九州大学大学院総合理工学府報告第 34 巻 第 1 号 1-6 頁 平成 24 年 9 月

Engineering Sciences Reports, Kyushu University
Vol. 34, No. 1, pp. 1-6, SEP, 2012

Deuterium trapping properties of plasma sprayedtungsten layers irradiated by low energy and high flux
plasmaAleksandr RUSINOV*1†, Mizuki SAKAMOTO*2, Hideki ZUSHI*3, Ryohei
OHYAMA*1, Koichiro HONDA*1, Tetsuo TANABE*1, Naoaki YOSHIDA*3（平成 24 年 5 月 14 日 受理，平成 24 年 5 月 15 日 掲載決定）†E-mail of corresponding author: rusinov@triam.kyushu-u.ac.jp
Deuterium retention and trapping in tungsten plasma sprayed layers after low energy plasma
irradiation was investigated by means of thermal desorption technique. Plasma irradiation at lowtemperatures (<650 K) showed the retention and trapping similar to polycrystalline tungsten
material. On the other hand, irradiation at high temperatures (>650 K) showed significant increaseof the retention in the plasma sprayed layers contrary to the polycrystalline material. Thermal
desorption spectra showed extremely high temperatures of the peak positions which are non-typicalfor the tungsten materials. The possible reasons for such high deuterium retention in the plasma
sprayed layers after irradiation at high temperatures and the possible trapping mechanisms arediscussed.
Key words: plasma sprayed tungsten, thermal desorption, retention, plasma irradiation

1. Introduction
Tungsten material is one of candidates
for the divertor plates in the future
thermonuclear experimental reactor ITER.
Because of difficult machining and welding ofbulk tungsten, plasma sprayed (PS) tungsten(W) layer, which has been used in ASDEX andother tokamaks, is also considered as plasma
facing material. PS-W has been used in thedivertor plates and inner guard limiters of theQUEST spherical tokamak in Kyushu
University1). The main advantages of PS-W
layer are low cost and possibility of coating oflarge areas even of non-planar shapes2).
Although a lot of studies on hydrogen isotope
retention have been done with polycrystalline
tungsten, there are few studies for the PS-W
layers. Literature data show large scattering ofdata for plasma exposed PS-W, retention
mechanisms have not been investigated indetails. Because divertor materials will be*1 Interdisciplinary Graduate School of EngineeringSciences, Kyushu University, Japan*2 Plasma Research Center, University of Tsukuba,Japan*3 Research Institute for Applied Mechanics, Kyushu
University, Japan

exposed to high flux plasma and heated to hightemperatures, investigations under these
conditions are of great interest.
It is necessary to study the retention
characteristics of the PS-W layer in thelaboratory conditions in order to predict its
behavior in the plasma confinement machines.
In this study, the deuterium (D) retention inPS-W layers exposed to low energy D plasma
was investigated at different irradiation
temperatures (Tirr) and fluences by means ofthermal desorption spectroscopy (TDS).

2. Experimental details
VPS (vacuum PS) and APS (atmosphericPS) W samples were used in this study. The
thickness of W layer on 1 mm SS substrate was
150 m. The samples were produced by
TOCALO corp. Experiments have been
conducted with as received samples as well aswith polished ones. The latter were polishedmechanically and electro-chemically and thenoutgassed by linear heating from 300 K up to1473 K with the rate of 1 K/sec. Polycrystalline
W (PC-W) sample with 99.99% purity and10×1 mm dimensions, produced by A.L.M.T.

2Deuterium trapping properties of plasma sprayed tungsten layers irradiated by low energy and high flux plasma

corp. was used in the experiments forcomparison. It had been polished mechanicallyto a mirror-like surface and annealed at 1173 K
for 1 hour in vacuum prior to the first
irradiation.
The samples were exposed to low energy
D plasma in the Advanced PWI Simulation
Experimental Device and Analysis System(APSEDAS). The scheme of the installation isshown in Fig. 1. The plasma was produced by
13.56 MHz RF field in a steady state magnetic
field. The samples were mounted on a copper
water-cooled stage and exposed to the various
fluences up to 1026 D/m2 under the constantflux of about 3×1021 D/m2sec. The stage waselectrically grounded. The energy of ionscoming from the plasma to the surface of thesample was estimated to be about 30 eV by theLangmuir probe measurement. During theirradiation the samples were heated by intense
plasma particle fluxes and the surface
temperature was measured by an infrared
thermometer through a pyrex window.
Additional substrates were inserted between
the sample and the water cooled stage in order
to fix the sample surface temperature at adesired level.

stand. A sample was put on a tantalum bed. A
thermocouple was attached on the backside of
the bed. Then the bed was evacuated inside aquartz tube and heated by an infrared ovenfrom 300 K up to 1473 K with the rate of
1 K/sec. The desorption rates of various species
were measured by a quadrupole mass
spectrometer. The calibration of the sensitivityof the spectrometer was performed in eachexperiment using standard He leak taking into
account the ratio of ionization cross-sections

