J. Plasma Fusion Res. SERIES, Vol. 11 (2015)

Influence of He implantation on deuterium trapping at defects induced in W byirradiation with MeV-range W ions
Vladimir Kh. ALIMOV1,2*, Yuji HATANO1, Kazuyoshi SUGIYAMA3, Beata TYBURSKA-PÜSCHEL3,
Makoto OYA4, Yoshio UEDA4, Kanetsugu ISOBE5, Akira HASEGAWA6
Hydrogen Isotope Research Center, University of Toyama, Gofuku 3190, Toyama-shi, Toyama 930-8555, Japan
National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Moscow 115409, Russia
3
Max-Planck-Institut für Plasmaphysik, D-85748 Garching, Germany
4
Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan5
Tritium Technology Group, Japan Atomic Energy Agency, Tokai-mura, Ibaraki 319-1195, Japan
6
Department of Quantum Science and Energy Engineering, Tohoku University, 6-6-01-2, Aramaki-aza-Aoba, Aoba-ku,
Sendai 980-8579, Japan
1

2(Received: 28 August 2014 / Accepted: 19 January 2015)

The effect of helium (He) ion irradiation on the concentration of the ion-induced defects preliminary generated intungsten (W) was investigated. Polycrystalline mechanically-deformed W samples were damaged by irradiation with 20
MeV W ions at room temperature to 0.5 displacements per atom (dpa) at the damage peak situated at a depth of 1.35 μm,
followed by 1 keV He ion irradiation at 473, 623, and 823 K to fluences in the ranges of (3.7−5)u1022 He/m2. The samples
were then loaded with deuterium (D) by D2 gas exposure at a pressure of 100 kPa at 673 K for 10 h. Concentration of
deuterium trapped in the W-ion induced defects was determined by nuclear reaction analysis using the D(3He,p)4He
reaction. It has been found that the effect of following He irradiation on the concentration of the W-ion-induced traps
responsible for D trapping is negligible.
Keywords: Deuterium gas exposure, Deuterium retention, Displacement damage, He ion irradiation, Tungsten
damaged with 20 MeV W ions [8]. Figure 1 taken from
Ref. [8] shows the concentration of deuterium (given inD/W) at the depth of the damage peak, damageCD, in thedamaged mechanically-deformed W exposed to pure D
and He-seeded D plasmas at various temperatures. After
exposure to the He-seeded D plasma at temperatures
above 450 K, damageCD is always lower than that for thepure D plasma.

1. Introduction

D/W atomic ratio at damage peakDue to its favorable physical properties, such as low
erosion yield and high melting temperature, tungsten (W)is a candidate material for plasma-facing high heat-flux
structures in future fusion reactors. As plasma-facing
material, W will be subjected to intensive fluxes ofenergetic deuterium (D), tritium (T), helium (He)particles as well as 14 MeV neutrons (n) from the D–T
fusion reaction. Neutron irradiation causes modificationof the W microstructure by creating displacements in thebulk [1, 2]. These processes lead to concerns about
tritium inventory in the n-irradiated W after long-term
exposure to D-T plasmas.
One of the ways to investigate the influence ofn-produced defects on the hydrogen isotope inventory is
to simulate displacement damage in tungsten byirradiation with ions at energies in the range from several
tenths to several tens of MeV and then to load thedamaged W with deuterium. It has been experimentallyshown that irradiations of W materials with 0.2−20 MeV
H, He, Si, and W ions lead to an increase of deuterium
retention in comparison with undamaged materials[3-11].
It has been shown recently that seeding of He ions
into deuterium plasmas significantly reduces thedeuterium retention at elevated temperatures both inundamaged W materials [12-16] and W preliminary_________________________

26

2

Pure D and He-seeded D plasmas () = 3u10 D/m ) o W
damaged to 0.89 dpa

-2

10

-3

10

D plasma
D+He plasma

400

500

600

700

Exposure temperature (K)Figure 1. Concentration of deuterium (D/W) retained at a depthof the damage peak in damaged (20 MeV W ions at 300 K to
0.89 dpa) mechanically-deformed W exposed both to pure D
plasma (76 eV D2+) and He-seeded D plasma [76 eV (D2+ +10% He+)], as a function of exposure temperature [8].

*author’s mail: vkahome@mail.ru

1©2015 by The Japan Society of Plasma
Science and Nuclear Fusion Research

V. Kh. Alimov et al., Influence of He Implantation on Deuterium Trapping at Defects Induced in W by Irradiation with MeV-Range W Ions

the defect concentration should be clearly detected if it
occurred.
In this work polycrystalline mechanically-deformed
W was mostly used, and recrystallized W was also usedin some of experiments for comparison. It should be
noted that after irradiation with 20 MeV W ions, the Dconcentration in the damage zone doesn’t depend on theinitial microstructure of these W materials [9].

