Tungsten ‘fuzz’ growth re-examined: the dependence on ion fluence in non-erosive and erosive helium plasma

Abstract

The thickness x, of tungsten fuzz layers are measured for non-varying helium (He) plasma exposure conditions spanning four orders of ion fluence Φ 10 24 − 10 28 m-2 and flux Γ 10 19 − 10 23 m-2 s-1 , at 1000−1140 K under low energy He ion impact (50 − 80) eV. The data obtained are complemented by previously published data of similar growth conditions, and collectively analysed. The new analysis allows for the reconciliation of fast high flux growth with commonly observed slower growth at lower flux. It is demonstrated that the standing t 1/2 time dependence is a special case of a more general expression for determining the layer thickness, x(Φ) = (C(Φ − Φ 0)) 1 2 , that depends on Φ, an incubation fluence Φ 0 , and the growth constant C = 2.36 +1.54 −0.56 × 10-38 m 4 , which is temperature dependent. The incubation fluence, which must be exceeded before the observation on the onset of fuzz surface morphology is determined to be Φ 0 = 2.5 +1.5 −1.0 × 10 24 m −2. In fuzz growth-erosion regimes, characterized by an erosion constant fuzz , that is proportional to the sputter yield, an analytic solution for x(Φ) has been found, by solving the growth-erosion equilibria problem of prior work with the Lambert W function. Simple limit expressions follow from the solution for determining the equilibrium fluence and fuzz thickness; the predictions of such being in good agreement with previous fuzz growth-erosion equilibria results in the literature. * Values of P disch. pertain to maximum Γ. Values of Te and ne are conditions at mid-range Γ. † Calculated from ∼27 h (10 5 s) of exposure time.

Pulsation Effects of Incident Ion Energy on W Fuzz Growth

“…1 (a) A schematic of the temporal evolution of the incident ion energy and SEM micrographs of the samples exposed to the He plasmas at T s of (b) 1100 and (c) 1300 K. The irradiation condition was as follows: E L = 7 -8 eV and Φ H = 1.5 -1.8 × 10 25 m −2 and D = 10 %. [7]. Recent comparison of the thickness of fuzz layer grown in the gaps of castellated W in PISCES-A between experiments and particle simulation have suggested the accuracy of the value [8].…”

“…where C is a coefficient; following fit values were obtained [7]: C = 2.36 (1) using a contribution factor α. The red dotted line in Fig.…”

Morphology Changes of Platinum and Tungsten Carbide by He Plasma Irradiation

“…Figure 5 summarizes the relationship between the morphology changes and the irradiation conditions. When the surface temperature was lower than 1100 K and the flu-3406074-3 reported that the thickness of fuzz layer depended on square root of fluence, and the fluence had threshold, which was called incubation fluence, in order to form fuzz structure [18]. On W, nanostructures were formed on the entire surface at the fluence of ∼ 10 25 m −2 , which was about one order of magnitude lower than that of Pt [1,18]; also, the incubation fluence of Pt was likely to be about one order of magnitude higher than that of W.…”

Object kinetic Monte Carlo simulations of irradiated tungsten for nuclear fusion reactors

“…Recent experimental results reported in Ref. [42] indicate that fuzz growth following the square root dependence only starts after accumulating an incubation fluence. Although the experiments were carried out at different irradiation conditions (He ion energy, flux and temperature), the incubation fluence is qualitatively comparable to the simulation results presented in this Chapter.…”

“…In general, not only large He bubbles but also He trapped in few vacancies can cause microstructural changes [37,38], even at low energy. Indeed, void formation and underdense W nanostructures (so called "fuzz") have been observed in irradiation conditions similar to those that will have to face the divertor in MCF [39][40][41][42].…”