Impact of helium nanobubbles on tungsten fuzz nanostructure conductivity

Abstract

Explosive electron emission splashes occurring in plasma interactions with nanostructured surfaces can be induced by Joule energy release under reduced conductivity. Distribution of helium nanobubbles inside tungsten at an elevated energy of implanted helium ions has been studied via numerical molecular dynamic modeling, and its impact on the nanostructure conductivity has been estimated. Average concentration and size of nanobubbles were calculated to be about n = 1020 cm−3 and r = 3 Å, respectively, at the helium fluence of about 2 × 1015 cm−2. The distribution of nanobubbles becomes more flat and extends deeper into the bulk material with increasing impact energy. At energies below about 300 eV, most nanobubbles form within a depth of 10 nm, corresponding to the characteristic size of the nanowires. At higher energies, a significant portion of the nanobubbles forms deeper in the material. An approach for estimating the electron scattering frequency in tungsten by helium nanobubbles has been proposed, taking into account the volume porosity of the W-fuzz nanostructure and scaling the helium content to the experimentally measured one. The resulting value for the tungsten fuzz nanostructure was 2.27 × 1016 s−1 and the resistivity was 1.92 mΩ cm that is 364 times larger than the normal tungsten resistivity, the conductivity of 521 S/cm was 0.275% from the normal tungsten one. The corresponding estimation of thermal conductivity via Wiedemann–Franz law gave 3.8 mW cm−1 K−1, which agrees well with recent measurements.