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

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 similar to 0.4x10-4 , and varied helium ion fluence from 0.18 to 3.4x1026 m-2. Fuzz thickness was measured by cross-sectional scanning electron microscopy to increase from 0.22 to 15 mu 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 similar to 3.4x1026 m-2, and varied W/He ion flux ratio from 0.26 to 3.0x10-4 . Fuzz thickness increased from 7.5 to 120 mu 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 0.18x1026 m-2, and W/He ion flux ratio of 2.2x10-4 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 104 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.