J. Plasma Fusion Res. SERIES, Vol. 10 (2013)

Study on plasma – tungsten surface interactions using the new experimental
device EXPRESSRyo Miura1)*, Tetsuo Fujishima1), Yuto Miyahara1), Tomohisa Taguchi1),
Masahiro Tanaka2), Akio Sagara2),Yasuhisa Oya 1), and Kenji Okuno1)
1) Radioscience Research Laboratory, Faculty of Science, Shizuoka University
2) National Institute for Fusion Science(Received: 25 May 2012 / Accepted: 5 December 2012)

An experimental apparatus named as EXPRESS (Effluence and Exchange Probe for Recycling Estimation atNIFS and Shizuoka University) has been designed and established in Shizuoka University to acquire laboratory data(well-conditioned data) needed for the design of an actual reactor. The behaviors of deuterium retention and thesurface chemical state after EXPRESS plasma operation were evaluated and compared with that for LHD (LargeHelical Device, NIFS) hydrogen plasma exposed sample. It was found that a thin impurity deposition layer with thethickness less than 0.3 nm was formed on the surface, which was one thirteenth thinner than that on LHD plasma
exposed tungsten. The TDS experiment showed that major deuterium desorption stage for EXPRESS plasma
exposed tungsten was located at the lower temperature less than 550 K, although that for LHD plasma exposed one
was found at the temperature between 550 K – 1000 K. These facts suggest that the impurity concentration in thedeposition layers have a large influence on the deuterium retention behavior in tungsten.
Keywords: deuterium plasma exposure, TDS, XPS, impurity deposition layer, tungsten(Large Helical Device, NIFS) plasma exposed tungsten
and only D2+ implanted tungsten for the evaluation of fuelrecycling during plasma-surface interactions in
EXPRESS.

1. Introduction

Tungsten will be used as a plasma facing material in
future fusion devices. Many studies have been devoted toelucidate fundamental tritium retention and transport
behaviors in tungsten under well-controlled condition [1-3].
However, it is quite difficult to apply these data obtained
under well-controlled condition to actual and larger-scale
plasma devices or fusion devices where conditions aremuch more complex. For example, a wall temperature
gradient will arise when the plasma is controlled. Therefore,
the key parameters should be acquired to complete
laboratory data (well-controlled condition data) needed forthe design of an actual reactor design. The design of a new
plasma experimental device, namely EXPRESS (Effluenceand Exchange Probe for Recycling Estimation at NIFS andShizuoka University) was designed to clarify the hydrogenisotope recycling dynamics. This devise has thesefollowing features: i) many samples can be placed on thesample stage for plasma exposure and the sample
temperature gradient can be controlled, and ii) 1 GBq / dayof tritium can be handled.
As a preliminary experiment, the deuterium plasma
exposure to pure tungsten was performed in this study tocharacterize and evaluate the behaviors of the deuterium
retention and the surface chemical state after EXPRESS
plasma operation at R.T. These experimental results
obtained at EXPRESS were compared with those in LHD

2. EXPRESS Apparatus

The recently designed plasma experimental device toinvestigate hydrogen isotope recycling behavior andplasma surface interaction, EXPRESS consists of threeparts: i) a plasma generator and plasma exposure system,ii) a fuel storage and purification system, and iii) a fuelrecovery-supply systems. Figure 1 shows the schematic
view and picture of the plasma generator and plasma
exposure system. The plasma generator and plasma

Fig. 1 Schematic view and picture of the plasma
generator and plasma exposure system inEXPRESS.

e-mail: r0232027@ipc.shizuoka.ac.jp85©2013 by The Japan Society of Plasma
Science and Nuclear Fusion Research

