J. Plasma Fusion Res. SERIES, Vol.3 (2000) 284-287

Tokamakium in TRIAM-IM and its lmpacts on PlasmaSurface Interaction
HIRAI Takeshi, MIYAMOTO Mitsutaka, FUJIWARA Tadashi, TOKUNAGA Kazutoshi, YOSHIDA Naoaki*,
KOMORI Akior, MOTOJIMA Osamur, ITOH Satoshi and the TRIAM groupResearch Institute for Applied Mechanics, Kyushu University, Kasuga 816-8580, Japan
National Institute for Fusion Science, Toki 509-5292, Japan(Received: 2l January 2000 / Accepted: 2 March 2000)

Abstract
Tokamakium in TRIAM-IM was consisted of very fine defective Mo crystals; the grain size wasabout lnm and the crystalline structure is not normal bcc but fcc-like because of co-deposition withoxygen. Due to the co-deposition, retention of hydrogen isotope is changed considerably. Thermal
desorption of implanted deuterium indicates that the retention is much larger and stronger than that ofnormal Mo. The defective structure and the micro-cavities contribute to the large hydrogen retention.

Keywords:
TRIAM-lM, tokamakium, deposit, microstructure, hydrogen retention, plasma-wall interaction

such as graphite have never used for in-vessel

1. lntroduction
Inner walls of plasma confinement devices are
more or less covered by the deposits of impurities

components. To examine the surface modification byimpurity deposition, impurity collectors (pre-thinned304SS disks) mounted on the head of a surface probe

emitted from the plasma facing materials due sputtering
and/or evaporation. Such phenomena have been
observed in many tokamaks and the deposited material
is named "tokamakium" tl-81. Especially in carbon
dominant devices, strong co-deposition of carbon withhydrogen isotope was observed [2-4]. Formation of thetokamakium changes physical and chemical properties

system [9] were exposed to the high-ion temperaturemode plasma uOl at the electron drift side (E-side) in
the scrape off layer (SOL). Duration of each discharge
was about I minute and the total exposure time was 31.5
minutes. Typical plasma parameters were following: 1p= 20-25 kA, n" - 2 x 7012 cm-3. zr = 1.5-2.5 kev. Afterof the plasma facing surface. Particularly, change ofthe exposures microstructure and composite elements ofhydrogen retention properties may have large impacts
on hydrogen recycling under long pulse operation.
In the present work, therefore, micro-structure of
the tokamakium in TRIAM-lM was observed and its
properties concerning with hydrogen retention wereexamined by using comparable vacuum-depositedthe deposits on the thin 304SS disks were examinedwith a transmission electron microscope (TEM) with
EDS. By using dark-field imaging technique one canobtain the image of the deposits without any serious
effects of the substrate.

material.

of the tokamakium in the TRIAM-IM was Mo [7,8],

According to the result that major metallic elements
vacuum-deposition experiments of Mo were carried out

2. Experiments
TRIAM-lM is a superconductive high fieldin various atmosphere to simulate it. Taking intotokamak with a vacuum vessel made of 304SS, divertor@2000 by The Japan Society of Plasma
Science and Nuclear Fusion Researchplates of Mo and limiters of Mo. Non-metallic materials
284

Hirair. et al., Tokamakium in TRIAM-IM and its Impacts on plasma-Surface Interacuon
account that major gas components during a plasma

diffraction condition have white image. Numerous small
dot images indicate that the deposit is consisted of finecrystal grains around only I nm in diameter. The radius
and intensity of the diffraction rings indicate that crystal

discharge is hydrogen and in addition some oxygen andwater exist as residual impurities, the vacuumdepositions were carried in a high vacuum (<10-4 pa)

structure is fcc-like, though its major metallic

and low-pressure hydrogen or oxygen atmospheres.
Partial pressures of hydrogen and oxygen wete Px2=l0-3 Pa and l0-2 Pa, Ps2 = 5 x l0-a Pa,5 x l0-3 pa,component is Mo, which normally form bcc structure.
One should note that the crystal structure must be very
defective in such fine grains, because more than half ofthe constituting atoms is expected to locate on or near
the grain boundaries. As shown in the picture (right) ofrespectively. Microstructure of the deposits wasexamined with a TEM.
Mo-deposits formed in oxygen atmosphere (Poz = 5x 10-3 Pa; were implanted with 6 keV-D3* deuterium

