J. Plasma Fusion Res. SERIES, Vol.7 (2006) 9-13

Observation of Forbidden Magnetic Dipole Transitions
Emitted from Highly Charged Argon in LHD Discharges
KATAI Ryuji, MORITA Shigeru1 and GOTO Motoshi1
Department of Fusion Science, School of Physical Science, Graduate University for Advanced Studies, Toki 509-5292, Japan
1 National Institute for Fusion Science, Toki 509-5292, Japan（Received: 4 October 2004 / Accepted: 28 April 2005）

Abstract
Vacuum ultraviolet (VUV) and visible spectra have been measured using a 3 m normal incidence and two
50 cm Czerny-Turner spectrometers with CCD detectors, respectively, in order to observe forbidden magneticdipole (M1) transitions in 2s2 2p x (x = 1 to 5) ground and 2s2p excited configurations of highly charged argon
produced in argon discharges of Large Helical Device (LHD). One of observed VUV lines was precisely
identified as an M1 transition of Ar XII (2s2 2p3 4 S3/2 − 2 P3/2 : 649.03 ± 0.02 Å) by analyzing Doppler
broadening. Four M1 transitions of Ar X (2s2 2p5 2 P3/2 − 2P1/2 : 5533 Å), Ar XI (2s2 2p4 3 P2 − 3 P1 : 6917 Å),Ar XIV (2s2 2p 2 P1/2 − 2 P3/2 : 4413 Å) and Ar XV (2s2p 3 P1 − 3 P2 : 5944 Å) have been observed in
the visible spectral region. Temporal behaviors of the VUV and visible lines were examined for reliable
identification, and a clear diﬀerence was confirmed in comparison with those of Ar lines in lower-ionizedstages. Wavelengths of the obtained M1 transitions were determined with high accuracy and compared withprevious experimental and calculated values.
Keywords:
vacuum ultraviolet, forbidden line, magnetic dipole transition, Doppler broadening, highly charged argon
1．Introduction
Forbidden lines arising from M1 transitions of highly
charged ions are useful in plasma diagnostic applications because their long wavelengths compared with allowed transitions enable to observe Doppler broadening
for ion temperature, T i , and Doppler shift for plasma
rotation. The M1 transitions of several highly charged
ions have been studied in many laboratory and spaceplasmas, e.g., Ti, Cr, Fe, Ni, Cu and Ge in PLT (Princeton Large Torus) [1-3], Ti in PDX (Poloidal DivertorExperiment) [4], Ti, Cr, Fe, Co and Ni in DITE (Divertor Injection Tokamak Experiment) [5], Ti, V, Cr,Fe, and Ni in TFR (Tokamak de Fontenay-aux-Roses)[6], Cu, Zn, Ga, As, Kr and Y in TEXT (Texas Experimental Tokamak) [7], Ti in ATF (Advanced ToroidalFacility) [8], Ni, Ge, Kr, Zr and Mo in JET (Joint European Torus) [9] and Fe in coronal plasmas using Hubble Space Telescope [10]. However, no reports on argon
M1 transitions in VUV spectral region have been madein the field of high-temperature plasmas [11].
The M1 transitions in 2s2 2p x (x = 1 to 5) ground and2s2p excited configurations of highly charged argon
have been identified using electron beam ion trap
(EBIT) in visible spectral region [12,13], but the studyof those transitions in VUV spectral region has not been

done so far.
Observation of the M1 transitions is generally diﬃcult, because the M1 transitions are much weaker thanallowed transitions. Argon is often used as radiation
media for heat reduction in the divertor region of tokamaks [14]. In LHD, pure argon discharges have been
carried out for ion heating experiments. In such discharges visible and VUV emissions from Ar ions drastically increased and the M1 transitions were observed
for the first time in LHD. The central electron temperature is very high (4 keV) in the argon discharges.
The M1 transitions of argon are emitted from the edge
plasma due to a relatively small ionization potential of
2s2 2p x , 2s2p configurations, although x-ray lines from
H- and He-like argon are measured as a probe of coreplasma ion temperature [15]. In this paper, the first observation of the Ar M1 transition in LHD is reported
and results of the analysis are discussed.
2．Experimental setup
Argon discharges were produced by neutral beam
heating with injection power up to 12 MW. The discharges were done at a standard configuration of LHD
C 2006 by The Japan Society of Plasma

