富山大学水素同位体科学研究センター研究報告29 ･ 30 : 71‑81, 2010.

Three‑Dimensional Morphology of Blister‑Like Structures and Deuterium

Retention in Tungsten Exposed to Low‑Energy, High‑Flux D Plasmav.Kh･ Alimovl, S･ Lindig2, M･ Balden2, K･ Isobel, W･M･ Shu3, J･ Roth2, T･ Yamanishi1
1 Tritium Technology Group, Japan Atomic Energy Agency, Tokai, Ibaraki, 319‑1 195･ Japan

2 Max‑Planck‑Institutfiir Plasmaphysik, EURA TOMAssociation, D‑85 748 Garching, Germany
3 ITER International Organization, CEA Cadarache, 131 08 Saint Paul‑lez‑Durance･ France

Three‑dimensional morphology of blisteト1ike structures and deuterium retention inrecrystallized tungsten have been examined after exposure to a low‑energy (38 eV/D), high‑nux (1022 D/m2S) deuterium plasma at ion nuences in the range from 1026 10 1027 D/m2andvarious temperatures･ The methods used were scannlng electron microscopy equlpped withfocused ion beam, thermal desorption spectroscopy, and the D(3Iie,p)4He nuclear reaction at
3日e energleS Varied from O･69 to 4･O MeV･ Blister‑like structures with various shapes andsizes depending on the exposure nuence and temperature are fわrmed on the W surface･ In

doing so, crackswith distorted areas (<600 K) and large cavities (100‑1000 pm3) at grainboundaries (≧500 K) were observed beneath the surface. The surface blister‑like structures

and the defects underneath are correlated along crystallographic orientation of the W grains inaccordance to the low‑indexed sliding systems･ The defects are mobile and accumulate under
D plasma exposure･ Samples exposed near room temperature do not fわrm such large cavities

even by subsequent heating up to 1300 K･ The D plasma exposure at temperatures above
700 K does not lead to formation of blister‑like structures. At ionfluences of 1026‑1027 D/m2,
the D retention increases with the exposure temperature, reaching Its maximum value or

about 1022 D/m2 at 480‑530 K, and then decreases to about 1019 D/m2 al 800 K･

Keywords: blisterlng; deuterium; deuterium retention; high ion nux; tungsten

Ⅰ. Introductiom

hydrogen isotope retention after long‑term

Due to its favourable physical properties,

deuterium‑tritium plasma exposure･ Available

like low erosion yield and high melting

data (reviews l113] and refTerences therein) havetemperature, tungsten (W) is fわreseen as ashown that the hydrogen isotope retention in W

candidate material for plasma‑facing high heat‑materials exposed to high‑nux hydrogen plasmas

nux structures infusion reactors in operation and

diffTers from that for ion implantation. In addition,

under construction. As plasma‑facing material,

there is evidence of blisterlng OCCu汀ed ontungsten (W) Will be subjected to bombardment

tungsten surface exposed to hydrogen plasmas

with low‑energy, high nux deuterium (D) andwith ion energleS Well below the displacement

tritium particles including helium (He) ash･ This

threshold l418]. However, not much is known

implantation process leads to concems about

about the hydrogen retention in different W

71

Ⅴ･Kh･ Alimov, S･ Lindig, M･ Balden, K･ Isobe, W･M. Shu, ∫. Roth, T. Yamanishi

materials irradiated with low‑energy hydrogendeliverlng a plasma beam comparable to the edge

ions and exposed to low‑energy, high‑flux

plasma at ITER divertor, the W samples werehydrogen plasmas･ Especially, the temperature

exposed to the D plasma highly enrichedwith

dependence of the hydrogen isotope retention

species or D2十to over 80% [141. The bias voltage

after exposure to the plasmas at ion nuences ≧1026

or‑80 V was applied to the W sample resulting lnions/m2 was not studied systematically･

an incident energy of 38

quite o触n the D retention is accompanied

eV/D, taking into

account the plasma potential or about‑4 V

by formation of blister‑like surface structures ([3]measured by a Langmuir probe･ The incident

and references therein, [6, 8, 9‑12]).Asit wasdeuterium ion flux was fixed at 1022 D/m2S, andshown recently, an appearance or two‑types or

the samples were exposed to ion nuences of 1026

blister‑like structures, small ones (around 1 pm)

and 1027 D/m2･ The sample was passively heated

and large ones (10‑30 pm), aRer exposure to low‑by the plasma itself and the exposure temperatureenergy D plasma at temperatures around 500 K,

was set by the the‑al contact between the sample

Coexists at the maximum in the D retention [6, 8].

and the cooled holder.

