Blistering and Retention in the Near-Surface Region of Tungsten
Exposed to High Flux Deuterium Plasmas of Tens of eV
W.M. Shu
Tritium Engineering Laboratory, Japan Atomic Energy Research InstituteTokai-mura, Naka-gun, Ibaraki-ken 319-1195, Japan
Tungsten plates annealed at 1473 K were exposed to deuterium plasmas with incident
energies ranging from 7 to 98 eV and a fixed flux of 1022 D/m2/s, and the blistering andretention in the near-surface region were investigated with a variety of techniques, such as
scanning and transmission electron microscopy (SEM and TEM), thermal desorption
spectroscopy (TDS), nuclear reaction analysis (NRA), elastic recoil detection (ERD),
secondary ion mass spectroscopy (SIMS) and X-ray diffraction (XRD),. Blisters with the
maximum diameter of about 2 microns (comparable to grain size) were formed on thetungsten surfaces after plasma exposure, and small blisters with a diameter of around 30
nm and microcracks were formed in the near-surface region before the formation of larger
blisters. Within the experimental error there was a zero change in the lattice parameter
after the plasma exposure, implying that deuterium does not exist in the lattice interstitialsites, but instead forms a deuterium-vacancy complex and then clusters and further bubbles(predominantly in the form of molecules in vacancy clusters and voids) in the near-surface
region. After deuterium plasma exposure, deuterium was retained in the depth up to a few
microns from the surface, and the maximum atomic ratio of deuterium against tungsten
reached as high as 1-2% in the near-surface region. These evidences suggest that crystal
defects like vacancies should be generated due to lowering of the formation energy ofvacancies by the intrusion of a great number of hydrogen isotope atoms into thenear-surface region of tungsten.
Keywords: tungsten, retention, blistering, hydrogen isotopes
I. Introduction
Resent researches [1-8] revealed thatblistering can occur on tungsten surface, even if
the ion energy is too low to create displacement
damage such as vacancies. Sze and co-workers
observed macroscopic blisters with thediameters of a few to a few hundreds microns on
the surfaces of tungsten foils exposed to high ion
flux (1022 D+/m2/s) and low ion energy (about
110 eV) deuterium plasma at 400-500 K [1-2].
Haasz et al. found blisters with the diameters ofa few to a few tens microns on the surfaces oftungsten pre-annealed at 1473 K after exposure
to deuterium plasmas with the ion flux of about
1020 D+/m2/s and ion energy of 500 eV at 500 K[3]. Venhaus et al. reported that un-annealed
samples and samples annealed to 1473 K
revealed extensive blistering with many blister
caps removed and minor blistering, respectively,
while samples annealed to 1273 K showed noblistering, after exposure to deuterium andtritium ions with high ion flux of 1.7×1022
ions/m2/s and low ion energy of 100 eV at 423 K[4]. Wang et al. [5] reported that the blistersize increased and the number decreased withthe deuterium fluence from 1019 to 1021 D+/m2 at
the energy of 1 keV and room temperature andno blisters were found at elevated temperatures
between 600 and 800oC. Tokunaga et al. [6]found that blisters were formed on the powdermetallurgy tungsten by deuterium irradiation
with ion flux ranging from 4.8×1021 to 1.2×1022
D+/m2/s and low ion energy of 100 eV attemperature of 708 to 843 K, and the amount of
blisters and their average size increased with theincreasing fluence from 7.5×1025 to 3.0×1026
D+/m2. Ye et al. [7] observed blister formation
on tungsten surface under a low energy (about
90 eV) and high flux (2×1021 H+/m2/s) hydrogenplasma exposure at the surface temperature lessthan 950 K and the fluence higher than 3×1024
m-2. Ueda et al. [8] investigated the impact ofcarbon impurities in hydrogen plasmas ontungsten blistering at a hydrogen ion flux ofabout 4×1021 H+/m2/s and hydrogen ion energyof 100 to 1000 eV.
The above researches have clearly
demonstrated that hydrogen isotope exposure
with energy as low as 100 eV can definitely
produce blistering on tungsten surface.
However, the mechanism of blistering is notwell understood yet and requires future
investigations. In this work, tungsten plates
annealed at 1473 K were exposed to deuterium
plasmas with incident energies ranging from 7 to98 eV and a fixed flux of 1022 D+/m2/s, and theblistering and retention in the near-surface
region were investigated with a variety oftechniques, such as scanning and transmission
and scanning electron microscopy (SEM andTEM), thermal desorption spectroscopy (TDS),
nuclear reaction analysis (NRA), elastic recoil
detection (ERD), secondary ion mass
spectroscopy (SIMS) and X-ray diffraction(XRD).
II. Experimental
II. A. Samples and Linear Plasma Generator
Tungsten plates were prepared bypowder-metallurgy and hot-rolled reduction, andthen annealed at 1473 K for 30 minutes. The
plates were subsequently cut and double-sided

