Probing damage in fusion reactor materials using laser-induced transient gratings

Tungsten is the material of choice for plasma facing components in future fusion reactors. During operation, these components receive extreme heat and radiation loads. The radiation and ion infusion from the plasma modify the tungsten lattice, altering its material properties such as the thermal diffusivity. Ion-implantation is a useful proxy for studying neutron irradiation and plasma exposure. However, it gives a thin damaged layer that is difficult to probe using conventional thermal transport measurement techniques.
Transient grating spectroscopy (TGS) is a novel tool for measuring the thermal diffusivity and surface acoustic wave (SAW) speed in thin ion-implanted layers. We present new developments of the TGS technique that enable the measurement of large area 2D maps of thermal diffusivity and SAW speed with targeted measurements and accommodating samples with increased roughness. These capabilities are demonstrated by recording large TGS maps of deuterium implanted tungsten, linear friction welded aerospace alloys and high entropy alloys. The results illustrate the ability to view grain microstructure in elastically anisotropic samples, and to detect anomalies, for example due to irradiation and previous measurements. They also point to the possibility of using TGS to quantify grain size at the surface of polycrystalline materials.
The thermal diffusivity and SAW speed in tungsten exposed to different fluences of deuterium plasma is studied. Scanning electron microscopy (SEM) shows the formation of surface blisters that have similar morphology for all fluences considered. A significant reduction in thermal diffusivity and SAW speed occurs as a result of plasma exposure. A saturation of the thermal diffusivity reduction with fluence is seen. Deuterium ion flux density appears to play a more important role in thermal diffusivity reduction than total fluence.
TGS is used to study the thermal diffusivity of tungsten exposed to different levels of 20 MeV self-ion irradiation. Damage as low as 3.2 x 10-4 displacements per atom (dpa) causes a measurable reduction in thermal diffusivity. Doses of 0.1 dpa and above, up to 10 dpa, give a degradation of ~55% from the pristine value at room temperature. Using a kinetic theory model, the density of irradiation-induced point defects is estimated based on the measured changes in thermal diffusivity as a function of dose. These predictions are compared with point defect and dislocation loop densities observed in transmission electron microscopy (TEM). Molecular dynamics (MD) predictions are combined with the TEM observations to estimate the density of point defects associated with defect clusters too small to be probed by TEM. When these “invisible” defects are accounted for, the total point defect density agrees well with that estimated from TGS for a range of doses spanning 3 orders of magnitude. Kinetic theory modelling is also used to estimate the thermal diffusivity degradation expected due to TEM-visible and invisible defects. Finely distributed invisible defects appear to play a much more important role in the thermal diffusivity reduction than larger TEM-visible dislocation loops. This work demonstrates the capability of TGS, in conjunction with kinetic theory models, to provide rapid, quantitative insight into defect densities and property evolution in irradiated materials.

Keywords: Transient grating spectroscopy
Self-ion Implantation
Advisor: Hofmann, F