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Retention and Surface Pore Formation in Helium Implanted Tungsten as a Fusion First Wall Material

S. J. Zenobia, G. L. Kulcinski

Fusion Science and Technology / Volume 56 / Number 1 / Pages 352-360

July 2009


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Single- and polycrystalline tungsten samples were implanted with 30 keV3He ions to fluences of 5e16, 4e17 and 5e18 He/cm2 at temperatures ranging from ~850 - 1000 °C. After implantation tungsten's retention characteristics were studied using 3He(d,p)4He nuclear reaction analysis (NRA) and 3He(n,p)T neutron depth profiling (NDP). Morphological analyses included scanning electron microscopy (SEM), focused ion beam (FIB) milling, and X-ray diffraction on the single crystalline W samples (XRD).

SEM analysis showed that the threshold forsurface pore formation occurs in both single-crystalline tungsten (SCW) and polycrystalline tungsten (PCW) between ~5e16 - 4e17 He+/cm2. Both surface and sub-surface pore formation is observed to increase with higher implant fluences. Focused ion beam (FIB) milling revealed a sub-surface porous layer in both SCW and PCW, which increased in depth with implanted fluences. NRA measured the retained He fluence in SCW between 1.1e16 - 1.1e17 He/cm2 and in PCW between 1.3e17 - 1.5e17 He/cm2. NDP analysis measured the retained He fluence in SCW between 2.0e16 - 2.7e17 He/cm2 and in PCW between 4.1e16 - 3.2e17 He/cm2. Both of these analysis techniques reveal that the retained helium saturates in both single and polycrystalline W at ~4e17 cm-2. The NDP analysis showed that the peak helium concentration shifted deeper into the specimens as the dose was increased, indicating a decrease in the effective density of the surface layer with an increased dose. Average retained helium concentrations were found to range from 0.7 - 8.6 at% in SCW and from 1.3 - 11.4 at% in PCW.

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