C1

C2

C3

Filament

Langmuir probe

GV1

Sample holder

Gas inCooling water
Thermocouple

Filament(10V100A)

Bias(240V12.5A)

Discharge(240V75A)

GV2
Insulation
10cm

Fig. 1

AV1
SP

TMP

The schematic view of the linear
plasma generator.

polished into samples of 10 × 10 × 2 mm. The
tungsten material has a purity of 99.99 wt% andprincipal impurities (in weight ppm) of Mo andFe ~10, C and O <30. The polished samples
were cleaned in an acetone ultrasonic bath prior
to placing into the deuterium exposure chamber.
The schematic view of the linear plasma
generator used in this study is shown in Fig. 1.
The apparatus consists of sections of vacuum
chamber and pumping, cooling water, gas
admittance, power supply, plasma generation,
plasma delivery, sample holder, and plasma
diagnosis. The vacuum system was composed of
two turbo molecular pumps (TMP1, 300 l/s and
TMP2, 70 l/s), a scroll pump (SP, 250 l/min),two gate valves (GV1 & 2), an angle valve(AV1), and tubing. The achievable backgroundpressure is lower than 5×10-6 Pa. The plasma is
ignited by an electron-emitting filament andmaintained by an arc discharge power supply
applied between the filament (cathode) and thegrounded chamber wall (anode), being assistedby the confinement coils (C1~C3). The filamentelement is made of LaB6 shaped into a hollow
dual-spiral, and heated directly with a low
voltage and high current power supply. The
coils (C1~C3) are not only necessary totransport the plasma onto the sample holder, butalso helpful to the plasma ignition and stability.
The water-cooled sample holder is isolated fromthe grounded chamber wall so that the sample
can be negatively biased to adjust the energy ofions impinging onto the sample. And a single

Langmuir probe is equipped to obtain basic
plasma parameters. The ion species can becontrolled by adjusting the operationalparameters of the plasma generator. With
suitable adjustment, we can achieve plasma
beams highly enriched with a single species of
D+, D2+, or D3+, to a ratio over 80%. A fixed
flux of 1×1022 D+/m2/s and varying incident
energies from 7 to 98 eV were used in theexperiments. The comparison between this workand the previous researches [1-8] is made in
Figure 2, in which the ion flux is plotted against
ion energy, and the coverage of both flux andenergy for ITER divertor dome, baffle and liner[9] is also shown.
1023
Ion flux (ions/m2/s)

ITER divertor liner
1022
21

10

This
work

Sze et al. [1, 2]
Tokunaga et al. [6]ITER
Venhaus et al. [4]divertor Ye et al. [7]dome &
Ueda et al. [8]baffleHaasz et al. [3]2010Wang et al. [5]1910Fig. 2

100

101
102
103
Ion energy (eV)104

A summary of flux and energy used inrecent researches on W blistering. For
comparison, the design values of ITER
divertor are also shown.

II. B. Observations and measurements
Blister formation on the surface of tungsten
exposed to deuterium plasmas was observed bya scanning electron microscopy (Real Surface
View Microscope, KEYENCE VE-7800) at a tilt
angle of 45º.
Thin foils for transmission electron
microscopy (TEM) were prepared with afocused ion beam (FIB) microsampling system(HITACHI FB-2000A). The system enabled not
only the broad cross-sectional TEM observation,
but also the detailed study of irradiatedmicrostructure. Procedures to prepare thin foils