Science and Nuclear Fusion ResearchCorresponding author’s e-mail: katai@nifs.ac.jp9Observation of Forbidden Magnetic Dipole Transitions Emitted from Highly Charged Argon in LHD Discharges

under a magnetic filed strength of 2.75 T and a magnetic
axis of 3.60 m. The density was built up by Ar gas puﬀ
at the beginning of the discharge and maintained during
2 − 3 s with electron densities up to 2 × 1019 m−3 . Almost pure argon discharges were performed remaininga small amount of hydrogen.
Vacuum ultraviolet (VUV) spectra have been measured using a space-resolved VUV system, consistingof a 3 m normal incidence spectrometer with a 1200
grooves/mm grating and with a reciprocal linear dispersion of 2.75 Å/mm, CCD detector (1024 × 1024 pixels, 13µm × 13µm/ − pixel) and a pair of two focusingmirrors [16]. Since emissions of the M1 transitions are
essentially weak, a pair of two mirrors for radial profile measurement equipped in front of an incident slitof the spectrometer was removed and VUV emissions
were directly measured to make the system brighter. A
space-resolved slit set between the entrance slit and thegrating was fully opened.
Visible spectra have been measured using two 50 cm
Czerny-Turner type spectrometers equipped with CCD
detectors (1024 × 256 pixels, 26µm × 26µm/ − pixel).
Low-resolution 100 and 150 grooves/mm gratings wereselected for monitoring a wider spectral band. The
plasma visible light was collected by a focusing lens
set at a diagnostic port of LHD and transmitted by optical fibers with a core diameter of 100 µm. Another side
end of the fibers was coupled on the incident slit of thespectrometers. The CCD detectors were cooled down to−20°C to decrease a thermal noise. As a result, the thermal noise count of the CCD detectors became smaller
than 1 count/pixel. The CCD detectors were operated in’full-binning mode’, which means a full summation ofvertical pixels, in order to take a better time resolution.
The exposure time per a frame of the CCD detector was
set to 50 ms.

Fig. 1 VUV spectrum obtained from argon plasmas using 3 m normal incidence spectrometer. Italic fonts
indicate Ar M1 line.

Fig. 2 Energy level diagram shows Ar XII M1 transitionsin 2s2 2p3 ground configuration. Italic fonts indicate M1 transition obtained in this work. Widths ofarrows show rough amplitudes of transition rates.

(649.03 Å) has the strongest transition rate in the diagram.
In order to confirm the identification, Doppler broadening for argon emissions was examined and compared.
Figure 3 shows temporal behaviors of the ion temperature, Ti, deduced from Doppler broadening of Ar VII(3s3p 3 P2 − 3p2 3 P1 : 644.37 Å), Ar XII M1 transition and Ar XVI Li-like resonance line (1s2 2s 2 S1/2 −1s2 2p 2 P1/2 : 389.14 Å). The second order light wasused for the Ar XVI to increase the spectral resolution. The Ar VII and Ar XII are taken from the samedischarge, and the Ar XVI is taken from a similar discharge. A full width at half maximum (FWHM) of theinstrumental width was checked using Ar III in lowionized stage and determined to be 6.7 pixels. Since
the observed spectral width of Ar VII is close to the instrumental width, the measured T i has a relatively large

3．Experimental results and discussion
The M1 transition of Ar XII (2s2 2p3 4 S3/2 − 2 P3/2 :649.03 Å) was identified in the VUV spectrum, as
shown in Fig. 1. Most of the lines are also identified as indicated in the figure, although two Ar unclassified lines and five unknown lines are remained.
The unknown lines have a narrow line width, and thenshould be arisen from lower ionization stages. Theenergy level diagram in 2s2 2p3 ground configurationof the Ar XII transitions is illustrated in Fig. 2 with
their wavelengths calculated by the multi-configuration
Dirac-Fock method [11]. The Ar XII M1 transitions areemitted in the VUV or microwave region. The arrow
widths express relative amplitudes of calculated transition rates [11]. The observed Ar XII M1 transition
10

Observation of Forbidden Magnetic Dipole Transitions Emitted from Highly Charged Argon in LHD Discharges

Fig. 4 Visible spectrum obtained from argon plasmas using 50 cm Czerny-Turner type spectrometer. Italic
fonts indicate Ar M1 lines.

