22C12
Wideband High-Resolution Spectroscopy of Metallic Pellet
Ablation Plasmas in LHD
LHD中金属ペレット溶発プラズマの広波長帯域同時高分解分光計測
Hirotaka Tanaka1, Keisuke Fujii1, Taiichi Shikama1, Motoshi Goto2,Shigeru Morita2, and Masahiro Hasuo1
田中博隆1, 藤井恵介1, 四竈泰一1, 後藤基志2, 森田繁2, 蓮尾昌裕1
1, Graduate School of Engineering, Kyoto University
Yoshidahonmachi, Sakyo-ku, Kyoto 606-8501, Japan
2, National Institute for Fusion Science322-6, Oroshi-cho, Toki 509-5292, Japan1, 京都大学大学院工学研究科 〒606-8501 京都市左京区吉田本町
2, 核融合科学研究所 〒509-5292 土岐市下石町322-6
We have developed an echelle spectrometer for simultaneous observation of a wide spectral range of390-770 nm with high resolution (about 0.1 nm at FWHM) and measured emission spectra of ablation
clouds of metallic pellets injected into high-temperature plasmas produced in the Large Herical Device(LHD). For an alminum ablation cloud, more than 50 emission lines of Al ions are observed and many ofthem show Stark broadening. Electron temperature of the cloud is determined from the population
distribution of Al+ against the excitation energy from the ground state, which is estimated from observed line
intensities. Electron density is estimated from the Stark broadening of the Al II 3p2(1D)-3s4p(1P) (466.3 nm)
line. From these results, we evaluated Stark broadening coefficients of other Al II lines.

1. Introduction
Pellets of various elements are injected into amagnetically confined fusion plasma for thepurposes of investigating impurity transport. A
pellet plunged into the plasma is immediatelyablated due to heat flux from the plasma and forms acloud in its vicinity from which luminous radiation
is emitted for several milliseconds [1].
Emission spectra of the ablation cloud of acarbon pellet injected into a Large Helical Device(LHD) plasma has been observed and it has been
revealed that a high density plasma (ne ≃ 1020-1023
m-3, Te = 2.5-3.0 eV) is produced in the cloud [1].
Recently,
wedeveloped
awideband
high-resolution echelle spectrometer for visible
region and observed emissions from ablation cloudsof various metal pellets in LHD plasmas. Here, wereport the results of an aluminum-pellet ablation
plasma.
2. Experiment
Figure 1 shows a schematic illustration of theexperimental set-up and the echelle spectrometer.
An aluminum pellet used in the present study has acylindrical shape whose diameter and length are 0.8mm. The pellet is injected from an outer port of theLHD by a pneumatic pipe-gun system with a heliumgas of 18 atm pressure [1].

LHD Top view

Top view
xy

trajectrypelletinjector

M1z

M2

entrance
slit
mirrorfield ofview

optical
fiber
prism

LHD plasma
to spectrometerechellegratingCCD

Fig. 1. A schematic illustration of the experimental
set-up (left) and the echelle spectrometer (right).

Emission from the pellet ablation cloud iscollected by a lens and introduced to thespectrometer through an optical fiber having a core
diameter of 300 µm. The observation direction is set
along the trajectory of pellet as shown in Fig. 1. The
emission introduced from the entrance slit of thespectrometer is collimated by a concave mirror (M1;
focal length: f = 304.8 mm). The parallel light rays
are diffracted in a plane perpendicular to the x-yplane by an echelle grating (46.1 grooves/mm, 32°blaze) according to their wavelength. Overlapping
30th-58th diffraction rays are spatially separated in adirection parallel to the x-y plane by a 60° quartz
prism and focused on a CCD detector (Andor, DVHIHe I

He I

101

He I

Al II
Al IV

He I

102

Al I
Al III

HIHe I

Intensity (arb. units)

103

100
10-1

400

500

600
Wavelength (nm)700

Fig. 2. Emission spectrum of an aluminum pellet ablation cloud in LHD. The relative intensity is calibrated.

3. Results and Discussion
Figure 2 shows an example of the observed
spectra. The relative intensities of all the emission
lines are calibrated. More than 50 of Al I, Al II, Al
III and Al IV lines are resolved, which are identified
with the NIST database [2].

N/g (arb. units)107
measured
fitted106

Al II
Te = 3.4 eV
105
1213

14
15
16
Energy (eV)1718

Fig. 3. Populations of Al+ excited levels per unit
statistical weight as a function of the excitation energy.

Figure 3 shows excited level populations of Al+
per unit statistical weight estimated from theobserved line intensities. They are plotted against
their excitation energies from the ground level.
Here, we assume that the aluminum ablation plasma
is in local thermodynamic equilibrium (LTE) state
as verified for the carbon pellet ablation plasma in asimilar condition [1]. By fitting the data in Fig. 3
with a straight line, we determine the electron
temperature (Te) to be 3.4 eV from the slope.
Figure 4 shows the enlarged profile of the Al II
3p2(1D)-3s4p(1P) (466.3 nm) line. Remarkable line
broadening is seen. The broadening would be
mainly due to Stark broadening approximated by aLorentzian function and due to the instrumental
broadening approximated by a Gaussian function.
From the fit with a Voigt function to the observed
profile with a known instrumental width (WG=0.062

nm at 466.3 nm), the Stark width (WL) is
determined to be 0.045 nm.
Intensity (arb. units)

435-BV) by another concave mirror (M2; f = 304.8mm). The wavelength range from 390 to 770 nm is
simultaneously recorded with resolution of about
0.1 nm.
We set an exposure time of the CCD to be much
longer than the period of luminous radiation fromthe ablation cloud.

10010Al II
3p2(1D)-3s4p(1P)
WG = 0.062 nm
WL = 0.045 nm

1465.5

466.0
466.5
467.0
Wavelength (nm)467.5

Fig. 4. Al II 3p2(1D)-3s4p(1P) emission line profile. The
solid line is the result of Voigt function fit. The dottedline indicates the background signal level for the fit.

The Stark broadening coefficient of this line has
been determined to be 7.88 x 10-25 nm m3 in thecase of Te = 40000 K (corresponding to about 3.4eV) [3]. From dividing WL by this value we obtainne = 5.7 x 1022 m-3 for the ablation cloud. Using thiselectron density, the Stark broadening coefficientsof other two Al II lines are estimated from theobserved WL. The results are in agreement with theprevious report [4] as shown in Table I.
Table I. Stark broadening coefficients of Al II lines at
Te=3.4 eV.
Wavelength Config. (Term)
CS (10-24 nm m3)(nm)
This work [4]624.3
3s4p(3P)-3s4d(3D)
3.1
3.40
683.7
3s4p(3P)-3s5s(3S)
3.2
3.26

Acknowledgments
The support of LHD experiment group is greatly
acknowledged.
References[1] M. Goto, S. Morita and M. Koubiti: J. Phys. B: At.
Mol. Opt. Phys. 43 (2010) 144023[2] NIST Atomic Spectra Database. ver. 4.
http://physics.nist.gov/PhysRefData/ASD/lines_form.html[3] A. Allen, M. Blaha, W. Jones, A. Sanchez and R.
Griem: Phys. Rev. A 11 (1975) 477[4] H.Griem: Spectral Line Broadening by Plasmas(Academic Press, 1974).