Fig. I, thermal stability of the deposit is quite low.
Grain growth and simultaneous change of crystalions (2 keV-D*) at room temperature up to a fluence ofI x lO22 ions/m2. It is likely that the majority of theimplanted ions were stopped within the deposit, because

structure from fcc-like to bcc occurred abruptly by theisochronal annealing (100 K steps for 10 minutes) up to

its thickness of about 50 nm was larger than the973 K. Diffraction pattern shows no oxidation. Theseresults indicate that the structure and the characters ofprojected range of 2 keV-D* ions (<30 nm). Thermal
desorption of molecules composed of deuterium (DH,D2, DHO, D2O) under heating with a ramping rate oflK,/s was measured with quadruple mass spectroscopy(QMS). QMS signals of DH, D2 and DHO, D2O werequantified by using He standard leak. Thermal
desorption spectra from the bulk Mo (99.957o purity)and bulk 304SS were also obtained as references.

the tokamakium formed in TRIAM-lM is completelydifferent from the normal vacuum-deposited Mo, which
are polycrystalline consisted of bcc grains of about 200
nm in size [8].
As reported in ref. [8] residual oxygen strongly
affects structure of vacuum-deposited Mo. If the ratio ofthe arrival rate of oxygen atoms and Mo atoms, Ao/AMo,is about 2-3, its structure is normal large bcc grains
while if the ratio is about 20, fine fcc-like grain
structure, similar to the deposit of TRIAM-lM, appear.
It is likely that oxygen atoms adsorbed on the surface
suppress free migration and crystallization of thedeposited Mo atoms and result in refinement of thecrystal grains. Moreover, oxygen atoms involved in theMo lattice may prevent to form normal bcc structure andcause distortion of lattice due to its large atom size.
Similar fcc-like fine grain deposit was observed in Fe
formed in oxygen atmosphere [11]. It was shown thatthe structure changed successively from large bcc
grains, fine fcc-like grains, fine FeO grains to fineFe2O3-Fe3Oa, with increasing partial pressure of oxygen

3. Results and Discussion
3.1 Microstructure of the deposits
Figure I shows dark field images andcorresponding electron diffraction patterns of thetokamakium deposited on a thin 304SS specimen inTRIAM-IM. The images were obtained from a part of
the first broad diffraction ring. At this imaging
condition, only the crystal grains satisfying the Bragg

during the vacuum-deposition.
Hydrogen atoms, in contrast, did not show strong
effects on the structure of the vacuum-deposited Mo.
The structure is large bcc grains even if the ratio Ag/
Ayo is about 200 [8]. Weaker chemical interaction with
Mo and its smaller atomic size may reasons why theeffects of hydrogen is not so strong as oxygen.

Though TRIAM-lM is an ultra-high vacuum
machine, some oxygen exist as residual impurity. It is
likely that co-deposition of the residual oxygen with MoFig. 1 Electron diffeaction patterns and mictostructuresof the deposition on the stainless steel specimen

exporsed to tokamak discharges (left) and thatannealed at 973 K for 10 minutes (right). White
image in these micrographs shows individual

brough the peculiar defective structure.

crystal grains.
285

HiraiT. et al., Tokamakium in TRIAM-I M and its Impacts on Plasma-Surface Interaction