The three‑dimensional (3D) morphology of the

The D concentration in the plasma‑exposed

defects is characterized by cavitiesinside large

W samples was measured by means of theblisters and cracks along grain boundaries [8, 1 1,

D(3He,.α)H reaction, where both, the α particles

12]･ Detailed analysis of the sub‑surface structure

and protons were analyzed･ To determine the D

belonglng tO the blisterl1ike surface structure wasconcentration at larger depths, an analyzlng beamfulfilled recent)y for clarification of theof3He ionswith energleS Varied from 0.69 to 4.0

underlying mechanisms for morphology changed

MeV was used･ The proton yields measured atby D impact [13]. It has been fわund that there isdifferent 3He ion energies allow measurlng the Dstrong variation on the surface topography and 3D
morphology depending on nuence, temperatureand grain orientation.

depth profile at depths of up to 7 LLm l15].

Deuterium retention in the W samples wasmonitored ex‑situ uslng thermal deso叩tion

thespectrometry (TDS). An infrared heater was usedtemperature dependence of sub‑surface

to heat the samples at a ramp rate of O･5 Kノs andmorphology and D retention in recrystallized W

the sample temperature was raised to 1300 K. HD

exposed to low‑energy, high nux D plasma at

and D2 molecules released during TDS run werehigh ion nuences (≧1026 D/m2).

monitored by a quadrupole mass spectrometer

This

work

fわcuses

on

study

of

(qMS). To calculate the relative contribution of
II. Experimental

the recorded HD and D2 masses tO the total release

Polycrystalljne tungsten (A.LM.T. Corp.,
Japan)with a purityof 99.99

wt.%

of deuterium, the partial currents of the ()MS

fully

were normalized as described in Ref. [16].

recrystallized a鮎r cutting and polishing was used.

Standard D2 leak with an inaccuracy lower thanThe recrystallization process was perfわrmed by

10% was employed to calibrate the QMS after

annealing at 2070 K fわr 1 hour in pure hydrogen

each TDS analysIS.

(protium) atmosphere. The W grain size was 20‑

The surface topography was examined by

200トLm･ tn the linear plasma generator used f♭rscanning electron microscopy (SEM), whereas the72Three‑Dimensional Morphology of Blister‑Like Structures and Deuterium Retention in Tungsten Exposed to Low‑Energy, High‑Flux D Plasma

three‑dimensional sub‑surface morphology of thewith the HKL Nordlys II detector and "HKL

plasma‑exposed W samples was analyzed by aChannel 5" analysis software l13]･

field emission SEM combined with a focused ion
beam (HELIOS NanoLab 600, FIE) [131･
To reveal co汀elation of obsen/ed blister‑

1ike structures to the crystallographic orientation

III. Results

III･A･ Surface topography and sub‑surface
morphology

of individual grains, the grain orientation was

After exposure to a nuence,功, of 1026 D/m2

determined by electron backscatterlng diffraction

at temperatures, Texp,inthe range from 320 to(EBSD). A 20 keV electron beam with an incident

370 K, only sparse low‑dome blisters with sizes ofangle of 700 to the normal of the analyzed surface

a few microns or less are observed (Fig. la).

area was typlCally used f♭r the EBSD analysts

At Texp ‑ 400‑500 K, the blisters become

Fig･ 1･ SEM images of recrystallized W exposed to low‑energy, high ion nux D plasmawith ion
nuences of 1026 D/m2 (a, C, e ‑ left part of the figure) and 1027 D/m2 (b, d, f‑ right part of the
figure) at temperatures of 360K (a, b), 530K (C), 460K (d), 600K (e), and 595 K (f)･ The

magnirlCation of all images is the same･ The surfaces were tilted at an angle of450 to the electron
beam.

