Simultaneous Observation of Tungsten Spectra of W⁰ to W⁴⁶⁺ Ions in Visible, VUV and EUV Wavelength Ranges in the Large Helical Device

Abstract

Spectroscopic studies for emissions released from tungsten ions have been conducted in the Large Helical Device (LHD) for contribution to the tungsten transport study in tungsten divertor fusion devices and for expansion of the experimental database of tungsten line emissions. Tungsten ions are distributed in the LHD plasma by injecting a pellet consisting of a small piece of tungsten metal wire enclosed by a carbon tube. Line emissions from W⁰, W⁵⁺, W⁶⁺, W²⁴⁺–W²⁸⁺, W³⁷⁺, W³⁸⁺, and W⁴¹⁺–W⁴⁶⁺ are observed simultaneously in the visible (3200–3550 Å), vacuum ultraviolet (250–1050 Å), and extreme ultraviolet (5–300 Å) wavelength ranges and the wavelengths are summarized. Temporal evolutions of line emissions from these charge states are compared for comprehensive understanding of tungsten impurity behavior in a single discharge. The charge distribution of tungsten ions strongly depends on the electron temperature. Measurements of emissions from W¹⁰⁺ to W²⁰⁺ are still insufficient, which is addressed as a future task.

1. Introduction

Tungsten (W) is considered as a candidate material for plasma-facing components (PFCs) in the divertor region of the international thermonuclear experimental reactor (ITER) and future fusion reactors because of its high melting point, low sputtering yield, and low tritium retention [1,2,3]. On the other hand, there is a concern that tungsten ions with a large atomic number of Z = 74 will cause large energy loss by radiation and ionization when the plasma is contaminated by the W impurity. Therefore, it is very important to investigate the behavior of tungsten in high temperature plasmas in order to control tungsten transport and establish reliable operation scenarios for fusion reactors. Since tungsten is high-Z and can take a wide range of charge states, the wavelength range to be covered by spectroscopy is also wide. Therefore, efforts to compile a database of the emission spectra of tungsten ions with a wide range of charge states are actively conducted [4], and spectroscopic diagnostics to further accumulate spectral data have been continuously carried out in basic experiments such as the electron beam ion trap (EBIT) and magnetic field confinement plasma experiments [5,6,7,8,9,10].

Spectroscopic studies for emission from W ions in combination with a tungsten pellet injection technique have been intensively conducted on the Large Helical Device (LHD) for contribution to tungsten transport studies in tungsten divertor fusion devices represented by ITER and for expansion of the experimental database of tungsten line emissions [11,12,13,14]. The electron temperature, T_e, of the LHD core plasmas with a tungsten pellet injection ranges from 0.5 keV to 3.5 keV, which is close to that of the edge plasmas in ITER around the last closed flux surface, including the scrape-off layer. Thus, observation of tungsten lines in LHD could improve the tungsten diagnostics in ITER edge plasmas. The current status of tungsten emission lines observed in LHD using visible, vacuum ultraviolet (VUV), and extreme ultraviolet (EUV) spectroscopy can be summarized as follows. The line emissions from the neutral atoms, W⁰, as well as the singly ionized ions, W⁺, were observed using visible spectroscopy in the wavelength range of 4000–4400 Å [11]. The visible spectroscopy has also observed magnetic dipole (M1) forbidden transition lines from W²⁶⁺ and W²⁷⁺ in the wavelength range of 3300–3900 Å [15,16,17]. The line emissions from tungsten ions in low charge states, W²⁺–W⁶⁺, have been identified in the VUV range of 500–1500 Å [18]. Recently, several M1 lines of W²⁹⁺–W³⁹⁺ were successfully observed in the VUV wavelength range of 500–900 Å [19]. Additionally, in the EUV range of 5–500 Å, tungsten ions in low charge states, W⁴⁺–W⁷⁺, medium charge states, W²⁴⁺–W³³⁺ in the structures of the unresolved transition array (UTA), as well as high charge states, W⁴¹⁺–W⁴⁶⁺, have been identified [20,21,22]. This paper is dedicated to summarizing the tungsten spectra in the visible, VUV, and EUV wavelength ranges based on the progress of line identifications. Temporal evolution of emissions from tungsten ions in various charge states that are observed simultaneously will also be demonstrated for comprehensive understanding of behavior of tungsten impurity.

