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.