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1
Effects of simultaneous impurity ion irradiation
on tritium behavior near tungsten surface
Y. Ueda, M. Fukumoto, H. Kashiwagi, Y. Ohtsuka (Osaka University)
R. Akiyoshi, H. Iwakiri, N. Yoshida (Kyushu University)
9th International Workshop on Hydrogen Isotopes in Fusion Reactor MaterialsJune 2 -3, 2008
Salamanca, Spain
Osaka University
2Surface phenomena affecting T behavior
• Deposition layer– Trapping site for T– Diffusion barrier for T
• Mixing layer– Trapping site for T– Diffusion barrier for T– Desorption barrier
• He bubble layer– Trapping site for T– Diffusion barrier for T
• Radiation damage by n– Trapping site for T– Diffusion barrier for T
C, Be
DHe
Ne, Ar
deposition layer
mixing layer(collision mixing)
mixing layer(diffusion mixing)
He bubbles
Ero
sio
n
T
TT
T
WT
TT
O
w
Diffusion barrier
n
Radiation damage
T
3
Osaka University
Steady-State High-Flux Dual Ion Beam
Flux : ~1020 m-2, Energy: 0.15~3 keVBlanket first wall condition
4Enhancement of blister formation by carbon impurity
C concentration in H beam increases
No No blistersblistersFormation of blistersFormation of blisters
Carbon depositionCarbon deposition(no blisters)(no blisters)
Small amount of carbon (less than 1%) in ion beam can enhance blister formation on W.
C layerW
Beam irradiation area
Beam Energy: 1keV H3+, Flux : (3-4)x1020 Hm-2s-1
Temperature : 653 K
Sample : pure W with mirror polished
Experimental conditions
Osaka University
5Mechanism for blistering
Implantation of H ( a few nm ~ 20 nm )
grain ejection
Accumulation of Hat grain boundaries
Dome-like blisters
> 1 µm
H
Cross section of blister(K-dope W)
Osaka University
6W and C mixing layer reduced desorption
• C depth distribution– broader than ion implantation range
– Due to recoil implantation by H
• High C (~0.9% in the beam) case– WC layer reduced recombination of H
– Enhance bulk diffusion of H
– Enhance blister formation
• Low C (~0.1% in the beam) case– Low surface C concentration
– no significant reduction of recombination
1 keV HC: ~0.9%
1 keV HC: ~0.1%
Atomic composition in tungsten
W
W
C
C
O
O
Blistering
no Blistering
Osaka University
PISCES
PISCES
From 300-700 K, thin and thick layers of Be suppresses blister formation.
• Blistering & exfoliation of blister caps is a concern for certain varieties of W.
• Increased retention is associated with the trapping of hydrogen in blisters.
E.g. K Tokunaga et al. J. Nucl. Mater. (2004) 337–339, 887.
• At 550 K a blistered surface is prevalent after exposure to D2 plasma.
• A thin layer of Be as little as a few 10’s of nm, or thicker, is found to suppress blister formation. D+ ion fluence ~1x1026 m-2
M. Baldwin et al. PSI 18(2008)
8Blister formation under H&He irradiation
• Small amount of He affected blistering– He : ~0.1% has strong effects
• Suppression of blisters at T>653 K– 0.1% He did not change surface mixing layer
much.
He : 0.1% He : 0%
473 K
653 K
753 K
500 µm 500 µm
500 µm 500 µm
20 µm 20 µm20 µm
Energy : 1 keV (H3+, H2
+ , H+)Carbon : ~0.8%Fluence : ~7.5 x 1024 m-2
Osaka University
9
Osaka University
He bubble could affect H diffusion
• 1 keV He has slightly longer range than1 keV H (mixed).– He bubbles could be formed around
the end of ion ranges.• He bubbles in W and C mixed layer.
• He bubbles could be a diffusion barrier for H into the bulk.– Stress field affects diffusion?
Ion range
10Flux dependence of blistering
• Flux dependence of blister formation– Blistering still appeared by reducing the flux
by about 3 ( (2.10.8) x 1020 /m2s ).• The number density of blisters decreased.
• Surface mixing layers (WC) were similar for these cases and formed in the early stage of ion irradiation.
• He effects on effective flux reduction– Since addition of 0.1% He+ to H ion beam
completely suppressed blistering, He irradiation corresponded to the case with the flux, lower by more than a factor of 3.