2. Tungsten Pellet Injection Experiment in LHD

LHD is a heliotron-type plasma confinement device which has the major/minor radii of 3.6/0.64 m in the standard configuration with a maximum plasma volume of 30 m³ and toroidal magnetic field of 3 T [23]. The coil system consists of a set of two continuous superconducting helical coils with a poloidal pitch number of two and a toroidal pitch number of 10 and three pairs of superconducting poloidal coils. Figure 1a illustrates the top view of the shape of the plasma in the LHD device together with schematic drawings of the neutral beam injection (NBI) for heating, the impurity pellet injection, and the spectroscopic diagnostics consisting of two flat-field grazing incidence EUV spectrometers (denoted as “EUV Short” [24] and “EUV Long” [25]), a normal incidence 20 cm VUV spectrometer (denoted as “VUV 109L” [26]), and an astigmatism-corrected Czerny–Turner-type 30 cm visible spectrometer (denoted as “MK300” [27]). Neutral hydrogen atoms are used as beam particles in the experiments presented in this paper. NBIs #1, 2, and 3, which have negative ion sources (n-NBI), are injected tangentially to the magnetic axis, while #4 and 5 with positive ion sources (p-NBI) are injected perpendicular to the magnetic axis. Tungsten ions are distributed in the NBI-heated LHD plasma by injecting a pellet consisting of a small piece of tungsten metal wire enclosed by a carbon or polyethylene pellet with the shape of a cylindrical tube [13]. Figure 1b,c illustrate the cross sections of the magnetic surfaces, where the optical axes of the VUV/EUV and visible spectroscopy systems are located, respectively, together with the field of view of each system. The EUV Short, EUV Long, and VUV 109L spectrometers cover the wavelength ranges of 5–60 Å, 100–300 Å, and 250–1050 Å, respectively. CCD detectors (1024 × 256 pixels, pixel size 26 × 26 μm², Andor DO420-BN) are placed at the positions of the exit slits of the spectrometers. A CCD data acquisition operational mode applied in this experiment is called “full-binning” mode, in which all CCD-pixels aligned in the vertical direction are replaced by a single channel, and the vertical spatial resolution is entirely eliminated. The time resolution for the spectra measurements is 5 ms in the full-binning data acquisition mode. The MK300 visible spectrometer covers the wavelength range of 3200–3550 Å. A CCD detector (1024 × 1024 pixels, pixel size 13 × 13 μm², Andor DU934-N) is placed at the position of the exit slit of the spectrometer and is operated in the sub-image data acquisition mode with a sampling time of 100 ms, including an exposure time of 61.55 ms. Although this spectrometer usually divides the field of view into 40 observation chords to measure the spatial distribution of the emission [28], in this paper we used the spectra obtained by integrating all 40 observation chords.