R. Miura et al., Study on Plasma-Tungsten Surface Interactions Using the New Experimental Device EXPRESS

exposure system can be evacuated to 1 × 10-5 Pa by a scroll
pump (SP) and a turbo-molecular pump (TMP)
combination. The pressure in the system is measured by
Baratron and ionization gauge (I.G.). The source gas issupplied through a mass flow controller (M.F.C.) at a flowrate of 0.1-5.0 sccm into the plasma generation tube madeby quartz with a size of 150 mm in length and 42 mm indiameter. The excitation frequency is fixed at 13.56 MHz
by a RF matching box. The bias voltage can be changed
from 0- 200 V between the chamber wall and the sample
stage to control implantation energy similarly larger-scale
plasma devices. The size of sample stage is 100 mm indiameter, which is made of tungsten. A heating unit isinstalled beneath the center of sample stage to generate atemperature gradient up to 200 K between the center andthe edge of the sample stage as shown in Fig. 2. Thisapparatus is used to investigate the study ofplasma-surface interactions on the first wall of actual
reactor. The heating unit can heat up to 873 K at the centerof stage. The ion flux can be measured by a faraday cup
installed in the plasma exposure system.
㻌900

at maximum heating

850800㻌Temperature / Kenergy for the stable plasma operation. The ion flux was3.4 × 1019 D+ m-2 s-1and the fluence was 1.0 × 1024 D+ m-2
at R.T. This sample was named as EXPRESS sample.
After plasma exposure, TDS (Thermal Desorption
Spectroscopy) were performed from R.T. to 1173 K with aheating rate of 0.5 K s-1 to investigate the deuterium
desorption and retention behaviors. The chemical states
and the thickness of the impurity layer on tungsten wereevaluated by XPS (X-ray Photoelectron Spectroscopy)(ESCA1600 system, ULVAC-PHI Inc.) using a Mg - KD
X-ray source (1253.6 eV) and 1 keV Ar+ sputtering
technique [4]. The thickness of impurity deposition layer
was defined as the depth where impurity concentration wasreached to be half compared to that on the surface. The
sputtering rate by Ar+ implantation was estimated by AFM(Atomic force microscope).
Two types of tungsten samples, namely D2+ imp.
sample and LHD sample, were prepared to compare thedeuterium retention behavior. In the D2+ imp. sample, 3.0
keV D2+ implantation was performed for tungsten withflux of 1.0 × 1018 D+ m-2 s-1 and fluence of 1.0 × 1022 D+
m-2 at R.T. In the case of LHD sample, the sample wasexposed to 134 shots (exposure time 271 seconds) ofhydrogen plasma in LHD, with the ion energy of about
10-100 eV and the temperature of less than 373 K, at afirst wall position in the plasma operation campaign of2011.

750

4. Results and discussion

Figure 3 shows the TDS spectra for EXPRESS andLHD and D2+ imp. samples. The TDS spectrum for the D2+
imp. sample consists of two desorption stages at 400 K and550 K, attributing to the desorption stages of deuterium
adsorbed on the surface and/or trapped by dislocation loops(Peak 1), [5,6] and deuterium trapped by vacancies (Peak2) [7] respectively. In the D2 desorption stages for theEXPRESS sample, Peak 1 was the dominant stage which issimilar for the D2+ imp. sample, but the ratio of Peak 1 to700
650
05 10 15 20 25 30 35 40 45 50

Distance from center / mm

Fig. 2 The maximum temperature profile on the sample
stage in EXPRESS.

3. Experimental

Peak 1-1-2
171.5

EXPRESS sample
LHD sample+D2 imp. sample

3.0
2.5
2.0

Peak 31.5

1.0

1.00.5

Peak 4

0.5-2500

700
900
Temperature / K

1100

0.0-10.0
300

Fig. 3 TDS spectra for the EXPRESS, LHDand D2+ imp. samples

8618Desorption rate / 10 D m s

2.0

Peak 2

Desorption rate / 10 H m s

A disk-type polycrystalline tungsten withstress-relieved conditions (heated at 1173 K for 1 hour),which was purchased by Allied Material Co. Ltd, wasused for all experiments. The sample size was 10 mmdiameter and 0.5 mm thickness. The samples were
polished mechanically to the roughness of less than 1 Pm
by SiC abrasive papers and 1-3 Pm diamond suspensions,and preheated at 1173 K for 30 minutes under ultrahighvacuum (< 10-6 Pa) to remove the impurities and damages
introduced during the polishing processes.
After preheating, the sample was introduced into theplasma exposure system of EXPRESS. The deuterium gas
was supplied at a flow rate of 3.0 sccm and the pressure of3.5 Pa. The D plasma was exposed to the samples with theD2+ energy of 63 eV to implant at the lowest possible

