7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

Line emission spectra of tungsten impurity ions across visible, VUV, EUV, andX-ray wavelength ranges observed in a magnetically confined high-temperatureplasma experiment
T. Oishi1, R. Nishimura1, I. Murakami2,3, D. Kato2,4, H. A. Sakaue2,M. Goto2,3, Y. Kawamoto2,3, T. Kawate,2,3 H. Takahashi1 and K. Tobita1
1
Department of Quantum Science and Energy Engineering, Tohoku University,
2
National Institute for Fusion Science, National Institutes of Natural Sciences
3Graduate Institute for Advanced Studies, SOKENDAI
4Interdisciplinary Graduate School of Engineering Sciences, Kyushu University
e-mail (speaker): tetsutarou.oishi.a4@tohoku.ac.jp

Tungsten is a candidate material for plasma-facing
components in ITER and future fusion reactors becauseof its high melting point and low sputtering yield.
However, there is a concern that tungsten ions with alarge atomic number of Z = 74 will cause large energy
loss by radiation and ionization when the plasma is
contaminated by the tungsten impurity. Therefore, it isimportant to understand and control tungsten impurity
transport in high temperature plasmas for establishingreliable operation scenarios for fusion reactors.
Spectroscopic measurements of tungsten ions andatoms have been conducted in the Large Helical Device(LHD) using tungsten pellets to investigate the transport
and to expand the database of tungsten line emissions.
We have succeeded in a measurement of multiple charge
states, from W0 to W46+ via visible, vacuum ultraviolet(VUV), extreme ultraviolet (EUV) and X-rays
spectroscopy [1]. In LHD, the lines from the neutral
atoms, W0, and the magnetic dipole (M1) forbidden
transition lines fromW26+ and W27+, were observed usingvisible spectroscopy in the wavelength range of 3300–3900 Å. The lines from low charge states, W2+–W6+,
and the several M1 lines from W29+–W39+ were observedin the VUV wavelength range of 500–1500 Å. In theEUV range of 5–500 Å, the lines from low charge states,
W4+–W7+, medium charge states, W24+–W33+ in thestructures of the unresolved transition array (UTA), andhigh charge states, W41+–W46+, were observed. W46+
lines were also observed with high wavelength
dispersion in the X-ray wavelength range of 3.87-3.90 Å.
On the other hand, a problem of the lack of line
spectral data for a range of W10+ to W20+ has not beensolved. These ions in low charge states are considered
to be distributed in the edge plasma that connects theplasma-facing components and the core plasma in fusion
reactors. Since impurity transport in the edge plasma
greatly affects the characteristics of the core plasma,
establishment of measurements for tungsten ions in low
charge states is required. In order to observe theselow- charged ions at LHD, we searched for spectra
during the time period of a discharge when the central
electron temperature is from 0 to about 0.3 keV. Figure
1 shows EUV spectra for a central electron temperature,
Te0, of almost zero, 0.22 keV, and 0.34 keV. These
spectra were obtained in a single discharge in a timesequence. As shown in Figure 1(a), when Te0 was

almost zero, line spectra with very low charge states,
such as W6+ at 216.2 Å and 261.4 Å, and W7+ at 197–202
Å, were observed. Thereafter, as shown in Figure 1(b)(Te0 = 0.22 keV), W6+ and W7+ disappeared and the line
spectra of W13+ were observed at 243.1 Å, 247.6 Å,248.3 Å, and 249.1 Å. Although these W13+ line
spectra were observed for the first time in fusion plasma
experiments, they were tentatively identified as W13+
because the shape of the spectra is very similar to that ofW13+ observed in a Compact Electron Beam Ion Trap
(CoBIT) experiment [2]. As Te0 was further increased,the W13+ peaks disappeared as shown in Figure 1(c) (Te0= 0.34 keV). The temperature range in which the lowcharged ions appear is narrow, and this is thought to beone of the reasons why it has been difficult to observe
them in high-temperature plasma experiments.
This work was partially supported by JSPS
KAKENHI grant number JP20K03896.
References[1] T. Oishi et. al., Atoms 9, 69 (2021).
[2] M. Mita et al., Journal of Physics: Conf. Series 875,012019 (2017).

Figure 1. EUV spectra measured in LHD for Te0 of (a)almost zero, (b) 0.22 keV, and (c) 0.34 keV.