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

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

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

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

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

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

E.g. K Tokunaga et al. J. Nucl. Mater. (2004) 337–339, 887. From 300-700 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

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

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

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

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

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.

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

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 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 diffusion of T and D from CX. Ranges of He and T ions are comparable. He effects? Edge Ti & Te CX Neutrals

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

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.