73

V.Kh. Alimov, S. Lindig, M. Balden, K. Isobe, W.M. Shu, J. Roth, T. Yamanishimuch denser and the domes of blisters become

exposure temperature of 460K (Fig. ld). At

higher･ Peculiar changes occur at Texp ‑ 5201

higher exposure temperatures, the temperature

570 K, where two kinds or blisters appear: large

dependence of blistering at

low‑dome blisters with sizes of a few tens of(Fig. lf) is similarto thatat 1026 D/m2.

microns and small cone‑shaped blisters withdiameters of several micrometers (Fig. lc). At

0 ‑ 1027

D/m2

For both nuences, no blisters appear at the

exposure temperatures above 700 K.

exposure temperatures in the range from 580 to

According to EBSD analysts, Shape and670 K, the small blisters disappear, whereas the

position orthe blisters fbmed on each individual

large blisters become sparser with the increaslng

grain are correlated to the grain orientation

temperature (Fig. le).

(Fig. 2). The elongation directions of the blisters

The increaseinnuence to 1027 D/m2 leads

coincide with the projections of the (I 10) highto the appearance of dense and low‑dome blisters

symmetry planes of the W crystal onto the surface

with sizes up to 15 Llm even at Texp ‑ 320‑370K[131.

(Fig･ lb)･ Two kinds of blisters appear already atThree‑dimensional sub‑surface morphology

Fig1 2･ Various elongated nat and high‑dome blisters on the surface of recrystallized W exposed tolow‑energy, high ion nux D plasmawith an ion nuence of 1026 D/m2 at 600 K. The orientation of
the grains was determined by EBSD and is represented by the inserted cubes. The projected (1 10)
sliding planes are partly marked･ The surface was tilted at an angle of 700 to the electron beam. The
image was stretched in vertical direction in order to correct the tilt shortening (it was not done in

Figs. 1 and3).

74

Three‑DimensionalMorphology of Blister‑Like Structuresand Deuterium Retention in Tungsten Exposed to Low‑Energy, High‑Flux D Plasmaof blister‑like structures at ￠ ‑ 1027 D/m2 ranges

from fine intragranular crack‑like structures for

the W exposed at temperatures around room
temperature (Fig. 3a) to large cavities fbmed a鮎r

exposure at elevated temperatures (Fig. 3C). The
cross‑section of the small blisters is correlatedwith crack‑like defTects withwidth of〜0. 1トIm andlength of 〜5トIm Situated mainly inside thecrystallites and some times at grain boundaries
beneath the surfTace at depths of at least 10 llm(Figs. 3a and 3b).
The cross‑section of

the large blisters

formed at elevated exposure temperatures exhibits

always a cavitydeep beneath the surface, which
has contact with a grain boundary (Fig. 3C). The
血ickness of the material above the cavity easily
reaches 10 トun, SOme Ofthe cavities extent even to

more than 50 pm beneaththe surface. The volumeof these cavities is roughly equal to the volume of

the blister above the surtTace and istypically in therange of 10011000 pm3 [13]. The shape of thesurface topography is a transfわrmed image of theFig. 3. Cross‑sectional images of recrystallized

cavity shape. The cavities are in most cases not

W exposed to low‑energy, highion nun D

exactly belowthe blisters. The lateral shiftis

plasma with an ion mlenCe Of 1027 D/m2 atcorrelated to血母 graln Orientation.

360K (a), 480K (b), and 595 K (C). Take note

Figure3b highlights another importantfeature of the D plasma‑exposed W surface: largethat priorthe crossISeCtionlng the surfaces were
coated with a Pt‑C film. The sur払ces were tilted

distorted reglOnS around the cracks, visible asat anangle of 520 to the electron beam anddifferent greyscales inside a grain (due to thecrossISeCtion respective by l3 80.

channelling contrast of the backscattered electrons,
areas of equal crystallographic orientation, i.e.

grains, appear with the same greyscale). In a fTew

cavities, as occur in samples exposed at elevatedcases,

temperatures [ 1 3] ･suchdistorted

subsur払ce

reglOnS

arepresent without any significant indication on thesurface l13].

