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

Implantation Energy Dependence on Deuterium Retention Behaviors for the
Carbon Implanted Tungsten

1)

Yasuhisa Oya1)*, Makoto Kobayashi1), Naoaki Yoshida2), Naoko Ashikawa3), Akio Sagara3),
Yuji Hatano4), and Kenji Okuno1)

Radioscience Research Laboratory, Faculty of Science, Shizuoka University, 836, Ohya, Suruga-ku, Shizuoka 422-8529,
Japan.
2)

Institute for Applied Mechanics, Kyushu University, Fukuoka 816-8580, Japan.
3)4)National Institute for Fusion Science, Gifu 509-5292, Japan.

Hydrogen Isotope Research Center, University of Toyama, 3190, Gofuku, Toyama 930-8555, Japan.
(Received: 9 May 2012 / Accepted: 8 August 2012)

Effects of energetic carbon implantation on deuterium retention behavior in tungsten were studied. The
dislocation loops and vacancies were introduced in tungsten as irradiation damages by energetic carbon
implantation. The density of irradiation damages was almost saturated by C+ implantation below 1 dpa according to
TEM observations. The retentions of deuterium trapped by dislocation loops and vacancies were observed in TDS
measurements, which were increased as increasing the carbon implantation energy. The deuterium desorption at
higher temperature above 600 K was also found, corresponding to the desorption of deuterium retained in the
intrinsic cavity in bulk region. However, this trapping was refrained by energetic C+ implantation, concluding thatthe retained carbon would play a role as the diffusion barrier of deuterium, which would prevent deuterium
penetration through bulk.
Keywords: Tungsten, TDS, TEM, Carbon, Irradiation damage, Diffusion, Simultaneous implantation

introduction of irradiation damages was clearly observed
because 10 keV C+ implantation induced higher
displacement damage in tungsten than that by deuterium[6]. On the other hand, it is reported that the carbon
retained in tungsten plays a role of diffusion barrier forhydrogen isotopes toward the bulk, resulting in thedecrease of hydrogen isotope retention in tungsten [7].
Recent ion-driven permeation experiments under carbon deuterium simultaneous implantation indicate that
deuterium permeation is promoted by simultaneous
carbon implanted tungsten [8, 9]. These results indicate
the estimation of hydrogen isotope retention in tungsten
is complicated under fusion environment. Therefore,
deuterium retention in tungsten implanted with carbon
was investigated in this study to understand the effect ofcarbon implantation on deuterium retention behavior intungsten. For this objective, two types of implantation
were carried out. One was the simultaneous implantation(C+ - D2+ sim. imp.) where carbon and deuterium were
implanted into tungsten simultaneously. In thesimultaneous implantation circumstance, deuterium wasretained in tungsten with formations of irradiation defects
and W-C mixed layer. The other implantation was thesequential implantation (C+ - D2+ seq. imp.), meaning that

1. Introduction

Tungsten is a candidate for plasma facing materials(PFMs) in fusion devices due to its low hydrogensolubility, low sputtering yield and high melting point [1].
The usage of carbon fiber composite (CFC) is also
considered in the high heat load region of divertor infusion reactor, because of its high melting point [2].
These materials will be exposed to energetic particles
from plasma during reactor operation. It is well known
that carbon materials are easily eroded by energetichydrogen isotopes by chemical or physical sputtering
processes. Then, sputtered carbon will be implanted into
tungsten with hydrogen isotopes during plasma operation,
which will form the tungsten-carbon (W-C) mixed layeron the surface region of tungsten [3-5]. Hydrogenisotopes including tritium will be implanted
simultaneously with formation of W-C mixed layer, andwill dynamically interact with carbon in tungsten. In theview point of the fuel recycling, it is important toelucidate the retention behavior of hydrogen isotopes
retained on tungsten materials in the fusion environment.
In our previous studies, deuterium retention behavior intungsten with simultaneous 10 keV C+ implantation was
evaluated. Enhancement of deuterium retention by theauthor’s e-mail: syoya@ipc.shizuoka.ac.jp

76©2013 by The Japan Society of Plasma
Science and Nuclear Fusion ResearchY. Oya et al., Implantation Energy Dependence on Deuterium Retention Behaviors for the Carbon Implanted Tungsten

carbon was implanted into tungsten prior to deuterium. In
the sequential implantation, deuterium was implanted into
tungsten after formations of irradiation defects and W-C
mixed layer. In addition, implantation energy of carbon
was changed to investigate dependences of the
distribution of carbon and damage on deuterium retention
behavior. The deuterium retentions were evaluated bymeans of Thermal Desorption Spectroscopy (TDS)
measurements. The irradiation damages were also
observed by Transmission Electron Microscopy (TEM) at
Kyushu University.

