W nano-fuzzes: A metastable state formed due to large-flux He+ irradiation at an elevated temperature

Wu, Yunfeng; Liu, Lu; Ni, Weiyuan; Liu, Dongping

doi:10.1016/j.jnucmat.2016.10.018

Cited by 10 publications

(7 citation statements)

References 13 publications

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“…This trend of reversible flow was confirmed by annealing of the fuzzy W surface to a high temperature in absence of Helium irradiation. Hence, obvious reduction in the W-fuzz height was reported [48][49][50].…”

Section: B Effect Of He + Ion Fluxmentioning

confidence: 99%

Low energy helium ion irradiation induced nanostructure formation on tungsten surface

Al-Ajlony

Tripathi

Hassanein

2017

Journal of Nuclear Materials

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We report on the low energy helium ion irradiation induced surface morphology changes on tungsten (W) surfaces under extreme conditions. Surface morphology changes on W surfaces were monitored as a function of helium ion energy (140-300 eV), fluence (2.3 × 10 24-1.6 × 10 25 ions m-2), and flux (2.0 × 10 20-5.5 × 10 20 ion m-2 s-1). All the experiments were performed at 900 o C. Our study shows significant effect of all the three ion irradiation parameters (ion flux, fluence, and energy) on the surface morphology. However, the effect of ion flux is more pronounced. Variation of helium ion fluence allows to capture the very early stages of fuzz growth. The observed fuzz growth and morphology changes were understood in the realm of various possible phenomena. The study has relevance and important impact in the current and future nuclear fusion applications.

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Section: B Effect Of He + Ion Fluxmentioning

confidence: 99%

Low energy helium ion irradiation induced nanostructure formation on tungsten surface

Al-Ajlony

Tripathi

Hassanein

2017

Journal of Nuclear Materials

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“…The observed incubation fluence (the minimum fluence before fuzz growth) is required for bubbles to form [21] which are required for fuzz growth [22]. At high temperatures, due to the low thermal conductivity of the fuzz [7,23], tendrils heat up and reintegrate back into the bulk. Irradiation by impurities or high-energy He ions (> 120 eV) can lead to sputtering, the emission of W atoms from the sample, which erodes surface features [24].…”

Section: Conditionsmentioning

confidence: 99%

“…In a thick sample (relevant to fusion reactors), He bubbles are required for fuzz initiation; this could be to drive adatom diffusion. Finally, annealing fuzz [23] leads to He release and subsequent destruction of the fuzz nanostructure; without the stresses from He bubbles, the metastable fuzz disappears when heated. Fiflis et al [30] replaced He + ions with Ne + which did not lead to fuzz.…”

Section: Adatom Diffusionmentioning

confidence: 99%

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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“…However, their accumulation in the subsurface region alters the material properties and surface morphology, significantly degrading the beneficial properties of tungsten. Numerous studies [6,[9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24] have demonstrated that a tungsten surface with surface temperature T s 1000 K, when exposed to a high flux of low-energy He + ions ( 20 eV) and fluences Φ 10 24 He/m 2 in linear plasma devices and tokamak plasmas, forms fine fiber-like nanostructures, often referred to as 'fuzz'. The thickness of the fuzz layer has been observed to increase as the square root of helium fluence after a delayed onset (incubation fluence) [25].…”

Section: Introductionmentioning

confidence: 99%

“…The fuzz layer can grow to thicknesses of a few micrometers, degrading the mechanical and thermal properties of exposed tungsten [26]. Furthermore, fuzz is fragile, thermally unstable, and can easily be sputtered [21,22]. Therefore, during sudden stress or high-heat loading, it can easily detach [27,28], forming dust that can cool the plasma, thereby adversely affecting the stability of the fusion plasma.…”

Section: Introductionmentioning

confidence: 99%

Effect of helium flux on near-surface helium accumulation in plasma-exposed tungsten

Nandipati¹,

Hammond²,

Maroudas³

et al. 2021

J. Phys.: Condens. Matter

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We report results of object kinetic Monte Carlo (OKMC) simulations to understand the effect of helium flux on the near-surface helium accumulation in plasma-facing tungsten, which is initially pristine, defect-free, and has a (100) surface orientation. These OKMC simulations are performed at 933 K for fluxes ranging from 1022 to 4 × 1025 He/m2 s with 100 eV helium atoms impinging on a (100) surface up to a maximum fluence of 4 × 1019 He/m2. In the near-surface region, helium clusters interact elastically with the free surface. The interaction is attractive and results in the drift of mobile helium clusters towards the surface as well as increased trap mutation rates. The associated kinetics and energetics of the above-mentioned processes obtained from molecular dynamics simulations are also considered. The OKMC simulations indicate that in pristine tungsten, as the flux decreases, the retention of implanted helium decreases, and its depth distribution shifts to deeper below the surface. Furthermore, the fraction of retained helium diffusing into the bulk increases as well, so much so that for the flux of 1022 He/m2 s, almost all of the retained helium diffused into the bulk with minimal/negligible near-surface helium accumulation. At a given flux, with increasing fluence, the fraction of retained helium initially decreases and then starts to increase after reaching a minimum. The occurrence of the retention minimum shifts to higher fluences as the flux decreases. Although the near-surface helium accumulation spreads deeper into the material with decreasing flux and increasing fluence, the spread appears to saturate at depths between 80 and 100 nm. We present a detailed analysis of the influence of helium flux on the size and depth distribution of total helium and helium bubbles.

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W nano-fuzzes: A metastable state formed due to large-flux He+ irradiation at an elevated temperature

Cited by 10 publications

References 13 publications

Low energy helium ion irradiation induced nanostructure formation on tungsten surface

Low energy helium ion irradiation induced nanostructure formation on tungsten surface

A review of late-stage tungsten fuzz growth

Effect of helium flux on near-surface helium accumulation in plasma-exposed tungsten

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