J. Plasma Fusion Res. SERIES, Vol. 11 (2015)

Structural analysis of carbon co-deposited layer in LHD
N. Ashikawaa,b, Suk-Ho Hongc, Y. Hamajid, Y. Torikaie, Y. Uedad, K. Nishimuraa and the LHD
experiment groupaab

National Institute for Fusion Science, Gifu, 509-5292, JapanThe Graduate University of Advanced Studies, Gifu, 509-5292, Japan
cNational Fusion Research Institute, Daejeon, 305-333, Korea
dOsaka University, Suita, 565-0871, Japane
University of Toyama, Toyama, 930-8555, Japan(Received: 21 October 2014 / Accepted: 19 January 2015)

The deposition layers that were formed at two different locations of LHD were analysed. Carbon is the major element
within the layers. The structures in the deposited layers were analysed with XPS and Raman spectrometry. The sp2/sp3
hybridized orbital ratio of carbon was shown to be 0.8-0.95, which implies disordered carbon bonding. On the deposition
layers, two-dimensional carbon bonding, as observed in the graphite structure, was not maintained because these structures
were broken during the sputtering process. The ellipsometry analysis shows single C-H bonds in these deposition layers.
These analytical results show a detailed amorphous structure in the deposited layers of LHD.
Keywords: Carbon deposited layer, Hydrogen retention, Raman spectroscopy, XPS, hydrocarbon bindings
deposited layers is insufficient. For carbon depositedlayers, some references presented optimized analyticalmethods [5] in which the relation between the amounts ofretained hydrogen isotopes and the structure of the carbon
layers, such as amorphous hydrogenated carbon (a:C-H),
were investigated. However, for amorphous hydrogenatedcarbon, the parameters vary widely. In addition, it isdifficult to clearly identify the C-H bonding. Hence, theanalysis parameters are not sufficient for carbon
deposited layers in fusion devices.
The present study shows the analysis results ofdeposited layers in LHD. In particular, the analyticalresults of a disordering of carbon bonding, sp2-sp3
hybridized orbitals and hydrocarbons are shown in carbon
deposited layers.

1. Introduction

Investigations of hydrogen isotope inventories inplasma-facing walls are important to control fuelrecycling and in-vessel tritium inventories in fusion
devices. However, the removal processes of retainedhydrogen isotopes from the walls have not been
optimized and are serious problems in ITER and DEMO.
In particular, more hydrogen isotopes are retained indeposition layers than in bulk materials [1]. The retainedhydrogen isotopes that originate from energetic hydrogenisotopes during plasma discharges and the molecularhydrogen isotopes have different depth profiles in thetarget materials [2-3]. In Tokamak devices, high-leveltritium retentions are observed in the dome regions
because molecular hydrogen isotopes are trapped in thedeposition layers [3]. Thus, the trapped molecularhydrogen isotopes must also be investigated.
In ITER, plasma-facing materials are designed usingtungsten and beryllium. In DEMO, some different
materials may be considered for plasma-facing materials.
Because carbon and metallic deposited layers havedifferent hydrogen-trapping mechanisms, different
analytical methods are required for the analysis. The
characterizations of different deposition layers are
particularly important to consider effective methods toremove hydrogen isotopes from the layers [4].
For metallic and mixed metal-carbon depositedlayers, analytical methods are not optimized, and theunderstanding of hydrogen isotope trapping sites in the2. Experimental

Two types of deposition layers were produced on
stainless steel (SS) 316 target samples S2 and S3, whichwere set near the graphite divertor targets, as shown in
Fig. 1, during an experimental campaign in LHD.
Samples S2 and S3 faced different directions on the same
holder. The minimum distance from the graphite divertor
targets to the sample holder was approximately 20 cm.
Sample S2 faced the divertor targets, and sample S3
faced in the opposite direction. Accordingly, sample S3
did not directly see the divertor targets. S2 and S3 had
identical neutral gas pressures and temperatures.
The thicknesses and compositions of the depositedauthor’s e-mail: ashikawa@LHD.nifs.ac.jp

15©2015 by The Japan Society of Plasma
Science and Nuclear Fusion Research

N. Ashikawa et al., Structural Analysis of Carbon Co-Deposited Layer in LHD

photodiode. The response of the sample to the polarized
light was measured as two ellipsometric angles: < and '.
After the analyses using these parameters, two parameters
were obtained: refractive index (n) and extinction
coefficient (k) [9].

