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

Spectroscopic study of tungsten ions in LHD

I.Murakami1,2, T. Oishi3, D. Kato1,4, H. A. Sakaue1, Y. Kawamoto1,2, T. Kawate1,2, M. Goto1,2,A. Sasaki5
1
National Institute for Fusion Science, National Institutes of Natural Sciences2
Graduate Institute for Advanced Studies, SOKENDAI
3
Department of Quantum Science and Energy Engineering, Tohoku University
4
Interdisciplinary Graduate School of Engineering Sciences, Kyushu University
5
Kansai Photon Science Institute, National Institutes for Quantum Science and Technology
e-mail (speaker): Murakami.izumi@nifs.ac.jp
Tungsten, used as a plasma-facing material in fusion
devices, is sputtered by plasma particles and transported
into central core plasma, causing electron temperature
reduction due to high radiation power [1]. Tungsten
behavior in plasmas should be examined byspectroscopic methods. Tungsten spectra in wide
wavelength ranges have been measured in various fusion
devices and electron beam ion traps (EBITs) (e.g. [2,3]).
Spectral line identifications and spectral model
calculations have been done. Theoretical studies on
atomic structure and collision cross sections of tungsten
ions also have been done but such data for low- andmid-charged ions are still missing.
We have measured spectra of W – W46+ ions fromextreme ultraviolet (EUV) to visible wavelength ranges
in Large Helical Device (LHD) by tungsten pellet
injection [4]. LHD plasma is stable with such impurity
injection. We can measure the time evolution of tungsten
ion charge states due to the change in electron
temperature caused by large radiation of tungsten ions.
For quantitative analysis of tungsten behavior, wehave developed collisional-radiative (CR) models fortungsten ions [5,6]. The atomic data necessary for the CR
models are calculated with the HULLAC atomic code [7].
The CR model calculations give the population densityof excited states under the quasi-steady-state assumption,
which is valid for plasma in which the time scale of thepopulation changes is much shorter than the time scale ofplasma changes. To explain the so-called unresolved
transition array (UTA) feature at 4.5-7nm region, weincluded recombination processes in the CR model inwhich fine-structure levels for lower excited states andrelativistic configuration average levels for higher
excited states are considered.
Figure 1 shows a typical waveform of the W pellet
injection experiment in LHD. The central electron
temperature decreases due to the large radiation oftungsten ions after pellet injection. The measured EUV
spectrum at 5.0s is shown in Fig.2 and the UTA feature isseen. Using the CR model calculations, we cansynthesize EUV spectrum with ion abundance which isdetermined by fitting to the measured spectral peaks at
3-4nm. These peaks correspond to W25+ to W22+ spectral
peaks [3]. The synthesized UTA profile looks very close
to the measured one, although the profile wavelength isshifted short-wards, and profile heights are larger. This is
big progress from previous work which cannot represent
the measured UTA feature [5,6].

Detailed analysis of tungsten spectra with CR model
calculations helps to understand W behavior in LHD
plasmas.
References[1] D. E. Post et al., At. Data Nucl. Data Tables 20,397-439 (1977).
[2] R. Neu et al., Nucl. Fusion 45. 209-218 (2005).
[3] H. A. Sakaue et al., AIP Conf. Proc. 1430, 91 (2012).
[4] T. Oishi et al., Atoms 9, 69 (2021)[5] I. Murakami et al., Nucl. Fusion 55, 093016 (2015).
[6] I. Murakami et al., Nucl. Mater. Energy 26, 100923(2021).
[7] A. Bar-Shalom et al., Quant. Spectrosc. Radiat.
Transfer 71, 169 (2001)

Figure 1. Waveform of W pellet injection experimentin LHD: (a) heating power of ECH, n-NBI, andp-NBI, (b) plasma stored energy, Wp, and total
radiation power, Rad, and (c) central electron
temperature. A W pellet was injected at 4.1s.

Figure 2. Measured EUV spectrum at 5.000s for thedischarge #170347 (red line) and synthesized
spectrum (blue line).