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

Observation of tungsten UTA spectra in the EUV wavelength range
around 200 Å in the Large Helical Device
R. Nishimura1, T. Oishi1, I. Murakami2,3, D. Kato2,4, H. A. Sakaue2, H. Ohashi5,
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
3
Graduate Institute for Advanced Studies, SOKENDAI
4Interdisciplinary Graduate School of Engineering Sciences, Kyushu University
5
Institute of Liberal Arts and Sciences, University of Toyama
e-mail (speaker): nishimura.ryota.s5@dc.tohoku.ac.jp

Tungsten (W) is a candidate material for plasma-facing
components in the divertor region of nuclear fusion
reactors because of its excellent properties, such as highmelting point and low sputtering yield. On the other hand,a large energy loss due to radiation and ionization willcause if the plasma is contaminated by tungsten ionswith a large atomic number of 74. Therefore, it isessential to understand the behavior of tungsten ions andcontrol impurity transport for achieving steady-state
operation of nuclear fusion reactors.
Spectroscopy has been used for diagnostics of tungsten
ions in the fusion plasma experiments. Tungsten ions inhigh temperature plasmas can take a wide range ofcharge states. So far, spectroscopic studies on tungsten
ions have been intensively carried out on Large Helical
Device (LHD) with tungsten pellet injection technique[1]. However, there is a lack of data on emission lines
emitted from tungsten ions in low charge states of W10+ W25+ [2]. In the presence of such low charge state ions,the resulting spectrum may be a pseudo-continuum thatcannot be resolved further, called unresolved transition
array (UTA). In this study, we attempted to understandthe charge states of the ions included in UTA byconducting analyses from multiple angles.
Waveforms of (a) heating power of neutral beam
injections (b) central electron temperature, Te0, andelectron density, ne0, (c) plasma stored energy, Wp, andradiation power, Prad, of tungsten injection experiment in
LHD are shown in Fig. 1. A tungsten pellet was injected
at 4.1s. Immediately after the tungsten pellet injection,
there was a sharp increase in Prad with a corresponding
drop in Wp. After that, Te0 decreased from 1keV to
0.3keV by 5.3s, while ne0 increased from 2×1019m-3 to
4×1019m-3. Spectroscopic measurements were performed
with multiple spectrometers, covering the wavelength
range from EUV to visible light. Since the UTA spectraof tungsten were observed around 200 Å in the EUV
region, which has not been identified so far, we focusedon these UTA spectra in this study.
Figure 2 shows the EUV spectra around 200 Å
including tungsten UTA measured at 4.95-5.30s. During4.95-5.30s, Te0 decreased from 0.83 keV to 0.31 keV. An
overall shift of the UTA toward longer wavelengths wasobserved following the decrease in Te0. Therefore, it isconsidered that the UTA is composed of the ions in low
charge states, such that the emission is enhanced atelectron temperatures of a few hundred eV, and that thelonger range of wavelengths contains components withlower charge states. The UTA spectral shape of the5.20-5.25s and 5.25-5.30s is characterized by thesuperposition of several peaks (e.g., 200-204 Å, 204-209Å) with finite wavelength width. Since each peakpossibly corresponds to a different charge state, we
attempted to extract the behavior of specific charge statefrom the quasi-continuous spectrum by dividing the UTA
into smaller wavelength regions and investigating themseparately.
This work was partially supported by JSPS KAKENHI
grant number JP20K03896.
References[1] T. Oishi et. al., Atoms 9, 69(2021).
[2] A. Kramida et. al., http://physics.nist.gov/asd for
NIST Atomic Spectra Database (ver. 5.10).

Figure 1. Waveforms of (a) heating power of neutral beam
injections, (b) central electron temperature and electron density,(c) plasma stored energy and radiation power in a tungsten
pellet injection experiment in LHD for the discharge #170347.

Figure 2. EUV spectra including tungsten UTA around 200 Å
measured at 4.95-5.30s.