Professor Felix Hofmann, Oxford Engineering Science

Tungsten armour for fusion reactors: Ion-implantation damage and its effect on material properties
When Nov 23, 2015
from 02:00 PM to 03:00 PM
Where LR8
Contact Name
Contact Phone 01865-273925
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http://hofmanngroup.org

Nuclear fusion has the potential to provide an environmentally friendly, sustainable energy source. A hurdle to the development of commercial fusion reactors is the availability of sufficiently resistant materials. Plasma-facing components will be exposed to high temperatures, intense neutron flux and bombardment with hydrogen and helium. Tungsten-based materials are the main candidates for divertor components, which will experience the harshest conditions. To assess the integrity of these components a detailed understanding of the influence of neutron irradiation and ion-implantation damage on material properties is essential. Unfortunately it is not yet possible to recreate the conditions that will be experienced by divertor components. Here we instead use ion-implantation as a proxy to study the interaction of injected helium with displacement damage. This approach avoids sample activation and allows large implanted doses to be reached quickly. However, due to limited ion penetration, implanted layers are only a few microns thick. Furthermore the defects generated are too small to be resolved by TEM and hence must be probed using alternative approaches.

 

In this talk I will discuss the use of synchrotron X-ray micro-diffraction to measure the lattice strains that arise due to helium-ion implantation in tungsten. By combining these measurements with density functional theory calculations we can elucidate the underlying defect microstructure, which appears to be dominated by helium-filled Frenkel pairs. Subtle changes in the elastic modulus of the implanted material, measured using the transient grating technique, are consistent with such a defect population. The transient grating method also allows the thermal transport properties of the implanted layer to be measured. We find that even a modest concentration of implanted helium leads to a substantial decrease in thermal diffusivity. Using a kinetic theory model this effect can be captured and shown to be consistent with the underlying defect microstructure. Importantly the changes in thermal properties are not a trivial function of implanted ion dose, but appear to depend on other factors, such as impurities in the material. Measurements of lattice strains in samples heat-treated after ion-implantation show significant evolution of the defect microstructure. Indeed they suggest that at elevated temperatures defects migrate deeper into the material bulk. Thus we can start to form a joined-up picture of ion-implantation-induced damage in tungsten and its diverse effects on mechanical and transport properties. This is a first step to assessing the anticipated evolution of properties in armor components of future fusion reactors.