24P072-P
Serious Modification of Plasma Facing Surfaces in LHD by Glow Discharge
Cleaning at Start-up Phase
LHD における初期グロー放電洗浄におけるプラズマ対向面の変質
Naoaki Yoshida1, Hironori Harada2, Yohta Kimura2, Reiko Yoshihara1, Tadashi Fujiwara1,Masayuki Tokitani3, Tsuyoshi Akiyama3, Suguru Masuzaki3, Yasuhisa Oya4, Kenji Okuno4, Yuji Hatano5
吉田直亮1、原田裕孝 2、木村陽太 2、吉原麗子 1、藤原正 1、時谷政行 3、秋山毅志 3、増崎貴 3、
大矢恭久 4、奥野健二 4、波多野雄治 5
1

RIAM, Kyushu Univ., Kasuga 816-8580 Japan, 2IGSES, Kasuga, Kyushu Univ., 3NIFS, Toki,
4FS, Shizuoka Univ. ,Shizuoka, 5HIRC, Univ. Toyama, Toyama
1九大応力研、福岡県春日市春日公園 6-1、2 九大総理工、3 核融合研、4 静大理、5 富山大水素セ
In case of LHD, intensive glow discharge cleanings using Ne, He and H are carried out successively at thestart-up phase of each experimental campaign. Though the amount of the adsorbed gas at the internalsurfaces of the torus decreases enough, its structure and character are changed seriously, and their features,
especially at the sputtering dominant area, were maintained through the experimental campaign. Mixing
with co-deposited metallic elements, C, O and He at the surface, which was induced by the plasma
bombardment, changed the character of the surface to that with very high ability for H retention.

1. Introduction
In order to improve plasma performance,
especially to achieve very long duration discharges,
active control of hydrogen recycling will beindispensable. One of the key processes controlling
the recycling is retention and desorption ofhydrogen through the plasma facing surfaces underthe operation. We have been studying modificationof plasma facing surfaces of LHD continuously
since 2008 (Cycle 12) by paying special attention to
the change of physical and chemical properties ofthe surface such as ability of hydrogen retention
and its nano-structure. As reported so far, it became
clear that even if the plasma facing surface placedat the sputtering dominant area its ability forhydrogen retention is unexpectedly high due to theformation of a mixing layer on the surface. Though
its thickness is only about 10nm, very complicateddefective structure of the mixing layer results invery high hydrogen retention.
In LHD, many different types of plasma
discharges are performed day by day. Not only
main plasma discharges with H or He but also glow
discharge cleaning (GDC) are carried out time totime by using H, He or Ne. We have reported thateach type of discharges have their own interaction
with wall materials. In order to control hydrogen
recycling process actively, it is important to knowwhat type of discharges play major role todetermine the nature of the plasma facing surface.

In the present work, therefore, modification ofplasma facing surfaces under the glow dischargecleaning at the start-up phase of the Cycle 14(2010) were examined by inserting probe materials
at the wall position, because vigorous modificationof the plasma facing surface under the GDC can be
expected from the relevant fundamental studies
performed so far.
2. Experimental Procedures
Three sets of material probe samples consistingof thin platelet samples and pre-thinned TEM
samples made of SUS316L, W and Mo wereexposed to GDC at the start-up phase, followinghydrogen main discharges (106 shots, #98201~98306) and both of them, respectively, by using theretractable material prove transfer system installedat the 4.5L port. GDC was performed with Ne gas
for 12h, H gas for 122h and He gas for 24h
successively. Energy of the plasma bombarding thesurfaces was estimated about 200eV. The samples
were quickly taken out from LHD after finishing
the plasma exposures and then modification of thesurface and its ability for hydrogen retention wereexamined by means of TEM, XPS, GD-OES andTDS.
3. Experimental Results and Discussion
3.1 Modification of surface by GDCAs shown by a TEM micrograph and thecorresponding electron diffraction pattern of apre-thinned Mo in Fig.1, the surface is covered byheavily modified layer consisting of fine crystals
with nano-size of Mo and oxide of Fe and Mo.
Chemical analysis using XPS and GD-OES
indicated that not only metallic elements of thesubstrate (Mo in this case) and the wall (SUS316L),
but also large amount of C and O exist. These
results indicate that physically sputtered Mo atoms
under GDC with Ne and He returned to the surface
near-by and deposit together with Fe, Cr, O and C
sputtered form the wall and the divertor made ofgraphite. In case of SUS316L substrate, the surface
was covered by thick modified layer consist of finecrystals of Fe and Fe-oxide together with C. As
reported so far, co-deposition of metallic atoms
together with O and C leads to pronounced
refinement of crystal grains and, as a result, thedeposited layer get very high ability for H trapping.
Decreasing of O and C is effective to keep theability of the metallic deposition at low level.
Another important feature of the modified layer
is effects of He bombardment. A large number offine He bubbles (white dot images in Fig.1a) areformed together with dislocation loops in thesubstrate. As shown in Fig.2, a cross-sectional viewof a W sample exposed to the GDC, the He bubbles
are mainly formed in the modified layer of about
8nm-thick. It is noteworthy that nanoscopic
structure and chemical state of the modified layer
formed by the GDC performed during the start-up
phase are quite similar to those of the samples
exposed to the whole plasmas in the experimental
campaign. It is considered that heavy GDC at theFig.1 Microstructure of Mo exposed to GDC at thestart-up phasestart-up phase plays very important role todetermine the nature of the plasma facing surfaces
located at the sputtering dominant area.
It was also observed that deposition of carbon on“shadowed” area, where sputtering by Ne and He
was difficult, was very remarkable, because thesputtered C can drift in the torus by forming C-H
complexes. This phenomenon also affects opticalproperties of first mirrors for plasma diagnostics.
3.2 Modification by the following H plasma
discharges
It was observed that the ratio of metallic oxides,
which have been formed in the GDC phase, decreased.
It means that the oxides were deoxidized by reacting
chemically with bombarding H plasma. It was also
remarkable that the surface was covered by Cdeposition. In LHD, chemical sputtering of C and it
re-deposition under the H main discharges is also
important PWI process.
4. Conclusions
At present, structure and character of the plasma
facing surfaces of LHD, especially at the sputtering
dominant area, are almost formed by the plasma
bombardments under the GDC in the start-up phase.
The present work indicates that minimizing the GDC
and reduction of C and O is necessary to reduceability for hydrogen retention, which will be effective
to improve the plasma performance of long duration
discharges. It is considered that replacing the present
graphite divertors by W ones would be very effective
to reduce contents of C and O and would promote
reactor-relevant studies with LHD furthermore.

Fig. 2 Cross sectional view of W exposed to GDC at thestart-up phase