R. Miura et al., Study on Plasma-Tungsten Surface Interactions Using the New Experimental Device EXPRESS

surface and thickness of impurity deposition layers on theEXPRESS and LHD samples. Large amount of oxygenand carbon were accumulated on the surface for bothsamples and its impurity concentrations were almost thesame. However, the thickness of the impurity deposition
layer for the EXPRESS sample was about one thirteenththinner than that for the LHD sample. It can be said thatthe thin impurity deposition layer was achieved byEXPRESS device and the formation of impurity deposition
layer hardly affected on deuterium retention as O-D bonds
for the EXPRESS sample, resulting in decrease of Peak 3
in the TDS spectrum of the EXPRESS sample compared tothe LHD sample.
Figure 6 shows C-1s XPS spectra of EXPRESS,
LHD and D2+ imp. samples. Three chemical states were
found at the binding energies of 286.1 eV, 284.8 eV and283.3 eV, attributing to C-O, C-C and C-W bonds,
respectively [13-15]. C-O and C-C bonds were major
chemical states in the LHD sample, although C-C bondwas formed by plasma exposure for the EXPRESS sample.
However, implantation depth of carbon was limited within
the surface region and the total amount of carbon for theEXPRESS sample was quite small compared to that for theLHD sample indicating that carbon would hardly affect thebehavior of hydrogen isotope retention in the EXPRESS
sample.
It was concluded that the EXPRESS sample had thethinner impurity deposition layer compared with LHD
sample and hydrogen isotope retention is affected bycarbon and oxygen chemical states and the amounts ofimpurity in deposition layer. In the EXPRESS sample,
most of the impurities were adsorbed on the top surface

Peak 2 was quite different among these samples, It can besaid that the plasma exposure hardly produces theirradiation defects in tungsten due to lower D+ implantationenergy according to SRIM code calculation [8], indicatingthat the formation of irradiation defects was limited forEXPRESS sample, and most of deuterium would beaccumulated on the sample surface. In the LHD sample,
additional desorption stages were observed at around 750
K (Peak 3) and 900 K (Peak 4). It was reported that theW-O bond in WO3 was one of the stable deuterium
trapping sites with a desorption temperature of around 750
K [9]. In our previous study, the deuterium implanted into
HOPG was trapped as C-D bond after dissociation of C-C
bond, which was desorbed at 900 K [10]. Therefore, thedesorption stages of deuterium at these higher temperaturesin the LHD sample may be assigned to that bound tooxygen as O-H (Peak 3) and carbon as C-H bonds (Peak 4),
respectively.
Figure 4 shows W-4f XPS spectra for the topsurfaces of the EXPRESS, LHD and D2+ imp. samples.
The W- 4f7/2 XPS spectra consisted of two chemical states
located at 30.8 eV for metallic W and 35.8 eV for WO3.
[11,12]. The abundance of WO3 for total W-4f XPS peakarea was 23% for the EXPRESS sample, and 63% for theLHD sample as shown in Table 1. In the EXPRESS sample,
the major chemical state of tungsten was the metallic state
which was different from the LHD sample, indicating thatmost of deuterium for the EXPRESS sample would beinteracted with metallic W rather than WO3. Figure 5shows the depth profiles of W, WC, WO3 and C for theEXPRESS sample. The impurities were accumulated
within the depth of ~ 0.3 nm for the EXPRESS sample.
Table 2 summarizes atomic concentrations on the top

WO3 㻌

1.0

4f5/2

0.8

4f7/2

4f5/2 4f7/2

0.6

LHD sample
EXPRESS sample
D imp. sample

0.4㻌

Intensity / -Metalic W
W
mtallic

0.2
0.0

40

3530

Binding energy / eV

Fig. 4 W-4f XPS spectra for the EXPRESS,
LHD and D2+ imp. samples

Fig. 5 Depth profiles of W and C for theEXPRESS sample

Table 1 Ratios of W and WO3 on the top surfaceof W

Table 2 Atomic concentrations for the surface of theEXPRESS and LHD samples and thethickness of the impurity deposition layers.