2. Experimental
A disk-type polycrystalline tungsten with stress-relieved
conditions (heated at 1173 K), which was supplied byAllied Material Co. Ltd was used. The sample size was
10 mm diameter and 0.5 mm thickness. The Sample waspolished mechanically to the roughness of less than 1 μm
by SiC abrasive papers and mono-crystalline diamondsuspensions, and preheated at 1173 K for 30 minutes
under ultrahigh vacuum to remove the impurities anddamages introduced during the polishing processes. Then,simultaneous or sequential carbon ion (C+) and deuteriumion (D2+) implantations were performed in the C+-D2+
simultaneous implantation apparatus at room temperature.
In this apparatus, the sample can be irradiatedindependently by two different ion guns, namely C+ andD2+ guns. In the C+ gun, CO2 was used as the C+ sourcegas to get rid of hydrogen impurities. The E × B mass
separator was equipped at the head of the C+ gun to
extract pure C+. The incident angles for the C+ and D2+guns were 15 degrees from the sample surface normal,
forming an angle of 30 degrees between the two beams.
The energy of C+ was changed from 5 to 10 keV and thatof D2+ was set to be 2 - 3 keV. The C+ and D+ flux were
0.1 × 1018 C+ m-2 s-1 and 1.0 × 1018 D+ m-2 s-1,
respectively. Therefore, for C+-D2+ simultaneous
implantation, the ion flux ratio was set to be C+/D+ = 0.1.
The C+ fleunce was constant to be 0.1 × 1022 C+ m-2 in
C+-D2+ sequential implantation. D+ fluence was changedin the range of (0.1 - 1.8) × 1022 D+ m-2. The only D2+
implantation was also performed to compare thedeuterium retention in C+ implanted tungsten. The
chemical states of carbon and tungsten were studied by
X-ray Photoelectron Spectroscopy (XPS). The TDS
measurements were performed from room temperature to1173 K with a heating rate of 0.5 K s-1. The TEM
observations were also performed to investigate theirradiation damages formed by the ion implantations. The
depth profile of deuterium was analyzed by Glow
Discharge Optical Emission Spectroscopy (GD-OES).

Fig. 1 TEM images of tungsten implanted with 3 keV
D2+ or 10 keV C+ at various ion fluences.
keV D2+ with the fluence of 1.0 × 1022 D+ m-2 andtungsten with 10 keV C+ with different C+ fluences in therange of (0.001-0.1) × 1022 C+ m-2. For all the samples,lots of dislocation loops were introduced in tungsten byion implantations, which was quite different from puretungsten where its density was low. In addition, since theirradiation vacancy and dislocation loop are pair defects,
the formation of dislocation loops indicated theintroduction of vacancies. It was also found that evenhigher fluence of D+, density of irradiation defects intungsten with 10 keV C+ implantation was higher thanthat with 3 keV D2+ implantation, indicating that
irradiation damages were easily introduced in tungsten by
10 keV C+ implantation compared to that by 3 keV D2+
implantation due to higher implantation energy, higher
mass and larger atomic radius. It was also found thatdensity of irradiation damages in tungsten induced by 10
keV C+ implantation was increased in the C+ fluence
range of (0.001-0.01) × 1022 C+ m-2, but its density wasseen to be the same with C+ fluence larger than 0.01 ×

1.5

 C-C bond (XPS)

D (GD-OES)

1.0

30
25
20
15

0.5
0.0

10
5

20
30
40
50
depth / nm
Fig. 2 Depth profiles of D observed by GD-OES andW-C and C-C bonds by XPS. The TEM image wasalso shown in this figure.

