Helium induced nanoscopic morphology on tungsten under fusion relevant plasma conditions

Abstract

Polished W discs exposed to pure He plasma in the PISCES-B linear-divertor-plasma simulator at 1120 and 1320 K are found to develop deeply nanostructured surface layers consisting of a conglomerate of amorphous ‘nanorods’. The growth of the thickness of the nanostructured layer is explored for exposure times spanning 300–(2.2 × 104) s in He plasmas of density n e ∼ 4 × 1018 m−3 and temperature T e ∼ 6–8 eV where the average He-ion surface-impact energy is ∼60 eV, below the threshold for physical sputtering. A nanostructured layer in excess of 5 µm thick is observed for the longest exposure time explored. The kinetics of the layer growth are found to follow Fick's law, characterized by an effective diffusive mechanism with coefficients of diffusion: D 1120 K = 6.6 ± 0.4 × 10−12 cm2 s−1 and D 1320 K = 2.0± 0.5 × 10−11 cm2 s−1. The diffusion of He atoms in W is considered too rapid to explain the observed growth of surface modification and points to the interplay of other mechanisms, such as the availability of thermal vacancies and/or the slower diffusion of He through the forming nanostructured layer.

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

“…A systematic study of W fuzz layers grown in PISCES linear plasma device has resulted in an empirical growth formula [1]. Using this t 1/2 -dependent growth rate formula from Baldwin et al [1] we can calculate the predicted W fuzz layer depth expected from the Alcator C-Mod exposure.…”

“…The growth of tungsten (W) nano-tendrils (or "fuzz") has been well documented in a wide range of linear plasma devices [1][2][3]. The growth conditions used for W fuzz in these devices are elevated surface temperatures (T s = 1000-2000 K) of a bare tungsten substrate while implanting with helium ions at sufficient incident energies (E He+ > 20 eV).…”

“…In future devices, there will be sections of the divertor that reach surface temperatures >1000 K and He ions will always be present in D-T burning devices, thus the growth of W fuzz in future devices is a possibility. The impact of W fuzz on plasmasurface interactions and tokamak operations is still largely unknown but is expected to be significant given that fuzz layers have been grown to large depths (>5 μm) [1] and therefore dramatically alter the plasma-surface interaction region. A major concern is that the growth of W fuzz will result in a large increase in the production of W dust via the mechanical failure of the individual nano-tendrils and, in turn, perhaps compromising the viability of a W divertor.…”

Molecular dynamics of single-particle impacts predicts phase diagrams for large scale pattern formation

“…Because the average helium ion energy is only ~60 eV and the threshold energy for sputter removal of tungsten is ~100 eV, this material has been considered impervious to the effects of erosion. However, despite a sputter erosion rate that is essentially zero, plasma-facing tungsten, nevertheless, develops under some conditions a nanoscale topography leading to surface degradation 8 -an outcome that appears mysterious within the erosion-based paradigm. The crater function-based analysis presented here suggests that such degradation may originate in impactinduced target atom redistribution effects.…”

“…5), have stimulated interest in self-organized pattern formation as a means of sub-lithographic nanofabrication 6 . On the other hand, extended exposure to energetic particle irradiation can lead to the structural degradation of fission and fusion reactor components 7,8 . Hence, it is important to be able to predict the expected behaviour within a given environment.…”