for TEM observation are: (a) W deposition (toreduce damage during the specimen fabricationin the region for observation), (b) removal ofmaterial with a fine Ga+ (20 keV) beam to obtaina foil, (c) welding of a W needle to the foil by W
deposition, (d) cutting to separate the foil fromthe irradiated disk, (e) welding the foil to a meshby W deposition, and (f) removal of thedamaged layer near the foil surface formed
during FIB processing by an Ion Miller(GENTILE MILL, TECHNOLOGY LINDA).
These efforts successfully enabled a TEM (JEOL
JEM-2000FX)
examinationofplasmaexposure-induced microstructural defects.
Patterns of X-ray diffraction (XRD) were
measured to examine the crystallographic
change in the near surface region of tungsten
before and after deuterium plasma exposure.
XRD patterns were recorded by an X-ray
diffractometer (Phillips PW1700, Toyama
University). The probe was the Cu Kα line at
0.1572 nm operated with 1200 W power. The
incident angle of X-rays was fixed at 1.5° to thesurface of the sample to measure the lattice
constant in the near-surface region, and thescanned 2θ ranged from 30° to 136°.
Thermal desorption spectroscopy (TDS) wasused to evaluate the deuterium retention in thetungsten samples after deuterium plasma
exposure via integrating the deuterium release
rate with respect to time. A standard deuterium
leak with an inaccuracy lower than 10% was
employed to calibrate the quadrupole mass
spectrometer (QMS) prior to each TDS analysis
so that the calibrated release rate during TDS
could be obtained. During TDS, an infrared
heater was used to heat the irradiated samples at
a ramp rate of 5 K/s and the sample temperature
was raised to ~ 1500 K and then held for 2 min,being monitored by a type R thermocouple.
Depth profile of deuterium in thenear-surface region of tungsten after plasma
exposure was measured by nuclear reaction

analysis (NRS) and elastic recoil detection(ERA). In the NRS measurement, a beams of 2
MeV 3He+ ions with a current of 5 nA (250nA/cm2) from a Pelletron 5SDH-2 accelerator(Kobe University) were incident on the sample
with an incident angle of 60° to the surface
normal through an aperture with diameter of 1.0
mm. A solid state detector (SSD) with solid
angle of 2.1×10−3 sr located at 50° to the surface
normal was used to detect alpha (α) particles
produced by the nuclear reaction D(3He,α)H.
Before the SSD, an aluminum film 8 mm thick
was placed to prevent the scatted 3He+ ions fromintervention. The output signals from the SSD
were analyzed with a multi-channel analyzer(MCA). On the other hand, in the ERD
measurements, a beams of 4 MeV 4He+ ions witha current of 0.5 nA (25 nA/cm2) from the sameaccelerator were incident on the sample with anincident angle of 75° to the surface normal andthe recoiled hydrogen isotopes were detectedwith the SSD at a forward angle of 60° to thesurface normal.
The depth profiles of deuterium, hydrogenand impurities (oxygen and carbon) in the nearsurface region of tungsten before and after
deuterium plasma exposure were analyzed bysecondary ion mass spectroscopy (SIMS)(ULVAC-PHI ADEPT1010 Dynamic SIMS
System, JT-60 Facilities Division II), using
cesium ion (Cs+) as the primary ion with anenergy of 5 keV and a beam current of 100 nA at
45° to the surface normal. The beam size wasIII. Results and Discussion
III. A. Blistering
A typical SEM image is shown in Fig. 3,
where blisters with diameters of 0.1 to about 2µm are observed. This sample was exposed todeuterium plasma with the energy of 98 eV andthe fluence of 1025 D/m2 at room temperature.
Subsequently, a cross-section of a large blisterwith a diameter of about 2 µm on this sample
was fabricated by FIB technique. Before
removing the material with Ga+ ions, aprotection thin layer of tungsten was depositedon the blister. The cross-sectional SEM imageof this blister is shown in Fig. 3, where thethickness of the blister cap was determined to beabout 0.4 µm.

Fig. 3Blisters formed on W surface
image at a tilt angle of 45o).

(SEM

Fig. 4

Cross-sectional SEM image of ablister viewed at a tilt angle of 45o.

about 32 µm and the rastering area was set at
400 × 400 µm2. Depth profiling with aprofilometer showed the sputtering rate to bearound 0.55 µm/h. In order to eliminate theeffect of crater edges, the signal of the secondary
ion was collected only from the center of therastered area. The negative secondary ion
intensities of deuterium and hydrogen werenormalized by that of tungsten for comparison.

The maximum blister size observed in thisstudy is 1-2 orders smaller than that of blisters
formed on tungsten exposed to 100-1000 eVhydrogen isotope plasmas reported before [1-8].
Fore instance, Sze and co-workers observedmuch larger blisters with the diameters up to afew hundreds microns on tungsten foils exposed
to high ion flux (1022 m-2s-1) and low ion energy(about 110 eV) deuterium plasma at 400-500 K[1-2], and Ye et al. observed blisters with thesize of a few tens to a few hundreds microns
formed on tungsten surface under a low energy(about 90 eV) and high flux (2×1021 H+/m2/s)
hydrogen plasma exposure at the surface
temperature less than 950 K [7]. It seems thatthe maximum size of blisters is independent of
the incident energy and the exposure
temperature, but instead it must be strongly
influenced by the microstructures like grain size.
Thus, the behavior of blistering on tungsten
exposed to high flux and low energy plasma was
firstly investigated by FIB/TEM in this study.

