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 Materials
June 2 -3, 2008
Salamanca, Spain
Osaka University

2. Surface phenomena affecting T behavior
Deposition layer
Trapping site for T
Diffusion barrier for T
Mixing layer
Desorption barrier
He bubble layer
Radiation damage by n w
Ne, Ar D T O He
C, Be
mixing layer
(collision mixing) T
deposition
layer T
Erosion 　 T T T
He bubbles
mixing layer
(diffusion mixing) T T W
Diffusion barrier　
Radiation damage

3. Steady-State High-Flux Dual Ion Beam
Flux：~1020 m-2, Energy: 0.15~3 keV
Blanket first wall condition
Osaka University

4. Enhancement of blister formation by carbon impurity
C concentration in H beam increases
C layer W
Beam irradiation area
Carbon deposition
(no blisters)
Formation of blisters
No blisters
Small amount of carbon (less than 1%) in ion beam
can enhance blister formation on W.
Experimental conditions
Beam Energy: 1keV H3+, Flux : (3-4)x1020 Hm-2s-1
Temperature : 653 K
Sample : pure W with mirror polished
Osaka University

5. Mechanism for blistering
Implantation of H （a few nm ~ 20 nm）
grain ejection
Accumulation of H
at grain boundaries
Dome-like blisters
> 1 µm H
Cross section of blister
(K-dope W)
Osaka University

6. W and C mixing layer reduced desorption
Atomic composition in tungsten
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 H
C: ~0.9% W
Blistering C O W
1 keV H
C: ~0.1%
no Blistering C O
Osaka University

7. E.g. K Tokunaga et al. J. Nucl. Mater. (2004) 337–339, 887.
From K, thin and thick layers of Be suppresses blister formation.
M. Baldwin et al.
PSI 18(2008)
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

8. Blister 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.
Energy ：1 keV　(H3+, H2+ , H+)
Carbon　 ：~0.8%
Fluence　：~7.5 x 1024 m-2
753 K
500 µm
500 µm
653 K
500 µm
500 µm
473 K
20 µm
20 µm
20 µm
He : 0.1%
He : 0%
Osaka University

9. 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
Osaka University

10. Flux dependence of blistering
C: 0.85%, He：none
T = 653 K
Flux dependence of blister formation
Blistering still appeared by reducing the flux by about 3 ( (2.10.8) 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.
High
2.1×1020 /m2s
500 µm
1.3×1020 /m2s
500 µm
Flux
0.8×1020 /m2s
500 µm
Low

11. TEM 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 K
Fluence : 4.1 x 1024 m-2
TEM observation of near surface structure

12. swelling rate estimation swelling rate vs. temperature
He bubble volume (swelling rate)
swelling rate estimation
swelling rate vs. temperature 6 5 4 3 2 1
Swelling rate (%)
1100
1000
900
800
700
600
Temperature (K) ━
He 0.1%
He 1%
Sample
20nm
Swelling rate = He bubble volume / total volume
Hydrogen diffusion greatly suppressed by only 2% He bubbles.

13. Effects of He energy on blistering
(a)　no He
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
no Blistering
(b) He：0.6 keV
(c) He：1.0 keV
Blistering
（0.6 keV He）
No blistering
(1keV He, 1.5keV He）
Osaka University
Ion range in tungsten

14. He effects in ITER (tungsten FW)
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 (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 diffusion of T and D from CX.
Ranges of He and T ions are comparable. He effects?
Edge Ti & Te
CX Neutrals

15. He effects in fusion reactors (divertor): ITER
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.
Normal incidence
0.5 m
Ion ranges for the edge plasma
condition (Temp.~15 eV)
Te & ne profiles near divertor SP

16. Summary 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.