Figure 5 is a micrograph at low magnification of
the Mo-deposit implanted 4 keV-H2* hydrogen ion atroom temperature to a fluence of 7.2 x 102r H/m2. The
substrate was an Al thin foil. It is clear that the3.2 Hydrogen retention in the defective Modeposition
Due to their defective structure, large and strong
retention of hydrogen is expected for the tokamakium in

TRIAM-lM and the Mo-deposits formed in a low
pressure oxygen atmosphere. To estimate the retentionof deuterium, implantation of deuterium ion was carriedax1o2l D/m2,C5E
F4oout. Figure 2 shows thermal desorption spectra (TDS) ofD2 (m/e = 4) and D2O (m/e = 20) from the deuterium

ion implanted Mo-deposit, which was formed in thex

atmosphere of 5 x l0-3 Pa 02. TDS data for the bulk Mo

D2 (m/e=4)![,-ro,nc.

oand 304SS are also plotted in the figure. In case of D2O,III

g2

large desorption occurred only for the Mo-deposit. At
least four peaks exist above 450 K and desorption still
continues even at 900 K. This large desorption indicatesthat quite a large amount of oxygen are involved in theo:v,aq)

E1Nogodefective Mo-deposit.

g" q,-As shown in Fig. 2, retention of implantedc)tr
gDFoL
1r=deuterium in the Mo-deposit is much larger and muchstronger than bulk Mo. For the Mo-deposit, two large
desorption peaks of D2 are distinguished at around 550
K and 720 K, while only a small peak at around 530 K
for the bulk Mo. The thermal desorption spectrum from
the Mo-deposit is also different from that of bulk 304SS.
This indicates that majority of the implanted deuterium
are retained in the deposited layer.
Retention of deuterium in deuterium ion implantedMo-deposit, bulk Mo and 304SS are plotted in Fig. 3
against deuterium ion fluence.
Each data is the sum of the desorbed deuterium as
DH and D2, DHO and D2O molecules. In contrast to theoJla.lo0-400 500 600 700 800 s00 1000 1100
Temperature [K]300

Fig. 2 Thermal desorption spectra of D, (m/e = 4) andDrO (m/e = 20) obtained from the Mo-deposit inoxygen atmosphere lPo, = 5 x 10-s Pa), Mo and
304SS specimens implanted with 6 keV-D.- at aflu.ence3x1021 D/m2.

aweak retention in bulk Mo, the defective Mo-deposit
shows strong retention. It is worth to note that trappingof deuterium in the Mo-deposit is even larger thanstainless steel, which is known as a strong hydrogen
trapping material. The ratios of D/Mo and O/Mo at thefluence of I x 1022 D/m2 were estimated to be 0.35 and<\En 1021

Mo+O depo.

rl

Ma Bul*a

SU$ Bulk.ssi

g.9coo 1 0200.04, respectively.
Microstructure of Mo-deposit implanted 4 keV-H2+
hydrogen ion at room temperature to a fluence of 7.2 xIt"""' - --f"-L^MoEJILol02t H/m2 is shown in Fig. 4. The micrographs arebright field images taken atjust-focus, under-focus and:Jo 10te

oover-focus conditions, respectively. The white image inthe under-focus condition reverses to black one in theover-focus condition as indicated by arrows in thefigure. Such reversible change of the image contrast
indicates that the defects are micro-cavities. It seems
that the defective structure and rather easy formation of

101s

1 020

1021

1022

Fluence [m-21Fig.3 Fluence dependence of the deuterium retention
for Mo-deposit. Deuterium retention in bulk Mo
and 304SS are also plotted. Specimens wereimplanted with 6 keV-Dr. at room temperaturesmall cavities are the reasons for larse and strons
retention of hydrogen.

before the TDS experiments.
286

Hirair. et al., Tokamakium in TRIAM-IM and its Impacts on plasma-surface Interacuon
but also as D2O and DHO. The defective structure andthe micro-cavities contribute to the large hydrogen
retention The ratio of D/Mo reached 0.35 at hish
fluence.

