Plasma and Fusion Research: Regular Articles

Volume 11, 2402019 (2016)

Estimation of Partial Carbon Radiation at Each Ionization Stage ofC2+ to C5+ Ions in Large Helical Device∗)
Hongming ZHANG1) , Shigeru MORITA1,2) , Tetsutarou OISHI1,2) , Izumi MURAKAMI1,2) ,Xianli HUANG1) and Motoshi GOTO1,2)
1)

Graduate University for Advanced Studies, Toki 509-5292, Japan
2)
National Institute for Fusion Science, Toki 509-5292, Japan(Received 27 November 2015 / Accepted 26 January 2016)

Detached plasma has been successfully achieved with use of m/n = 1/1 resonant magnetic perturbation(RMP) coils in a large helical device (LHD) without any additional impurity gas puﬃng. Study of the impurity
radiation is then important to clarify the physical mechanism triggering the RMP-assisted detachment. Resonancelines of CIII (977.02 Å, 2s2p–2s2 ), CIV (1548.2 Å, 2p–2s), CV (40.27 Å, 1s2p–1s2 ), and CVI (33.73 Å, 2p–1s)
measured by vacuum ultraviolet (VUV) and extreme ultraviolet (EUV) spectrometers are used to estimate theradiation power from C2+ –C5+ ions because carbon is the most abundant impurity element in LHD. For thepurpose the spectral intensity from the VUV spectrometer is absolutely calibrated using the EUV spectrometer,with which the spectral intensity has already been calibrated. The partial carbon radiation at each ionization stageof C2+ to C5+ ions, Prad (Cq+ ), can be estimated for attached and detached plasmas by calculating the ratio of thepartial carbon radiation to the resonance line based on ADAS atomic code. It is found that the radiation fromC3+ ions existing near a radial location of ι/2π = 1 in the ergodic layer increases up to 40% of the total radiation
loss and becomes a dominant origin to trigger the detached plasma, whereas carbon radiation is negligible in theattached plasma.
c 2016 The Japan Society of Plasma Science and Nuclear Fusion ResearchKeywords: EUV spectroscopy, VUV spectroscopy, detached plasma, radiation loss, carbon radiation
DOI: 10.1585/pfr.11.2402019

1. Introduction
Steady operation of detached plasma has beenachieved without additional impurity gas puﬃng in theLarge Helical Device (LHD) by forming an m/n = 1/1
magnetic island at ι/2π = 1 in the ergodic layer with resonant magnetic perturbation (RMP) coils [1]. Carbon released from graphite divertor plates is a dominant intrinsic impurity in high-density LHD discharges [2]. Then, it
seems that the carbon radiation during the plasma detachment is also important for the eﬀective edge plasma cooling triggering the detachment. In LHD, C2+ and C3+ ionswith low ionization energies (Ei = 48 and 65 eV) are located near the outer boundary in the ergodic layer, whereas
C4+ and C5+ ions with relatively high ionization energies(Ei = 392 and 490 eV) are located near the last closed flux
surface (LCFS) [2]. In the RMP-assisted plasma detachment, therefore, it is very interesting to compare the radiation power from C2+ and C3+ ions located outside ι/2π = 1
with that from C4+ and C5+ ions located inside ι/2π = 1.
The partial radiation power at each ionization stage
in the carbon ions of C2+ to C5+ , Prad (Cq+ ), is analyzed
from the line intensities of CIII–CVI resonance transitions,which can be measured with extreme ultraviolet (EUV: 10 author’s e-mail: zhang.hongming@nifs.ac.jp∗) This article is based on the presentation at the 25th International Toki
Conference (ITC25).

500 Å) and vacuum ultraviolet (VUV: 300 - 2400 Å) spectrometers. The intensity ratio of Prad (Cq+ ) to the resonanceline can be used to estimate Prad (Cq+ ). For the purposethe electron temperature at the radial location where thecarbon ion exists has to be evaluated because the intensity ratio is sensitive to the electron temperature. In addition, the spectral intensity has to be absolutely calibrated
for the VUV spectrometer. Thus, the Prad (Cq+ ) analyzed
with the intensity ratio are compared between attached anddetached plasmas.

