  Tungsten surface enrichment of EUROFER steel by 250 eV deuterium sputtering
is in-situ measured using low energy He$^{+}$ ion scattering spectroscopy. The
samples are irradiated at various temperatures between 300 K and 800 K with a
deuterium atom flux of 2e18 m$^{-2}$s$^{-1}$ and maximum fluence up to 1.1e23
m$^{-2}$. The measurements at room temperature show a clear increase of
tungsten surface density, but already at 520 K the observed enrichment is only
half as large. At a temperature of 800 K no tungsten surface enrichment is
detectable. The obtained data allows to determine an upper limit of 1.6 eV for
the diffusion activation energy of tungsten in EUROFER.