For W samples exposed around room

III.B. Depth profiles
Deuterium

depth

profilesinthetemperature, subsequent heating up to 1300 K

recrystallized W exposed to the D plasma at(e.g., during TDS experiments) does not anneal

various temperatures are shown in Fig. 4 fわr ion

the distorted reg10nS and does not fom large

fluences of 1026 and 1027 D/m2.

75

V.Kh. Alimov, S. Lindig, M. Balden, K. Isobe, W.M. Shu, ∫. Roth, T. Yamanishi

In the recrystallized W exposed at Texp ‑330K to ♂‑ 1026 D/m2,the D depth profile is

38eVDplasma‑>W,￠=lx1026Dlm2

characterized by a sharp near‑surfTace

1事,i‑"i†叫‑iiiiiiii

concentration

maximum

of

I‑2

at.%,

and,

‑鞄;i轡等舶=暗合鰯書籍昔諾‡▲ 唱:ii放‑ILI‑:‑‑0‑†一汁叶叶すat

仰 げ ば 吋1 0. 1 0 灯

depths above I pm, by a concentration of aboutlO 2 at.% slowly decreasing into the bulk (Fig. 4a).

(%.It!)UO葛Jlu8uODQ

岩芸等十什什什 ニ;≡……;:l問

一丁=330K ■■ ｢ムーT=505K 一p

As the exposure temperature increases up to▲▲AT.1ー aゝ⊥11‑i/ ■■ 鳩

530 K,the D concentration in the near‑surface

I ★★十十★､★★77 耶

i/卜HlLEH11Lll■■decreases, whereas the concentration at depths of
38eVDplasma‑>W.￠=1X1027D/m2

1‑3 pm reachesthe maximum value of about 1

lU･▲▲章二:Ai蒜東雲票轡

at･%･ Further increase of the exposure temperatureleads to a unifbrmdecrease of the D concentration･1ｩ)｣ﾓ3# ｲ Tｵ

ﾔ ﾕCﾓC ｲ 二二竺吐叶 ‑ローT=620K(Fig. 4a).
▲▲叶1t ▲ゝ L＼ b.

Asthe ionfluence increases to 1027 D/m2 atVﾅ

ｨ爾ﾕCﾓS3Tｲw

IQ ●XEI★1‑†★‑耶FFH …γ………■日 lll ■■■Texp = 320 K, the D concentration at depths of 1‑2
pm is significantly increased (Fig･ 4b)･ At Texp ‑
0.0

0.2

480 K, the D profile demonstrates,inaddition to

1

2

3

4

5

6

the near‑surfTacepeak, the concentration of about lFig. 4. Depth profiles of deuterium retained

at.% withno decrease into the bulk up to a depth

in recrystallized W exposed to low‑energy,of7 pn.

highion nu又 D plasma withion nuences ofFor both ion nuences,the maximum D
concentration is about 1 at.%, and this Dconce山rationis

temperatures ofachieved

at

the

7

Depth (l⊥m)

1026 (a) and 1027 D/m2 (b) at various
temperatures ･

exposure

480‑535K. This highDconcentration could be due to accumulation of D2
molecules in cavities created during the D plasma
expo Sure ･peaks at temperatures Tpeak = 500 and 650K
(Fig. 5).
At ￠ ‑ 1027 D/m2 and Texp ‑ 330K,thepositions of these peaks are shifted towards higher

A氏er D plasma exposure at temperatures ortemperatures and are located at Tpeak = 590 and

725‑745 K, the D depth profiles demonstrate

780 良. The amount of deuterium released as theconcentration minimum at depths of 0.5‑2 pm

second TDS peak (Tpeak = 780 K) increases by a

(Fig･ 4a,b). Unfわrtunately, we have no clear

factor of about 10 as compared to that for a

explanation of reasons fわr this local decrease influence of 1026 D/m2･ This fact denotes strong W

the D concentration.

structure modification due to plasma exposure atnear‑room temperatures at ◎ ≧1026 D/m2･ At

III･C･ Thermal desorption spectra

exposure temperatures above 500 K, as a fLuence

Thermal desorptlOn Spectrum Obtained alter

increases from 1026 to 1027 D/m2, the amount of

exposure to a凡uence ￠ ‑ 1026 D/m2 al ㌔詫P ‑deuterium released during TDS run increases by a

320 K demonstrates two strongly pronounced

factor of about 3.