2. Experimental Setup
In LHD, the m/n = 1/1 magnetic island formed by theRMP coil system was originally used for the local island
divertor experiment to realize an eﬃcient particle exhaust.
The O-point of the m/n = 1/1 magnetic island is then positioned in the outboard side of the horizontally elongatedplasma cross section at the #6-O or #7-O toroidal section.
In the present study it is called 6-O or 7-O island configuration. Since the detached plasma is realized at the magnetic
axis position of Rax = 3.90 m, the m/n = 1/1 island is located in the ergodic layer due to the low edge rotationaltransform.
The EUV and VUV spectrometers are installed on
the #10-O diagnostic port of LHD, as shown in Fig. 1 (a).

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c 2016 The Japan Society of Plasma
Science and Nuclear Fusion ResearchPlasma and Fusion Research: Regular Articles

Volume 11, 2402019 (2016)

Fig. 2 Absolute intensity calibration factor of VUV_109L andVUV_106R in the wavelength range of 400 - 1600 Å.

Fig. 1 (a) Schematic view of space-resolved EUV andVUV spectrometers (EUV_Short2 and EUV_Long2)
and high-time-resolution EUV and VUV spectrometers (EUV_Long, VUV_109L and VUV_106R) and(b) observation area of EUV_Long, VUV_109L and
VUV_106R.

In the present study, two EUV spectrometers named
EUV_Short (10 - 100 Å) and EUV_Long (50 - 650 Å) andtwo 20-cm normal incidence VUV spectrometers named
VUV_109L (300 - 1000 Å) and VUV_106R (970 - 1900Å) are used for simultaneous high-time-resolution measurements (5 ms) of the carbon resonance lines, i.e. CIII
(977.02 Å, 2s2p–2s2 ), CIV (1548.2 Å, 2p–2s), CV (40.27Å, 1s2p–1s2 ) and CVI (33.73 Å, 2p–1s) [3, 4]. Twospace-resolved EUV spectrometers named EUV_Short2and EUV_Long2 are used for simultaneous vertical profile measurements of CIII (386.2 Å), CIV (384.17 Å),CV (40.27 Å) and CVI (33.73 Å) in the range of −0.6 ≤Z ≤ 0.6 m with a time resolution of 100 ms [5–7]. The
EUV_Long, VUV_109L and VUV_106R have a similar
observation range, as shown in Fig. 1 (b).

3. Calibration of VUV Spectrometers
In order to obtain the absolute intensities of the CIII
and CIV resonance lines, the spectral intensity measured
by VUV_109L and VUV_106R has to be calibrated intoabsolute intensity. The wavelength ranges of EUV_Long
and VUV_109L overlap between 300 and 650 Å. Because
the spectral intensity of EUV_Long has already been calibrated on the basis of the bremsstrahlung profile measurement in previous studies [3, 8], the spectral intensity of
VUV_109L can be easily calibrated by directly comparing
the same spectrum from both spectrometers in the samewavelength range of 400 - 650 Å. The result is plotted inFig. 2 with open circles.
In order to calibrate the intensity of the VUV spectrain the long wavelength range, e.g., 650 - 1600 Å, the in-Fig. 3 Intensity ratios of CIII (977.02 Å/386.2 Å) and CIV(1548.2 Å/312.4 Å) calculated with ADAS atomic code.

tensity ratio method is applied. As shown in Fig. 3, twointensity ratios of CIII (977.02 Å/386.2 Å, 2p–2s/3p–2s)
and CIV (1548.2 Å/312.4 Å, 2p–2s/3p–2s) are calculatedusing ADAS atomic code as a function of electron temperature [9]. The contribution of ne to the ratio can beneglected because the ratio is practically insensitive to ne .
Here, the absolute intensities of CIII (386.2 Å) and CIV(312.4 Å) are obtained using the EUV_Long spectrometer.
The electron temperature at the radial location where
the C2+ –C5+ ions exist is determined by analyzing the vertical profile of CIII–CVI emissions, as shown in Fig. 4. The
vertical profiles of CIII (386.2 Å), CIV (384.02 Å), CV(40.27 Å) and CVI (33.73 Å) are simultaneously observedwith EUV_Long2 and EUV_Short2, as shown in Fig. 4 (a).
A sharp peak appears in the vertical profile at 0.4 < Z <0.5 m because the chord length passing through the edge
plasma is long at the top edge in the horizontally elongatedplasma cross section. The intensity in CIII–CVI profiles
is normalized at each edge peak. Local emissivity pro-2402019-2

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Fig. 5 Ratios of partial carbon radiation power at each ionization stage to radiation power of resonance line calculatedwith ADAS atomic code: (a) Prad (C2+ )/PCIII (2p–2s), (b)Prad (C3+ )/PCIV (2p–2s), (c) Prad (C4+ )/PCV (2p–1s) and(d) Prad (C5+ )/PCVI (2p–1s).