It has been proposed that seeding of He ions into the
D plasma leads to nano-scale helium bubble formationand development of an open porosity in the near-surface
layer. As a consequence, short-circuit paths to the surface
are created to enhance the D re-emission and limiting the
D diffusion into the bulk [13, 17]. Accordingly, the Dconcentration in a solid solution reduces significantly andconsequently the D concentration in the damage zone
decreases [8, 10].
At temperatures sufficiently high to activate thedetrapping process, trapping-detrapping equilibrium canbe attained, and the fraction of traps, θt, occupied with
deuterium at temperature T is expressed asθt/(1 − θt) = θL exp(Ebin/kT),(1)
where θL is the fraction of occupied interstitial sites, Ebinis the binding energy (the enthalpy difference between
deuterium atom in a solid solution state and in a trappedstate), and k is the Boltzmann constant [18]. This means
θt varies with the D concentration in the solid solution
state, CSS, being proportional to θL. Note that
CSS = CL θL,(2)
where CL is the concentration of interstitial sites.
Under exposure to D plasmas, CSS is determined by
the balance between penetrating flux αϕin and re-emitted
flux ϕr of deuterium. Here, α is the sticking coefficientand ϕin is the incident flux. In the first approximation, in a
steady state
α ϕin = ϕr = 2V kr CSS-S2,(3)
where CSS-S is the concentration of solute D atoms
beneath the surface, V is the surface roughness factor [19],and kr is the recombination rate constant. Hence,
CSS-S = (α ϕin/V kr)1/2,(4)
and CSS-S decreases as the surface roughness increases.
It should be noted that when tungsten is exposed toD2 gas, CSS is determined by Sieverts’ law:
CSS = k0 exp (−ES/kT)P1/2,(5)
where k0 is the solubility constant, ES is the heat ofsolution and P is the D2 pressure.
However, there is another assumption that formationof He bubbles and accompanying emission ofself-interstitial atoms and dislocation loops [20, 21] canreduce the concentration of W-ion-induced defects
responsible for trapping of D atoms in the damage zone.
The objective of this work is, therefore, to study theeffect of He ion irradiation on the concentration of theion-induced defects preliminary generated in W. W
samples damaged with 20 MeV W ions were irradiatedwith 1 keV He ions and then loaded with D by exposure
to D2 gas at 673 K and 100 kPa. In contrast to plasma
exposure, D loading by gas exposure is an equilibrium
process. The concentration of solute D and consequently
trap occupancy by D are determined solely by D2 gaspressure and temperature as formulated above, but open
porosity has no influence on it. Therefore, the change in

2. Experimental
Mechanically-deformed polycrystalline W from
A.L.M.T. Corp., Japan, having a purity of 99.99 wt.%
was used for irradiation with He ions. The microstructureof this W material consists of anisotropically elongatedgrains along the deformation axis. The grain size is 1-3
μm in section and up to 5 μm in length. Individual
elongated cracks observed between grains are due to thedeformation treatment [22]. Square-shaped samples,
10u10 mm2 in size and 2 mm in thickness, were cut frommechanically-deformed polycrystalline W rods annealed
at 1173 K for 30 min in a hydrogen atmosphere to relieveinternal stresses occurred during the manufacturing
process. The samples were prepared such that theirradiated surfaces were perpendicular to the deformation
axis, which corresponds to the ITER specification [23].
The samples were mechanically polished, cleaned in an
acetone ultrasonic bath, and then annealed in vacuum at1173 K for 30 min to relieve stresses induced in thepolishing process.
The W samples were irradiated with 20 MeV W
ions to a fluence of 8u1017 W/m2 at room temperatureusing a tandem accelerator at IPP Garching. As a result, thenear-surface layer of the samples was damaged to 0.5
displacements per atom (dpa) at the damage peak situatedat a depth of 1.35 μm. The damage profile was calculatedusing the program SRIM 2008.03 [24], “full cascade
option”, with a displacement threshold energy of Ed =90 eV [25]. In what follows the W-ion-irradiated W
samples will be designated as “damaged” W samples, anda value of the damage level will be indicated as a numberof dpa at the damage peak.
The damaged W samples were irradiated with 1 keV
He ions at temperatures of 473, 623, and 823 K using the
HiFIT device at Osaka University [26]. The ion flux was(2.7−3.7)u1019 He/m2s, and the fluences were 3.7u1022
and 5u1022 He/m2. The sample was heated during theirradiation by using an infrared heater placed behind thesample. Temperature was measured by a type K
thermocouple attached at the front surface of samples.
The depth distribution of implanted He atoms wascharacterized by the projected range of about 4 nm andlongitudinal straggling of about 7 nm [21].
After He ion irradiation the damaged W samples
were exposed to D2 gas with a pressure of 100 kPa at