76Three‑Dimensional Morphology of Blister‑Like Structures and Deuterium Retention in Tungsten Exposed to Low‑Energy, High‑Flux D Plasma

It should be noted that the tenfbld increasehightemperature is required)and then atoms

in an ionfluence at temperatures in the range fromdiffuse to the surface wherethey recombine and

320 torelease as molecules.

620K results in shiftof TDS peaks

towards higher temperatures (Fig. 5). This fact

It follows from the TDS spectra (Fig. 5a)polntS tO a Plasma‑induced evolution of defects,

that for D ionfluence of 1026 D/m2,the main TDS

which are responsible fわr trapplng Of deuterium.
A丘er plasma exposure at temperatures andpeak is shifted toward higher temperature as the
exposure temperature increases. This effect is also

are formed, the TDS

observed for ionfluence of 1027 D/m2 after D

spectra demonstrate bursting release (Fig. 5).

plasma exposure at 330and 480 K･ However, at

Numerous burstlng peaks indicate that deuterium

exposure temperatures from 515 to 620 Kthe

accumulates in tungsten in the fbm or molecules

main TDS peak is positioned practically at theinside cracks and cavities, co汀eSpOnding to thesame temperature Tpeak = 950 K･ It should be notes

fTormation of blisters. Thus, during TDS heatingthat this peak temperature is highest among all

deuterium releases directly when blisters burst or

TDS spectra

firstly dissociates from molecules into atoms (thus

exposure temperatures inthe range from 320 tofluences when blisters

for both ion

fluences and all

620 K.
38eVDplaSma‑'W,￠=1X1028D/m2

Ill.D. Total retention●丁=320K tヽP ∇丁=405K

ln the recrystallized W exposed to the D●叩 ▲丁=500K ttP □丁=550K Lq) tT=620K I 唯 $ﾂ
6 4 20 0 0 0(sN∈＼QトLO )￠t EJOSea一巴∈n.58naQ

plasma at ♂ ‑ 1026 D/m2, the deuterium retentionis (2‑3)×1020 D/m2 at Tex, ‑ 320K and, as

exposure temperature increases, rises to its
maximum of about 7×1021 D/m2 at Texp ‑ 530 K

and then decreases down to about 1019 D/m2 at
3

02

0

0

㌔xp‑ 775 K (Fig･ 6)･

Increaslng the nuence t0 1027 D/m2 at

exposure temperatures inthe range from 320 to500K results in significant increase of the Dretention.

At

￠

‑

1027

D/m2

the

maximum

retention of about 1022 D/m2 is observed at Texp ‑
300

500

700

900

1 1 00480 K, At exposure temperatures of 550‑650 K,

Temperature (K)the D retention at ￠ ‑ 1027 D/m2 decreases but

Fig.5. Thermal desorptlOn Spectra Of

remains higher by a factor of about 3 thanthat atdeuterium released from recrystallized W￠ ‑ 1026 D/m2･ At Texp above 750K,the D

exposed to low‑energy, highionflux D

retention is about 1019 D/m2 fわr both ion fluences.

plasma with ion nuences of 1026 D/m2 (a)and 1027 D/m2 (b) at various temperatures.
TDS heating rate was 0.5 K/S.