Figure 2 shows a typical waveform of the tungsten pellet injection experiment in a hydrogen discharge with the position of the magnetic axis, R_ax, at 3.6 m at a toroidal magnetic field, B_t, of 2.75 T in the counter-clockwise direction. In this discharge, the length and diameter of a tungsten wire enclosed in a carbon pellet were 0.7 mm and 0.1 mm, respectively. Then, the number of tungsten atoms enclosed in a pellet, N_W, was 3.5 × 10¹⁷. As shown in Figure 2a, the plasma was initiated by electron cyclotron heating (ECH), and further heated by the neutral hydrogen beams. Figure 2b–e show the central electron temperature, T_e0, the line-averaged electron density, ${\overline{n}}_e$, the plasma stored energy, W_p, and the total radiation power, P_rad, respectively. In order to obtain T_e0, the electron temperature measured by Thomson scattering at the location of −0.1 < r_eff/a₉₉ < 0.1 was averaged, where r_eff is the effective minor radius and a₉₉ is the plasma edge, defined as the effective minor radius in which 99% of electron stored energy was enclosed [29]. After the tungsten pellet injection at 4.1 s, T_e0 and W_p quickly decreased, while n_e increased. ECH was superposed for 4.2–4.7 s; then, T_e0 recovered up to around 3 keV. After the ECH was turned off, T_e0 decreased and kept the value around 0.6 keV for 5.0–5.3 s. The NBI heating scheme was switched from the n-NBIs to p-NBIs at 5.3 s. Then, T_e0 decreased down to a very low level close to zero for 5.4–5.9 s, followed by a recovery to a value of around 1.4 keV due to continuous heating by p-NBIs. In this paper, spectral data were obtained at four different timings with different T_e0, namely t₁ = 4.5 s (T_e0 ~ 3.0 keV), t₂ = 4.7 s (T_e0 ~ 1.7 keV), t₃ = 5.0 s (T_e0 ~ 0.6 keV), and t₄ = 5.6 s (T_e0 ~ 0 keV), as indicated in Figure 2, to demonstrate the effect of the electron temperature on the observable charge states of the tungsten ions.

Figure 3 shows temporal evolutions of radial profiles of the electron temperature, T_e, and the electron density, n_e, Figure 3a,b for 4.0–4.8 s, Figure 3c,d for 5.0–5.8 s, and Figure 3e,f for 6.0–6.8 s, plotted against r_eff/a₉₉. After the pellet injection at 4.1 s, T_e decreased over the entire region within the plasma edge, and then recovered in the central region of the plasma at about −0.6 < r_eff/a₉₉ < 0.6 due to the ECH superposition for 4.2–4.7 s, as shown in Figure 3a. n_e increased after the pellet injection, keeping a flat radial profile as shown in Figure 3b. When the NBI heating scheme was switched from the n-NBIs to p-NBIs at 5.3 s, both the T_e and n_e profiles became extremely hollow, as shown in Figure 3c,d. In this paper, the emission spectra with the central electron temperature close to zero are presented as “the spectrum with T_e0 = 0 keV”, but note that the actual emission is from the peripheral part of the plasma with a finite electron temperature, which surrounds the central part with a very low electron temperature. Thereafter, both of the central T_e and n_e recovered due to a continuous heating by p-NBIs, as shown in Figure 3e,f. The large variation of T_e after the pellet injection as shown in Figure 2 and Figure 3 can provide us with an excellent opportunity to observe tungsten line emissions in various kinds of charge states as a function of discharge time.

3. Tungsten Line Emissions in the Visible, VUV, and EUV Wavelength Ranges

The results of the spectroscopic observations are summarized in this section. The identifications of charge states and the transitions are taken from the NIST database [30]. Figure 4 shows visible spectra including W⁰, W²⁶⁺, and W²⁷⁺ line emissions in the wavelength range of 3200–3550 Å measured using the MK300 spectrometer. T_e0 is also indicated for each timing of the data acquisition. W²⁶⁺ 3337.05 Å (4d¹⁰4f^{2 3}F, J = 4 → 4d¹⁰4f^{2 3}F, J = 3), W²⁶⁺ 3357.61 Å (4d¹⁰4f^{2 3}F, J = 4 → 4d¹⁰4f^{2 1}G, J = 4), and W²⁷⁺ 3377.42 Å (4d¹⁰4f ²F°, J = 7/2 → 4d¹⁰4f ²F°, J = 5/2) were observed in the spectra with T_e0 = 3.0 keV and 1.7 keV, as shown in Figure 4a,b, respectively. These W²⁶⁺ and W²⁷⁺ lines have already been identified as the M1 forbidden transition lines [16,17]. These lines became less significant in the spectrum with T_e0 = 0.6 keV as shown in Figure 4c, and completely disappeared in the spectrum with T_e0 = 0 keV as shown in Figure 4d. On the other hand, Figure 4d indicated that several W⁰ lines appeared at the wavelengths of 3205.54 Å, 3229.26 Å, 3346.19 Å, 3427.03 Å, and 3461.26 Å in the spectrum with T_e0 = 0 keV [30].

