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Yoshida2005-He-on-W.pdf
Journal of Nuclear Materials 337–339 (2005) 946–950
www.elsevier.com/locate/jnucmat
Impact of low energy helium irradiation on plasma
facing metals
N. Yoshida *, H. Iwakiri, K. Tokunaga, T. Baba
Research Institute for Applied Mechanics, Kyushu University, 6-1 Kasugakoen, Kasuga-shi, Fukuoka 816-8580, Japan
Abstract
Effects of helium ion irradiation for tungsten and other metals have been studied extensively as functions of ion
energy, temperature and fluence, for a wide range of burning plasma conditions, using not only ion accelerators, but
also large-sized plasma confinement devices such as TRIAM-1M and LHD. In this paper, recent results on blistering,
erosion and many other irradiation effects such as internal damage evolution, change of mechanical properties and heat
load resistance, and synergetic effects with neutron irradiation, are comprehensively reviewed for better understanding
of the performance of tungsten under helium plasma bombardment. It is emphasised that helium irradiation is a serious
issue for tungsten as a plasma facing material under burning plasma condition.
2004 Elsevier B.V. All rights reserved.
PACS: 52.40.H; 61.80
Keywords: Tungsten; Helium; Bubbles; Plasma facing materials; Radiation effects
1. Introduction
Under burning plasma conditions, plasma-facing
materials (PFM) suffer irradiation of helium in addition
to that of hydrogen isotopes. Sputtering and blistering
by helium ions with energy above a few keV were studied extensively many years ago [1]. More recently, the
effects of helium ion irradiation on tungsten and other
metals have been studied for reactor relevant plasma
conditions as functions of ion energy (eVs to keVs), temperature (300–3000 K) and fluence (1 · 1019–1 · 1027
He+/m2), using both accelerators and large-sized plasma
confinement devices such as TRIAM-1M and LHD. In
these studies, not only blistering and erosion, but also
many other irradiation effects were examined such as
internal damage evolution, change of mechanical properties and heat load resistance, retention and desorption
of gas, etc. In this paper we review recent results to gain
a better understanding of the performance of metallic
PFMs under helium plasma bombardment relevant to
burning plasma conditions. The main focus will be on
tungsten.
2. Radiation effects by low energy helium ions
2.1. Distinctive features of helium irradiation effects
*
Corresponding author. Tel.: +81 92 583 7716; fax: +81 92
583 7690.
E-mail address: [email protected] (N. Yoshida).
The behaviour of helium in metals is characterized
by its fast thermal migration through the lattice and
very strong attractive interaction with defects such as
0022-3115/$ - see front matter 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jnucmat.2004.10.162
N. Yoshida et al. / Journal of Nuclear Materials 337–339 (2005) 946–950
947
vacancies, vacancy clusters, impurity atoms and even
themselves. In the case of irradiation by helium with
keV range energy, the number of helium atoms implanted into the material is comparable to the number
of radiation induced point defects (vacancies and interstitials). Moreover, the helium implantation and the
resultant displacement damage are localized in the subsurface region of about some 10 nm or less. The radiation induced defects and helium atoms are accumulated
there by cluster formation.
2.2. Fundamental defect possesses under helium ion
irradiation
If the energy of the incident helium is higher than the
threshold value for the displacement damage (0.5 keV
for tungsten), interstitials and vacancies with the same
number are formed in the narrow projected range beneath the surface. Due to the very low migration energy
(0.08 eV for tungsten) the interstitials migrate thermally
even at room temperature and form interstitial type dislocation loops at the start of the irradiation. Continuing
the irradiation, the loops grow further and the less mobile vacancies are highly accumulated in the narrow
damaged area. The majority of vacancies trap helium
atoms. The behaviour of the vacancies and the vacancy-helium complexes depends on the specimen
temperature, i.e., at low temperatures where thermal
migration of the vacancies and the vacancy-helium complexes are scarcely expected, very dense fine helium bubbles (about 1 nm in diameter) are formed by absorbing
more and more helium. On the other hand, large helium
bubbles are formed at high temperatures where vacancies and helium bubbles can migrate thermally [2].
Radiation damage also occurs for irradiation with
very low energy helium (less than about 0.5 keV for
tungsten), where displacement damage is not expected
to occur due to the absence of the required knock-on
energy. Helium atoms, once injected into the material,
aggregate by themselves and grow as bubbles by pushing
out the host atoms from their lattice sites (formation of
interstitials) and/or interstitial loops. We note that preexisting vacancies are not necessary for the formation
of the helium bubbles. Of course, a supply of vacancies
(radiation induced vacancies and thermal vacancies) is
very helpful for bubble formation. Details of defect
formation processes under helium ion irradiation are
discussed in [2]. Such type of damage accumulation for
the sub-threshold energy condition has not been observed in electron beam irradiations and hydrogen ion
irradiations at relatively low fluxes [3].
