J. Plasma Fusion Res. SERIES, Vol.7 (2006) 14-17

Visible Spectral Analysis for Bremsstrahlung Measurementin High-Temperature Plasmas on LHD
NOZATO Hideaki, MORITA Shigeru1 and GOTO Motoshi1
Science and Technology Center for Atoms, Molecules and Ions Control,
Graduate School of Engineering, Osaka University, Suita 565-0871, Japan
1 National Institute for Fusion Science, Toki 509-5292, Japan（Received: 4 October 2004 / Accepted: 24 May 2005）

Abstract
Visible bremsstrahlung in hydrogen and helium plasmas has been measured using a visible spectrometer.
In LHD, an investigation of visible spectrum for correct bremsstrahlung measurement becomes importantbecause emission lines as well as visible continuum are emitted. For bremsstrahlung measurement, contributions of visible emission lines were estimated by visible spectroscopy. As a result, it was found that theeﬀect of emission lines could not be negligible in the range of 2 × 10 19 ≤ ne ≤ 6 × 1019 m−3 . A dependenceof eﬀective charge (Ze f f ) on the line-averaged electron density is evaluated, and the Z e f f values evaluated byspectrometer are mainly comparable to those evaluated by an interference filter at n e ≤ 5 × 1019 m−3 .
Keywords:
visible bremsstrahlung, visible spectroscopy, eﬀective charge, carbon impurity,hydrogen and helium plasmas, LHD
1．Introduction
Bremsstrahlung emission for fusion plasmas gives
the eﬀective charge, Ze f f , of plasmas and the value becomes important for studies on impurity behavior [1]and plasma collisionality. The emission process of thebremsstrahlung is given by Coulomb collisions between
electrons and ions, and the emissivity per unit wavelength λ (nm) is expressed by∆P 1.89 × 10−33 n2e g f f Ze f f=∆λT e1/2 λ2
1240exp −(Wm−3 nm−1 ),Teλplasma with high density roughly equal to the core density because of the existence of the ergodic layer. Then,a careful investigation on the influence of emission lines
is particularly required in LHD in order to measure thecorrect bremsstrahlung signal. For the purpose, the visible spectroscopy was done to observe the visible emission lines from LHD plasmas. In this paper, the contribution of emission lines to the bremsstrahlung measurement is reported with discussions on influences ofvarious wall conditionings such as Ti gettering (TiG),
boronization and glow discharge cleanings with H2 , Heand Ne gases.

(1)

2．Bremsstrahlung diagnostics
The visible continuum emitted from LHD plasmas
is monitored by two diagnostic systems, which are a
monochromatic system [5] and a visible spectrometer.
The monochromatic system consists of an interference
filter (λ = 536.6 nm, ∆λ1/2 = 6.2 nm, peak transmission
rate = 0.69) and 80 optical fibers connected to focusing lenses and photomultipliers. The data are acquiredby camac system with a sampling rate of 10 kHz. A
50 cm visible spectrometer with a charge-coupled device (CCD; 1024 pixels) is used to monitor visible lightfrom 510 to 590 nm. The CCD camera was cooled

where ne (m−3 ), T e (eV), Ze f f and g f f stand for theelectron density, the electron temperature, the eﬀective
charge and the free-free Gaunt factor, respectively. The
value of Ze f f is defined as
Ze f f =2s ns Zsne,(2)where Z is the charge state and the subscript s stands for
all ionic species in plasmas.
In high-temperature plasmas, a visible spectrum consists of continuum bremsstrahlung and emission lines.
The presence of the emission lines disturbs a correct
bremsstrahlung measurement [2-4]. The LHD coreplasma is surrounded by a thick low-temperature

C 2006 by The Japan Society of Plasma

Science and Nuclear Fusion Research

Corresponding author’s e-mail: nozato@ppl.eng.osaka-u.ac.jp

14Visible Spectral Analysis for Bremsstrahlung Measurement in High-Temperature Plasmas on LHD

down to −20 ◦ C, at which the noise level was negligible. In the present experiment, observation positions
in both diagnostics are nearly close along the equatorial plane of LHD. The spatial resolution defined by thefocal lenses was 5 cm at plasma center for both cases.
The contribution of emission lines can be, thus, evaluated from the wavelength dependence of λ−2 exp(1/λ)
as known in eq. (1).

