  Tungsten is a promising candidate material for plasma-facing armour
components in future fusion reactors. A key concern is irradiation-induced
degradation of its normally excellent thermal transport properties. In this
comprehensive study, thermal diffusivity degradation in ion-implanted tungsten
and its evolution from room temperature (RT) to 1073 K is considered. Five
samples were exposed to 20 MeV self-ions at RT to achieve damage levels ranging
from 3.2 x 10-4 to 3.2 displacements per atom (dpa). Transient grating
spectroscopy with insitu heating was then used to study thermal diffusivity
evolution as a function of temperature. Using a kinetic theory model, an
equivalent point defect density is estimated from the measured thermal
diffusivity. The results showed a prominent recovery of thermal diffusivity
between 450 K and 650 K, which coincides with the onset of mono-vacancy
mobility. After 1073 K annealing samples with initial damage of 3.2 x 10-3 dpa
or less recover close to the pristine value of thermal diffusivity. For doses
of 3.2 x 10-2 dpa or higher, on the other hand, a residual reduction in thermal
diffusivity remains even after 1073 K annealing. Transmission electron
microscopy reveals that this is associated with extended, irradiation-induced
dislocation structures that are retained after annealing. A sensitivity
analysis shows that thermal diffusivity provides an efficient tool for
assessing total defect content in tungsten up to 1000 K.