The cross-sectional TEM images are shownin Fig. 5 to Fig. 8 for a sample exposed to 7 eV
deuterium plasma with a fluence of 1025 D/m2.
At the exposure conditions, visible blisters could
not be observed by the SEM with a solubility of
about 0.1 µm.
As shown in Fig. 5, the microstructures oftungsten are almost isotropic with a maximum
grain size of about 2 µm, and a deformation
layer around 0.1 µm thick is formed in thenear-surface region.

100 nmFig. 6

XTEM image of microcrack/void along
grain boundary.

1 µm
A modification layer

100 nmFig. 7

XTEM image of microcrack along grainboundary and void at grain corner.
30 nm

Fig. 5

XTEM image of W sample after 7 eV
deuterium plasma exposure.

Fig. 8

XTEM image of small blisters(around 30 nm) and microcrack.

size of blisters (about 2 µm) is limited by
microstructures, especially by grain size.
Much larger blisters observed by the previous
researchers like Sze et al. [1-2] should be causedby the crystal texture of the tungsten foils usedin their study.
III. B. Retention
The deuterium retention in the exposed
samples was measured using thermal desorption
spectrometer (TDS). The TDS spectrum of
deuterium shows a peak around 770-970 K at aheating rate of 5 K/s, implying a predominantrelease in its molecular form from the blisters.
The relationship between the amount of retained
deuterium and the fluence shows a sudden drop
at a certain fluence, indicating that deuterium
retention is limited by rupture of blisters. The
maximum retention in samples exposed to theplasma at room temperature is around 1020 D/m2.
In addition, the relationship between the amountof retained deuterium and the plasma exposure
temperature shows the maximum retention ataround 400~500 K.
Above 700 K, theretention decreased to a quite low level, andblisters disappeared at the exposure of 900 K.
The NRA/ERD results are summarized in Fig.
9, where averaged values of NRA data wereplotted for 3 sets of the data measured at thecenter, and at 2 mm above/below the center.
The results show that deuterium diffuses to the0.020

Atomic ratio, D/W

NRA-38eV
ERD-38eV

0.015

NRA-68eV
NRA-98eV

0.010

0.005

0.000
-0.2 0.0 0.2

0.4

0.6

0.8

1.0

1.2

Depth (µm)

Fig. 9Depth profile of D/W measured by
NRA and ERD.

depth deeper than around 1 µm. The maximum
atomic ratio of D/W reaches as high as 1-2% in
the near-surface region.
A preliminary SIMS result is shown in Fig.
10. As pointed out by Alimove et al. [10], theappearance of the SIMS D- signal is attributed tothe existence of separate D atoms within thematrix. In other words, SIMS only detects theatomic hydrogen isotopes in the material.
Since the efficiency transferred to negative ionsfor deuterium is almost the same as that ofhydrogen, deuterium retains in atom form with avery small fraction. It is also seen in this figurethat the background of hydrogen signal is machlarger than that of deuterium. On the other
hand, in comparison with the un-exposed sample,
deuterium in tungsten exposed for 1000 seconds
at the flux of 1022 D/m2/s, the energy of 98 eV
and room temperature diffuses to the depth
deeper than around 1 µm.
510

H in un-exposed W

Intensity (arb. unit)

Microcrack/void was generated at the depthof about 100 nm along the grain boundary
parallel almost to the surface (Fig. 6), and void
was generated at the grain corner at the depth ofabout 300 nm and microcrack extended along
the grain boundary (Fig. 7). Besides, small
blisters with a diameter of around 30 nm andmicrocracks formed in the near-surface region
before the formation of larger blisters withdiameters of up to a few microns (Fig. 8).
The observations indicate that the maximum

H in W exposed to 1025 D/m2
@ 38 eV104
3

10

D in W exposed to 1025 D/m2
@ 38 eV2

10

D in un-exposed W

101
0

100 200 300 400 500 600 700
Depth (nm)Fig. 10

Depth profiles of H- & D- intensities
in exposed and un-exposed samples.

Lattice constant (n m)

0.320
0.318
0.316
0.314
0.312
0.310
0

3x10231025
2

Fluence (D/m )Fig. 11

Lattice constants of un-exposed andexposed W samples.