The present work indicates that re-deposition ofsputtered elements on plasma facing surface is a serious
problem even in a metallic tokamak. Due to its defective

structure and involved oxygen, the deposited layer must
trap large amount of hydrogen and may affect hydrogen

Fig.4 Microstructure of Mo-deposit implanted with 4recycling. One should pay more attention to thekeV-Hr. at room temperature to a fluence ol 7.2 x1021 Hm-2. Each micrograph was taken at theconditions of just-, under- and over-focus.

modification of plasma facing surface by the impurity
deposition and its effects on the hydrogen behavior. Thisis particularly important for long pule and steady state
plasma.

Referencestll

R. Behrisch, M. Mayer and C. Garcia-Rosales, J.
nucl. Mater. 233-237,673 (1996).

[2] R. Behrish, A.P. Martinelli, S. Grigull, R.
Groetzschel, U. Kreissig, D. Hildebrandt and W.
Sshneider, J. Nucl. Mater. 220-222, 590 (1995).

t3l K. Krieger, J. Roth, A. Annen, W. Jacob and C.S.
Picher, J. Nucl. Mater.24l-243,684 (1997).

[4] E. Gauthier, A. Grosman and J. Valter, J. Nucl.
Mater. 220-222, 506 ( 1995).
t5l N.Yoshida, K. Tokunaga, T. Fujiwara, T. Muroga,
S. Itoh and the TRIAM group, J. Nucl. Mater. 258263,4rs (1998).

Fig. 5 Blisters formed in Mo-deposit by hydrogen ion

implantation at room temperature.

t6l N.Yoshida, A. Nagao, K. Tokunaga, K. Tawara, T.
Muroga, T. Fujiwara, S. Itoh and the TRIAM
group, Radiation Effects and Defects in Solids 124,

deposited layer form blisters by the accumulation ofimplanted deuterium at the interface between thesubstrate and the deposit layer. Similar blistering was[7] T. Hirai, K. Tokunaga, T. Fujiwara, N. Yoshida, S.

observed in the tokamakium formed in TRIAM-IM [7].

Itoh and the TRIAM group, J. Nucl. Mater. 25E-

Present work indicates that re-deposition ofsputtered impurities on plasma facing surface causes

t8l T.Hirai, T. Fujiwara, K. Tokunaga, N. Yoshida,99 (1992\.

263,1060 (1998).

serious problem such as large retention of hydrogenisotopes even in metallic tokamak as well as carbon
tokamak [2-4].

A.Komori, O. Motojima, S. Itoh and the TRIAM
group, in J. Nucl. Mater. (ICFRM-9, 1999), to bepublished.

[9] T. Hirai, T. Fujiwara, K. Tokunaga, N. Yoshida, S.
4. Gonclusions

Itoh and the TRIAM group, in Fusion Energy 1998
(Proc.l7th, Int. Conf., Yokohama, 1998), IAEAcN-69/FT2l3.

A major element of the tokamakium in TRIAM-lM
is Mo but its structure is peculiar; crystal grain size isabout lnm and their crystal structure is not normal bcc
but fcc-like because of co-deposited oxygen, which

tlOl H. Zushi, K. Nakamura, A. Itoh, K. Sato, M.
Sakamoto, K. Hanada, E. Jotaki, K. Makino, S.
Kawasaki and H. Nakashima, in Fusion Energy
1998 (Proc.17th, Int. Conf., Yokohama, 1998),
IAEA.CN.69IEXP2I16.
ll ll N. Yoshida and F.E. Fujita, J. Phys. F: Metal Phys.,
2, roo9 (1972).

exists in plasma as residual impurities. The Mo-deposits
formed in the low pressure oxygen atmosphere, whichhave the same structure of the deposit in TRIAM-lM,

show very large and strong hydrogen retention.
Implanted deuterium is desorbed not only as D2 and DH287