24P076-P
Surface characterizations of low-activation ferritic steel alloy exposed toplasma (2)低放射化フェライト鋼へのプラズマ暴露による表面特性変化に関する研究（２）Naoko Ashikawa1), Yoshi Hirooka1) and Takeo Muroga1)
1)1)

芦川直子 ，廣岡慶彦 ，室賀健夫

1)National Institute for Fusion Science, 322-6, Oroshi-cho, Toki, Gifu 509-5292,Japan
核融合科学研究所 〒509-5292 岐阜県土岐市下石町 322-6
A low activation ferritic steel, F82H (8Cr-2W), was exposed to hydrogen plasma in Vehicle-1.

Oxide and other impurity layers with a thickness of less than 1 nm have been observed bothbefore and after plasma exposure in F82H. In the case of target samples irradiated up tofluencies of 7-17x1018 hydrogen plasma in Vehicle-1, the amount of retained hydrogen has
been estimated to be 0.9-11x1015 atoms/cm2. This amount is comparable to that in SUS316.
Two trapping sites were observed and were considered to be inner defects.
1. Introduction
It is widely recognized in the fusion researchcommunity that the extensive use of low-activation
alloys is of critical requisite for the structural andplasma facing components of future DEMO, such
as FFHR [1]. Worldwide efforts on the evaluationof candidate materials are currently in progressbecause the successful candidate is envisaged to be
used for the plasma-facing side of the blanketstructure. However, it is also true that little has
been known about edge-plasma interactions withthese alloys such as surface modifications andparticles retention. In previous studies, the JFT-2M
tokamak [2] and related material R&D groups [3] inJAEA have carried out material characterizations,such as outgassing, magnetization effects andoxide/impurity contaminations. But investigationsof plasma wall interactions, such as surface
characterizations of oxidation/impurities andretained gasses, were not sufficient performed yet.
In this work a low activation ferritic steel
alloy, F82H (89at%Fe, 8at%Cr, 2at%W, 0.1at%C)
developed by Japan Atomic Energy Agency(JAEA), has been bombarded with steady-statehydrogen plasma under some of the conditions
relevant to the first wall environment, using theVEHICLE-1 facility [4].
2. Experiment

The Vehicle-1 facility employs a 1 kW 2.45
GHz ECR plasma source which can generate
steady state plasmas. These plasmas aremagnetized at around 300 G, flowing linearly

down to interact with the target samples. The
surface temperature of target samples weremeasured by a thermocouple, and plasma
parameters, such as an electron density, an ion
flux, and electron and ion temperatures, weremeasured by a movable Langmuir probe.
The plasma density is 3~5x109 cm-3 and theelectron temperature is 3-4 electron volts, resulted
in the ion flux of the order of 1016 ions s-1cm-2. The
ion bombarding energy is set at 100eV by applying
a negative DC-bias onto the target assembly.
Two target samples were used in this study.
One is a ferritic steel alloy, F82H, and the other isan austenitic stainless steel (SUS) 316 (18Cr-8Ni),
whose sizes as the targets are 10 x 10 x 1 mm.
The surface in F82H was treated by amechanical polishing with mirror finishedsurface and the surface in SUS316 was treatedby electropolishing. After these polishing, an
acetone cleaning was done for the both sanples
and were installed in TDS system. In the firstTDS, outgassing processes were done to reduce
residualgassesofas-received
andsurface-treated samples.
The chemical bonding on the surface wasanalyzed by X-ray Photoelectron Spectroscopy(XPS) with Mg X-rays of 400W. An argon ion gun
is used to etch the target samples for depth profilingwith the beam voltage of 4 kV.
Retained gasses in target samples are measured
by thermal desorpstion spectroscopy (TDS). An
infrared heater is located at the upper side of this

Atomic Concentration (%)

3. Results and summary
Figures 1 show atomic concentration and theirrations of metal oxides. Metal oxides ofCr (x10)604030FeC20

FexOx
10O
CrxOx (x10)2000

Atomic Concentration (%)

4000.75

W0.25

0.5
Depth (nm)After (F82H)

80(b)

80

Cr (x10)
Fe60

60

4040

FexOx
20OC0020

CrxOx (x10)

W0.25
0.5
Depth (nm)00.75

Partial pressure of hydrogen (Pa)

Fig.1. Atomic concentrations on the surface in F82H(a) before and (b) after plasma exposure.
10F82H-530min, 255℃
42min, 273℃
10-610-7600

20min, 252℃
400

800

1200
Time (s)1600

Temperature (C)Acknowledgments
This work was supported by the NIFS budget
UFFF028. The authors would like to thank Mr.Y.

(a)

Atomic Concentration : Cr x10

chromium and iron are also plotted in thisfigure. Compositions of oxides were calculated
by a fitting analysis and two kinds of ratios,
without oxide and with oxide were obtained. At
the depth of 0.75 nm from the top surface inFigs.1, iron oxides seem to still remain to about
30-40%. This can be a reason why highbackground intensities were remained at higher
bonding energy region from the iron peak. By
comparison of F82H before and after theplasma exposure, the peak positions of iron andchromium dies not change significantly by theexposure. There data indicates a sputtering andimpurity contaminations during plasma
exposure are negligible. In addition, hydrogen
trapping sites are expected to exist in this bulk
target sample.
In target samples irradiated up to fluencies of
7-17x1018 hydrogen plasma in Vehicle-1, theamount of retained hydrogen has been
estimated to be 0.9-11x1015 atoms/cm2. As
shown in Fig. 2, two distinctive desorption
peaks at around 420 and 500 ℃ have been
exhibited, which indicated the presence oftrapping sites with their respective trapping
energies. These temperatures are the same as
the second and the third desorption temperaturepeaks, respectively, in a previous work in ref. 3.
From surface characterizations on F82H
measured by XPS, thicknesses of oxide andother impurity layers did not changes byplasma exposure and these thicknesses wereless than 1nm. In this experiment, influences ofthe surface contamination and oxide layers islimited but
the bulk trapping sites,
presumably bulk residual defects resulted in thepeaks.

Before (F82H)

80

Atomic Concentration : Cr x10

system and a quartz rod assists a heat transfer fromheater to the target samples located on the bottomside. These samples were heated from room
temperature to 873 K with a ramp rate of 0.48K/s. A
quadrupol mass spectrometer (QMS) measured thepartial pressures.

30
2000

Fig.2. Partial pressures of hydrogen with different

Miyo, Mr. J. Yagyu and Dr.Sakasai in JAEA for
their material preparation. The authors are also
thankful to Dr. T. Nagasaka and Dr. T. Tanaka inNIFS for their expertise in materials.
References[1] S.Sagara et al., Fusion Engineering and Design
83 (2008) 1690-1695[2] K.Tsuzuki, et al., J. Nucl. Mater. 329-333(2004)721.
[3] K.Shiba et al, JAERI-Tech 97-038.
[4] Y.Hirooka et al., J. Nucl. Mater. 337–339(2005) 585.
[5] K.Yamaguchi, et al, J. Vac. Soc. Jpn. (2003)
449.