Fig. 4 (a) Normalized chord-integrated vertical profiles of CIII
(386.2 Å), CIV (384.02 Å), CV (40.27 Å) and CVI (33.73Å) and (b) normalized local emissivity profiles of CIII–CVI and T e profile as a function of minor radius.

files of CIII–CVI are obtained by reconstructing the chordintegrated profiles measured along the vertical direction
based on the Abel inversion method. The magnetic surface necessary for the Abel inversion is calculated with thevariational moments equilibrium code (VMEC), including
the eﬀect of the finite plasma pressure. Although no magnetic surface exists in the ergodic layer, virtual magnetic
surfaces are assumed at ρ > 1 by extrapolating the magnetic flux surface at ρ = 1. This assumption is generally
used in LHD when the analysis is carried out for the ergodic layer [6], as shown in Fig. 4 (b). The emissivity profile is also normalized at each peak value. Figure 4 (b) also
shows the T e profile measured by the Thomson scatteringsystem. Then, the electron temperature at radial locationsof C2+ –C5+ ions is determined from the peak position inthe CIII–CIV emissivity profiles, i.e., 15 eV for C2+ , 20 eV
for C3+ , 210 eV for C4+ and 360 eV for C5+ .
The intensity ratio in Fig. 3 is determined with theelectron temperatures mentioned above. Thus, the spectral
intensities from VUV_109L and VUV_106R spectrometers can be absolutely calibrated at wavelengths of 977.02
Å and 1548.2 Å based on the absolute intensities of CIII
(386.2 Å) and CIV (312.4 Å) and the intensity ratio. The
results obtained from the intensity ratio method are shownin Fig. 2 with open squares. Finally, the analyzed calibra-tion factor is fitted with a smooth curve in the wavelength
range of 400 - 1600 Å, as shown in Fig. 2 with solid line.

4. Estimation of Partial Radiation
Prad (Cq+ )The radiation power of the resonance transition generally occupies a considerably large part in the radiation
power from impurity ions in a certain ionization stage. Because the absolute intensities of the resonance lines of CIII
(977.02 Å), CIV (1548.2 Å), CV (40.27 Å) and CVI (33.73Å) are measured with the EUV and VUV spectrometers,
the ratio of Prad (Cq+ ) to resonance line intensity in Cq+ions can be calculated with the ADAS atomic code. Theratio of Prad (Cq+ ) to the resonance line intensity is shownin Figs. 5 (a)-(d) as a function of electron temperature withthe parameter of electron density. Because the ratio is practically insensitive to ne , as shown in Fig. 5, the contributionof ne can be neglected in the present analysis. As the valueof T e for each ionization stage of carbon ions is already determined from the profile analysis done in Fig. 4 (b), Prad(Cq+ ) can be evaluated from the intensity ratio in Fig. 5.

5. Prad (Cq+ ) in Attached and Detached Plasmas
The partial carbon radiation of Prad (C2+ )–Prad (C5+ )is analyzed for attached and detached plasmas. Figure 6
shows a comparison of the discharge waveform between
attached (Figs. 6 (a)-(f)) and detached (Figs. 6 (g)-(l)) plasmas. These two discharges are operated at the magnetic
axis position of Rax = 3.90 m and heated by negative-ion-2402019-3

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Fig. 6 Time behaviors of (a) NBI port-through power and line-averaged electron density, (b) total radiation loss (Prad ), partial carbon
radiation power of (c) Prad (C2+ ) and Prad (C3+ ) and (d) Prad (C4+ ) and Prad (C5+ ) and ratios of Prad (Cq+ ) to Prad of (e) Prad(C2+ )/Prad and Prad (C3+ )/Prad and (f) Prad (C4+ )/Prad and Prad (C5+ )/Prad in the attached plasma without RMP. Graphs (g)-(l) show
the time behaviors for detached plasma with 6-O RMP island in the same meaning as those of graphs (a)-(f).

