Material mixing during fuzz formation in W and Mo

Patino, M. I.; Nishijima, D.; Tokitani, M.; Nagata, Daisuke; Doerner, R. P.

doi:10.1088/1402-4896/ab4c1f

Cited by 8 publications

(12 citation statements)

References 31 publications

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“…This explains why the supply of new material is from the bulk. It was also observed that there is greater concentration of bulk material at the edge of tendrils [36,44]. This supports adatom migration over viscoelastic flow.…”

Section: Adatom Diffusionmentioning

confidence: 54%

“…The original theory suggested adatoms would form on the side of tendrils due to He bombardment. However, experimental studies with W isotope films [36] on W, thin Mo films on W (Mo forms similar fuzz) [30,44,45], and thin W films on Mo [45] have found that the surface and bulk materials mix in fuzz formation. Thus, bubble growth, rupture and loop punching (discussed in Sect.…”

Section: Adatom Diffusionmentioning

confidence: 99%

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A review of late-stage tungsten fuzz growth

Wright

2022

Tungsten

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Tungsten will be used as the plasma-facing divertor material in the International Thermonuclear Experimental Reactor (ITER) fusion reactor. Under high temperatures and high ion fluxes, a ‘fuzz’ nanostructure forms on the tungsten surface with dramatically different properties and could contaminate the plasma. Although simulations and experimental observations have provided understanding of the initial fuzz formation process, there is debate over whether tungsten or helium migration is rate-limiting during late-stage growth, and the mechanisms by which tungsten and helium migrations occur. Here, the proposed mechanisms are considered in turn. It is concluded that tungsten migration occurs by adatom diffusion along the fuzz surface. Continual helium migration through the porous fuzz to the tungsten bulk is also required for fuzz growth, for continued bubble growth and rupture. Helium likely migrates due to ballistic penetration, although diffusion may contribute. It is difficult to determine the limiting process, which may switch from helium penetration to tungsten adatom diffusion above a threshold flux. Areas for further research to clarify the mechanisms are then considered. A greater understanding of the fuzz formation mechanism is key to the successful design of plasma-facing tungsten components, and may have applications in forming porous tungsten catalysts.

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Section: Adatom Diffusionmentioning

confidence: 54%

Section: Adatom Diffusionmentioning

confidence: 99%

A review of late-stage tungsten fuzz growth

Wright

2022

Tungsten

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“…Similar accumulation of substrate material at the surface within the uppermost ∼50 nm was observed by reference [32] when exposing a 15 nm 182 W-rich film on W substrate with natural isotopic distribution to He plasma at 1100 K to form fuzz. As discussed in reference [23], accumulation of the substrate material at the fuzz tip suggests material migration upwards and fuzz growth from the fuzz tip outwards. Fuzz growth from the surface outwards was also observed by references [33,34] when comparing the height measured by cross-sectional SEM of the fuzzy-exposed and smoothunexposed regions.…”

Section: Temperature Dependence (W Film On Mo)mentioning

confidence: 85%

“…Previously we investigated the transport of near-surface W and Mo atoms during fuzz formation using molybdenum (Mo) thin (∼45 nm) films deposited on W substrates and exposed to He plasma at T s = 838-1075 K, Φ He = 3.4 × 10 25 m −2 , and E i < 40 eV [23]. We found that the developed fuzz contained both Mo and W at all temperatures, with the fuzz thickness (and size of surrounding bubbles) increasing with temperature.…”

Section: Introductionmentioning

confidence: 99%

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Material migration in W and Mo during bubble growth and fuzz formation

Patino¹,

Nishijima²,

Tokitani³

et al. 2021

Nucl. Fusion

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Growth of helium (He) induced bubbles and fuzz in tungsten (W) and molybdenum (Mo) is investigated using samples of W films on Mo substrates and Mo films on W substrates exposed to He-containing plasma in the temperature range of 340 to 1075 K, fluence range of 1.0–14 × 1025 He·m−2, and incident ion energy of <50 eV. No fuzz (only up to 2 nm diameter bubbles) and no material transport occur in W films at ⩽750 K, while precursors-of or fully-developed fuzz and material mixing occur in W and Mo films at ⩾800 K. This suggests that fuzz forms in multi-material systems as long as one material meets the conditions for fuzz formation, namely T s/T m ∼ 0.27–0.5 where T s and T m are the sample exposure and material melting temperatures, respectively. Larger He bubbles, more material mixing, and further-developed fuzz occur at higher temperature due to increased mobility of He atoms and small He clusters. Accumulation of substrate material at the surface of fuzzy W and Mo thin-film (<80 nm) samples suggests fuzz growth by material transport from the bubble layer in the bulk up to the fiber tip, likely by a two-step process: (i) diffusion of punched dislocation loops in the bulk toward the fuzz base and (ii) diffusion of adatoms along the fuzz base and fiber surface (with effective transport of adatoms upwards due to trapping of adatoms at curved surfaces of fiber tips and/or due to the continuous generation of adatoms at the fuzz base). While the bubble size and fuzz thickness increase with reduced W concentration in Mo thin-film samples at 838 K likely due to an increase in trap mutation and dislocation loop punching in Mo compared to W, the fuzz thickness decreases with reduced W concentration at 1075 K despite an increase in the bubble size likely due to slower diffusion of interstitial loops in Mo.

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He bubble-driven growth of W fuzz during the interaction between H2/He plasmas and W materials

Fan

Niu

et al. 2021

Tungsten

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Material mixing during fuzz formation in W and Mo

Cited by 8 publications

References 31 publications

A review of late-stage tungsten fuzz growth

A review of late-stage tungsten fuzz growth

Material migration in W and Mo during bubble growth and fuzz formation

He bubble-driven growth of W fuzz during the interaction between H2/He plasmas and W materials

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