Figure 5 shows VUV spectra, including W⁵⁺, W³⁷⁺, and W³⁸⁺ line emissions in the wavelength range of 250–1050 Å, measured using the VUV 109L spectrometer with T_e0 for each timing of the data acquisition. W³⁷⁺ 646.3 Å (4p⁶4d ²D, J = 5/2 → 4p⁶4d ²D, J = 3/2), W³⁸⁺ 532.2 Å (4p⁵4d (3/2, 5/2)°, J = 3 → 4p⁵4d (3/2, 3/2)°, J = 3), and W³⁸⁺ 559.3 Å (4p⁵4d (3/2, 5/2)°, J = 3 → 4p⁵4d (3/2, 3/2)°, J = 2) were observed in the spectrum with T_e0 = 3.0 keV as shown in Figure 5a [19]. These W³⁷⁺ and W³⁸⁺ lines have been already identified as the M1 forbidden transition lines. These lines completely disappeared in the spectrum with T_e0 = 1.7 keV, as shown in Figure 5b. In the spectrum with T_e0 = 0.6 keV, a UTA-like broad peak was observed around 300 Å, as shown in Figure 5c. Identification of the charge states and the transitions of this UTA will be a subject of future studies. In the spectrum with T_e0 = 0 keV, two clear peaks of W⁵⁺ emission with 6p → 5d transition were observed at 639.62 Å and 677.34 Å, as shown in Figure 5d [18,26,30].

Figure 6 shows EUV spectra, including W⁶⁺, W⁷⁺, and W⁴¹⁺~W⁴⁵⁺ line emissions in the wavelength range of 100–300 Å, measured using the EUV Long spectrometer with T_e0 for each timing of the data acquisition. W⁴¹⁺ 131.15 Å (3d¹⁰4s²4p^{3 2}D°, J = 5/2 → 3d¹⁰4s²4p^{3 2}D°, J = 3/2), W⁴²⁺ 129.31 Å (3d¹⁰4s²4p^{2 1}D, J = 2 → 3d¹⁰4s²4p^{2 3}P, J = 0), W⁴³⁺ 126.25 Å (3d¹⁰4s²4p ²P°, J = 3/2 → 3d¹⁰4s²4p ²P°, J = 1/2), W⁴⁴⁺ 121.84 Å as the second order of W⁴⁴⁺ 60.93 Å (3d¹⁰4s4p (1/2,3/2)°, J = 1 → 3d¹⁰4s^{2 1}S, J = 0), and W⁴⁵⁺ 127.06 Å (3d¹⁰4p ²P°, J = 1/2 → 3d¹⁰4s ²S, J = 1/2) were observed in the spectrum with T_e0 = 3.0 keV, as shown in Figure 6a [20,30]. The W⁴¹⁺ and W⁴³⁺ lines are M1 forbidden transition lines and the W⁴²⁺ line is an electric-quadrupole (E2) transition. These W⁴¹⁺–W⁴⁵⁺ lines completely disappeared in the spectrum with T_e0 = 1.7 keV, as shown in Figure 6b. In the spectrum with T_e0 = 0.6 keV, a broad UTA was observed around 170–200 Å, as shown in Figure 6c. It has already been reported that this UTA is primarily formed by n = 5 → 5 transitions of W⁷⁺–W²⁷⁺ ions [31]. In the spectrum with T_e0 = 0 keV, two clear peaks of W⁶⁺ emission with 5d → 5p transition were observed at 216.17 Å and 261.31 Å, as shown in Figure 6d. These lines are recognized as useful tools to evaluate tungsten influx in tokamak experiments [32]. Moreover, several small peaks were found at around 198–202 Å in Figure 6d. They are probably W⁷⁺ lines because the spectral shape is similar to that of the W⁷⁺ spectrum identified in EBIT experiments [33,34]. The emission of W⁷⁺ ions in this wavelength region has been identified in detail through vacuum spark experiments with high-resolution spectroscopic diagnostics [35,36]. The spectral identification of W⁷⁺ ions in LHD will be improved by comparing the observed peaks with these previous studies.