2

V. Kh. Alimov et al., Influence of He Implantation on Deuterium Trapping at Defects Induced in W by Irradiation with MeV-Range W Ions

at a depth of the damage peak, damageCD, is roughly
constant regardless of prior He irradiation or the He
irradiation temperature, while damageCD becomes slightly
lower after He ion irradiation at 823 K.

673 K for 10 h in a gas exposure apparatus at Universityof Toyama. Damaged W sample without He ion
irradiation was also exposed to D2 gas for comparison.
The sample was placed inside the quartz tube connectedto the high-vacuum pumping system and heated in a
vacuum with the use of an external ohmic heater. The
temperature was monitored using a type K thermocouple
contacted directly to the sample inside the tube. As the
sample temperature reached the required value, a valve
between the tube and the pumping system was closed andthe tube was filled with D2 gas. The background pressure
inside the tube was measured with an ionization gauge,
whereas the D2 gas pressure was controlled with a
Baratron capacitance manometer. After reaching required
exposure duration, D2 gas evacuation and sample coolingstarted simultaneously. D2 gas was evacuated in several
seconds, while the sample was cooled down in several
minutes.
After the D2 gas exposure, the near-surface
deuterium depth profiles in the W samples were
determined by nuclear reaction analysis (NRA) at IPP
Garching. The D(3He,p)4He reaction was utilized, andboth the D particles and protons were analyzed. An
analyzing beam of 3He ions with energies varied from0.69 to 4.0 MeV was used, which allows to determine theD concentration to depths of 6 μm. For the deconvolutionof the proton yields measured at different 3He ion
energies, the program SIMNRA [27] was used. A
deuterium depth distribution was assumed taking into
account the near-surface depth profile obtained from theD particle energetic spectrum, and the proton yield was
calculated as a function of incident 3He energy. The shapeof the D depth profile was then varied using an iterativetechnique until the calculated curve matched themeasured proton yields [28].

D2 gas (673 K @ 100 kPa @ 10 h) o W

10

0

10

-1

10

-2

10

-3

no He ion irradiation
after 1 keV He ion irradiation
222
5.0u10 He/m at 473 K
222
3.7u10 He/m at 623 K
222
3.7u10 He/m at 823 K

damage profile

0

1

2

3

Depth (Pm)

4

5

6

10

0

10

-1

10

-2

10

-3

Damage (dpa)

D concentration (at.%)damaged to 0.5 dpa and then irradiated with He ions

D/W atomic ratio at damage peakFigure 2. Depth profiles of deuterium retained in damaged (20
MeV W ions at 300 K to 0.5 dpa) mechanically-deformed W
samples without and with following irradiation with 1 keV He
ions, after exposure to D2 gas (at 673 K and 100 kPa for10 h). The He ion irradiation temperatures and fluences areindicated in the legend. Damage profile is plotted with the useof right ordinate axis.

3. Results and discussion

D2 gas (673 K @ 100 kPa @ 10 h) oW damaged to 0.5 dpa
and then irradiated with He ions or annealed in vacuum

0.005

D2 gasno He ion
irradiation

exposure

0.004
no annealing

0.003

0.002

damaged md W without He ion irradiation
damaged md W irradiated with 1 keV He ions
damaged rc W without annealing
damaged rc W annealed in vacuum for 6 h

300

400

500

600

700

800

900

1000

Annealing / irradiation temperature (K)Deuterium depth profiles in damaged (20 MeV W ions at
300 K to 0.5 dpa) W samples (Fig. 2) show that in thecourse of D2 gas exposure (at 673 K and 100 kPa for 10h) D atoms dissolve into W matrix, diffuse through themetal lattice, and become trapped at defects (or traps) likevacancies and vacancy clusters [29-31] generated fromthe W-ion-induced displacement damage. Note that the Dconcentration beyond the damage zone (at depths
t 3 μm) is (3−6)u10-3 at.% that is significantly higher
than the solute D concentration. Obviously, the intrinsic
defects such as intergranular cracks in the bulk of themechanically-deformed W serve as traps for diffusing D
atoms.
Figure 3 shows the D concentration (given as D/W)
at the damage peak (at a depth of ~1.35 μm) after D2 gas
exposure at 673 K as a function of annealing / Heirradiation temperature. Below 623 K the D concentration

Figure 3. Concentration of deuterium (D/W) retained at a
depth of the damage peak in (i) damaged (20 MeV W ions at
300 K to 0.5 dpa) mechanically-deformed (md) W samples,
without and with following irradiation with 1 keV He ions,and (ii) in damaged (20 MeV W ions at 300 K to 0.5 dpa)
recrystallized (rc) W samples, without and with following
annealing at 973 K for 6 h, after exposure to D2 gas (at 673 K
and 100 kPa for 10 h), as a function of the exposure andannealing temperature.