layers were measured using Auger electron spectroscopy(AES). In AES, Ar+ ions with an energy of 3 keV wereset for sputtering. The amounts of retained hydrogen were
measured using thermal desorption spectrometry (TDS).
The target samples were heated from room temperature to1273 K at a rate of 0.5 K/s and maintained at 1273 K for
15 min [6]. The desorbed gasses from the heated targetsamples were measured using quadrupole mass
spectrometry (QMS).
The sp2-sp3 hybridized orbitals of carbon on thedeposition layers were measured using X-ray
photoelectron spectroscopy (XPS). A Mg X-ray source of
400 W and a spot size of 800 Pm were used. The
background intensities of these narrow profiles wereremoved using the Shirley method [7]. A peak binding
energy of C1s was calibrated using a highly orderedpyrolytic graphite (HOPG) target. An argon ion gun wasused to etch the target samples and analyse the depth
profiles of the binding energies.
Raman spectra were measured (800 ~ 2000 cm-1)
using an Ar laser. The carbon deposition layers show abroad G peak (at ~1570 cm-1), a D peak (at ~1350 cm-1)
and a continuous photo luminescence (PL) background[8]. Using a fitting analysis, the G and D peaks wereseparated, and the ratio of the two peak intensities of theG and D peaks and the full width at half maximum(FWHM) of the G peak were obtained.
The optical characteristics, which are connected tohydrocarbon bonding in deposited layers, were measured
using ellipsometry. The commercial ellipsometer VASE
(variable angle spectroscopic ellipsometer) from J. A.
Woollam Inc. was used. The ellipsometer has ameasurement range of 193-2000 nm, and the angle ofincidence can be varied between 15 and 90°. The linearlypolarized incident light beam was reflected by a sample,and elliptically polarized light was detected by a

3. Results and Discussion
According to the AES analysis, the thickness andcomposition of the deposition layer are 382 nm (80 %Cand 15 % Fe) and 22 nm (60 %C, 25 %Fe and 5 %B) for
samples S2 and S3, respectively. The amounts of retainedhydrogen in the deposition layers are 1.03x1022 mol/m2
and 1.10x1021 mol/m2 for S2 and S3, respectively [4].
Hydrogen was mainly trapped in the deposition layers
during the main plasma discharges and glow discharges
in LHD. Because carbon is the major element of thedeposition layers in the two samples, most hydrogen
trapping sites are assumed to be carbon in the layers. The
estimated hydrogen-carbon (H/C) ratio in the S2 and S3layers are 0.23 and 0.55, respectively.
A comparison of the H/C ratio and retainedhydrogen in the S2 and S3 samples shows that S3 hasmore hydrogen per carbon atom than S2.
The sp2-sp3 hybridized orbitals of carbon in thedeposition layers were measured using XPS. A fitting
analysis for the separation of sp2 and sp3 carbons is
performed as shown in Fig. 2. A peak due to an sp2
carbon was observed at 285.59 eV using the HOPG target,
and a peak due to an sp3 carbon was observed with ashifted binding energy from sp2 in Ref. 10. An sp2
satellite was estimated to be negligible and acarbon-dioxide bond is observed on sample S3. The XPSdepth profiling shows weak variation of the sp2/sp3 ratiowith depths for both S2 and S3: 0.95-0.8 for S2 and0.9-0.8 for S3.

(b)
S4S3S2(a)First wallSample holder

S1Graphite divertor
targetFigure 1. Setups of the sample holder and their location in LHD. (a) The sample holder was installed near the 6.5
port section. (b) The holder has two facings, the sample S2 facing to graphite divertor target and sample S3 facing
to the opposite direction of sample S2.

16N. Ashikawa et al., Structural Analysis of Carbon Co-Deposited Layer in LHD

peaks and the full width at half maximum (FWHM) ofthe G peak are shown in Fig. 3. In Ref. 11, the same
parameters of the net erosion and net deposition areas inTEXTOR were reported and are plotted in the samefigure. For isotropic graphite (IG-430U), ID/IG isapproximately 0.1 because a clear graphite structure near
the G-band is observed. The FWHM of G-band isapproximately 10 for this material. The FWHM of theG-band is related to the sp2 hybrid orbit as a graphite
structure on a two-dimensional plane, and the high
FWHM of the G-band indicates highly disorderedcarbon-carbon bonds. For the deposition layers on the S2and S3 samples of LHD and the graphite limiter ofTEXTOR, the observed FWHMs of the G-band are120-160.
In general, graphite structures are broken because ofthe erosion-deposition processes, and it is difficult torecover this structure on deposited layers. Hence,
deposition layers in LHD and TEXTOR show similar
disordered carbon-carbon bonds.
The ID/IG ratio is inversely proportional to the crystal
size of carbon, which was measured using X-ray
diffraction [13]. For carbon deposited layers in fusion
devices, the ID/IG ratios disperse, as shown in Fig. 3.
Hence, the ID/IG ratios do not show the characteristics ofthe deposition layers in fusion devices.
Figure 4 shows the refractive index that wasmeasured using ellipsometry. By comparing the data for
LHD deposition layers and known materials, the types of
C-H bonds were determined. Four targets were used as