From the comparison of the TDS and NRA
data (Fig. 6) it is clearthat at ￠‑ 1026 D/m2 andirradiation temperatures up to 700 K about 50% of
retained deuterium is localized in the sub‑surfTace

77

V.Kh. Alimov, S. Lindig, M. Balden, K｣sobe, W.M. Shu, ∫. Roth, T. Yamanishi

the generation of vacancies, vacancy complexes
and microscopic Cavities at depths of several tens(Z∈[凸)POU!etaJ∈n!JetnaQor micrometers and the concu汀ent accumulationof diffusing deuterium･ It may be suggested thatthe stress‑induced plasticityof tungsten appears at
high concentrations of solute hydrogen andincreases as the temperature grows.

On the other hand, according to the theoryof fわrmation of superabundant vacancies in the
400

500

600

700

800

Exposure temperature (K)presence of interstitial hydrogen atoms l19], theformation energy of a vacancy is decreased owlng

Fig･ 6･ Deuterium retention in recrystallized

to vacancy‑hydrogen cluster fbmation and theW exposed to low‑energy, high ion nux D

configurational entropy of the system at highplasma with ion nuences of 1026 and 1027
D/m2,

as

a

functionofthehydrogen concentration. Recently Shu lll]

exposureconsidered that the mechanism of vacancy

temperature. The totaldeuterium retention

formation due to the lowerlng Of the vacancy

was dete‑ined by themal desorption

formation･ energy by trapping Of deuterium canspectrometry (TDS) (solid points), whereas

also be valid for tungsten exposed to low‑energy,

the D retention up to a depth of 7 pm was

high‑flux D plasma･ The deuterium‑vacancy

measured by nuclear reaction analysis (NRA)

clusters may diffuse deeply into the bulk and

I (Open points).

agglomerate resulting ln microscoplC Cavities.

At long‑term irradiation the dimlSing D
layer up to 7 pm･ However, at ♂‑ 1027 D/m2 theatoms

recombineonthecavitysurfaces,

fraction of deuterium retained at depths above 7

increaslng thus the D2 gas Pressure inside thesepm significantly increases.

cavities･ The cracks, distorted areas, and large andsmall cavities beneath the su血ce (Fig. 3) are

IV. I)iscussion
caused by stress exceeding the toughness of the

For tungsten irradiated with D ions atmaterial･ The stress is may be increased by the gas
energleS Well below the displacement threshold,the mechanism of plastic defbmation due tooverpressure inside the cavities. At elevatedtemperatures exceeding the brittle‑to‑ductile‑

deuterium

super‑saturation

【17]

must

be

transition temperature, the stress can be relaxed byconsidered for modification of the sub̲surfTace

dislocations movlng along lattice planes throughstructure and fb‑atiom of trapplng Sites f♭r

deuterium l9, 1 8]･ During exposure to low‑energy,

high‑fhx D plasma, the D concentration in thethe whole crystallite leading to the cavities at thegrain boundaries l13, 20]. This corresponds to theobserved material migration above the surface, I.e,

implantation zone greatly exceeds the solubility
the blister‑like surface topography. It may belimit and stresses the matrix lattice until plastic
suggested that the overpressured large and small
defamation occurs to alleviate these tensions 【1 7】.

cavities are responsible for appearance of large
This defb‑ation is assumed to be responsible f♭r

low‑dome and small cone‑shaped blisters,78

Three‑Dimensional Morphology of Blister‑Like Structures and Deuterium Retention in Tungsten Exposed to Low‑Energy, High‑Flux D Plasmaco汀eSpOndingly･ It should be noted that fわr all

increased, and therefore, the transient D

small blisters, deep holes or pits were fb‑ed inconcentration is strongly lowered.

the internal blisters l1 1]. Thus, formation of small

The co‑incidence of the maximum of the Dcone‑shaped blisters at Texp ‑ 5201570 K may beretention in the temperature dependency (Fig･ 6)

explained by deuterium gas mlgration toward theWith the appearance of highest denslty Of

surface at conditions of high plasticity

structural changes (Fig. 3) is evident. Deuteriumofrecrystallized W.