87R. Miura et al., Study on Plasma-Tungsten Surface Interactions Using the New Experimental Device EXPRESS

1.0

㻌
C-O C-C C-W
LHD sample
EXPRESS sample
D imp. sample

References

P.S. Krstic et al,.Nucl. Inst. and Meth. B 267,704(2009).
K. Ohya et al., Fusion Eng. Des.85,1167(2010).
Y. Oya et al,, J. Nucl. Mater. 390, 622 (2009).
K. Okuno et al.,Fusion Eng. Des. 85, 2328(2010).
H. Eleveld et al., J. Nucl. Mater, 191 433(1992).
H. Iwakiri et al., J. Nucl Mater., 307 ,135(2002).
M. Poon et al., J. Nucl. Mater., 374, 390(2008).
Ziegler J F 2006 SRIM code.
N. Matsunami et al., J. Nucl. Mater. 390 , 693(2009)
T. Suda et al., Fusion Eng. Des. 82 , 1762(2007)
B.F. Dzhurinskii et al., Russian J. Inorganic Chem., 20,2307(1975).
[12] D.R. Chopra et al.,J. Vac. Sci. Technol. B, 10, 1218(1992).
[13] B. Rousseau et al .,J. Phys. Chem., 53, 419(1992).
[14] A. Fahlman et al.,J. Phys. Chem. Solids, 30, 1835(1969).
[15] CH. Cardinaud et al., Applied Surface Science, 68,595(1993).
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
[11]

0.6
㻌

Intensity / -0.8

0.4
0.2
0.0
288

286

284

282

280

278

Binding energy / eVFig. 6 C-1s XPS spectra for the EXPRESS,
LHD and D2+ imp. samples.
after deuterium plasma exposure. However, the amounts ofimpurities beneath the surface were too small to affect thedeuterium retention behavior. In addition, the deuterium
was hardly trapped by irradiation defects because theformation of defects was limited due to low energy ion
irradiation. Therefore, the deuterium was only retained bysurface adsorption during deuterium plasma exposure inEXPRESS.
In further experiments, longer time (higher fluence)
plasma exposure and oxygen and carbon contaminatedplasma exposure will be needed to make impurity
deposition layers that have a variety of thickness andchemical state in order to clarify the influence of impurity
deposition layer on deuterium retention behavior.

5. Conclusions

A new equipment to acquire laboratory data(well-conditioned data) needed for the design of an actual
reactor named EXPRESS has been designed and isestablishing. The deuterium plasma exposure was
performed to characterize and evaluate the behaviors of
deuterium retention and surface chemical states after theEXPRESS plasma operation. The deuterium retention
behaviors for tungsten after EXPRESS plasma exposure
was studied and these results were compared to those forthe LHD and D2+ imp. samples. It was found that most ofdeuterium was desorbed at the lower temperature less than
550 K for the EXPRESS sample, which was almost thesame with the D2+ imp. sample. However, the TDS
spectrum for the LHD sample was quite different from thatfor the EXPRESS and D2+ imp. samples, which would becaused by the formation of thick impurity deposition layeron the surface for the LHD sample. These facts indicatethat the amounts of impurity in the deposition layer could
affect on the deuterium retention behavior in tungsten usedin actual plasma devices and fusion reactors.

Acknowledgement

This study was supported by NIFS collaboration
program, NIFS11KOBA025 and the Center for
Instrumental Analysis at Shizuoka University.

88