In the field of plasma processing to produce
diamond-like carbon (DLC), abundant data collections
and analyses are performed [5]. The H/C and sp2/sp3
ratios for S2 and S3 were plotted in a ternary phase
diagram of bonding in amorphous carbon-hydrogenalloys [12], and the type of amorphous carbon in the two
samples was estimated. A hard type a-C:H was suggested
as the type of hydrogenated carbon layer in LHD.
Disordered carbon-carbon bonds were measured
using Raman spectroscopy. Raw data for the G- andD-bands were analysed, and the intensity ratio of the two

Figure 2. Narrow profile of C1s peak by XPS
at (a) a depth of 31.8nm on sample S2 and (b)a depth of 4.5 nm on sample S3. A fitting
analysis for the separation of sp2 and sp3
carbons is performed.

FWHM

G200150100Sample S2 and S3
deposition area(LHD)

Net deposition area(TEXTOR)Net erosion area
(TEXTOR)50
0Amorphization

Disordered structuresof carbon

Single vacancy
IG-430U

00.5

1I /I

Amorphization

1.52D G

Figure 3. Analytical data of Raman
spectroscopy for deposition layers on S2 and S3
samples of LHD (this work), IG-430U graphitetarget (this work) and deposition and erosion
areas on graphite limiter of TEXTOR [11].

Figure 4. Refractive index measured by the theellipsometry analysis (a) for reference datausing known materials, (b) for LHD samples,S2 and S3.

17

N. Ashikawa et al., Structural Analysis of Carbon Co-Deposited Layer in LHD

known materials, as shown in Fig. 4 (a):CH4-polymer-like, CH4-DLC, C2H2-polymer-like andC2H2-DLC.
The
threetargets
methane(C‫آ‬H4)-polymer-like, methane DLC and acetylene (H-C≡C-H)-polymer-like are in the same parameter range ofrefractive indices as those for S2 and S3. The common
characteristicofCH4-DLC,
C2H2-DLCandC2H2-polymer-like is a single C-H bond. Thus, the major
type of bonding in the carbon deposited layers of S2 andS3 may be single C-H.

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[14] G. Katagiri, et al., Carbon 26 (1988) 565-571.

4. Summary
The relations of retained hydrogen and analyticalparameters were investigated for the deposition layers at
two positions in LHD: facing and not facing the divertor.
Similar carbon characteristics were shown for thetwo deposition layers. The obtained sp2/sp3 hybridized
orbital ratio of carbon and H/C were 0.8-0.95 and 0.2-0.5,
respectively, for the two layers. From the relationshipbetween sp2/sp3 and H/C and according to the databasefor DLC, a hard type a-C:H is expected for the type ofcarbon bonding in the two deposition layers.
Raman spectroscopy shows disordered carbon
bonding. On the deposition layers, two-dimensional
carbon bonding, such as that seen in the graphite structure,
were not maintained because these structures were broken
during the sputtering process on graphite targets. Thus,
carbon deposited layers show higher parameters ofFWHMG, and similar types of disordered carbon bonds
are shown in different locations.
Based on the ellipsometry analysis, C-H bonds in thecarbon deposited layers were suggested. The analysed
data for the deposition layers are in the same parameter
range of CH4 polymer-like, CH4 DLC and C2H2polymer-like structures, which contain single C-H bonds.
These analytical results show a detailed amorphous
structure in the deposited layers of LHD.
5. Acknowledgements
This research was partly supported by theJSPS-NRF-NSFC A3 Foresight Program in the field ofPlasma Physics (NSFC: No.11261140328, NRF:No.2012K2A2A6000443) and NIFS budget KUHR010,
ULFF004, KOBF031. The authors would like to thankProf. S. Morita for organizing the A3 program.
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