2.1×1020 /m2s
500 µm
1.3×1020 /m2s
500 µm
0.8×1020 /m2s
500 µm
C: 0.85%, He : noneT = 653 K
Flu
x
High
Low
11TEM observation of He bubbles
• He:1.0%, ~2 nm He bubbles
• He:0.1%, 1~2 nm He bubbles– He fluence : 4.1 x 1021m-2.
– From erosion depth (~300 nm) and ion range (~10 nm), effective He fluence was ~1020 m-2.• Only this fluence affected hydrogen
diffusion
• Bubble size and bubble number density had weak dependence on He% and C%.
• He bubbles were formed in WC layer for C:~0.8%.
T = 653 KFluence : 4.1 x 1024 m-2
TEM observation of near surface structure
12
He bubble volume (swelling rate)
Hydrogen diffusion greatly suppressed by only 2% He bubbles.
20nm
swelling rate estimation swelling rate vs. temperature
Sample
Swelling rate = He bubble volume / total volume
6
5
4
3
2
1
Swelling rate (%)
11001000900800700600
Temperature (K)
━ He 0.1%━ He 1%
13Effects of He energy on blistering
Main Ion Beam(1.5 keV : H+C:0.8%)(a) no He ion beam Blistering
(b) 2nd He beam :0.05% ( 0.6 keV ) Blistering
(c) 2nd He beam :0.05% ( 1.0 keV ) *
2nd He beam :0.05% ( 1.5 keV ) * *angle of incidence ~ 40 deg
(a) no He
(b) He : 0.6 keV
(c) He : 1.0 keV
Ion range in tungsten
no Blistering
Blistering( 0.6 keV He )
No blistering(1keV He, 1.5keV
He )
Osaka University
14He effects in ITER (tungsten FW)
1019
1020
1021
1022
1023
1024
0 5 10 15 20
E
1
10
100
reference distance [m]
iter585: c-x neutral flux and average energy E
eV
m–2s–1a b c d e f g
1
10
100
0 5 10 15 20
TiTe
reference distance [m]
iter585: plasma temperatureseVa b c d e f g• Energy of ions
– CX neutrals have relatively high energy ( D,T 、 ~600eV ) with the flux of mid 1019 m-2s-1.
– Fuel ions (D,T) have relatively low energy (~200 eV , ~3kTe+2kTi) with the flux of 1020 m-2s-1 .
– He ions have energy (~300 eV, 3ZkTe +2kTi) with the flux of ~1018 m-2s-1
• (R. Behrisch et al., JNM 313-316 (2003) 388.)
• Ion ranges (normal incidence)– CX neutral (T) 8.4 nm (600 eV)– T ions 4.2 nm (200 eV)– He ions 3.1 nm (300 eV)– He implantation may enhance inward diff
usion of T and D from CX.– Ranges of He and T ions are comparable.
He effects?
CX Neutrals
Edge Ti & Te
15
Ion ranges for the edge plasma condition (Temp.~15 eV)
He effects in fusion reactors (divertor): ITER
Te & ne profiles near divertor SP
0.5 m
30
25
20
15
10
5
0
Dis
trib
utio
n (x
10-3
Ato
ms/
Ang
/ion
s)
1086420Depth (nm)
120 eV He
75 eV T
Normal incidence
• Divertor plates– He ranges are shorter than T.
• Te, Ti = 15 eV, He2+
– He bubbles are also desorption barrier?
• Enhancement of T retention?• H bubble size ~ range
– He bubble could not be important.
16Summary and conclusion
• Simultaneous irradiation of impurity ions (C, He, (Be)) significantly affects hydrogen behavior in tungsten.– Surface mixing layer affects hydrogen-isotope behavior
• Its effects are determined as a balance between reduction of surface recombination and reduction of diffusion into the bulk.
– He bubble layer can be a diffusion barrier• Stress field could reduce diffusion?
– He effects strongly depend on energy.• He ion range H ion range Reduction of bulk diffusion
• He ion range < H ion range Enhancement of bulk diffusion
– More study is needed under edge plasma conditions (He ion energy less than ~300 eV).
• No displacement damage and short ion ranges compared with out experiment.
• This effect should be properly evaluated and included in T retention estimation in W.