traced by thick solid lines. In hydrogen plasmas afterTiG, many C I emission lines and a charge-exchange
recombination line of C VI (528.9 nm) are seen in thespectrum. In helium plasmas after TiG, the He II line isstrongly emitted at λ = 541.2 nm, while the C VI line
becomes considerably weak. In hydrogen plasmas afterboronization, however, the strong carbon emission lines
are disappeared.
For quantitative evaluations, the bremsstrahlung
emission and the visible line contribution estimated
1.02spectra

Intensity (103 counts)

CVI

brems.
filter

0.8
0.6

10.4
0.2

(a) H plasma
21.0

Intensity (10 3 counts)(b) He plasma
HeII0.6
10.4
0.2
0021.0Intensity (103 counts)

20.610.40.20520

103
450500 550 600 650
Wavelength (nm)0.8CIIHeICIIHHCIII

104HeII

H

Intensity (counts)

106

0.8

Filter transmission rate

0Filter transmission rate

20(c) H plasma (Bor.)

105

Filter transmission rate

3．Results of spectral analysis and discussions
In LHD, almost all visible emission lines are radiated in the ergodic layer located outside the last closed
flux surface [6]. The visible lines consist of many elements such as fuel gases, carbon, oxygen, metals andNe and Ar puﬀed for diagnostic use. A typical example of visible spectra measured by the spectrometer with150 grooves/mm grating is presented in Fig. 1, which istaken in a hydrogen plasma with Ti-gettering (TiG). Analyzing the visible spectra, it is suggested that the wavelength range of 530 to 550 nm or the vicinity of 630 nm
is a unique candidate for the visible bremsstrahlung
measurement. The free-spectral range of 530-550 nm
is wider than other ones. The wavelength range of
530-550 nm seems to be the best choice for the visible
bremsstrahlung measurement. In order to analyze thecontribution of emission lines in detail, high-resolution
visible spectroscopy was made using 600 grooves/mm
grating for various discharge conditions.
Figure 2 shows visible spectra recorded from (a)hydrogen and (b) helium plasmas after TiG and(c) hydrogen plasma after boronization. Here, thebremsstrahlung emissions denoted by dashed lines areapproximated as λ−2 exp(1/λ), which is derived by Eq.
(1). The transmission rate of the interference filter is530

540 550 560
Wavelength (nm)

570

0580

Fig. 2 Visible spectra (thin solid lines) in (a) hydrogenand (b) helium plasmas after TiG., and (c) hydrogen plasmas after boronization. Dashed and thicksolid lines indicate bremsstrahlung emissions andtransmission rate of interference filter, respectively.

700Fig. 1 Typical example of visible spectrum in hydrogenplasmas after TiG.

15

Visible Spectral Analysis for Bremsstrahlung Measurement in High-Temperature Plasmas on LHD

from visible spectra are plotted in Fig. 3 as a functionof electron density. The solid line in the figure indicates the square dependence of bremsstrahlung signal.
Since the intensity of visible emission lines cannot benegligible in a range of the electron densities, the truebremsstrahlung emission can be obtained by evaluating
the dependence of visible continuum on the wavelength.
Thus, the bremsstrahlung measurement only using thepresent interference filter seems to be diﬃcult. Under
the present situation, the spectroscopy of visible light isneeded for an exact Ze f f evaluation.
Line-averaged Ze f f are, thus, obtained as a functionof electron density for H2 and He plasmas after TiG andboronization as shown in Fig. 4. The Ze f f values estimated from only the visible spectrum are shown in