2.3. Temperature dependence of internal damage
Formation of helium bubbles in tungsten at elevated
temperatures was examined for impact energies of
Fig. 1. Temperature dependence of bubble formation in
tungsten due to 0.25 keV He+ irradiation.
0.25 keV and 8 keV, corresponding to cases with and
without displacement damage, respectively [2]. Fig. 1
shows TEM micrographs of damage accumulation at
different temperatures for 0.25 keV He+ (flux: 1018
He+/m2 s; fluence: 1021 He+/m2). Though the efficiency
of damage accumulation is lower for 0.25 keV due the
absence of vacancies supply, the fundamental damage
processes are similar. Fine bubbles of about a few nanometer in diameter are formed densely at room temperature. The phenomenon does not change much up to
873 K, where thermal migration of vacancies is still inactive. The temperature range below 873 K is noted here
as Ôlow temperature regimeÕ.
In contrast, the number of bubbles decreases but the
individual bubbles grow larger at temperatures where
sufficient thermal migration of vacancies is expected
(1073 K and 1273 K). This temperature range is noted
here as Ôhigh temperature regimeÕ. It was found by
Nishijima et al. [4] that large bubbles of 2 lm in diameter were formed at 2600 K by a very low energy
helium plasma (10 s eV). In the high temperature regime,
coalescence of bubbles through the thermal migration
process plays a major role for the growth of the bubbles.
A similar phenomenon was dynamically observed by
TEM in a Fe–Cr–Ni alloy under helium irradiation at
high temperatures [5]. Due to very high binding energy
with helium [6], the bubbles can survive even at such
high temperatures. This type of bubble formation in
the high temperature regime has not been observed for
very high fluence irradiations with hydrogen plasmas
[7] and hydrogen ions [8]. We note that the formation
of bubbles in a wide temperature range is a distinctive
phenomenon of helium irradiation.
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N. Yoshida et al. / Journal of Nuclear Materials 337–339 (2005) 946–950
2.4. Correlation between internal damage and surface
structure
Blistering by He+ irradiation at relatively high energies (> a few keV) was extensively studied about 20–30
years ago [1], and it was well established that inter-bubble cracking through the highly pressurized fine bubbles
formed at or near the projected range of the incident
ions causes blistering in the low temperature regime.
Recent helium glow discharge studies in LHD
showed that blistering occurred even by helium ions with
only 200 eV, less than the threshold energy for displacement damage. In addition to blistering, very heavy
damage was accumulated in the sub-surface region; see
Fig. 2 [9]. Various size bubbles (1–25 nm diameter) were
formed together with dense dislocation loops. The TEM
images also indicate the formation of nano-size cracks.
It is considered that some of the cracks link the bubbles
to the surface. After erosion by blistering at around
102122 He+/m2, erosion due to sputtering and the
formation of bubbles progressed continuously. With
increasing fluence, a thick damage layer is formed as
steady state which is reached by balancing sputtering
erosion and helium injection. In the case of SUS316L,
the thickness of the layer was about 45 nm, which is
much deeper than that of the projected range of the incident helium (1 nm). Some of the bubbles appear at the
surfaces as holes caused by sputtering and some are
linked to the surface through nano-cracks. Because of
the formation of holes and cavities the effective surface
area will increase. In fact, the highly damaged layer
may act as good trapping sites for gas such as helium,
hydrogen, oxygen, etc., and may play an undesired role
for particle control of the plasma. Details of helium and
hydrogen trapping in the damage layer have been discussed in [10] and [11], respectively.
The relation between the surface morphology and the
internal damage in the high temperature regime is com-
Fig. 3. Surface modification and underlying internal damage
formed by 8 keV He+ irradiation at 1273 K at a fluence of
1.5 · 1022 He+/m2. (a) and (b) image of surface taken with
atomic force electron microscopy; (c) TEM image.
pletely different. Migration and growth of bubbles play
essential roles. Fig. 3 shows the surface morphology
and corresponding internal damage of tungsten irradiated at 1273 K with 8 keV He+ to the fluence of
1.5 · 1022 He+/m2. Comparable size surface bulges (a,
b) and bubbles inside the specimen (c) indicate that the
bulges are formed by the bubbles directly underneath.
It is expected that such type of surface modification
may change not only the optical properties such as
reflection coefficient but also the thermal conductivity
at the surface.
It was reported that cyclic heat loads with 14 keV
He+ cause a peculiar morphology [12]. The surface,
reaching 2600 K at each cycle, is fully covered with small
projections just as the inner surface of the small intestine. It is clear that migration and coalescence of the helium bubbles play an essential role for the formation of
such peculiar structure.
2.5. Influence on mechanical properties and heat load
resistance
Fig. 2. Bubbles formed in SUS316L and W at room temperature irradiated by LHD helium glow discharge plasma of
200 eV at a fluence of 4 · 1022 He+/m2.
