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Tungsten as a plasma-facing materials in fusion devices : behavior under helium high-temperature irradiation

par Caroline CHAMPENOIS - publié le

séminaire du laboratoire
vendredi 3 février 2017, 14:00 service 322 campus Saint-Jérôme

Élodie Bernard
IRFM, CEA Cadarache

abstract :
Choice of plasma-facing materials for next generation fusion machines, such as ITER and DEMO, is strongly related to the intensive fluxes of light elements, such as He and H isotopes, which the first wall materials will be subjected to. This irradiation can let to important damages at the surface, affecting the properties and life span of the materials, hence the efficiency of the reactor. For W, one of the most promising candidates, incident He particles can drastically affect the surface, with observed formation of dislocation loops, bubbles or W-fuzz.

One key parameter to examine in this aim is the material temperature : indeed, W operation temperature in fusion can reach up to 1000 ºC. Temperature affects vacancy and interstitial mobility in the material, and preliminary studies in laboratory highlighted that has a strong impact on the final micro structure of the material.

The WHIrr project focuses on the study of helium impact on tungsten and its consequences for the material properties, with various irradiation setups, and high temperature irradiation as the main parameter of interest. A dedicated temperature controlled material probe designed for the exposure of W samples to He plasma in the LHD (Large Helical Device) allows the study of the material micro structure change in a fusion machine, i.e. in a complex exposure spectra closer to the ones in future machines such as ITER and DEMO. Various laboratory devices from ion beam irradiation to plasma machine exposure completed the study with an extended range and more controlled conditions, hence providing reference samples to the complex LHD set of conditions.

TEM (Transmission Electron Microscopy) analysis was the main technique used to evaluate the impact of He irradiation under high temperatures on W microstructure ; in particular, bubbles were observed much deeper than the heavily damaged surface layer, rising concerns about the consequences for the material properties conservation. This technique was coupled with Positron Annealing Spectroscopy (PAS) to map the material defects at a lower scale and Grazing Incidence Small Angle X-ray Scattering (GISAXS) for bubble mapping at a larger scale. To investigate potential additional trapping due to the material change, hydrogen retention in pre He exposed W samples was investigated, notably through tritium gas loading and desorption.

contact : Thierry Angot

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