The results of small angle XRD at a fixed
incident angle of 1.5o are summarized in Fig. 11
for the W samples un-exposed and exposed tofluence of 3×1023 and 1025 D/m2 at 38 eV. The
lattice constant was determined to be
0.31584±0.00008 nm, indicating that within theexperimental error there was a zero change inthe lattice parameter after the plasma exposure.
This implies that deuterium does not exist in thelattice interstitial sites, but instead forms adeuterium-vacancy complex and then clusters
and further bubbles (deuterium molecules invacancy clusters and voids) in the near-surface
region.
Ogorodnikova et al. [11] introduced two
kinds of traps to describe the deuterium retention
in tungsten exposed to high-flux and low-energy
plasmas.
One is low-temperature traps:
intrinsic defects (dislocations, grain boundaries,
some impurities, presence of bulk oxide) with atrapping energy of 0.85 eV distributed over thewhole sample thickness; and another ishigh-temperature traps: “ion-induced” traps
associated with deuterium agglomeration inmolecules and bubbles near the implanted
surface and deuterium trapping in vacancies
with a trapping energy of 1.45 eV which fom
and grow during implantation, distributed near
the surface and correlated with the implantation
range. Many of the other studies such as thoseby van Veen et al. [12, 13] and Sakamoto et al.
[14] have shown that hydrogen decorates
pre-existing voids and can create new ones ifthey do not already exist. Deuterium implanted
into tungsten will diffuse back to the surface ordeeper into the material.
The hydrogen
diffusing deeper into the material will eventuallyfind vacancies, dislocations, and voids into
which they are trapped (unless the temperature ismaintained at a temperature above which thetraps are thermally depopulated). Creating areasof high pressure voids in the very near-surface
region may create microcracks to release thishydrogen to the surface. In this case, bubbles
would be created deep into the material leading
eventually to blister formation.
Causey [15] argued that using tungsten in aplasma-facing application may eventually resultin moderate inventories of hydrogen isotopes inthe bubbles and blisters (unless the tungsten is
maintained at very high temperatures). Notethat the bubble formation may also eventually
result in tungsten being released into the plasma.
Tungsten can be released into the plasma either
through grain ejection [1] or by evaporation due
to the creation of open blister caps that lose their
thermal contact with the material below.
Even many researches have been carried out,some questions about the mechanisms ofblistering and retention still remain.
Forinstance, how the “ion-induced” traps are
generated? And why could blistering occur inannealed tungsten and even single crystal
tungsten [16] exposed to deuterium plasmas
with energies much smaller than that requiredfor generating displacements? In a future work,
we will propose a new model to describe thegeneration of vacancies due to lowering of theformation energy of vacancies by the intrusionof a great number of hydrogen isotope atoms
into the near-surface region in tungsten, and theformation of hydrogen isotopes-induced
vacancies in the near-surface region of tungsten

will be further confirmed directly by positron
lifetime and Doppler broadening measurementswith slow positron beams.
IV. Conclusions(1) Blisters with the maximum diameter ofabout 2 microns (comparable to grain size) wereformed on the tungsten surfaces after plasma
exposure, and small blisters with a diameter ofaround 30 nm and microcracks were formed in
the near-surface region before the formation oflarger blisters. The maximum size of blisters islimited by the microstructures, especially by thegrain size.
(2) After deuterium plasma exposure,
deuterium was retained in the depth up to a few
microns from the surface, and the maximum
atomic ratio of deuterium against tungsten
reached as high as 1-2% in the near-surface
region. There was a zero change in the lattice
constant after the plasma exposure, implyingthat deuterium does not exist in the latticeinterstitial sites, but instead is trapped byvacancy clusters and voids in the near-surface
region predominantly in the form of molecules.
These evidences suggest that crystal defects likevacancies should be generated due to loweringof the formation energy of vacancies by the
intrusion of a great number of hydrogen isotope
atoms into the near-surface region of tungsten.

Acknowledgements
The author would like to appreciate thecontributions of Dr. G.-N. Luo (JAERI) for
plasma exposure experiments and TDS
measurements, Dr. T. Sawai (JAERI) for FIB
and TEM observations, Mr. Y. Aihara(KEYENCE) for SEM observations, Drs. N.
Kubota and K. Ochiai (JAERI) for NRA andERD measurements, Drs. Y. Ishimoto and N.
Miya (JAERI) for SIMS measurements and Dr.

M. Hara and Prof. M. Matsuyama (Toyama
University) for XRD measurements. Special
thanks are due to Drs. M. Seki, S. Seki, H.
Takatsu, M.F. Nishi and T. Yamanishi (JAERI)
for their continuous encouragement and supportof this study.
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