Figure 3 also shows the D/W at damage peakobtained from damaged (20 MeV W ions at 300 K to 0.5dpa) recrystallized W, without and with after-damaging
annealing in vacuum at 973 K for 6 h, followed by D2 gas
exposure under the same exposure conditions (at 673 K
and 100 kPa for 10 h) [32]. Comparison of these data

3

V. Kh. Alimov et al., Influence of He Implantation on Deuterium Trapping at Defects Induced in W by Irradiation with MeV-Range W Ionsdiffusing into the bulk decreases [13, 17]. As this takes
place, the D concentration in the solid state is reduced ascompared to the case of pure D plasma exposure. As aresult, the D retention decreases in W materials, bothwithout and with W-ion-induced damages.

suggests that the damageCD is more influenced by theannealing temperature rather than the prior He ion
irradiation. In other words, the effect of irradiation with
He ions on the concentration of the W-ion-induced traps
responsible for trapping of hydrogen isotopes is virtually
negligible. It appears that the self-interstitial atoms of W
and dislocation loops produced beneath the surface by He
implantation did not diffuse to deeper regions.
Recently, ion-driven permeation of D through W
under simultaneous He–D irradiation at temperatures in
the range from 800 to 1050 K was studied by Lee et al.
[33]. Reduced deuterium permeation from 1 keV He ion
implantation was observed, and the permeation flux
reached steady state following an implanted He fluenceof ~1021 He/m2. It has been concluded that undersimultaneous He–D irradiation a reduction in diffusive Dconcentration takes place [33]. However, the results
reported in [33] don’t allow distinguishing effects of (i)enhancement of the D re-emission and (ii) retardation of
D diffusion caused by trapping of D atoms at defects
generated by 1 keV He ion irradiation.
As may be inferred from the D depth profiles
shown in Fig. 2, trapping of deuterium at the
He-ion-induced defects is negligible. Otherwise, after D 2
gas exposure, the near-surface D concentration (i.e.,
within the range of He ions) in the He-ion-irradiateddamaged W would be higher than that for the damaged
by only W self-irradiation. In fact, the near-surface D
concentration in the He-ion-irradiated damaged W isslightly lower.
It should be also noted that in damaged W theion-induced traps are filled by D atoms diffusing from thesurface [11], and, as the D loading time increases, theboundary separating layers unsaturated and saturated with
deuterium migrates from the surface into the bulk. Thus,
the plateau-like D depth profiles shown in Fig. 2 indicate
saturation of the damage zone with deuterium.
A remark is in order that the plateau-like D depth
profiles observed in damaged mechanically-deformed W
after exposure to He-seeded D plasma [8] also suggestthat reduction of the D concentration in damaged zone
caused by seeding of He ions into D plasma can not be
explained by retardation of D diffusion due to trapping of
D atoms at W-ion-induced defects.
Thereby, significant decrease of the Dconcentration in the damage zone of W materials exposed
to He-seeded D plasmas at elevated temperatures [8] canbe explained by significant reduction of the D soluteconcentration [10]. It is safe to assume that under
exposure of W materials to the He-seeded D plasmas at
temperatures above 400 K, a mechanism of nano-scale
helium bubble formation leads to the development of anopen porosity in the near-surface layer and creates
short-circuit paths to the surface. As a consequence, the D
re-emission flux is enhanced and the flux of D atoms

4. Summary
It has been revealed that irradiation of W,
beforehand damaged with 20 MeV W ions, with energetic
He ions doesn’t influence the concentration of theion-induced defects.
Significant decreasing of the D concentration in thedamage zone of W materials exposed to He-seeded D
plasmas at elevated temperatures can be, therefore,
explained by significant reduction of the D concentration
in a solute solution due to formation of He-ion-induced
short-circuit paths to the surface enhancing the D
re-emission flux.
Acknowledgements
The authors would like to thank J. Dorner and
M. Fußeder (IPP Garching, Germany) for irradiation of
W samples with W ions and technical assistance with the3
He beam analyses. This work was partly supported JSPS
KAKENHI Grand Number 24246151.
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