3. Results and discussion
Fig. 1 show TEM images of tungsten implanted by 3

77

0

10



 W-C bond (XPS)XPS Intensity / arb. unit

GD-OES Intensity/ arb. unit20nm

Y. Oya et al., Implantation Energy Dependence on Deuterium Retention Behaviors for the Carbon Implanted Tungsten

Peak 2

Peak 1
4.0

Peak 322

D fluence (x 10 )+-2
0.3 D m+-2
1.8 D m-2
17+

Only D2 imp.
+

Desorption rate / 10 D2 m s-11022 C+ m-2, indicating that density of irradiation defects
were saturated in the C+ fluence of 0.01 × 1022 C+ m-2. By
the SRIM (Stopping range of Ions in Matter) code
calculation [10], irradiation defects in tungsten implantedby 10 keV C+ with the C+ fluence of 0.01 × 1022 C+ m-2
were distributed within 30 nm beneath the surface, withthe peak at around 5 nm (1.3 dpa), indicating that thedensity of irradiation defects would be saturated below
1.3 dpa. The depth profiles of D observed by GD-OES at
University of Toyama and, W-C and C-C bonds
evaluated by XPS were shown in Fig. 2. The TEM image
was also overlapped in this figure. It was found that mostof D was retained in W-C mixed layers and major
chemical state of C in this layer was found to be W-C
bond.
Fig. 3 shows D2 TDS spectra for only 3 keV D2+
implanted tungsten and 10 keV C+ - 3 keV D2+
sequentially implanted one with various D+ fluences.
These spectra were consisted of three desorption stages ataround 370, 480 and 620 K which were named as Peaks 1,
2 and 3, respectively. In lower D+ fluence of 0.1 ~ 0.3 x1022 D+ m-2, the deuterium retentions as Peaks 1 and 2
were clearly increased by C+ implantation, indicating thatthese desorption stages would be related to the deuterium
interacted with the irradiation damages. The density ofirradiation damages induced by 10 keV C+ implantation
was higher than that by 3 keV D2+ implantation as shownin Fig. 1, resulting in the higher deuterium retention for
the 10 keV C+ implanted tungsten. In general, trapping
energy of deuterium by vacancy would be higher thanthat by dislocation loop [11,12], indicating that Peaks 1
and 2 could be assigned to the desorption stages of
deuterium trapped by dislocation loops and vacancies,
respectively. In higher fluence of 1.8 × 1022 D+ m-2, thedeuterium retentions as Peaks 1 and 2 for C+ implanted
W were almost the same as that without C+ implantation,
indicating that deuterium trappings by irradiation
damages should be almost saturated. However, thedeuterium retention as Peak 3 for only D2+ implanted
tungsten was higher than that for 10 keV C+ - 3 keV D2+
sequentially implanted one. The amount of trapping site
as Peak 3 was not induced by ion implantation because
the retention of deuterium as Peak 3 was not increased by
10 keV C+ implantation. Therefore, the promisingtrapping site for Peak 3 would be intrinsic sites such as
cavities, grain boundary and/or oxygen impurity [13,14].
Lower deuterium retention as Peak 3 for 10 keV C+ - 3
keV D2+ sequentially implanted tungsten would indicatethat deuterium diffusion in tungsten was refrained by 10
keV C+ implantation.
The retentions of deuterium retained as Peaks 1, 2 and 3
for tungsten with 10 keV C+ - 3 keV D2+ sequential
implantation or simultaneous implantations werecompared in Fig. 4. The deuterium retentions for the
samples with the D+ fluence of 1.8 × 1022 D+ m-2 were

3.0

+

+

C -D2 seq. imp.

2.0

+

22

D fluence (x 10 )+-2
0.1 D m+-2
1.8 D m

1.0
0.0
300

500
700
Temperature / K

900Fig. 3 D2 TDS spectra of 3 keV D2+ implanted
tungsten and 10 keV C+- 3 keV D2+ implanted one.

1.5

Peak1
Peak2
Peak3

20

Deuterium retention / 10 D m-2compared. In the case of C+- D2+ simultaneous
implantation, the retention of deuterium retained as Peak
3 was decreased by C+ implantation, which was almost
the same with both of sequential and simultaneous
implantations. However, the deuterium retentions as
Peaks 1 and 2 for the simultaneous implantation wereincreased compared to those for the sequential
implantation. The deuterium trappings as Peaks 1 and 2
for C+- D2+ simultaneously implanted tungsten werehigher than that for sequentially implanted tungsten,
indicating that simultaneous implantation effects were
contributed to the increase of deuterium retentions inthese peaks.
To clarify the simultaneous implantation effects on
deuterium retention behavior in tungsten, incident energy
of C+ was changed for both of sequential or simultaneous
implantations. The implantation energy of D2+ was fixed
to be 2 keV. Figs. 5 (a) and (b) show the D2 TDS spectra
for C+ - D2+ simultaneously implanted tungsten and C+ D2+ sequentially implanted one as a function of C+
incident energy, respectively. Both of D+ and C+ fluenceswere set to be 1.0 x 1022 D+ m-2 and 0.1 x 1022 C+ m-2,
respectively. The D2 TDS spectra were slightly changed
in simultaneous implantation case although little changes

1.0

0.5

0.0

+

Only D2++

C -D2 seq.

++

C -D2 sim.

Fig. 4 Comparison of deuterium retentions as Peaks 1, 2
and 3 for tungsten implanted with various implantation
procedure.