is retained mainly at vacancy‑type defTects and in

At near‑room temperatures the stressgas‑filled cavities and cracks, both in the form of

relaxation results in plastic defbmation and crack

D2 molecules and D atoms. According to van

fTormation inside the grains. Thus, the temperature∨een et al. [21], the D2 gas inside the voids isdependence of the structure modification

expected to be released during annealing at 400‑

correlates well with the temperature dependence

600 K (detrapping energy矢〜LO eV), whereas theoffracture toughness for W l20, 21].

D atoms bound on the inner surface of voids

The fact that annealing of sampleswith high(binding energy

矢〜1.7eV) is expected to beamount of the retained D does not lead toreleased atformation the large cavities, as it does during

where the voids are generated. On the other hand,

exposure to the plasma, indicates that just thedeuterium TDS peaks at 640 and 520 K are welltransient D concentration maintained during

explained by trap energleS Or 1.34 and l･07 eV,

implantation is responsible fわr the stress state･ It

which is related to trapplng Ofa丘rst and second D

can be expected that the mobilityof vacancies,

atom at a vacancy, respectively [22]･ TDS spectra

voids, and dislocations in tungsten is affected byfor the plasma‑exposed W (Fig･ 5) demonstrate athe transient D inventory during exposure to low‑variety of trapping energies fわr deuterium and,energy, high nu又 D plasma･

therefore, a variety of vacancy defTects and700‑1000K, depending on depths

From the known low‑indexed sliding

cavities/cracks fわrmed in the sub‑surface layer.

systems [20], the lateral location of the cavity andObviously that during D plasma exposure atsurface structure as well as the direction of thetemperatureselongation of the blisters could be explained (Figs.

accumulated in the fbm of D2 molecules and D

2 and 3C) [13], but further detailed investigations

atoms, whwreas at temperatures above

are neCeSSary･

deuterium is retained as D atoms solely.

below

600K

deuterium

is

600 K

The absence of blister‑like structures as well

In closlng it is pertinent to note that theas the low D retention at temperatures above

nuence and temperature dependence of the D

700 K indicates that the requlrement fわr extended

retention is a consequence of the sub‑surface

defect production is not fulfilled･ Possibly, it

structure modification

could be explained by the strongly increased

deuterium in the cracks and cavities.

and accumulation ofsolubilityand diffusivityll] avoiding that thetransient D concentration exceeds the solubility.

V. Summary

Also it could be speculated that the defects in W

Significant temperature dependence of

anneal at these temperatures, the reemission issurface topography, sub‑surface morphology anddeuterium retention is found fわr recrystallized W

79

V･Kh Alimov, S. Lindig, M. Balden, K. Isobe, W.M. Shu, J. Roth, T. Yamanishi
exposed to low‑energy (38 eV/D), high‑nux (1 022

deuterium‑vacancy clusters are suggested as

D/m2S) D plasma at ionfluences of 1026 and 1027

mechanisms

D/m2･ At temperatures of 320‑400 K, only sparse

microscoplC Cavities at depths of several tens ofblisterswith diameters of 0.5‑2 pm are formed on

micrometers･ At long‑term plasma exposure, thethe W surface･ In this case the D depth profilesare

di仇lSing D atoms recombine on the cavity

characterized by a near‑surface concentration

surfaces, lnCreaSlng thus the gas pressure insidemaximum of 1‑2 at.% and, in the sub‑surface

these cavities, which may support the fbmation or

layer

blister‑like structures.

(atdepths

from

1

to

7

LLm),

by

a

fわr

nucleation

andgrowthofconcentration of O･02‑0･2 at･% slowly decreaslng
into the bulk･ As the exposure temperature

Acknowledgements

increases, the blisters become much denser. Two

The

portion

of

this

work

perf♭medatkinds ofblisters appear: large blisters with sizes or

JAEA was supported by MEXT/Japan through

10‑30トLm and small cone‑shaped blisterswith

JSPS (Kagaku‑kenkyuhi‑hqjyokin, No. 1 9055009)

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and ITER Intemational Organization under Task

place, the D concentration in the sub‑su血ce layer

Agreement ITER/TA/08/490.

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above 700 K, and the D concentration at depths ofRefe r甲CeS

severalmicrometers is about 1 0‑3 at.%.

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