The dependence of tungsten fuzz layer thickness and porosity on tungsten deposition rate and helium ion fluence

Abstract

Fuzz formation on a heated tungsten surface in the presence of a helium-containing plasma and tungsten deposition source was investigated. Tungsten samples were exposed at 1123 K to pure helium plasma with ion incident energy of 76 eV, W/He ion flux ratio of ${\sim}0.4\times10^{-4}$ , and varied helium ion fluence from 0.18 to $3.4\times10^{26}$ m ^−2 . Fuzz thickness was measured by cross-sectional scanning electron microscopy to increase from 0.22 to 15 µ m with increasing helium ion fluence. No indication of saturation in fuzz thickness at high fluence was observed, in contrast to fuzz produced on a tungsten surface without tungsten deposition. Additional tungsten samples were exposed at 1123 K to pure helium plasma with ion incident energy of 76 eV, helium ion fluence of ${\sim}3.4\times10^{26}$ m ^−2 , and varied W/He ion flux ratio from 0.26 to $3.0\times10^{-4}$ . Fuzz thickness increased from 7.5 to 120 µ m with increasing W/He ion ratio. A final sample exposed at 1123 K to a mixed helium-deuterium plasma with ion incident energy of 76 eV, helium ion fluence of $0.18\times10^{26}$ m ^−2 , and W/He ion flux ratio of $2.2\times10^{-4}$ developed nearly identical fuzz structures to that developed in a pure He plasma. As a function of deposited tungsten fluence, all results were found to trace out a single layer-growth curve given by a power law relation, indicating that fuzz thickness is independent of the W/He ion flux ratio in the range investigated and independent of any deuterium present in the plasma. As a result, for tungsten plasma facing walls in magnetic fusion devices at 1000–2000 K with 10 ^−4 W/He ion flux ratio, fuzz with thicknesses greater than hundreds of microns may form in as little as 10 ^4 s (in the absence of ELM-induced erosion or annealing), and may more significantly affect its thermophysical properties than fuzz generated without a tungsten deposition source.

Keywords

• helium
• tungsten
• deposition
• fuzz
• large fiberform nanostructures
• plasma-material interactions