source-based NBIs. The total radiation loss, Prad , shows
similar values for both plasmas, i.e., 20% - 25% to the portthrough NBI power, PNBI . In the attached plasma, the lineaveraged electron density is constantly maintained (∼ 5 ×1013 cm−3 ) during t = 4 - 8 s with Prad = 1400 kW. The contributions of Prad (C2+ ) and Prad (C4+ ) to Prad are less than
1%, whereas those of Prad (C3+ )/Prad and Prad (C5+ )/Pradare 8% and 2%, respectively.
In the discharge with detached plasma, the m/n = 1/1
island is formed in the ergodic layer and the O-point of theisland is located at the outboard side of the #6 toroidal section. With increase in the electron density, plasma detachment occurs at t = 5.0 s, as shown in Figs. 6 (g)-(l). The
electron density starts to increase after the occurrence ofdetachment at t = 5.0 s but saturates at t = 6.0 s. The plasma
detachment is steadily maintained at a high density of ne =12 × 1013 cm−3 during t = 6 - 8 s until the end of the NBI
pulse. The radiation loss also reaches the maximum valueof Prad = 1700 kW during this period. Compared with theattached plasma, it is found that the Prad (C2+ )/Prad and Prad(C3+ )/Prad significantly increase to 3% and 40% during thedetachment phase, respectively, whereas the increment ofPrad is only 300 kW. In contrast, Prad (C4+ )/Prad and Prad
(C5+ )/Prad do not significantly change compared to the attached plasma.

The drastic increase in Prad (C2+ ) and Prad (C3+ ) during the detachment phase is very clear from the presentanalysis. Prad (C3+ ) expresses a dominant portion in thetotal carbon radiation in the detached plasma. A flattening of the T e profile appears during the detachment phasein edge T e of 10 - 20 eV at the radial location of the m/n= 1/1 magnetic island [1]. Radial locations of C2+ andC3+ ions are also broadened reflecting the T e flattening asshown in Fig. 4 (b). The expansion of the radial location
significantly enhances Prad (C2+ ) and Prad (C3+ ) in the detached plasma. Therefore, it is concluded that a change inthe edge transport of C2+ and C3+ ions located near the m/n= 1/1 island brought by the RMP field plays an importantrole in achieving the present plasma detachment. In addition, the carbon transport near the LCFS is not aﬀected by
the supplied RMP field because Prad (C4+ ) and Prad (C5+ )do not change in both discharges with attachment and detachment.
Before the transition to the detached plasma, the divertor heat flux increases with density, but the edge radiation loss increases much larger than the divertor heat flux.
This divertor heat flux level has been usually observed inrelatively high-density discharges with attached plasmas.
Therefore, the present detachment is certainly triggered by
the enhanced edge radiation loss.

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6. Summary
2+5+The partial carbon radiation of C –C ions has been
estimated for attached and detached plasmas by analyzing the intensities of the CIII–CVI resonance lines. It isfound that the Prad (C3+ ) is much stronger than Prad (C2+ ),Prad (C4+ ) and Prad (C5+ ) in both the attached and detachedplasmas. Prad (C3+ ) is much higher in the detached plasma
with the 6-O island (40% to Prad ) compared to that in
the attached plasma (8% to Prad ). Prad (C3+ ) is therefore
thought to be the dominant origin for triggering the RMPassisted detached plasma in LHD.

Acknowledgements
The authors thank all members of the LHD experimental group for their technical supports. This work
was partially carried out under the LHD project finan-cial support (NIFS14ULPP010). This work was also
partly supported by the JSPS KAKENHI Grant Number
23340183 and JSPS-NRF-NSFC A3 Foresight Programin the field of Plasma Physics (NSFC: No.11261140328,
NRF: No.2012K2A2A6000443).
[1] M. Kobayashi et al., Phys. Plasmas 17, 056111 (2010).
[2] M.B. Chowdhuri et al., Phys. Plasmas 16, 062502 (2009).
[3] M.B. Chowdhuri et al., Rev. Sci. Instrum. 78, 023501(2007) and Erratum: 84, 109901 (2013).
[4] T. Oishi et al., Plasma Fusion Res. 10, 3402031 (2015).
[5] C.F. Dong et al., Rev. Sci. Instrum. 81, 033107 (2010).
[6] H.M. Zhang et al., Jpn. J. Appl. Phys. 54, 086101 (2015).
[7] X.L. Huang et al., Rev. Sci. Instrum. 85, 043511 (2014).
[8] C.F. Dong et al., Rev. Sci. Instrum. 82, 113102 (2011).
[9] H.P. Summers et al., Plasma Phys. Control. Fusion 48, 263(2006).

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