[12]. This indicates that emissions from the Ar plasmas
are really brighter than from collision experiments.
Figures 5 (a)-5 (f) show temporal behaviors of Ar
VII, Ar X, Ar XI, Ar XII, Ar XIV blended with Ar
II and Ar XV. Only a line of Ar VII is allowed transition and other lines are the M1 transition. All theemissions rapidly increase after Ar gas puﬀ suggestingthat the indicated signals are from Ar ions. The CCD
temporal resolution (50 ms) is not enough for distinction of each emission from diﬀerent ionization stages
at the increasing phase of intensities after gas puﬀ wasturned on. The electron temperature gradually increases
and reaches 4 keV at t = 1.4 s and the density keeps
4 × 1018 m−3 . The Ar VII emission having a low ionization potential of 124 eV rapidly decreases after increaseof the emission, indicating a typical behavior as an edge
particle influx of Ar ions. On the contrary, behaviorsof Ar XIV and Ar XV emissions having larger ionization potentials of 756 eV and 855 eV are much diﬀerent from the Ar VII, especially in the decay time of thesignals. These long decay times suggest a core confinement of such highly ionized Ar ions, and of course, it isconfirmed that the present identification of Ar XIV andAr XV is the unquestionable result.
The obtained M1 transitions were compared withprevious works. Results are listed in Table 1 with theoretically calculated values. The wavelength of Ar XII
VUV line was determined with high accuracy as presented in the table. The measured wavelength (649.03±0.02 Å) obtained in the present study shows a goodagreement with the theoretical value (648.93 Å). Measured wavelengths of the four visible M1 transitions (ArX, Ar XI, Ar XIV and Ar XV) do not have a good accuracy because the spectral resolution was poor due tothe usage of low-resolution gratings. The high-accuracy

Fig. 3 Temporal behaviors of ion temperature measuredfrom (a) Ar VII (3s3p 3 P2 – 3p2 3 P1 : 644.37 Å),(b) Ar XII M1 transition (2s2 2p3 4 S3/2 – 2 P3/2 :649.03 Å), (c) Ar XVI (1s2 2s 2 S1/2 – 1s2 2p2 P1/2 :389.14 Å). Raw data are indicated in (d), (e) and(f) taken from (a), (b) and (c), respectively, (opencircles: data points, solid lines: fitting by Gaussianprofile).

error bar (see Fig. 3 (a)). In contrast, the broadeningof Ar XII and Ar XVI are much wider than Ar VII, as
shown in Figs. 3 (d), 3 (e) and 3 (f). High Doppler temperatures are obtained for Ar XII and Ar XVI, although
the Ti of Ar XVI is a little higher than that of Ar XII(see Figs. 3 (b) and 3 (c)). In argon discharges the central ion temperature reaches 8 to 10 keV. These results
are very reasonable taking into account the ionization
potentials (I.P.), which roughly mean the radial location; I.P. = 124 eV for Ar VII, 618 eV for Ar XII and918 eV for Ar XVI.
A visible spectrum is shown in Fig. 4 representing
four M1 transitions of Ar X (2s2 2p5 2 P3/2 − 2 P1/2 :5533 Å), Ar XI (2s2 2p4 3 P2 − 3P1 : 6917 Å), Ar XIV(2s2 2p2 P1/2 − 2 P3/2 : 4413 Å) and Ar XV (2s2p 3 P1 −3P2 : 5944 Å). The Ar XIV line is blended with Ar II.
Those spectra were taken with an exposure time of 50ms, whereas in the EBIT experiment an exposure timeover 1 hour was necessary to get the same count rate

11Observation of Forbidden Magnetic Dipole Transitions Emitted from Highly Charged Argon in LHD Discharges

Table 1 Wavelengths of Ar M1 transitions determined from the present work in comparison with previous data.

4．Conclusions
Forbidden magnetic dipole (M1) transitions of Ar
ions were found in argon discharges of LHD. One
VUV lines was identified as the M1 transition of Ar
XII (2s2 2p3 4 S3/2 − 2 P3/2 : 649.03 Å) by analyzing
the Doppler broadening. Four visible emissions of Ar
X, Ar XI, Ar XIV and Ar XV were also identified as
the M1 transition from the analysis of time behaviors.
The emissions of the Ar M1 transitions were relatively
bright, and then a regular study on the M1 transitionsis possible in LHD. Further examination on the spectral
structure of M1 transitions will be intentionally done inaddition to the plasma diagnostics application.

measurement is possible also for the visible region.
More precise measurements will be done in near future.

Acknowledgements
The authors would like to thank the LHD experimental group members for their eﬀort to support the experiments in LHD.

[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
Fig. 5 Temporal behaviors of (a) Ar VII (b) Ar X (M1), (c)Ar XI (M1), (d) Ar XII (M1), (e) Ar XIV (M1) blended
with Ar II, (f) Ar XV (M1), (g) line-averaged electron
density and (h) central electron temperature.

[13]
[14]
[15]

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