the figure, which are denoted with ’Spectrometer’ inthe bracket. ’Filter’ indicates the Ze f f evaluated fromthe monochromatic measurement, and the values arealso plotted as a reference only in high electron density regime. Interference filter with 3.0 nm of FWHM isnormally used for visible bremsstrahlung measurement[3,7,8]. However, an interference filter with 6.2 nm of
FWHM is adopted because of roughly 50 % light loss
by a 10 cm quartz glass and 100 m optical fiber band inour bremsstrahlung diagnostics. As a result, the Ze f fvalues by ’Filter’ are comparable to those by ’Spectrometer’ at ne ≤ 5×1019 m−3 . The low-density limit for
the Ze f f measurement seems to be 2 × 1019 m−3 , and thebonized wall-coating in such a density regime would beeﬀective for a reduction of high-Z impurity contamination. Moreover, it is a little surprise to see in the figure that the Ze f f have the same values for H2 and Hedischarges. As one of possible reasons, the carbons inhydrogen plasma increase much greater than that in helium plasmas. As known in Fig. 2(a), the strong emission line of C VI (528.9 nm) is observed, which shows
the amount of fully ionized carbon ions in core plasma.
Emission lines from carbon molecules are also observedin hydrogen discharges.

5Visible line contribution
Estimated bremsstrahlung

Intensity (104 counts)

432

14．Summary
For visible bremsstrahlung measurement in LHD,
the wavelength spectrum was obtained from visible
spectroscopy, and the contribution of emission lines wasanalyzed in various discharge conditions. Strong emission lines such as C VI (528.9 nm) and He II (541.2 nm)
were observed in hydrogen and helium plasmas, respectively. The contribution of emission lines, which occupied 10 − 50 % of visible emissions through an interference filter at 2 × 1019 ≤ ne ≤ 6 × 1019 m−3 , was estimated by high-resolution spectroscopic measurement.
The line contribution was found not to be negligible
for the visible bremsstrahlung measurement using theinterference filter. Moreover, any large improvement
was not observed for the variety of discharge cleaning
conditions, although only the boronization reduced thespectral line contribution.
In future work, the radial distribution of the line contribution has to be measured in order to take the Ze f fprofile, especially for the edge Ze f f profile. An alternative idea may be needed e.g. changing the filter wavelength to infrared light.

00

1

2

3
4ne (1019 m-3)

56Fig. 3 Intensities of bremsstrahlung and emission lines,
estimated from visible spectrum, as a function ofline-averaged electron density.

5H TiG (Filter)
H TiG (Spectrometer)
H Bor. (Spectrometer)
He TiG (Filter)
He TiG (Spectrometer)

Zeff

4321024619ne (10 m-3)

810Fig. 4 Line-averaged Zeff against line-averaged electrondensity (○, ●, +: H2 plasmas after TiG andboronization, ■, □: He plasmas after TiG). Each’Spectrometer’ and ’Filter’ indicates Zeff values,
which are estimated by visible spectrometer andmonochromatic measurement using an interference filter.

[1][2]
16References
H. Nozato, S. Morita, M. Goto et al., Phys. Plasmas 11, 1920 (2004).
M.E. Foord, E.S. Marmar and J.L. Terry, Rev.

Visible Spectral Analysis for Bremsstrahlung Measurement in High-Temperature Plasmas on LHD

[3]
[4]
[5]

Sci. Instrum. 53, 1407 (1982).
F. Orsitto, M.R. Belforte, M. Borra et al., Rev.
Sci. Instrum. 68, 1024 (1997).
S. Morita and J. Baldzuhn, Max Planck Institute
IPP-report (Garching, Germany), No.199 (1995).
H. Nozato, S. Morita, M. Goto et al., J. Plasma
Fusion Res. Series 5, 442 (2002).

[6]
[7]
[8]

17

M. Goto and S. Morita, Phys. Rev. E 65, 026401(2002).
E.S. Marmar, R.L. Boivin et al., Rev. Sci. Instrum. 72, 940 (2001).
D.G. Whyte, M.R. Wade, D.F. Finkenthal et al.,Nucl. Fusion 38, 387 (1997).