Formation of helium bubbles brings changes in hardness at the sub-surface region [13]. Fig. 4 shows the surface hardening of tungsten irradiated by He+ at 300 K
and 873 K. Once the helium bubbles are formed the
hardness increases remarkably. The hardness becomes
N. Yoshida et al. / Journal of Nuclear Materials 337–339 (2005) 946–950
949
the narrow ion range but expand much deeper. It is
likely that the helium diffused far beyond the projected
range, causing embrittlement of a rather thick sub-surface area, which then exfoliated by thermal shock.
2.6. Synergistic effects with neutron irradiation
Fig. 4. Surface hardening by He+ irradiation at room temperature and 873 K.
more than 4 times higher than that of the un-irradiated
material at a fluence of 2 · 1022 ions/m2.
It was also reported that surface erosion by high heat
loads was greatly affected by helium pre-injection [14].
Fig. 5 shows the temperature and erosion of the surface
of helium pre-injected tungsten for a heat load of
13 MW/m2 for 30 s as a function of helium fluence.
The pre-injection was done at room temperature with
8 keV He+. Once the helium bubbles and blisters are
formed, the surface temperature increases due to the
reduction of thermal conductivity at the surface. The
weight loss at 1 · 1022 He+/m2 is about 0.3 mg, corresponding to an estimated erosion depth of 0.8 lm,
based on the specimen size. This value is about 10 times
larger than the helium ion range, indicating that the irradiation effects at very high fluence are not restricted in
Fig. 5. Effects of helium irradiation on heat load resistance
(indicated by surface temperature rise) and surface erosion
measured by weight loss.
Some of the injected helium, which can successfully
evade the trapping sites such as vacancies and bubbles
localized in the damaged zone, will migrate into the bulk
until it gets trapped. For the simulation of radiation
damage of plasma-facing materials in reactors, accumulation of helium and point defects under simultaneous
irradiations by helium and neutrons has been calculated
based on rate theory by considering the diffusion of the
point defects and helium. The probability that one
vacancy located deep in the material meets with a helium
atom diffusing from the incident surface is comparable
to the probability of interstitials and vacancies – produced homogeneously by the neutron irradiation –
meeting [15]. This means that the behaviour of the
vacancies, which result in void swelling and radiation
hardening for example, must be strongly controlled by
the helium from the plasma. Fig. 6 is an example showing the synergistic effect of diffusing helium. In case of
(a), tungsten was irradiated at 1073 K by only Cu2+ at
2.4 MeV for 3 dpa, while in (b) it was simultaneously
irradiated by Cu2+and He+ at 0.25 keV (1 · 1022 He+/
m2); here 0.25 keV He+ ions cannot form displacement
damage. Both dense interstitial loops (black images)
and fine voids (white images) were formed in (a) but only
sparse interstitial loops are seen in (b). The absence of
visible voids in (b) indicates that the vacancies cannot
form the voids, because they become immobile by
absorbing helium. Though this is only one example
demonstrating a synergistic effect, it is likely that other
synergistic effects may change the scenario of neutron
irradiation damage of the plasma-facing materials.
Fig. 6. Comparison of damage by (a) irradiation with high
energy Cu+ only, and (b) simultaneous irradiation with high
energy Cu+ and low energy He+.
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N. Yoshida et al. / Journal of Nuclear Materials 337–339 (2005) 946–950
3. Damage in large-size plasma confinement devices
Acknowledgment
To further study the phenomena of plasma-wall
interactions in large-size plasma confinement devices,
metallic specimens were exposed to helium plasma discharges in the scrape-off layer in TRIAM-1M [16].
Remarkable formation of dislocation loops and dense
fine bubbles was observed in tungsten facing the core
plasma after being exposed for only 125 s. It was concluded that the defects are formed mainly by the bombardment of charge- exchanged helium neutrals ejected
from the core plasma. According to recent experiments
in LHD, interaction with the divertor helium plasma
causes serious blistering for tungsten [17].
This study was supported by Grant-in-Aid for Science Research from Ministry of Education, Science
and Culture, Japan.
4. Summary
The most distinctive irradiation effect of helium in
tungsten is the formation of helium bubbles for very
wide conditions, i.e., above a few 10 eV, from very
low dose about 1019 He+/m2 and up to very high temperatures near the melting point. It is remarkable that
self-aggregation results in bubble formation without
pre-existing vacancies such as radiation induced ones.
Dense and fine bubbles and dislocation loops are
formed at low temperatures, where thermal migration
of vacancy-helium complex and bubbles are rather
low. They form the thick damaged layer beneath the surface and change the retention of gaseous atoms very
much. The helium bubbles induce serious hardening
and reduction of thermal conductivity which enhances
erosion under high heat load. At high temperatures,
internal structure such as large bubbles determined the
wavy and peculiar morphology of the specimen surface.
It was also pointed out that the synergistic irradiation effects of neutrons and helium plasma would play important roles for radiation damage of the plasma facing
materials. These recent results indicate that helium irradiation is a serious issue for tungsten as a plasma facing
materials in the burning plasma condition.
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