Figure 7 shows the EUV spectra, including W²⁴⁺–W⁴²⁺ UTA and W⁴⁶⁺ line emissions in the wavelength range of 5–60 Å, measured using the EUV Short spectrometer with T_e0 for each timing of the data acquisition. A W⁴⁶⁺ emission line was observed at 7.93 Å as the highest charge state of tungsten ions in LHD in the spectrum with T_e0 = 3.0 keV, as shown in Figure 7a [22]. This line is a blend of an E2 transition of W⁴⁶⁺ at 7.928 Å (3d⁹4s (5/2, 1/2), J = 2 → 3d^{10 1}S, J = 0) and a magnetic-octupole (M3) transition of W⁴⁶⁺ at 7.938 Å (3d⁹4s (5/2, 1/2), J = 3 → 3d^{10 1}S, J = 0). UTAs consisting of W²⁴⁺–W³³⁺ and W²⁷⁺–W⁴²⁺ also appeared at 19–33 Å and 46–53 Å, respectively. The identification of charge states of the UTA was performed by comparing the spectra obtained in LHD and the compact electron beam ion trap (CoBIT) with CR model calculations [11,37,38,39]. It is worth noting that the UTAs at around 50 Å have been used to evaluate tungsten ion concentrations in tokamak experiments [40]. More detailed discussion on determination of the tungsten ion density in LHD has also been provided using W²⁴⁺, W²⁵⁺, and W²⁶⁺ peaks in the UTA [41]. The spectral shape of the UTAs depended on T_e0 in such a way that the spectral structure of W³⁷⁺–W⁴²⁺ at 46–48 Å disappeared in the spectrum with T_e0 = 1.7 keV as shown in Figure 7b, and W²⁴⁺ at 32.5 Å became prominent in the spectrum with T_e0 = 0.6 keV as shown in Figure 7c. Finally, no emission lines of W ions can be seen in the spectrum with T_e0 = 0 keV as shown in Figure 7d.

It is worthwhile to summarize the wavelengths of the emission lines observed in this study for useful tools in future spectroscopic studies. The wavelengths of W⁰, W⁵⁺, W⁶⁺, W²⁴⁺–W²⁸⁺, W³⁷⁺, W³⁸⁺, and W⁴¹⁺–W⁴⁶⁺ line emissions observed in this study are summarized in

Table 1. Wavelength list of W⁰, W⁵⁺, W⁶⁺, W²⁴⁺–W²⁸⁺, W³⁷⁺, W³⁸⁺, and W⁴¹⁺–W⁴⁶⁺ line emissions observed in this study.