σHe+/σD2+, which was measured experimentally.
The amount of desorbed deuterium wascalculated as an integral of D2 and HD
desorption signals over the time. HD
calibration factor was assumed to be the sameas that of D2.
There might be a certain concern about
the temperature distribution in the PS-W layer
during the plasma irradiation and also during
the TDS heating, since the thermal
conductivity of the PS-W layer seems to be low.
The temperature of a deeper position in thePS-W layer might differ from that on thesurface. From a simple calculation, themaximum temperature difference in the PS-Wlayer is considered to be only 2–3 degree evenfor the thermal conductivity of 100 times
smaller than that of polycrystalline W.
Therefore,
errorsinthetemperature
measurement during the irradiation andduring the TDS heating seem to be small.

3. Results and discussion

Fig.1 APSEDAS PWI simulator scheme

Measurements of thermal desorption
from samples were performed in a separate

The thermal desorption spectra ofdeuterium for PCW, VPS-W and APS-W afterirradiation at different Tirr up to 800 K at thefluence of 2×1025 D/m2 are shown in Fig. 2.
At low Tirr (<600 K), TDS spectra for
APS-W and VPS-W are similar to that for PCW.
The contribution of the low temperaturepart (below 900 K) in the TDS spectra
decreases for all materials with the increase ofTirr. On the other hand, the contribution ofhigh temperature part (above 900 K) increases
with Tirr for PS W and is not changed for PCW.
The structure of TDS spectra from APSW and VPS-W in the high temperature平成 24 年度

九州大学大学院総合理工学報告

desorption region is complicated. It consists ofmany desorption peaks with different
amplitudes. The peaks are overlapped andform wide spectrum. Though both materials
revealed similar desorption peak positions inthis region, the structure of the spectra isdifferent. In other words, for a given Tirr thedistribution of the peak amplitudes in thespectra are different for APS-W and VPS-W.
Figure 3 shows TDS spectra ofdeuterium from APS-W irradiated at variousfluences up to 4.6×1025 D/m2 at Tirr = 760 K. It
is seen that all peak amplitudes grow with thefluence. The relative amplitudes of the spectra
are not changed with the fluence much. The
expression “relative amplitudes” can be applied
to a set of spectra and means following. If wenormalize a set of TDS spectra and all
amplitudes of peaks located at the same
temperatures become the same then we saythat the relative peak amplitudes in the set ofthe spectra are the same. The normalization
should be performed so that the amplitudes of
one chosen peak in every spectrum from the set
became the same.
Figure 4 shows the dependence of thedeuterium retention on Tirr in PC-W, APS-W
and VPS-W, where the fluence was 2×1025 D/m2.
In the low Tirr region (near 500 K) the retention
in PS-W is 2-3 times larger, than that in PCW.
In the high Tirr (above 650 K) the retention for
PS W layers is much higher compared to PCW.
The retention in PCW quickly decreases
with increase in Tirr, while that in PS-W
increases up to two orders larger than that of
PC-W at Tirr = 750 K. There seems to be no signof saturation of the retention in APS W up toTirr = 800 K.
Figure 5 shows fluence dependence ofthe retention in APS-W calculated from TDS
spectra in Fig. 3. The retention increases as
the square root of the fluence up to 5×1025 D/m2
and then it saturates. Irradiation at thefluence of 9.8×1025 D/m2 showed even smaller
retention compared to that at the fluence of
4.6×1025 D/m2. Note that all points in thisdependence had been measured consequently
with the same sample. Moreover, six monthes
after a set of experiments with this sample at

第 34 巻第1号

3Tirr = 750 K the repetitive measurements of theretention at the fluences of 2×1025 D/m2 and4×1025 D/m2 have been conducted. Thesemeasurements showed 4 times less values andFig.2 TDS spectra of D2+0.5HD from PCW, VPS W and
APS W samples irradiated up to the fluence of
2×1025 D/m2. The irradiation temperatures are shown inthe plots.

than that of earlier measurements performed
after irradiation at similar fluences (Fig. 5),though the peak positions of the corresponded

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Deuterium trapping properties of plasma sprayed tungsten layers irradiated by low energy and high flux plasma

TDS spectra as well as their shape remained
the same.

the pores and voids are enriched by the
impurities, which are accumulated on thesurface layer of each grain during thedeposition process.
Usually, the desorption of hydrogenisotopes implanted by ion beams in PC-W as
well as by plasma has been observed within
300 K–1000 K region3)-6). Contrary to thepolycrystalline
tungstenmaterial,
hightemperature (>1000 K) desorption from plasma
sprayed tungsten layers has been reported inseveral studies7)-9).

Fig.3 TDS spectra of D2+0.5HD from APS W irradiated by
D plasma at the irradiation temperature 760 K. The
fluences are shown in the plot in D/m2.