78

6.0

+

+

+

+

+

-2

5.0

+

7 keV C -2 keV D2

17

5 keV C -2 keV D2

4.0

+

3.0

only 2 keV D22.0(a) C+-D2+ sim. imp

1.0
0.0
300

500
700
Temperature / K

6.0

-1

10 keV C -2 keV D2Desorption rate / 10 D2 m s

17-2

Desorption rate / 10 D2 m s-1

Y. Oya et al., Implantation Energy Dependence on Deuterium Retention Behaviors for the Carbon Implanted Tungsten

900

+

+

10 keV C -2 keV D2

5.0

+

+

+

+

7 keV C -2 keV D2

4.0

5 keV C -2 keV D2

3.0+

only 2 keV D22.0(b) C+-D2+ seq. imp

1.0
0.0
300

500
700
Temperature / K

900Fig. 5 D2 TDS spectra of tungsten with (a) C+- 2 keV D2+simultaneous and(b) C+- 2 keV D2+ sequential implantation with various C+ incident energies

2.0

-2

(a) C+-2 keV D2+ sim. imp

20

2.5

Peak1
Peak2
Peak3

1.5
1.0
0.5
0.0

the simultaneously implanted tungsten was resulted from
distribution of irradiation damages. Deuterium trappings
by irradiation defects in deeper region (20 - 30 nm
beneath surface) would be contribution to these increasesof deuterium retention as Peaks 1 and 2. However in thecase of C+ - D2+ sequential implantation, deuterium
retention was not increased with increasing C+ incident
energy, considering that retained carbon would prevent
deuterium penetration into bulk region. In C+ - D2+
sequential implantation, the 2 keV D2+ was implanted
into tungsten after C+ implantation. Therefore, implanted
deuterium should be affected by retained carbon intungsten. On the other hand in C+- D2+ simultaneous
implantation, several portion of deuterium was implanted
into tungsten before retaining carbon enough to prevent
deuterium diffusion into bulk.
In summary, the retention of deuterium trapped byirradiation damages was enhanced by energetic carbon
implantation. The deuterium retention by intrinsic cavity
in bulk region was also found and was decreased byenergetic carbon implantation. Therefore, it wasDeuterium retention / 10 D m

20

Deuterium retention / 10 D m-2

were observed in that for C+- D2+ sequentially implanted
tungsten. Fig. 6 summarizes the retentions of deuterium
retained as Peaks 1, 2 and 3. The deuterium retention as
Peak 1 was clearly increased for C+- D2+ simultaneously
implanted tungsten. It was found that the no difference of
deuterium retention for C+- D2+ sequentially implanted
tungsten was observed nonetheless C+ incident energy
was changed from 5 to 10 keV. By changing the C+
incident energies from 5 to 10 keV C+, the density ofirradiation damages was estimated to be above 10 dpa,
indicating that the density of irradiation defects was
saturated in all the samples. However, distributions ofirradiation defects induced by C+ implantation werechanged with incident energy from SRIM code
calculation. With increasing C+ incident energy,
distribution of irradiation damages was expanded toward
the depth. The irradiation damages induced by 10 keV C+
were extended from surface to 30 nm of depth althoughthose by 5 keV C+ were limited with 20 nm from thesurface, indicating that the increase of deuterium
retention with increasing C+ incident energy observed for

0

2
4
6
8
+
C incident energy / keV

10

2.5
2.0

(b) C+-2 keV D2+ seq. imp.
Peak1
Peak2
Peak3

1.5
1.0
0.5
0.0

0

2
4
6
8
+
C incident energy / keV

Fig. 6 Deuterium retention for each peak of tungsten with (a) C+- D2+simultaneous and(b) C+- D2+ sequential implantation with various C+ incident energies.

79

10

Y. Oya et al., Implantation Energy Dependence on Deuterium Retention Behaviors for the Carbon Implanted Tungsten

considered that implanted carbon would play a role as thedeuterium diffusion barrier leading the reduction of
deuterium penetration toward the bulk. However, several
experiments showed deuterium ion-driven permeation
was promoted by carbon simultaneously implanted into
tungsten. These facts can be explained by the project
range of carbon and deuterium ions. In our experiments,
the project range of 5 - 10 keV C+ were almost the sameas that of 2 - 3 keV D2+. As the project range of carbon
reported in ref. [8,9] was shorter than that of deuterium,
higher portion of deuterium was implanted deeper regionof tungsten than carbon. The implanted deuterium waseasily permeated through tungsten because of thedecreased diffusivity in the implanted surface by carbon.

[14]

4. Conclusions
The effects of energetic carbon implantation on
deuterium retention behavior in tungsten were
investigated in this study. Dislocation loops andvacancies were formed in tungsten as irradiation defects
by energetic carbon implantation. The density ofirradiation damage was saturated below about 1 dpa by
TEM observations. Deuterium retentions by dislocation
loops and vacancies were observed in TDS measurements.
Deuterium retentions by irradiation defects wereincreased by energetic carbon implantation. Deuterium
retention by intrinsic cavity in bulk region was also found
and was decreased by energetic carbon implantation. It
was concluded that retained carbon would play a role as
the diffusion barrier of deuterium, preventing deuterium
to penetrating into bulk.
Acknowledgement
This study was supported by JSPS Kakenhi No.
22360389, NIFS collaboration program NIFS11KEMF022,
University of Toyama collaboration program and theCenter for Instrumental Analysis at Shizuoka University.

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