Wq+	IE (eV)	λNIST (Å)	λobs (Å)	λNIST − λobs (Å)	Lower-Level Configuration, Term, J	Upper-Level Configuration, Term, J	Remarks	References
W0	7.9	3205.50	3205.54 ± 0.06	−0.04	5d46s2, 3D, 1	-		[30]
W0	7.9	3229.23	3229.26 ± 0.07	−0.03	5d5(4D)6s, 5D, 2	-		[30]
W0	7.9	3346.11	3346.19 ± 0.06	−0.08	5d5(6D)6s, 7S, 3	5d46s(6D)6p, 5D°, 4		[30]
W0	7.9	3426.88	3427.03 ± 0.06	-	5d46s2, 3D, 1	-	blended	[30]
		3427.72			5d46s2, 3G, 3
W0	7.9	3461.36	3461.26 ± 0.06	0.1	5d5(4G)6s, 5G, 4	-		[30]
W5+	64.8	639.68	639.62 ± 0.09	0.06	5s25p65d, 2D, 5/2	5s25p66p, 2P°, 3/2		[18,26,30]
W5+	64.8	677.72	677.34 ± 0.08	0.38	5s25p65d, 2D, 3/2	5s25p66p, 2P°, 1/2		[18,26,30]
W6+	122.0	216.22	216.17 ± 0.06	0.05	5s25p6, 1S, 0	5s25p5(2P°1/2)5d, (1/2,3/2)°, 1		[30,32]
W6+	122.0	261.39	261.31 ± 0.06	0.08	5s25p6, 1S, 0	5s25p5(2P°3/2)5d, (3/2,5/2)°, 1		[30,32]
W24+	734.1	-	~27.3	-	-	-	UTA	[11,21,37,38,39,41]
W25+	784.1	-	~28.4	-	-	-	UTA	[11,21,37,38,39,41]
W26+	833.4	-	~29.7	-	-	-	UTA	[11,21,37,38,39,41]
W26+	833.4	-	3337.05 ± 0.09	-	4d104f2, 3F, 3	4d104f2, 3F, 4	M1	[16,17]
W26+	833.4	-	3357.61 ± 0.08	-	4d104f2, 1G, 4	4d104f2, 3F, 4	M1	[16,17]
W27+	881.4	-	~31.0	-	-	-	UTA	[11,21,37,38,39]
W27+	881.4	-	3377.42 ± 0.06	-	4d104f, 2F°, 5/2	4d104f, 2F°, 7/2	M1	[16,17]
W28+	1132.2	-	~32.5	-	-	-	UTA	[11,21,37,38,39]
W37+	1621.7	646.7	646.3 ± 0.1	0.4	4p64d, 2D, 3/2	4p64d, 2D, 5/2	M1	[19,30]
W38+	1829.8	532.9	532.2 ± 0.2	0.7	4p5(2P°3/2)4d, (3/2, 3/2)°, 3	4p5(2P°3/2)4d, (3/2, 5/2)°, 3	M1	[19,30]
W38+	1829.8	559.1	559.3 ± 0.1	−0.2	4p5(2P°3/2)4d, (3/2, 3/2)°, 2	4p5(2P°3/2)4d, (3/2, 5/2)°, 3	M1	[19,30]
W41+	1994.8	131.22	131.15 ± 0.06	0.07	3d104s24p3, 2D°, 3/2	3d104s24p3, 2D°, 5/2	M1	[20,30]
W42+	2149.1	129.41	129.31 ± 0.06	0.10	3d104s24p2, 3P, 0	3d104s24p2, 1D, 2	E2	[20,30]
W43+	2210.0	126.29	126.25 ± 0.06	0.04	3d104s24p, 2P°, 1/2	3d104s24p, 2P°, 3/2	M1	[20,30]
W44+	2354.5	60.93 × 2	121.84 ± 0.06	0.02	3d104s2, 1S, 0	3d104s4p, (1/2,3/2)°, 1	second order	[11,20,30]
W45+	2414.1	127.00	127.06 ± 0.06	−0.06	3d104s, 2S, 1/2	3d104p, 2P°, 1/2		[20,30]
W46+	4057	7.928	7.93 ± 0.02	-	3d10, 1S, 0	3d94s, (5/2,1/2), 2	E2 + M3 blended	[22,30]
		7.938			3d10, 1S, 0	3d94s, (5/2,1/2), 3

. The first and the second columns give the charge states and the ionization energies, IEs, respectively. The third and the fourth columns give the wavelengths of line emissions from the NIST database, λ_NIST, and the present observation, λ_obs, respectively. A discrepancy between λ_NIST and λ_obs is shown in the fifth column. The lower- and upper-level configurations from the NIST database are given in the sixth and the seventh columns, respectively. The eighth column gives remarks on the blended lines, the unresolved transition arrays (UTAs), or the transition types for forbidden lines. References for the line identifications or the previous observations are given in the ninth column. If no adequate item is registered in the NIST database, “-“ is indicated.

