  Tungsten is the main candidate material for plasma-facing armour components
in future fusion reactors. Bombardment with energetic fusion neutrons causes
collision cascade damage and defect formation. Interaction of defects with
helium, produced by transmutation and injected from the plasma, modifies defect
retention and behaviour. Here we investigate the residual lattice strains
caused by different doses of helium-ion-implantation into tungsten and
tungsten-rhenium alloys. Energy and depth-resolved synchrotron X-ray
micro-diffraction uniquely permits the measurement of lattice strain with
sub-micron 3D spatial resolution and ~10-4 strain sensitivity. Increase of
helium dose from 300 appm to 3000 appm increases volumetric strain by only ~2.4
times, indicating that defect retention per injected helium atom is ~3 times
higher at low helium doses. This suggests that defect retention is not a simple
function of implanted helium dose, but strongly depends on material composition
and presence of impurities. Conversely, analysis of W-1wt% Re alloy samples and
of different crystal orientations shows that both the presence of rhenium, and
crystal orientation, have comparatively small effect on defect retention. These
insights are key for the design of armour components in future reactors where
it will be essential to account for irradiation-induced dimensional change when
predicting component lifetime and performance.
