the boronizationis planned in LHD to suppress metallic impurities [6].
The contribution of line radiation can be investigated in

20 cm, respeclively, at the horizontally elongated crosssection. As sh,rwn in Fig. 6, the additional peak for theRu*= 3.75 m case is larger than that for the R* = 3.6 mcase. The two reason that these additional peaks appear
inside p=0.2 c,rn be considered. Firstly, the accumulated
error in Abel inversion contributes as noted above.
Secondary, as known in Fig. 1, emissions from theprivate region (inside P=0.2) are also detected and thecontribution is not taken into account in Abel inversion
at present. The, ratio of the integrated additional peak at
the plasma center, A/B, is usually 1.93 and different
from the ratio (20113-1.54) of path-lengths between theRu*= 3.75 m and 3.6 m configurations. Even if each
additional pear< includes the accumulated error in Abelinversion, it can be concluded that the cause for the peakis the contribution from the emission in private region.
The region has an electron density to some extent and is
not understooc. in the masnetic structure.

more detail, since it is expected that the line contribution

would be reduced by boronization.
The authors thank Prof. H. Yamada (NIFS) for his
help on magnetic surface calculation programs and all

members of the LHD experimental group for theirsupported.

References

lll D.P. Schissel et al.,Phys. Fluids 31, 3738 (1988).
t2l M.E. Foord, US DOE Rep. DOE-ET 51013210,
159 (1986).
t3l M.Fujiwara et al.,Plasma Phys. Control. Fusion
4r,Brs7 (1999).

[4] S. Morita et al., Proc. of l2th International
Stellarator Workshop, Masion (1999).
t5l K. Narihara et al., Phys. Rev. Lett. 87, 135002(2001).

[6] S. Morita et al., IPP-report No.199 (Garching),(1995).

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