As an example of an application of the summarized wavelengths, the temporal variation of emission of each charge state during the discharge is demonstrated. Figure 8 shows the temporal evolution of: Figure 8a, the heating power of ECH, n-NBI, and p-NBI, Figure 8b, the central electron temperature and line-averaged electron density, W⁰–W⁴⁶⁺ intensities integrated over the wavelength ranges of Figure 8c, 3426.2–3427.9 Å for W⁰, Figure 8d 637.8–641.2 Å for W⁵⁺, Figure 8e 261.0–261.5 Å for W⁶⁺, Figure 8f 32.15–32.30 Å for W²⁴⁺, 30.73–31.69 Å for W²⁵⁺, 29.29–30.40 Å for W²⁶⁺, Figure 8g 28.58–28.69 Å for W²⁷⁺, 27.35–27.78 Å for W²⁸⁺, Figure 8h 645.3–647.1 Å for W³⁷⁺, 558.6–560.3 Å for W³⁸⁺, Figure 8i 131.0–131.3 Å for W⁴¹⁺, 129.2–129.5 Å for W⁴²⁺, 126.1–126.5 Å for W⁴³⁺, 126.9–127.3 Å for W⁴⁵⁺, and Figure 8j 7.89–7.95 Å for W⁴⁶⁺. The vertical axes in Figure 8c–j show the number of counts detected by the CCD, and the corresponding spectrometer is indicated by subscripts such as “I_MK300”, “I_{VUV 109L}”, “I_{EUV Long}”, and “I_{EUV Short}”. The signal levels in neighboring wavelength ranges with no significant line emissions were subtracted from the tungsten line intensities as background levels mainly consisting of bremsstrahlung emissions.

As shown in the figure, the emissions from medium charge states of W²⁴⁺–W²⁸⁺ first appeared just after the pellet injection at 4.1 s. As T_e0 increased from 2 to 3 keV by the superposition of ECH from 4.2 to 4.7 s, W²⁴⁺–W²⁶⁺ decreased while W³⁷⁺, W³⁸⁺, and W⁴¹⁺–W⁴⁶⁺ increased. After the termination of ECH superposition at 4.7 s, W³⁷⁺, W³⁸⁺, and W⁴¹⁺–W⁴⁶⁺ suddenly disappeared, while W²⁴⁺–W²⁸⁺ recovered for some time and then began to decrease as T_e0 decreased from 3.0 to 0.6 keV. When the NBI heating was switched from n-NBIs to p-NBIs at 5.3 s and T_e0 approached 0 keV, W⁵⁺ and W⁶⁺ appeared first, then W⁰ became dominant; subsequently W⁰ decreased and W⁵⁺ and W⁶⁺ increased. Since the heating by the p-NBIs continued until the end of the discharge, W⁵⁺ and W⁶⁺ disappeared and W²⁴⁺–W²⁸⁺ appeared sequentially as T_e0 recovered up to 1.4 keV. It has been clearly demonstrated that the dominant charge state varied sequentially, together with T_e0, which is a reasonable relationship between the electron temperature and the ionization energy. This is progress toward comprehensive understanding of the behavior of tungsten impurities in plasmas, but on the other hand, spectroscopic data from W¹⁰⁺ to W²⁰⁺ are extremely insufficient. It is our future task to measure these charge regions for further understanding.

4. Summary

Spectroscopic studies of emissions released from tungsten ions have been conducted in LHD for contribution to the tungsten transport study in tungsten divertor fusion devices and for expansion of the experimental database of tungsten line emissions. Tungsten ions have been distributed in the LHD plasma by injecting a pellet consisting of a small piece of tungsten metal wire enclosed by a carbon tube. Wavelengths of W⁰, W⁵⁺, W⁶⁺, W²⁴⁺–W²⁸⁺, W³⁷⁺, W³⁸⁺, and W⁴¹⁺–W⁴⁶⁺ line emissions observed in the visible, VUV, and EUV wavelength ranges have been summarized. The temporal evolution of line emissions from these charge states has been compared for comprehensive understanding of tungsten impurity behavior in a single discharge. The charge distribution of tungsten ions strongly depends on electron temperature. Measurements of emissions from W¹⁰⁺ to W²⁰⁺ are still insufficient, which is a subject for future research.