The evolution of He nanobubbles in tungsten under fusion-relevant He ion irradiation conditions

Abstract

He-induced W nanofuzz growth over the W divertor target is one of the main limiting factors affecting the current design and development of fusion reactors. In this paper, based on He reaction rate model in W, we simulate the growth and evolution of He nanobubbles during W nanofuzz formation under fusion-relevant He+ irradiation conditions. Our modeling unveils the existence of He nanobubble-enriched W surface layer (<10 nm), formed due to the He diffusion in W crystal into defect sites. At an elevated temperature, the growth of He bubbles in the W surface layer prevents He atoms diffusing into the deep layer (>10 nm). The formation of W nanofuzz at the surface is attributed to surface bursting of high-density He bubbles with their radius of ~4 nm, and an increase in the surface area of irradiated W. Our findings have been well confirmed by the experimental measurements.

Tensile stress-driven cracking of W fuzz over W crystal under fusion-relevant He ion irradiations

“…Our modelling based on He reaction rates in W showed the existence of a He nano-bubble enriched W surface layer (~10 nm) due to 100 eV He + implantations into W and He diffusion into defect sites in W, which was well confirmed by our TEM measurements [32]. After the surface bursting of He nano-bubbles, nano-sized loops were generated in the edge of steps (figure 2(b)).…”

“…This indicates that during the W fuzz growth, the diameter of the fuzz may be significantly affected by the size of He nano-bubbles. Increasing the irradiation temperature may significantly contribute to the He diffusion in W and the growth of He nano-bubbles [32], thus potentially leading to an increase in the mean diameter of the W fuzz. However, further investigation needs to be performed to analyze the effect of temperature on the W fuzz growth.…”

“…With increasing He + fluence, the number density of He nano-bubbles bursting over the surface is greatly increased, accompanied by a decrease in their distance (a) [32].…”

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

“…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].…”

Modelling of hydrogen reflection on tungsten fuzzy surface in an erosive hydrogen plasma

“…Studies on the hydrogen reflection coefficient from non-irradiated tungsten surfaces have been conducted widely through both simulations and experiments [2]. However, tungsten's surface morphology can be strongly modified in the intense radiation environment a These authors contributed equally to this work and should be considered co-first authors of fusion devices [3][4][5][6][7][8][9]. In particular, experiments performed in linear and tokamak devices with helium-containing plasma revealed that a fibrous tungsten nanostructure called 'fuzz' is formed [10][11][12][13][14][15] under the following conditions: surface temperature (1000-2000 K), incident helium ion energy (20-250 eV) and sufficient helium fluence (~10 25 m −2 > the incubation fluence of 1.5 ~4.0 × 10 24 m −2 ) [8,16,17].…”