TEM observation of the growth process of helium nanobubbles on tungsten: Nanostructure formation mechanism

Comparison of tungsten nano-tendrils grown in Alcator C-Mod and linear plasma devices

“…This layer depth is in-line with the calculations from the Baldwin formula and proof that a significant layer of W fuzz can grow over a short timescale. It must be mentioned that the Baldwin formula is based on data only at surface temperatures of 1120 K and 1320 K. Some work has shown this formula to overestimate growth rates for T surf > 1400 K [3,12]. It has been suggested that a factor dependent on He ion energy might be missing from the formula [12] and that re-crystallization effects might be competing with the surface changes at higher temperatures [3].…”

“…It must be mentioned that the Baldwin formula is based on data only at surface temperatures of 1120 K and 1320 K. Some work has shown this formula to overestimate growth rates for T surf > 1400 K [3,12]. It has been suggested that a factor dependent on He ion energy might be missing from the formula [12] and that re-crystallization effects might be competing with the surface changes at higher temperatures [3]. Despite concerns about the formula raised in the literature, the comparison with the Alcator C-Mod results, which were also performed at T surf > 1400 K, and the Baldwin formula resulted in good agreement.…”

“…Despite concerns about the formula raised in the literature, the comparison with the Alcator C-Mod results, which were also performed at T surf > 1400 K, and the Baldwin formula resulted in good agreement. It is also worth noting that the data for the Baldwin formula [1] as well as from [3] and [12] were taken at steady-state surface temperatures over one continuous plasma exposure while the CMod exposures had transient surface temperatures and were the integration of 14 short discharges.…”

“…If we assume temperature of 1350 K as an example where we know nano-tendrils grow well, then we can estimate the equilibrium He pressure in the 10 nm diameter bubbles (see Fig. 5) at ~1 GPa using the same method as was used in [12]. From a simple ideal gas law calculation, we can estimate the density of the He in that void to be ~5.4 x 10 28 He/m 3 , or ~85% of the number density of the surrounding tungsten.…”

Grain size threshold for enhanced irradiation resistance in nanocrystalline and ultrafine tungsten

“…Over an ion dose threshold, this is considered to be the main phenomenon responsible for severe microstructural changes and the formation of fuzz (fiberform morphology) which occur at high CONTACT O. El-Atwani oelatwan25@gmail.com Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA ion doses [7]. If a major driver for these microstructural changes is indeed the high bubble densities in the grain matrices as has been suggested [8], it can then be hypothesized that the high grain boundary area in a nanocrystalline W microstructure will increase the ion dose threshold required to form these microstructures at temperatures where no grain growth (which changes sink strength) is possible.…”