An example of scanning electron
microscope (SEM) picture of chemicallypolished surface of APS-W is shown in Fig. 6. It
is seen from the figure that the material
consists of tungsten beads with the voids
between them. Sometimes the beads contain a

Fig.5 Fluence dependence of D retention in APS W
exposed to D plasma at 760 K (triangles). The repetitive
points are shown by squares, the square root behavior isshown by the line

Fig.4 Irradiation temperature dependence of D retention
in PCW, APS W and VPS W irradiated by D plasma up to
the fluence of 2×1025 D/m2

lot of small bubbles and voids. The net of voids
forms the pores or paths for the deuterium
atoms and molecules from the surface into thebulk. Thus, deuterium can migrate in the formof molecules or atoms along the internalsurfaces toward the top surface. Since closed
pores and voids exist in the material,
deuterium gas penetrated and accumulated
inside them needs to overcome the surface
barrier to release to the vacuum or to the open
pores. Most probably, the internal surfaces ofFig.6 SEM picture of APS W chemically polished surface

In ref. 10, Otsuka et al. have measured
tritium depth profiles in plasma exposed APSW and VPS-W layers deposited on F82H bymeans of tritium imaging plate technique. Ithas been shown that the contribution of theF82H substrate in the total deuterium
retention is small and decreases with Tirr. Evenat the Tirr = 453 K tritium distributes

平成 24 年度

九州大学大学院総合理工学報告

throughout whole 1 mm deposited APS andVPS layers. This depth is extremely large
compared to deuterium permeation insidepolycrystalline W material at these conditions.
Deep tritium migration was attributed totritium diffusion (penetration) through open
pores and/or migration along grain boundaries
followed by adsorption on grain surfaces facing
the pores and dissolution of T in the grains.
TDS spectra evolution with Tirr (Fig. 2)
from APS-W, VPS-W and PCW suggest that
PS-W layers have high energy trapping sites
additionally to lower energy ones, typical for
PC-W. Complicated shape of TDS spectra from
PS-W in the high temperature desorption
region indicates that there are multiple highenergy traps with different detrapping
energies. Similar desorption peak positions inthe high temperature region of TDS spectrafrom APS-W and VPS-W layers leads to theconclusion that the both materials have similar
high energy trapping sites. Different relative
peak amplitudes in the TDS spectra from APSW and VPS-W can be explained by different
concentrations of high energy traps in thematerials. At low Tirr the trapping mechanisms
in PS-W layers are similar to those in the PCW,
but still small amout of D atoms is trapped in
the high energy traps. The role of the highenergy traps increases substantially in PS-W
layers with Tirr increase. This allows us tosuggest that at higher Tirr D atoms can reachthe regions which they cannot reach at low Tirr
and can be trapped there.

Fig.7 EDS spectra of plasma irradiated surface of nonpolished APS W, VPS W and PCW. CK, OK, WK – theenergy level of X-rays corresponded to the K shells of C, O

第 34 巻第1号

5and W atoms.

Surface analysis of plasma irradiated
VPS-W and APS-W layers was performed by
Energy Dispersive Spectroscopy (EDS). The
both surfaces were contaminated by oxygen.
The oxygen concentration was higher than thatof PCW as shown in Fig. 7. It seems that thesurface of each W drop in PS-W is enriched byimpurities, mainly oxygen, bounded to thebeads during the deposition process. Other
studies have also showed the presence ofoxygen on the surface of W beads in PS-W
layers, e.g. ref. 9.
At high plasma irradiation temperatures
plasma sprayed layers can store huge amountof hydrogen isotopes. In order to remove it fromthe material heating up to high temperatures
is necessary. Probably, the internal structure of
the layers plays crucial role in the hydrogenisotopes
accumulation.
Infurtherinvestigations the D retention in PS-W layers
deposited in different conditions should beinvestigated. The deposition conditions shouldbe changed to keep lower concentration andsize of pores and cracks between W beads andlower contamination level of the W beads
surfaces.

4. Conclusion
Deuterium trapping at low energy
plasma irradiation in plasma sprayed layers
was investigated by means of thermal
desorption spectroscopy. It was observed that
retention in the plasma sprayed layers strongly
increases for the irradiation temperatureshigher 650 K, contrary to polycrystalline
tungsten. The high temperature part (>1000 K)
of TDS gives a large contribution in the total
retention after high temperature irradiations.
High oxygen concentration on the surfaces ofPS-W layers was observed.
High storage ability of hydrogen andhigh detrapping energies could be negative forusing this material as plasma facing in thefusion reactor especially at elevated divertor
working temperatures. It is proposed that thecontamination level and the porous structure
have strong influence on the retention

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Deuterium trapping properties of plasma sprayed tungsten layers irradiated by low energy and high flux plasma

properties in the PS-W layers. In order toimprove their characteristics it is necessary toreduce the number of voids and cracks andcontamination level in plasma sprayed layers
by appropriate deposition procedure.

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